Transcript
Page 1: Swimming Pool Design & Build

Swimming Pools

Copyright 2000 Philip H Perkins

Page 2: Swimming Pool Design & Build

Swimming Pools

Fourth edition

Philip H Perkins

London and New York

Copyright 2000 Philip H Perkins

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This edition published in 2000 by E & FN Spon11 New Fetter Lane, London EC4P 4EE Simultaneously published in the USA and Canada by Routledge29 West 35th Street, New York, NY 10001 This edition published in the Taylor & Francis e-Library, 2003. E & FN Spon is an imprint of the Taylor & Francis Group First edition 1971Second edition 1978Third edition 1988 (Elsevier Applied Science Publishers Ltd) © 2000 Philip H Perkins

All rights reserved. No part of this book may be reprinted or reproduced or utilised in any formor by any electronic, mechanical, or other means, now known or hereafter invented, includingphotocopying and recording, or in any information storage or retrieval system, withoutpermission in writing from the publishers.

The publisher makes no representation, express or implied, with regard to the accuracy of theinformation contained in this book and cannot accept any legal responsibility or liability forany errors or omissions that may be made. British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data

A catalog record for this book has been requested ISBN 0-203-47788-X Master e-book ISBN ISBN 0-203-78612-2 (Adobe eReader Format)ISBN 0-419-23590-6 (Print Edition)

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Contents

Preface

1 The planning and layout of swimming pools

General considerations

1.1 Introduction1.2 Basic requirements for all swimming pools1.3 Pools for private houses, clubs, hotels and schools1.4 Covered pools for private houses, hotels, clubs and schools1.5 Teaching/learner pools1.6 Public swimming pools1.7 Floor gradients1.8 The drainage of walkways and wet areas1.9 Hydrotherapy pools1.10 Pools used for sub-aqua activities1.11 Facilities for the disabled 1.12 Swimming pools with movable floors1.13 Wave-making machines

Recommended procedure for getting a pool built: contracts anddealing with disputes

1.14 Introduction1.15 Contracts: how to proceed1.16 Dealing with disputes

Further reading

2 Basic characteristics of the materials used in the construction ofswimming pools2.1 Introduction2.2 Portland cements2.3 Aggregates from natural sources for concrete and mortar2.4 Admixtures

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2.5 Additions2.6 Water for mixing concrete, mortar and grout2.7 Steel reinforcement2.8 Spacers2.9 Non-ferrous metals2.10 Bimetallic corrosion2.11 Curing compounds for concrete and mortar2.12 Polymers2.13 Reactive resins2.14 Joint fillers2.15 Joint sealants2.16 Ceramic tiles2.17 British standards and euro codes

References

Further reading

3 Factors affecting the durability of reinforced concrete andcement-based materials used in the construction of swimming pools

3.1 Introduction3.2 Corrosion of steel reinforcement in concrete3.3 Carbonation of concrete3.4 Chloride-induced corrosion of reinforcement3.5 Deterioration of the concrete3.6 Chemical attack on cement-based mortar3.7 Swimming pool water and chemicals used in water treatment3.8 Moorland water and the Langelier Index3.9 Alkali-silica reaction

Further reading

4 Construction of swimming pool shells in insitu reinforced concrete

4.1 Introduction4.2 Site investigations4.3 Under-drainage of site4.4 Flotation (uplift) of the pool shell4.5 General comments on design and construction4.6 Concrete construction in cold weather4.7 Concrete construction in hot weather4.8 Plastic cracking4.9 Thermal contraction cracking4.10 Swimming pools with floor slabs supported on the ground4.11 Construction of the walls of the pool

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4.12 Construction of walkway slabs and floors of wet changingareas

4.13 Curing the concrete floor and walls of the pool4.14 Construction of suspended pool shells4.15 Thermal insulation of swimming pool shells4.16 Under-water lighting and under-water windows

Further reading

5 Construction of swimming pool shells in reinforced sprayedconcrete and other materials

Reinforced sprayed concrete (shotcrete)

5.1 Introduction5.2 Design and specification5.3 Methods of application5.4 Execution of the work5.5 Thermal insulation5.6 Pipework5.7 Testing for watertightness5.8 Under-water lighting

Swimming pools constructed with reinforced hollow concreteblock walls and insitu reinforced concrete floor

5.9 Introduction5.10 Construction of the floor5.11 Construction of the walls5.12 Pipework5.13 Under-water lighting5.14 Curing the concrete and protecting the blockwork5.15 Testing for watertightness5.16 Back-filling around the walls5.17 Thermal insulation

Sandwich type construction with insitu reinforced concretecore wall and concrete blocks as permanent form work

5.18 Introduction5.19 Construction of the floor5.20 Pipework5.21 Construction of the walls5.22 Under-water lighting5.23 Finishes to floor and walls5.24 Testing for watertightness5.25 Back-filling around the walls

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5.26 Thermal insulation

Other methods of construction

5.27 General comments5.28 Pools constructed with mass (gravity) type walls5.29 Curing the concrete5.30 Testing for watertightness5.31 Pools constructed in very stable ground such as chalk or rock5.32 Pools constructed of precast post-tensioned concrete units5.33 Pool shells of steel

Further reading

6 External works

6.1 General considerations6.2 Paving6.3 Surface water drainage6.4 Walling

Further reading

7 Finishing the pool shell and associated structures; problems withpool hall roofs

Finishing the pool shell and associated structures

7.1 Cement-sand rendering to insitu concrete walls7.2 Cement-sand rendering to sprayed concrete walls7.3 Cement-sand rendering to concrete block walls7.4 Cement-sand screeds on insitu concrete floors7.5 Cement-sand screeds on sprayed concrete floors7.6 Ceramic tiles and mosaic7.7 Walkways and wet changing areas7.8 Testing the completed tiling7.9 Marbelite7.10 Coatings and paints7.11 Sheet linings to swimming pools7.12 Glass-fibre polyester resin linings7.13 Finishes to walls of pool halls

The roofs of swimming pool halls

7.14 General considerations7.15 Pressurised roof voids7.16 The warm-deck roof

Further reading

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8 Water circulation and water treatment

Water circulation8.1 Flow-through pools8.2 Pools where the pool water is in continuous circulation8.3 Ducts for pipework

Water treatment

8.4 Layout of treatment plant8.5 Filtration and filters8.6 Chemical dosing of the pool water8.7 The disinfection of pool water8.8 Chlorination8.9 Ozone8.10 Bromine8.11 Chlorine dioxide8.12 Metallic ions (silver and copper)8.13 Ultra-violet radiation8.14 The base-exchange process for softening pool water8.15 Sulphates in swimming pool water

Further reading

9 Notes on heating swimming pools and energy conservation

9.1 Heating open-air swimming pools9.2 Heating the water in indoor swimming pools9.3 Heating and ventilation of pool halls and adjoining areas9.4 Solar heating of swimming pools

Further reading

10 Maintenance and repairs to swimming pools

Maintenance of swimming pools10.1 General considerations10.2 Routine supervision: smaller pools10.3 Shut-down periods10.4 Algal growths: prevention and removal10.5 Foot infections

Repairs to external works: paving

10.6 Remedial work to insitu concrete paving for pedestrians10.7 Remedial work to insitu concrete paving for light commercial

vehicles10.8 Remedial work for precast concrete flag paving

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10.9 Remedial work to precast concrete block paving10.10 Remedial work to clay pavers10.11 Remedial work to slippery paving10.12 Preventing trips and falls

Repairs to external works: walling

10.13 Remedial work to free-standing walls10.14 Remedial work to earth-retaining walls

Remedial work to pools under construction

10.15 General comments10.16 Remedial work to thermal contraction cracks10.17 Remedial work to drying shrinkage cracks10.18 Remedial work to honeycombed concrete10.19 Inadequate concrete cover to the reinforcement

Remedial work to existing pools: tracing leaks and investigations

10.20 Introduction10.21 Tracing leaks10.22 General investigations

Remedial work to existing pools: repairs following leak tracingand investigations

10.23 Remedial work to leakage10.24 Improving support to the pool floor10.25 Structural lining to the pool shell10.26 Remedial work to finishes

Further reading

Appendix 1 Conversion factors and coefficients

Appendix 2 Testing swimming pools shells, walkway slabs and otherwet areas for watertightness. Commissioningswimming pools

IntroductionTesting new poolsTesting existing poolsThe leakage test procedureGeneral comments on testingWatertightness test for walkway slabs and other wet areasCommissioning swimming pools (filling and emptying)

Appendix 3 Investigations, sampling and testing

General considerations

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Sampling and laboratory testingCover-meter survey

Appendix 4 The consultant/designer as an expert witness

IntroductionThe form of the Expert’s ReportThe expert witness and the Construction Act 1996

Appendix 5 Notes on safety in swimming pools

IntroductionWater depths for divingSigns for water depths in the poolOther safety signsOutlets for water in the pool floorWater slides and play equipmentSlipping and tripping on floors of walkways,changing rooms etc.Chemicals in water treatment

Appendix 6 List of organisations relevant to this book

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Preface

Since the third edition of this book was published in 1988 there have been nostartling changes in the materials used for the construction of swimming pools. Asimilar comment can be made about the design of reinforced concrete swimmingpool shells.

The number of swimming pools has continued to increase both in the publicand private sectors. This is particularly so with private club leisure centres whichoffer a wide range of activities.

There has been significant developments in the field of National Specificationsand Code of Practice relating to construction due to the intensive work on thepreparation of Euro Standards and Codes and the issue of Directives from theEEC. The latter set out minimum quality standards for a wide range of constructionalmaterials, and establish the responsibility of suppliers and designers.

Of particular importance are The Construction (Design and Management)Regulations 1994 which became completely effective in December 1995. TheseRegulations make people assess risks and take precautions rather than waiting todeal with problems when they occur. They target the health and safety of thosewho build, maintain, install and demolish buildings and plant.

The Construction Products Regulations came into force at the end of 1991 toimplement the Construction Products Directive. The potential scope of theRegulations is very wide indeed as they are applicable to all types of productwhich are intended for permanent incorporation in buildings and civil engineeringworks. The Regulations provide for the application of the European Communityregulatory mark—the CE mark—to construction products. The Building ResearchEstablishment Information Paper IP. 11/93 gives information on Ecolabelling ofbuilding materials and building products.

The British Standards Institution emphasise that the Kite Mark will continue toensure that the level of quality is above the minimum legal requirements.

Health and Safety Regulations have been extended and tightened up and thereis increasing awareness of the need for a more enlightened and professionalapproach to treatment of swimming pool water. The Committee which producedthe publications for the Department of the Environment on the purification ofswimming pool water is no longer in existence. It has virtually been replaced bythe independent Pool Water Treatment Advisory Group.

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It is important to observe recognised safety precautions when using certainmaterials, and also all types of plant and equipment.

Concrete itself is not a hazardous material; however, Portland cement whenmixed with water is highly alkaline (it has a pH of about 13.5) and is considered acaustic alkali. It can cause burns to the skin, particularly to people who are vulnerableto dermatitis. A safety warning is included as an Appendix in all British Standardsfor Portland cement. It recommends that precautions be taken to prevent dry cemententering the eyes, nose or mouth, and prevent skin contact with wet cement.

Polymer resins are now widely used in construction and there are hazardsassociated with the use of some of these compounds. Users should obtaininformation from the manufacturers and be aware of the requirements of thepublications of the Health and Safety Executive relating to the use of substanceshazardous to health.

The corrosion of steel reinforcement continues to be the number one cause ofdeterioration in reinforced concrete structures. Research Focus, No. 37, May 1999,states that: ‘Corrosion of reinforcing steel in concrete structures…is estimated tobe costing the UK £550 million a year. Many of these structures continue to requiremaintenance or replacement…’

It is therefore surprising that the protection of rebars by properly formulatedand applied epoxy resin coatings (see BS 7293 and ASTM Specification A775) isstill only used on a comparatively small scale in the UK.

The author acknowledges with gratitude the encouragement, and many usefulcomments, he has received from his wife. He also records the help he has beengiven by numerous people, organisations and firms, and in particular, David Butlerof the Sports Council, Andrew Alphick of the Pool Water Treatment AdvisoryGroup, Ralph Riley of the Institute of Baths and Recreation Management, GeoffreyRoberts and Jim Gordon of Buckingham Swimming Pools Ltd.

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Chapter 1

The planning and layout ofswimming pools

GENERAL CONSIDERATIONS

1.1 Introduction

In the United Kingdom, the construction of the shell of a swimming pool (withoutancillary buildings such as plant house, changing rooms etc.) is unlikely to requirea Building Permit under the Building Regulations, but planning permission maybe required. It is therefore advisable for any one wishing to build a swimming poolto consult their Local Authority, and also the water supply company as there maybe special requirements, such as metering of the supply, restriction on the amountof water used etc.

While there are regulations relating to swimming pools open to the public, thelegal control over the purity of water in pools for private houses, clubs and hotelsis minimal. Recommendations for the treatment and quality of swimming poolswater have been issued by the Pool Water Treatment Advisory Group (PWTAG),namely the Pool Water Treatment and Quality Standards. The PWTAG is anindependent body supported by all the organisations involved in the operation ofswimming pools.

In the United States, the position is different; for example in Californiaregulations are in force which apply to all swimming pools except private poolsmaintained by an individual for use by his family and friends. The regulationsspecifically apply to pools belonging to hotels, clubs, schools and healthestablishments. Important aspects of design, layout, operation and maintenanceare detailed and clear directions given. Requirements for the chemical andbacteriological quality of the water are included.

1.2 Basic requirements for all swimming pools

The recommendations given below are intended to apply to all swimming poolsconstructed of what may be termed ‘long-life’ materials such as concrete. 1. The pool shell (floor and walls) must be structurally sound.2. The shell must be watertight against loss of water when the pool is full or

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partially full, and if constructed below ground level, against infiltration ofground water when the pool is empty or partly empty.

3. The internal surface of the floor and walls must be finished with a smooth,reasonably impervious, easily cleaned, attractive material. The water must bemaintained at a proper standard of purity and clarity.

4. A walkway of adequate width (minimum about 1.5 m), with a non-slip, easilycleaned and durable surface should be provided around the pool.

5. A safety step (or ledge) should be provided on all the walls of pools used byyoung children and non-swimmers. This safety step should be located notmore than 900 mm (0.9 m) below top water level (Figure 1.1).

6. A diving board should not be provided unless the dimensions of the divingarea and the water depth comply with the recommendations of the AmateurSwimming Association (ASA). For pools used for international divingcompetitions, the regulations of the Federation Internationale de NatationAmateur (FINA) should be followed.

Figure 1.1 Sketch showing safety step.

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1.3 Pools for private houses, clubs, hotels andschools

1.3.1 Open-air pools: location

With pools in this category, there is generally a limited choice of location as theyusually have to be built on the same plot as the main building. An exception isschool pools as these may form part of sports ground facilities which are likely tobe some distance from the school.

For open-air pools for private houses, and hotels, the following points shouldreceive consideration. 1. A position should be selected which receives as much sun as possible,

particularly in the afternoon.2. The vicinity of large trees or potentially large trees should be avoided. Tree

roots can cause damage to foundations, and to drains and other pipelines.Leaves can cause discolouration of the pool water and staining of the poolfinish which is difficult to remove.

3. It is advantageous to utilise a natural wind-break, such as a thick hedge, gardenwall, or part of the main building, and if it does not exist, to provide one aspart of the landscaping.

4. The position of existing drainage, water supply, electricity and gas supplylines is important.

5. Depending on the method of construction of the pool (see Chapters 4 and 5),access for materials and plant required for the construction can be critical.

6. A small building (or room in the main building) will be needed for plant andequipment and storage of cleaning materials and the chemicals used for watertreatment.

7. It is desirable for the distance from the changing accommodation to the poolto be as short as practical bearing in mind the points mentioned above.

8. For private houses and hotels, landscaping of the area in which the pool is tobe located should be given careful thought and professional advice is usuallyworthwhile.

9. People often find it difficult to envisage from a two-dimensional sketch whatthe completed three-dimensional project will look like. The cost of a simplemodel and/or an isometric drawing could be justified.

Figures 1.2 and 1.3 illustrate alternative positions for a private pool.

1.3.2 The shape and dimensions of swimming pools

The shape and dimensions of a swimming pool are mutually interdependent. Theprimary use of the pool will be a major factor in determining both shape anddimensions.

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If the primary use is for training and swimming, then a rectangular shape isnormally chosen. The length should be a simple fraction of 100 m, and the widtha number of swimming lanes which are usually to be 2.0 m wide (ASA for 25 mpools).

The materials used in the construction of the pool shell will also influence itsshape. Pools constructed in insitu reinforced concrete can be of any shape, but thecost of a free-formed pool would be very high due to the cost of the formwork,compared with a pool constructed in sprayed concrete (shotcrete). But this costdifferential is influenced by the size of the pool, it being greater for smaller poolsthan for larger ones. The smaller domestic and hotel pools, constructed in sprayedreinforced concrete can be any shape, with little difference in cost betweenrectangular and free-formed.

Figure 1.2 Pool adjacent to building.

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Figure 1.3 Pool near boundary of plot.

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As these pools are likely to be used by children, non-swimmers and weakswimmers, the provision of a safety step around the pool at a depth not exceeding900 mm (0.9 m) below top water level is strongly recommended. This is a standardfeature of hotel pools in Switzerland (Figure 1.1).

1.3.3 Requirements for swimming

Even the smallest pool should be large enough for a swimmer to take severalstrokes; the minimum size would be about 6.00 m long by about 4.00 m widewith a minimum water depth of 1.00 m. However, a water depth of 1.00 m is notsufficient from a safety point of view for even a very flat dive. For generalcomfort, there should be an allowance of about 4.5 m2 for each person whowants to swim.

1.3.4 Requirements for diving

The depth of water and the dimensions of the diving area for competitive diving arecovered in the UK by the requirements of the ASA. For international events thesematters are covered by the world governing body, the Federation Internationale deNatation Amateur (FINA). There are minor differences between these two sets ofregulations but both provide adequate safety for diving in properly designed pools.The relevant publications of both organisations should be consulted and followed bythe designers of any swimming pool which is intended to include a diving board. Thedesigner should check and comply with the latest recommendations.

It is emphasised that the dimensions given are essential for safe diving from aposition not more than 1.00 m above the water level in the pool.

A natural question is ‘What about diving from the sides of the pool?’. Theonly form of dive recommended into shallow water from the pool sides is what isknown as a flat racing dive, which can only be safely executed by experiencedswimmers; even then the minimum depth of water is 1.50 m, which must be

Table 1.1 Examples of rectangular swimming pools for private houses, hotels, clubs andschools

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maintained forward for a distance of 7.6 m and the water level in the pool shouldnot be more than 0.38 m below the pool edge. These recommendations are givenby the courtesy of the Institute of Baths and Recreation Management.

Diving should not be permitted nor attempted into pools which do not meetthe above recommendations. Should an accident occur to a person diving into apool from a diving board which does not meet authoritative safetyrecommendations, the pool owner/manager may be faced with a claim thatwould be difficult to contest.

1.4 Covered pools for private houses, hotels,clubs and schools

There are obviously many advantages in having a covered swimming pool insteadof an open-air one. A covered pool can be used in comfort 365 days a year comparedwith the ‘season’ for an open-air pool of about 150 days. The conditions underwhich the pool has to operate are much less onerous; problems arising from freeze-thaw do not arise, staining of the walls and floor is much reduced, and discolourationof the water from leaves and air-borne dirt will be eliminated.

See Figures 1.5–1.6 for views of private house pools, and Figures 1.7–1.10 forviews of hotel, club, and school pools.

A major problem with covered pools is the occurrence of condensation on thewalls, windows and ceiling, and, depending on the method of construction, withinthe roof space.

The environment in the hall of a heated indoor swimming pool can be consideredas particularly hostile to many building materials; the air temperature is relativelyhigh—probably about 28 °C to 30 °C, and the relative humidity is also high, say,70–75%. The surfaces in contact with the air in the pool hall will generally have alower temperature than the temperature of the air in the hall; if the air is saturated

Figure 1.4 Section through 25 m pool with diving pit.

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Figure 1.6 Indoor private house pool. Courtesy, Buckingham Swimming Pools Ltd.

Figure 1.5 Pool with Roman end and steps and fully automatic cover. Courtesy, Buckingham Swimming Pools Ltd.

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Figure 1.8 Open-air pool at private club leisure centre. Courtesy, Buckingham SwimmingPools Ltd.

Figure 1.7 Indoor hotel deck-level pool with spa pool. Courtesy, Buckingham SwimmingPools Ltd.

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Figure 1.9 Indoor, 25 m school pool. Courtesy, Buckingham Swimming Pools Ltd.

Figure 1.10 Indoor hotel pool, Switzerland.

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with water vapour, condensation will occur on the contact surfaces. The temperatureat which condensation occurs is known as the dew point.

The design and detailing of the roof requires special attention and this is discussedbriefly in Chapter 7.

1.5 Teaching/learner pools

This section deals with general principles relating to layout and dimensions ofteaching pools irrespective of whether they belong to a school or form part of alarge swimming pool complex (leisure centre) run by a local authority.

The first principle is that the pool must be absolutely safe for non-swimmers. The pools are usually rectangular on plan with an almost levelbottom. The water depth generally varies from 0.80 m to 1.00 m. A useful sizeis 12.00 m by about 7.0 m.

There are often shallow steps into the pool extending the full length of theshort side.

There are different opinions as to whether the walkway around the pool shouldbe lower than the deck to enable the teacher to carry out his duties without havingto bend down, or whether the pool shell should be elevated similar to thehydrotherapy pool shown in Figure 1.15 and briefly described in Section 1.9.

In the UK, it is customary for the teaching pool to be quite separate from themain swimming pool so that the two different types of use do not interfere witheach other. If the teaching pool is in a separate enclosed part of the main building,it is usual for the temperature of the water and the air in the pool hall to be a fewdegrees above that in the main part of the building.

1.6 Public swimming pools

1.6.1 Introduction

In the UK and most countries in the temperate zone, all new large swimmingpools which are publicly owned are covered to enable them to be used throughoutthe year.

Table 1.2 Examples of dimensions of teaching pools

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There are a number of large open-air pools in the UK which are owned bylocal authorities, but these were built many years ago, generally before theSecond World War. These are only in operation for four or five months in theyear, usually from May to September. A few of these are heated. They arerectangular on plan and some contain sea water which contributes to a high rateof general deterioration.

In Europe, mainly in Switzerland and Germany, in spas, there are large open-air heated pools, some with wave-making machines.

Figures 1.11–1.13 are examples of pools in public leisure centres.

1.6.2 Location

It is not possible to lay down detailed rules for the location of publicswimming pools, but the following are matters which should receive carefulconsideration:

1. provision of adequate public transport;2. provision for adequate car parking;3. provision of public sewers (foul and surface water), water supply, electricity,

gas and telephone;4. adequate access for emergency services, fire brigade and ambulance;

Figure 1.11 View of part of pool at Bletchley Leisure Centre.

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Figure 1.12 View of pool in Rushcliffe Leisure Centre. Courtesy, British Cement Association.Photographer, T.Jones.

Figure 1.13 View of pool in Swansea Leisure Centre with wave machine in operation.Courtesy, British Cement Association. Photographer, T.Jones.

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5. sub-soil conditions including water table levels, presence of aggressivechemicals and presence of contaminated ground. This is of particularimportance in view of the alleged shortage in the UK of ‘good building land’,which in some cases exerts pressure to build on land-fill sites. TheEnvironmental Protection Act 1990 should be studied. Requirement C2 ofSchedule 1 of the Building Regulations 1991 states that ‘precautions shall betaken to avoid danger to health and safety caused by substances found on or inthe ground covered by the building…’ Approved Document C, 1992 edition,sets out detailed requirements for dealing with containments. Reference canalso be made to Section 4.2, and the list of Further Reading at the end of thischapter.

1.6.3 Types, shapes and dimensions

When the first edition of this book was published in 1971, the standard shape ofpublic swimming pools in the UK was rectangular or L-shaped. Some large poolshad two shallow ends. In L-shaped pools, the long leg can be used for swimmingand the short leg for diving.

However, with the advent of the leisure centre, the shape, size and use of poolshave changed considerably. Figures 1.11–1.13 show examples of public swimmingpools in leisure centres. In these centres, it is usual for the main pool to be free-formed and incorporate a sloping ‘beach’ and the installation of a wave-makingmachine which is switched on for relatively short periods several times a day.

It is emphasised that for competitive swimming, diving and aquatic sports, therequirements of the ASA (for national events) and FINA (for international events)must be fully complied with. The requirements mentioned in this book are only afew of the very detailed requirements laid down by these two organisations. Thefollowing are examples of some of these requirements: 1. For competitive swimming (national events), the water depth in front of the

starting blocks must not be less than 1.80 m and this must extend forward fora distance of 6.00 m.

2. Stairs and steps must be accommodated outside the pool dimensions, i.e. theymust be recessed.

3. For water polo, the minimum depth of water over the whole playing area mustnot be less than 1.80 m; the playing area must not exceed 30 m× 20 m andmust not be less than 20 m×8 m.

4. For life saving certificates, a water depth of 2.0 m is required and this mustextend for a length of 6.00 m over the full width of the pool.

The provision of a diving pit as part of the main pool is deprecated as diving intoa pool in which persons are swimming is unpleasant and can be dangerous. Thereare many advantages in having a separate diving pit which is used only for diving.

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By having a separate diving pit, swimming and diving events can be heldsimultaneously, and the same applies to training.

Pools intended for competitive swimming are normally 25 m or 50 m long witha width based on a number of swimming lanes, either 2.0 m for national events or2.5 m for international events. Reference must always be made to the latest editionof the relevant regulations.

1.7 Floor gradients

1.7.1 The pool

The floor of the pool must be laid with a fall (slope) towards the outlet with such agradient that the pool can be effectively emptied. However, the gradient should notbe so steep that non-swimmers and learners can lose their balance and/or slip. It isgenerally considered that the depth of water at which boyancy is likely to affect aperson’s balance is about 0.75 of the person’s height. The steeper the gradient, thesooner a person will reach the point of over-balance. As the point of over-balancevaries with the height of the person, it is suggested that the maximum gradient forthe floor of a pool used by children and non-swimmer/learners should be 1 in 40(25 mm in 1.00 m).

For efficient emptying of the pool, hydraulic considerations require a gradientof about 1 in 80 (25 mm in 2.00 m). The gradient should be uniform betweenclearly marked locations and depth markers on all walls are essential. A furthersafety precaution is to provide non-slip tiles on the floor.

The above comments on gradient do not apply to the steep slope down to adiving pit as shown in Figure 1.4.

Water outlets in the floor at the deep end of the pool should be fitted with smallaperture gratings. See Section 8.2.6.

1.7.2 Walkways and wet areas

In this context ‘wet areas’ include all those areas, such as changing areas, walkwaysaround the pool, shower cubicals etc., which are made constantly wet by poolusers and by cleaners. It is in these areas that injuries resulting from slipping aremost likely to occur. There is a conflict between the need for a non-slip (or slipresistant) surface and the need for easy cleaning and efficient drainage (run-off).At the time of writing, there does not appear to be any formal and recognisedgradients for floors in these areas.

There is also the problem of ponding. To avoid ponding, a gradient of about 1in 40 (25 mm in 1.00 m) is normally required, but for safety a gradient of 1 in 60(25 mm in 1.50 m) is probably needed. The frictional characteristics of the finishedsurface when in contact with the bare feet of pool users are relevant. It should benoted that the gradient suggested here is considerably less than for the floor of thepool in Section 1.7.1.

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1.8 The drainage of walkways and wet areas

Where walkways and wet areas form part of a covered pool complex, thedrainage of these areas should not discharge into the water circulation systemof the pool but should be connected to the main drainage system of thebuilding.

1.8.1 General comments

In some cases the public sewers are designed on the ‘separate’ system in whichsurface water is carried in surface water sewers, but in other areas, the seweragesystem is ‘combined’ and the foul and surface water is carried in the same sewer.It is important that an adequate number of inspection chambers/man-holes shouldbe provided and drainage system laid to gradients which will provide a self-cleansingvelocity.

With open-air pools, the surrounding paving should slope away from the pool.

1.9 Hydrotherapy pools

The advantages of carrying out special exercises under water have been known tothe medical profession for many years; the weight of the body is reduced by theweight of water displaced and thus movements are made much easier with lessmuscular effort.

While spas in the UK have declined in popularity, this has not happened on thecontinent of Europe. There, special health resorts, with names starting with ‘Bad’in Germany, Switzerland and Austria, continue to flourish and attract large numbersof visitors/patients. The spas are mainly situated in beautiful country and make avery pleasant location for a holiday.

Many of the special baths and swimming pools contain naturally heated highlymineralised water from springs, which in some places is slightly radio-active. Thereare often a number of pools which operate at different temperatures and possesstherapeutic properties. Under-water massage by powerful jets located at differentdepths below the water surface is a common feature of many of these pools. Thelength of stay in the pool is strictly limited.

Figure 1.14 shows a pool at a spa in Switzerland.Figure 1.15 shows a hydrotherapy pool at a school for pupils with severe

learning difficulties. The pool is 10 m×5.00 m and the depth varies from 0.90m to 1.50 m. It is a deck level pool and the water temperature is maintainedat 32°C.

In the UK there are many therapeutic pools but these are mainly attached tohospitals, recuperation homes and similar institutions and are used for treatmentprescribed by a physician. They seldom form part of a holiday resort.

Special features to be taken into account in the design of such pools include thefollowing:

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Figure 1.14 Open-air pool with wave machine in operation Bad Vals, Graubunden,Switzerland.

Figure 1.15 Hydrotherapy pool in school for pupils with severe learning difficulties. Courtesy,Buckingham Swimming Pools Ltd.

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1. The floor should have a flat gradient of about 1 in 80 (25 mm in 2.00 m)which should be adequate for emptying.

2. The floor of the pool and all wet areas should be finished with non-slip ceramictiles or ceramic mosaic.

3. If the pool is not deck-level, then glazed ceramic scum channels should beprovided as these are more efficient in providing good water circulation thanskimmer outlets.

4. The turn-over period (the time required to completely circulate all the waterin the pool) should not exceed 3 hours; some pools operate on a 1½hour cycle.See Section 8.2.1.

5. The water temperature and the air temperature in the pool hall and changingareas should be maintained at a higher temperature than in normal swimingpools. A water temperature of 30–32°C and an air temperature of 33 °C isadopted in many pools.

6. All fittings should be corrosion resistant (austenitic stainless steel or phosphor-bronze).

7. As these pools will certainly be used by disabled persons, special arrangementsshould be included to enable such persons to enter and leave the pool easily;see also Section 1.11.

8. If the pool contains saline water, then a detailed chemical analysis shouldbe obtained, including information on any variations in the concentrationand type of dissolved salts. This is essential in order to decide whetherspecial protective measures are needed for the pool shell, finishes andfittings.

1.10 Pools used for sub-aqua activities

Sub-aqua activities have become very popular in all parts of the world.Training in the sea, lakes and rivers in the UK and other countries in the temperate

zone is often difficult owing to low temperatures, low visibility, currents etc. Thus,there are many advantages in carrying out training in a swimming pool. The BritishSub-Aqua Club (BSAC) requires that every beginner should receive basic trainingin a swimming pool.

The use of public swimming pools is not always permitted by Baths Managersowing to interference with public use of the pool and possible damage to the finishof the pool and walkways by the divers equipment. With reasonable care, theequipment used by the Club’s members should not cause damage if the finishesare high quality ceramic tiles or ceramic mosaic.

In any event, these finishes require maintenance and repair in the course oftime. Small damaged areas can be repaired under-water which eliminates the needto lower the water level or empty the pool.

The BSAC have published a booklet giving detailed information on allaspects of aqualung diving—see Further Reading at the end of this chapter.

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The BASC Code of Conduct contains some 21 directions, including emphasison the prohibition of dropping heavy equipment in the pool and anywhere inthe pool premises.

As far as the pool itself is concerned, the requirements for sub-aqua activitiesare very modest. The minimum dimensions required for a group lesson are 3.60m×5.00 m, with a minimum depth of water over this area of 1.50 m. Theserequirements can be increased with advantage, with special reference to waterdepth to 3.50 m and if possible 5.50 m.

Designers should contact the BSAC for their latest recommendations. Figure1.16 shows sub-aqua training.

1.11 Facilities for the disabled

The absolute need to provide satisfactory arrangements for disabled persons to usepublic swimming pools is now recognised.

The specification and design of the necessary facilities require special studyat the design stage as it can be difficult and costly to provide these facilities at alater date.

Work in this field is done by a number of organisations and reference should be

Figure 1.16 Aqualung training in public pool. Courtesy, British Sub-Aqua Club.Photographer, T.Jones.

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made to BS 5810 1979 Code of Practice for Access for the Disabled to Buildingsand to the publications of the Thistle Foundation.

1.12 Swimming pools with movable floors

The desirability of having separate pools for swimming, diving and teaching hasbeen mentioned earlier in this chapter. Such separation entails additional capitalinvestment and increased operating and maintenance costs; also, the teaching anddiving facilities are only used from time to time.

This led to the development of hydraulically operated movable pool floors andseparating walls. The depth of water can be reduced over part of the pool by raisinga section of the floor thus forming a teaching/learner area.

This feature has proved more popular in Europe than in the UK where thenumber of public pools with movable floors is small and very few have beenconstructed in recent years. Figure 1.17 shows a movable floor in the raisedposition.

Figure 1.17 View of movable floor in public pool. Courtesy, Buckingham Swimming PoolsLtd.

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1.13 Wave-making machines

1.13.1 Introduction

The provision of equipment which generates artificial waves in swimming poolshas become increasingly popular in recent years. When the first edition of thisbook was published in 1971, there was only one pool with wave-making equipmentin operation in the UK, namely the large open-air Portobello pool at Edinburgh,which was installed in 1936.

During the past 25 years, wave-making equipment has been installed in manynew leisure centre pools.

The wave-making machines are usually switched on at stated times for about15–20 minutes.

There are several methods of creating artificial waves in swimming pools, themain methods being by (1) swing arm equipment, and (2) compressed air.

1.13.2 Swing arm equipment

The makers usually make a model of the pool so that they can assess all importanthydraulic features, such as wave height, location of ‘breaking’ point, backwash,cross currents etc. The shape and sloping floor create the effect of a sloping beachwith the waves breaking naturally. The shape also provides adequate area of shallowwater for non-swimmers.

The wave-making equipment consists of two swing arms which operate togetherbut not in complete unison. For example, one arm oscilates at 17.5 oscillations perminute and the other at 18.0 oscillations per minute. A specially designed screen isprovided in front of the wings.

1.13.3 Compressed air equipment

There are a number of patented systems using compressed air to create artificialwaves.

The creation of waves of the desired height and distance from crest to crest(wave-length) is not a simple matter and all relevant factors must be taken intoaccount. This usually includes the making of a scale model.

Figures 1.13 and 1.14 show pools with a wave machine in operation.

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RECOMMENDED PROCEDURE FOR GETTING APOOL BUILT: CONTRACTS AND DEALING WITHDISPUTES

1.14 Introduction

The recommendations which follow are intended mainly for private persons,club committees and owners of small hotels, although it is hoped that evenlarge hotel groups and local authorities will find some of the points mentioneduseful.

1.15 Contracts: how to proceed

It is not recommended that a swimming pool should be built on a do-it-yourselfbasis. Although there is a theoretical saving in capital cost the dividing linebetween success and failure is a very narrow one and the cost saving does notjustify the risk.

The two procedures recommended are: 1. To engage a qualified professional person with proven experience in swimming

pool design and construction who will take responsibility for the preparationof the design, drawings, specification and other contract documents, andobtaining all necessary permits. The consultant should recommend a list of,say, three contractors, send out the invitations to tender, recommend to theclient the adjudication of the contract, certify the contractor’s accounts, andinspect the work at appropriate stages.

The consultant should be a Chartered Civil or Structural Engineer orChartered Architect. In the case of large contracts for public swimming pools/leisure centres, there will be several professional firms involved responsiblefor structural and civil design, heating and ventilating, electrical and mechanicaland architectural, and quantity surveyors; the Architect is usually the head ofthe team.

2. To employ a consultant to advise on the selection of a suitable ‘package deal’contractor. The names and addresses of swimming pool contractors can beobtained from the Swimming Pool and Allied Trades Association. Theconsultant, in discussion with the client (referred to as the Employer in thecontract), should prepare a clear brief setting out the requirements (see Section1.2). It is important that the contractors should submit a list of recentlycompleted pools; these should be checked by site visits by the consultant,with the client.

The financial standing of the selected contractors should also be checked,and information obtained on the extent the contractors employ sub-contractors.A test for watertightness should be clearly described and included in the

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contract; see Appendix 2 at the end of this book for details of this test.The consultant’s brief should also include for periodic site inspections to

help ensure that the contractor is carrying out the work in accordance with thecontract.

1.15.1 Insurance-backed guarantees and warrantees

In recent years there have appeared on the market ‘insurance-backed guarantees’.These are offered by contractors/sub-contractors and material suppliers claimingthat should the work prove defective, then the insurance company will provide thefunds to have remedial work put in hand in the event of the contractor/supplierfailing to do so. This suggests that the client will avoid the necessity of legal actionto obtain redress. These ‘guarantees’, which are sometimes referred to as‘warrantees’, are stated to be valid for periods of 10–20 years from the completionof the work.

A careful scrutiny of these guarantee/warrantees will usually reveal that theycontain many anomalies and uncertainties. Such documents should be examinedby a solicitor experienced in that particular field. A consultant would be unwise torecommend reliance on such a guarantee without first taking competent legal advice.

1.16 Dealing with disputes

The above may appear to be exaggerated, but experience suggests that caution andattention to detail is the best approach. Irrespective of which procedure is adopted,if things do go wrong, such as work unduly delayed, poor workmanship, the use ofsub-standard materials, the failure of the pool to pass the leakage test etc., thebuilding owner will find he is faced with the following limited choice: 1. He can accept the situation, which he would be most unwilling to do, or2. He can instruct the contractor to put things right, in accordance with the terms

of the contract, and if he fails to do so he can follow the procedure laid downin the Conditions of Contract.

If the faults are serious, it is unlikely that even after completion of the remedialwork, the finished job will be as satisfactory as if it had been done properly thefirst time.

One of the worst things that can happen is for the contractor to go intoliquidation during the contract. The cost of employing another contractor tocomplete the project will be very high and the chance of obtaining financialcompensation from the original contractor is extremely small. This is why it isimportant for the consultant not to feel obliged to recommend the acceptanceof the lowest tender even though the tenders are from a list which he hasdrawn up. There can be many reasons why a contractor will submit an

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exceptionally low price. The client would be unwise to disregard hisconsultant’s advice in this matter.

The value of an experienced consultant, a properly drawn-up contract, and carein the selection of the contractor cannot be over emphasised.

1.16.1 General comments

The method of dealing with disputes which may arise during or aftercompletion of a contract will depend mainly on whether the contract comeswithin the scope of the Housing, Grants, Construction and Regeneration Act1996 which came into force on 1 May 1998 (known as the ConstructionAct 1996).

If the contract falls outside the scope of the Act then dealing with disputeswould follow established procedure. However if the contract falls within theAct, entirely new procedures would have to be followed. Unfortunately, thewording of the Act in a number of important matters is lacking in clarity andat the time of writing this book, there is little reported experience in theoperation of the Act.

The Act covers a very wide field, but the construction of a private swimmingpool is likely to be outside the Act as it would most probably come under theexemption given to residential contracts.

The coming into force of the Construction Act has necessitated the revision ofthe Standard Forms of Building Contract and Sub-Contract, and the ICE Conditionsof Contract.

The February 1999 issue of Construction Briefing issued by Merricks, Solicitors,London, states:

‘There has been a fundamental review of the legal system in this country overthe last three years. A review carried out by Lord Woolf (“Access to Justice”) willculminate this year in major changes to come into effect on 26 April 1999 whichwill affect the speed and cost of legal proceedings.

The changes can be divided into three areas: (a) The restriction of legal aid;(b) The expansion of contingency fee arrangements (‘No win, No fee’) for all

proceedings except crime and family;(c) Fundamental changes to court proceedings which should decrease costs and

increase speed. In the main, all Personal Injury work will be excluded from the new Legal Aidstructure…’ Except in certain cases, ‘the case will be dealt by another route, e.g. aconditional fee arrangement, or via alternative dispute resolution.’ (This change indealing with personal injury cases is likely to affect claims arising from accidentsin swimming pools.)

‘Even before litigation starts there will be changes in the manner in which

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parties conduct themselves. There will be a series of preaction protocolsdealing with various categories of litigation…There is a single objective to thenew court rules—to enable the court to deal with the cases justly. Lord Woolffelt that the two evils of modern litigation were delay and disproportionatecosts…

Expert evidence will be more restricted and may not be adduced without leaveof the court. Normally experts will not be allowed to give oral evidence but willprovide a written report and answers to written questions put to them by the opposingparty…’

There will be considerable control of costs. ‘Prior to taking interlocutory stepsdetails of the costs must be given to the other side…’

All the above changes face the test of practical use and there may well be furtherchanges in the light of experience.

1.16.2 Notes on procedure for contracts outside theconstruction Act 1996

As stated in Section 1.16.1 above, the situation may arise where the owner isfaced with the choice of accepting an unsatisfactory swimming pool or takinglegal action against the contractor, and/or the consultant. A Solicitor experiencedin construction disputes would be able to advise the client on the appropriateprocedure. The action to be taken is generally laid down in the GeneralConditions of Contract.

Action against the consultant could arise if it was considered by the employer’sSolicitor that he had been guilty of professional negligence.

The majority of construction contracts (prior to the 1996 Act), contain aprovision for referring disputes as a last resort to arbitration. But subject tocertain conditions, a party can apply to the Court to have the matter settled byCourt action.

It is important to remember that Arbitration can be more expensive than Courtaction as the Arbitrator has to be paid (Arbitrators fees are high), and payment hasto be made for the hire of the arbitration room. Costs usually ‘follow the event’which means that the losing party may have to bear his own legal costs and thoseof the other party. Court judgments can be quite surprising. This, together with thehigh cost of litigation, is no doubt why so many disputes are settled out of Court(about 75–80%).

In the event of Arbitration or Court action, the employer would beadvised by his Solicitor to engage a professional person to act as an ExpertWitness. Some information of the duties of an Expert Witness are given inAppendix 4.

In Court proceedings, difficult technical considerations can arise if adefence of ‘Limitation’ is put forward. Such a defence is only likely to arisesome years after the completion of the pool. A defence of limitation would

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involve the Limitation Act 1980 and the Latent Damage Act 1984, and theexpert witness would be asked for his opinion on the following two issues:

1. When did significant damage first occur?2. What was the earliest date on which the Plaintiff had both the knowledge

required to bring an action for damages in respect of the relevant damage andthe right to bring such action?

Such questions give rise to very complex technical considerations to which thereis unlikely to be a clear-cut technical answer.

Due to the enormous cost of High Court actions and the very considerabledelay which occurs between the time of the issue of the Writ and the handingdown of the judgment, proposals have been made in recent years to find alternativemethods of settling disputes. This is generally known as ‘Alternative DisputeResolution (ADR)’.

While the majority of cases in the Official Referee’s court and in arbitrationsettle before trial, few do so early enough to avoid the substantial costs incurred inthe preparations leading to trial.

The essence of ADR is to create a framework in which the parties involved in adispute can reach a solution for themselves. This usually requires the assistance ofa neutral third party.

There are a number of ADR techniques which include:

Conciliation;Mediation;Mini-trial;Expert fact finding and adjudication.

The success of ADR depends entirely on the willingness of all parties to resolvetheir dispute in a mutually satisfactory way, and this requires considerable giveand take. Some references on ADR are given under Further Reading at the end ofthis chapter.

1.16.3 Notes on procedure under the construction Act1996

The comments which follow are intended to supplement those made inSection 1.16.1.

The Act gives parties to a construction contract the right to refer a disputearising under the contract to adjudication in accordance with a clearly definedprocedure. The procedure is intended to provide a fast-track method ofresolving disputes. A party to a construction contract has a right, but not anobligation, to refer a dispute for adjudication under the procedure laid down inthe Act.

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The Adjudicator must then be appointed within seven days of the issue of anotice by the party wishing for adjudication. The Adjudicator must reach adecision on the dispute within 28 days of referral, unless both parties agree toan extension of time. With the consent of the referring party, the Adjudicatormay extend the 28–day period by up to 14 days.

The decisions of the Adjudicator are binding until the dispute is finallydetermined by legal proceedings or by arbitration or by agreement between theparties. The orders of the Adjudicator must be complied with and they are bindinguntil the dispute is finally determined.

The Act contains numerous new concepts and conditions and anyoneintending to have work carried out by contract after 1 May 1998 shouldseek legal advice on whether the contract will come within the scope ofthe Act.

Reference can usefully be made to the publication Construction Briefings,issued by Merricks, Solicitors, Chelmsford and London, and Notes of aSeminar on the Construction Act, given by Lawrence Graham, Solicitors,London.

Further reading

General

British Sub-Aqua Club. Pools for Sub-aqua Use.Cottam, G. Adjudication under the Scheme for Construction Contracts. Thomas Telford,

London, 1998.Department of the Environment. Building Regulations (Amendment) Regulations

1998, S.I. 2561—Revision to Part M, Access and Facilities for DisabledPeople.

Institute of Baths and Recreation Management. Practical Leisure CentreManagement, Vol. 2.

Institute of Baths and Recreation Management. Diving in Swimming Pools.International Board for Aquatic, Sports and Recreation Facilities. International Standards

Swimming Pools: Part B, Construction, Finish and Equipment, 1977.Sports Council. Safety in Swimming Pools, 1998.State of California, Department of Public Health. Laws and Regulations Relating to

Swimming Pools, excerpts from the California Health and Safety Code and theCalifornia Administrative Code.

Swimming Pools and Allied Trades Association. Swimming Pool Guide, 1995.

Construction Act 1996

Merricks, Solicitors. Construction Briefing—The Housing Grants, Construction andRegeneration Act 1996, Merricks, Chelmsford and London, May 1998.

Lawrence Graham, Solicitors. Four papers at a seminar on The Housing Grants,Construction and Regeneration Act 1996, Lawrence Graham, London, July 1998.

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Health and safety and environmental lawFink, S. Health and Safety Law for the Construction Industry, Thomas Telford, London,

1997.Health and Safety Executive. Slips and Trips HS (G) 155.Stubbs, A. Environmental Law in the Construction Industry, Thomas Telford,

London, 1998.

Alternative dispute resolutionMcKenna and Company, Solicitors. Law Letter, Autumn/Winter 1989, McKenna, London,

pp. 14–15.McKenna and Company, Solicitors. Alternative dispute resolution, Litigation Update, May

1995, McKenna, London, pp. 6–8.Hollands, D.E. Alternative dispute resolution, Journal CIArb, February 1992, pp. 57–9.Grove, J.B. The role of arbitration in an ADR environment, Journal CIArb, November 1997,

pp. 244–5.

The expert witnessNewman, P. Professional liability of expert witnesses, Journal CIArb, August 1993, pp.

173–81.Lord Taylor. The Lund lecture—The expert witness, Journal CIArb, May 1995, pp. 113–17.The Times, Law Report: 6 Oct. 1999: Court of Appeal Judgement 27 July 1999; Stevens v

Gullis (Pile third party).

EEC construction legislationDepartment of the Environment. Construction products directive, Euronews, Construction ,

Special supplement, September 1991.Kay, T. and Wyatt, B. European Standards for protection and repair, J Concrete, September

1997, pp. 11–17.Taylor Joynson Garrett, Solicitors. The Construction (Design and Management) Regulations,

1994, Construction Review, Issue No. 1, 1995.

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Chapter 2

Basic characteristics of thematerials used in theconstruction of swimmingpools

2.1 Introduction

The objective of this chapter is to provide information on the principal materialsused in the construction of swimming pools, including those used in external worksdescribed in Chapter 6. The materials are:

Portland cements;Aggregates from natural sources for concrete and mortar;Admixtures;Additions;Water for mixing the concrete/mortar;Steel reinforcement including stainless steel;Spacers;Non-ferrous metals;Curing compounds;Polymers and reactive resins;Joint fillers and joint sealants;Ceramic tiles;Notes on bimetallic corrosion;Notes on British Standards and Euro Codes.

The information given in this chapter is intended to be of a general nature andspecifiers and users should always refer to the latest edition of the relevant NationalStandard.

Work is continuing at the British Standards Institution on the revision of existingStandards and Codes and the production of new Euro Standards and Codes. It istherefore essential that anyone wishing to incorporate into a contract requirementsfor compliance with British Standards should ensure that they are still valid and havenot been replaced by a Euro Standard. Reference can also be made to BRE Digest397, September 1994: Standardisation in Support of European Legislation.

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2.2 Portland cements

It is made by burning at high temperature a mixture of chalk and clay in a rotarykiln. The clinker is ground and gypsum (calcium sulphate) is added to control theset. British Standard BS 12 limits the amount of sulphur (expressed as SO

3) to

3.5%. The fact that Portland cement contains sulphate is important wheninvestigating the possibility of sulphate attack on concrete or mortar which isdiscussed in Chapter 3.

The addition of water to the cement results in a complex reaction accompaniedby the evolution of heat.

Revised British Standards for cements were published in 1996. The newdesignations for Portland cements likely to be used for the construction of swimmingpools and external works are as follows:

Portland cement class 42.5 to BS 12 1996 (CEM 1);Portland cement class 52.5 to BS 12 1996;Portland cement class 42.5R to BS 12 1996;Sulphate-resisting Portland cement class 42.5 to BS 4027 1996;Portland Masonry cement to BS 5224 1995, ENV 413.1.

The letter R denotes high early strength.The revisions were mainly concerned with methods of test and terminology

and were intended to agree with the European Standard for cement, ENV 197–1.Minor changes in composition were also introduced.

In the early 1990s a complete and major revision was carried out to BS 5328Concrete, and this was issued in four parts:

Part 1 Guide to Specifying Concrete 1995;Part 2 Methods for Specifying Concrete Mixes 1991;Part 3 Specification for the Procedures to be Used in

Producingand Transporting Concrete 1990;Part 4 Specification for the Procedures to be Used in Sampling,

Testing and Assessing Compliance of Concrete 1990.

In 1993, BSI issued a Published Document PD 6534 1993 Guide to the Use in theUK of DD ENV 206 1992.

The principal characteristics of Portland cement are: 1. A very fine powder, particle size 1–50 microns (1 micron equals 0.0001 mm).2. The paste (cement and water) is highly alkaline, having a pH of about 13.5.

The high alkalinity is relevant to the protection of steel reinforcement, andalso to the occurrence of alkali-aggregate reaction. The interaction

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betweenalkalis in the cement and certain types of silicious aggregates isdiscussed in Section 3.9.

3. The setting time (initial and final) is in the range of 45 minutes to 10 hours.4. The compounds which are principally responsible for the cementing action of

the cement paste are mainly the calcium silicates (the C2S and the C3S).5. It is the hydration products of the cement which, other things being equal,

determine the strength of the concrete/mortar. The hydration products are verycomplex chemical compounds. The principal compounds are calcium silicategel, calcium hydroxide (about 20%) and tricalcium aluminate hydrate. Thecalcium hydroxide (Ca(OH)

2) is liberated by the hydrolysis of the calcium

silicates. The various hydration products hydrate at different rates, but thehydration is rapid to start with and then slows down.

6. The three major factors which influence the rate of gain of strength are thechemical composition, the fineness, and the temperature of the hydrating mix.With modern cements, the increase in strength after the first 28 days is likelyto be very small and can generally be ignored.

7. The amount of water in the mix (usually referred to as the water/cement ratio,w/c) is a vital factor in determining the strength, permeability, absorption anddurability of the concrete/mortar. Generally, other factors being equal, thehigher the water/cement ratio the lower the strength and the higher thepermeability and absorption. This is why it is often necessary to use a plasticizerin the mix when high quality concrete is required.

8. Due to its high alkalinity, Portland cement is very vulnerable to attack byacids. The reaction between the cement and the acid takes place immediatelythe two are in contact. See Section 3.5.2.1.

2.2.1 Sulphate-resisting Portland cement

Sulphate-resisting Portland cement (SRPC) should be specified as Low AlkaliSulphate-resisting, class 42.5 complying with BS 4027 1996.

The cement is similar in its strength and other physical properties to OrdinaryPortland cement (OPC), but the tricalcium aluminate content (the C3A) is limitedin the relevant British Standard (BS 4027) to a maximum of 3%. It is the C3Awhich is attacked by solutions of sulphates of various bases. This can have importantconsequences as the reaction products are expansive in character and is discussedin Section 3.5.2.2.

The low alkali content, not exceeding 0.6% equivalent sodium oxide, is usefulin minimising the risk of alkali-silica reaction; this is discussed in Section 3.9.

2.2.2 Blended cements

Blended cements consisting of mixtures of Portland cement and pulverised fuelash (pfa) and Portland cement and ground granulated blast furnace slag (ggbs) areused in concrete for special purposes such as reduction of heat of hydration and to

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improve sulphate resistance. Generally, these additions slow down the rate of gainof strength of the concrete.

2.2.3 Portland masonry cement

This should be specified as Portland masonry cement class MC 12.5 complyingwith BS 5224 1995. It contains an air-entraining agent which increases resistanceto freeze-thaw conditions. The inclusion of additives to impart workability andimprove water retention are particularly useful. It is used for mortar for brickworkand blockwork and for external rendering.

2.3 Aggregates from natural sources for concreteand mortar

The relevant British Standard for concreting aggregates, which is still valid at thetime of writing, is BS 882, and covers gravel, crushed rock and sand.

The British Standards for fine aggregate (sand) for mortars and external renderingare BS 1199 and BS 1200 1976/1996, and these cover sand for mortar for plainand reinforced brickwork, blockwork and masonry.

The Standards should be referred to for their detailed requirements, whichinclude grading limits, flakiness, shell content, and limits on clay, dust andchlorides. Regarding durability, BS 882, Appendix B makes the point that ‘Nosimple tests for durability and resistance to frost or wear of concrete can beapplied; hence, experience of the performance made with the type of aggregatein question and a knowledge of their source are the only reliable means ofassessment.’

With sea-dredged aggregates, special attention should be paid to the shell andsalt (mainly sodium chloride) contents.

In the UK, aggregates from some sources in Scotland and the north of Englandpossess high shrinkage characteristics. When there is any doubt about an aggregate,reference should be made to BS 812 Testing Aggregates; this is in 23 parts publishedbetween 1985 to 1995.

Part 120 details test methods for determining drying shrinkage of mortar prismsmade with the suspect aggregates and recommendations are given for theinterpretation of the results.

There are different opinions among experienced engineers on the effect ofabsorption of aggregates on the permeability of concrete used for water-retainingstructures. The relevant Code, BS 8007 1987 Code of Practice for the Design ofConcrete Structures for Retaining Aqueous Liquids, places a limit of 3% on theabsorption of aggregates. However, published information on properlyconducted tests which would justify this restriction are conspicuous by theirabsence.

The Standards for sands for mortar are BS 1199 and 1200: Building Sands fromNatural Sources.

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2.4 Admixtures

2.4.1 Introduction

An admixture can be defined as a chemical compound that is added in comparativelysmall quantities to concrete, mortar, or grout at the time of batching or mixing, toproduce some desired characteristic in the mix and/or in the mature concrete, mortaror grout.

While the use of admixtures in the UK has increased significantly in recentyears, this country lags behind continental Europe, the USA and other developedcountries.

The main types of admixtures in general use are:

Water-reducers, plasticizers/workability aids;Superplasticizers;Accelerators;Set retarders;Air-entraining admixtures.

The general use of admixtures is covered by various Codes and by BS 5328Parts 1 and 3 and by ENV 206 (draft European Standard). The ENV puts anupper limit on the use of admixtures in a mix at 5% by mass of the cement anda lower limit of 0.2%. The ENV also requires that when the dosage ofadmixtures in liquid form exceeds 3 litres/m3 of concrete, this shall be takeninto account when calculating the water/cement ratio of the mix.

The British Standards are performance specifications. The USA Standardfor admixtures for concrete is ASTM C494–86. It should be noted that BS8110 Structural Use of Concrete refers to pigments as an admixture, but PD6534 1993 Guide to the Use in the UK of ENV 206 1992 Concrete, clause 4.5includes pigments under the heading of Additions and this practice has beenfollowed in this book.

2.4.2 Water-reducing admixtures/workabilityaids/plasticizers

For concrete, these admixtures are covered by BS 5075 Part 1. This type ofadmixture is a compound which increases the workability of a concrete mixwith a constant w/c ratio, or permits the w/c ratio to be reduced withoutreducing the workability of the concrete. It should not increase the air contentof the mix.

The Standard also covers ‘accelerating water/reducing’ admixtures and ‘retardingwater-reducing’ admixtures.

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2.4.3 Superplasticizing admixtures

These admixtures are covered by BS 5075 Part 3, and by the draft European StandardENV934–2.

This type of admixture is a compound which when added to a concrete miximparts very high workability to the mix, or allows a large decrease in the w/c ratiowhile maintaining a given workability.

The very high workability obtained (150–200 mm) slump ensures that theconcrete is virtually self-compacting. However, the super workability onlylasts for a limited period, usually in the range of 2–4 hours. In the context ofthis book, this type of admixture can be very useful for placing concrete inpositions where compaction is very difficult, e.g. in members containingcongested reinforcement and repairs to honeycombed concrete.

The two main basic types of superplasticizers are sulphonated naphthalene-formaldehyde condensates, and sulphonted melamine-formaldehydecondensates.

2.4.4 Accelerators

These are covered by BS 5075 Part 1 and draft European Standard ENV 934–2.There is no British Standard for accelerators for mortar and grout.

This type of admixture increases the rate of reaction between the cementand water in a concrete mix, and thus accelerates the setting and rate of gainof strength of the concrete. Some accelerators contain chlorides as an activeingredient and the British Standard requires that the chloride content must bestated by the manufacturer. Standards for concrete now strictly limit thechloride ion content of concrete which contains ferrous metals, see BS 5328Parts 2 and 3.

2.4.5 Set retorders

This type of admixture is covered by BS 5075 Part 1. It is a compound that reducesthe rate of reaction between the cement and water in a concrete/mortar, thus reducesthe rate of setting of the concrete/mortar.

The relevant British Standard for set retarders for mortar is BS 4887 Part 2; thedraft European Standard is ENV 934–2. The British Standard covers buildingmortars and rendering, but not mortars for floor screeding. Suppliers of ready-mixed building mortars make extensive use of this type of admixture.

2.4.6 Air-entraining admixtures

This type of admixture is covered by BS 5075 Part 2. It is a compound which whenadded to a concrete mix incorporates air during the mixing; it should not significantlyaffect the setting of the concrete.

The draft European Standard is ENV 934–2. For mortars, the relevant BritishStandard is BS 4887 Part 1.

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The presence of the entrained air increases the resistance of the concrete(and mortar) to frost attack (freeze-thaw conditions). However, there is areduction in the compressive strength compared with a control mix of thesame mix proportions and W/C ratio; this is important in concrete and shouldbe allowed for. The actual reduction in compressive strength depends on anumber of factors, but a figure of 4% for each 1% of entrained air is oftenused as a guide. The size of the bubbles of entrained air is about 50 microns(0.05 mm). It should be noted that with cement contents in excess of about350 kg/m3 difficulties are likely to arise in entraining the air. This type ofadmixture is normally specified for all external insitu concrete paving toprevent damage by frost and freeze—thaw conditions.

2.5 Additions

2.5.1 Introduction

These are materials which are added to a mix (concrete or mortar or grout) inmuch larger quantities than admixtures. The reason for the use of Additions issimilar to that for the use of admixtures, namely to impart some desirablecharacteristics to the mix and/or the concrete, mortar or grout.

The materials described below are generally considered as Additions.

2.5.2 Pulverized fuel ash

Pulverized fuel ash (pfa) is covered by BS 3892 Part 1 1997 Pulverized-fuel Ashfor Use in Concrete, and European Standard BS EN 450 1995 Fly Ash forConcrete—Definitions, Requirements and Quality Control.

Pulverized fuel ash is also classed as a cement replacement and in fact that is itsprincipal use in the concrete industry. The material is a by-product of pulverizedcoal-fired electricity generating stations. It is a fine powder, and the approximatecomposition is:

50% silicon (SiO2);

28% alumina (Al2O

3);

11% iron oxide (Fe2O

3);

11% oxides of calcium, magnesium, sodium and potassium.

British Standard BS 3892 Part 1 covers pfa for use as a cementitious compound instructural concrete. Part 2 covers pfa for use in grouts (excluding grout used inprestressing ducts) and for miscellaneous uses in concrete.

The two main differences in the BS EN 450 compared with BS 3892 lies in thepermitted fineness; the former allows a much coarser material. The increase infineness results in a decrease in strength (other factors being equal). This is discussedin some detail in Research Focus No. 31, November 1997.

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The main advantages claimed for the inclusion of pfa in concrete are: 1. reduction in heat of hydration;2. improved workability with constant w/c ratio;3. increased resistance to sulphate attack;4. reduced permeability to liquids;5. long-term increase in compressive strength. There appears to be some reason to believe that the inclusion of pfa in the concretemay render it more resistant to alkali-silica reaction.

Reference should also be made to BS 6588 Specification for PortlandPulverized Fuel Ash Cements which lays down requirements for composition,strength, chemical and physical properties for two combinations of Portlandcement and pfa.

While the presence of pfa in hardened concrete can be determined by microscopicexamination of thin sections, it is not possible by chemical analysis to determinethe proportion of pfa in concrete, mortar or grout. The pfa content in fresh concretecan be determined by the chemical method described in BS 6610 Specification forPozzolanic Pulverized Fuel Ash Cement, or by the particle density method describedin Annex D in Part 128 of BS 1881 Methods for the Analysis of Fresh Concrete.

2.5.3 Ground granulated blastfurnace slag

This material should comply with BS 6699 Specification for Ground GranulatedBlastfurnace Slag (ggbs) for use with Portland cement.

The slag is a waste product produced in steelworks. It can be used as an aggregatefor concrete or as an addition to Portland cement for concrete. When used incombination with OPC, it increases the resistance of the concrete to sulphate attack,and to alkali-silica reaction by limiting the alkali content of the binder (cementplus ggbs). It also reduces the heat of hydration.

The proportions used with OPC depends on the required characteristics of thehardened concrete. Generally, mixes containing 40% ggbs and 60% OPC, to 65%ggbs and 35% OPC are used.

2.5.4 Condensed silica fume

Condensed silica fume is used as an Addition in concrete, although there is noBritish Standard for this material at the time this book was written, but a EuropeanStandard (an EN) is in course of preparation.

Condensed silica fume is a waste product of the ferrosilicon industry. It consistsof about 88% silicon dioxide (SiO

2) with very small percentages of carbon, ferric

oxide, aluminium oxide (alumina) and oxides of magnesium, potassium and sodium.It is a very fine greyish powder with a specific surface about fifty times that of

normal Portland cement and is a highly reactive pozzolan.

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The addition of condensed silica fume to a concrete or mortar mix has asignificant effect on the properties of the plastic mix as well as on the hardenedconcrete or mortar. The dosage is usually in range of 2% to 10% by mass of thecement, and it imparts a number of beneficial characteristics to the concrete/mortar;these are: 1. increased cohesion;2. reduced permeability;3. increase in compressive strength;4. increase in resistance to sulphate attack, except possibly ammonium sulphate;5. increase in resistance to a number of aggressive chemicals. The very small particle size increases the water demand of the mix and can resultin premature stiffening if placing and compaction is delayed. It is normally usedwith a superplasticiser.

In the UK and USA, it is mainly marketed as a stabilised slurry which containsa plasticiser or superplasticiser. The Agrement Certificate for a proprietary slurrymarketed in the UK states that the pH is 5.5 plus or minus 1.0.

It is not possible to determine by chemical tests the percentage of condensedsilica fume in a mix.

2.5.5 Pigments

The relevant British Standard is BS 1014 Pigments for Portland Cement andPortland Cement Products. The USA Standard is ASTM C979–82.

Table 1 of BS 1014 lists seven pigments of which four are oxides of iron, one iscarbon black, one is chromic oxide and one is titanium dioxide. The principalpigments used are the oxides of iron and are in the form of very fine powders,having a particle size of about 0.1 microns. This can be compared with Portlandcement 1.0–50.0 microns and sand 150–5000 microns (1 micron= 0.001 mm).Some comments on their use are in Section 6.2.

2.6 Water for mixing concrete, mortar and grout

The relevant British Standard is BS 3148 Methods of Test for Water for MakingConcrete.

Water used for mixing concrete, mortar and grout should be free fromcompounds which adversely affect the setting and hardening of the mix and/orhave an adverse effect on the properties of the hardened concrete/mortar/grout.The impurities may be organic or inorganic. Sulphates in solution (as SO

3) should

not exceed 1000 ppm.Water which is fit for drinking (potable water) is suitable for making concrete/

mortar/grout, but water which is unfit for drinking may be quite suitable.

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2.7 Steel reinforcement

Steel reinforcement for concrete is covered by the following British Standards:

BS 4449 Carbon Steel Bars for the Reinforcement of Concrete;BS 4482 Cold Reduced Steel Wire for the Reinforcement of

Concrete;BS 4483 Steel Fabric for the Reinforcement of Concrete;BS 4486 Hot Rolled and Processed, High Alloy Steel Bars

for the Prestressing of Concrete;BS 5896 High Steel Wire and Strand for the Prestressing of

Concrete;BS 7295 Fusion Bonded Epoxy Coated Carbon Steel Bars

for the Reinforcement of Concrete;BS 6744 Austenitic Stainless Steel Bars for the

Reinforcement of Concrete.The coefficient of thermal expansion of plain carbon steel is 12×10-6.

All reinforcing steel should meet the tests prescribed by CARES (UKCertification Body for Reinforcing Steels) and purchasers should check thatsuppliers hold the Quality Assurance Certificate issued by CARES.

2.7.1 Galvanised reinforcement

At the time of writing, there is no British Standard specifically for galvanizedreinforcement, but there is a Standard BS 729 for hot dipped galvanized coatingsfor iron and steel articles.

The author is indebted to the Galvanisers Association for the followinginformation. Galvanised reinforcement was first used in Bermuda in the 1930s; itbecame widely used during the war years when sea-dredged aggregates were usedfor concrete structures. In the UK, it appears to be mainly used in precast concreteunits for large building projects.

When Portland cement concrete/mortar is placed around galvanised rebars,there is a chemical reaction between the zinc coating and the calcium hydroxide inthe hydrating cement paste. The zinc surface is passivated with the evolution ofhydrogen. The passivation occurs with the initial formation of a layer of zinchydroxide; further chemical reactions follow, resulting in the formation of acomplex stable zinc compound, zincate. For durability in aggressive conditions, itis essential that this passivated film on the zinc be undamaged.

The protection of the steel provided by the zinc coating is mainly dependent onthe thickness of the coating and therefore the thickness should be specified to meetthe anticipated exposure conditions.

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The presence of a very small concentration of chromate (about 20 ppm) in thecement will inhibit the reaction between the cement paste and the zinc and thuslimit the formation of hydrogen.

2.7.2 Fusion-bonded epoxy coated reinforcement

This method of protecting steel reinforcement from corrosion has been in use inthe USA since the early 1970s, but in the UK its acceptance has been much slower.

The American Standard is ASTM A775 Standard Specification for Epoxy-coatedReinforcing Bars. The UK Specification is BS 7295 Parts 1 and 2.

The coating is an epoxy powder specially formulated to resist impact andabrasion, and to possess a sufficient degree of flexibility to accommodate bendingstresses in the bars and to possess high bond to the bars.

The epoxy resin is defined in BS 7295 as a thermosetting epoxy powderconsisting mainly of epoxy resin plus curing agent and pigments.

Two conditions are paramount for the coating to protect effectively the steelrebars: 1. high bond strength to the surface of the rebars;2. toughness to reduce damage to the coating during transport and fixing and

this includes an adequate coating thickness which latter should be in the range130–300 microns.

2.7.3 Stainless steel reinforcement

The relevant British Standard is BS 6744 Specification for Austenitic StainlessSteel for the Reinforcement of Concrete. Of the three basic types of stainless steel,martensitic, ferritic and austenitic, the latter steel types 302, 315 and 316 are by farthe most resistant to attack by concentrations of chlorides. Tests by the BuildingResearch Establishment, UK, have shown that austenitic steel embedded in concretecontaining 3% chloride by mass of the cement, showed no sign of corrosion after17 years.

Type 316 steel contains 18% chromium, 10% nickel, and 3% molybdenum.However, even with this steel if it is exposed to warm humid conditions and is veryhighly stressed, corrosion can occur. This was illustrated by the failure of stainlesssteel hangars supporting a reinforced concrete ceiling slab in a swimming pool inSwitzerland in 1985.

Stainless steel is much more expensive than ordinary carbon steel and its usefor reinforcement is only justified in special cases.

The coefficient of thermal expansion of austenitic stainless steel 18×10-6

compared with 12×10-6 for carbon steel.

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2.8 Spacers

The use of correct spacers is an essential part of the construction of reinforcedconcrete. There is no British Standard for spacers, but in 1991 the Concrete Societypublished a manual Spacers for Reinforced Concrete.

Spacers are used to ensure correct cover to the rebars. Cover less than thatrequired by the relevant Code of Practice can increase considerably the risk ofcorrosion of the rebars resulting in premature deterioration of the concretemember. Spacers are normally made of plastics, but some are made of fibre-reinforced cement based material which has the advantage of bonding to thesurrounding concrete.

2.9 Non-ferrous metals

Only a limited number of non-ferrous metals are likely to be used in the constructionand fitting-out of swimming pools, and these are phosphor-bronze, gunmetal andcopper.

2.9.1 Phosphor-bronze

Bronze is an alloy of copper and tin, and phosphor-bronze contains phosphorus ascopper phosphide.

These alloys are corrosion resistant and are used for fittings and fixings. Thecoefficient of thermal expansion is about 20×10-6. Reference should be made tothe comments in Section 2.10.

2.9.2 Copper

Copper is resistant to most conditions met in building construction. For many yearsit was used as water bars in concrete water-retaining structures but has been entirelysuperseded by PVC. Copper is corroded by solutions of chlorides and by solutionsof ammonium salts which latter may be present in some organic adhesives usedfor floor coverings. BS 2870 1980 covers copper and copper alloys for sheet, stripand foil.

Reference should be made to the comments in Section 2.10.

2.10 Bimetallic corrosion

When using dissimilar metals, it is important to ensure that either they arenot in contact with each other, or that bimetallic corrosion will not takeplace. Comprehensive recommendations on this subject are contained in aBSI publication, PD 6484 1979 Commentary on Corrosion at BimetallicContacts and its Alleviation. For example, mild steel can be seriouslycorroded if it is in contact with copper, phosphor-bronze or stainless steel(Figure 2.1).

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2.11 Curing compounds for concrete and mortar

The efficient curing of concrete and mortar is essential to ensure strength,resistance to shrinkage cracking, and resistance to abrasion, and long-termdurability.

Materials used for curing are in three forms:

Spray-applied membranes;Sheet materials;Wet/water curing.

2.11.1 Spray-applied membranes

The relevant British Standard for testing spray-applied membranes is BS 7542Methods of Test for Curing Compounds for Concrete. The USA Standard isASTM 309–81 Standard Specification for Liquid Membrane-formingCompounds.

These compounds are generally either water-based or resin-based and shouldbe applied as soon as possible after completion of compaction and finishing. When

Figure 2.1 Corrosion of mild steel rebar caused by direct contact with phosphor-bronzetie.

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concrete is cast in formwork, curing should commence as soon as the formwork isremoved. Many of these compounds gradually weather away in the course of time,but if it is intended to apply a coating or other layer to the concrete (e.g. renderingor bedding for tiles etc.) then the suppliers of the curing compound should beconsulted as most curing compounds adversely affect bond at the interface withthe base concrete.

2.11.2 Sheet materials

The principal sheet material used for curing concrete is polyethylene sheeting (tradename polythene). This is very effective in reducing moisture loss provided it is laidas soon as practical after casting the concrete or mortar, and the sheets are helddown around the edges, and kept in position for at least four days. Also, 1000gauge sheeting should be used; this gauge material is 250 microns (0.25 mm)thick.

2.11.3 Wet/water curing

The curing of concrete and mortar by water spray is only carried out in specialcases mainly when it is required to keep the temperature of the concrete undercontrol. It is specifically recommended for high alumina cement concrete.

2.12 Polymers

The term polymers includes a wide range of materials, but in the context of thisbook they are materials used in a concrete or mortar mix to provide some desirablecharacteristics to the mix, such as: 1. improved workability of the mix with constant w/c ratio or reduced w/c ratio

with constant workability;2. increased bond with the substrate;3. reduced permeability and absorption;4. improved resistance to carbonation;5. some limited increase in resistance to chemical attack. They are mainly available in liquid or powder form. The liquids are dispersions(also referred to as latexes) and are generally whitish in colour. The solid contentand the viscosity vary; the solid content is usually in the range of 40–70%. Thepolymers in most general use include:

Modified polyvinyl acetates (PVAs);Ethylene vinyl acetate (EVAs);Acrylics;Styrene butadiene rubber (SBR);Styrene acrylics.

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When these polymers are added to a mix, the proportions recommended by thesuppliers should generally be followed. In the case of SBRs, the proportions areusually in the range of 15–25% by mass of cement, depending on the reason forthe addition. It is assumed that the dispersion weighs 1 kg/litre. All polymers areexpensive and so the amount used justifies careful consideration.

2.13 Reactive resins

These materials are generally used for protective coatings, and when mixed withselected fine aggregates, for thin bonded repairs. The main resins used for mortarsfor the repair of concrete are:

Epoxies;Polyesters;Polyurethanes.

2.13.1 Epoxy resins

These resins are by-products of the petrochemical industry. The basic resin is aliquid with a fairly high viscosity and will remain in this condition almostindefinitely. For use, it must be mixed with a hardener/accelerator. The hardenerreacts chemically with the epoxy and changes it from a liquid to a solid. Adequatemixing by mechanical means is essential to ensure effective dispersion of thehardener in the epoxy. The great advantage with epoxies is that they can beformulated to suit particular conditions of application and end use.

The principal properties of epoxy resins are: 1. high bond strength to many materials, with special reference to concrete and

steel;2. very low shrinkage during curing;3. high resistance to a wide range of chemicals;4. high resistance to water penetration;5. high compressive, tensile and flexural strength when used with selected

aggregates;6. coefficient of thermal expansion of sand-filled epoxy is about three to four

times that of concrete made with natural aggregates;7. epoxies suffer loss of compressive strength with increase in temperature; with

temperatures above about 75 °C the loss of strength can be considerable.

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2.13.2 Polyester resins

These resins are similar in many respects to epoxies; the main differences are: 1. coefficient of thermal expansion is about 1.5 times that of epoxies;2. shrinkage during curing is appreciably higher than that of epoxies;3. the shelf life of the basic resin is strictly limited. The curing of polyester resins

is adversely affected by the presence of moisture.

2.13.3 Polyurethane resins

These resins are used mainly for floor sealants and protective coatings. There is alarge variety of polyurethane resins, all of which possess specific qualities; it istherefore important when specifying these resins that the performance characteristicsbe clearly defined. They have a number of excellent characteritics includingflexibility, resistance to abrasion and resistance to chemical attack. They bondwell to concrete and mortar, but the surface of the substrate must be dry to avoidformation of blisters.

2.14 Joint fillers

These materials are used in ‘in-house’ design joints and are sometimesreferred to as back-up materials. They provide support to the sealant, butshould not bond to it; they also help prevent the entry into the joint of stonesand debris during construction as the sealant is usually applied later in thecontract.

The materials used for joint fillers should fulfil the following requirements: 1. It must be durable under service conditions; ideally, the service life should be

the same as that of the structure in which it is inserted.2. It must be chemically inert and non-toxic.3. It should be resilient but should not extrude so as to interfere with the sealant.4. It should not bond to the sealant; if it is liable to do so, a bond breaker should

be used.5. It must be formed easily and be inserted readily into the joint. The main materials used for joint fillers include: 1. cork granules bonded in a resin which is resistant to long-term immersion in

water;2. wood fibre with bitumen (not suitable for use in damp conditions);3. cork granules bonded in bitumen (may be unsuitable for use in contact with

potable water).

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2.15 Joint sealants

These materials can be divided into two basic groups:

Insitu compounds;Preformed compounds.

Materials in both groups should possess the following characteristics: 1. For external use and in liquid retaining structures, the material must be virtually

impermeable.2. Ideally, the service life should be the same as that of the structure in which it

is used.3. It must bond well to the side of the joint, but should not bond to the joint filler

(a bond-breaker may be required).4. It should deform in response to the movements in the structure without

extruding and without losing its integrity. For detailed advice on the use of sealants in structures, reference should bemade to BS 6213 Guide to the Selection of Constructional Sealants. Thisdocument deals mainly with the selection of insitu sealants. Table 4 indicatesthat the only sealant recommended as suitable for use in swimming pools is aflexible epoxy.

Sealants in joints in swimming pools have to function in particularly severeconditions and consequently few materials can be relied upon to give satisfactoryservice.

Another relevant British Standard is BS 6093 Code of Practice for Design ofJoints and Jointing in Building Construction.

2.15.1 Insitu compounds

The insitu sealants can be divided into two main classes:

Hot applied sealants;Cold applied sealants (pouring-grade and gun-grade).

As far as swimming pools and ancillary structures are concerned, the sealants ingeneral use are the gun-grade sealants. The relevant British Standards are:

BS 5215 One-part Gun-grade Polysulphide-based Sealants;BS 5889 One-part Gun-grade Silicone-based Sealants;BS 4254 Two-part Polysulphide-based Sealants;BS 5212 Cold-applied Joint Sealant Systems for Concrete

Pavements.

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The first three Standards listed above have been declared obsolete and not replacedat the time of writing this book. In the circumstances, it is recommended thatreference be made to International Standards Organisation, Standard ISO 11600Building Construction Sealants, Classification and Requirements.

2.15.2 Preformed sealants

Preformed sealants suffer from one practical disadvantage, namely the sides of thejoints have to be smooth and even, as the preformed sealant does not accommodateitself well to out-of-true surfaces. This may require that the sides of the joint haveto be trued-up with an epoxy resin mortar; cement/sand mortar is not recommendedfor this type of repair.

Neoprene and EPDM (ethylene-propylene diene monomer) are particularlyresistant to a wide range of chemicals and this makes them suitable for use infloors of stores holding chemicals for water treatment and in swimming pools.

2.16 Ceramic tiles

Ceramic tiles are covered by BS 6431–EN87 Ceramic Wall and Floor Tiles, whichis published in 23 parts.

There are two basic categories of tiles used for swimming pools in the UK.The difference arises mainly from the method of manufacture, and they areusually described as pressed tiles and extruded tiles. In each category, thereare many divisions and full details can be obtained from the manufacturers.Generally, the pressed tiles are thinner and the body of the tile is relativelymore absorbent, but the dimensional tolerances are smaller so that there isvery little variation in the declared size of the tiles. This means that the jointscan be narrower. Extruded tiles are thicker and the body is wholly or partlyvitrified, and dimensional tolerances are larger. This results in the absorptionbeing much lower and the joints being wider. The thicker tiles generally havedeeper indentations or ‘frogs’ on the back. Generally, extruded tiles with verylow absorption are recommended for swimming pools. For outdoor pools thetiles must be frost resistant.

Further information on ceramic tiles is given in Section 7.6.

2.17 British standards and Euro codes

Frequent reference is made in this book to British Standards and Codes of Practice,and to Euro Codes.

Theoretically, once a Euro Code or Standard has been finally approved andpublished, the equivalent British Code or Standard will be withdrawn. However,in practice, it is likely that certain sections of some of the British Codes andStandards will be retained on the grounds that they have special application toconditions in the UK.

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The following terminology is in general use:

CEN European Committee for Standardisation;CPD Construction Products Directive;EN European Standard;prEN Draft European Standard;prENV Draft European PreStandard.

References

American Society for Testing Materials (ASTM):Portland Cement, C150.Steel Wire Plain for Reinforcement of Concrete, A82.Steel Welded Wire Fabric for Reinforcement of Concrete, A185.Steel Wire Deformed, for Reinforcement of Concrete, A496.Aggregates for Concrete, C33.Chemical Admixtures for Concrete, C494.Pigments for Concrete, C979.Epoxy Coated Reinforced Bars for Concrete, A775.Silica Fume, C1240.

British Standards Institution. Building Sands From Natural Sources, BS 1199and 1200.British Standards Institution. Corrosion-resistant Stainless Steel Fastners, BS 6105.British Standards Institution. Rolled Copper and Copper Alloy Sheet, BS 2870.

Further reading

American Concrete Institute. Chemical Admixtures for Concrete, ACI Committee 252, 1991.American Concrete Institute. Superplasticisers for Concrete, ACI Committee 252, 1993.American Concrete Institute. Aggregate for Concrete, ACI Committee 211, 1997.Brown, B. Aggregates for concrete, Concrete, May 1998, pp. 12–14.Concrete Society. Polymers in Concrete, Technical Report No. 39, 1994.Concrete Society. Guidance on the Use of Stainless Steel Reinforcement, Technical Report

No. 51, November 1998.Dennis, R. Polymer dispersions, in Construction Materials Reference Book, Ed. D.K. Doran,

Butterworth Heinemann, 1992.Ryle, R. Technical aspects of aggregates for concrete, Journal of Quarry Management, April

1988, pp. 27–31.Walters, D.G. Comparison of latex-modified Portland cement mortars, ACI Materials Journal,

July/August 1990, pp. 372–7.

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Chapter 3

3.1 Introduction

When one considers the need for a structure to be durable the following questions arise:

1. What maintenance is likely to be required?2. The time lapse between construction and the need for repairs?3. What is the likely useful life of the structure before partial or complete

replacement is considered appropriate?

The following terms are important when dealing with the above questions:

1. Durability. A material can be considered durable if it fulfils its intended dutyfor the whole of its design life with an acceptable amount of maintenanceincluding general repair.

2. Design Life. This is the length of time which the designer estimates the materialwill remain durable.

3. Service Life. This is the actual length of time the material remains durable.4. Maintenance. The work and materials which when applied to a structure enables

the structure to fulfil its duty during its service life. Maintenance should includecleaning, minor repairs, decorating and replacing parts when required.

This is a very important subject and guidance is given in two British Standards towhich reference should be made:

BS 7543 Guide to the Durability of Buildings, Building Ele-ments and Components;

BS 8210 Guide to Building Maintenance Management.

Factors affecting thedurability of reinforcedconcrete and cement-basedmaterials used in theconstruction of swimmingpools

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The suggestions/recommendations put forward are linked to the anticipated level ofmaintenance (including repair) applicable to the type of structure underconsideration. Unfortunately, the two publications referred to above do not includespecific reference to swimming pools, but the basic principles are applicable.

The main causes of deterioration of reinforced concrete and the commonly usedcement-based materials are discussed in this chapter, with particular reference toswimming pools.

Consideration should be given to how maintenance and repairs can be carriedout. This is particularly important with swimming pools as generally these areconstructed in the ground and inspection and repair to the pool shell can be difficultand disruptive to the use of the pool.

It is necessary to distinguish between the causes of the deterioration of theconcrete and of the steel reinforcement.

The corrosion of steel reinforcement is the most serious durability problemaffecting concrete structures; in other words, steel reinforcement is the ‘AchillesHeel’ of reinforced concrete. However, in swimming pool shells which are finishedon the inside with rendering, screed and tiling/high-quality decorative coating, thechance of reinforcement corrosion is reduced.

Unfortunately, concrete and cracking are often closely associated in peoples’minds. Cracks can result in corrosion of steel reinforcement due to the admissionof moisture, air and agressive chemicals in solution. For this to occur the crackmust be of a certain minimum width at the interface of the concrete and the rebars.It is usually assumed that if the width of the crack at the surface of the concrete isnot more than 0.3 mm wide, corrosion is unlikely to occur; this is valid if theconcrete is good quality and the cover to the rebars is adequate taking into accountthe exposure conditions.

Cracks which penetrate right through the wall or floor of a swimming pool arelikely to form a source of leakage and need attention. However, even in thesecircumstances, very fine cracks may be self-healing (known as autoginous healing).

The types of cracks which normally occur in the floor and walls of swimmingpools are discussed in Chapters 4 and 5. Remedial work to cracks which are foundto be a source of leakage is described in Chapter 10.

3.2 Corrosion of steel reinforcement in concrete

3.2.1 Introduction

The corrosion products of steel, known generally as rust, consist of oxides of iron.For rusting to occur, moisture and oxygen must be present and for steel embeddedin the vast majority of concrete structures, both are present to a greater or lesserdegree.

Some chemicals used in water treatment are acidic in solution, e.g. sodiumbisulphate, and aluminium sulphate; see Section 3.7. Steel does not corrode whenit is surrounded with concrete or cement-based mortar, unless special factors are

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present. The high alkalinity of the cement paste passivates the steel due to theformation of a protective film of oxidation products, such as ferric oxide. As longas this film is maintained, further oxidation of the steel is inhibited, and the steeldoes not corrode. It therefore follows that subsequent corrosion (oxidation) of thesteel must be due to a breakdown of the passivating film on the surface of the steel.This loss of passivation can be caused by a number of factors, of which the principalare set out below: 1. physical damage to the concrete surrounding the steel, resulting in exposure

of the steel;2. development of cracks in the concrete extending down to the rebars, of sufficient

width to allow the ingress of moisture, oxygen, or aggressive chemicals, e.g.chlorides. Such cracks are normally due to shrinkage and/or stress. See Sections3.2.2.1–3.2.2.3;

3. high permeability/porosity of the concrete surrounding the rebars allowingingress of moisture etc.;

4. inadequate thickness of the cover coat of concrete or mortar. The recommendedthickness/depth of cover will depend on the exposure conditions and thepermeability/porosity of the cover coat. See BS 5328 Part 1, with specialreference to Table 6.

5. The presence in the concrete of chlorides in excess of the ‘safe’ recommendedconcentration as laid down in Standards and Codes of Practice. See BS 5328Part 1 Clause 4.2.2.

Carbon dioxide is present in the air, and while it does not damage the concrete itlowers the pH of the cement paste to about 9.5 which can result in depassivation ofthe steel; this is known as carbonation; see Section 3.3.

3.2.2. Causes of cracking in concrete

All concrete contains micro-cracks and these do not adversely affect theperformance of the concrete. However, macro-cracks, if they extend down to therebars, can result in loss of passivation leading to corrosion of the rebars.

The main types of cracks likely to be found in swimming pool shells andassociated structural members are:

Shrinkage cracks;Flexural cracks;Thermal contraction cracks.

3.2.2.1 Shrinkage cracks

Shrinkage cracks can result from a high water/cement ratio, or the use of shrinkableaggregates (see Section 2.3), and/or inadequate curing of the concrete. Commentson water/cement ratio and curing are given in Sections 4.5 and 4.13.

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3.2.2.2 Flexural cracks

The width of this type of crack is limited in BS 8110 to 0.3 mm; the width ismeasured at the surface of the member and decreases in width with depth. It isconsidered that cracks of this type and width are unlikely to result in corrosion ofthe rebars. See also BS 8007 1987 Code of Practice for the Design of ConcreteStructures for Retaining Aqueous Liquids, which is the main Code for the designof swimming pool shells.

3.2.2.3 Thermal contraction cracks

This type of crack is not uncommon in reinforced concrete swimming pool walls.They are caused by the cooling of the concrete after the removal of the formworkwhen inadequate distribution steel has been used to control this type of cracking.This is discussed in Section 4.9.

3.3 Carbonation of concrete

Carbon dioxide in the air reacts with the calcium hydroxide in the hydrating cementpaste to form calcium carbonate:

Ca(OH)2+CO

2=CaCO

3+H

2O

The reaction results in a significant reduction in the pH of the cement paste (fromabout 12.5 to about 9.5).

The surface of concrete exposed to the air carbonates very rapidly, forming acarbonated layer of micron thickness. In good quality concrete, the rate ofpenetration is very slow and depends on many factors, the principal ones beingporosity, permeability and moisture content. The floor and walls of the majority ofswimming pools are not exposed directly to the air once the back-filling has beencompleted. It is therefore reasonable to assume that the carbonation of the concreteshell is unlikely to endanger the reinforcement during the lifetime of the pool.

3.4 Chloride-induced corrosion of reinforcement

3.4.1 General considerations

Corrosion of rebars can occur in un-carbonated concrete due to the presence ofchlorides. Chlorides are present either because they were added to the concretemix, or were present in the aggregates, and/or the mixing water, or they hadpenetrated into the hardened concrete from an outside source.

When present, chlorides are in solution in the pore water. When a salt is dissolvedin water it is immediately split up into electrically charged particles known as ions:

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NaCl=Na++Cl-

It is the negatively charged chloride ions which destroy the passivity (the layer offerric oxide) on the surface of the rebars.

In practice, the chloride ions present in the pore water exist in two forms, freechloride ions and combined chloride ions. The combined chloride ions are combinedwith the hydration products in the cement paste, mainly the tricalcium aluminate(C3A). It is generally agreed that it is the free chloride ions which damage thepassivity of the steel, resulting in the corrosion of the rebars.

It can thus be seen that the higher the concentration of C3A in the hydratingcement paste, the higher the percentage of chloride ions which will be ‘locked up’and not free to attack the steel. From the point of view of dealing with potentialchloride attack on reinforcement, the use of a cement with a high C3A content is tobe preferred. Ordinary Portland cement has a C3A content in the range 8–12%,compared with sulphate-resisting Portland cement which has a C3A content notexceeding 3.5% (see BS 4027).

The formation of rust by chloride attack can cause cracking and spalling of theconcrete due to the considerable increase in volume of the steel when it is convertedinto rust (an increase of three to four times the original thickness of steel).

There are two main types of corrosion of rebars, general corrosion and localcorrosion (pitting). The general corrosion is more likely to cause cracking andspalling of the concrete, but local corrosion can be more serious due to significantreduction in the diameter of the rebars at the ‘pits’. These pits may penetrate therebars by more than 50% of the bar diameter; even a careful visual examination ofthe concrete may not detect localised/pitting corrosion.

Localised/pitting corrosion is more likely to be the result of chloride ions in theconcrete in contact with the steel than carbonation of the concrete in contact withthe rebars.

3.4.2 Chlorides in swimming pool water

Concern is sometimes expressed about the durability of reinforced concrete incontinuous contact with chlorinated swimming pool water. A careful literaturesearch has not revealed any authorititive research or detailed study of this problem.This suggests that significant corrosion of steel reinforcement in pool shells due tochlorides introduced into the pool water for disinfection of the water has so far notbeen detected and/or has not caused serious concern. However, a brief discussionon the subject is considered justified.

A great deal has been written about the durability of reinforced concrete marinestructures, and consideration of the characteristics of sea water is worthwhile asthis water contains a high percentage of dissolved salts, mainly chlorides.Comparatively few swimming pools contain sea water; the majority of pools in theUK contain water of drinking quality. For reasons of hygiene, the water in the poolis filtered and treated with various chemicals including a disinfectant. The

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disinfectant in most general use in public swimming pools in the UK is chlorinewhich is generally produced by dosing the water with sodium or calciumhypochlorite.

Sea water contains a high percentage of dissolved salts. Figures given by Lea,in The Chemistry of Cement and Concrete, 3rd edition, are quoted below:

This Table can be converted to:

The dosage of chlorine into swimming pool water depends on a number of factorsas the aim is to maintain the ‘free chlorine residual’ at about 0.5–1.0 ppm. If it isassumed for the purpose of this discussion that the actual dosage of chlorine is ashigh as 5 ppm, when the bathing load is heavy, then it can be seen that theconcentration of chloride ions in swimming pool water is less than 0.01% of thatin sea water. On this basis, the chance of chloride-induced corrosion ofreinforcement occurring in a properly constructed swimming pool shell can beconsidered as insignificant, and can be reasonably disregarded until authenticresearch proves otherwise.

3.5 Deterioration of the concrete

3.5.1 Physical damage

The usual causes of physical damage to concrete structures are unlikely to be relevantto swimming pools with the possible exception of open-air pools, which could bein possible danger from frost (freeze-thaw conditions). However, all swimmingpools are provided with a finish, i.e. a decorative coating or tiling/mosaic whichwould protect the concrete from the effects of frost, although the finish itself maybe damaged.

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3.5.2 Chemical attack on the concrete

This is a wide field for discussion as concrete is vulnerable to a range of chemicalcompounds in solution. However, concrete swimming pools are only likely to comeinto contact with a limited range of aggressive agents and these are discussedbelow.

3.5.2.1 Acids and industrial chemicals in the subsoil

All Portland cement-based materials will be attacked by acids (acidic solutions).Generally speaking, inorganic acids such as the three listed below are moreaggressive than organic acids:

Sulphuric acid (H2SO

4);

Hydrochloric acid (HCl);Nitric acid (HNO

3).

These are only likely to be found in sub-soil contaminated by use as anindustrial tip and should be detected by a proper site investigation, seeSection 4.2.

Some salt solutions are acidic, e.g. sodium bisulphate, and alum which are usedin the treatment of swimming pool water to adjust the pH; this is referred to inSection 3.7. Organic acids are present in moorland water and if this class of wateris used in a swimming pool without pretreatment, quite significant attack on exposedconcrete and cement-based mortars can take place.

Acidic solutions have a pH below the neutral point of 7.0 and alkaline solutionshave a pH above 7.0. A solution with a pH of 5.0 has 100 times the hydrogen ionconcentration than a solution with a pH of 7.0.

The pH of a liquid can be readily measured by the use of indicator papers or bya pH meter.

The pH alone does not define the type nor the amount of acid present; it measuresthe intensity of the acidity. It is important to remember that the same concentrationin solution of different acids will give different pH values.

3.5.2.2 Sulphates in solution

As far as the reinforced concrete shell of the pool is concerned, the most likelysource of sulphate in sufficient concentration to cause deterioration of the concreteis from sub-soil and ground water. However, it must be pointed out that if closecontrol of the addition of chemicals to the pool water is not adhered to, sulphatescan build up in the pool water and attack mortar joints between tiles, the tile bedding,and the screed and rendering. See Section 3.6 and Chapter 8. An exception iswhere the concreting aggregates are contaminated by sulphates, e.g. the ArabianGulf or the gauging water contains sulphates in solution as would be the case ifbrackish/saline water had to be used for mixing.

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Sulphates and acids in solution in the sub-soil and ground water are dealt within some detail in Building Research Establishment Digest 363 1991 Sulphate andAcid Resistance of Concrete in the Ground.

Sulphates in solution react with the hydrates of tricalcium aluminate (C3A) inthe cement forming calcium aluminium hydrate (known as ettringite). This reactionis expansive in character and this expansion can cause disruption of the concrete.Figure 3.1 shows a concrete surface attacked by sulphates in solution.

If the sulphate in the sub-soil and/or ground water exceeds the concentrationsgiven in BRE Digest 363, it is advisable to use a Portland cement with a low C3Acontent such as sulphate-resisting Portland cement in which the C3A content islimited to 3.5% (BS 4027). Ordinary Portland cement can contain up to about 12%C3A. Increased sulphate resistance can also be obtained by reducing the w/c ratioand increasing the cement content; also by the inclusion of pfa or ggbs as an additionto the mix. See also Section 2.5.

Portland cement contains gypsum (calcium sulphate) expressed as SO3, by mass

of the cement as this is added during manufacture to control the setting of the

Figure 3.1 Concrete surface attacked by sulphates in solution.

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cement. There is reason to believe that because gypsum is added during manufactureit does not form ettringite on the hydration of the cement paste.

The formation of ettringite may be considered as ‘standard’ sulphate attack.Early in April 1998 the technical press carried reports of severe deterioration ofconcrete bridge foundations and this was claimed to be due to sulphate attackinvolving the formation of the mineral thaumasite.

Thaumasite is a calcium silicate sulphate carbonate hydrate. When this form ofattack occurs the surface of the concrete becomes very soft, rather like lime putty.The reaction needs wet cold conditions and a source of carbonate (which is usuallyfound in calcareous aggregates) and the presence of sulphates or sulphides in thesub-soil in contact with the concrete. Laboratory tests showed that attack on concretedue to the formation of thaumasite could be produced by continuous exposure tohigh concentrated sulphate solutions using limestone aggregate. The fact that thisreaction could take place had been known for many years, but only about fourcases had been reported world-wide prior to the report on the UK motorway bridgefoundations.

The quality of concrete recommended for swimming pool shells is likely topossess considerable resistance to general sulphate attack. See Chapter 4 forrecommended concrete mixes.

3.6 Chemical attack on cement-based mortar

It can be seen from what has been written so far that it is the cement which is theingredient in concrete and mortar that is most vulnerable to chemical attack.

Hand-applied mortar, rendering, bedding for tiles and mosaic and building mortarhave appreciably higher porosity and permeability than concrete and thereforemay suffer attack in conditions where concrete would be virtually immune.

3.7 Swimming pool water and chemicals used inwater treatment

The water used in the vast majority of swimming pools is taken from a publicsupply. While such water is quite fit for drinking, the chemical characteristics ofthe raw water can vary considerably. In the UK, such water can be assumed to fallinto the following general categories: 1. soft, slightly acidic water, low in alkalinity and total dissolved solids; pH in

the range 5.0 to 6.5;2. waters with a pH range between 6.5 and 7.5; alkalinity about 200 ppm;3. waters with higher alkalinity and a pH in the range of about 7.5 to 8.5. Chapter 8 gives information on the basic principles of water treatment which willnot be repeated here. The following compounds are in common use for the treatmentof swimming pool water. Which compounds are used and the dosage will depend

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on a number of factors, including the characteristics of the ‘raw’ water and thebathing load:

Sodium hypochlorite (NaOCl);Calcium hypochlorite (Ca(OCl)

2);

Sodium bisulphate (Na2SO

4);

Hydrochloric acid (HCl);Aluminium sulphate (alum) (Al

2(SO

4)

3);

Carbon dioxide (CO2).

The hypochlorite (sodium and calcium) are both strongly alkaline chemicals andtend to raise the pH of the pool water. They are used to provide chlorine for thedisinfection of the pool water. As the pool water should have a pH in the range 7.2to 7.8, an acidic compound usually has to be added to adjust the pH to the requiredlevel.

The hypochlorite is not aggressive to cement-based materials in theconcentrations used in water treatment. A concentrated solution of sodiumhypochlorite will attack concrete slowly but this is only likely to occur from spillagein storage areas. As previously stated, hydrochloric acid is very aggressive to allcement-based materials and protective measures should be adopted in storage areaswhere spillage is liable to occur. Sodium bisulphate is acidic in reaction (it is oftenreferred to as ‘dry acid’), but is significantly less aggressive to cement-basedproducts than hydrochloric acid. Nevertheless, in areas where spillage of theconcentrate may occur, protective measures to concrete floors and cement/sandscreeds should be adopted. Its use in the pool water (to correct the pH) can result inthe slow build-up of sulphate to undesirable levels. For this reason it is advisablefor cement-based bedding and jointing for tiles and mosaic to be formulated toresist sulphate attack. Reference should be made to Chapter 7.

Aluminium sulphate (alum) is strongly acidic and is used mainly to provide a‘floc’ for the efficient filtration of the pool water. It is also used to adjust the pH ofthe water. The concrete floors of storage areas should be protected by the provisionof a suitable coating based on epoxy or polyurethane resins.

Carbon dioxide is a gas and when dissolved in water forms carbonic acid whichis a weak acid and it is used to adjust the pH of the pool water necessitated by theuse of hypochlorite. When not properly controlled, it can cause etching of cement-based mortar joints in tiling.

Even though the pH of the pool water may be above the neutral point of 7.0 (i.e.slightly alkaline), it may still be ‘lime dissolving’ and therefore needs specialattention. This is discussed in the next paragraph.

3.8 Moorland water and the Langelier Index

Soft moorland waters can be particularly difficult to deal with in their raw state astheir characteristics can vary considerably through the year. It is not unusual to

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find that the pH of the raw water drops appreciably after heavy rain, and may evengo as low as 3.5.

The characteristics of these waters are:

Low calcium hardness;Low alkalinity;Some dissolved carbon dioxide and often some organic acids;Low total dissolved solids (tds).

These waters are aggressive to cement-based materials and Figure 3.2 shows attackon a concrete water channel in a moorland area. The Langelier Index was developedin the USA in the 1930s by Dr Langelier to assess the characteristics of boiler feedwater. A positive Langelier Index indicates that the water is lime depositing, whilea negative index indicates that it is lime dissolving. Lime in this context is calciumcarbonate. The chemistry behind the calculation of the Index is complicated, but itcan be summarised by accepting that if the Index is negative the water will be limedissolving and consequently potentially aggressive to Portland cement. If the Indexis positive the water will be lime depositing and will not be aggressive to Portlandcement. There is a scarcity of published guidance on the practical interpretation of

Figure 3.2 Severe etching of concrete water channel by moorland water.

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the Index. An ISO (International Standards Organisation) document, N18E, ofFebruary 1983 indicates a classification of the Langelier Index for water as follows:

Highly aggressive -2.0 and lowerModerately aggressive 0.0 to -2.0Non-aggressive any positivevalue

If swimming pool water has a negative Langelier Index, it is recommended thatmeasures be adopted to increase the resistance to attack on the bedding mortar andgrouted joints between tiles and mosaic. It is unlikely that the concrete shell of thepool will suffer attack.

Figure 7.6 shows severe erosion of grouted tile joints by a soft moorland water.

3.9 Alkali-silica reaction

Although a library search has not disclosed any reports of alkali-silica reaction(ASR) affecting swimming pools, it is considered that some basic information onthis important subject is justified, as ASR was confirmed some years ago in theconcrete of a water reservoir in the UK.

The environmental conditions in which swimming pools operate (relatively hightemperature and high humidity) are favourable to the occurrence of ASR, but muchmore than this is needed for attack to take place, and this is discussed below.

Alkali-silica reaction was first reported in 1940 in the USA. Since then it hasbeen identified as the cause of expansion and cracking in concrete in many countries.

There are differences of opinion on the number of confirmed cases of ASR inthe UK, and an even greater difference on the extent to which concrete affected byASR suffers loss of strength, load carrying capacity, frost resistance, and increasedrisk of rebar corrosion.

Agreement is general that the effects of ASR are long term and about five yearsis the minimum period required for any visible signs to appear.

Alkali-silica reaction arises from chemical reaction between alkalis in theconcrete and certain types of siliceous aggregates. The alkalis generally originatein the cement and are present in the pore fluid. This reaction results in the formationof an alkali-silica gel. This gel in contact with water expands and causes visiblecracking.

The situation is complicated by the fact that with aggregates in the UK there isa maximum percentage of reactive silica beyond which expansion decreases. Thismaximum amount (the ‘pessimum’), varies from one type of aggregate to another.

For the reaction to take place, there must also be a high moisture level withinthe concrete.

The main precautions which can be taken to avoid ASR damage is to either usea non-siliceous aggregate or use a low alkali cement, or both.

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Also, steps should be taken to prevent solutions of alkalis coming into contactwith and penetrating the concrete. This latter is most unlikely to occur withswimming pools, except from a highly alkaline ground water in an industrial tip,or external concrete paving treated with de-icing salts.

The alkali content of cement is expressed as ‘equivalent sodium oxide’ (Na2O).

At the time of writing this book, the recommended limit in the UK for equivalentsodium oxide is 3 kg/m3 of concrete.

General guidance on ASR is given in:

BS 5328 Part 1 1991;BS 8110 Part 1 1985, clause 6.2.5.4;BRE Digest 330 1988;Concrete Society Technical Report No. 30 1987.

Further reading

American Concrete Institute. A Guide to the Use of Waterproofing, Damp-proofing, Protectiveand Decorative Barrier Systems for Concrete, ACI 515–1R–79 43, 1979, reviewed 1985.

American Concrete Institute. Guide to Durable Concrete, ACI 201–2R–77, reaffirmed 1982.American Concrete Institute. Permeability of Concrete, 11 papers, 1988.American Concrete Institute. Corrosion of Steel in Concrete, 1996.British Cement Association. Minimum Requirements for Durable Concrete, Ed. D.W. Hobbs,

1998.British Standards Institution. Guide to the Durability of Buildings, Building Elements and

Components, BS 7543.British Standards Institution. Guide to Building Maintenance, BS 8210.Building Research Establishment. Sulphate and acid resistance of concrete in the ground,

Digest 363, 1991.Concrete Society. Alkali-silica Reaction: Minimising the Risk of Damage to Concrete,

Technical Report 30, 3rd edition, 1999.Department of the Environment. The Thaumasite Form of Sulphate Attack: Risks, Diagnosis,

Remedial Works and Guidance on New Construction, January 1999.Langelier, W.H. The analytical control of anti-corrosion water treatment, Journal AWWA,

28(1), October 1936.

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Chapter 4

Construction of swimmingpool shells in insitu reinforcedconcrete

4.1 Introduction

This chapter sets out to discuss a number of important matters which are generallyapplicable to concrete water-retaining structures but with special reference toswimming pools.

It is not possible to draw a clear dividing line between design, specification andconstruction. Designers should possess a working knowledge of how the structurewill be built and the problems the contractor is likely to face on site. Equallynecessary, the contractor should understand the basis of the design and the reasonsfor the specification requirements.

Detailed treatment of the structural design of the pool shell is outside the scopeof this book as the design of concrete liquid-retaining structures are covered by anumber of well-known publications, some of which are included under FurtherReading at the end of this chapter.

The three most important factors in the construction of swimming pool shellsin insitu reinforced concrete are: structural stability, durability, and watertightness.

For durability, the steel reinforcement must be protected against corrosion duringthe life-time of the structure. This requires that the concrete cover to the rebarsshould be dense and virtually impermeable. It is difficult to ensure that the concretecover to the rebars (usually specified as 40 mm) is fully compacted and there is noloss of grout (cement and fines) through joints in the formwork.

General (standard) type formwork consists of timber, fibre-glass panels, or steel.The type of formwork used determines the type of finish to the concrete when theformwork is removed. It is important to ensure that the correct type of releaseagent is used, and also that the joints between the formwork panels are grout tight.

The use of what is known as ‘controlled permeability formwork’ can help toachieve the necessary quality of the concrete cover to the rebars. This type offormwork was originally developed in Japan in the 1980s and is now used in Europeand other developed countries.

The formwork is designed to be permeable to water and air but not to cementparticles. The forms consist of an outer shutter, a filter and a drain.

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Work by the Cement and Concrete Association (now the British CementAssociation) showed that about 90% of the pressure on the formwork by the plasticconcrete is caused by the pore water. Therefore, by removing a substantial proportionof the pore water, the pressure on the formwork will be substantially reduced.

On removal of the formwork, the concrete surface has relatively few blow holesand a fine textured finish.

It is recommended that the use of this type of formwork be considered for largeinsitu reinforced concrete pools.

The pool shell should be tested for watertightness, as detailed in Appendix 2,before any finishes are applied.

4.2 Site investigations

4.2.1 General comments

It is essential that site investigations should be carried out in the early stage of thedesign process.

The work should only be entrusted to experienced firms and they should beasked in the brief to give practical interpretation and advice on the results of theinvestigation. The theory and practice of sub-soil surveys falls within the provinceof soil mechanics and is outside the scope of this book. Three important publicationsrelevant to this subject, particularly to larger projects are:

BS 5930 1981 Code of Practice for Site Investigations;BS 8004 1986 Code of Practice for Foundations;DD 175 1988 Code of Practice for the identification of

contaminated land and its investigation.

4.2.2 Reasons for site investigations

There are many reasons for carrying out a site investigation of which the followingthree are the most important:

1. to obtain information on the sub-soil, its physical and chemical characteristics,to enable the designer to decide on the type and dimensions of the foundationsand other parts of the structure below ground level;

2. to ascertain whether the sub-soil and/or ground water is likely to be aggressiveto the concrete in contact with it, including information on whether there arehazardous and/or toxic substances present;

3. to obtain sufficient information for the contractor to appreciate the problemsinvolved in carrying out the work below ground level and to price his contractaccordingly, including the time required to carry out the work.

It is essential that there should be an adequate number of trial pits or boreholes to ensure thatthe information obtained gives a clear picture of sub-soil conditions over the whole site.

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4.2.3 Interpretation of results

In all sub-soil investigations, except rock, the pH of the soil and ground watershould be given. If the pH is below 6.5 or above 9.5, a chemical analysis should becarried out to ascertain the composition of the compounds present which give riseto the low (acidic) or high (alkaline) pH. Information should be obtained (ifpossible), on the likely variations in water table level.

The following factors should be taken into account: 1. There are many practical difficulties in obtaining truly representative samples

of sub-soil and ground water, and in carrying out the analysis.2. All test results should be viewed with caution and carefully considered with

all other relevant information. For example, ground water may be practicallystatic, i.e. the velocity of flow may be very slow, as in clay, or appreciable asin gravels.

3. Due to excavation, and to the control of ground water by pumping subsurfaceconditions can change appreciably and these changes can be significant duringthe construction period, and the life-time of the structure. Where aggressivechemicals come from a specific deposit, e.g. an industrial tip on adjacent land,their quantity may be limited and perhaps removed altogether if the adjoiningsite is developed.

4. The degree of exposure to attack from aggressive chemicals should be assessedin the site investigation report. The following conditions are listed in increasingpotential for attack on the concrete:(a) conditions, above water table level;(b) conditions below static ground water level;(c) conditions below flowing ground water level.

5. Seasonal fluctuations in ground water level and changes in its chemicalcomposition, as well as its direction and velocity of flow can have an importantbearing on the severity of attack. For example, in peaty sub-soils, the pH can fallsignificantly after heavy rain and then rise again.

It may be worthwhile to consider the practicality of permanently lowering thewater table by means of a properly designed sub-soil drainage system. Some generalrecommendations on sub-soil drainage are given in the next paragraph. It may benecessary to lower the water table level to enable the pool to be constructed in thedry. This can be effected by well point dewatering or water collecting pits. However,it is necessary to take into account the effect of lowering the water table on thefoundations of adjacent/nearby structures.

4.3 Under-drainage of site

Under-drainage of sites are carried out for two main reasons: 1. to permanently lower the water table for the reasons given in the precedingparagraph;

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2. to prevent the build-up of ground water pressure which may result in uplift(flotation) causing serious damage to the structure.

4.3.1 Materials and layout

The pipes generally used for sub-soil drainage are: 1. unglazed clayware field drainage pipes to BS 1196 1989;2. plastics pipes for sub-soil field drains to BS 4962 1989. It is also possible, and in some types of fine-grained soils desirable, to use denseconcrete pipes to BS 5911 Part 3 1982 or vitrified clay pipes to BS 65 1991 withopen joints, the joints being surrounded with graded aggregate. Dense concretepipes may be attacked by an aggressive groundwater unless protected by a suitablecoating.

The layout of the under-drainage system will depend on site conditions. Onmany sites, the provision of a simple perimeter drain with inspection chambers/manholes at each change of direction would provide adequate access for periodicinspection and clearing.

4.4 Flotation (uplift) of the pool shell

It is unusual for flotation to be a serious problem with the construction of insituconcrete swimming pools because the dead weight of the concrete shell, withouttaking into account the friction between the ground and the walls, is usually morethan adequate to counter the upward pressure of the groundwater even when thewater table is high.

In any case, the design should be checked for the posibility of flotation.

4.5 General comments on design andconstruction

4.5.1 Introduction

Anyone undertaking the design and/or construction of a reinforced concreteswimming pool is recommended to become acquainted with the relevant provisionsin BS 8007 Code of Practice for the Design and Construction of Concrete Structuresfor Retaining Aqueous Liquids, BS 8110 The Structural Use of Concrete, and BS5328 Concrete Parts 1–4.

The basic publication in the UK is BS 8007, and the equivalent document in theUS and Canada is the American Concrete Institute publication ACI 350 R-89.

Essentially, BS 8007 sets out recommendations for the quality of theconcrete and the structural design of the structure to meet the design life underservice conditions, which is stated to be in the range of 40–70 years. However,

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it is pointed out that some components such as jointing materials have ashorter life than the concrete and are likely to require replacement at intervals.See comments in Sections 2.14 and 2.15 which discuss joint fillers andsealants.

British Standard BS 8007 is linked to the principal structural design Code BS8110. Where the two Codes differ on a specific point, the recommendations of BS8007 take precedence.

British Standard BS 8007 recommends that the minimum conditions ofexposure should be taken as ‘severe’ (see Table 3.2 of BS 8110 and Tables 5 and 6of BS 5328 Part 1). Calculated crack widths are set at 0.2 mm and 0.1 mmdepending on conditions of exposure. This is the width of cracks at the surface ofthe concrete. Cracks in reinforced concrete members are caused by applied loads,by thermal effects (heat of hydration of the hydrating cement and by externaltemperature changes), and by drying shrinkage which occurs as the concretematures. The crack width and crack spacing is controlled by reinforcement and thelocation and type of joints.

4.5.2 Joints

The selection of the type of joints and their location is fundamental to thecalculation of the distribution steel for crack control. There is some conflictbetween the wish to keep the amount of distribution steel within ‘reasonable’limits and the known fact that joints are the major source of leakage in aconcrete water retaining structure. The author’s experience is that jointsshould be kept to a practical minimum.

In the case of swimming pools which are finished with tiles/mosaic, it isstrongly recommended that there should be close co-operation between thesuppliers of the tiles/mosaic and the designer of the pool shell. The reason forthis is that joints in the pool shell which may have to accommodate movementshould be carried through the tiling/mosaic and the cutting of tiles should beavoided as far as this is practical.

The joints considered here are:

Full movement joints (also known as expansion joints);Contraction joints;Partial contraction joints;Stress relief joints;Construction joints (also known as day-work joints);Sliding joints.

4.5.2.1 Full movement joints

This type of joint is designed to cater for both expansion and contraction of theconcrete on each side of the joint. Figures 4.1 and 4.2 show these joints.

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It is desirable that a water bar be provided and that the joint should be sealed. Itshould be noted that this type of joint is about 15 mm to 20 mm wide. Reinforcementdoes not cross the joint. However, if it is decided that there may be non-uniformsettlement across the joint, it may be desirable to insert dowel bars (as in a roadslab), but this is unusual. It is important that the wall should be of adequate thicknessto enable the concrete to be thoroughly compacted around a centrally located waterbar. The thickness is likely to be 300 mm to 400mm.

4.5.2.2 Contraction joints

This type is shown in Figure 4.3. There is no initial gap left between the adjacentconcrete, but no attempt is made to secure bond at the interface. The reinforcementis stopped off each side of the joint.

Figure 4.1 Full movement joint in floor slab.

Figure 4.2 Full movement joint in wall.

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The joint becomes a Partial Contraction Joint if some of the reinforcement (up to 50%)crosses the joint.

The joint should be sealed, and provided with a water bar which is centrallylocated in a wall, and located on the underside of a floor slab; see note above onminimum wall thickness for installation of centrally located water bars.

4.5.2.3 Stress relief joints

These joints are formed in the freshly placed concrete in floor slabs, and sometimesin walls.

A crack inducer is fixed at a predetermined position in the underside of the slaband directly over this a slot is wetformed in the top surface of the slab to a depth ofone-third of the overall depth of the slab. The surface slot can be sawn, but thecorrect time after casting the concrete to carry out the sawing is very difficult todetermine. If too early, the concrete is likely to ravel and, if too late, a crack may

Figure 4.3 Contraction joint in floor slab.

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form which is unlikely to be straight; in either event, sealing will be very difficult.Reinforcement crossing the joint is generally reduced by 30–50%. The principle isthat thermal contraction and drying shrinkage will cause a crack to form throughthe slab in a straight line which can be sealed.

The joint should be sealed and provided with a water bar as for a contractionjoint.

An alternative type of stress relief joint in a wall is shown in Figure 4.4. Thereinforcement crossing the joint is reduced by 30–50%. The diameter of the voidshould be about one-third of the wall thickness.

4.5.2.4 Constructionlday work joints

These joint are formed to facilitate the construction of the pool shell; also, theybecome necessary if there is a serious delay in the supply of concrete. Such jointsshould be formed with a stop-end and all necessary steps taken to ensure maximumbond between the ‘old’ concrete and the newly placed concrete when placing startsagain. The surface of the hardened concrete should be lightly bush hammered orother means used to expose the coarse aggregate, followed by removal of all loose

Figure 4.5 Concrete surface at construction/day-work joint prepared to secure bond.

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grit and dust. Reinforcement should be carried across the joint. The objective is tomake the concrete at the joint as monolithic as possible. Figure 4.5 shows a concretesurface prepared.

4.5.3 The concrete

Since the third edition of this book was published in 1988, the recommendationsfor concrete mixes to meet specific requirements for buildings and civil engineeringstructures have been greatly extended. These requirements are included in BS 5328Concrete Parts 1–4.

The contractor has the option of deciding on site mixing or using ready-mixedconcrete. The use of ready-mixed concrete is recommended. At the time this bookwas being revised, about 80% of concrete used in the UK was supplied by ready-mixed concrete firms. It is recommended that ready-mixed concrete should besupplied by a QSRMC Registered Company from a plant holding current QSRMCertification for Product Conformity. The QSRMC Certification Mark should beon all quotations and delivery tickets.

For swimming pool shells, the concrete should be either a designed, a prescribed,or designated mix as detailed in Part 2 of BS 5328.

The main differences in the selection of the type of mix referred to in BS 5328relate to: 1. the responsibility for selection of mix proportions;2. the terms in which the mix is specified;3. the main parameters used for judgement of conformity. For a designated mix, the concrete should have a characteristic strength of 35 N/mm2, with a maximum w/c ratio of 0.50 (but a lower w/c ratio plus the use of aplasticiser is recommended by the author.

Aggregates should be 20 mm maximum size, well graded and complying withBS 882 Aggregates from Natural Sources for Concrete. The workability should beadequate for full compaction (say a nominal slump of 75 mm).

It is essential that the supplier of the concrete, whether this is a ready-mixedconcrete firm or a contractor who wishes to mix the concrete on site, is given thenecessary information so that it is clear exactly the type and standard of concreterequired. Tables in Part 2 of BS 5328 provide detailed information for thespecification requirements of designed, prescribed and designated mixes.

It was emphasised in Section 1.2 that the pool shell should be watertight againstloss of water when the pool is full and against ingress of ground water when thepool is empty. This necessitates a practical water test, and the details of a suitabletest are given in Appendix 2.

Concrete has a pore structure and water can very slowly move through it, butwith good quality concrete and proper design, this permeability is unlikely to havean adverse effect on the durability of the structure.

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British Standard BS 8007 emphasises this and states that the concrete shouldpossess low permeability which is one of the important characteristics required toensure durability of the structure (see also Chapter 3). Permeability may be definedas the characteristic of a material which allows fluids to pass through it underdifferential pressure. Low permeability helps to ensure resistance to chemical attack,and protection of the steel reinforcement. To secure low permeability, the mixproportions of the concrete have to be carefully designed and this is emphasised inthe Code.

Consideration should be given to the effect of the heat of hydration on themaximum temperature likely to be reached by the concrete, which is importantwhen the concrete is cast in timber formwork, particularly in hot weather. This canresult in thermal contraction cracking and the Code deals with this in some detailunder Temperature and Moisture Effects. Moisture effects in this context refer tothe drying out of the concrete after casting, and these ‘effects’ consist of dryingshrinkage which can result in cracking. The thermal effects (cracking) will occurin the early age of the concrete, generally within a few days after removal of theformwork, while shrinkage cracks are likely to appear later. This is the reason forthe suggestion given above to reduce the w/c ratio and thus reduce the amount ofwater in the mix when it is placed, and this in turn reduces the risk of dryingshrinkage cracking.

Thermal contraction cracking can be controlled by specific design of thereinforcement, e.g. by increasing the amount of distribution steel, and/or by reducingthe length of wall or floor slab between joints. The reduction of the amount ofPortland cement in the mix by replacing, say, 20% with ground granulatedblastfurnace slag (ggbs), or pfa, will also help.

The type of aggregate used also has a significant effect on the thermal movement(expansion or contraction) of the concrete. The coefficient of thermal expansionof concrete made with a flint gravel is generally taken as 12–14× 10-6 per °C, whilefor the same mix using a limestone aggregate, it would be about 7–8×10-6 per °C.It has recently been suggested that the use of limestone aggregate concrete in contactwith a high concentration of sulphates in the ground water may trigger the occuranceof thaumasite attack.

It is important that the specified nominal cover to all reinforcement is maintainedby the use of spacers and careful fixing of the reinforcement. It is recommendedthat the cover to reinforcement in walls be checked by means of a cover-metersurvey as soon as practical after the removal of the formwork. A similar exerciseshould be carried out on the floor slab as soon as practical after casting and finishingthe concrete. Some information on cover meter surveys is given in Section 10.22.2and in Appendix 3.

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4.6 Concrete construction in cold weather

4.6.1 General considerations

The information given here is intended to bring out important principles whichshould be followed when concreting in cold weather. For more details on thissubject, readers are referred to Further Reading at the end of this chapter.

The important factors involved in using Portland cement concrete when airtemperatures are near freezing point are set out below: 1. When the temperature of the setting and maturing concrete is lowered, the

chemical reaction between the cement and the mixing water is slowed downand the rate of gain of strength decreases. As the temperature of the concreteapproaches freezing point, the hardening process practically ceases.

2. However, if the concrete is not saturated with water and if it has reached acompressive strength of not less than 3 N/mm2, and has a reasonable cementcontent (300 kg/m3), then even if the concrete does freeze it is unlikely thatpermanent damage will result. When the temperature of the concrete risesagain the maturing process (hardening) will recommence and will continue ata rate proportional to the temperature of the concrete.

3. It is the temperature of the concrete which is the key factor and the concreteshould not be placed unless it has a temperature of at least 10 °C.

The precautions to be taken to prevent frost damage to maturing concrete must bedirected towards maintaining the temperature of the concrete as high as practical,by providing thermal insulation and/or using heated concrete. However, thetemperature of the heated concrete should not exceed about 30 °C at the time ofplacing.

4.6.2 Recommended precautions to be taken

The following simple rules, if properly applied, will enable concreting to proceedin the severe weather likely to be experienced in the UK. 1. Frozen aggregates and icy water must not be used.2. Concrete must not be placed on frozen ground nor in frozen formwork. Wet

curing should not be used. The cement content should not be less than 300 kg/m3. It can be advantageous to increase the cement content by, say, 1½ bags(75 kg) per m3 unless there is a specific reason not to do so. Or to use rapid-hardening Portland cement. Sulphate-resisting Portland cement generally hasa slower hardening rate than ordinary Portland cement, and if sulphate-resistantPortland cement is required to resist sulphate attack, it may be necessary tosuspend concreting until the air temperature rises.

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3. The temperature of the concrete at the time of placing should not be lowerthan 10 °C. This can be achieved by ordering heated concrete from a ready-mixed concrete plant which has the equipment to produce this type of concrete.The usual way of producing heated concrete is to heat the mixing water, butthe maximum temperature should not exceed about 50 °C.

4. The concrete must be well insulated as soon as the finishing processes havebeen completed. This is particularly important when casting slabs on theground, as the exposed surface area to volume ratio is high. The degree ofexposure of the site should also be given consideration.

4.7 Concrete construction in hot weather

4.7.1 General considerations

It may be thought that, in temperate climates (such as in the UK), there would beno problems arising from concreting in the summer months. However, withtemperatures in the high twenties and low thirties, difficulties can be experienced,including premature stiffening of the concrete which makes placing and compactiondifficult, and an increased risk of plastic cracking and thermal contraction cracking.The adverse consequences of the neglect of proper curing and protection againstdirect sunshine and strong wind will be increased.

It has been mentioned in the preceding paragraph that increase in the temperatureof the concrete speeds up the chemical reaction between the mixing water and thecement. In the summer, the increase in ambient temperature provides this additionalheat. For example, concrete placed at a temperature of, say, 10 °C may reach atemperature of about 30 °C in 24 hours, while the same mix placed under the sameexternal conditions but with a casting temperature of 20 °C may reach a temperatureof about 55 °C after 20 hours. Also, the heat of hydration of the cement type usedwill affect the temperature rise of the concrete.

4.7.2 Recommended precautions to be taken

The following matters should be given careful consideration: 1. If the specified cement content exceeds 330 kg/m3, it may be advantageous to

reduce this, but the need to maintain the specified strength and low permeabilitymust be kept in mind.

2. The possible change in the cement type from OPC to a cement containing pfaor ggbs (BS 6588 or BS 4246), but such a change may increase the strikingtime for formwork.

3. The aggregate stock piles can be sprayed with water as this will lower thetemperature of the aggregate.

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4. The mixing water should be as cool as possible. Water drawn from a publicsupply main will be reasonably cool, but if kept in a black painted steel tank itwill become quite hot, and water storage tanks should be painted white.

5. The use of a retarder is likely to be essential when ready-mixed concrete hasto be transported over a long distance.

6. Special attention to proper curing is essential and consideration may have tobe given to the use of tentage for floor and roof slabs.

4.8 Plastic cracking

There are two types of plastic cracking, namely plastic shrinkage cracking andplastic settlement cracking. The former is more common, and the latter is unlikelyto be encountered in the construction of a swimming pool shell and associatedfloor slabs.

4.8.1 Plastic shrinkage cracking

This type of cracking may occur on the surface of floor and roof slabs while theconcrete is still plastic. Investigations by various authorities have established thatthe principal cause is the rapid evaporation of moisture from the surface of theconcrete while it is still in a plastic or semi-plastic state. When the rate of evaporationexceeds the rate at which water (known as bleed water) rises to the surface of theconcrete, plastic shrinkage cracking is very likely to occur.

Figure 4.6 Plastic shrinkage cracks in floor slab. Courtesy, G.F.Blackledge.

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The cracks are usually very fine and are often not noticed until the next day(Figure 4.6).

These fine cracks are usually straight and short and transverse in direction, andrarely extend to the slab edge. They are sometimes parallel to each other and thespacing can vary from about 50 mm to 300 mm; the cracks are shallow and seldomextend to below the top layer of reinforcement. Figure 4.6 shows plastic shrinkagecracking in the floor slab of a swimming pool.

While these cracks generally occur in hot weather, they can also appear oncooler days if the concrete is exposed to a strong wind. They should begrouted in with a Portland cement grout, preferably including a styrenebutadien (SBR) emulsion, say 10 litres to 50 kg cement. The treated surfaceshould be covered with polythene sheeting, held down around the edges withplanks or blocks.

The following are recommended precautions to be taken when conditions arelikely to be suitable for this type of cracking to occur: 1. The formwork (if any) should be well damped down prior to placing the

concrete. Slabs cast ‘on the ground’ should be separated from the sub-base by1000 gauge polythene sheeting.

2. The aggregates, particularly if they are dry, should be sprayed with water. Itmay be advisable to use a slightly finer grading of sand.

3. An air entraining admixture can often be used with advantage. The mean aircontent of the mix, when using 20 mm maximum size aggregate should be5.5% by volume of the fresh concrete (BS 5328 Part 1 clause 4.3.3). Theallowable tolerance on the 5.5% is given in Part 4 of the Standard, clause 3.6.

4. The placing, compacting and finishing should be proceeded with as quicklyas possible without delay between each operation.

5. Curing should be commenced as soon as possible after finishing is completeand the surface of the concrete should be protected from hot sun and strongwind.

4.9 Thermal contraction cracking

4.9.1 General considerations

This is a common type of cracking in the walls of reinforced concrete water retainingstructures including swimming pools.

During the setting and early hardening of concrete, considerable heat is evolvedby the chemical action between the mixing water and the cement which results ina rise of temperature of the concrete; this has been referred to in Section 4.7.

The actual rise, the peak temperature and the time taken to reach the peak andthen to cool down depend on a large number of factors of which the following arethe most important:1. the temperature of the concrete at the time of placing;

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2. the type of formwork used (whether timber, steel or plastic) and the time theformwork is kept in position;

3. the ambient air temperature;4. the volume/exposed surface area of the concrete;5. the thickness of the section cast;6. the type of cement and the cement content of the mix;7. the method of curing;8. the amount and detailing of crack control reinforcement, see BS 8007. As the temperature of the concrete rises it expands and when it cools down itcontracts. By the time it starts to cool, the concrete has already started toharden so that tensile stress set up by the contraction can only beaccommodated without crack formation if the tensile strength of the concreteand/or the bond strength with the distribution reinforcement is not exceeded.It must be kept in mind that the walls are restrained at the base as the jointbetween the kicker and the wall panel is specially prepared to securemaximum bond and minimum permeability.

Figure 4.7 shows a thermal contraction crack in the wall of swimming pool. This type of crack is usually narrow, seldom exceeding 1.0 mm in width, but it can pass

Figure 4.7 Thermal contraction crack in wall.

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through the wall and is a potential source of leakage. They are often present when theformwork is stripped but are frequently not noticed until some time later when their‘discovery’ causes consternation to designers and contractors alike.

Once the formwork is removed, drying shrinkage will start to take effect,and unless controlled by careful curing will tend to widen the existing thermalcracks.

Cracks of ‘design’ width, i.e. 0.1 mm and 0.2 mm, are unlikely to allow waterto reach the rebars provided the specified cover (normally, 40 mm) has beenprovided.

Experience shows that some cracks seal themselves by ‘autogenous healing’,but this cannot be relied upon.

4.10 Swimming pools with floor slabs supportedon the ground

4.10.1 Casting the floor slab

The floor slab should be cast on a sliding layer consisting of two sheets of 1000gauge polythene laid on a compacted granular sub-base, not less than 100 mmthick.

The concrete should be thoroughly compacted by a vibrating tamper workingoff the side forms. The type of finish required should be clearly stated in thespecification. The tamper can be a single or double beam (twin beam compactor).Poker vibrators should be used around the perimeter (next to the side forms) tohelp ensure adequate compaction.

The concrete should be placed with a surcharge of about 50 mm to allow forreduction due to compaction. The actual surcharge will depend on the slump of theconcrete and the thickness of the slab. The surface left by the tamper will be ribbed.To provide a smoother finish, the tamper should be taken back every 1.5 m to 2.0m and then moved forward slowly over the compacted surface to smooth out theridges and furrows left by the first pass of the beam. The finish can be furtherimproved by the use of a power float followed by a power trowel. To achievesatisfactory results, the concrete must have reached the right degree of stiffnessbefore the power floating. The use of the power float and power trowel should notbe necessary if the floor slab is to receive a screed. However, the power floatingand trowelling would be desirable if the finish is to be a proprietary coating orPVC sheeting.

4.10.2 Slab reinforcement

The reinforcement for the main floor slab could be either high yield deformed barsor high tensile fabric, and is normally located in the top of the slab with 50 mmcover. There is usually a perimeter bay which forms the heel of the wall. Thedetailing of reinforcement in the perimeter bay at the four corners of a rectangular

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pool requires careful thought and experience, as the wall reinforcement has to besecurely anchored into this slab.

4.10.3 Joints in the floor slab

The selection of the types of joints and their location is an essential part of thedesign, and has been discussed in some detail in Section 4.5.2.

4.10.4 Pipework through the floor slab

The main outlet pipe for the pool is located at the deep end and special care isrequired to ensure that leakage does not take place at this point. In some specialsystems of water circulation, the inlets for the treated water are located longitudinallyon the centre line of the floor.

There are two ways of carrying the pipes through the floor (and in fact the sameapplies to the walls), namely by boxing-out, or building-in, as the work proceeds.

There are differences of opinion as to which method gives the better resultsfrom a watertightness point of view, and this is discussed below. It should be notedthat the majority of the pipes used for water circulation are now plastic instead ofsteel or cast iron.

4.10.4.1 Boxing-out

This method has to be adopted when for one reason or another it is not practical toinsert the pipe at the time the slab is cast, e.g. the pipe may not be available or itsexact position has not been finally determined.

The boxed-out hole must be of adequate size to accommodate the pipe andallow for compaction of the concrete around it. For small diameter pipes, upto, say, 75 mm diameter, mortar can be used instead of concrete. The mortarshould have a low w/c ratio and a styrene butadiene emulsion should be addedto the mix (10 litres of SBR to 50 kg cement). Thorough compaction isessential.

The pipe should be cleaned and all dirt and coating/paint removed prior tofixing. A flange should be provided on the water face of the floor slab. Figure 4.8shows a detail of a boxed-out steel pipe with a flange on the water face of the floorslab or wall.

If plastic pipe is used it will be difficult to obtain a good bond between thepipe and the concrete/mortar. The surface of the pipe should be roughened anda specially formulated resin bonding coat applied. This should be sprinkledwith coarse sand while it is still tacky and then allowed to harden, the objectbeing to assist bond with the concrete/mortar. An alternative is toaccommodate the pipe in a galvanized steel sleeve which would overcome theproblem of bond.

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4.10.4.2 Building-in

This method should be adopted whenever possible and involves casting-in thepipe as the concrete is cast. The same precautions should be taken to ensure goodbond between the pipe and the concrete. A flange should be provided on the waterface of a floor slab, but can be centrally located in a wall panel which in fact is theusual location for what is known as a ‘puddle’ flange.

4.11 Construction of the walls of the pool

4.11.1 Casting the concrete

The walls are cast within formwork firmly secured at the base to the kicker whichforms part of the perimeter bay of the floor slab. The thickness of the wall will bedetermined by the design, but it must be thick enough to allow for thoroughcompaction of the concrete, bearing in mind that there will be four layers ofreinforcement and two lots of cover.

Also, the designer may have decided to specify the use of water bars incontraction joints as well as in expansion joints. This means that the minimumthickness of the wall would be 300 mm without a water bar, and probably 400 mmwith water bars. Figure 4.2 shows a PVC water bar in an expansion joint in a wall.The PVC water bar must be securely fixed to the reinforcement otherwise if mayinterfere with the placing and compaction of the concrete. Displaced water barsare a source of leakage which is very difficult to rectify.

The top surface of the kicker should be carefully prepared so thatmaximum bond is obtained at this position where stress is likely to be at amaximum. This preparation can consist of bush hammering to lightly expose

Figure 4.8 Boxing-out for pipe through wall.

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the coarse aggre gate, or the use of high-velocity water jets. If the surface isscabbled then all grit and dust must be removed before the concrete is cast onit. The surface should be well damped down but care taken to avoid standingwater between the pieces of exposed aggregate. Figure 4.9 shows the topsurface of a kicker prepared for the casting of the wall panel. The use of awater bar in the kicker is not recommended as it is very easily displacedduring the placing and compaction of the concrete.

The wall panels should be cast to their full height in one lift.

4.11.2 Joints in the walls

The question of type and location of joints has been discussed in Section 4.5.2.The joint between the formwork and kicker must be grout tight otherwise

honeycombing at the base of the wall is likely to occur resulting in leakage whenthe water test is applied. The same comment applies to joints in the formwork.

4.11.3 Execution of the work

The reinforcement should be checked for specified cover and for general condition.It should be free from grease and loose scale rust; light powdered rust does no hardand may in fact improve bond with the concrete.

Figure 4.9 Top surface of kicker prepared to receive placing of concrete for wall.

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The problem of thermal contraction cracking has been discussed in Section 4.9and this is relevant to the type of formwork used (whether timber or steel) and thelength of time it is kept in position. If there is any fear about the probable occurrenceof thermal contraction cracks, it is better to leave the formwork in position for aday or so longer. The time lost and the cost of repairing these cracks can beconsiderable.

4.11.4 Pipework through walls

Pipes for the water circulation system will pass through the walls below top waterlevel and the precautions outlined in the previous paragraph for the floor of thepool also apply to the walls. However, in the case of the walls, if the pipes are castin as the work proceeds, a puddle flange can be provided in the centre of the wall.A flange on the water face is not required unless the pipes are sleeved. The omissionof the surface flange would simplify the formwork.

4.12 Construction of walkway slabs and floors of wet changing areas

4.12.1 Introduction

If these slabs are suspended to provide space below for some specific purpose,such as storage, plant rooms etc., then it is essential that they should be designedand constructed so as to be completely watertight. They should be tested forwatertightness as described in Appendix 2.

Slabs uniformly supported on the ground need not be designed for watertightness.The design and method of construction will depend on whether the pool is openair or enclosed.

4.12.2 Suspended slabs

The slabs should be designed to the ‘water retaining’ Code (BS 8007). This maysound obvious, but it is surprising the number of cases where this has not beendone. This omission can result in serious leakage through the slab into the utilisedarea below (Figure 4.10).

The leakage is often only reported some time after the completion of theswimming pool complex and can only be rectified at considerable cost andserious dislocation of the use of the pool. The standard of watertightness ofthe slab itself should be the same as for the roof slab of a building. Accountshould not be taken of applied finishes such as screeds and non-slip ceramictiles or mosaic. The slab should be tested for watertightness, as described inAppendix 2.

Seepage is likely to occur through cracks and joints, and pipework passingthrough the slab such as drainage channel, outlets etc. This requires careful detailing

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and consultation between the designer and the suppliers of the fittings and pipeworkis desirable. The provision of a flange on the top and/or bottom surface of the slabis likely to provide a practical solution. The precautions described in the previousparagraph for pipes passing through the pool floor are applicable here.

These slabs can be designed as flat slabs and all interior joints are detailed asconstruction joints so that the whole slab is virtually monolithic.

The perimeter of the slab can be tied to the supporting structure, but this restraintmay introduce complex stresses and therefore a sliding joint is often preferred(Figure 4.11).

If the joint is tied, any movement joints in the supporting walls should be carriedthrough into the suspended slab and this will increase the problem of ensuringcomplete watertightness.

4.12.3 Slabs supported on the ground

If the pool is an open air one, then the walkways can be constructed as described inChapter 6. If the pool is enclosed, then the walkways and other ‘wet’ areas can beconstructed as normal ground floor slabs as required by the Building Regulationsand the relevant Approved Documents made thereunder.

Figure 4.10 Seepage through walkway slab into plant room below.

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4.13 Curing the concrete floor and walls of thepool

Some general information on the materials used for curing concrete has been givenin Section 2.11.

Recommendations for the curing of the concrete of swimming pool shells isgiven below.

Concrete swimming pools are normally provided with a decorative and easilycleaned finish consisting of either a proprietary coating or ceramic tiles/mosaic,or, in the case of private pools, an insitu terrazzo known as marbelite. All thesefinishes are required to bond strongly either directly to the base concrete or to anapplied rendering which also has to bond well to the base concrete. For this reason,the use of a spray-applied curing membrane is not recommended as this is likely tointerfere with the bond at the interface of the finish with the concrete. Wet curingwith water is also not recommended as it is very difficult to ensure proper control.

A practical solution is to use polythene sheeting securely fixed so that windcannot blow underneath it. The sheeting should be kept in position for a minimumof four days after completion of the casting of the concrete floor.

For the walls, special frames are needed to secure the sheeting and these shouldbe kept in position for four days after removal of the formwork. If the weather is‘favourable’ (mild and damp), it may be practical to keep the formwork in positionfor a few extra days and omit special curing procedure.

Figure 4.11 Sliding joint between walkway slab and supporting structure.

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4.14 Construction of suspended pool shells

4.14.1 Introduction

Very few pools are designed and constructed as completely suspended structures;when this is done it is usually required to utilise the space below the pool itself.Examples are large public pools when it is required to locate plant rooms, storesetc. beneath the pool shell.

A pool located on the upper floor of a building has to be designed as a suspendedstructure and is constructed in a special structural void.

The standard of watertightness required is much higher than required for anormal pool located on or in the ground. This creates a number of special problemsof which the most important is the need to ensure that there is no moisturepenetration through the pool shell into the areas below. While a damp patch may

Figure 4.12 Sketch showing suggested arrangement for reinforced concrete pool on upperfloor of a building.

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be tolerated on the ceiling of a plant room, it would not be accepted on the ceilingof a hotel room or similar.

The type of finish required, i.e. a decorative waterproof coating or ceramictiles/mosaic, will influence the design and method of construction of the poolshell.

The precautions outlined below may be considered as rather exaggerated, butthought should be given to how a leak can be located and repaired.

4.14.2 Methods of construction

It is assumed that the pool shell is located in a structural void in the building so thatthe outside of the walls and floor are available for inspection. It is also recommendedthat the void should be tanked so that should leakage occur it will be contained andwill not penetrate to other parts of the building. All pipework should be in accessibleducts (Figure 4.12). Suggested methods of construction are: 1. Insitu reinforced concrete post-tensioned to ensure that the walls and floor of

the pool are in a state of permanent compression (Figure 4.13). It would be logical to post-tension the walkway slab and the floor slab of

Figure 4.13 Reinforcement with nominal prestressing for pool on upper floor of a buildingin London. Courtesy, S.B.Tietz & Partners, Consulting Engineers and Scarlett,Burkett, Griffiths, Chartered Architects.

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wet changing areas, but this is a matter of design. If joints, other than day-work joints, can be eliminated, this would be very advantageous from thepoint of view of watertightness.

2. Insitu reinforced concrete with sandwich type membrane. This increases thedead load considerably.

3. Insitu reinforced concrete finished with either fully bonded PVC or similarsheeting or a decorative waterproof coating applied by spray.

In methods 2 and 3, the walkways around the pool, and wet changing areas if thereare any, should be provided with a membrane fully bonded to the structural floorslab. The membrane can be sheet material or a waterproof coating applied by spray;the important point is that the membrane must be fully bonded to the base concreteand be sufficiently tough to withstand the laying of the finishes on top.

An alternative is to form the pool shell out of steel. However, there can beproblems with the application of finishes, the flexing of the pool shell, and corrosion.See paragraph 5.33.

The void in which the pool is located should be large enough for men to enter,carry out a detailed inspection, and if necessary repair any leakage. Permanentlighting and power points should be provided in this working space.

There should be a regular routine inspection, say, every six months which shouldbe recorded.

4.14.3 Additional matters for consideration

When the pool is located on the upper floor of a residential building special attentionshould be given to: 1. The method to be adopted for the disinfection of the pool water. Chlorine, due

to its penetrating smell is not suitable and some other method should be selectedsuch as ozone, or metallic ions. See Chapter 8.

2. An efficient system of mechanical ventilation to ensure that the warm humidair in the pool hall does not find its way into other parts of the building. Analternative is complete aerial disconnection between the swimming pool halland associated rooms, and the rest of the building.

4.15 Thermal insulation of swimming pool shells

It is sometimes suggested that thermal insulation should be provided to thepool shell.

By far the greatest loss of heat is from the surface of the water with only a smallpercentage through the floor and walls to the surrounding ground unless the watertable is high. With a permanently high water table, the loss of heat from the heatedwater in the pool may be significantly increased, in which case the installation of

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thermal insulation is worth considering. To be effective, the insulation must bevirtually impermeable to water.

4.16 Under-water lighting and under-waterwindows

The provision of under-water lighting can be very attractive, and it can improvesafety for the bathers. Under-water windows require an access corridor or similarfor use by viewers, and should be installed between two under-water lights.

It is essential that recesses for the light fittings should be formed when thepool shell is being cast, and the same applies if the light fitting is installed in awatertight tube.

If it is decided to install the fittings after the pool shell has been constructed, itcan prove very difficult indeed to make such openings watertight.

The whole installation should be corrosion-proof and water-proof. Thewindow(s) must be made of high-quality safety glass.

The equipment and wiring etc. should be installed and tested in accordancewith the latest edition of Regulations for Electrical Installations, published by theInstitution of Electrical Engineers (IEE). The light fittings should preferably beinstalled so that bulbs can be changed without having to lower the water level inthe pool.

The fittings themselves are proprietary items and recommendations forinstallation should be obtained from the manufacturers.

The International Board for Aquatic Sports and Recreational Facilitiesrecommend that, in swimming pools, the lights should be installed at a depth ofabout 0.90 m below the water surface as this will prevent dazzle to swimmers andspectators. In deep pools, it may be desirable to install a second row of lights. Thelongitudinal distance between the lights is usually 2.0 m to 3.0 m. In diving pools,lights should only be installed in the side walls.

Inspections and tests should be carried out at intervals prescribed by the IEE,and these should be recorded.

Further reading

American Concrete Institute. Environmental Engineering Concrete Structures, ACI350-R-89.

American Concrete Institute. Testing Reinforced Concrete Structures for Watertightness,ACI Committee 360/AWWA Committee 400–511.

American Concrete Institute. Practitioners Guide to Hot Weather Concreting, 1996.American Concrete Institute. Cold Weather Concreting, ACI Committee 306, 1988.American Concrete Institute. Practitioners Guide to Cold Weather Concreting, 1997.American Concrete Institute. Guide to Sealing Joints in Concrete Structures, ACI 504-R-90.American Concrete Institute. Control of Cracking in Concrete Construction, ACI Committee

224, 1989.

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Anchor, R.D. Design of Liquid Retaining Concrete Structures, 2nd edition, Edward Arnold,London, 1992.

British Cement Association. Concrete on Site No. 11; Winter Working, 1993.British Standards Institution. Code of Practice for Site Investigations, BS 5930.British Standards Institution. Code of Practice for Foundations, BS 8004.British Standards Institution. Code of Practice for Building Research Establishment, Sulphate

and Acid Resistance.British Standards Institution, Code of Practice 4: Indentification of Contaminated Land and

its Investigation, DD175, 1988.Concrete Society. Formwork—A Guide to Good Practice, CS 030, 1995.Concrete Society. Joints in insitu concrete, Digest No. 10, CS 053, 1988.Concrete Society. Non-structural Cracks in Concrete, Technical Report 22, 1992.Concrete Society. Curing concrete, Digest No. 3, 1985.Deacon, R.C. Watertight concrete construction; Cement and Concrete Association, 46.504

1980.Harrison, T.A. Early-age crack control in concrete. CIRIA Report 91, 1981.International Standards Organisation. Building Construction Sealants, Classification and

Requirements, ISO 11600.Pink, A. Winter Concreting, British Cement Association, 1978.Price, W. Recent developments in the use of controlled permeability formwork, Concrete,

March 1998, pp. 8–10.Shirley, D. Concreting in Hot Weather, British Cement Association, 1980.

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Chapter 5

Construction of swimmingpool shells in reinforcedsprayed concrete and othermaterials

REINFORCED SPRAYED CONCRETE (SHOTCRETE)

5.1 Introduction

The term gunite was originally used in the UK, for pneumatically applied mortar(cement, sand and water); in the US this was known as shotcrete. With the increasinguse of a coarse aggregate in addition to the sand, the term now generally used in theUK is sprayed concrete. The American Concrete Institute (ACI), defines shotcrete as‘mortar or concrete pneumatically projected at high velocity onto a surface’.

The Concrete Society (UK), published a specification for sprayed concrete in1979, and Guidance Notes on the measurement of sprayed concrete in 1981.

The UK Code for the design of concrete water-retaining structures (BS 8007)only devotes one short clause (6.7) to pneumatically applied mortar and does notoffer any detailed advice or information on the use of this material. The Codecomments: ‘It is a specialist operation…’ and ‘the designer should agree a fullspecification with the contractor for materials, mix proportions, mixing, placing,equipment and curing…’ The implication of this statement is that structuresconstructed of pneumatically applied mortar are outside the scope of the Code.No reference is made in the Code of pneumatically applied concrete, and this isunfortunate as sprayed concrete has been used satisfactorily for many years forswimming pools, both large and small. The material is also used for repair andstrengthening reinforced concrete structures and for lining tunnels.

In the US and Canada the relevant Code for concrete water retaining structuresis ACI 350 R-89 Environmental Engineering Concrete Structures. This is similarin many respects to the UK Code (BS 8007), and does not refer to the constructionof water retaining structures with shotcrete.

The use of sprayed concrete has a number of advantages:

1. Formwork (which is very expensive) is virtually eliminated.2. The pool can be formed to any desired shape without undue difficulty and

significant increase in cost.

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3. High speed of construction is possible.4. The pool can be constructed on a congested site where access for materials

and equipment is severely restricted, because the delivery hose to the gun canbe at least 100 m long.

5. The only joints normally found in sprayed concrete pools are plain, buttconstruction joints which are usually not provided with a sealing groove andsealed with sealant.

There are a number of disadvantages with shotcrete pools:

1. The design and construction is not specifically covered by the UK Code (BS

8007).2. The problem of flotation can be a real one and this should be checked in all

cases, and where necessary, suitable precautions should be taken. See Section4.4 and 5.4.1

3. The usual design incorporates a wide cove angle at the junction of the walland floor; this prevents the use of ceramic tiles as a finish. The alternative is touse ceramic mosaic, or to design the pool shell to eliminate the wide cove atthe junction with the floor (Figures 5.1 and 5.2). Pools constructed for national and international competitions should

Figure 5.1 Reinforcement prepared for construction of pool shell in sprayed concrete.Courtesy, Cement Gun Co.

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have a right angle at the junction of the walls and floor in order to complywith ASA and FINA requirements.

4. Special skill and care is required by the gun operator to ensure that all therebars are properly embedded. It is particularly difficult to ensure fullembedment of the rebars in the floor slab and at wall junctions.

5. Sprayed concrete construction can be more vulnerable to low wintertemperatures than insitu concrete due to the virtual absence of formwork. SeeSection 5.5.

5.2 Design and specification

As mentioned above, the design of liquid-retaining structures constructed in sprayedconcrete are not specifically referred to in the UK Code of Practice (BS 8007).However, if the design procedure set out in the Code is followed it could reasonablybe claimed that the Code had been complied with. It appears that some experiencedcontractors specialising in swimming pool construction have evolved their owndesigns with the object of eliminating movement and stress relief joints, and thishas generally proved satisfactory in practice.

Figure 5.2 Construction of pool shell in reinforced sprayed concrete. Courtesy, BuckinghamSwimming pools Ltd.

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The specification for the concrete mix should be suitable for the type ofequipment used, and also the concrete should possess low permeability and lowshrinkage and have an adequate cement content. It is usual for shotcretingcontractors to use ready-mixed concrete. It is recommended that the ready-mixedconcrete should be supplied by a QSRMC Registered Company from a plant holdingcurrent QSRMC Certification for Product Conformity. The mix should containnot less than 325 kg of Portland cement per cubic metre, and the aggregates shouldcomply with BS 882 Aggregates from Natural Sources for Concrete.

5.3 Methods of application

There are two methods of application, the wet mix and the dry mix. 1. In the dry mix method, the cement and aggregates are weigh batched without

the addition of water and the mix is then conveyed pneumatically to the ‘gun’which consists of a mixing manifold and nozzle. It is here that the water isadmitted by the gun operator. The operator has thus complete control over theamount of water in the mix. The mix is then conveyed at high velocity intoplace. Volume batching should not be used.

2. In the wet mix method, the cement and aggregates are weigh batched and apredetermined quantity of water is added. It is usual for the concrete to beready-mixed to a specification prepared by the designer or by the package-deal contractor. The mix is then pumped to the nozzle where compressed air isadmitted which conveys the mix at high velocity into place.

The Building Research Establishment in the UK carried out tests on the compressivestrength of cores taken from both dry mix and wet mix sprayed concrete. The drymix cores gave a compressive strength range of 50–72 N/mm2, and the cores fromthe wet mix gave a compressive strength range of 37–40 N/mm2. These figures arenot necessarily accepted by experienced contractors as being representative ofgood quality wet mix construction.

The properties and performance of sprayed concrete depends largely on theexperience and skill of the operators, but the mix proportions, grading of the sandand coarse aggregate and the type and condition of the equipment used are alsoimportant.

It should be particularly noted that the mix proportions of the sprayed concretein place are likely to be different to the proportions at the time of batching. This isdue principally to what is known as ‘rebound’. The amount of rebound is affectedby the w/c ratio, grading of the sand and placement velocity, and the placementfactor, i.e. whether the shotcrete is applied to a floor, wall or a ceiling. Experiencein the US (see ACI 506-R-90 Guide to Shotcrete) gives the following figures forrebound.

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5.4 Execution of the work

The majority of swimming pools are constructed wholly or partly below groundand the comments given in Chapter 4 on site investigations, under-drainage of siteand flotation are applicable for pools constructed in sprayed concrete with specialreference to flotation/uplift.

5.4.1 Flotation of the pool shell

The thickness of the floor and walls of sprayed concrete is less than a conventionallydesigned insitu reinforced concrete pool. Also, the density (mass per unit volume)may be rather less than for well compacted insitu reinforced concrete. Consequently,the weight of the shell may be appreciably less than a reinforced concrete pool ofthe same size. With a high water table, the danger of uplift can be very real and thenecessary precautions should be taken. If a calculation shows that flotation mayoccur, then the mass of the shell should be increased by thickening the floor slab toensure a reasonable factor of safety, or by installing pressure relief valves in thefloor. Many pool contractors install these valves as standard procedure. When apressure relief valve operates, it admits ground water into the pool when the groundwater pressure is higher than the pressure of the water in the pool.

These valves, being mechanical devices, can fail to operate, and then the poolshell may be damaged by uplift. An increase in the mass of the pool shell inaccordance with flotation calculations has obvious advantages.

A further point for consideration is that the ground water may be contaminated,or may become contaminated, and if admitted to the pool through pressure reliefvalves, this could constitute a health hazard.

A third solution to the problem of uplift is to control ground water level bymeans of under-drainage and level controlled pumps. The effect on the foundationsof adjacent structures by the lowering of the water table must be considered andexpert advice taken before a decision is made.

5.4.2 Application of the reinforced sprayed concrete

One of the most difficult, but at the same time most important, problems is toensure that the sprayed concrete is of consistent density throughout and that thereare no voids or sand pockets behind the reinforcement. Corners and the floor requirespecial care in this respect. Reinforcing bars should be fixed so that they are at

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least four bar diameters apart or 50 mm which ever is the greater, to help ensurefull embedment of the steel.

It is important to check for the presence of hollow-sounding areas behindreinforcement during the construction of the pool shell. This can be done by simpletapping with a light hammer or rod. For large projects, ultra-sonic pulse velocityequipment or impulse radar can be used.

The nominal cover should be 40 mm. The cover should be checked with a covermeter as soon as practical after completion of the placing of the shotcrete (seeSection 10.22.2 and Appendix 3 for information on cover meter surveys).

In swimming pools, it is usual for the walls and adjoining bays of the floor to begunned first, followed by the central portion of the floor. The walls are usuallygunned in panels to their full height and thickness, followed by the adjoining floorbay, in one operation. This means that the size of the panel is largely governed bythe capacity of the equipment, the organisation of the work and the output of thegun operator.

Formwork is not used for the walls but only for a ring beam at the top of thewalls, and for any intermediate wall beams which may be included in the design ofdeep pools.

On the outer face of the walls, hessian or hardboard is fixed to a timber framewhich forms the background to which the sprayed concrete is applied. If theexcavation has been cut accurately and is stable, it is sometimes used as the ‘backshutter’.

Prior to the commencement of the gunning, the hessian is sprayed with mortar(cement, sand and water) to stiffen it. Hardboard and plywood can be used insteadof the hessian, but plaster-board should not be used as it is composed largely ofgypsum (calcium sulphate) and the sulphates may, in the course of time, migrateinto the sprayed concrete resulting in sulphate attack.

The sub-base is often formed of large shingle (known as ‘rejects’), laid to adepth of 150–200 mm. Compacted granular material of adequate thickness can beused as a sub-base. On this the reinforcement is fixed and the sprayed concreteapplied to the thickness required by the design. Figures 5.1 and 5.2 show swimmingpools being constructed of sprayed concrete. It can be seen that the amount ofreinforcement is considerable.

Movement joints are not normally provided in walls and floor and thus the poolshell is virtually monolithic. This is discussed in the next paragraph.

5.4.3 Joints

Although movement joints are not normally provided in the shell of sprayed concretepools, even a small private house pool will not be gunned in one continuousoperation. This requires the use of day-work/construction joints.

There are differences of opinion about how these joints should be formed. TheACI in their Recommended Practice for Shotcrete recommends featheredging, butin the UK it is considered good practice to form a plain butt joint down to the

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reinforcement, and below this the joint is tapered. The object is to make the jointsmonolithic and to assume that movement across the joint does not take place to adegree likely to cause cracks exceeding about 0.2 mm wide at the surface as theweight of reinforcement normally used would ensure adequate crack control. Thevirtual absence of formwork will make a major contribution to reducing temperaturerise in the sprayed concrete.

Cracks of this width (0.1–0.2 mm) are most unlikely to be seen on the roughsurface of sprayed concrete, unless carefully and specifically looked for. The widthof very fine cracks on such a surface are very difficult to measure.

5.4.4 Curing the sprayed concrete

Proper curing, as described in Section 4.13 is essential, although there is a tendencyto neglect it or even omit it altogether. Lack of careful curing can result in seriousdrying shrinkage cracks which may penetrate down to below the reinforcementand result in corrosion of the steel.

5.4.5 Finishing the sprayed concrete

Sprayed concrete can be left ‘in the rough’ straight from the gun, or it can beworked over with a wood float, depending on what subsequent finishes are required.The time after ‘gunning’ for the application of the wood float must be carefullyjudged by an experienced operator; if too early, it will disturb the newly gunnedmaterial, and if too late it will be ineffective in providing a relatively smooth, evensurface.

The ‘as gunned’ material provides a good key to rendering, and screed, but thesurface must be well brushed down, to remove all loose material (this cleaning canalso be carried out by compressed air), prior to the application of the rendering/screed. For pools which are finished with tiles or mosaic, it is normal good practiceto apply rendering in order to ensure a true and even background for the tiling/mosaic.

5.4.6 Construction in cold weather and hot weather

The problems associated with placing concrete in cold weather with air temperatureclose to or below 0°C, and in hot weather with air temperature above about 28°C,have been discussed in Sections 4.6 and 4.7. The basic principles involved apply tothe application of sprayed concrete. Sprayed concrete walls of swimming poolsare particularly vulnerable to low temperature as the inside (water) face does nothave any formwork and the outer face has only hessian or hardboard or thinplywood. The cement content of sprayed concrete is usually higher than insituconcrete and the resulting increase in the heat of hydration is advantageous.However, in very cold weather, the use of heated concrete and efficient thermalinsulation are likely to be essential. If these cannot be provided then it would beprudent to suspend concreting until the weather improves.

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5.5 Thermal insulation

The provision of thermal insulation to the floor and walls of swimming pool shellsconstructed below ground level in insitu reinforced concrete has been discussed inSection 4.15 to which readers should refer.

A sprayed concrete pool is likely to have thinner walls and floor than an insituconcrete pool and therefore thermal insulation would be comparatively moreeffective. However, the basic fact that the heat loss from the pool surface issubstantially greater than through the walls and floor applies and this is why thermalinsulation is seldom used.

5.6 Pipework

Recommendations for dealing with pipes which pass through the floor, and wallsbelow top water level have been given in Sections 4.10.4 and 4.11.4. The commentsand recommendations made can be considered as generally valid for poolsconstructed in reinforced sprayed concrete. An exception is the use of puddle flangesfor pipes passing through the walls. The presence of a flange within the wallthickness would create problems in gunning behind the flange and therefore it isgenerally better to provide the flange on the inner surface of the wall and floor.

5.7 Testing for watertightness

Theoretically, the water test described in Appendix 2 should be carried out beforeany finishes such as rendering, screed etc. are applied. However, the finish to thesprayed concrete is usually not acceptable to tilers, or suppliers of proprietarycoatings and sheet vinyl linings, and this necessitates the application of renderingand screed. This constitutes a reason for carrying out the water test after applicationof rendering and screed.

5.8 Under-water lighting

The provision of under-water lighting has been discussed briefly in Section 4.16to which readers are referred, as the same principles apply to sprayed concretepools.

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SWIMMING POOLS CONSTRUCTED WITHREINFORCED HOLLOW CONCRETE BLOCK WALLSAND INSITU REINFORCED CONCRETE FLOOR

5.9 Introduction

This is a popular method of constructing small swimming pools for private houses.British Standard BS 5628 Part 2 1985 The Structural Use of Reinforced Masonry,deals with the design of laterally loaded walls based on limit state principles and itis recommended that these design principles be followed. Figure 5.3 is a sketchshowing a suggested section through a reinforced blockwork wall and insitureinforced concrete floor of a small swimming pool with a maximum depth of1.50 m, and 8 m×4 m on plan.

The blocks should be dense aggregate two core blocks, to BS 6073, having aminimum compressive strength of 10 N/mm2.

All batching of concrete should be by weight, but mortar is normally batchedby volume.

The method of construction described here does not comply with the

Figure 5.3 Sketch through wall and floor of pool constructed with reinforced blockworkwalls and insitu reinforced concrete floor.

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recommendations of the Code of Practice for concrete water retaining structures(BS 8007).

The joint between the concrete block walls and the insitu concrete floor isparticularly vulnerable to leakage and the provision of a substantial cove in cement/sand rendering is recommended.

5.10 Construction of the floor

The floor is cast first with a 100 mm high kicker on which the wall is built, asshown in Figure 5.3. The top surface of the kicker must be horizontal toaccommodate the blockwork.

The floor slab for these small pools (dimensions on plan should not exceedabout 8 m×4.0 m) can be cast without a transverse joint provided the reinforcementis designed accordingly. If a joint is provided it can be considered as a contractionjoint and detailed as shown in Figure 4.3.

The reinforcement can consist of high tensile fabric to BS 4483, and the type ofmesh should be determined by the dimensions of the slab. The fabric should befixed so that the cover (to the top surface) is 50 mm. The cover should be checkedwith a cover meter as soon as practical after casting. See Section 10.22.2 andAppendix 3 for information on cover meter surveys.

The mix proportions for site mixed concrete would be 360 kg OPC, 550 kgsand, and 1100 kg coarse aggregate (20 mm maximum size).

The w/c ratio should not exceed 0.50 and the nominal slump should be 75 mm.To ensure full compaction of the concrete a plasticiser may be required.

If the concrete is ready mixed then the order for the concrete should be:Designated mix to BS 5328, characteristic strength of 35 N/mm2, w/c ratio notexceeding 0.50, and nominal slump of 75 mm.

The concrete should be cast on a slip membrane consisting of two sheets ofpolythene laid on either 50 mm of oversite concrete or 75 mm of compacted granularmaterial blinded with sand. The floor slab should be not less than 150 mm thick.

The floor slab should be cured by covering it with polythene sheeting helddown around the perimeter and kept in position for at least four days.

The floor would normally be finished with a cement/sand screed, having mixproportions of 1:4½ cement to concreting sand, medium grading (BS 882),preferably pre-packed. For additional information on floor screeds see Chapter 7.Any joints in the floor slab should be carried through the screed and any rigidfinish such as tiles etc.

5.11 Construction of the walls

5.11.1 Reinforcement

The vertical rebars must be located as accurately as practical and securely fixedinto the floor slab.

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Reliance has to be placed on the infill concrete and the rendering to the blockworkwalls to protect the rebars from corrosion as the concrete blocks have relativelyhigh permeability, and to ensure watertightness.

5.11.2 The hollow concrete block walls

The two-core hollow concrete blocks for the walls should comply with BS 6073Precast Concrete Masonry Units, having a compressive strength of 10 N/mm2.The blocks should be 440 mm×215 mm×215 mm.

The top surface of the kicker should be roughened and all grit and dust removedprior to the commencement of the blockwork so as to provide good bond to thebuilding mortar. Horizontal reinforcement should be provided in each course ofblockwork as shown in Figure 5.3 and the diameter of these rebars should bedetermined by the length of the wall.

The mortar mix for the blockwork would be determined by the design of thewall, as required by BS 5628.

The joints of the blockwork should be raked out to a depth of 10 mm to improvethe key for the rendering.

The walls should be finished on the inside with two coats of a cement/sand rendercontaining 10 litres of SBR to 50 kg of OPC; the mix should be 1:3½ cement to sand.The thickness of the first coat should be between 12 and 15 mm, and the thickness forthe second coat should be between 5 and 8 mm. The second coat should not be appliedsooner than four days after the completion of the first coat to allow the first coat tomature. The final coat can be finished with a wood float if ceramic tiles or mosaic isspecified. However, if a proprietary coating is specified, then the finish to the rendershould comply with the recommendations of the coating supplier. A substantial coveshould be formed at the junction of the wall and floor.

As a precautionary measure, it is recommended that the outside of the wallsbe given two coats of a proprietary waterproofing compound. If a bituminousbased material such as Liquapruf is used this should be protected byhardboard to prevent damage by the back-filling. An alternative is to renderthe outside of the walls.

5.11.3 The concrete infill

The concrete infill for the blocks should have mix proportions of 1:2:2½ bymass, using 10 mm maximum size aggregate, with a slump of about 150 mm,and a w/c ratio not exceeding 0.5; this would require the use of a plasticiser orsuperplasticiser.

5.11.4 The ring beam

The wall is finished at the top with a reinforced concrete ring beam; the verticalreinforcement in the walls is extended into the ring beam. The mix for the ringbeam can be the same as that used for the floor or the infill for the blocks.

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5.12 Pipework

Pipework which has to pass through the floor slab should be carried out asrecommended in Section 4.10.4. The passing of pipework through the blockwalls below top water level presents considerable difficulty and special care isneeded. A flange should be provided on the inside (water) face.

5.13 Under-water lighting

For under-water lighting, reference should be made to Section 4.16, but it must beemphasised that with the walls constructed in reinforced blockwork, there are seriouspractical difficulties in inserting lighting fittings.

5.14 Curing the concrete and protecting theblockwork

The floor slab should be cured as described in Section 5.10. The rendering andscreed should be cured as recommended in Sections 7.1.4.2 and 7.4.5.

It is unlikely that the blockwork walls will require curing unless the weather isparticularly hot and windy. The mortar joints in the blockwork are vulnerable tofreezing temperatures and building the walls should be suspended during verycold weather.

5.15 Testing for watertightness

The pool should be required to pass the water test described in Appendix 2 withthe following modifications: 1. The test should be carried out after the rendering and screed have been applied

and allowed to mature for at least 14 days, and before the back-filling aroundthe walls is carried out.

2. The ‘initial soakage’ period should be 21 days as the precast concreteblocks have higher absorption than insitu reinforced concrete or sprayedconcrete.

3. The maximum permitted water loss over the test period of seven days shouldtheoretically, not exceed 10 mm, but from a practical point of view, a somewhathigher figure may have to be accepted.

5.16 Back-filling around the walls

This should be carried out after the pool has passed the watertightness test, and notsooner than 28 days after completion of the walls.

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The back-fill material should be carefully placed and compacted in 150 mmthick layers; see also the recommendation in Section 5.11.2.

5.17 Thermal insulation

Thermal insulation has been discussed in Section 4.15 and in Section 5.5, to whichthe reader is referred.

SANDWICH TYPE CONSTRUCTION WITH INSITUREINFORCED CONCRETE CORE WALL ANDCONCRETE BLOCKS AS PERMANENT FORMWORK

5.18 Introduction

This method of construction is favoured by many package-deal contractors. Asgenerally constructed, it does not comply with the Code of Practice, BS 8007 forconcrete liquid retaining structures. However, when properly designed andconstructed, it can give satisfactory service.

Figure 5.4 Sketch through wall and floor of pool constructed with insitu reinforced concretecore wall with concrete blocks as permanent formwork.

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The main question that arises concerns the walls. The concrete blockswhich act as permanent formwork to the insitu core wall cannot be consideredas forming a structural part of the wall because the blocks themselves do notcomply with the requirements for concrete quality in the Code. This difficultycan be overcome by designing the insitu core wall to comply with the Coderequirements without taking into account the concrete blocks, and then thesuggested restrictions on size and depth in Section 5.19 would not apply.However, this is likely to be uneconomic compared with taking account of theblockwork in the design.

The recommendations which follow are based on the assumption that the designis an empirical one and does not comply with the BS 8007.

Figure 5.4 shows a section through the wall of this type of pool.

5.19 Construction of the floor

The floor should be constructed in insitu reinforced concrete and provided with akicker 100 mm high which forms the base on which the wall is constructed asshown in Figure 5.4. The top surface of the kicker should be horizontal so that theblockwork bed joints are also horizontal.

The size of the pool should be limited to about 10.0 m×6.0 m×2.0 m deep.For a pool of this size, the insitu reinforced concrete floor can be cast withoutmovement joints, provided the reinforcement is designed accordingly. Shouldthe contractor decide to cast it in two bays then the transverse central jointcan be a contraction joint formed by a stop end, and detailed as shown inFigure 4.3.

Starter bars for the wall should be securely fixed in the floor slab.The main reinforcement for the floor is usually a high tensile fabric to BS

4483, located 50 mm from the top surface of the slab which should have aminimum thickness of 150 mm. The cover to the reinforcement should bechecked with a cover meter as described in Appendix 3.

The concrete mix should be as recommended in Section 5.10; and curing shouldbe carried out as described in Section 5.14. The construction of the floor slabshould be as described in Section 5.10.

5.20 Pipework

For the walls, the pipes can be either cast-in or boxed-out. For pipework passingthrough the floor, reference should be made to Section 4.10.4.

5.21 Construction of the walls

The reinforcement should be securely fixed so that the minimum cover of theinsitu concrete is 40 mm.

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The concrete blocks should be solid dense aggregate blocks complying withBS 6073, with a minimum strength of 10 N/mm2.

The mortar mix should be 1 part OPC, ½ part lime and 4½ parts of sand TypeS to BS 1200, and the gauging water should contain 10 litres of SBR to 50 kgcement. As an alternative, masonry cement can be used as this eliminates theneed to use lime. The mix would then be 1 part masonry cement to 3 parts sandType S to BS 1200.

As the height of the wall is limited to 2.0 m, the blockwork can be built to thefull height and then the concrete for the core wall can be cast in one pour not lessthan 7 days after completion of the blockwork. Both skins of blockwork shouldbe supported while the core wall is being cast and the supports should be left inplace for 48 hours after completion of casting.

A suitable mix for the concrete core wall would be:

360 kg cement; 550 kg sand; 1100 kg coarse aggregate (20 mm maximumsize); Maximum water cement ratio, 0.50; Nominal slump, 100 mm.

The use of a plasticiser is likely to be required in order to obtain the 100 mm slumpwith the w/c ratio of 0.50.

Galvanised wall ties should be inserted in each course at 450 mm centres andstaggered vertically.

5.22 Under-water lighting

The procedure suggested in Section 5.20 for pipework should be adopted for under-water light fittings. Reference should also be made to Section 4.16.

5.23 Finishes to floor and walls

The floor would normally be finished with a cement/sand screed and theinside surface of the walls with two coats of cement/sand rendering. It isrecommended that the outside of the walls be given two coats of a proprietarywaterproofing compound. If a bituminous-based compound is used, it wouldbe desirable for the coating to be protected by plywood or hardboard toprevent damage by back-filling. A cement/sand rendering could be usedinstead of a proprietary waterproofing compound.

The final finish can be ceramic tiles/mosiac, or chlorinated rubber paint. Packagedeal contractors often use marbelite, see Section 7.9.

5.24 Testing for watertightness

The pool should be tested for watertightness not less than 14 days after completionof floor screed and rendering, as described in Appendix 2, modified as recommendedin Section 5.15.

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5.25 Back-filling around the walls

The back-filling should be carried out as recommended in Section 5.16.

5.26 Thermal insulation

This has been discussed in Sections 5.5 and 5.17, to which the reader is referred.

OTHER METHODS OF CONSTRUCTION

5.27 General comments

Chapter 4 and parts of this chapter are intended to cover the methods of constructionadopted for the vast majority of swimming pools. There are however other methodswhich may be suitable and convenient in special circumstances. For these methodsof construction, the pools should be limited in size to about 5 m×4 m with amaximum depth of water of 1.5 m. Also, for these pools, other than the precastpost-tensioned type, a higher rate of water loss than the figure of 10 mm drop inlevel given in Appendix 2 may have to be accepted. Problems arise with joints inthe walls.

The installation of inlets and outlets and the circulating pipework are likely tocreate serious problems.

When these factors have been taken into account, it is probable that the cost andtime for construction exceeds that of a conventional pool.

Where reinforced concrete is used for the floor, the nominal cover to the rebarsshould be 50 mm and this should be checked with a cover meter after casting theconcrete; see Appendix 3 for information on cover meter surveys.

5.28 Pools constructed with mass (gravity) typewalls

5.28.1 The walls

The walls must be structurally stable by their own weight when subjected to waterpressure from the inside when the pool is full and when subjected to sub-soil andground water pressure when the pool is empty.

Mass (gravity) type walls should be constructed on independent foundations.The walls can be mass concrete with mix proportions of about 1:3:6 by mass

using 40 mm maximum size aggregate. The walls can be finished with cement/sand rendering on the inside carried out as described in Section 5.11.

The walls can often be conveniently constructed by casting against the sides ofthe excavation which has been covered with polythene sheeting.

The insertion in the base/foundation of a steel flat as a waterbar is recommended.

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An alternative method is to use dense aggregate two-core hollow blocks (to BS6073), 440×215×215 mm, with a minimum compressive strength of 10 N/mm2.The hollow blocks are filled with insitu concrete, and thus the wall is basicallysimilar to the pool wall described earlier in this chapter, with the verticalreinforcement omitted. A waterbar cannot be used at the junction of the wall andthe foundation.

The wall must be stable by its mass; it is a gravity type wall and the thickness isdetermined by calculation. Such a wall should be finished with cement/sandrendering.

The mortar for the blockwork should have mix proportions given inSection 5.11.

5.28.2 The floor

The floor can be insitu reinforced concrete, 150mm thick, constructed as describedin Section 5.10.

5.29 Curing the concrete

Curing of the mass concrete, the insitu reinforced concrete floor and thecement/sand rendering should be carried out as recommended in Section 5.14.

5.30 Testing for watertightness

The pools described here should be tested for watertightness as recommended inSection 5.15, but a higher water loss may have to be accepted.

5.31 Pools constructed in very stable groundsuch as chalk or rock

Such pools can be satisfactorily constructed by the application of a lining ofreinforced sprayed concrete direct to the sides of the excavation. The excavationshould be carried out carefully to line and level so as to ensure a reasonably uniformthickness of the sprayed concrete. The principle is similar to that used for thelining of rock tunnels and similar work and therefore would be suitable for a flow-through pool as briefly described in Section 8.1.

The application of the sprayed concrete should be carried out generally asdescribed earlier in this chapter. The thickness of the sprayed concrete and theamount of fabric reinforcement required would have to be decided in the lightof site conditions, but the thickness should not be less than 75 mm. Thesprayed concrete should be finished with cement/sand rendering and screed asdescribed earlier in this chapter. A final finish with chlorinated rubber paint isrecommended.

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Figure 5.6 Precast post-tensioned concrete units under erection. Courtesy, Dickerhoff &Widmann in association with IBACO International.

Figure 5.5 Section through pool constructed in chalk or rock and lined with reinforcedsprayed concrete.

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The pool should be tested for watertightness as described in Section 5.7 but asomewhat higher water loss would not be unexpected.

If the pool is intended to have water treatment, then the problems associatedwith the installation of the circulating water pipework as previously mentionedwould apply. Figure 5.5. shows a section through such a pool.

5.32 Pools constructed of precast post-tensionedconcrete units

This type of construction has been used on a number of public swimming pools inGermany and Switzerland. Figure 5.6 shows such a pool under construction atChur, Switzerland. As far as the author is aware, this method has not been used forswimming pools in the UK, and in Europe it has not progressed as originallyanticipated due to the popularity for pools which are not rectangular on plan. Theprecast units are only suitable to rectangular pools.

5.33 Pool Shells of Steel

With the object of ensuring complete watertightness and reducing dead load, asmall number of pool shells have been constructed of welded steel sheets (carbonsteel and stainless steel). Carbon steel shells have given poor performance, mainlydue to problems of corrosion and deterioration of the finishes. In recent yearsstainless steel has been used for a few small pools. A valid assessment can only bemade after the pools have been in use for several years.

Further reading

American Concrete Institute. Guide to Evaluation of Shotcrete, ACI-506–4R-94.American Concrete Institute. Guide to Shotcreting, ACI-506R-90.American Concrete Institute. Specification for Shotcrete, ACI-506–2–95.Austin, S.A. and Robbins, P.J. (eds). Sprayed Concrete—Properties, Design and Application

Whittles Publishing, Scotland, 1995.British Standards Institution. Precast Concrete Masonry Units, BS 6073, Parts 1 and 2.Building Research Establishment. Sprayed Concrete Tunnel Support Requirements and the

Wet Mix Process, Current Paper CP18/77.Concrete Society. Specification for Sprayed Concrete, CS 021, 1979.Concrete Society. Guidance Notes on the Measurement of Sprayed Concrete, CS 022, 1981.Tomsett, H.M. The practical use of ultra-sonic pulse velocity measurements in the assessment

of concrete quality, Magazine of Concrete Research, Vol. 32, No. 110, March 1980.

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Chapter 6

External works

6.1 General considerations

External works on a large scale are only likely to be required for majorswimming pool projects such as leisure and sports centres. Nevertheless, someexternal works will be needed for even a small private pool, and it is importantto remember that good landscaping can transform a rather dull uninterestingsite into a most attractive one. The advice of an experienced landscapearchitect is generally worth the cost.

The term external works comprises paving, walling and surface water drainage.As most open-air pools are wholly or partly below ground level, the excavated

material can be used to adjust levels and form terraces and embankments, whichrequire proper compaction.

On a sloping site, the pool can be constructed into the side of the hill, but caremust be taken to ensure that the whole of the pool is built on solid undisturbedground or is uniformly supported in some other way. The floor can be designed asa suspended slab supported on reinforced concrete columns or load-bearing walls,but this can be very expensive and is seldom adopted except for large projects.

Paved areas laid on compacted fill can present problems arising from long-termsettlement, and this is discussed in Section 6.2.2.

Natural depressions in the ground can be useful in saving excavation for thepool. With unsymmetrical sites, the construction of a free-formed pool instead ofa rectangular one can assist in producing an attractive layout. Consideration canbe given to the construction of a circular pool for private houses when seriousswimming is not contemplated.

Large covered pools which form part of a leisure centre are sometimes designedwith the plant rooms, stores etc. below the pool shell and/or below walkways andchanging rooms. This is discussed in Sections 4.12 and 4.14.

For outdoor pools, it is advisable for the edge of the pool and the surroundingpaving to be raised slightly, say, 50–75 mm above the level of the adjoining ground.This will help to keep insects from crawling into the pool and will generally helpin maintaining cleanliness in the pool. Paved areas adjacent to the pool should belaid to slope away from the pool; a minimum gradient of 20 mm in 1.00 m (1 in 50)is adequate; this is to prevent water used for washing down the paving and rain

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water, from draining into the pool. A proper system of surface water drainage maybe required, depending on the layout and size of the project. See also Section 6.3.

6.2 Paving

6.2.1 Introduction

There are a number of materials and methods of construction which can be usedsatisfactorily:

Insitu concrete: plain or reinforced; pigmented; pattern-imprinted; chemicallystained;Precast concrete flags—plain or pigmented, or chemically stained;Natural stone flags;Precast concrete paving blocks—plain or pigmented;Clay paving bricks (pavers);Asphalt or coated macadam, plain or pigmented.

All the above can be used for paving for pedestrians and for vehicles but the enduse must be taken into account when specifying the material and the method ofconstruction.

Precast and insitu terrazzo, and natural marble, while very attractive, are notsuitable for paving around a swimming pool nor for any form of external paving asthe finished surface is very smooth resulting in a real danger of slipping, particularlywhen wet. Action to provide a non-slip surface to these materials is likely to spoiltheir natural attractive appearance.

It is not possible in this book to give a detailed specification for each ofthe above types of paving laid on various sub-soil conditions, but it is hoped

Fgure 6.1 Illustration of terms used in concrete paving.

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that the information which follows will be useful. Figure 6.1 illustrates theterms used.

The finished level of paving close to buildings must be at least 150 mm lowerthan any damp-proof course (dpc) in the walls of the building.

6.2.2 Paving for pedestrians

If the paving has to be laid on compacted fill, then the use of insitu concretecan give rise to problems, particularly for small jobs when heavy compactingplant is not available. Even well-compacted fill is liable to settle in the courseof time resulting in cracks, and unevenness across the cracks which is verydifficult to rectify without complete relaying. Owners can be bitterlydisappointed when they find their new paving has developed an unevensurface and a considerable amount of cracking which can only be rectified athigh cost.

The use of compacted hardcore to make up levels is likely to give appreciablybetter results, but care must be taken to ensure that the hardcore does not containmaterial such as gypsum, which can attack concrete paving laid on it.

The hardcore should be broken up into pieces not exceeding 50 mm in size and‘blinded’ with a thin layer of sand.

6.2.3 Insitu concrete

For the reasons given in Section 6.2.2, this type of paving is notrecommended if it has to be laid on filled ground. It is better to use concreteflags or concrete or clay paving blocks as settlement of the sub-base can bereadily corrected.

The following recommendations apply when the insitu paving is laid onundisturbed ground.

Top soil must be stripped down to the appropriate level and shape, to the finishedlevel of the surface of the paving. On clay/peat, it is advisable to lay 75 mm ofcompacted gravel or similar material as the sub-base.

A separating layer of 1000 gauge polythene sheeting should be laid on the sub-base. On natural ground such as gravel or sand, the separating layer can be laiddirectly on the prepared ground (known as the sub-grade).

Plain unreinforced concrete should be 100 mm thick and laid in bays notexceeding 2.5 m×2.0 m. The length should not exceed 1.5 times the width.

If ready-mixed concrete is used, this should be specified as GEN 4 in accordancewith the relevant clauses in BS 5328: Concrete.

The concrete must be thoroughly compacted, and after finishing must be curedfor not less than four days by covering with polythene sheeting held down aroundthe edges.

For site-mixed concrete, the mix proportions recommended are:

1 bag (50 kg) cement (about 1¼ cubic feet or 0.036 m3);

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2.5 ft3 (0.072 m3) coarse, clean concreting sand;3.75 ft3 (0.108 m3 coarse (20 mm) aggregate.

This gives mix proportions by volume of 1:2:3. Sufficient water should beadded to make a workable mix which can be easily spread, compacted andfinished.

6.2.3.1 Pigmented concrete

Many building owners do not like the grey colour of concrete and want a coloured(pigmented) concrete. The amount of pigment should be decided by trial and thepigment added to the mix before water is added. The result can be disappointingdue to variations in the tone (intensity of colour).

It is not possible to obtain the same uniformity of colour as is given by apigmented coating. These variations in pigmented insitu concrete arise frominevitable variations in mix proportions, standard of mixing, amount of water inthe mix, variations in compaction and finishing. The pigments used should complywith BS 1014 Pigments for Portland Cement and Portland Cement Products.However, some colours are adversely affected by ultra-violet light (mainly bluesand greens).

There is one further problem with pigmented concrete and that isefflorescence or lime-bloom. This is a whitish discolouration which canappear on the surface of the concrete irrespective of whether it is pigmented orplain. It is much more noticeable on pigmented concrete, particularly if darkpigments have been specified. It wears off in time, but can be very disfiguringbefore it finally disappears.

Some information on pigments is given in Section 2.5.5.

6.2.3.2 Pattern-imprinted concrete

Pattern-imprinted concrete has been in use for many years but on a smallscale. The colour is imparted to the surface of the plastic concrete in the formof a ‘colour-hardener’ which is sprinkled evenly by hand in two applications.The first application uses about 60% of the total dosage. The colourhardeners usually consist of mixtures of Portland cement, pigments, fine hardaggregate, and an admixture to assist the imprinting process. The imprintingtools consist of rubber mats of various shapes and patterns, which are pressedinto the surface after the second application of the sprinkled colour-hardener.The final operation consists in the application of a wax or acrylic typesealant. The process should only be entrusted to specialist contracters with aproven record of successful jobs. The basic principles of construction givenin Section 6.2.3 should be followed. Figure 6.2 shows a garage drive ofpatterned concrete.

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6.2.3.3 Chemical staining of concrete

Chemical staining of concrete is carried out after it has hardened for at least amonth. These stains are water-based solutions of metallic salts and are claimed toreact with the free lime in the concrete and form stable coloured deposits in thesurface layers of the concrete. Concrete to be treated by this process should not begiven a floated finish as this will significantly reduce the penetration of the metallicsalt solution into the concrete. There will be variations in the intensity of the surfacecolour and unless this fact is accepted, the results can be very disappointing.

Only specialist contractors should be employed with a proven record ofsatisfactory work.

6.2.4 Precast concrete paving flags

These can be obtained in a range of standard sizes, rectangular and square. Thethickness varies from 50 mm to 70 mm. These flags should comply with BS 7263Precast Concrete Flags, Channels, Kerbs, Edgings and Quadrants, Part 1

Figure 6.2 Garage drive in imprinted concrete.

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Specification. They should be laid in accordance with Part 2 Code of Practice forLaying. The Precast Concrete Paving and Kerb Association (Interpave) have issueda number of explanatory leaflets giving detailed information on these productsand how they should be laid.

The precast flags can be pigmented and, as they are factory made, variations incolour intensity should be much less than with insitu concrete. There is still apotential problem with change in colour intensity (fading) due to ultra-violet light,and efflorescence as referred to in Section 6.2.3.1.

Generally, the flags should be laid on a bed of mortar, sand, or crushed rockfines, 25 mm compacted thickness.

The bedding should be laid on a properly prepared sub-base. The flags must beuniformly bedded and well punned down onto the sub-base which should consistof Type 1 Granular Material, cement-bound material or wet lean concrete. TheInterpave Information Sheets give details of these alternative materials suitable forthe sub-base.

The joints between the flags can be narrow (2–4 mm) and filled with finesand, or they can be wide (5–10 mm) and filled with mortar (1:4½ cement tosand).

British Standard BS 7263 requires that the difference in level between adjacentflags must not exceed 3 mm. This is important, particulary for paving around aswimming pool where people walk with bare feet. It is not unusual for this 3 mm‘lip’ between adjacent flags to increase with time due to consolidation of the beddingand sub-base. When this occurs, it is relatively easy to take up the offending flagsand rebed them. This is a sound reason for using precast flags instead of insituconcrete.

6.2.5 Natural stone flags

These are best specified to have a riven finish to help prevent slipping. They can belaid in the same way as precast concrete flags, but the minimum joint width is 6mm to allow for slight variations in the slab surface. The joints should be madewith cement-sand mortar, having mix proportions of 1 part cement to 4½ parts offine sand. Difference in level between adjacent flags should not exceed 3 mm. SeeBS 5385 Part 5 Wall and Floor Tiling.

6.2.6 Precast concrete paving blocks

This type of paving has become very popular for both pedestrian areas andcar parks, garage drives and access roads for commercial vehicles (Figures6.3 and 6.4).

The blocks should be manufactured to BS 6717 Precast Concrete Paving BlocksPart 1. Specification for Paving Blocks, and laid in accordance with Part 3 Code ofPractice for Laying.

The blocks are generally rectangular, 200×100 mm, but can be obtained in

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Figure 6.3 Concrete block paving around garden pool. Courtesy, Redland Precast Ltd.

Figure 6.4 Concrete block paving for light commercial traffic. Courtesy, Marshalls plc.

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a number of proprietary shapes and sizes. They are made in a range of thicknessesand colours and are frost resistant.

Where the block paving is intended for pedestrian use only, then the full designprocedure recommended by the Code (BS 7533) and detailed in the InterpaveInformation Sheet Concrete Block Paving, Structural Design is not required.Informed consideration must be given to the type of sub-grade (California BearingRatio, CBR), and the thickness of the Type 1 Granular Material used for the sub-base. The level of the water table must also be taken into account.

A block thickness of 50 mm would generally be sufficient for pedestrian useonly. The recommendations on the Information Sheets issued by Interpave relatingto installation and detailing should be followed.

6.2.7 Clay paving bricks (pavers)

The relevant Standards and Codes are:

BS 7533 Guide for Structural Design of Pavements Con-structed with Clay or Concrete Block Pavers;

BS 6677 Clay and Calcium Silicate Pavers for FlexiblePavements Part 1 Specification for Pavers; Part 2Code of Practice for Lightly Trafficked Pavements;Part 3 Method for Construction of Pavements.

Figure 6.5 Flexible clay paving under construction in herringbone pattern. Courtesy, BrickDevelopment Association.

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The Brick Development Association have issued a Design Note (No. 9) FlexiblePaving with Clay Pavers, and this, together with the Standards and Codes referredto above should provide the information required for the design and laying of thistype of paving (Figures 6.5 and 6.6).

6.2.8 Paving for light vehicular traffic

This is intended to cover parking areas and access roads for private cars and lightcommercial vehicles.

The materials discussed in this section are:

insitu reinforced concrete;precast concrete paving blocks;clay paving bricks (pavers);asphalt and coated macadam.

6.2.8.1 Insitu reinforced concrete for use by private cars and lightcommercial vehicles

For this type of use, the following is recommended, based on the use of ready-mixed concrete laid by a local contractor.

Figure 6.6 Decorative clay pavors in large pedestrian area. Courtesy, Brick DevelopmentAssociation.

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The order to the supplier should state the purpose for which the concreteis to be used. The mix should comply with BS 5328 Designated Mix PAV 1Table 13 Part 1 and Table 6 of Part 2. The concrete should be air-entrained(mean 5.5% entrained air); see Chapter 2 for information on air entrainingadmixtures. A minimum thickness of 150 mm is recommended, laid on a slipmembrane of 1000 gauge polythene sheeting, laid on a compacted sub-basenot less than 100 mm thick. The slab should be reinforced unless it is laidin bays not exceeding 3.0 m×2.5 m. The use of fabric reinforcement located50 mm from the top surface of the slab would allow the bays to be increasedin length and width, the dimensions depending on the weight ofreinforcement used.

The transverse joints can be stop-end joints with the reinforcement stoppedback 75 mm each side of the joint. Tie bars are sometimes used in these joints.

The use of saw-cut joints in this class of work can result in practical difficultiesover timing of the sawing. Tie bars are recommended at longitudinal (warping)joints; they are usually located at 500 mm centres and are located at the neutralaxis, and are bonded throughout their length.

The concrete should be laid between side forms, well compacted, and properlycured for four days by covering with polythene sheeting or by the application of aresin-based curing compound. OCCASIONAL USE BY HEAVY COMMERCIAL VEHICLES

Paving subject to occasional use by heavy commercial vehicles (e.g. petrol bowsers)should be designed, specified and laid on the same principles as those adopted forhighways, which are laid down in the publications of the Department of Transportin the UK and to the following authorities in the USA and Canada:

American Association of State Highways and Transportation Officials(AASHTO);The American Concrete Institute;The Portland Cement Association (USA);The Ministry of Transportation and Communications (Ontario);The Portland Cement Association (Canada).

The work should be carried out by an experienced contractor under reasonable sitecontrol.

6.2.8.2 Precast concrete paving blocks

This type of paving is suitable for light and heavy commercial vehicles providedthe recommendations of BS 7533 Guide for the Structural Design of PavementsConstructed with Clay or Concrete Block Pavers, supplemented by the InformationSheets issued by Interpave, are followed.

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Essentially, the procedure is to establish the CBR value for the sub-grade,followed by the selection of the sub-base material and determination of its thickness.The paving blocks should be laid in accordance with BS 6717 Precast ConcretePaving Blocks Part 3 Code of Practice for Laying.

It should be noted that if the sub-grade material is susceptible to frost attackthen the thickness of the sub-base would have to be substantially increased andexpert advice should be sought.

6.2.8.3 Clay pavers

The paving should comply with BS 6677 Clay and Calcium Silicate Pavers forFlexible Pavements Parts 1, 2 and 3, supplemented by the detailed recommendationsgiven in the Brick Development Association (BDA) Design Note No. 9 FlexiblePaving with Clay Pavers. In brief, it is necessary to establish the CBR value, andfrom this select the material for the sub-base, i.e. Type 1 Granular Material, leanconcrete, soil cement, or cement-bound granular material (all in accordance withthe DoT Specification for Highway Works) and decide its thickness based on theCBR value. If the sub-grade is frost susceptible, the sub-base should not be lessthan 450 mm thick. Care should be taken to prevent the water table rising to lessthan 600 mm from the pavement surface. This may require sub-soil drainage.

6.2.8.4 Asphalt and coated macadam

This material is suitable for garage drives, car parks for light commercial, andheavy commercial vehicles. It can also be used for the resurfacing of deterioratedconcrete paving; some information on this is given in Section 10.7.

It has the advantage of being relatively easy to lay to close tolerances. RelevantBritish Standards are BS 594 Hot Rolled Asphalt for Roads and Other Paved Areas,and BS 4987 Coated Macadam for Roads and Other Paved Areas.

The Quarry Products Association have issued a set of Information Sheets givingdetails of this type of construction for pavements and roads. Details of thesepublications are given under Further Reading at the end of this chapter.

6.3 Surface water drainage

To help eliminate ponding, cross falls to all types of paving should not be less than1 in 60 and drainage channels should have an appropriate longitudinal fall; factory-made channels are now available on the market.

Surface water drainage for a large paved area normally consists of a channel orchannels which collect the run-off from the paving and these channels dischargeto road gulleys located at predetermined positions. The gulleys are connected tothe main drainage system which is either a surface water sewer, a combined sewer,water course or soakaways. The design and construction of soakaways need carefulconsideration and reference should be made to BRE Digest 365, September 1991Soakaways and CIRIA Report No. 156 Infiltration Drainage Manual of Good

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Practice 1996. The information in these publications is largely based on theoreticalconsiderations. New research by the Hydraulics Research Station at Wallingford,UK, will be based on how soakaway systems actually work, taking into accountrainfall, and the run-off from various types of surface.

The design of large drainage systems should be based on therecommendations in BS 6367 Code of Practice for Drainage of Roofs and PavedAreas. The details of surface water drainage design and construction are outsidethe scope of this book.

For the small projects, reference to BS 8301 Code of Practice for BuildingDrainage is recommended.

6.4 Walling

6.4.1 Intoduction

For external works associated with swimming pools there are essentially two typesof walling, namely free-standing walls and earth-retaining walls.

6.4.2 Free-standing walls

These walls are usually constructed in clay bricks and one of the best-knownreferences is the Brick Development Association’s publication Design of Free-standing Walls DG12. Also relevant is BS 5628 Part 3 Code of Practice for the Useof Masonry Materials, Components, Design and Workmanship.

The most important factor in the design of such walls is the correctassessment of the exposure conditions and reference should be made to theDriving Rain Index map or the Wind Zone map of the UK which is included inBS 5628, and the BSI publication DD 93 Methods for Assessing Exposure toWind Driven Rain.

Ordinary clay bricks to BS 3921 Clay Bricks and Blocks, are generally suitablefor use in sheltered and moderate zones, provided the wall is provided with anoverhanging coping and has a damp-proof course of two courses of engineeringbricks or two courses of slates half-lapped and bedded in mortar, located 150 mmabove adjacent ground level.

It is recommended to use sulphate-resisting Portland cement for the mortar forthe full height of the wall. A mix of 1:¼:3 up to dpc level. Then a mix of1:½:4½(cement, lime, sand) to coping level, or masonry cement and sand mix 1:3by volume.

For sites exposed to freeze-thaw cycles, frost resistant bricks should bespecified.

Copings should be provided with a throat and bedded on a bituminous felt damp-proof coursing and can be precast concrete or stone, and should project notless than 50 mm beyond the face of the wall on both sides.

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Movement joints should extend the full height of the wall (from foundation totop of the wall including the coping). The location of these joints depends mainlyon the design of the wall (Figure 6.7); see Brick Development Associationpublication DG 12 for details.

The wall should be structurally designed in accordance with the procedure setout in the BDA publication DG 12. As an alternative for small projects, the BuildingResearch Establishment publication Building Brick or Blockwork Free-standingWalls, Good Building Guide 14 gives useful practical advice based on ‘rule ofthumb’ procedures for this type of wall.

6.4.3 Earth retaining walls

These can be constructed in reinforced concrete, mass concrete, stone, concreteblocks, or clay bricks.

The first question which arises is under what circumstances should a gardenretaining wall be ‘structurally designed’. There is no clear answer to this, but it issuggested that reference should be made to the Building Research Establishment’s

Figure 6.7 Boundary wall (free-standing) in contrasting facing bricks. Courtesy, BrickDevelopment Association.

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Good Building Guide GBG 27 which provides rule of thumb guidance for the safeconstruction of brick and block earth retaining walls, up to a maximum retainedheight of 1.75 m.

For masonry, clay bricks or concrete blocks, the relevant Code is BS 5628Code of Practice for the Use of Masonry. Specific information on the use ofclay bricks is given in the BDA publication Design of Brickwork RetainingWalls, DG 2. It is recommended that low sulphate clay bricks be used with asulphate-resisting Portland cement (SRPC) mortar. ‘Weep holes’ should beprovided near the base of the wall at about 2.0 m centres longitudinally. Themortar mix should be by volume, 1 part SRPC to ¼ lime to 3 sand (type S toBS 1200).

For concrete block retaining walls, useful practical advice is given in the BREpublication GBG 27 Building Brickwork and Blockwork Retaining Walls. The mortarmix should be the same as for clay bricks. See Figure 6.8 for the view of an attractiveclay brick retaining wall.

Insitu reinforced concrete would only be used for larger projects andshould be designed to BS 8110 Structural Use of Concrete. Usefulinformation is contained in Civil Engineering Code of Practice No.2 1951Earth Retaining Structures.

Figure 6.8 Reinforced clay brick retaining wall, 3 m high and 337 mm thick. Courtesy, BrickDevelopment Association.

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Further reading

Bennett, D. Chemical stain, concrete terrazzo and exposed aggregate finishes, Concrete,November/December 1992.

British Standards Institution. Code of Practice for Assessing Exposure to Wind-driven Rain,BS 8104.

Department of Transport. Specification for Highway Works, Parts 3 and 7, HMSO, London,1986/89.

Department of Transport. Structural Design of New Road Pavements, HD 14/87, withamendments, HMSO, London, December 1989.

Department of Transport. Joints, New Civil Engineer Concrete Engineering, November 1997,pp. 32–3.

Precast Concrete Paving and Kerb Association. Concrete Block Paving—Structural Designof the Pavement.

Precast Concrete Paving and Kerb Association. Concrete Block Paving—Detailing.Precast Concrete Paving and Kerb Association. Paving Flags—Techniques for Laying.Precast Concrete Paving and Kerb Association. Kerbs and Footways, Model Specification

Clauses.Quarry Products Association—Asphalt Information Service. Construction and Surfacing of

Car Parking Areas, Information Sheets 1, 2 and 3.Quarry Products Association—Asphalt Information Service. Decorative and Coloured

Finishes for Asphalt Surfacings, Information Sheet 4.Roeder, T. Update on pattern-imprinted paving, Concrete Quarterly, Autumn 1992, pp. 14–

15.Transport Research Laboratory. Design for Road Surface Dressing, Road Note 39, 4th edition.Whitehead, T. Rain relief, New Civil Engineer Concrete Engineering, November 1997, pp.

36–7.

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Chapter 7

Finishing the pool shell andassociated structures;problems with pool hall roofs

FINISHING THE POOL SHELL AND ASSOCIATEDSTRUCTURES

For the purpose of this chapter, the term associated structures means walkwayslabs and floors of wet changing areas. The substrates to which the finishes are:applied are:

Insitu concrete;Sprayed concrete;Precast concrete blocks.

The finishes considered here in some detail are cement/sand rendering andscreed followed by ceramic tiles and mosaic. Brief mention is made ofmarbelite (an insitu white terrazzo), coatings, sheet linings, and glass-fibrepolyester resin linings.

While ceramic tiles and mosaic can be applied successfully direct to insituconcrete, this should only be attempted by experienced concreting contractorsdue to the practical difficulties in obtaining the required surface tolerances onthe concrete so that it is suitable for the laying of the tiles or mosaic. Thesetolerances depend on the type of bedding used. The Code is BS 5385 Part 4 andthis requires that with thin-bed cement-based adhesives the gap beneath a 2.00m straightedge must not exceed 3 mm; with thick-bed adhesives, the ‘gap’ mustnot exceed 6 mm.

It will be noted in Sections 7.1.2 and 7.1.3 that Sulphate ResistingPortland cement (SRPC) is recommended instead of Ordinary Portlandcement. The reason for this is that the author has investigated a number ofcases of deterioration of tile bedding and cement-based rendering andscreeds caused by sulphate attack. On ‘the balance of probabilities’, thesource of the sulphate has been the pool water.

The temperature of the pool water 26 °C to 28 °C assists the chemical reactionbetween the sulphates in solution and the tricalcium aluminate (C3A) in OrdinaryPortland cement. See Sections 2.2.1, 3.5.2.2, 3.6, 3.7, and 8.8.

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British Standard BS 5385 Part 4 sets great importance on providing an adequateperiod for drying out of the pool shell when insitu concrete or sprayed concrete isused for walls and floor. A period of six weeks is recommended between the curingof the pool shell and the commencement of rendering or screed. The curing of thepool shell is likely to take about seven days after the removal of the formwork, orcompletion of the application of the sprayed concrete.

It is assumed that pool shells constructed in insitu concrete or sprayed concretehave been successfully tested for watertightness (as described in Appendix 2) beforethe rendering and screed are applied. Compliance with the recommended periodsresult in the following time sequence:

From casting to test (for structural reasons): 28 daysFilling for test, 2 days; preliminary soakage, 7 days;leakage test, 7 days: 16 daysDrying out after completion of test including 2 days foremptying (2+42) 44 days

Total: 88 Days

This is a total period of almost 3 months from casting the pool shell to the start ofthe rendering/screed.

7.1 Cement-sand rendering to insitu concretewalls

7.1.1 Preparation of the base concrete

It is essential to obtain maximum bond between the base concrete and the renderingas bond failure is probably the most frequent cause of serious trouble (failure)with rendering. The surface of concrete prepared for application of screed andrendering should be checked and accepted by the supervising officer before thescreed/rendering is allowed to proceed.

The most effective way of ensuring a high standard of bond is to prepare thesurface of the concrete so that the coarse aggregate is slightly exposed. Thistype of surface is shown in Figure 7.1 and can be obtained by the followingmeans: 1. percussion tools such as bush hammers and Kangos;2. grit blasting (wet and dry);3. high-velocity water jets. Of the above, method 3 is much preferred.

A depth of exposure of the coarse aggregate of 3 mm is adequate; excessive useof percussion tools can cause fracture of the coarse aggregate resulting in a weaksurface.

Methods 1 and 2 can be create considerable dust and fine grit and

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method 1 considerable noise. All grit and dust must be removed before thefirst coat of rendering is applied. Percussion tools should not be usedearlier than 21 days after completion of the casting of the concrete; in coldweather this may have to be increased, see notes on concreting in coldweather in Section 4.6.

Methods 2 and 3 can be started seven days after casting the concrete in ‘normal’weather conditions. However, there is some advantage in delaying the exposure ofthe coarse aggregate until after the completion of the water test.

The amount of water used with high-velocity water jets is comparatively small,about 50 litres per minute per jet, of which about one-third is dissipated as mistand spray.

The pressure at the nozzle is in the range of 25 N/mm2 to 30 N/mm2. The jettingleaves the concrete clean and damp and very suitable for the application of renderingand screed (Figure 7.2).

An alternative to exposure of the coarse aggregate, but rather less satisfactory,is to use a spatter-dash coat direct on the concrete, as described in Section 7.1.2.

It should be noted that very high-pressure water jets can be used for cuttingconcrete (Figure 7.3).

It is recommended that the preparation of the base concrete to receive screed

Figure 7.1 Close-up view of concrete surface prepared for application of rendering or screed.

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and rendering be carefully checked to ensure that the preparation is satisfactorybefore the application of the screed/rendering.

7.1.2 Spatter-dash coat

A spatter-dash coat is not required if the concrete surface has been preparedby the exposure of the coarse aggregate as described in Section 7.1.1. It isnecessary, however, if rendering is to be applied to a smooth dense concretesurface.

Before the spatter-dash is applied, it is necessary to brush down the surface toremove all dust, dirt and the remains of the release agent and curing membrane (ifthe latter has been applied). Then immediately before the spatter-dash is appliedthe concrete surface should be well damped down.

The object of this coat is to provide a firm rough surface with reasonably uniformsuction, and this should ensure a good bond with the first coat of rendering. Themix proportions by volume should be:

1 part SRPC (Sulphate Resisting Portland cement) class 42.5 to BS 4027 1991;2 parts of sharp clean dry sand. If damp sand is used allowance should be

made for ‘bulking’ up to 25%. The grading of the sand should comply with

Figure 7.2 The use of high-velocity water jet to expose the coarse aggregate in preparationfor application rendering.

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Type A of Table 1 of BS 1199 Building Sands from Natural Sources forExternal Rendering.

The cement and sand should be mixed with sufficient water to give aconsistency of a thick slurry. It is applied as a very thin coat not exceedingabout 2 mm thick. About an hour after the application, the spatter-dash shouldbe lightly sprayed with water to ensure adequate hydration of the cement.Also, it must be protected from hot sun and/or strong winds by properlysecured covers. About 36–48 hours after application, the first coat ofrendering can be applied. The timing depends on the weather. Figure 7.4shows a spatter dash coat being applied.

7.1.3 The first and subsequent coats of rendering

The number of coats, and to some extent the thickness of each, will dependon the total thickness of the rendering, and this will depend on the accuracyof the as-cast surface of the base concrete. If, owing to inaccuracies in the

Figure 7.3 Concrete wall cut by very high-velocity water jet.

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surface of the concrete, the rendering has to exceed 25 mm in thickness,then it is advis able for a lightweight steel mesh to be provided asreinforcement. The mesh should preferably be stainless (austenitic) steel.The mesh must be pinned into the wall by stainless steel fixings, care beingtaken to ensure that the stainless steel does come into contact with thereinforcement of the concrete otherwise bimetallic corrosion is likely tooccur (see Section 2.10).

The thickness of the rendering is critical on the short end walls if the pool isused for competitive swimming. In the UK, the Amateur SwimmingAssociation (ASA) requires a tolerance on the length of the pool of plus 30mm, but with no minus tolerance. Unfortunately, the ASA does not say how thelength should be measured, nor how a zero tolerance can be achieved inpractice.

The batching of rendering is generally done by volume in spite of the knowninaccuracies involved. The UK Code of Practice is BS 5262 1991, and the BritishCement Association publication External Rendering, reference number 47.102,

Figure 7.4 Application of spatter-dash to concrete wall. Courtesy, British CementAssociation.

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1992, give useful and practical advice. The mix proportions recommended herefor the first coat are:

1 part SRPC class 42.5 to BS 4027 1991;3 parts clean dry sharp sand, graded to Type A, Table 1 of BS 1999 1976.

The addition of 10 litres of SBR latex to 50 kg cement is recommended; the SBRacts as a plasticiser and thus improves workability; it also reduces permeabilityand the leaching of lime from the mix if the pool water has a negative LangelierIndex (see Section 3.8).

For the second coat, a slightly weaker mix is recommended, e.g. 1:3½ by volume,also with the inclusion of SBR.

All mixing should be by mechanical pan mixer as an ordinary concrete mixergives considerable variations in the actual proportions of the mixed material.

For rendering onto dense concrete, the overall thickness should ideally notexceed 15 mm, except for small localized areas. The first coat would be about10 mm thick and the second about 6 mm thick. It is important that the secondcoat should be appreciably thinner than the first coat. If for any reason a thirdcoat has to be applied, this must be thinner than the second coat. So that fora total thickness of, say, 20 mm the first coat could be 12 mm thick and thesecond 8 mm. With a total thickness of 25 mm, the third coat could be 5 mm.

The first coat should be scraped or scratched to provide a key for the secondcoat, and the same applies to the second coat if a third coat is required (Figure 7.5).

If the final finish is ceramic tiles or mosaic, then the final coat of renderingshould be finished with a wood float, and/or lightly scratched to provide a keyfor cement-based adhesive. If an organic-based adhesive is used, advice shouldbe obtained from the adhesive supplier on the recommended finish to therendering.

Each undercoat should be allowed to mature for several days before a subsequentcoat is applied; the exact time between coats has to be determined by weatherconditions. In cold wet weather, the period should be increased.

7.1.4 Application of the rendering

7.1.4.1 Panel sizes and joints

If the pool has been designed and constructed without full or partial movementjoints, then the rendering can be applied in panels for the full height of thewalls and with a length not exceeding about 5.0 m. Full movement joints in thepool shell must be carried through the rendering and any rigid applied finishsuch as ceramic tiles/mosaic. These joints are usually 15–20 mm wide and thiswidth should be carried right through all applied finishes. Partial movement

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joints (contraction and stress relief joints) should also be carried through, butthese can be of a width to coincide with the joint widths of the tiles. The jointsin the rendering can be saw cut or wet formed. A discussion withrecommendations for tiling is given in Section 7.6. The part of a full movementjoint within the thickness of the rendering should be filled with a suitable back-up material such as resin-bonded cork.

It is recommended that apart from the desirability of lining-up movement jointsin the pool shell with joints in the rendering and tiling, movement joints in thetiling should be carried down through the rendering to the base concrete. Thewhole question of making provision for movement in the pool shell, rendering andtiling is complicated and it has to be decided by experience, and practicalconsideration. Joint decisions between all parties concerned should be undertakenas early as possible in the design process.

7.1.4.2 Curing the rendering

There is no need to wet-cure rendering apart from the spatter-dash coat asrecommended in Section 7.1.2.

However it is most important that each coat of rendering should be protectedfrom hot sun and drying winds for a period of not less than 48 hours after application.

Figure 7.5 Combing freshly applied undercoat to provide key for subsequent coat. Courtesy,British Cement Association.

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It is strongly recommended that rendering should not be carried out in very coldweather as the 10–15 mm thickness is very vulnerable to low temperature especiallywhen it is applied to cold concrete.

When swimming pools are constructed inside buildings, it is sometimes thoughtthat there is no need to protect the rendering as recommended above. The fact is thatin a building under construction it is likely that doors and windows will not be fixedat the time the pool shell is rendered. A funnel effect can be created and the resultingblast of air can have a most adverse effect on the rendering resulting in serious dryingshrinkage cracking, and debonding.

7.2 Cement-sand rendering to sprayed concretewalls

The ‘as gunned’ surface of sprayed concrete is rough and only requiresbrushing down to remove all loose material, and damping, to be fit to receivethe rendering.

In most cases, two coats should be adequate and the mix proportions should beas set out in Section 7.1.3 and curing in Section 7.1.4.2. A limited amount ofdubbing-out may be required.

Sprayed concrete pools are normally constructed without movement joints, andconstruction joints are intended to be monolithic. The panel lengths can thereforeline up with the panel sizes for the tiling, namely at about 4.5 or 5.0 m centres, andat other locations as recommended in Section 7.6.2.3. The joints can be wet formedor cut with a disc.

7.3 Cement-sand rendering to concrete blockwalls

Dense aggregate concrete blocks provide a good key for cement-sandrendering provided they are first well brushed down to remove all grit andloose particles and then damped down immediately prior to the application ofthe rendering.

The only other preparation needed is for the mortar joints to be recessed about10 mm as the wall is built. Dense, very smooth faced blocks should be given aspatter-dash coat, but this type of block is unlikely to be used for the walls ofswimming pools.

The first coat of rendering should be 10–15 mm thick and each subsequent coatshould be thinner than the preceding one. Generally, there should be at least twocoats.

The mix proportions recommended by volume are:

First coat:1 part SRPC;3.5 parts clean sharp sand Class A, to BS 1199 Table 1;10 litres of SBR to 50 kg cement.

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The addition of the SBR (styrene butadiene rubber latex) improves workabilityand reduces water penetration.

Second coat:1 part SRPC;4 parts sand to BS 1199 (as for first coat);10 litres of SBR to 50 kg cement.

The rendering should be finished, protected and cured as recommended forrendering on to insitu concrete. See paragraph 7.1.

7.4 Cement-sand screeds on insitu concretefloors

7.4.1 Introduction

Screeds are laid to provide a true and level surface with good suction on which tolay ceramic tiles and mosaic. High-quality dense concrete does not provide adequatesuction to secure the standard of bond needed in a swimming pool if ordinarycement-sand mortar is used for bedding the tiles. Also, insitu concrete floors aregenerally not finished with sufficient accuracy for the laying of tiles and mosaicdirectly on them if thin or thick bed cement-based adhesives are used for tilebedding.

Reference can usefully be made to BS 5385 Part 4 1986 Wall and Floor Tiling;Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions. Section13 of the Code covers swimming pools.

7.4.2 Preparation of the concrete

The surface of the concrete should be prepared by exposure of the coarse aggregateas detailed in Section 7.1.1.

7.4.3 Mix proportions and laying

The mix proportions recommended are set out below, and whenever possible thebatching should be by weight/mass.

1 part SRPC;4 parts clean concreting sand to grading limits C or M in Table 3 of BS 882.

The w/c ratio should not exceed 0.5. The addition of 10 litres of SBR latex to 50 kgcement is recommended; this will assist bond, reduce permeability and improveworkability.

The mixing should be by pan mixer as concrete mixers give wide variations inthe proportions of the resulting mix. The mortar should be laid between screedingboards and well tamped down either by a mechanical tamper or other suitable

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means to secure good bond with the base concrete and to ensure good compaction.The screed can be finished with a screed board, but this is not adequate for use asa compacting tool.

7.4.4 Joints

Movement joints, both full and partial, should be carried through the screed.Monolithic joints can be ignored. Movement joints in tiling should be carried downthrough the screed; these joints are usually at 4.5–5.0 m centres.

7.4.5 Curing the screed

The screed should be properly cured for a minimum period of seven days bycovering with polythene sheeting held down around the perimeter and placedin position as soon as possible after the finishing operations are completed.

7.5 Cement-sand screeds on sprayed concretefloors

The recommendations given in Section 7.4 apply here with the exception of thepreparation of the surface of the sprayed concrete floor.

The ‘as gunned’ surface of the sprayed concrete provides an adequate key forthe mortar screed provided all dust, loose particles are first removed by light wetblasting.

The laying of the screed, mix proportions, compaction, treatment of joints andcuring are all as recommended for screeds laid on insitu concrete.

7.6 Ceramic tiles and mosaic

7.6.1 Introduction

It is important that before any tiles/mosaic are fixed that the substrate (renderingand screed) should be carefully checked for bond to the pool shell. This can bedetected by tapping with a light hammer or rod; a hollow sound indicates loss ofbond (adhesion). The tapping should be done systematically, with particularattention to the edges of the bays. All hollow sounding areas should be clearlymarked and the extent of each defined. Areas of defective bond, provided they arenot extensive, can be grouted in with a low-viscosity polymer resin. Any cuttingout of the rendering or screed should be kept to a minimum and should be done bysawing, percussion tools should not be used as the vibration is liable to reduce thebond in adjacent areas.

For all practical purposes, the recommendations relating to the installation ofceramic tiles also apply to the installation of ceramic mosaics. Significant differencesare drawn to the readers attention in Section 7.6.3.

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7.6.2 The tiles

7.6.2.1 General considerations

The relevant British Standard is BS 6431 EN87 Ceramic Wall and Floor Tiles,which is a comprehensive document in 23 parts. Also relevant is BS 5385 Walland Floor Tiling. Part 4 of this Standard is a Code of Practice for CeramicTiling and Mosaics in Specific Conditions, Tables 3 and 4 list the suitability ofwall and floor tiles in specific conditions. For tiles continuously immersed(e.g. swimming pools), users are advised to ‘refer to manufacturers to confirmsuitability’.

For swimming pools, the tiles should be vitrified extruded tiles with a waterabsorption not exceeding 3% by mass when measured in accordance with Part11 of BS 6431 Ceramic Floor and Wall Tiles. This type of tile is frost-proofand can be used for open-air pools. The floor tiles in the shallow end of thepool should have a slip resistant finish; this recommendation also applies to thewalkways and wet changing areas and other areas which are frequently washeddown and used by bathers with bare feet. There are various finishes which areslip-resistant and the choice is largely a matter of experience and personalpreference.

Reference should be made to Section 2.16 for additional information onceramic tiles.

7.6.2.2 Laying the tiles

The tiles should be laid with joints 3–4 mm wide and fixed with a proprietarycement-based adhesive suitable for continuous immersion, such as Ardex Arduflex5000 or BAL High polymer modified adhesive.

For greater resistance to sulphate attack and leaching of lime from thecement as a result of a negative Langelier Index, a proprietary epoxy-basedadhesive, such as Ardipox WS or BAL Epoxy LV, could be used. The cost ofan epoxy-based adhesive is significantly higher than a cement-based one, butmay be worth the extra compared with the cost and great inconvenience ofclosing down a pool for extensive remedial work to the tiling, should the poolwater prove more aggressive than originally anticipated. The adhesive shouldbe mixed as directed by the manufacturers. It is emphasised in Section 8.6.1that the pH of the pool water should be strictly maintained in the range 7.2 to7.8 for effective water treatment. If the pH falls below 7.0, there is a risk thatacid attack on the grouted joints between the tiles will take place (Figure 7.6).

The tiles must be fully bedded, the adhesive being applied to the back of thetiles with a toothed and notched trowel, and the tiles being firmly pressed andtamped into position. The thickness of the bed will depend on the regularity of thesubstrate to which the tiles are fixed. Thin-bed adhesives should be used when thesubstrate checked with a 2 m straight edge does not reveal any gaps behind thestraightedge which exceed 3 mm in depth. Suitable thick-bed adhesives should be

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used when the gaps are between 3 mm and 6 mm. It can be seen that the regularity/trueness of the substrate is of great importance.

Grouting the tiles should not be carried out earlier than three days aftercompletion of fixing. The grout should be a proprietary product from the samemanufacturer as the adhesive and compatible with the adhesive.

Figures 7.7 and 7.8 show pools completed with high-quality ceramic tiles andmosaic.

7.6.2.3 Movement joints

Movement joints should be provided in the tiling at 4.5 m to 5.0 m centres,carried down through the rendering/screed to the structural pool shell.Movement joints are also required at all internal angles and changes ofdirection, and in the floor, at changes of gradient. Movement joints in thetiling are normally 6 mm wide.

As movement joints in the pool shell must be carried up through the rendering/screed and tiling, it follows that there must be close co-operation between the tilesuppliers, tile layers and pool designer if these basic requirements are to beproperly met.

All movement joints must be sealed with a suitable sealant. The relevant BritishStandard is BS 6213 1982 Selection of Constructional Sealants. According to Table

Figure 7.6 View of erosion of grouted joints by pool water of low pH (acidic).

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Figure 7.7 View of leisure centre pool finished with high-quality ceramic mosaic. Courtesy,Pilkington’s Tiles Ltd.

Figure 7.8 View of part of leisure centre pool and walkway slab finished with high-qualityceramic mosaic and ceramic tiles. Courtesy, Pilkington’s Tiles Ltd.

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4, the only material recommended without reservation is a flexible epoxy. Othermaterials are recommended, subject to reference to the manufacturers for theirsuitability.

This indicates a fairly wide choice, but Table 2 of BS 6213 suggests that the‘expected service life’ of the silicone, polysulphide and polyurethane typesealants is about 20 years. The estimated ‘life’ of the flexible epoxy sealantsis not given, but it would be reasonable to anticipate an appreciably longer lifethan 20 years.

The British Standards for polysulphide and silicone based sealants are listed byBSI as obsolete. Reference should be made to Section 2.15.

The provision of movement joints recommended for ceramic tiling is alsoapplicable to ceramic mosaics.

7.6.2.4 Tolerances on finished surface

Acceptable tolerances on surface regularity are:

1. 3 mm under a 2 m straightedge;2. difference in level across joints including movement joints: 1 mm for joints

not exceeding 6 mm wide; 2 mm for joints exceeding 6 mm wide.

7.6.2.5 Scum channels and deck level pools

Ceramic scum channels provide better circulation of the pool water thanskimmer outlets. They should be securely bedded on the shelf formed in thepool wall. The perimeter channel which are an essential feature of deck levelpools should be either glazed ceramic or finished with a smooth, imperviousand durable coating such as chlorinated rubber paint (see Section 7.10.4), orepoxy resin. The internal surface of the concrete channel should not be leftwith the bare concrete as this becomes dirty and almost impossible to keepclean.

With deck level pools, the perimeter channel usually discharges to a balancingtank and it is strongly recommended that the internal surface should also be finishedwith a suitable coating.

Water circulation is dealt with in Chapter 8.

7.6.3 Mosaic

There are two types of mosaic, ceramic mosaic and glass mosaic, the formerbeing the type mostly used. It is particularly suitable for free-formed poolsand pools constructed in sprayed concrete when there is a wide cove at thejunction of the walls and floor. The ceramic mosaic are fully vitrified and are

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therefore frost proof. The tesserae (pieces of mosaic) can be obtained in avariety of shapes, sizes and colours; standard sizes are 20 mm×20 mm and 25mm× 25 mm.

The basic recommendations for laying ceramic mosaic are similar to those forceramic tiles and are covered by BS 5385 Part 4 Code of Practice for CeramicTiling and Mosaic in Specific Conditions.

The experience of the author is that the use of a nylon or other mesh embeddedon the back of the mosaic can reduce the bond between the mosaic and the substrate(screed and rendering). It is better if the sheets of mosaic have paper on the facewhich is removed after laying.

Very attractive patterns can be formed with mosaic as can be seen in Figure 7.8.For the best results, the work should only be entrusted to experienced firms.

7.7 Walkways and wet changing areas

The recommendations given for screeds and tiling for insitu reinforced concretepool shells apply to walkways around the pool and wet changing areas and showerrooms.

It is essential that the surface of these areas should be ‘non-slip’, i.e. slip resistant,and when the floor slabs are suspended and use made of the space below, thefloors must be completely watertight to the same standard as the roof of a building.See Section 4.12.

The non-slip requirement can be readily met by the provision of slip resistanttiles, but the requirement for complete watertightness requires special attention todesign, specification and execution.

The design should be based on the water retaining Code (BS 8007). Inview of the serious trouble which has occurred in a number of public poolswhere these floor areas have not been watertight, it is recommended that awaterproof membrane be incorporated in the floor. An insitu brush or sprayapplied coating in two coats is recommended. The second coat can besprinkled with coarse sand to provide a key for the cement—sand screed.Because the key provided is not as good as that obtained by exposure of thecoarse aggregate in the concrete slab, the screed should be not less than 30mm thick and laid in bays not exceeding 3 m in width. The length of the bayis less important than the width as the fine transverse cracks which occurcan be readily grouted in before the tiles/mosaic are laid.

The membrane should be carried up walls which are built directly from theslab, and all openings in the slab for pipes and gulleys must be carefully detailedso as to be watertight. If a sheet membrane is used this will completely debond thescreed from the base concrete and the screed should be not less than 75 mm thickand should be concrete with 10 mm maximum size coarse aggregate. Such aconstruction would increase the dead load of the floor.

The floors must be laid to falls to drainage channels which discharge to thedrainage system and not to the water circulation system of the pool. A gradient of

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1 in 40 (25 mm in 1.00 m) should be adequate to prevent ponding, but for safetyreasons (to prevent slipping) a gradient of 1 in 60 (25 mm in 1.50 m) isrecommended.

7.8 Testing the completed tiling

The whole of the tiling should be tested for adhesion (detection of hollow soundingtiles by tapping with a rod) in the same way and for the same reason as recommendedfor rendering and screeds.

Difficulty can arise in the interpretation of results and the fixing of a line betweenacceptance and rejection, particularly as the defects in adhesion may only extendover part of the area of individual tiles. The decision requires considerableexperience as the vibration caused by the removal of defective tiles may increasethe debonded area.

7.9 Marbelite

Marbelite is a white terrazzo applied insitu to the walls and floor ofswimming pools in the private sector. Package deal swimming poolcontractors appear to favour this material for small pools, especially forprivate houses (Figure 7.9).

It is composed of white Portland cement, white marble chippings, usually gradedfrom 3 mm down and should be free from dust. The mix proportions are generallyin the range of 1 part cement to 2–2½ parts marble. There is no Code of Practicefor the use of insitu terrazzo as a finish to swimming pools, but the NationalFederation of Terrazzo, Marble and Mosaic Specialists have issued a specificationfor the material, mainly for use as flooring.

The marbelite should be applied to cement-sand rendering and screed to ensurean even base.

The surface of the substrate should be combed or scratched to provide a key forthe marbelite which is usually applied in one coat to a finished thickness of 6 mmon the walls and about 10 mm on the floor.

A minimum period of 21 days should elapse between the completion of therendering/screed and the application of the marbelite.

It should only be entrusted to contractors who specialise in the application ofinsitu terrazzo, and with a good ‘track record’ for the use of the material in swimmingpools.

It is not advisable to have the work carried out by an ordinary plasterer.The following list outlines some of the important points in its application:

1. The marbelite should be applied in one coat to the required thickness (6–10

mm).

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2. Some applicators will try to finish the whole of a small pool in one day, whichis not advisable. The marbelite should be applied in panels, square if possible,otherwise the length/height should not exceed 1.5 times the width. Shrinkagecan then take place along the line of the panel joints instead of forming randomcracks. Even a 1:2.5 mix is very rich in cement and liable to shrinkage crackingand crazing.

3. The marbelite must be protected for at least 48 hours by means of damp sackingor polythene sheets properly secured to prevent sun and wind impinging onthe newly applied marbelite.

4. Care must be taken to ensure that the surface of the floor and of steps leadinginto the pool are not too smooth, as polished merbelite is very slipperyparticularly to wet feet.

Marbelite stains easily because, in spite of the polishing, the surface is absorbentand stains are very difficult to remove and usually require grinding. This appliesparticularly to open-air pools; the stains from leaves and other organic matter arevirtually impossible to remove without grinding.

To combat staining, it is usual to provide a row of ceramic tiles or mosaic about

Figure 7.9 Application of marbelite to wall of private swimming pool.

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300 mm wide at the water line as these are less liable to become stained, and if thisdoes occur, cleaning is relatively easy.

Marbelite is particularly vulnerable to acid attack which can occur if the pHof the pool water is not maintained consistently in the recommended range of7.2 to 7.8.

The marbelite should be subjected to a test to detect defective bond as describedin Sections 7.1 and 7.8.

Figure 7.9 shows marbelite being applied to the wall of a small pool.

7.10 Coatings and paints

7.10.1 Introduction

In cases where shortage of funds prevent the use of ceramic tiles/mosaic, thealternative is to use a proprietary coating (also referred to as paints). There is a fairlywide choice of materials, and for swimming pools the desirable characteristics are setout below:

7.10.2 Desirable characteristics

The main features include the following: 1. The coating must be attractive in colour and appearance.2. It must have a smooth impermeable surface which can be readily cleaned.3. The coating must be capable of forming a good bond to the substrate.4. The normal substrate is cement based (concrete, rendering, screed) and

the coating must not be adversely affected by the alkalies in the cement.5. The coating must be durable under the conditions in which it has to

exist, namely warm chlorinated water containing solutions of chemicalsused in water treatment. In the case of open-air pools, the coating mustbe resistant to weathering, ultra-violet light, and the effect of frost.

6. It should possess some degree of elasticity (elastomeric), as the pool shell towhich it is applied will move to some extent during filling and emptying,changes in temperature and foundation movement.

Reference should be made to BS 3900 Methods of Tests for Paints. The followingParts are of special interest:

Part C3 Through Dry Test for Multi-coat Systems;Part C5 Determination of Film Thickness;Part E2 Scratch Test;Part E6 Cross-cut Test;Part E10 Pull-off Test for Adhesion.

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7.10.3 Material types

The basic types of available materials may be summarised as follows: 1. Chlorinated rubber paint;2. Polymers such as acrylics, epoxies and polyurethanes;3. Cement-based paints. Some types of coatings are vulnerable to blistering if there is moisture or vapourtrapped behind the coating.

A reasonable period must elapse between the completion of the substrateand the application of the coating and also before the admission of water intothe pool. Directions for these two periods should be obtained from themanufacturers.

7.10.4 Chlorinated rubber paint

This type of paint is very popular for swimming pools, both covered and open air.The paint should be the high-build type and generally consists of chlorinated

rubber, a plasticiser, inert pigments and a thickening or thixotropic agent. It shouldbe applied to a dry film thickness of 0.10–0.15 mm.

The best results are obtained when the paint is applied to good qualitycement/sand rendering/screed which has been finished with a wood float; thewood float gives a dense even surface without laitance. The substrate must beclean and dry and it is advisable to neutralise the surface by means of a diluteacidic wash (1 part hydrochloric acid to 10 parts water). Immediately prior tothe application of the acid, the surface should be washed down with water asthis will help prevent the acid being absorbed into the substrate. After aboutfive minutes, the surface should be well washed down with water and the dampsurface tested by a pH indicator; the pH should be in the range recommendedby the paint supplier. The paint should be applied in three coats, the secondcoat being applied at right angles to the first to eliminate pin holes. A minimumof 24 hours should elapse between coats, and 48 hours is better. A period of14 days should elapse between the completion of the final coat and the fillingof the pool with water.

One disadvantage with chlorinated rubber is that the makers usually recommendthat it be applied within a fairly narrow range of temperature and humidity; thiscan be particularly difficult with open-air pools.

The above information is intended as a general guide, and the detailed directionsof the supplier should be followed.

The useful life of chlorinated rubber is very difficult to predict and depends onmany factors such as whether the pool is indoors or open-air, the standard of watertreatment and cleaning and maintenance, and the amount of back-pressure (if any)arising from moisture and/or vapour trapped behind the coating. However, a useful

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life of four years would be a reasonable average for open-air pools and seven yearsfor indoor pools.

Figure 10.8 shows a pool completed with chlorinated rubber paint.

7.10.5 Epoxy resins and polyurethanes

For basic information on epoxies and polyurethanes the reader is referred toSection 2.13.

These paints can be obtained in a standard range of colours and when properlyapplied give very satisfactory results and should have a useful life of 5 to 7 yearsfor open-air pools and 7–10 years for indoor pools.

The following factors are important: 1. The material must be of the highest quality and a statement should be obtained

from the manufacturer on the percentage of resin in the paint. Epoxies andpolyurethanes bond strongly to concrete, rendering and screeds.

2. The correct preparation of the substrate is of primary importance and themanufacturer’s directions should be carefully followed. However, the followingindicates the steps which are generally recommended:

(a) Grit blasting is advisable, but this should be very light as it is only requiredto remove the thin layer weak laitance. With good-quality concrete about1 mm is all that should be removed.

(b) A minimum of two coats is required, but three coats are preferred. Twocoats should eliminate ‘holidays’ (pin holes), but three coats will certainlydo so. When applied by brush, the second and subsequent coats shouldbe applied at right angles to the preceding coat. It is usual to apply aprimer before the first coat.

(c) Epoxy resins can now be formulated to bond to damp concrete and it isadvisable to select such a resin for external application.

(d) Manufacturers supply information on the temperature and ‘shelf life’ ofthe resin and this information should be noted and adhered to.

(e) If the material is two pack (resin plus accelerator), proper mechanicalmixing is essential. These resins are normally used with a primer whichshould be supplied with the resin.

There is a distinct advantage if the supplier of the material is also responsible forits application as this avoids divided responsibility if something goes wrong.

7.10.6 Cement-based paints

These paints are based on white or pigmented Portland cement, to which areusually added accelerators, waterproofers, and inert fillers. As normally used,they give a matt finish and are available in a range of colours including white.By the addition of a glaze, a gloss can be imparted to the surface.

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This type of paint is particularly suitable for cementitious backgrounds such asconcrete, dense concrete blocks and rendering and screeds.

The paint is mostly used for private and club pools as it has the great merit oflow cost and ease of application and it can be applied over many types of existingdecorated surfaces provided these are clean and sound.

In the case of dusty, friable or absorbent surfaces, it is advisable to apply first apriming coat or stabilising liquid supplied by the paint manufacturer. Themanufacturers always give detailed directions for the mixing and application ofthe paint which should always be followed.

Approximate coverage on different surfaces is also given. A minimum of twocoats is recommended. A minimum period of 24 hours should elapse betweensuccessive coats and in cool weather this may have to be extended to 48 hours. Itis not possible to say how long such a coating should last as many factors areinvolved such as the location of the pool (open-air or indoors), the number ofpersons using the pool and the standard of maintenance. It is likely that an open-air pool would need repainting each year if the owner required a high standard offinish.

Reference can be made to BS 4764 Specification for Powder-cement Paints.

7.11 Sheet linings to swimming pools

There are various types of flexible sheeting materials which can be used to providea waterproof lining to small swimming pools. However, the type of material whichis used almost exclusively in the private swimming pool market is polyvinyl chloride(PVC). These pools are known as liner pools.

The pool shell must be structurally sound and if ground water rises abovethe level of the underside of the floor of the pool, the pool shell should also bewatertight against infiltration of ground water. Unless this is taken intoaccount, then if the pool is emptied, ground water pressure may force the linerout of position and it will prove very difficult, if not impossible, to rectify this.When the liner is originally fitted, it is ‘stretched’ and fits tightly against theinside surface of the pool shell.

The PVC liner can be obtained in a variety of attractive colours and patterns.The ‘life’ of a high-quality PVC liner which has been correctly installed andcarefully used is likely to be in the range of six to ten years. When a new liner isfitted, a different colour and/or pattern can be selected. For satisfactory service, itis essential that the liner should be supplied and fitted by the same firm as thiseliminates divided responsibility. The suppplier of the liner should also providedetailed instructions for cleaning the pool and for advice on the treatment of thepool water.

Figure 7.10 shows a pool completed with PVC sheet lining.

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7.12 Glass-fibre polyester resin linings

This type of lining was originally introduced into the UK in the 1960s and wasfirst used for the renovation of old swimming pools.

The use was extended to new private and club pools and then it was used for fora limited number of new public pools. As far as public pools were concerned, theresults were not up to the original expectations in most cases.

The pool shell should be watertight against loss of water from the pool andagainst infiltration of ground water when the pool is empty.

This type of lining is intended to be fully bonded to the pool shell (floor andwalls) and this normally requires the shell to be constructed of insitu reinforcedconcrete or sprayed reinforced concrete. This requirement does not apply whenthe material is used to renovate an old existing pool. The concrete surfaceshould be prepared by grit blasting to lightly expose the coarse aggregate. Allloose grit and dust must be removed. The following is a brief description ofhow the material is generally applied, but the details vary from one specialistapplicator to another.

The polyester resin, glass fibre and a catalyst are applied by compressed airthrough a three-nozzle gun, to a first coat thickness of about 1 mm. It is consolidatedby metal rollers to ensure that the glass fibres are completely embedded in the

Figure 7.10 Pool completed with liner of PVC sheeting. Courtesy, P.G.Sales and ServicesLtd.

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resin. The first coat is immediately followed by the application of glass tissuesheet about 0.3 mm thick to seal in completely any glass fibres which may beprojecting through the resin. The next stage consists of the application of a gel coatof polyester resin; this coat is allowed to semi-cure (the time taken depending onthe formulation of the resin) and can be modified to suit the execution of the work.A second gel coat is then applied and contains pigments. The overall thickness ofthe laminate is between 3–5 mm.

When properly executed this type of lining can be very attractive, but certainfacts should be kept in mind when considering its use. These are summarised below: 1. Polyester resins have high shrinkage characteristics and this can lead to a

build-up of stress at angles, corners and points of entry of pipework, resultingin formation of cracks.

2. There is a considerable difference in the coefficient of thermal expansion ofthe resin and the glass fibres, resulting in internal stresses in the laminate.

3. It can be difficult to secure good bond between the concrete/ rendering/screedand the laminate. If water penetrates through cracks in the lining, debondingcan occur and spread over a considerable area.

4. As the gel coats are of different composition to the body of the laminate,blistering and flaking sometimes occurs.

Briefly, while this type of laminate lining has performed well in many cases, therehave been some instances of serious trouble. Special care should be taken in drawingup the contract documents to help ensure guarantees of satisfactory performance.

7.13 Finishes to the walls of pool halls

7.13.1 General considerations

The selection of attractive and durable finishes to the inside surface of swimmingpool halls requires careful consideration due to the conditions under which thefinishes have to operate. The notes which follow are intended to highlight theproblems and offer practical solutions.

7.13.2 The use of the natural (unprotected) surface ofthe structural material

The increase in cost of both labour and materials in recent years has led to thesearch for structural materials which can be produced with a finish which isaesthetically satisfying and has a long maintenance-free life. This appliesparticularly to insitu and precast concrete. Good-quality Portland cement concretewill not suffer deterioration by the atmosphere in swimming pool halls. The highhumidity, relatively high temperature and the presence of chlorine compoundswill have no corrosive effect on the concrete. Some staining will occur but this will

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not generally be noticeable if it is above eye level, and the effect can be mitigatedby the application of a good-quality silicone water repellent at about five-yearlyintervals.

However, this type of finish is not recommended in any position where itcan come into contact with persons using the pool as serious staining by greaseand dirt will result from such contact. The untreated surface of concrete isabsorbent and this type of staining cannot be removed without grinding andthis changes the light reflecting properties of the surface making the areasclearly visible.

Figure 7.11 shows the wall of a pool hall left in its ‘natural state’. For the reasonsgiven above this is not recommended.

7.13.3 Applied finishes

The finishes briefly described here are all considered suitable for the walls, columnsand similar structures of swimming pool halls and some for the walls of showerrooms:

Figure 7.11 Wall of pool hall left in its ‘natural state’. This is not recommended for reasonsgiven in the text.

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Ceramic tiles and mosaic;Terrazzo;Marble.

7.13.3.1 Ceramic tiles and mosaic

The relevant UK Code of Practice is BS 5385 Part 4 Code of Practice for CeramicTiling and Mosaics in Specific Conditions. It is recommended that this Part of themain Code be followed rather than Part 1 which gives recommendations for normalinternal tiling. Part 4 contains detailed recommendations for tiling which will haveto operate under wet and damp conditions, which covers pool halls, shower roomsetc. The principal points to note are: 1. The tiling should be fixed to concrete or dense aggregate concrete blocks, or

to cement/sand rendering. The structure behind the tiles must be watertight.This ‘tanking’ must be carried up from the floor to an appropriate height. Inthe case of the walls of pool halls, not less than 2.5 m; in shower rooms, forthe full height of the walls.

2. Cement-based and organic-based adhesives can be used.3. It is advisable, but not essential if the walls are ‘tanked’, for the grout used for

the tile joints to be impervious. Detailed advice for waterproofing/tanking the substrate to which the tiles are fixedcan be obtained from the manufacturers of the tiles and adhesives.

7.13.3.2 Terrazzo and marble

The same basic principles as described in Section 7.13.3.1 apply to terrazzo andmarble.

Thin bed adhesives should not be used, and organic-based adhesives are notrecommended.

Generally, these materials can be used on the walls of the pool hall but not inshower rooms. For detailed recommendations for the fixing of terrazzo and marble,reference should be made to the National Federation of Terrazzo, Marble and MosaicSpecialists. See the note in Section 7.13.3.1 on tanking the walls behind the tiles.

7.13.4 Toppings and coatings for plant rooms and storesfor chemicals

High-quality insitu or precast concrete is not really suitable as a finish for thefloors of plant rooms and chemical stores. It is better from the point of view oflong-term durability to provide an applied finish (coating or thin topping) of apolymer resin such as epoxy or polyurethane. Coatings normally do not exceedabout a millimetre in thickness are likely to be adequate for private and hotel pools.These are applied in two coats on a primer, and can be obtained in a range of

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colours. The surface of the concrete should be wire brushed and all grit and dustremoved prior to the application of the primer.

For larger pools (clubs and public pools), the extra expense of providing a resin-based topping 3–4 mm thick is usually justified. The 3 mm thick toppings areoften self-levelling. The material consists of selected aggregates and fillers and theresin and accelerator/activator. The base concrete must be good quality and laitenceshould be removed and the surface left free of dust and grit.

If, for reasons of economy, it is decided to use a coating, then the tests for paintsset out in BS 3900 Part E6 Cross-cut Test, and E10 Adhesion, should be includedin the Contract Specification.

THE ROOFS OF SWIMMING POOL HALLS

7.14 General considerations

As mentioned in Section 1.6, there are serious problems associated with roofs ofswimming pool halls arising from the comparatively high air temperature and highhumidity in the pool hall. If chlorine is used in the water treatment process, thiswill aggravate the situation from the point of view of corrosion of any unprotectedferrous metal.

The result can be condensation in the roof space, leading to corrosion of ferrousmetals, deterioration of electrical equipment and timber used in the roof structure.Cases have been reported where stainless steel has suffered serious corrosion whenhighly stressed in roof voids.

Thermal insulation is required to prevent unacceptable heat loss and a vapourbarrier is needed to avoid interstitial condensation. The principle is that the vapourbarrier should be on the warm side of the thermal insulation. The details ofconstruction depend on the design of the roof structure.

In a swimming pool hall this problem can be overcome in two basic ways:(1) The provision of a pressurised roof void, or (2) The provision of a ‘warmdeck’ roof.

7.15 Pressurised roof voids

The provision of a pressurised roof construction is only suitable for largeinstallations. In this system, the air in the roof space/void is maintained at a pressurein excess of that in the pool hall so that the air moves outwards into the hall andthus prevents the warm humid air in the hall moving up into the roof void. Such aninstallation requires very expert design and necessitates the use of fans which mustoperate continuously. A vapour barrier must be provided at ceiling level so as toreduce to a practical minimum the size and power consumption of the fans.

A high standard of operation and maintenance, plus regular and carefulinspection are required. Records of all such inspections should be kept.

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7.16 The warm-deck roof

For the majority of cases, warm-deck construction of the roof is a satisfactory andpractical solution. In this method, thermal insulation is located above the roofdeck, and a vapour barrier is located between the deck and the thermal insulation.

This method of roof construction is basically different to that known as thecold-deck roof in which the thermal insulation is located below the roof deck andit is not recommended for the roofs of swimming pool halls.

In the warm-deck roof, the deck provides a satisfactory surface on which thevapour barrier can be laid, with the insulation above it, followed by the waterproofmembrane for the roof.

A major design objective is to ensure that the temperature at the vapour barrieris not below the dew point temperature. This can be achieved by increasing the airtemperature in the pool hall and improving the ventilation/dehumidification.

If there is a roof void then it is essential that this should be inspected at regularintervals, say, every 12 months, to check for signs of deterioration; the result of allsuch inspections should be recorded.

Readers are referred to Building Research Establishment Digests, and BS 5250Code of Practice for Control of Condensation in Buildings, listed under FurtherReading at the end of this chapter.

Further reading

British Standards Institution. Code of Practice for Control of Condensation in Buildings,BS 5250.

Building Research Establishment. Swimming Pool Roofs, Digest 336, 1988.Building Research Establishment. Flat Roof Design, the Technical Options, Digest 312,

1986.Building Research Establishment. Flat Roof Design, Waterproof Membranes, Digest 373,

1992.International Standards Organisation. Building Construction Sealants—Classification and

Requirements, ISO 11600.

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Chapter 8

Water circulation and watertreatment

WATER CIRCULATION

For swimming pools, an efficient system of water circulation is essential for thehealth and safety of the users, to ensure relative freedom from pathogenic bacteriaand maintenance of a high standard of clarity in the pool water.

There are two basic systems of water circulation:

Figure 8.1 Diagram of flow-through pool.

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1. The simplest method is where a pool draws its water from a stream, lake or thesea, so that the water in the pool is continuously changed. These are oftenreferred to as flow-through pools (Figure 8.1).

2. The normal method for the vast majority of pools is the provision of a systemof pumped water circulation. This requires inlets to the pool and outlets fromthe pool connected to a pump so that the pool water is kept in continuouscirculation, and fresh water is only added to make up ‘losses’. At predeterminedintervals which can vary from a year or longer, the pool is emptied for generalcleaning, detailed inspection and repairs.

See Figure 8.2 for general layout of treatment plant for small pools.

8.1 Flow-through pools

Even with this simple type of water circulation, certain principles should beadhered to.

The inlets and outlets should be located so that as far as practical, the whole ofthe water in the pool is changed at a calculated rate, and there are no ‘dead’ pocketsof uncirculated water. Screens should be provided at appropriate locations. Thescreens require regular inspection, with special attention after heavy rain (for poolsfed from a stream) and after storms for sea water pools.

It is essential that a reasonable standard of clarity of the water in the pool bemaintained for the safety of the bathers. See Section 8.5.1.

Figure 8.3 shows a stream used as a swimming pool in Switzerland.

Figure 8.2 Diagram of typical layout of water treatment plant for small pool. 1, outlet mainfrom pool; 2, strainer; 3, circulating pump; 4, coagulant dosing; 5, pH regulator;6, filter(s); 7, heater; 8, aerator; 9, water disinfecting equipment; 9A, alternativeposition for disinfecting equipment; 10, treated water main to pool.

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Figure 8.3 View of a natural flow-through pool in Switzerland. Courtesy, EdwardSchwartz.

Figure 8.4 View of one of a series of three main circulating pumps for pools in a privateleisure centre. Courtesy, Pool Water Treatment Advisory Group.

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8.2 Pools where the pool water is in continuouscirculation

It is a basic principle that the inlets and outlets should be designed and located sothat the circulation is as complete as possible and that there are no pockets of‘dead’ water. The maximum contamination is in the surface water and in the areaof the pool where the bathing load is heaviest.

The methods adopted to achieve adequate circulation will depend on the sizeand shape of the pool and the use to which it is put. That is, whether the pool isused by members of one family and their friends, when the maximum bathing loadis likely to be very light, or whether it is a hotel, club, school or public pool whenthe loading can vary from very light to very heavy.

Success depends largely on the experience of the designer; the system must bein reasonable balance, i.e. the inflow of water must keep pace with the withdrawalof water. With heated pools, particularly open-air ones, the even distribution of theheated water throughout the whole pool is important for the comfort of the bathers(Figure 8.4).

The principal factors relating to the efficiency of water circulation in swimmingpools are: 1. the turn-over period. See Section 8.2.1;2. the pool loading. See Section 8.2.4;3. the amount of make-up of fresh water used. See Section 8.2.5;4. the hydraulic design of the system (size of pumps and size and layout of

pipework);5. the type and location of outlets and location of inlets. See Section 8.2.6 and

8.2.7.

8.2.1 The turn-over period

Typical turn-over periods are set out below:

Private house pools 6–8 hoursHotel and club pools 2.5–4 hoursSchool pools 2.0–3 hoursPublic pools 2.0–3 hoursTeaching/learner pools 1.0–1.5 hoursDiving pools 4–6 hours

The circulation rate is the volume of water in the pool divided by the turn-overperiod.

The effect of the turn-over period is largely governed by the mixing efficiencywhich depends on the location and number of the inlets and the method of draw-off from the pool to the filters.

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Figure 8.5 Sketch of skimmer outlet.

Figure 8.6 Sketch of standard scum channel. Courtesy, Pilkington’s Tiles Ltd.

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8.2.2 Circulation systems for small pools for privatehouses

The bulk of the contamination in the pool water is in the surface layers and this iswhy it is most important to remove effectively this surface water. Usually the shallowend of the pool is the more heavily loaded and consequently the water is morecontaminated.

The system in general use consists of skimmer-weir outlets and one outlet inthe floor at the deep end; the incoming water from the treatment plant is distributedthrough spreader inlets. Provided there is an adequate number of inlets and outletsproperly located, this constitutes an effective method of circulation.

Figures 8.5 and 8.6 show a skimmer-weir outlet and a scum channel.The following are suggestions for location of outlets and inlets for relatively

small private pools using skimmer outlets. However, the circulation system mustbe in balance and the number of outlets and inlets should be calculated by thedesigner of the circulation system and treatment plant. 1. Rectangular Pools

(a) Water area 40 m2 say 10 m×4 m (i) Outlets: Two skimmer-weirs in each long wall towards the shallow

end of the pool (should be a reasonable distance from the inlets) andone skimmer outlet in the centre of the short wall at the deeper end ofthe pool. One outlet in the floor at the deeper end of the pool. Total:five surface outlets and one floor outlet.

(ii) Inlets: Two inlets in each long wall towards the shallow end and onein the short wall at the shallow end. The inlets should not be close tothe outlets as this can result in short-circuiting. Total: five inlets.

(b) Water area 133 m2, say 16.67 m×8.00 m(i) Outlets: Three skimmer-weirs in each long wall towards the shallow

end, one skimmer outlet in the short wall at the deeper end of thepool, and one outlet in the floor at the deeper end of the pool. Total:seven surface outlets and one floor outlet.

(ii) Inlets: Two inlets in the short wall at the shallow end, and two in eachlong wall towards the deeper end of the pool. Total: six inlets.

2. Free-Formed PoolsThe outlets and inlets should be located in accordance with the principle that theheaviest contamination is in the area of shallow water, that short circuiting shouldnot occur, and that the turn-over period is generally as given in Section 8.2.1.

8.2.3 Circulation systems for larger pools for hotels,clubs and schools and public pools

For hotels, clubs and schools, it is recommended that either scum channels areused for the outlet of the pool water or the pool is designed as a deck-level pool

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with a properly designed perimeter channel and balancing tank. The inlets for thetreated water would discharge the water through spreaders.

Scum channels, and the provision of a perimeter channel in deck-levelpools are appreciably more effective in removing surface contamination thanskimmer weirs. Their use is recommended for pools where a heavy bathingload is anticipated.

Scum channels consist of heavy glazed ceramic units (Figure 8.6).Unfortunately, the perimeter channel of deck-level pools is sometimes left

as unlined concrete which is certainly not satisfactory. The best finish isobtained by the use of special glazed ceramic units; a rather less satisfactorymethod is to form the channel in insitu concrete, finished with a smoothsurface which is then finished with two coats of chlorinated rubber paint or anepoxy-based coating.

It has been mentioned previously in this chapter that it is of the utmostimportance that the system for the withdrawal of contaminated water and thedistribution of the purified water should ensure that the whole of the pool wateris circulated during the turn-over period. The water treatment system (filters andwater disinfection plant) should maintain the whole of the pool water at therequired standard of purity (and temperature if the water is heated). Someinformation on water treatment is given elsewhere in this chapter. Notes onheating swimming pools and energy conservation are given in Chapter 9.

It is recommended that the design of the water circulation system and thewater treatment plant should be the responsibility of one firm, either as a‘package deal’ or by a firm of independent consultants experienced in this field.

The water circulation system for a large free-formed pool incorporatingwavemaking equipment requires careful design by an experienced firm.

8.2.4 Pool loading

It is obvious that the number of persons using the pool at any one time is directlyrelated to the contamination entering the pool water, and the removal of thiscontamination is related to the turn-over period/circulation rate, filters and treatmentplant.

The amount of this contamination affects the quality and clarity of the poolwater. Bathing loads should be controlled under two main headings, physical safetyof those using the pool, and the maintenance of water quality.

For an acceptable standard of physical safety: 1. The maximum number of persons in the pool at any one time should be limited.

The HSE booklet Managing Health and Safety in Swimming Pools recommends3 m2 per person.

2. A high standard of clarity is maintained in the pool water. The clarity of waterfrom a public supply is not necessarily adequate for use in a swimming pool.The term clarity includes turbidity and colour. It is essential that a bather who

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is in trouble on the floor of the pool should be clearly seen from the poolsides.

8.2.5 Make-up water

An important factor in the water purification sytem is the amount of ‘make-up’water used; this is the amount of fresh water introduced into the pool at intervals.This is referred to by the Institute of Baths and Recreation Management as‘progressive dilution’. In Europe, the amount of fresh water is appreciably greaterthan that used on the average in the UK.

Depending on the design of the system, the admission of large quantities offresh water can increase the heating cost.

8.2.6 Inlets and outlets

While with smaller pools (referred to in Section 8.2.2), inlets for fresh waterare invariably located in the pool walls. In larger pools for local authorities,inlets are sometimes located along the centre line of the floor, but this canresult in the incoming water which is under pressure from the circulatingpumps finding its way under the floor screed resulting in lifting and damage tothe tiling.

Outlets should be in the form of either scum channels or a deck-level poolwith a perimeter channel. These systems are much more efficient in removingthe heavily contaminated surface water than individual skimmer outlets usedfor smaller pools. There must also be an outlet or outlets in the deeper end ofthe pool which are also used for emptying the pool for general maintenanceand cleaning.

The scum channel/deck-level overflow should take at least 60% of thecirculating water, the remaining 40% (maximum) being removed through theoutlet in the floor at the deep end of the pool. Sometimes more than one flooroutlet is provided to help ensure that dangerous suction does not develop.Gratings must have small openings to prevent injury to bathers’ toes.

8.2.7 Deck-level pools

This type of pool has become very popular, particularly in leisure centres. Thecirculating water flows over the side of the pool into a continuous perimeterchannel. This channel discharges into a balancing tank. There is no generallyrecognised method for calculating the size of the balancing tank; treatmentplant manufacturers develop their own design and are reluctant to divulge thedetails. The amount of water discharging to the perimeter channel can varyconsiderably due to wave action which depends on the activities of thebathers; a large group doing exercises would create more wave action thanpersons swimming. For the actual dimensions of the tank, an adequateallowance should be made for ‘free-board’, say, 300 mm.

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The dimensions and gradient of the perimeter channel has also to be calculatedas it forms an essential part of the circulation system. The perimeter channel isclosed at the top with a removable grating which is usually made of extruded PVC,but stainless steel (austenitic) is sometimes used in high-class installations. Theopenings in the grating must be toe and finger ‘proof’ (Figure 8.7).

Both perimeter channel and balancing tank should be finished with a smooth,durable coating or glazed ceramic units.

8.3 Ducts for pipework

These days, pipework for the circulating water system is almost always unplasticisedPVC; reference should be made to BS 3505 Unplasticized PVC Pressure Pipes forCold Water and BS 3506 Unplasticized PVC Pipe for Industrial Uses, and CP 312Code of Practice for Plastics Pipework (Thermoplastics Material).

It is recommended that all pipework should be in accessible ducts unlessotherwise laid/fixed so as to be reasonably accessible for inspection and repair.

Figure 8.7 Sketch of perimeter/circulating channel for deck-level pool. Courtesy, Pilkington’sTiles Ltd.

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Although this may add a significant amount to the first cost of the installation,this increase is far less than that incurred for repairs/replacement of leaking pipesnecessitating breaking up of floors etc. This cost has to include the loss suffered bythe closure of the pool for a fairly long period. A further point is that it is extremelydifficult to repair an opening made in the floor or wall of a water-retaining structureso that it is watertight.

WATER TREATMENT

It was stated in Chapter 1 that in the UK legislation directly relating to the purity ofwater in swimming pools only applies to pools which are open to the public, mainlylocal authority pools. There are no detailed unambiguous standards laid down bylaw which apply to all classes of swimming pools.

However, there can be no doubt about the moral obligation of every oneresponsible for the operation of a swimming pool to ensure that the water in thepool is clear and is good quality. Also that the combination of chemicals used inthe treatment of the water does not result in distress to the pool users, and in theconcentrations used, is not aggressive to the materials of which the pool isconstructed and finished, including the pipework and fittings.

Figure 8.8 Diagram of complete treatment control for swimming pool water. Courtesy, USFStranco.

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Figure 8.9 View of three air blowers forming part of the complete water treatment plant forpools in a private leisure centre. Courtesy, Pool Water Treatment Advisory Group.

Figure 8.10 View of plant room with fully automatic water treatment equipment. Courtesy,Buckingham Swimming Pools Ltd.

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The size and type of plant required depends on the size and use to which thepool is put. At one extreme, there is the private house pool used only by the ownerand his family and friends. At the other extreme, there is the large public pool withheavy pool loading (see Section 8.2.4). However, all methods of satisfactorytreatment have much in common (Figures 8.8–8.10).

It will be appreciated that the proper control of swimming pool water iscomplicated even for small pools with a low bathing load. For the large publicpools, considerable practical experience supported by sound theoretical knowledgeof the chemistry of water treatment is necessary.

Reference should be made to the publication Swimming Pool Water-treatment and Quality Standards prepared by the Pool Water TreatmentAdvisory Group, and to the publications of The Institute of Baths andRecreation Management.

Many of the chemicals used in water treatment are potentially hazardous tohealth and special care is needed in their use and storage. Reference should bemade to the requirements of the Health and Safety Executive and to the briefcomments in Appendix 5.

8.4 Layout of treatment plant

The equipment recommended for a small pool is shown in the diagram at Figure8.2. The plant would consist of: 1. strainer;2. circulating pump and electric motor;3. coagulant dosing equipment;4. pressure filter;5. disinfecting equipment;6. heater.

The coagulant dosing equipment may be omitted for small private house pools,and the heater may not be included in the owners brief.

8.4.1 Plant rooms

The equipment is expensive and needs to be properly maintained. Therecommendations which follow are intended to refer to plant rooms generally,irrespective of size.

All the above should be installed in a properly constructed plant house/room, which should also provide space for the storage of chemicals used in thecoagulant dosing equipment and the disinfecting equipment and as well astools and spares.

The plant room should have a concrete floor, clay brick or concrete block walls,

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adequate windows and permanent ventilation which in small plant rooms can consistof two louvered openings each about 250 mm×76 mm covered with wire mesh.

It must be designed with easy access and adequate floor space so that the plantcan be installed, serviced and removed without difficulty. The roof should be ofdurable materials and completely weathertight.

While many plant rooms have plain concrete floors, it is better if the concretefloor is finished with a high-quality durable paint, such as chlorinated rubber,polyurethane or epoxy which is resistant to the chemicals used as these are certainto be spilt on floor. The floor should be laid to a gradient of about 1 in 60 dischargingeither to the drainage system via a floor gulley or to outside the building, dependingon the circumstances of each case.

Electrical wiring and equipment should be of the best quality and there shouldbe an accessible control panel with fuses/circuit-breakers. The recommendationsof the Institution of Electrical Engineers should be followed.

For large installations, it may be necessary to install a gantry for the moving ofheavy items of plant. Bunds should be provided around tanks containing chemicalsin liquid form.

In large municipal pools, the plant room is often below the pool walkways andthe changing accommodation. The walkway slabs and floors of the wet changingareas must be completely watertight; see Sections 4.12 and 7.7.

8.4.2 Notes on circulating pumps

Centrifugal pumps are used for water circulation with directly coupled electricmotors, operating on AC 3-phase, usually 440 V, but very small capacity pumpsmay operate on 220 V. The pumps should be self-priming.

For large installations, the pumps are in sets of two, three or four operating inparallel. In this way, pumps and motors can be taken out of operation formaintenance without an undue effect on the water circulation.

With the larger pumps, it is an advantage if they are of the split casing type asthis enables the top half of the casing to be removed for inspection of the bearingsand impeller.

The ‘characteristics’ of the pumps should be such that delivery does not fall offsignificantly with increase in delivery head caused by build-up of deposits in thefilters.

For solution feed of chemicals, a different type of pump is used, usually a piston/displacement type.

8.5 Filtration and filters

The basic requirements for a satisfactory swimming pool water are closelyconnected and have been discussed in the Introduction.

The filters have two functions. They must ensure that the water leaving thefilters has a high degree of clarity by reducing the matter in suspension and, as

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these particles are mainly organic and some may contain micro-organisms,filtration assists the disinfection of the water. The filters are assisted by thestrainer, shown at 2 in Figure 8.2 as this holds back the coarser material insuspension.

8.5.1 Clarity of pool water

Clarity is reduced by suspended and colloidal matter in the water, and by colour.There are two principal reasons for requiring that water in a swimming pool

should possess a high standard of clarity: user safety and public health. In fact, theuser safety aspect can be more important as the cases of water-borne disease whichare established as originating from a swimming pool hardly ever occur in the UKand other developed countries with a similar climate. The same cannot be claimedfor fatalities due to bathing in a swimming pool containing water of sub-standardclarity.

If a bather gets into difficulties and sinks below the surface of the water, it canbe very difficult for other users to notice what has occurred and to locate the bodyunless the water has a high standard of clarity. In practice, the water should besufficiently clear that the bottom of the pool can be easily seen at the deepest partby persons on the walkway around the pool.

In the UK, the type of filter in general use is the pressure sand filter and theseare described briefly in Section 8.5.3. There is also the precoat type of filter whichis also commented on in Section 8.5.4.

8.5.2 Aids to efficient filtration

There are differences of opinion on the type of floculent/coagulant whichshould be introduced into the circulating water before it enters the filters.The purpose of these chemicals is to form a ‘floc’ (a gelatinous precipitate)which is retained in the upper layers of the filter and assists the filtrationprocess.

The material in general use is aluminium sulphate which when dissolved inwater forms an acidic solution. As acidic solutions are aggressive to ferrous metalsand to cement-based materials (see Sections 3.5, 3.6 and 3.7), pH correction isusually needed. This pH control can be manual or automatic. For public pools,automatic pH control should be adopted. The pH should be maintained in therange 7.2 to 7.8.

The pH can be measured approximately by indicator papers, or more accuratelyby a pH meter.

8.5.3 Pressure sand filters

The principal type of filter in use for the treatment of swimming pool water in theUK is the pressure sand filter.

Pressure sand filters use graded sand as the filter medium in circular steel

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or glass-reinforced plastic (grp) tanks. These vary in size from small singleunits for private house pools to a battery of large units for public pools. Thereare two types, vertical downward flow, and horizontal. The former areconsidered more efficient. The steel shell requires a high-quality protectivelining, and should comply with the relevant clauses in BS 5500 Steel PressureVessels, Unfired, Fusion Welded. In recent years, steel shells have beenreplaced by glass fibre shells which are cheaper in first cost but appear to havea shorter life.

For all except small pools for private houses, there should be at least two filters.See Section 8.4.1 for design of plant rooms with particular reference to provisionof adequate access for installation and removal.

Filters are normally rated on the basis of m3/m2/hour, and the rate is classifiedas low, medium and high. For club, hotel and private pools, high-rate filters areusually installed, while for public pools and school pools medium-rate filters areusually selected. High-rate filters operate in the range 30–50 m3/m2/hour andmedium-rate filters in the range 20–30 m3/m2/hour.

Pressure sand filters have to be ‘back-washed’; the frequency dependingmainly on the efficiency of the filter in removing suspended and coloidal matterand the bathing load. As the deposit on the filter increases, there is a loss of headthrough the filter which is measured by two pressure gauges on the two mainconnections to the filter, one near the top of the filter and the other near the bottom.In many installations, the back-washing is assisted by the agitation of the filtermedia (sand), either by mechanical rakes or by compressed air.

The amount of water used in back-washing filters can be considerable; an averageflow rate is about 25 m3/m2/hour. For a 2.5 m diameter filter, filter area 4.9 m2,medium flow rate, the back-washing would take about 8 minutes, would use about16 m3 (3590 gal) of water. The discharge to the drainage system would be about2000 litres/minute (440 gal/minute) per filter.

This figure is determined by the filter manufacturer and should be followed.Filters have a viewing window on the outlet and the clarity of the wash watershould be checked before back-washing is stopped.

8.5.4 Precoot filters

In a precoat filter, the filter medium is a very fine powder mixed with water anddeposited on ‘carriers’ known as candles. The only advantage with this type offilter is a considerable saving of space, which may be attractive for small installationsfor private houses, hotels etc. They have not found favour for use in public pools inthe UK nor in Europe.

Precoat filters also require cleaning from time to time as the coat on the candlesbecomes blocked. This cleaning is done by compressed air as directed by themanufacturers of the filter.

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8.6 Chemical dosing of the pool water

The addition of chemicals to the pool water (in addition to those needed to form afloc prior to filtration) is required for the following reasons: 1. to control the pH so that it is maintained in the range 7.2 to 7.8 (slightly

alkaline);2. to maintain the water in proper ‘balance’;3. to disinfect the water to ensure a reasonable standard of bacterial purity.

8.6.1 Control of the pH, alkalinity and a balanced watercontrol of the pH

The chemical characteristics of the incoming fresh water, usually from a publicsupply, may also influence the pH, see Sections 3.7 and 3.8.

The control of the pH is essential for efficient water treatment. The pH is thehydrogen ion concentration; the neutral point is 7.0; values less than 7.0 indicatean acidic solution and values above 7.0 indicate that the solution is alkaline. ThepH scale is logarithmic, so that a water with a pH of 5.0 has 100 times the hydrogenion concentration of a water with a pH of 7.0.

The main disinfecting agent used in swimming pools is chlorine. This may be

Figure 8.11 View of three small and one large dosing pumps for sodium hypochloritesolution for pools in a private leisure centre. Courtesy, Pool Water TreatmentAdvisory Group.

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in the form of chlorine gas, or derived from a salt containing chlorine such ascalcium or sodium hypochlorite or from an organic compound containing chlorine(Figure 8.11). When chlorine gas is dissolved in water, hypochlorous acid andhydrochloric acid are formed. Hypochlorous acid is very effective in destroyingbacteria and the formation of this acid lowers the pH. Therefore, the higher theconcentration of hypochlorous acid, the lower the pH and the more effective is thesolution in killing bacteria. The formation of hydrochloric acid (which is a strongacid) is undesirable as it lowers the pH still further, and it is not a very effectivebactericide.

Acidic solutions attack ferrous metals and cement-based materials andtherefore the pH must be controlled and kept within the range previouslymentioned, 7.2–7.8.

Both sodium and calcium hypochlorite are strongly alkaline, and if the pH israised too high and the water is hard, calcium compounds may be deposited.

Chlorine reacts with ammonia to form compounds known as chloramines. Theseare unstable and in the presence of chlorine break down to produce hydrochloricacid, which as stated above is undesirable as it is aggressive and is not effective indestroying bacteria. If aluminium sulphate (alum) is used as a coagulant (to forma floc) before the water enters the filters, the pH is lowered as alum in solution isacidic. To raise the pH to the required level alkali is added, usually in the form ofsodium carbonate (soda ash). The amount of soda ash added has to be determinedby the pH.

To counteract the effect of the high alkalinity of sodium and calciumhypochlorite, it is often necessary to add an acid salt such as sodium hydrogensulphate (known as ‘dry acid’).

It can be seen from the brief comments above that there are many factors involvedin effective treatment of pool water.

8.6.1.1 Alkalinity

Alkalinity is expressed as mg/litre (ppm) of equivalent calcium carbonate(CaCO

3), and indicates the amount of alkaline compounds in solution in the

pool water. The Pool Water Treatment Advisory Group recommend a generalminimum level of alkalinity at 75 mg/litre which is value required foreffective coagulation. High values can cause difficulty in maintaining the pHin the range of 7.2 to 7.8.

8.6.1.2 Maintaining balance in the pool water

There is need to maintain the pool water in ‘balance’ and the main factors whichdetermine whether or not a water is in ‘balance’ are the total hardness expressed ascalcium carbonate (CaCO

3), the total alkalinity, and the pH. All these are related,

but in a complex way.A balanced water is not corrosive to cement-based materials but will corrode

unprotected ferrous metals. There are two principal tests which can be used to

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determine whether a water is in balance. These are the Langelier Index, and thePalin test.

The Langelier Index has been briefly discussed in Section 3.8. The followingtable indicates a classification of water based on the Langelier Index by theInternational Standards Organisation (ISO). The Palin test is the simpler to apply and is favoured by pool operators. In thistest, three variables are considered: the pH; the total hardness (as calcium carbonate,CaCO

3); the total alkalinity.

The adverse effect of a soft moorland water on the cement-based groutedjoints in a public swimming pool is shown in Figure 7.6. The water was not inbalance and the joints were seriously attacked and large-scale remedial workwas required.

8.7 The disinfection of pool water

The words purification, sterilisation and disinfection are used for the process whichis aimed at the destruction of bacteria in the pool water. In this book, the worddisinfection is used. Purification really refers to the work of the whole treatment,while sterilisation suggests the complete elimination of all bacteria which is certainlynot practical nor necessary. The DoE publication The Treatment and Quality ofSwimming Pool Water states ‘that when coliforms are absent and a satisfactorylevel of free residual chlorine is maintained throughout the pool, the risk of infectionto bathers from the small number of organisms remaining in the pool water isminimal.’

The PWTAG in their treatise Swimming Pool Water and Quality Standardsrecommend that a reasonable bacterial standard for pool water is that the numberof bacterial colonies in 1 ml should not exceed ten and there should be no E. coli in1 millilitre.

It is generally agreed that the disinfecting agent used should remain active inthe pool water after passing through the treatment plant. This stipulation is necessarybecause as soon as the treated water enters the pool fresh contamination occursand this will remain (and increase) as the water is circulated in the pool until itagain passes through the treatment plant which may take several hours. See Turn-over Period in Section 8.2.1.

There are a number of methods for disinfecting swimming pool water and theseare briefly described in the following sections.

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8.8 Chlorination

The most effective disinfectant for pool water is chlorine as it is not only veryeffective in the destruction of bacteria, but it is also a powerful oxidisingagent and can deal effectively with organic matter in solution and insuspension.

With proper dosing, a ‘residual’ remains in the pool water. This residual is notelemental chlorine but consists of compounds containing available chlorine. Thisis usually expressed as ‘free residual’ chlorine.

The smell of chlorine in pool halls is not due to a very low concentration ofchlorine gas but arises from complex chlorine compounds. One such compound,dichloramine, can cause irritation to the eyes and throat of bathers.

Chlorine reacts with ammonia to form chloramines which are to a limited extentbactericidal but are slow reacting and are therefore more stable than free chlorinewhich reacts very rapidly. Ammonia is present in pool water as it is introduced bythe bathers by the decomposition of nitrogenous compounds.

In the UK, chlorine gas compressed in steel cylinders is no longer used for thedisinfection of pool water but is still used in Europe and in the USA, as it is veryefficient and effective. This virtual elimination of the use of gaseous chlorine leadto the extensive use of solution feed using sodium hypochlorite or calciumhypochlorite.

Sodium hypochlorite is normally supplied as a solution, while calciumhypochlorite is supplied as a powder. Both compounds are strongly alkaline,and acidic solutions have to be added to correct the pH and maintain it in therange 7.2 to 7.8. This correction of the pH is achieved by the use of eitherhydochloric acid, sodium hydrogen sulphate, or carbon dioxide.Hydrochloric acid is highly corrosive and if it comes into contact with the‘raw’ sodium or calcium hypochlorite chlorine gas is liberated which can bevery dangerous and special precautions must be taken to ensure that thiscontact does not occur, particularly as this can happen accidentally in astorage area.

A concentrated solution of sodium hypochlorite will attack Portland cementconcrete and it is advisable, if this compound is used, that the concrete floor of thestorage area be protected by a high-quality epoxy-based coating.

Modern dosing equipment makes control easy and safe. This equipmentautomatically controls the chlorine residual in the pool water at a predeterminedlevel, and regulates the pH of the water within acceptable limits.

8.8.1 Break-point chlorination

Chlorine dissolves in water forming hypochlorous acid and hydrochloric acid:

Cl2+H

2O=HOCl+HCl

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The hypochlorous acid reacts with ammonia in a complex reaction and in thepresence of excess chlorine, breaks down to form hydrochloric acid andnitrogen. The point at which the chloramines start to be broken down is calledthe ‘break-point’ and the technique of achieving this is called ‘break-pointchlorination’.

A free chlorine residual of 1.0 ppm should be adequate to maintain satisfactorybactericidal conditions in pool water. This may be increased to a maximum of 1.5ppm when the pool loading is very high.

8.8.2 Chlorinated isocyanurates

Another source of chlorine is compounds in which the chlorine is combined as inchlorinated isocyanurates. These are not used much in public pools but are popularin the private sector. The compounds can be in the form of tablets or as a solution,which are often fed by hand directly into the pool, a procedure which isunsatisfactory and is not recommended.

With both types, cyanuric acid is formed and this lowers the pH and adjustmentis required to obtain the necessary balance.

8.9 Ozone

Ozone (O3) is a very effective bactericide and a powerful oxidising agent. When

correctly used it produces a water with no unpleasant taste nor smell. It effects avery rapid ‘kill’ of bacteria and oxidises organic matter in the water as it passesthrough the plant but there is virtually no residual ozone left in the water when it isreturned to the pool.

When ozonised water enters the pool from the treatment plant, it starts to becomecontaminated by the bathers and as there is no residual, the newly introduced bacteriaand organic matter are not ‘dealt with’ until the water again passes through thetreatment plant.

This disadvantage of ozone can be overcome by the injection of a comparativelylow dose of chlorine; the chlorine is derived from sodium or calcium hypochloriteand is injected immediately before the treated water enters the pool.

An activated carbon filter is sometimes provided to remove excess ozone beforethe treated water enters the pool. With disinfection by ozone, the free residualchlorine can be maintained at a low level of about 0.5 ppm.

In the UK, the number of public pools using ozone as the main disinfectant hasincreased considerably in recent years, but reliable information on the number isnot available.

The disinfection of swimming pool water with ozone is very popular in Europe,e.g. Germany, Switzerland and France, and the USA. In Germany, it is mandatoryto provide for the injection of a small amount of chlorine to ensure a free residualchlorine in the water in the pool. In Switzerland, this is not generally considered

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entirely necessary provided the pool is properly operated, including generous useof make-up water. See also Section 8.2.5.

Ozone is a poisonous gas and therefore safety warning signals should be installedto operate when the concentration of ozone in the plant room exceeds apredetermined level. This safety system should include an automatic plant shut-down device.

8.10 Bromine

Bromine (Br2) is in the same group of elements as chlorine (the halogens) which it

closely resembles in chemical properties. While chlorine is a gas at normaltemperature and pressure, bromine is a liquid which freezes at -7.3 °C and itsboiling point is 58.8°C. It is a red liquid with a pungent smell and is very soluble inwater.

Bromine is claimed to be popular for use in small swimming pools for privatehouses, clubs and hotels, but is not used in public pools in the UK. It is a strongoxyidising agent and powerful germicide. In solution in water, it reacts withammonia to form bromamines (in a similar way to chlorine-formingchloramines).

The concentration of bromine residual is recommended by the PWTAG to be inthe range 4.0 to 6.0 mg/litre using DPD tests. It is claimed that the use of brominedoes not cause any irritation to the eyes, nose or throat and does not give rise toobjectionable odours. However, there is some reason to suspect that it can causeirritation to the skin of some bathers.

It can be dispensed into swimming pools by means of tablets introduced intothe pool water by a brominator.

8.11 Chlorine dioxide

Chlorine dioxide (ClO2) is a heavy yellow gas which in its pure form is unstable

and explodes violently on heating. It can now be prepared in patented stablesolutions. Two stable forms of chlorine dioxide are Ultrazon and Dichlor. It is astrong oxidising agent and is claimed to have powerful germicidal properties.

When used on its own, there is a tendency for the pool water to become rathercloudy and develop a yellowish-green colour. To overcome this, it is usual practiceto dose with chlorine as often as necessary to maintain the necessary clarity andgood appearance of the water. It is used to a limited extent in hotel and privatepools in Europe.

8.12 Metallic ions (silver and copper)

Probably the first reference to the use of metallic ions for the sterilisation/disinfectionof water was work by Dr. Krause of Munich in 1929. This method became knownas the ‘Catadyn Silver Process’. An account of experimental work on this process

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is contained in a paper by E.V.Suckling in the Proceedings of the British WaterWorks Association in 1932. It was used to a very limited extent in Europe forpurifying small quantities of water.

Ions are derived from atoms, but unlike atoms they possess electrical charges.Ions derived from hydrogen and metals have a positive charge, while ions fromnon metals and acid radicals have a negative charge.

In the early 1960s, it was used for treating water in the swimming pool ofa large hotel in Flims Waldhaus, Switzerland. At this time it was known as the‘Vellos Casanovas’ process and a brief description of the installation is givenbelow.

Water is drawn from the pool and passes through a strainer and then through aseries of copper plates. A pulsating electric current passes through the plates anddue to the difference in potential between the plates metallic ions are liberated intothe circulating water. The suspended and colloidal matter in the water are attractedto the liberated ions and form what the patentees term a micro-floc which is muchfiner than the floc formed by coagulants (see Section 8.5.2).

The micro-floc penetrates into the filter medium and this is claimed to increaseits efficiency so that the rate of flow is about double that through a high pressuresand filter.

After filtration, the water is passed through a battery of silver plates similar tothe copper plates used for the micro-floc formation. The electric current liberatessilver ions and these have a strong sterilising/disinfecting effect. It is also claimedthat the silver ions remain in the water as it is returned to the pool and thus have aneffect similar to that of free residual chlorine. There is no simple test for detectingcopper and silver ions in pool water.

It is important that the pH of the water is controlled within fairly narrow limitsand for small pools for private houses, hotels and clubs this can present practicaldifficulties in overall control. There is very little information on the use of thissystem of swimming pool water treatment in the UK.

A considerable amount of work has been carried out in Switzerland on thebactericidal effect of silver ions in water. Bulletins issued by the Federal Institutefor Water Supplies in Zurich showed that silver ions do have a significant destructiveeffect on E. coli in water.

8.13 Ultra-violet radiation

Ultra-violet (UV) radiation for the sterilisation of small quantities of water hasbeen known since the early part of the 20th century.

An essential feature for the disinfection of water is that the UV radiation mustsecure maximum penetration of the water being treated. In addition, there is anoptimum wave-length band for effecting maximum kill of the bacteria and viruses.This optimum wave-length band is claimed by the suppliers of the UV equipmentto be 2500–2800 angstroms (250–280 nm).

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The need for maximum penetration of the water means that suspended andcolloidal matter must be at a minimum and the total dissolved solids (tds) mustalso be low, particularly iron salts and nitrates. This requires an effective check onthe chemical characteristics of the water supply to the treatment plant and thefilters must operate at maximum efficiency. The pH of the water should be in therange 7.2 to 7.8.

The advantages claimed for this method are:

Over-dosage is impossible;No chemicals are added to the water;When correctly designed the plant obtains a very high percentage of ‘kill’(over 90%).

A serious disadvantage for its use in the treatment of pool water is that there is noresidual in the water after the water has passed the treatment point. Also, compliancewith the tight control procedures can, in practice, prove difficult.

Nevertheless, it can be an attractive method of water disinfection for small poolsfor private houses, clubs and hotels.

The UV radiation is produced by low, medium and high pressure mercury vapourdischarge lamps; up to about 50 m3/hour (11 000 gal/hour) can be treated.

8.14 The base-exchange process for softeningpool water

The purpose of softening water is to reduce the hardness and this has manyadvantages for swimming pool water, especially if it is heated.

Hardness is due mainly to the presence in solution of bicarbonates and sulphatesof calcium and magnesium. Boiling will reduce the bicarbonate hardness but not thatdue to sulphates. The bicarbonate hardness is known as temporary hardness and thesulphate hardness as permanent hardness. Most domestic and small industrial watersofteners operate on the base-exchange (or ion-exchange) process which removesboth bicarbonate and sulphate hardness. In this process, the active material is a naturalor artificial zeolite, a sulphonated carbonaceous material, or a synthetic resin whichhas ion-exchange properties. Water flows through a bed of the active material and thecalcium and magnesium ions combine with the zeolite as shown:

Calcium bicarbonate+sodium zeolite=calcium zeolite+sodium bicarbonate

Magnesium sulphate+sodium zeolite=magnesium zeolite

+sodium sulphate After a time all the sodium zeolite is used and the softener needs to be regeneratedby the addition of common salt (sodium chloride) as follows:

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Sodium chloride+calcium zeolite=sodium zeolite+calcium chloride

Sodium chloride+magnesium zeolite=sodium zeolite+magnesium chloride

The calcium and magnesium chlorides are in solution in the wash water whichgoes to waste.

There is no change in the total dissolved solids (tds) in the water. Water ofalmost zero hardness can be obtained which is not desirable for most purposesincluding water for swimming pools. To prevent this happening, the softened watershould be blended with a percentage of the ‘raw’ water to give the required degreeof hardness.

The process is expensive and it is not used when large volumes of softenedwater are required.

For use in swimming pools the Langelier Index should be positive and the pHin the range 7.2 to 7.8.

8.15 Sulphates in swimming pool water

A further matter to be considered is the possible build-up of sulphates in the poolwater arising from the use of aluminium sulphate and sodium hydrogen sulphate,for reasons previously given.

Sulphates in solution are aggressive to Portland cement and therefore tile joints,rendering and screeds are vulnerable to attack. British Standard BS 5385 Part 4Code of Practice for Ceramic Tiling and Mosaics in Specific Conditions, clause13.1 states: ‘Ideally, the sulphate concentration (expressed as SO

3) in the water of

swimming pools should not exceed 300 ppm. Where this level cannot be achieved,consideration should be given to the use of impermeable adhesives and groutingmaterials that are not affected by sulphates.’

It is recommended that when compounds containing a sulphate radical are usedin the treatment process, regular testing for the concentration of the sulphate ionsshould be part of the control tests.

Recommendations for mitigating or preventing sulphate attack by the use ofappropriate materials in the finishes of the pool shell are given in Chapter 7.

Further reading

Amateur Swimming Association. Acceptability of Swimming Pool Disinfection by DifferentMethods, 1984.

Department of the Environment. Treatment and Quality of Swimming Pool Water, HMSO,London, 1984.

Elphick, A. Treatment of Swimming Pool Water with Sodium Hypochlorite, Wallace & Tiernan,Tonbridge, 1978.

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Institute Of Baths and Recreation Management. Practical Leisure Centre Management, Vol.2.

Langelier, W.H. The analytical control of anti-corrosion water treatment, Journal AWWA,28(1), October 1936, pp. 1500–21.

Pool Water Treatment Advisory Group. Swimming Pool Water Treatment and QualityStandards, 1999.

Wuhrman, K. and Zobrist, F. Investigations into the bactericidal action of silver in water,Information Bulletin No. 142, Federal Institute of Water Supplies, Zurich, 1958(translation).

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Chapter 9

Notes on heating swimmingpools and energy conservation

It is usual practice to provide heating for indoor swimming pools, both for the poolwater and for the pool hall, changing rooms etc. On the other hand, the heating ofthe water in open-air pools is rather less common in the UK.

In the UK and countries with a similar climate, an open-air pool can only beused in reasonable comfort for about 4–5 months during the year, and during thisperiod there are many days when only the most determined swimmers will bewilling to use the pool unless the water is heated, and wind protection provided.

The term pool heating means a properly designed and installed heating systemconnected to the water circulation system of the pool.

9.1 Heating open-air swimming pools

By far the greatest loss of heat is from the surface of the water, with only acomparatively small percentage through the walls and floor to the surroundingground, unless the ground water level is high. See Section 4.15.

The heat loss from the water surface depends on a large number of factors all ofwhich, except one, are closely associated with weather conditions. The exceptionalfactor is whether the pool has a thermal insulating cover for use at night and othertimes when the pool is not in use. Weather conditions include ambient airtemperature, wind velocity, and direction, hours of sunshine, all of which changeduring the day and from day to day. A formula which seeks to take into account allrelevant factors may well turn out to be more inaccurate than a simplified versionand experience.

The simplified calculation which follows assumes that the pool is covered atnight with a proper cover and thus the fall in temperature during the time when theheating is turned off is 3 °C. The calculation is intended as an illustration, and theselection of a suitable type of boiler should always be left to experienced firms.

If the pool is 16.67 m long, 8.0 m wide with a minimum depth of 0.90 m and amaximum depth of 1.50 m, the water surface will be 133 m2 and the volume ofwater about 160 m3. When the boiler is switched on in the morning, it will berequired to raise the temperature of the 160 m3 of water 3 °C in, say, 3 hours, i.e. 1°C per hour. Boiler capacity, assuming 80% efficiency, is:

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(160×l000×l.00)×4.18÷0.80=836 000 kJ=836 000÷3600=232 kWh (1calorie=4.18 J).

To this figure of 232 kWh should be added a percentage to cover heat loss duringthe warming-up period of, say, 5%, thus making an estimated boiler capacity ofsay 245 kWh (or 924 000 Btu/hour).

The boiler would be gas or oil fired.

9.2 Heating the water in indoor swimming pools

The temperature of the water in indoor swimming pools is generally higher than inopen-air pools. In private house, club and hotel pools, the temperature is often 30°C, while in public pools in the UK it is 26–28 °C; in hydrotherapy pools, thewater is usually maintained at about 32 °C. In Europe, in public pools, a watertemperature of 28 °C is considered a minimum.

9.3 Heating and ventilation of pool halls andadjoining areas

9.3.1 General considerations

For comfort, the air temperature in the pool hall and changing rooms should be atleast 1 °C above the water temperature, assuming this is not less than 26°C.

Mechanical ventilation is considered essential in indoor public swimming poolsas it helps to control condensation and adds to the comfort of the pool users. Seecomments about roof construction in Chapter 7.

The heating of the water and the heating and ventilation of the pool hall andadjacent rooms are all part of the same problem which has to be resolved byexperienced firms of consulting engineers, or by experienced and reliablecontractors on a package deal basis.

In Europe, it is quite usual to find that benches around the pool are heated andunderfloor heating is provided to the walkways, and floors of changing rooms.

The details of heating and ventilating systems vary from one building to anotherand to the requirements of the client who is naturally concerned with both thecapital cost and the operating costs.

In spite of the wide differences in design approach and client requirements, it isgenerally agreed that the following principles apply: 1. Condensation should be reduced to the maximum practical extent.2. Air pressure in the pool hall should be slightly lower than in adjoining areas

so as to induce a flow of air towards the pool hall. This will help reduce, butwill not eliminate the diffusion of ‘chlorine smell’ to other parts of the buildingwhen chlorine is used as the main disinfectant in the pool water. The ‘smell of

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chlorine’ is not caused by the presence of elemental chlorine, but by chlorinecompounds, such as nitrogen trichloride, and dichloramine.

3. When chlorine is used as the main disinfecting agent in the pool water, the airshould not be recirculated, but should be discharged, preferably in total, to theexternal air.

4. The air changes per hour (ventilation rate) will normally vary in differentparts of the building. For the pool hall, the ventilation rate will be closelyrelated to the area of the pool and the area of surrounding walkways as it isfrom these areas that evaporation takes place.

Heat is a form of energy and exists in a body in the form of motion of the molecules.Heat can be transferred from one body to another by conduction, when the bodiesare in direct contact, by convection through a liquid and by radiation by whichheat can be transferred through a vacuum.

There are two forms of heat, the latent heat of the fusion of ice and the latentheat of evaporation. The unit of heat is the amount of heat required to raise 1 g ofwater 1 °C and is known as a calorie, and this is equivalent to 4.18 J.

During the change of state (ice to water and water to steam), the temperatureremains constant. The latent heat of the fusion of ice is about 80 calories (360J) and the latent heat of evaporation of water is about 540 calories or 2260 J(2.26 kJ).

It can be seen that the amount of heat energy required to convert water to vapour/steam is very high.

All reasonable steps should be taken to reduce heat loss and thus reduceenergy consumption. The first principle is to ensure that the floor, walls androof have appropriate low U values. The Building Regulations 1985 ApprovedDocument L Conservation of Fuel and Power requires that the U value ofexposed walls, exposed floors and ground floors for industrial buildings shouldnot exceed 0.45 (W/m2K). For semi-exposed walls and floors, the U valueshould not exceed 0.6 (W/m2K).

As far as heating and ventilation is concerned, there are many systems availableto conserve energy. There is an excellent and comprehensive publication from theEnergy Efficiency Office entitled Energy Efficiency Technologies for SwimmingPools (details are given under Further Reading at the end of this chapter). It isclaimed in this publication that, in a typical indoor public swimming pool, theannual cost of energy consumed can be reduced by a significant figure by theadoption of well-tried techniques.

The main factor which controls the use of energy in maintaining satisfactoryconditions in an indoor swimming pool is the evaporation of water from the poolsurface. The energy used operates on two distinct levels, namely the heat used upin the evaporation process, and the energy used by the mechanical ventilation systemwhich is needed to reduce the relative humidity to an acceptable level, say, 60–70%. It has been established that the energy used at these two levels is over 60% ofthe total energy used for the whole building and its operation. There are a number

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of methods which will make a material contribution to the conservation of energyand these include the following: 1. The provision of a thermal insulating cover to the pool for use when the pool

is not in use, e.g. at night;2. The reduction of the mechanical ventilation (rate of air change) when the pool

is not in use and the pool hall not occupied. This can effect a saving of 10–12% in the energy consumed, with of course, a corresponding reduction inoperating cost. However, if the pool hall has a pressurised roof void, the closingdown of the ventilation system can cause problems (see Sections 7.14–7.16);

3. Accurate and effective control of temperature and humidity;4. The use of heat recovery and/or heat reclaim techniques.

9.3.2 Heat conservation techniques

Briefly, heat recovery uses heat exchangers, and heat reclaim uses heat pumps.Heat exchangers collect waste heat for reuse, while heat pumps reclaim andregenerate heat from lower energy sources. The installation of an efficient systemof energy conservation is said to reduce energy consumption for pool hall heatingby up to about 30%.

Heat pumps are ideal for heat energy conservation. A heat pump operates toextract heat from a low temperature heat source and up-grade it to a highertemperature. For example, a heat pump can be used to extract heat from a largevolume of relatively cool water and use this heat to raise the temperature of acomparatively small volume of water.

A heat pump is similar in principle to a refrigerator, but working in thereverse; it requires an external source of power, electricity or gas, to drive thecompressor.

A ‘simple’ heat exchanger will extract heat from warm air which is beingdischarged to waste, and transfer this heat to fresh incoming air, without externalenergy input, and the same principle applies to out-going and incoming water.More complex heat exchangers do the same thing but with an external energysource in addition.

9.4 Solar heating of swimming pools

The sun provides heat energy free of charge, the only cost being that required toput this energy to practical use.

It appears that the large-scale use of solar energy to heat water for domestic usewas probably started in Israel in the 1950s. As far as the UK is concerned, it wasnot until the oil crisis of the early 1970s that serious attention was given to thepossible use of solar heating for open-air swimming pools.

In 1986, the British Standards Institution published a Code of Practice for the

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Solar Heating of Swimming Pools. The Code makes recommendations forcomponents, design and installation of equipment, performance and commissioning.In addition, a great deal of useful information is included.

Contrary to general opinion, properly designed and installed solar panels cancollect a significant amount of heat energy on overcast days. The temperature ofthe water in an average unheated open-air pool in the UK during the four summermonths (mid-May to mid-September) is likely to be about 18 °C. With properlydesigned and installed solar heating, this could average about 23 °C. This isundoubtedly very useful from the point of conservation of energy (fuel) and money,but for those people who like warmer water (the 23 °C is an average figure), it isnecessary to install a conventional heating installation in addition to the solar heating.The boiler can have a smaller output and the operating costs would show aconsiderable saving compared with an installation without solar heating. Theconventional system should be considered as a back-up to the solar heating. Thetwo systems should be controlled thermostatically to obtain the best results.

The solar collectors are in the form of panels made from a patented form ofpolypropylene which has a black matt surface. To secure the best results, they haveto be correctly sited and orientated; they are connected to the water circulationsystem of the pool.

Further reading

Acoustics & Environmetrics Ltd. Some Ways of Saving Energy—the Nature of Heat andCold Energy, 1988.

British Standards Institution. Code of Practice for the Solar Heating of Swimming Pools,BS6785, 1986.

Department of the Environment. The Building Regulations 1985, Approved Document L,Conservation of Fuel and Power, 1989.

Energy Efficiency Office and Sports Council. Energy Efficiency Technologies for SwimmingPools, January 1985.

Sports Council. Energy Data Sheets 1–21.Towler, P.A. Protection of buildings from hazardous gases, Journal of the Institute of Water

and Environmental Management, 1993, No. 7, June, pp. 283–94.

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Chapter 10

Maintenance and repairs toswimming pools

MAINTENANCE OF SWIMMING POOLS

10.1 General considerations

The term swimming pool in this context includes paving and walling describedunder External Works in Chapter 6.

The recommendations relating to maintainance are intended mainly for the privatepool owner and the owners of swimming pools where full-time technical personnelare not available for daily supervision and maintenance of the pool and equipment.

The following factors are relevant to the operation of any of the types ofswimming pool referred to in this book: 1. whether the pool is open-air or inside a building;2. the length of time the pool is in use each day;3. the details of the water circulation system and the treatment plant with special

reference to the chemicals used;4. the type and quality of finish to the inside of the pool.

10.2 Routine supervision: smaller pools

There is no such thing as a fully automatic pool system which operates withoutregular attention, and the following routine is recommended: 1. A visit should be paid every day to the plant room to check the equipment

including the screen and circulating pump(s).2. The pool water should be checked for pH at least twice a day (morning and

evening). When the disinfectant is chlorine, metallic ions, or UV radiation,the pH should be in the range 7.2 to 7.8. If bromine is used as the disinfectant,the PWTAG recommend the pH should be in the range 7.8 to 8.2. The PWTAGis the Pool Water Treatment Advisory Group.

The maintenance of the correct pH value is fundamental to the efficientoperation of the treatment process. See Chapter 8, particularly Section 8.6.1.

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3. If chlorine is the main disinfectant, the water should be checked for ‘free’ chlorineby the use of a standard test kit which is normally provided by the suppliers ofthe equipment. The ‘free’ chlorine should be in the range 1.00 to 1.5 mg/litre.

4. If bromine is the main disinfectant, the PWTAG recommend that total bromineshould be maintained in the range 1.5 to 3.5 mg/litre.

5. The pool should be cleaned regularly; leaves and ferrous objects can causesevere staining. With a marbelite finish, such stains are very difficult toremove.

6. A watch should be kept for algal growths at and near the water line and onadjacent paving. Information on the removal of algal growths is given in Section10.4.

7. The checking and servicing of all equipment should be carried out asrecommended by the suppliers. The frequent blowing of a fuse indicates thatsomething is wrong and this should be attended to.

8. If pressure sand filters are installed, back-washing should be carried out asrequired to maintain a high standard of clarity in the pool water; see Section8.5. Detailed directions are usually provided by the suppliers.

9. Chemicals for water treatment and such items as fuse wire, and cleaning materialsshould not be allowed to go out of stock.

10. Thermal insulating covers should be installed wherever practical on all open-air pools. Covers are also very useful for indoor pools as the major heat loss isfrom the water surface. The installation and regular use of such coverssubstantially reduces evaporation and for indoor pools reduces humidity inthe pool hall.

10.3 Shut-down periods

While indoor public pools are normally open all the year round, they are usuallyshut down for general inspection etc. every 18 months to two years.

It may be convenient to close other pools for short or long periods. For shortperiods, e.g. for a few weeks, the following procedure would be satisfactory: 1. The pool should be thoroughly cleaned and given a strong dose of the

disinfectant used, and the filter(s) should be back-washed.2. All containers holding chemicals should be properly closed.3. All switches should be closed and fuses removed.4. Proper ventilation of the plant room should be ensured.5. The pool should be covered (assuming a cover is provided). For long periods, including winterisation, rather more precautions should be taken.The details depend on whether the pool is open-air or indoors, and location andconstruction of the plant room.

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10.3.1 Winterisation: open-air pools

Two important parts of winterisation are the closing-down and re-opening. 1. It is not advisable to leave the pool empty during the winter. The pool should

be given a heavy dose of algicide and then, after a period depending on thealgicide used, the pool should be thoroughly cleaned, emptied, and refilledand given a strong dose of the disinfectant used. The water level should be justbelow the outlets. The provision of an efficient thermal insulating cover wouldensure that thick ice does not form unless the winters are generally severe. Ifthick ice is anticipated, a ‘buffer’ of thick timber should be provided aroundthe perimeter to reduce the pressure of the ice on the walls.The emptying and refilling should be carried out carefully in accordance withthe recommendations given in Appendix 2.

2. An alternative to the above suggestions is to carry out the general cleaning,give a generous dose of disinfectant, and leave the whole installation ‘tickingover’ during the winter, with the heater operating on a thermostat set to ensurethe water temperature does fall below about 7 °C.

3. With a complete shut-down, the filter should be drained (after back-washing),the heater drained, and the disinfecting equipment dealt with as directed bythe suppliers. Unless the plant room is well ventilated and comparatively warm,it would be advisable to remove the pump motor, heater and disinfectingequipment to a warm dry store. All switches should be left in the closed positionand all fuses removed.

4. All movable pool equipment should be cleaned and carefully stored. If thereis a diving board it would be advisable to remove it. All metal fixtures andfittings should be cleaned and well greased.

10.3.2 Putting a pool back into operation

The pool should be emptied and thoroughly cleaned. The walls and floor shouldbe carefully inspected for damage and all defects made good.

Refilling should be done slowly and the temperature raised slowly, asrecommended in Appendix 2.

All plant and equipment removed for the winter should be inspected, cleanedand refitted.

The whole installation should be given a trial run well in advance of the ‘openingparty’.

10.4 Algal growths: prevention and removal

Algal growths have a habit of suddenly appearing on the upper part of the walls ofthe pool and on adjoining paving despite reasonable efforts to operate the poolsatisfactorily. Their presence in the pool does indicate some short-coming in the

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way the pool is operated, generally due to low chlorine residuals. If the ‘free’chlorine is maintained in the range 1.0 to 1.25 mg/litre, there is little chance ofalgae establishing themselves in the pool.

On paving, such growths cause the surface to be very slippery especially tobare feet. However, once found they should be removed by the use of analgaelcide of which there are a large number of proprietary compounds on themarket. An efficient and reasonably inexpensive method is to use a solution ofcopper sulphate crystals (the chemical formula is CuSO

4.5H

2O). It is the

copper which is the active part of the compound. The dosage depends largelyon the hardness and temperature of the pool water. For a warm soft water,about 0.30 mg/litre (0.30 ppm) of copper is required; this is equivalent to 1.2mg/litre of copper sulphate crystal. For a cool hard water, about 2.5 mg/litremay be needed.

The concentrations suggested for copper sulphate can be calculated as follows:One litre of water weighs 1000 g; 1 mg is 1/1000 of a gramme. One m3 of waterweighs 1000 kg. Therefore, 1 m3 of the cool hard water would require 2.5 g ofcopper sulphate.

Bleaching powder (calcium hypochlorite) solution is also effective and can beused in a strong solution on paving.

10.5 Foot infections

Various infections, mainly of the skin, can be picked up in swimming pools.The skin infections are usually on the feet, e.g. verrucae, and athlete’s foot.

These arise from contact with floor surfaces. Thorough and regular cleaning withmild disinfectant will help reduce the risk of infection but is unlikely to prevent itentirely.

It is very important that cleaning materials should be non-aggressive to thefinishes to the floors of walkways and changing rooms. Advice should be soughtfrom the suppliers of the finishes (tiles, grouted joints etc.).

The risk of infection from pathogenic bacteria is very small indeed in a properlyrun swimming pool, but the price of immunity from such infection is constantvigilance over all parts of the water treatment process.

REPAIRS TO EXTERNAL WORKS: PAVING

Information and recommendations for various types of paving in general use havebeen given in Chapter 6 and if these are followed the amount of repairs requiredshould be small. However, circumstances arise when repairs are necessary. Thedetails of the repair will depend largely on the area involved and the cause of thedefects.

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10.6 Remedial work to insitu concrete paving forpedestrians

If the concrete is badly cracked due to settlement/subsidence, the practical solutionis to remove and relay it in accordance with good practice.

Small areas damaged by frost attack (spalling of the surface) or wear can berepaired as follows: 1. All loose particles should be removed by the light use of a bolster and wire

brush down to sound concrete. The damaged area should be cut out as squareas practical (Figure 10.1).

2. A coat of cement/styrene butadiene (SBR) emulsion, 25–30 litres of SBRto 50kg cement should be well brushed into the cleaned area and thisshould be followed within 30 minutes with a cement/sand/SBR mortar;mix proportions of 1 part OPC, 3 parts clean concreting sand and 10 litresof SBR emulsion to 50 kg cement. The mix can be quite stiff and a w/cratio of 0.40 should be adequate as the SBR acts as a workability aid.

The repaired areas should be cured for at least three days by covering withpolythene sheets held down by concrete blocks or similar.

3. The surface of the concrete around the cut-out patches should be cleaned andwire brushed for a distance of at least 75 mm. As long as possible after thecompletion of the patch repairs, a coat of cement/SBR grout should be brushedinto the surface. The brush coat of grout should also be cured as above.

Figure 10.1 Sketch showing patch repair to insitu concrete paving.

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10.7 Remedial work to insitu concrete paving forlight commercial vehicles

Small isolated defective areas can be repaired by a similar method to that describedabove, but it may be advisable to use small aggregate concrete instead of a mortarif the depth of the cut-out areas exceed 50 mm.

With badly cracked areas, a practical solution is to remove and relay in insituconcrete, or change to precast concrete flags, precast concrete paving blocks, orclay pavers. Whichever is selected reference should be made to the relevantparagraphs in Chapter 6.

If a careful inspection and diagnosis indicated that the existing concrete canbe retained then the provision of a new surface of asphalt could be a practicalsolution. The following comments indicate the main points which requireattention.

The existing concrete should be cleaned and all potholes and defective areasrepaired. A bitumen emulsion tack coat should be applied by spray and must beallowed to ‘break’ (change from brown to black) before the wearing course is laid.See BS 434 Bitumen Road Emulsions. A base course of dense macadam to BS4987 would be suitable, followed by a wearing course of cloe-graded densemacadam to BS 4987.

More detailed information is contained in Information Sheet 3 Resurfacing ofRoads and Other Paved Areas Using Asphalt, issued by the Quarry ProductsAssociation.

10.8 Remedial work to precast concrete flags

Precast concrete flags are normally laid for pedestrian use and defects usuallyconsist of cracking of individual flags and ‘steps’ or ‘lipping’ between adjacentflags caused by uneven settlement of the sub-base.

Repair is relatively simple; cracked flags should be replaced and uneven flagsshould be removed and the bedding/sub-base adjusted and the flags relaid so thatthe difference in level between adjacent flags does not exceed 3 mm (see BS 7263Precast Concrete Flags Part 2 Code of Practice for Laying).

10.9 Remedial work to precast concrete pavingblocks

Precast concrete blocks complying with BS 6717 Precast Concrete Paving Blocksnormally only need adjusting due to settlement of the foundation on which theywere laid. Part 3 of BS 6717 makes recommendations for laying and these shouldbe followed when carrying out remedial work. Reference can also be made to therelevant sections in Chapter 6.

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10.10 Remedial work to clay pavers

Clay pavers, complying with BS 6677 Part 1, should only need adjusting in levelarising from settlement of the foundation on which they were laid. The relayingshould be carried out in accordance with BS 6677 Part 3, care being taken toremedy any faults in the sub-base and sub-grade. Reference can also be made tothe relevant sections in Chapter 6.

10.11 Remedial work to slippery paving

Paving around and giving access to the pool should have a non-slip (slip-resistant)surface. Persons walking with bare wet feet are more likely to slip than those withdry bare feet.

A considerable amount of work has been put into the problem of slipping atwork and means to assess a ‘safe’ coefficient of friction between various types offoot wear and different floor surfacing materials. At the time of writing, there is noauthoritative recommendation for friction coefficients between wet bare feet andfloor surfaces. The author’s experience is that the best surface is that provided byhigh-quality slip-resistant ceramic tiles. Riven natural stone paving is alsoreasonably slip-resistant.

Concrete, precast slabs and insitu, can become slippery with use and then stepsshould be taken to deal with it. The methods in general use are: 1. acid etching;2. slight roughening of the surface by mechanical scabbling, or grit blasting,

cutting of shallow grooves;3. provision of resin-based coatings. These methods are not suitable for paving used by bare feet.

10.11.1 Acid etching

The results can be quite satisfactory, but care is needed. It is inexpensive andrelatively easy to carry out.

Dilute hydrochloric acid (HCl) is used, one part of commercial acid to ten partswater. Rubber gloves and an eye shield must be worn. The diluted acid is appliedand brushed in and allowed to remain in contact with the concrete for about 10minutes. The area treated is then well washed down. The acid attacks the cementpaste (and the aggregate if this is calcareous). The treatment should be repeateduntil a satisfactory depth of exposure of the coarse aggregate is obtained; 1–1.5mm should be adequate. Thorough washing down after each application of theacid is essential.

Acid etching should not be used on marble nor on terrazzo. The latter is amixture of white cement and marble chippings and the acid will attack both cementand marble.

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10.11.2 Scabbling

Owing to the effect of impact and vibration caused by the hard steel heads, thismethod should only be used on good-quality concrete of adequate thickness. Therecommended minimum thicknesses for precast paving flags is 50 mm and forinsitu concrete 100 mm.

10.11.3 Grit blasting and high-pressure water jets

If the area to be treated is a large one, grit blasting and high-pressure water jets canbe a practical solution. Noise, vibration and dust is eliminated by the use of high-velocity water jets.

10.11.4 Grooving

This is an effective method for improving slip resistance of concrete paving. Figure10.2 shows an area treated in this way.

10.11.5 Slip-resistant resin-based coatings

These coatings are usually based on two pack epoxies or two-packpolyurethanes, with appropriate primers. The surface of the concrete has to beprepared first by light scabbling or grit blasting, and then all grit and dustmust be removed. The primer and resin must be applied in accordance withthe directions of the supplier. The resins can be pigmented.

When the final coat of resin is still tacky, a fine, hard grit is sprinkled onthe surface to provide the desired slip resistance. The whole operation isexpensive.

10.12 Preventing trips and falls

Uneven paved surfaces can result in trips and falls to persons walking on the paving.While this can occur to paving around an indoor pool it is much more likely tohappen with external paving around open-air pools, due mainly to subsidence, butcan be caused by uplift of the pool shell.

Unevenness in paving flags can be readily corrected by lifting the flags andadjusting the bedding so that the difference in level between adjacent flagsdoes not exceed 3 mm (see BS 7263 Code of Practice for Precast ConcreteFlags). The recommendation for floor tiles is 1 mm where the joint does notexceed 6 mm wide and 2 mm when the joint exceeds 6 mm wide. If tilingbecomes sufficiently uneven to cause tripping, it is a much bigger job tocorrect, as fairly large areas of screed would have to be replaced. Repairs toscreeds is discussed in Section 10.26.

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REPAIRS TO EXTERNAL WORKS: WALLING

10.13 Remedial work to free-standing walls

On the assumption that the wall has been built in accordance with good practice(see Section 6.4.2), the only repair likely to be necessary is repointing at intervalsdepending on the degree of exposure.

However, a very severe gale or a tornado (UK type) may cause part of the wallto become significantly out of plumb or even partly demolished. While completerebuilding may at first sight seem the only remedy, a careful assessment by anexperienced architect or engineer may conclude that part rebuilding is a reasonablesolution. A limited degree of out-of-plumbness can often be accepted provided aperiodic check is made to determine whether further movement has taken place.

10.14 Remedial work to earth-retaining walls

When correctly designed and constructed, these walls should be virtuallymaintenance free over many years. They sometimes develop a bow due to groundand ground water pressure, and the growth of the roots of large trees.

Figure 10.2 Concrete paving grooved to improve slip resistance.

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When this occurs, the extent of the displacement, both vertically andhorizontally, should be recorded at intervals to establish whether themovement is continuous, intermittent, or has become static. Trees on the earthside of the retaining wall need special attention, preferably by an experiencedperson. The height of the wall is an important factor in deciding what action isneeded.

REMEDIAL WORK TO POOLS UNDERCONSTRUCTION

Repairs may be needed to pools which are under construction, for example, failureto pass a leakage test; more extensive repairs may be required to existing poolsafter many years in service. There is a discussion on the former in Sections 10.15–10.19, and the latter in Sections 10.20–10.26.

10.15 General comments

Remedial work to the pool shell may be required as a result of failure to pass aleakage test on a new pool before any finishes are applied. This repair work wouldbe simpler to carry out than repair work to a pool shell to which finishes have beenapplied. It is recommended that provided it is practical to do so, back-filling aroundthe walls should not be carried out until the leakage test has been satisfactorilycompleted.

If the shell is constructed in sprayed concrete it may not be possible to do thisfor the reasons given in Chapter 5.

If the leakage test is satisfactory, it is reasonable to assume that there will be noinfiltration when the pool is completed and put into operation.

When defects are found in the shell of a new reinforced concrete pool they arelikely to be of the following types: 1. thermal contraction cracks in the walls;2. shrinkage cracks in the floor;3. areas of honeycombed concrete;4. deficiency of concrete cover to the rebars, detected by a cover meter survey.

10.16 Remedial work to thermal contractioncracks

Thermal contraction cracks penetrate right through the wall but are very narrow,seldom exceeding 1 mm. The amount of leakage through this type of crack isusually small, so that while it can be seen, it cannot be measured. Sometimes thesecracks are self-sealing (known as autogenous healing), but repair is recommended.

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The suggested method of repair is shown in the sketch at Figure 10.3. Thesurface of the crack should be opened up by light tapping with a hammer andbolster, and the surface of the concrete for a distance of 300 mm on both sides ofthe crack should be wire brushed. All grit and dust should be removed and anepoxy primer well brushed in, and this should be followed by two coats of epoxyresin. For cracks exceeding 1.00 mm wide, the use of crack injection may be required(Figure 10.4).

10.17 Remedial work to drying shrinkage cracks

These cracks are usually confined to floor slabs rather than walls. Remedialwork to drying shrinkage cracks will depend on whether the cracks are due toplastic shrinkage (as shown in Figure 4.1), or whether they are wider but lessfrequent. The former are very narrow and usually penetrate only a fewmillimetres. A suitable method of repair is to wire brush the surface of the areaover which cracking has occurred, remove all dust and grit and well brush in acement/SBR grout with a mix of about 25 litres of SBR to 50 kg cement. Therepaired area should be cured by covering with polythene sheeting held downaround the perimeter, for four days.

Wider cracks usually penetrate down to the reinforcement and in extreme cases,right through the slab. The shallower cracks can be repaired as described for thermal

Figure 10.3 Sketch showing repair of fine thermal crack in wall of pool.

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contract cracks in walls. The deeper cracks may require crack injection by aspecialist firm. See Section 10.23.3.

10.18 Remedial work to honeycombed concrete

Honeycombing of concrete is usually due to lack of care in compaction of theconcrete, or errors in mix design which are sometimes aggravated by loss of waterand fines at defective joints in the formwork.

The repair consists in the removal of weak, honeycombed concrete, andremoval of all grit and dust. For small isolated area(s), this can be done bypercussion tools but for larger and/or deeper areas of honeycombing, the useof high-velocity water jets is recommended. The latter has many advantages asvibration is eliminated and the concrete is left with a clean, damp exposedaggregate surface which is ideal for securing bond with the new mortar orconcrete. When concrete is used in a wall repair, a collapse slump with a loww/c ratio is required to help ensure full compaction. After removal of theformwork, the newly placed concrete should be cured for four days bycovering it with polythene sheets properly secured against wind. For shallowareas, a cement/sand/SBR mortar can be used after preparation of the concreteas described above and the application of a cement/SBR grout to assist insecuring a good bond (Figure 10.5).

Figure 10.4 Sketch showing repair of serious thermal crack in wall of pool by crack injection.

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10.19 Inadequate concrete cover to thereinforcement

It has been recommended in Chapters 4 and 5 that the prescribed cover toreinforcement should be checked by means of an electro-magnetic cover-meter,preferably as soon as practical after casting. The actual cover should not be lessthan the prescribed/nominal cover minus 5 mm.

The decision as to the action, if any, to be taken when the cover revealed by thecover-meter survey is less than the minimum required rests with the professionalman responsible for supervising the contract. The options are limited and usuallyare restricted to the selection of a suitable material to apply to the concrete torestore the protection lost by the inadequacy of the cover. The use of coatings isunlikely to be satisfactory if the finish to the pool is tiles, mosaic or marbelite asthe coating would seriously interfere with the bond at the concrete-finish interface.

A properly applied cement/sand rendering not less than 15 mm thick, containing10 litres of SBR to 50 kg cement should provide adequate protection unless theactual cover is grossly inadequate over large areas. In this case, it may be necessary

Figure 10.5 Sketch showing repair of honeycombed concrete in wall of pool.

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for a structural lining of sprayed reinforced concrete, 75 mm thick to be specified.However, such a lining on the walls will reduce the inside dimensions of the pool,and if on the floor will reduce the water depth.

The surface of the concrete should be prepared by high-velocity water jets aspreviously described.

Further information on the use of cover meters is given in Section 10.22.2.There may be some evidence of debonding of either the rendering, screed or

tiling while the work is still in progress; recommendations for testing the bond aregiven in Section 7.8, and reference should be made to Section 10.26 for remedialwork to this type of defect.

REMEDIAL WORK TO EXISTING POOLS: TRACINGLEAKS AND INVESTIGATIONS

10.20 Introduction

Remedial work to existing pools is usually initiated because leakage is found to betaking place and/or serious visible defects have appeared (Figures 10.6–10.7).

Figure 10.6 Defects in floor of old open-air pool. Courtesy, Colebrand Ltd.

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Figure 10.7 Defects in wall of old open-air pool. Courtesy, Colebrand Ltd.

Figure 10.8 View of old open-air pool after completion of repairs and decoration withchlorinated rubber paint. Courtesy, Colebrand Ltd.

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The first step is to ascertain the amount of leakage and whether it is loss ofwater from the pool or infiltration of ground water, or some combination of both.A careful investigation has to be put in hand which is likely to be time consumingand expensive.

If the leakage is serious it is usually accompanied by other defects in whichcase a detailed investigation should be carried out. The cost of investigating theloss of water can be high and before undertaking such an investigation, the ‘cost-benefit’ aspect should be given careful consideration. The recommendations whichfollow are based on the assumption that the cost of the investigation is consideredworthwhile.

10.21 Tracing leaks

Experience shows that leaks mainly occur through cracks and joints and lessfrequently through honeycombed concrete.

Unfortunately, there is no practical and simple method of locating points ofleakage unless there are clear visible defects on the inside of the pool. Figures 10.6and 10.7 show major defects in the floor and wall of an old open-air pool where infact serious loss of water was taking place.

In most cases, the location of leakage is difficult to establish. From time totime, ingenious suggestions are put forward for locating leaks by means of tracerdyes, concentrated salt solutions and radio-active tracers. These may be useful inspecial cases where the water loss is large and the ground water is below theunderside of the floor of the pool. It would be necessary to excavate inspection pitsat close centres around the pool carried down to below the pool floor.

When loss of water is suspected, the following procedure is recommended: 1. A drawing should be prepared showing the location and extent of all major

visible defects.2. A water test should be carried out as described in Appendix 2; but if the pool

has been empty for less than about three months at the time of the test, theinitial soakage period can be omitted. It is particularly important that the dropin water level should be recorded for each 24 hour period.

3. If the water level virtually ceased to drop below a certain level, then this wouldindicate that a major leak was at, or close to, this level. With a major leak inthe floor or lower part of a wall, the rate of fall in water level would decreaseas this level was approached due to the reduction in the head of water over theleak.

4. A decision has to be taken as to what loss of water can be accepted, and thisdepends on a number of factors including the age of the pool, its method ofconstruction, and whether the water is heated. It would be unrealistic to expecta pool which was more than 10 years old, constructed in other than reinforcedconcrete to achieve a standard of water loss similar to that of a new pool.

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5. For a new pool, 25 m×13m, the acceptable water loss excluding evaporationwould constitute a drop in level of 10mm in seven days; this is 3.225 m3=705gal, or 102 gal per day.

For an old pool of similar size, the drop in level could well be 25 mm in 24hours, resulting in an outflow from the pool of 8.125 m3 per day (1787 gal).

6. When the rate of fall of the water level indicates that a major source of leakagehas been reached, this should be recorded and the test should be continueduntil the rate of loss is considered acceptable, or the pool is virtually empty.The latter would indicate a major leak probably in the floor at the deep end.

Suggestions for repairing leaks are given in Sections 10.23–10.26.

10.22 General investigations

When it is considered that the sources of leakage have been established, it isnecessary to decide whether it is desirable to carry out investigations into the generalcondition of the pool. The details of such an investigation will obviously dependon the materials used for the construction of the pool, its age, and on the visibledefects.

For the purpose of this chapter it will be assumed that the pool was constructedin reinforced concrete (insitu or sprayed) and finished with cement/sand rendering/screed and ceramic tiles.

The tests briefly described in paragraphs 10.22.2, 10.22.3 and 10.22.4 are whatare known as Non-Destructive Tests (NDT). They are very useful and are nowaccepted as satisfactory methods provided they are interpreted by experiencedpersonnel and are verified by an adequate number of visual inspections whichwould usually need small holes down to the reinforcement, or in the case of animpulse radar survey, down to the level where inadequate support was indicated.

10.22.1 Checking for loss of bond between finishes andthe pool shell

On areas suffering from lack of adhesion, a hollow sound will be given out whenthe surface is tapped with a light hammer or rod. These areas should be marked onthe tiles and a drawing prepared showing their location. It will be necessary toestablish whether the loss of bond is located between the tiles and the substrate(rendering/screed) or whether it is between the substrate and the concrete shell.All relevant information must be recorded.

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10.22.2 Cover-meter survey

It is generally advisable to carry out a cover-meter survey as part of the generalinvestigation even though there are no visible signs of damage to the concretecaused by corrosion of the reinforcement.

An electro-magnetic cover-meter is used to check the depth of cover to thereinforcement. Detailed recommendations for the use of this equipment are givenin BS 1881 Part 201. The cover-meter consists of a search-head, a battery, a metershowing the depth of cover and a length of cable.

With an experienced operative, a correctly calibrated cover-meter should indicatethe depth of cover (the distance from the surface of the concrete or of the appliedfinishes, to the surface of the reinforcement to an accuracy of ±2 mm or ±5%whichever is the greater. However, when used on an average site by an averageoperative, a reduced accuracy of ±5 mm, or ±15% could be reasonable.

The nominal cover for water-retaining structures is generally specified as 40mm, but if the pool holds saline water, the nominal cover should be increasedto 50 mm.

10.22.3 Half-cell potential survey

If the results of the cover-meter survey are unsatisfactory, i.e. the concrete coveris substantially less than required to protect the reinforcement, or there are signsof spalling of the concrete and disruption of the tiling due to the rusting of therebars, it would be advisable to carry out a half-cell survey. Pitting corrosion canseverely damage rebars without spalling occurring. Reference should be made toSection 3.4.

This method appears to have been developed in the USA in the early 1960sand is covered by ASTM Specification C876–80 Standard Test method forHalf-cell Potentials for Reinforcing Steel. There is no British Standard for thistechnique but it is described in BS 1881 Part 204. It measures the potential ofan embedded rebar relative to a half-cell, and consists of a reservoircontaining a saturated solution of copper sulphate (CuSO

4). Secured centrally

within the container is a copper rod connected by an electric lead to a highimpedance voltmeter. At one end of the container is a sponge plug whichremains continuously saturated with the copper sulphate solution. Thesaturated sponge can be considered as the search head of the apparatus. Thevoltmeter is connected to a rebar in the concrete.

The surface of the concrete to be examined is divided into squares about 300mm×300 mm and this grid is marked in chalk on the surface of the concrete.Immediately prior to the test, the surface of the concrete is sprayed with water. Thesearch head is placed on the surface of the concrete in the centre of each grid andthe readings are recorded on a drawing. An alternative is to move the search headabout so as to locate lines of equal potential (contour lines).

The half-cell indicates the intensity at which corrosion is taking place at thetime of test; it does not indicate the rate of corrosion nor the amount of corrosion

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which has occurred. There is always some electro-chemical activity between thesteel and the concrete.

The implications of the readings are as follows: 1. For potentials less negative than -200 mV, there is a 90% probability that

corrosion is not taking place.2. For potentials between -200 mV and -350 mV, there is a 50% probability of

corrosion.3. For potentials numerically greater than -350 mV, there is a 90% probability

that corrosion is taking place. There are a number of factors which can influence the readings and these includethe moisture content of the concrete and the presence of salts in the concrete. It istherefore advisable to check a few readings by exposing the rebars and seeingwhether the readings follow the usual pattern.

This equipment should only be used in circumstances where the search headcan be in direct contact with the surface of the concrete.

Figure 10.9 Diagram showing part of assessment of the construction of an old swimmingpool using impulse radar. Courtesy, G.B.Geotechnics.

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10.22.4 Impulse radar survey

This technique is also known as pulsed radio echo sounding. At present, there isno British Standard for this, but it is briefly described in BS 1881 Part 201Section 2.12.

It was first introduced into the UK in the early 1980s and has a variety of usesincluding the location of voids in concrete and in the sub-grade below slabs.

With old pools, especially when leakage has occurred over a considerabletime, investigations by impulse radar, may reveal that adequate support to thefloor is missing in a number of locations. Figure 10.9 shows part of such asurvey of a large, open-air swimming pool. Areas of inadequate support areclearly revealed.

REMEDIAL WORK TO EXISTING POOLS: REPAIRSFOLLOWING LEAK TRACING AND INVESTIGATIONS

10.23 Remedial work to leakage

Repairs to points of leakage would normally follow after the completion of theInvestigation which has been detailed in Section 10.22.

It has been stated that when leakage has occurred it is usually found to betaking place through joints and cracks, and to a lesser extent through honeycombedconcrete. Recommendations for these repairs are given below.

Leakage can take place outwards from the pool (loss of water), or inwards intothe pool (infiltration) when the pool is empty or partly empty and the water level inthe pool is below the level of the ground water.

10.23.1 Controlling infiltration

To repair defects which allow infiltration, the inflow of ground water must besealed off first. The method to be employed to seal off the inflowing water willdepend on many factors including the rate of inflow and the hydrostatic head. Thework should only be entrusted to experienced contractors with a proven record ofsuccessful work in this field.

The methods used for this work include: 1. Control of ground water level by pumping so that the leaks can be repaired in

the ‘dry’. However, before a decision is taken to lower the water table, expertadvice should be obtained as this may have a serious effect on the foundationsof the pool and adjoining structures.

2. The use of ultra-rapid setting compounds or grouts which set almostinstantaneously when they come into contact with water. These can be applied

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by hand or by crack injection. Whichever method is adopted, it should only beentrusted to specialist firms. Unsuccessful crack injection by one firm canresult in another (more experienced firm) being unable to rectify the situation.The author has come across this unfortunate state of affairs on more than oneoccasion.

3. Grouting the sub-soil by the use of special grouts to form a ‘curtain’which, when successful, will greatly reduce the inflow of ground waterbut is unlikely to form a complete cut-off. The objective is to fill the voidsin the sub-soil with the grouted barrier; grouting can be suitable forcohesionless soils with a particle size in excess of about 0.002 mm (2microns).

Clay forms a major constituent of many of these grouts, and is a complexmaterial. Its important characteristics depend on the clay minerals. It has beenfound that for successful grouting, the calcium and sodium montmorillonites areparticularly useful in forming a gel which fills the voids in the sub-soil andsubstantially reduces the flow of water. Needless to say, this work is highlyspecialised.

When the infiltration has been sufficiently controlled, the joints, cracks or areas ofhoneycombed concrete can be dealt with as described below.

10.23.2 Remedial work to joints

A decision has to be taken on whether the defective joint allows movement totake place a (‘live’ joint) or whether it is static such as a construction or day-work joint.

10.23.2.1 Movement joints

They should be cleaned out and all old sealant completely removed. This workmay necessitate some repair to the sides of the joint before the new sealant isinserted; an epoxy mortar can be used for this repair work. Information on sealantsis given in Section 2.15. The sealant selected should bond to the sides of the jointbut should not bond to the back-up material and a separating strip may be required.

10.23.2.2 Construction/day-work joints

If it is assumed that movement does not take place across these joints, then a rigidrepair mortar can be used.

The joint should be opened up by sawing, and percussion tools, to a depth andwidth of about 10 mm. All dust and grit must be removed and the joint filled withan epoxy mortar, well trowelled in. If there is doubt about possible movementacross the joint then the repair material should be flexible and the repair carriedout in a similar manner to that described for cracks in the following paragraph.

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10.23.3 Remedial work to cracks

A decision should be made on whether there is movement across the crack, inwhich case it is considered as ‘live’ and the repair must take this into account.When dealing with old pools, it can be very difficult to know whether a crack is‘live’ or static and therefore when this doubt exists it is prudent to assume thatsome movement will occur across all cracks which need repair.

For ‘live’ cracks, the material used to seal the crack must possess some degreeof flexibility; a semi-flexible epoxy resin is usually suitable.

The crack should be opened up by light tapping with a chisel. The concreteon each side of the crack should be wire brushed for a distance of 300 mm andthen all grit and dust removed. A coat of low viscosity epoxy primer should bewell brushed into the crack and the prepared surface on each side. The semi-flexible epoxy should be applied to the crack and this should be followed by aglass-fibre mesh extending for the 600 mm width of the prepared concrete; themesh will be embedded in the first coat of resin and then a second coatapplied.

The mesh can be omitted if the crack is less than 0.3 mm wide. For crackswider than about 1.5 mm, it is worthwhile to consider crack injection usingspecially formulated epoxy resin which has low viscosity (for maximumpenetration into the concrete) and is semi-flexible. Prior to the crack injection thecrack and the surrounding concrete should be prepared in the manner describedabove (Figure 10.4).

10.23.4 Remedial work to honeycombed concrete

While it is unusual to find significant seepage through honeycombed concrete,even slight seepage over the long term can cause loss of bond between the baseconcrete and the finish (rendering, screed, tiling etc.).

Honeycombing can occur during construction of the pool shell and a method ofrepair has been described in Section 10.18.

10.23.5 Remedial work around pipework

Circulating pipework has to pass through the pool shell below top water leveland loss of water and infiltration can occur at these perforations in the poolshell. The same comment applies to under-water light fittings, and viewingwindows.

Outlets in the floor of the pool, especially at the deep end, are particularlyvulnerable to leakage due to the increase in hydrostatic head.

Cast iron and steel pipes have been largely replaced by plastic which has resultedin a decrease in bond between the concrete and the pipe surface.

When it is found that water is seeping past these pipes, repair can be very difficult.Pressure grouting or injection can be tried, otherwise it means cutting away theconcrete for the full thickness of the floor or wall, providing a new section of pipe,

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and then making good after steps have been taken to provide a roughened surfaceon the plastic pipe to increase bond.

The provision of a surface flange and the acceptance of a reduction in the seepage,instead of complete elimination, can be a practical solution.

10.24 Improving support to the pool floor

With old pools, especially when leakage has been taking place for some considerabletime, it is prudent to have the sub-base and sub-grade beneath floor checked bymeans of an impulse radar survey. This technique has been described briefly inSection 22.4, and illustrated in Figure 10.9.

The state of affairs shown in Figure 10.9 can often be rectified by pressuregrouting of the sub-base and sub-grade. If the seepage is considerable and the sub-grade is found to be unsuitable for pressure grouting, then consideration has to begiven to breaking out part(s) of the pool floor and filling in the voids with concreteand then providing a new structural lining of reinforced sprayed concrete over thewhole floor.

10.25 Structural lining to the pool shell

This is a costly undertaking as all the circulating water pipe connections have tobe remade. The whole system may require upgrading to bring it into line withpresent day standards. At the same time, an investigation into the water treatmentsystem may show that it does not meet present day requirements, and also needsupgrading.

As far as the sprayed concrete lining is concerned, reference should be made toChapter 5. A decision will be required on whether to bond the new shell to theexisting one, or whether to fix a slip membrane to the floor and walls and thuscompletely debond the new sprayed concrete from the old structure.

From the above, it will be seen that remedial work to old swimming poolsrequires very careful thought as all matters relating to the operation of the poolhave to be taken into account before large sums of money are spent on repairs tothe pool itself.

10.26 Remedial work to finishes

The method of carrying out the repair will depend on a correct diagnosis of thecause of the deterioration and its extent. Remedial work to finishes is only likelyto be required after the pool has been in use for some time and in this context,the term does not apply to the regular renewal of coatings which have a limitedlife compared with ceramic tiles and mosaic. Reference should be made toSection 7.10.

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10.26.1 Remedial work to tiling

The remedial work can vary from the refixing of a few tiles or the regrouting ofa few joints to complete removal of large areas of tiling, rendering/screed, and/or regrouting of a large number of tile joints.

A few loose or detached tiles can usually be refixed without the necessity oflowering the water level. Epoxy resins can be formulated so as to cure underwater.

If investigation shows that there is widespread loss of bond between the tilesand the substrate (rendering and/or screed), then the only practical solution is toremove all defective tiling. Assuming that the substrate is, overall, well bonded tothe concrete, the substrate should be repaired where it has been damaged by theremoval of the tiles, and all grit and dust and contamination removed and the tilesrefixed as described in Chapter 7. Care must be exercised in removing the tiles tominimise damage to the substrate.

If there is widespread loss of bond between the rendering/screed and the concreteshell, then all defective rendering/screed should be removed and relaid as describedfor new work in Chapter 7.

Percussion tools should not be used for the removal of the defective areas as thevibration caused by these tools will extend the loss of bond to adjacent, relativelysound areas. High-velocity water jets should be used in preference to percussiontools.

Sometimes cracks appear in the tiling and investigation shows that the tiles andsubstrate were laid over a joint or crack in the pool shell, across which movementhas taken place. A movement joint should be incorporated into the new tiling toline up with the joint, or crack, as far as this is practical.

The grouted joints between the tiles are sometimes eroded by chemical attackby the pool water. The main causes of this attack are a pH below about 6.5, or anegative Langelier Index, or a high concentration of sulphate in the pool water.The first action to be taken is to ascertain the reasons for the attack and to remedythe faults in the operation of the water treatment plant.

The affected joints should be cleaned out and regrouted using a specialproprietary polymer grout, or an epoxy resin-based grout.

10.26.2 Remedial work to marbelite

This material is briefly described in Section 7.9. The usual defects areshrinkage cracks, loss of bond with the substrate and severe staining.Marbelite is a special type of insitu terrazzo and is difficult to repair.Percussion tools should not be used for removal of defective areas as this willincrease the area of defective bond. Advice on repairs to marbelite should besought from the National Association of Terrazzo, Marble and MosaicSpecialists.

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10.26.3 Remedial work to coatings and sheet linings

When the pool shell has been finished with a decorative coating, such as chlorinatedrubber paint, and this has suffered premature deterioration it would be prudent tocontact the coating supplier for advice before attempting to carry out repairs unlessthe cause is obvious, such as physical damage.

The same comment applies to defects in the PVC lining of liner pools; see alsoSections 7.10 and 7.11.

Further reading

Bowan, R. Grouting in Engineering Practice, Applied Science, London, 1975.Construction Industry Research and Information Association. Civil Engineering Sealants in

Wet Conditions, Tech. Note 128, 1987.Construction Industry Research and Information Association. Water Resisting Basement

Construction—A Guide, Report 139, 1995.Domone, P.L.J. and Jefferis, S.A. (eds) Structural Grouts. E & FN Spon, London, 1993.Institute of Baths and Recreation Management. Practical Leisure Centre Management, Vol.

2.Perkins, P.H. Repair, Protection and Waterproofing of Concrete Structures, 3rd edn, E & FN

Spon, London, 1997.Pool Water Treatment Advisory Group. Swimming Pool Water Treatment and Quality

Standards, 1998.Swimming Pool and Allied Trades Association. Swimming Pool Guide, 1995.

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Appendix 1

Conversion factors andcoefficients

1 m2 = 10.7 ft2

1 ft2 = 0.093 m2

1 inch = 25.4 mm1 m = 3.28 ft1 ft = 0.305 m1 kg = 2.205 lb1 lb = 0.454 kg1 lb/in2 = 700 kg/m2

1 lb/ft2 = 4.86 kg/m2

1 ft3 = 0.0283 m3

1 m3 = 35.3 ft3

1 m3 = 220 gal (Imp.)1 gal (Imp.) = 1.20 US gal1 gal (Imp.) = 4.55 litres1 ft head of water = 0.435 lb/in2

1 N/mm2 = 102 m head of water1 N/mm2 = 145 lb/in2

1 N/mm2 = 65.75 kg/in2 = 9468 kg/ft2

1 N/mm2 = 1 MN/m2 = 1 MPa1 kgf = 9.8 N1 lbf = 4.45 N1 N/mm2 = 1 MPa

Density:1 lb/ft3 = 16.0 kg/m3

1 kg/m3 = 0.062 lb/ft3

Density of structural reinforced concrete made with natural aggregates:148 lb/ft3 = 2376 kg/m3 = 2400 kg/m3 (approximately)

Bulk densities of concreting materials (very approximate):Cement 1450 kg/m3 = 91 lb/ft3

Sand 1675 kg/m3 = 105 lb/ft3

Coarse Aggregate 1500 kg/m3 = 94 lb/ft3

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Conversion factors and coefficients 207

Temperature:°F to °C (°F-32)×5/9 = °C°C to °F (°C×9/5)+32 = °F

Heat:1 Btu = 1.055kJ1 Btu/hour = 0.293 W1 calorie = 4.18 J

Transmittance (U value):1 Btu/ft2/hour/°F = 5.678 W/m2/°C

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Appendix 2

Testing swimming pool shells,walkway slabs and other wetareas for watertightness.Commissioning swimmingpools

Introduction

A swimming pool is a water-retaining structure and must be watertight so thatthere is no unacceptable loss of water from the pool. If the pool is wholly or partlybelow ground, there must be no unacceptable infiltration of ground water whenthe pool is partly or completely empty.

The requirement for infiltration of ground water can be checked when the poolis empty and ground water level is not lowered by pumping.

For checking the loss of water from the pool, a clear and practical leakage testmust be carried out.

The two criteria mentioned above apply to new pools and existing ones, but theassessment of the results is likely to be different for new and existing pools.

Testing new pools

It is advisable for back-filling around the walls to be delayed until the water test hasbeen passed successfully. If the test is not passed, the location(s) of the leak(s) must betraced and this will be very difficult and time consuming if the outside of the walls arenot visible. Leaks in the floor are always particularly difficult to locate.

There is a difference of opinion among engineers, architects and contractors asto whether the leakage test should be carried out before or after the application ofrendering and screed and some contractors even prefer to carry out the test afterthe completion of applied finishes.

The recommendations are: 1. For pools constructed of reinforced insitu concrete or reinforced sprayed

concrete, the test should be carried out before the application of renderingand screed.

2. For pools constructed with walls of hollow concrete blocks and insituconcrete floor, and pools with the walls constructed of an insitu reinforced

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concrete core with concrete blocks as permanent formwork, and an insituconcrete floor, the test should be carried out after the application of therendering and screed, but before the final finishings, such as tiling,marbelite and decorative coatings are applied.

For details of the construction of the types of pools mentioned in 1 and 2 above,readers should refer to Chapters 4 and 5.

However, if the pool walls are designed to comply with the Code BS 8007, thenthe test should be as for a pool constructed in insitu reinforced concrete.

For information on rendering and screeds, see Chapter 7.The Code BS 8007 recommends the following water loss over a test period of

seven days as acceptable for reinforced concrete water retaining structures: 10mm, or 1/500 of the average water depth, or some other specified amount. Lossdue to evaporation, and addition due to rainfall must be taken into account.

It is recommended that a water loss of 10 mm over seven days be accepted,taking into account evaporation and rainfall.

Testing existing pools

Existing pools have to be tested with finishings intact as the objective is to find outwhether the pool as it stands is losing an unacceptable quantity of water, and/orthere is an unacceptable amount of infiltration of ground water.

The leakage test procedure

The recommended procedure is set out below. These recommendations, or otherswhich are deemed satisfactory, should be incorporated into the contract for newpools. When the result of a water test is satisfactory, no arguments arise. However,when the test result does not meet the specified figure, there is certain to be a ratherheated discussion which may end up in a legal dispute. Therefore the clearer thetest requirements are the better. 1. All valves on outlet pipes should be closed.2. Existing pools which have been in constant use should be filled to top water

level in preparation for the start of the test. However, if the pool has beenstanding empty for more than about three months, an initial soakage periodof seven days as described below for new pools should be adhered to.

3. It is advisable for the filling to take place slowly, about 0.75 m depth perday. The same precautions should be taken for new pools.

4. For new pools in insitu reinforced concrete with a design maximum crackwidth of 0.10 mm, the preliminary soakage period should be seven days; ifthe design maximum crack width is 0.20 mm, the soakage period can beextended to 21 days, at the discretion of the designer.

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5. For pools constructed in reinforced sprayed concrete and the other types ofconstruction described above, the soakage period can be 21 days.

6. During this soakage period, the water level in the pool should be kepttopped up daily to top water level. The amount the water level drops eachday should be recorded.

7. For existing pools which are in constant use, the soakage period can beomitted.

8. After the end of the soakage period, the water level should be raised to topwater level and carefully marked in some satisfactory way and all inletvalves closed. The test period then begins.

9. The test should last for seven days and during this period the water levelshould not drop more than 10 mm from causes other than evaporation, withan allowance for rainfall. Evaporation should be deducted from themeasured drop in level, while rainfall should be added.

10. During the period of test, the water level in the pool should be recordedeach day at the same time.

11. An allowance for evaporation loss should be made and a satisfactorymethod of measuring this is to anchor in the pool a drum or similarcontainer which is filled with water to within about 75 mm of the rim (thefreeboard); this is to prevent a strong wind blowing water over the rim ofthe container. The use of formula to calculate the evaporation loss isunlikely to provide a practical result as so many factors are involved. Thedrop in water level in the drum can be considered as due to evaporation.

12. The rainfall should be measured by a rain gauge, but this is unlikely to bepractical even for large public pools. Rainfall figures from the nearestmeteorological station should be obtained, and accepted as reasonably validfor the site in question.

General comments on testing

It is recommended that with new pools the outside of the walls should be inspectedduring the test. It is likely that signs of slight seepage at joints and cracks aredetected, and rather less likely, wet patches where the concrete has not beenthoroughly compacted. These are often self sealing. Even if they persist to the endof the test period, the actual loss of water is very small and cannot be measured.The permitted water loss of 10 mm is quite small but measurable. For a pool 25m×12 m, the loss on the 10 mm basis would amount to 3.0 m3 (660 gallons or 3000litres), which is about 93 gal (430 litres) a day.

For existing pools, it is most likely that a higher figure for water loss than thosegiven above would have to be accepted. The actual loss which is consideredreasonable will depend on a number of factors of which the following are the moreimportant:

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1. the age, method of construction and general condition of the pool;2. whether the water is heated, this is an economic consideration;3. the estimated cost of remedial work required to reduce water loss to an

acceptable figure, and the funds likely to be available;4. if the water circulation system and treatment plant does not meet present-

day requirements, consideration should be given to major up-grading as partof the refurbishment, and availability of funds should also be considered;

5. for closed pools, the condition of the pool hall and associated structures andequipment.

Watertightness test for walkway slabs and otherwet areas

In large pool complexes, the space below the walkways around the pool and belowthe wet changing areas is sometimes utilised for plant rooms, storage or otherpurposes which require a dry environment. These floor slabs should be as watertightas the roof of a building. Unfortunately, this important requirement is sometimesoverlooked, with the result that these floor slabs are not designed for watertightnessand are not subjected to a water test before acceptance. In such cases, seepage ofwater through the floors can appear some years after completion, and causesconsiderable consternation, especially when water is dripping onto plant andequipment.

Remedial work is difficult and costly as it usually entails closing down part ofthe centre.

The water test recommended is as follows: 1. Not less than 28 days after completion of the floor slabs and before any

screed or finishes are applied, flood the slab to a minimum depth of 25 mmand maintain this depth for 72 hours.

2. The floor slabs can be considered as satisfactory if no seepage or damppatches appear on the soffits during the test and for a period of seven daysafter completion of the test.

Commissioning swimming pools (filling andemptying)

Once the pool shell has passed the leakage test, it can be emptied slowly, at a rateof about 0.75 m depth of water per day. The remainder of the work can proceed butthe pool shell must be allowed to dry out before finishings are applied. Commentson this are given in Chapter 7.

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After all the finishings have been completed and given time to dry out andmature, the pool can be filled with fresh water, the rate of filling being the same aswhen it was originally filled for the leakage test, 0.75 m per day.

If the pool water is heated, the temperature should be raised slowly, at a rate notexceeding about 1 °C per hour.

Public swimming pools are generally emptied about once a year or 18 monthsfor general inspection and maintenance. This is usually done in winter whentemperatures are low. The pool has thus been filled with heated water (about 26–28 °C) for at least 12 months and the pool shell and finishings are correspondinglywarm. The emptying should be carried out slowly, say, at 0.75 m per day, and theair in the pool hall should be maintained at a reasonable temperature, say, 20 °C,for the duration of the maintenance work. The refilling should also be carried outslowly, the water level rising at about 0.75 m per day; the temperature of theincoming fresh water should likewise be raised slowly, about 1 °C per hour. Therecommendations for rate of emptying and refilling, and the restriction on the rateof increase in temperature of the fresh water, and the maintenance of a reasonableair temperature in the pool hall, are particularly important when the pool shell iselevated/suspended in a structural void in the building.

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Appendix 3

Investigations, sampling andtesting

General considerations

While the majority of defects in swimming pools involve leakage and/or infiltrationof ground water, if a dispute arises which results in litigation, a detailed investigationis likely to be required. This would include sampling and testing the concrete andmortar used in the construction of the pool.

The details of the investigation would depend largely on the nature and extentof the alleged defects, and the allegations made in the defence to the claim, andmay bring into question the specification and matters arising from the execution ofthe contract. Reference can be made to Chapter 1.

This Appendix will be confined to practical matters relating to sampling andtesting. Reference should be made to the comments in Chapter 10, paragraph 10.22on Non-Destructive Testing.

Sampling and laboratory testing

It is very important that every effort be made to obtain agreement by all the partieson the proposed sampling procedure, details of the testing, and the laboratory whichwill carry out the tests. The testing should be carried out by a laboratory accreditedby the United Kingdom Accreditation Service (UKAS).

The principal British Standard for testing concrete is BS 1881 Testing Concrete.For testing mortars, the Standard is BS 4551 Methods of Testing Mortars, Screedsand Plasters. Reference may also be made to BS 6089 Assessment of ConcreteStrength in Existing Structures.

The samples of concrete would normally be taken by means of 100 mm diametercores. The cores are tested to assess the compressive strength of the concrete andexamined visually to assess the voidage and standard of compaction.

The pieces of concrete resulting from the compressive tests can be used todetermine the cement content, chloride content and sulphate content of the concrete.

The type of tests should be restricted to obtaining information on the allegeddefects in the Statement of Claim. This may apply to tests on the concrete, and the

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mortar used for rendering, screed and building mortar, and possibly on othermaterials used in the construction of the pool.

Care and experience are needed in the interpretation of the test results. Chemicalanalysis to establish the probable cement content uses only about 5 g of powderedconcrete/mortar for the actual analysis which entails a considerable reduction insize from the original sample which is likely to weigh 1 kg or more. The samplesanalysed must be truly representative of the original combined samples and of theconcrete as a whole. There is testing variability on samples tested within onelaboratory and between laboratories even though they are accredited by UKAS.Concrete Society Technical Report 32 Analysis of Hardened Concrete suggestssampling variability of ±25 kg/m3 and testing variability as ±30 kg/m3, thus makinga combined variation of ±40 kg/m3 (the square root of 252+302=40).

The above emphasises the need to ensure that the sampling provides sampleswhich are truly representative of the concrete/mortar being investigated, and theinterpretation of the test results must take this into account.

The approximate grading of aggregate can be obtained by sieve analysis of thebroken concrete/mortar. The results should be used with caution to supplement ageneral assessment of the quality of the concrete/mortar. The actual grading of theaggregate used can only be obtained by sieve analysis of the aggregate stock pilesused for the concrete.

If the specification includes a compressive strength requirement for concreteblocks used in pool wall construction, then the testing should be carried out onblocks before they are incorporated in the construction.

If the quality of other materials used is suspect then these materials should besampled and tested in accordance with the relevant National Standard. For example,in the UK, ceramic tiles should be sampled and tested in accordance with BS 6431EN 121.

Cover-meter survey

A cover-meter survey to check the depth of cover to reinforcement is a standardpart of an investigation of a reinforced-concrete structure. In the case of a swimmingpool, the concrete is hidden from view by rendering, screed and tiling, mosaic,marbelite or a decorative coating. Detailed recommendations for the use ofelectromagnetic cover-meters are given in BS 1881 Part 204. A cover-meter usuallyconsists of a search head, a battery, a meter showing depth of cover and a cable. Acorrectly calibrated cover-meter should indicate the cover (depth from the surfaceto the rebar) to an accuracy of ±2 mm or ±5% whichever is the greater. However,the Standard emphasises that on site when used by an average operator, the accuracyis likely to be ±5 mm or ±15% whichever is the greater, for depth of cover notexceeding 100 mm.

Reference can also be made to Sections 10.15–10.22.

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Appendix 4

The consultant/designer as anexpert witness

Introduction

It is possible that a consultant who is experienced in the design, construction andoperation of swimming pools may be asked to act as an Expert Witness (EW) in acourt action or arbitration arising from a dispute over defects or shortcomings in aswimming pool. Provided he has had no connection with the project before thedispute arose, he should feel free to accept the appointment.

The following notes are intended to summarise the duties and responsibilitiesof an expert witness, in court and arbitration proceedings.

The information given on the duties of an expert witness are modified by thenew High Court procedures contained in the Woolf Report on ‘Access to Justice’.These reforms came into effect on 26 April 1999, and include an important changein the way evidence is given by experts; normally experts will not be allowed togive oral evidence but will provide a written report and answers to written questionsput to them by the opposing party.

The expert is usually, but not necessarily, instructed by a firm of solicitorsrepresenting one of the parties to the action, with the approval of the solicitor’sclient.

Sometimes the EW is called in by one of the parties to the dispute before legalaction is initiated, the object being to obtain an independent assessment of thetechnical issues. The EW would be prudent, when submitting his preliminaryfindings, to recommend his client to obtain legal advice from a firm of Solicitorsexperienced in construction disputes.

It is sometimes thought that the expert’s duty is to ensure that his report isworded as favourably as possible in the interests of his client. But this is not thecase; the expert’s duty is to provide a report which is impartial and which willassist the court or arbitrator in determining the case. This is of fundamentalimportance and has been emphasised on a number of occasions by High Courtjudges and barristers.

Another important fact which should be kept in mind is that the expert shouldbe experienced in the technical matters on which he is asked to report, and heshould be careful not to become involved in matters outside his field of expertise.

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The following is an example of procedure in a typical case. Resulting from apreliminary telephone enquiry which determined that the necessary expertise wasavailable, and the engineer or other professional was then invited by the solicitorsto act as an expert witness in a construction dispute. The letter of invitation outlinesthe case and may include a copy of the writ and a few of the more relevantdocuments.

The engineer replies, accepting the invitation and confirming that he or shepossesses the required level of expertise. They also provide information on theirfee scale which should be accepted in writing by the appointing solicitors. Thesolicitors then send to the expert additional documents relating to the case and setout the matters which they require the expert to consider in detail, express theiropinion, give conclusions. This often takes the form of questions on which thesolicitors want as far as possible, unambiguous answers.

The expert then prepares an initial report summarising the facts as he sees themand giving a preliminary technical opinion on the merits of the case, based on the‘balance of probabilities’.

The report should set out all the pros and cons of the technical issues in order toensure that the solicitors are fully briefed on the strengths and weaknesses of theirclient’s case.

In a case of any importance (usually judged on the amount of the claim and/orcounter claim), the expert would probably have one or more meetings with solicitorsand council to discuss various matters arising from his report.

It will also normally be appropriate for the expert to visit the site to carry outboth preliminary and detailed inspections. A comprehensive set of notes supportedby photographs will be helpful in both preparing the report and dealing with furtherquestions which inevitably arise.

If it is necessary to take samples of materials for testing then it is important toensure that they are representative and that when appropriate that sampling andtesting is carried out in accordance with recognised procedures. Each sample mustbe clearly labelled and its source precisely identified (see Appendix 3 on samplingand testing).

It is always desirable to try to arrange for the other parties to the dispute toagree on the procedure for sampling and testing, but such attempts are seldomsuccessful.

The reports from the various experts are ‘exchanged’ simultaneously on a datedirected by the judge/arbitrator which is usually a few months before the date ofthe hearing. Prior to the ‘exchange’, the experts are usually directed by the judge/arbitrator to meet and try to agree on as many relevant matters as possible in orderto limit the technical points at issue. It is usually found that agreement on importantmatters cannot be achieved, but such meeetings can be very useful.

In court actions, it is usual for the case to be heard by an official referee who isa judge with special experience in technical matters. Arbitrators should be selectedfor their technical knowledge of the matters which form the basis of the dispute, aswell as for their experience in arbitration.

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The form of the expert’s report

It is suggested that the adoption of a standard format is very useful. The paragraphsand pages should be numbered, and a contents list included. One short section ofthe report should provide a summary of the expert’s qualifications and experience.The report should list the instructions received from the solicitors and then proceedto deal with them in a logical order. The Conclusions can be at the beginning of thereport or at the end. The expert witness should not attempt to apportionresponsibility/liability as this is best left to the Court.

The expert witness and the Construction Act1996

The appointment of expert witnesses outlined above may be modified by theimplementation of the Housing, Grant and Regeneration Act 1996 (known as theConstruction Act) which came into force on 1 May 1998. This contains provisionsfor the appointment of an adjudicator when a dispute arises in a construction contractentered into after 1 May 1998. Brief information has been given on some of theimportant provisions of the Act in Section 1.16.3.

At the time of writing, there is little reported experience in the operation of thisAct which is unfortunate, as it will have far reaching implications to constructioncontracts.

It is reasonable to assume that an adjudicator, once appointed to a contract, willbe required to adjudicate on all disputes arising under the contract. For example,in a contract for a large leisure centre/swimming pool complex, disputes may arisefrom electrical and mechanical work, heating and ventilating work, structural design,site construction methods, etc. The adjudicator is unlikely to have extensiveknowledge and experience in all these subjects, and will need to have advice/opinions from experts in the relevant fields. The expert witness would then beappointed by or for the adjudicator; however, there is no provision in the Act forthe appointment by the adjudicator of experts to assist him in his duties.

If appointed by an adjudicator, under the 1996 Act he would be expected togive clear and impartial technical advice/opinion to the adjudicator on the particularmatter arising from his expert knowledge.

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Appendix 5

Notes on safety in swimmingpools

Introduction

For the purpose of these notes, the term swimming pools includes the pool, thepool hall, changing rooms and shower rooms, plant rooms, and external areas usedby the public, such as car parks and landscaped areas.

As far as can be ascertained, there is not a complete record of all accidents inswimming pools. The basic legal requirement to report accidents is limited in scope,and is contained in a Home Office publication Reporting of Injuries, Diseases, andDangerous Occurrencies Regulations. The enforcing authority is the Health andSafety Executive (HSE) for pools under the control of a local authority, and schoolpools. Hotel pools come under the control of the local authority. In practice, the HSEare only concerned with ‘serious’ accidents and they decide what is serious.Reference should be made to the HSE booklet Reporting an Injury of DangerousOccurrence HSE (11), revised.

In recent years, there has been a substantial increase in litigation arising out ofpersonal accidents in swimming pools. It has been said that in the past when someone tripped, slipped or fell, friends expressed their sorrow at the injured party’sbad luck; now the injured party is said to be lucky because he/she can sue someone alleged to be responsible for the accident.

An essential publication on this subject is Safety in Swimming Pools, issuedjointly by the Sports Council and the Health and Safety Commission.

The safety aspects which are dealt with here are:

1. water depths for diving;2. information signs for water depths in pools used by swimmers and non-

swimmers;3. other signs giving essential information on the proper use of the pool and its

facilities;4. outlets for water in the floor of the pool;5. water slides and play equipment;6. slipping and tripping on floors of walkways, changing rooms and shower

rooms;7. slipping and tripping on external paving forming part of the pool complex.

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Water depths for diving

These are covered by the regulations of ASA for National competitions, and by FINAregulations for International competitions. Reference should be made to Section1.3.4. The main objective of the regulations is to ensure safety of the persons diving.

Signs for water depths in the pool

The depth of water in the pool, particularly changes in depth must be clearly markedon both long sides of rectangular pools and at appropriate locations in free-formedpools. The signs should be clearly visible to persons using the pool as well as tothose intending to enter the pool.

Other safety signs

There are a number of signs required for safety purposes, such as prohibiting certaindangerous or undesirable activities and these signs should comply with BS 5378Parts 1, 2 and 3 Safety Signs and Colours. Reference should be made to the HSEbooklet HS(R) 7 A Guide to the Safety Signs Regulations.

Outlets for water in the pool floor

There is normally a comparatively large outlet for water in the deep end of the pool.This forms part of the water circulation system and also for the emptying of the poolwhen this is required. This is covered with a grating and can be a source of danger tobathers if they are in the pool when the outlet valve is fully opened, as it may be foremptying or lowering the water level. Bathers should be excluded from the pool whenthe outlet valves are being operated to adjust the water level. See also Section 8.2.6.

Water slides and play equipment

Water slides

These are now a standard feature of leisure centre pools and have introduced problemsof safety to the users who are often young children. The number of injuries, some ofthem serious, which have arisen from the use of water slides is cause for concern.

There is a British and European Standard for water slides over 2 m height, BSEN 1069–1 and BS EN 1069–2 1996. Part 1 of the Standard deals with safetyrequirements and test methods and Part 2 deals with instructions for safe operation.Both Parts give detailed recommendations for safe construction and use and areessential reading for designers and operators. Slides are divided into six typesaccording to whether they are individual (single) or multi-track, and the average

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and maximum speed of descent. The speed of descent can vary from about 7–14m/s (25–50 km/hour). Detailed recommendations are given for the structuraldesign of slides.

It is preferable for slides to discharge into a pool separated from the main pool(this is known as a splash-down area) and it should have the dimensions set out inthe Standard.

It is particularly important that the entrance to the slide should be undercontinuous experienced supervision.

Play equipment

There is a great variety of play equipment which is mostly used (or intended to beused) by young children. The water depth in the play area needs carefulconsideration. It should be sufficiently deep to act as a ‘buffer’ when children falloff the equipment to help prevent injury caused by hitting the pool floor with thehead; at the same time, it should not be too deep for children who cannot swim. Adepth of 1.00 m appears to be a reasonable compromise. There do not appear to beany authoritative recommendations for this.

Reference should be made to the publication of the Institute of Baths andRecreation Management A Suggested Code of Practice for the Use of PlayEquipment in Swimming Pools.

Slipping and tripping on floors of walkways,changing rooms etc.

At the time of writing, there was no recognised and authoritative guidance onacceptable slip resistance/coefficient of friction for floors of ‘wet’ areas. Inthe absence of such formal recommendations, it is suggested that thecoefficient of friction of slip resistant ceramic floor tiles when measured by aprescribed procedure should be used as the acceptable standard. Other typesof floor finishes, such as polymer resins, synthetic rubber etc., should berequired to meet this standard. Ceramic Research Limited of Stoke-on-Trent,UK, have carried out research in this area and have developed suitable frictionmeasuring equipment (Figure A5.1).

To combat tripping on uneven surfaces, the difference in level across jointsin precast/preformed units should not exceed that given in BS 5385 Part 3Code of Practice for Design and Installation of Ceramic Floor Tiles andMosaics, namely 1 mm across joints less than 6 mm wide and 2 mm acrossjoints exceeding 6 mm wide.

Another factor which should be taken into account in dealing with slipperiness isthe gradient of the floor surface. The floors of all ‘wet’ areas must be laid to falls sothat the water drains off to outlets. The problems arising from the conflictingrequirements for ‘no ponding’ and a ‘safe gradient’ have been discussed in Chapter 7.

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General unevenness should meet the detailed requirements set out in the Clause23.4 of BS 5385 Part 3.

Reference should be made to the Health and Safety at Work Act 1974 and to therecommendations of the Health and Safety Executive, specifically to the HSEpublication and Trips HSG 155. Appendix 1 of this booklet deals with floors, but theconcept of slipperiness is related to the floor surface and the type of footware used bypersons walking on the floor. There is no reference to persons walking bare-foot on awet surface.

Chemicals in water treatment

Many of the chemicals used in the treatment of swimming pool water can bepotentially dangerous, and special care is needed in storage areas and plant rooms.

Figure A5.1 View of TORTUS floor friction tester. Courtesy, Ceramic Research Ltd, Stoke-on-Trent.

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For example, if hydrochloric acid comes into contact with sodium or calciumhypochlorite chlorine gas is given off. Ozone which is used in many swimmingpools is poisonous in concentrations exceeding about 1 part to 50 000 parts of airby volume. See Section 8.9 and the publication by the Health and Safety ExecutiveControl of Substances Hazardous to Health 1994.

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Appendix 6

List of organisations relevantto this book

American Concrete Institute, P.O. Box 9094, Farmington Hills MI 48333–9094,USA.

American Society for Testing Materials, 100 Barr Harbor Drive, WestConshohocen, PA 19428, USA.

Brick Development Association, Woodside House, Winkfield, Windsor, Berks.SL4 2DX.

British Cement Association, Century House, Telford Avenue, Crowthorne, Berks.RG45 6YS.

Building Research Establishment, Garston, Watford, WD2 7JR.British Standards Institution, 389 Chiswick High Road, London W4 4AL.Ceram Research Ltd, Queens Road, Penkhull, Stoke-on-Trent ST4 7LQ.The Concrete Society, Century House, Telford Avenue, Crowthorne, Berks.

RG45 6YSConstruction Industry Research and Information Association, 6 Storey’s Gate,

Westminster, London SW1P 3AU.Federation of Terrazzo, Marble and Mosaic Specialists, P.O. Box 117, Leeds

LS18 2DZ.Federation for the Repair and Protection of Structures (FeRFA), Association

House, 235 Ash Road, Aldershot, Hants, GU12 4DD.Health and Safety Executive, Library and Information Services, Baynards House,

1 Chepstow Place, Westbourne Grove, London W2 4TF.Institute of Baths and Recreation Management, Gifford House, 36–38 Sherrard

Street, Melton Mowbray, Leics. LE13 1XJ.Pool Water Treatment Advisory Group, Field House, Thrandeston Near Diss,

Norfolk IP21 4BU.Quarry Products Association, 156 Buckingham Palace Road, London SW1W

9TR.Precast Concrete Paving and Kerb Association (INTERPAVE), 60 Charles Street,

Leicester LE1 1FB.Swimming Pool and Allied Trades Association (SPATA), Spata House, Junction

Road, Andover, Hants. SP10 3QT.The Sports Council, 16 Upper Woburn Place, London WC1H 0QP.

Copyright 2000 Philip H Perkins


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