Concrete Materials Problems

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Concrete Materials

Copyright 2003 by Taylor & Francis Group. All rights Reserved.


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To Evelyn



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Concrete Materials

Problems and solutions

M.Levitt

PhD, FIQA, MICT

E & FN SPONAn Imprint of Thomson Professional

London Weinheim New York Tokyo Melbourne Madras



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Published byE & FN Spon, an imprint of Thomson Professional,26 BoundaryRow, London SE1 8HN, UK

Thomson Science & Professional, 26 Boundary Row, London SE1 8HN, UK

Thomson Science & Professional, Pappelallee 3, 69469 Weinheim, Germany

Thomson Science & Professional, 115 Fifth Avenue, New York, NY 10003, USA

Thomson Science & Professional, ITP-Japan, Kyowa Building, 3F, 221Hirakawacho, Chiyoda-ku, Tokyo 102, Japan

Thomson Science & Professional, 102 Dodds Street, South Melbourne,Victoria 3205, Australia

Thomson Science & Professional, R.Seshadri, 32 Second Main Road, CIT East,Madras 600 035, IndiaFirst edition 1997

This edition published in the Taylor & Francis e-Library, 2003. M.LevittISBN 0-203-47676-X Master e-book ISBN

ISBN 0-203-78500-2 (Adobe eReader Format)

ISBN 0 419 21690 1 (Print Edition)

Apart from any fair dealing for the purposes of research or private study, orcriticism or review, as permitted under the UK Copyright Designs and PatentsAct, 1988, this publication may not be reproduced, stored, or transmitted, in anyform or by any means, without the prior permission in writing of the publishers,or in the case of reprographic reproduction only in accordance with the terms ofthe licences issued by the Copyright Licensing Agency in the UK, or inaccordance with the terms of licences issued by the appropriate ReproductionRights Organization outside the UK. Enquiries concerning reproduction outsidethe terms stated here should be sent to the publishers at the London addressprinted on this page.

The publisher makes no representation, express or implied, with regard to theaccuracy of the information contained in this book and cannot accept any legal

responsibility or liability for any errors or omissions that may be made.A catalogue record for this book is available from the British Library



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Contents

Preface

1 Concrete materials1.1 OPC, strength gain and sulfate and frost resistance1.2 OPC and curing1.3 Aggregates and frost damage1.4 Air-entraining agents and frost damage1.5 Alkali-silica reaction1.6 Calcium chloride

1.7 Aluminous cement1.8 Steel reinforcement: additional requirements1.9 Excess steel reinforcement1.10 GRC and alkali-glass reaction1.11 Fibre-reinforced sandwich panels1.12 Delayed ettringite formation

2 Health and safety2.1 Cement eczema

2.2 Cement burns2.3 Pumping grout

3 Concrete on site3.1 Covercrete dor k3.2 Spacers for rebars3.3 Tiling and moisture in floors3.4 Flatness of floors3.5 Flatness of formwork

3.6 Joints between precast paving and between kerbs3.7 Tolerances3.8 Rising damp and a chemical barrier3.9 Cracking in the non-visual zone

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3.10 Large-area scaling of floors3.11 Silanes

4 Specification problems

4.1 The CE mark4.2 Durability4.3 Concrete quality4.4 Specifying strength

5 Precast concrete5.1 Hydration staining5.2 Lime bloom5.3 Colour variations

5.4 Cracking and slenderness ratio5.5 Thermal cracking in pipes5.6 Tunnel segment impact damage5.7 Tesserae detachment

6 Testing6.1 Labcrete or realcrete6.2 Design or performance6.3 Camouflage testing

6.4 Repeatability and reproducibility6.5 Changes in testing6.6 Testing fixation6.7 Testing accuracy

Glossary

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Preface

In more than 40 years experience in the construction industry, I have spenta lot of time in what is commonly known as troubleshooting: on site withworks in progress, on completed projects, and sometimes even on anoccupied building or civil engineering project.

The advice I gave to the party raising the problem was usually based onmaterials science. My main aims were to explain the mechanisms that Ithought had caused the effect, and to suggest possible remedial measures.With the wide spectrum of problems that I have encountered, I felt that Icould make a useful contribution to the concrete industry by setting these

experiences down on paper.This book is the result. Each chapter deals with a particular problem

area, organised into sections that are generally relevant to that chapter.While I was deciding how best to organise the material, my attention wasdrawn to the Building Research Establishments Defect Action Sheets.These DASs ceased issue in March 1990, but their formatillustrating aproblem, dealing with its cause(s), and offering remedial proposalswasvery useful. Following this vein, I have described each problem on thebasis of personal experience, with sections discussing identification,

remedial measures, and avoidance. In most sections these three items formthe final paragraphs. However, in problem areas that generated secondaryproblems, the three items have sometimes been discussed with each of thesubproblems.

The book reflects personal experience, with a description of the bestspecific solutions that were found. The hardest part of problem solving isidentification, and this is often aggravated by there being more than onedamaging mechanism present. Therefore I make no guarantee that therecommendations in this book will always work.

A significant degree of overlap between chapters was unavoidable. If aproblem is described in a particular chapter, it does not mean that theproblem could not or does not occur elsewhere. For example, lime bloomis discussed in Chapter 5 (Precast concrete), but it is also known to occur



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with in-situ concrete. However, it is not as much of a problem as it is forprecast concrete products.

A basic knowledge and appreciation of aspects of materials science isessential in dealing with these problems. Architectural preferences and

engineering matters are not my discipline, and have been largely avoided.Even so, most problems that arose were probably due to a lack of mutualunderstanding between the various professions involved.

The causes of most problems are to be found in design, workmanship ormaterials. The division between design and workmanship is not verydistinct, but together these two account for over 80% of problems. It isunfortunate that materials have received, and continue to receive, too muchattention. When I analysed troubleshooting problems recently, I found thatmaterials accounted for only 18% of the total.

More often than not, the construction industry has problems withconcrete for the apparently simple reason that many procedures areundertaken in ignorance of the requirements of the other professions. Ingeneral, the solution is to identify the problem area and put the rightquestions to the right person(s). Although I have spent 20 years in theprecast concrete industry, and a similar time in general construction, theviews, recommendations and comments made in this book are mine alone.They do not necessarily reflect the views or policy of the precast concretetrade federations, nor of any companies within the Laing Group.

I have used the minimum number of references necessary to supportthe text. The advice given should stand alone, as being based upon goodfaith and experience.



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Concrete materials 1

INTRODUCTION

Although this chapter and Chapters 3and5are devoted to materials, innearly every case it was the deployment of these materials that gave rise toproblems, and not the materials themselves.

The introductions to the remaining chapters are much longer than thisone. This chapter covers a large range of materials problems, and eachsection has its own specific introduction. The introductions to theremaining chapters have a somewhat different function, in that each hasthe role of colouring in the background to what follows in the individualsections.

1.1 OPC, STRENGTH GAIN AND SULFATE AND FROSTRESISTANCE

A Concrete Society technical report (Concrete Society, 1987a) comparedcement property changes in the years 1960, 1974 and 1983. More recentdata has not been published, as far as I know, and I have made some

comments based upon individual cement test certificates that have comeinto my possession.The principal changes have been in the percentages of the two calcium

silicates that are the main contributors to strength through their formationof calcium silicate hydrate (CSH) in the cements chemical reaction withwater. These are tri-calcium silicate (C3S) and di-calcium silicate (C2S).Typical contents of these in the late 1940s were 40% and 35% respectively.Representative figures for 1990s production could be 60% and 15%. Thesepercentages are intended to illustrate medians covering a range of cement

plants and in-plant variations; I have a not-so typical cement test certificatein which the C3S and C2S contents are reported as 70% and 7% respectively.Because C3S hydrates much more rapidly than C2S, strength is built

up much more quickly, but the 28-day strength is not significantly

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affected. In effect, the shape of the strength-time curve has changed.This more rapid strength gain has had most effect in the in-situ concreteindustry, in that formwork can be stripped earlier, but the attractionhas been that the usual stripping times have been maintained while

using less cement than was the case before cements were so high inC3S.This is the main cause of the problems discussed in this section. The

decision to decrease the cement content for an equivalent early strengthbehaviour ignores the detrimental effect that it can have on resistance todurability risks such as sulfate and frost attack. Decreased cement contentpromotes more voids in the concrete matrix that would otherwise havebeen filled up with cement paste. These voids result in increased spacewithin the concrete or mortar for the ingress of sulfates, frost-prone water

and/or de-icing salts/chemicals.Changes over the years in the equivalent Na2O and alkali-silica reaction

are discussed more fully insection 1.5.Cement fineness also increased in the two decades reported from 1960

to 1983 (Concrete Society, 1987a), and this change also added to a morerapid hydration rate but with no significant effect upon the 28-day strength.The effect that this, coupled with the C3S and C2S changes, has had onthermal and moisture curing is outlined in section 1.2.

A problem that disappeared with the cessation of precast concrete pipe

manufacture by the spinning process was that too fine a cement resulted intoo much cement separating at the inside of the bore. Although cement hasa higher density than conventional aggregate as well as water for the veryfine powders that are spun, segregation is more a function of particle sizethan of density. Lighting columns are commonly made by the spinningprocess, but the problem of segregation of cement has not been brought tomy attention. The original problem with spinning pipes was solved by themanufacturer by purchasing a cement with a specific surface of about260m2/kg and sold as coarse-ground Portland cement. A typical current

OPC specific surface is about 330m2

/kg.In the following sections, sulfate attack has been discussed singularly,

but frost attack has been divided into two groups: direct frost attack withno other considerations, and attack accompanied by de-icing or anti-frostchemicals. Weathering due to the effects of wind and rain alone has alsobeen included.

1.1.1 IDENTIFICATION

(a) Sulfate attack

A softening and loss of the surface layers of the concrete is due to theformation of calcium alumino-sulfate (ettringite) from the reaction of



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ground or air/rain-borne sulfate with the tri-calcium aluminate (C3A)component of Portland cement. The reaction is expansive, because theettringite takes up more space than the C3A hydrate with which the sulfatereacted. Delayed ettringite formation is discussed insection 1.12.

(b)Frost attack without de-icing salts or anti-frost chemicals

The original international research (RILEM, 1977) explained the meaning ofcritical saturation and specified the relevant tests. In simple terms, when thewater content in the water-accessible void/capillary structure reaches acritical level, if frost occurs damage can take place because of ice formation.For example, if the critical saturation of a specific concrete is 80% and itstotal water-accessible space is 15% by volume (approximately equal to 6%

by mass), then frost damage is possible when the water saturation levelreaches 0.815=12% by volume (about 5% by mass) or more.Frost damage usually manifests itself as surface spalling and/or

softening, resulting in exposure of the aggregate. Less commonly, damageoccurs not by any apparent surface loss but by a decrease in the elasticmodulus, which for concrete used in structural applications could result indeflection beyond that designed.

Attempts to look for frost damage in open-textured concrete, such astypical concrete blocks and no-fines concrete, are generally fruitless. This

is probably because such concretes have plenty of time to drain out to belowcritical saturation level before frost occurs.

In addition, in very severe winters problem areas of frost damage needbe identified only when salts or chemicals have been used. This is because,for ordinary frost damage without salts or chemicals being involved, mildwinters tend to cause more problems as more freeze-thaw cycles areinvolved than during very cold weather.

The other area of frost damage identification that can safely be put torest is that of water in blowholes being subject to freezing. No case of frost

damage due to such action has ever been reported; ice in a blowhole has afree face out of which it can expand as water freezes.

(c)Frost attack with de-icing salts or anti-frost chemicals

Identification is generally in the form of severe surface spalling and acommon wet appearance. The use of salts or chemicals aggravates andaccelerates frost damage for two main reasons and one subsidiary one.

First, the use of salts (commonly salt as NaCl) and chemicals such as

ethylene glycol or urea depresses the freezing point to below 0C. Themethod of broadcasting these materials onto concrete results inconcentration variations, which passing traffic can exacerbate. Additionalfreeze-thaw cycles can take place with no change in external temperature.



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Second, when common salt dissolves into solution the reaction isendothermic, which can cause concrete just above freezing point to freezefor a while as it cools the surface.

Third, although ethylene glycol is not deliberately used to de-ice

concrete (it is used for spraying onto aircraft) it is an additional risk. Notonly can any chemical that drops onto the concrete cause a similar freezingpoint depression, but the glycol itself has a slow dissolution effect on thecalcium salts in the hydrated cement.

(d) Weathering

This is a dusting and softening of a 02mm depth of the surface, whichtakes place over many years, and is noticeable on unsheltered surfaces.

1.1.2 REMEDIAL

Remove all suspect and degraded concrete, and patch or full repair using(preferably) a polymer mortar (Concrete Society, 1984; Perkins, 1986).

1.1.3 AVOIDANCE

The cementitious content should be kept in the range 375450kg/m3.

1.2 OPC AND CURING

Hydration of cement is an exothermic reaction: heat is produced, and themore rapid the reaction process the more rapid the heat production. Thehigher contents of C3S now common in OPC (as well as the form of themore finely ground rapid-hardening Portland cement), together with thehigher fineness levels of modern cements, have caused concrete to emitheat more quickly than in previous years. These factors have resulted intwo potential problem areas that have given increased cause for concern.

The first of these is high thermal gradients, which can give rise to thermalcracking, particularly in thick sections of OPC concrete. This cracking isthought to be caused because the strain set up by the thermal gradientexceeds the strain capacity of the concrete in its hardening state (Harrison,1992). Thermal cracking can also arise when concrete is cast against a coldsubstrate such as older concrete.

The second risk area is too rapid a loss of moisture at the surface (Birt,1985). This can give rise to drying shrinkage cracks, and can also result insurface softening, caused respectively by shrinkage from too high a rate ofmoisture loss and by there being insufficient water in the surface area tohydrate the cement effectively.



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It may be seen, therefore, that the subject of curing problems alwaysneeds to be qualified by reference to whether the curing is for thermal ormoisture reasons or both.


(a) Thermal cracking

This is most noticeable when there is a maximum temperature gradientthrough the section, which for thick (over 0.5m) plain OPC concrete occurstypically at 4050 hours. Crack apertures up to 2mm wide are notuncommon. However, as the temperature gradient becomes less steep,these cracks largely close down to apertures of about 0.5mm. These crackpositions often coincide with the main rebars, and the cracks penetrate asfar as the steel. However, in an in-depth examination of marine-exposedconcrete I observed that the crack width tapered rapidly from the surfacedown to about 10mm deep, where it kept to a consistent value of about0.1mm. Chloride penetration from exposure to sea water did not occurbelow this 10mm depth in the crack, and was marginal in all other areas.

Thermal cracks occurring at the junction of new and old concreteinterfaces (the old concrete being known as a heat sink) may generally beseen to occur at right angles to the interface and horizontal and parallelfurther up the wall.

(b)Drying shrinkage cracking

This has typical crack apertures in the range 0.1-0.5mm, tapering down tozero at depths of about 10mm. The cracking commonly manifests itself as aseries of parallel lines about 100200mm apart and at right angles to the longestaxis of the section. For example, on a precast concrete column, the dryingshrinkage cracks would be seen as a number of bracelets along the unit.

As with crazing in cast stone and other concretes, drying shrinkagecracks can seal themselves by autogenous healing, and this effect isillustrated inFigure 1.1for the crazing on a cast stone pedestal.

1.2.2 REMEDIAL


A low-viscosity resin can be used (Concrete Society, 1984) if the cracks

are considered to be a risk. This can be assessed by estimating the crackdepth by means of the ultrasonic pulse velocity (UPV) method (BS 1881Part 203:1986). This method is practical only if the cracks are nominallyfree of water and the average crack width over all positions is at least



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0.2mm. The efficacy of the repair system used can be examined using the

initial surface absorption test (BS 1881 Part 208:1996). Repaired andunrepaired areas can be compared to eliminate the effect of the absorptioncharacteristics of uncracked concrete.


These cracks are generally aesthetic defects rather than a corrosion hazard,so as a rule no repair is required. If the effect needs to be masked, thensurface grinding followed by a flood application with a silicone (BS

6477:1992) is recommended. As far as the lifetime of this form of remedialmeasure is concerned, the longest in my experience was shown by caststone lintels on a school, which were treated in 1958. The lintels wereexamined in 1996 and found still to be in good condition.

1.2.3 AVOIDANCE


Use formwork with built-in thermal insulation and/or leave thermally

insulating covers in place until both the concrete surface temperature andthe temperature gradient have reached acceptable levels. As far as isknown, no quantitative guidance is available on concrete surfacetemperature, because much depends on air temperature, wind speed,

Fig. 1.1Autogenous healing of crazing on cast stone plinth.



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relative humidity and the possible risk of rain. Taking wind speed andwind chill into account, a difference of 10 deg C is probably the maximumacceptable.

As far as temperature gradient acceptability in the concrete section is

concerned, the recommendations suggested for precast concrete productsshould be similar to those for in-situ concrete (Levitt, 1982). Recommendedtargets are a maximum temperature gradient of 0.1 deg C/mm and amaximum cooling rate of 15 deg C/hr. These maxima could probably berelaxed (that is, increased) for lightweight aggregate or limestone aggregateconcrete, but might need to be more severe for flint gravel or igneous rockaggregate concretes.

As far as is known there is no published method for measuring thesurface temperature of concrete. My experience has shown that an ordinary

copper-constantan thermocouple spirally wound for its last 3050mm andstapled to the face of a hand-held cork is suitable.In addition to or in place of using thermally insulating formwork and/

or covering the surface, it is worth considering procedures such as theinclusion in the concrete mix of additives such as PFA (BS 3892 Part 1:1993)or GGBS (BS 6699:1992), or the use of a retarding admixture, which willboth modify the exotherm-time curve with less heat being produced in thecritical period.


Good practice in mix design and procedures to inhibit moisture loss (Birt,1985; Neville, 1995) inhibit or prevent this form of cracking. Typicalmethods of moisture curing are by covering with soundly fixed polythene,or with hessian kept wet for at least the first 48 hours. These methods ofcuring should not be used for visual concrete such as cast stone andexposed-aggregate concrete, because they can promote hydration staining,lime bloom and colour variations (seesections 5.1,5.2and5.3respectively).

Figure 1.2illustrates typical locations in construction of both thermaland drying shrinkage cracking.

The necessity for directing attention to curing being required for thermalor moisture reasons or both is repeated.

1.3 AGGREGATES AND FROST DAMAGE

Aggregates such as flint gravels, and igneous rocks such as granite andbasalt, are generally resistant to frost, but they have been observed to cause

pop-outs when de-icing salt has been used (Fig. 1.3).In my experience the occurrence of these pop-outs is a function

neither of concrete quality nor of the method of manufacture. It has been

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observed on in-situ concrete (both air-entrained and non-entrained) as wellas on wet-cast vibrated and hydraulically pressed precast concrete units,and for both flint gravel and granite aggregates.

The mechanism of frost damage and critical saturation has already beendiscussed insection 1.1,and it is possible that there are two processes thatsingly or jointly could exacerbate this mechanism.

First, concrete that has been broadcast with de-icing salt will havebecome more hydrophilic (attracting water) because of the presence of saltin its voids and capillaries.

Second, particles of aggregate of flint or granite and the like will have ahigher specific heat than the surrounding mortar, and will subsequentlyrespond more quickly to changes in temperature. This will result in theseparticles being colder nearer the surface of the concrete, and becomingwetter around their peripheries. These higher saturation levels would bedue to the moisture in the surrounding area moving towards the particles,

Fig. 1.2Typical locations for thermal and drying shrinkage cracks (not to scale).(After C.J.Turton)

Fig. 1.3Example of frost-induced pop-out at piece of aggregate.



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which because of their lower temperature have a lower vapour pressure.(This is why, in a refrigerator, food will dry out unless it is encased, becausemoisture will migrate towards the colder condenser area.)


These are aesthetic defects in the form of pop-outs typically 1020mm indiameter and 13mm deep, with the aggregate particle visible at the bottomof the pop-out. In general the pop-outs occur only at the largest pieces ofaggregate. For example, if the aggregate maximum size is 20mm, pop-outswould as a rule be observed at the near-surface 20mm particles.

1.3.2 REMEDIALThis is a difficult matter to address because, provided the presence of pop-outs can be accepted, no remedial action is necessary. Any attempt atremedial work in the form of a paint or mortar coating could exacerbatethe occurrence of pop-outs, because moisture could become trapped orinhibited from evaporation. A possible way of dealing with the aestheticproblem of these pop-outs would be to feature the aggregate exposurerather than try and hide it. This could be achieved by exposing theaggregate over the whole of the visual face, possibly using a flame

treatment or another suitable method.The use of de-icing salt can be minimised by ensuring well-maintained

drainage.

1.3.3 AVOIDANCE

Assuming that de-icing salts are likely to be used, and that efficientdrainage (including its maintenance) is unlikely to be installed, it can beadvantageous to select a coarse aggregate with a low specific heat, such as

limestone or sandstone. An alternative or addition to this precaution wouldbe to silicone (BS 6477:1992) the concrete on site, or to incorporate a water-repellent admixture such as 12% m/m cement of stearic acid powder. If awater-repellent admixture is being considered and another admixture isalso planned to be used, their compatibility needs to be assessed beforeconcrete production (see sections 5.2and5.3).

Some so-called limestones and sandstones are prone to frost attackbecause they have a significantly high water retention capacity of theirown, and a study of their track history is recommended. Flint gravels witha thick cortex surface coating (white, 13mm thick typical layer) can bealso prone to frost damage. Any such risk with these aggregates can beindicated by a simple test. Take exactly 100 pieces of the largest aggregate

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size to be used, soak them in tap water for about 24 hours, then drain offand place in a polythene bag. Seal the bag and place it in a deep freezeovernight; take the bag out the following morning, thaw, and recount. Ifthere are more than 100 particles then pop-outs can be predicted, and the

number above 100 gives an indication of the intensity of the pop-out frostdamage.

1.4 AIR-ENTRAINING AGENTS AND FROST DAMAGE

This section is solely concerned with the reliance placed upon the rathermisconceived idea that having the specified amount of air in the freshconcrete always means that the concrete, in its hardened state, has good orimproved frost resistance. I have been involved in several cases of

troubleshooting where frost damage, in the form of severe surface spalling,has occurred for concretes with the correctly specified fresh concrete aircontent.

It has been known for over 40 years (Powers and Helmuth, 1953) thatthe critical characteristics are optimum bubble sizes and spacing factors,and not the total amount of air in the compacted concrete. The freshconcrete air content test (BS 1881/106:1983) does not give any informationon bubble size, nor on the bubble-spacing factor. An added disadvantageis that the BS test does not differentiate between entrapped air and

entrained air; entrapped air contributes insignificantly to frost protection.These bubble geometries are usually verified by a microscopic method

(ASTM, 1990) on slices taken from samples (usually drilled cores) of thehardened concrete. In all the troubleshooting cases that used this USstandard to examine the air void structure, the total amount of air wasfound to comply with the specification, but the recommended bubble sizesand spacing factors did not obtain.

Reference can be made to the British Standard (BS 5075 Part 2:1982),both for the recommended bubble geometries and for a freeze-thaw test

on samples of labcrete. Typical diameters of the spherical air-entrainedbubbles would lie in the range 0.50.05mm and the spacing factor in therange 0.20.3mm. The freeze-thaw test in this standard does reflect thebubble geometries, but the test is on a specified laboratory concretelabcreteand not on realcrete.

A European standard (BS prEN 48011:1996) is in course of preparationthat specifies a microscopic test based upon the ASTM standard.

This section is solely concerned with the form (geometries) of theentrained air, and is not concerned with workmanship factors.

An example of misapplication is described in section 3.10, to whichreference may be made when large-area scaling occurs without theoccurrence of frost.

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This defect shows itself as surface spalling, typically to a depth of 1020mmwith exposure of the larger particles of aggregate. Sometimes the spallingis in the form of isolated areas, but at other times larger areas can spall.

Confusion with the pop-outs discussed in the previous section is unlikelybecause frost spalling does not, as a rule, result in conical craters asillustrated inFig. 1.3.Also, the large-area scaling discussed insection 3.10is unlikely to be blamed, because that scaling is typically in the form ofsheets 13mm thick and covering areas from 0.1m2up to complete baysize.

1.4.2 REMEDIAL

Remove all degraded material; wash the surface thoroughly, and refacewith a polymer mortar or a frost-resistant concrete overlay. Considercomplete concrete replacement only if there are doubts about the propertyof the apparently unaffected material.

1.4.3 AVOIDANCE

If an air-entraining admixture is to be used it should comply with BS 5075

Part 2, and should be assessed on the actual concrete to be used in theworks, taking into account all the variables that are likely to occur on site.The recommended method of assessment is by either microscopic or freeze-thaw tests on representative hardened concrete samples.

The alternative to using an air-entraining agent is to use a well-mixedand compacted and cured (thermal and/or moisture) concrete with acementitious content in the range 375450kg/m3.

The cost considerations of the selected path would need to be discussed.

1.5 ALKALI-SILICA REACTION

Although the few recorded cases of alkali-silica reaction have generallyresulted in severe cracking, leading to some loss of structural integrity, theproblem discussed in this section is not this (chemical) reaction. It is, rather,the human reaction of a large cross-section of those involved in constructionto the possibility of this chemically expansive reaction causing problemswith their concrete.

Alkali-silica reaction, or ASR as it is commonly known, is generally a

long-term chemical reaction that takes place between the alkali in thecement and other ingredients in the aggregate (Concrete Society, 1987b;Swamy, 1991). Although virtually all siliceous aggregates contain some ofthis alkali-prone material, it is generally those containing strained quartz

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such as opaline that cause most problems. The reaction that takes place isthe formation of sodium and potassium silicates by a slow and expansivereaction of the sodium and potassium hydroxides with the available silica.

As there will always be some available silica, whichever aggregate is

used, the thing that needs to concern those involved is the question: Is theASR expansion that will occur going to be a problem? The longevity of somany constructions in the UK without any manifestation of such problemsindicates that the general answer to this question is No.

There is a requirement for updating on concrete performance in respectof ASR, because CSTR29 (Concrete Society, 1987a) referred to OPC propertychanges only by comparing data from three years: 1960, 1974 and 1983.The alkali equivalent of cement from those data, given as Na

2Oequiv.,

remained constant at about 0.6%m/m. Not only did these data exclude the

period since 1983, but also they did not take into account the properties ofthe (then and now) cements imported from Greece and Spain. A notuntypical analysis of one of these imported cements gave a batch-to-batchNa

2Oequiv. of 1.2%.

Notwithstanding comparisons between UK-manufactured andimported cements in their Na

2Oequiv. contents, the few cases of damage

reported (Concrete Society, 1987a) indicate that ASR is a problem whenspecific aggregate location sources are likely to be used. Suppression ofthis reaction is relatively simple, and recommendations are made insection

1.5.3.The few cases reported seem to indicate that cements are not themain problem.


On the few occasions that ASR damage has been observed it takes the formof generally random cracking, with crack apertures up to 10mm. However,the crack widths taper rapidly with depth. Secondary cracking often occursin the form of map cracking with crack apertures typically up to 1mm. A

gel-like exudation similar to egg-glass (waterglass) may be seen at theapertures.

1.5.2 REMEDIAL

If the problem is with the specification wording, which could be somethinglike Not more than 3kg Na

2Oequiv. per m3concrete, draw the attention of

the parties concerned to the lowor zero-risk situation if that concrete hasalready shown a good track record.

If the problem is an actual risk, and damage has occurred, then eitherremoval and replacement of the damaged concrete and/or a protectivewaterproofing render will generally inhibit further damaging reaction.



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1.5.3 AVOIDANCE

If it is definitely known that there is a risk of damaging ASR then this canbe inhibited by specifying an additive such as PFA, GGBS or MS in theconcrete mix design.

It is worth observing that, many decades ago, ships delivering cargo toLondon from the USA went back with Thames Valley aggregates andcements as ballast, from which much of the currently standing New YorkHarbour was built. These concretes are still performing well, even thoughthey are subject to wetting and drying conditions and sea water effects.

1.6 CALCIUM CHLORIDE

The use of calcium chloride is now generally banned in specifications. Themain problem that used to be encountered was that corrosion of thereinforcement was accelerated if water and air penetrated to the rebars.Calcium-chloride-induced corrosion rarely occurs nowadays, but instancesstill come to light. Calcium chloride admixture accelerates both the settingand the hardening rates of Portland cement concrete. The main attractionof this was to the precast concrete industry, and the use of calcium chloridewas concentrated in the 1950s and 1960s. It enabled manufacturers toachieve adequate demoulding strengths at earlier ages and/or a decreasein the cement content. This latter route was often chosen, and led toconcrete that contained more voids than would otherwise have been thecase, thus giving less protection to the reinforcement.

Steel reinforcement has a passive oxide layer on its surface when it issurrounded by a highly alkaline environment such as uncarbonatedcement. This high alkalinity can be reduced by a carbonation front reachingthe steel or, much more rapidly, by chloride reaching the steel. Once thispassive oxide layer breaks down, owing to either or both forms of ingress,the steel becomes subject to corrosion.

CP 110, the old (and now superseded) code of practice for structuralconcrete, used to recommend a maximum of 1.5% anhydrous calciumchloride m/m cement. There also used to be a commercially available OPCinto which this level of calcium chloride had been added. The materialwas sold as 417 Cement. It is possible that if this admixture had beenaccompanied by a recommendation on the minimum cement content, andthis had been adhered to, the status quo might be different. The structuralconcrete division of one large precast manufacturer used to use calciumchloride regularly in structural concrete units. As far is known, no case ofcorrosion of reinforcement in any of the companys units has ever beenbrought to light. The reason for this probably lay in that companys strictattention to quality control, and in its use of a high minimum cementcontent.



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Although it is a dubious thought, because the addition of calciumchloride would accelerate the corrosion risk with mediocre or poor-quality concrete, it could be a useful means of accelerating trouble thatwould otherwise have occurred rather late in the day. The facade shownin Fig. 1.4 was of white cement precast concrete architectural units, whichbecame corrosion damaged at 23 years old. The word quality glimpsedin the lower right corner of this photograph obviously applied to thecars, but it has been misused as an adjective (seesection 4.3)instead of anoun.


This defect is seen as spalling and/or cracking, with cracks commonlymirroring the rebar positions, together with rust staining marks at the crackapertures. When pieces of concrete are broken off to expose rebars, a slimy

Fig. 1.4Chloride-accelerated corrosion in precast concrete cladding.



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green coating may be seen on the steel surface. This is ferrous chloride,which, within some seconds exposure to air, turns into the brown ferricchloride. An additional test for chloride is to rub a finger over the steel andtaste it, or even lick a piece of spalled concrete. This will generally give a

salty taste.

1.6.2 REMEDIAL

Following the Concrete Societys recommendations (Concrete Society,1984), effect a repair after removing all degraded and suspect concrete andapplying a protective coating to the rebars. Even concrete where there isno visible damage needs to be assessed, as those areas may have corrosionoccurring at the steel, and/or the quality of the covercrete might not be

acceptable.

1.6.3 AVOIDANCE

The answer is simple: do not use calcium chloride admixture. If excesschloride is likely to find its way into the mix, either by deliberate additionor by the use of contaminated materials, then the use of additives isrecommended. Where this is to be applied as a remedial action then thecementitious content should lie in the range 375450kg/m3and have 30%

PFA, 70% GGBS or about 7% MS cement replacement.

1.7 ALUMINOUS CEMENT

This is the updated description for what used to be known as high-aluminacement or HAC. The problem with this cement is that it is subject to whatis known as conversion. This is an additional chemical change (followingthe initial hydrates formation) in the calcium alumino-hydrates, promotedunder conditions of high humidity and temperature. These later changes

generally cause a significant loss in strength (2050%), andpossibly moreimportantlythis is coupled with an increase in the permeability.

Virtually all aluminous cement concretes were produced as precast unitsby three main manufacturers specialising in the production of prestressed,pretensioned concrete products. This production was undertaken mainlyin the years 19461975 but generally ceased at the time of the bans and partbans produced by the Government in the mid-1970s, following the partialcollapse of two constructions. The popularity of this cement was probablydue to its rapid build-up of strength: for example, 28-day equivalent OPCconcrete strengths were obtained after 23 days. The setting times ofaluminous cement concrete were (and are) not significantly different fromthose of OPC concretes; only the hardening times differed.



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Many organisations involved in consultancy or testing have carriedout examinations and tests on buildings where aluminous cementconcrete had been used. The prime purpose of these exercises was toassess the concrete from the structural viewpointthat is, to assess the

effect of any conversion on the strength of the concreterather than tostudy the permeability changes or any other design implications.Troubleshooting work has been undertaken in both the conversion andthe permeability areas.

In all, about 1000 constructions were examined (for this purpose I havecategorised a number of dwellings on a housing estate as a singleconstruction). Over the three decades referred to above about 15 000 000precast units were manufactured, and these were built into 60000constructions. The number of constructions would probably lie in the

several hundreds of thousands if each dwelling on a housing estate hadbeen separately counted. So a very small proportion of the total number ofconstructions was examined: about 1.5%.

About four constructions out of these estimated 1000 were foundwanting because of either loss of structural integrity or degradation of theconcrete by chemical action. Four out of 60 000 is a very small proportionof the total number of units manufactured.

Personal experience in site examination was directed towards examiningsamples of concrete in the laboratory, using differential thermal analysis to

estimate the degrees of conversion. Most analyses gave a range ofconversion levels of 7090% with no noticeable distress due to structuralcauses. Webs of T-section precast concrete beams were examined usingUPV but, again, problems were seldom found, and, when present, werenot at significant levels. The problems found were in two areas: detailingat supports and the previously mentioned chemical attack. This risk hasnever previously been emphasised in any warnings as far as is known.

As an illustration of the first problem area, visits to two schools with theprecast units open to view (not hidden by a suspended ceiling or similar)

showed that the beam ends sometimes rested on column haunch bedlengths of 12mm. This was observed with children and teachers in theclass. The classrooms so concerned were immediately evacuated, andremedial work was put in hand. No other undue distress or deflexions ofthese precast elements were observed.

The second problem area was highlighted in concentrating surveys inareas of construction where there were high humidities and warmconditions, such as kitchens, bathrooms and roof areas. It was in roof areasthat several cases of chemical degradation were observed. In each case,

woodwool slabs were used above the units, and condensation and/orleakage was taking place. It was ascertained that the wood fibres inwoodwool slabs were commonly stabilised (to prevent organic growth) bytreatment with calcium chloride. Effluent from condensate or leakage



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resulted in chloride attack on concrete made more prone to attack byconversion, causing increased permeability. The attack was mainly due tothe formation of calcium alumino-chloride, and the surface had becomesoftened to a depth of about 10mm. Although in some cases chloride was

found to have reached the prestressing wires, no problems had occurred.Although, like OPC concrete, aluminous cement concrete is subject tocarbonation (BRE, 1981a) this mechanism was not found to have caused aproblem.


Problems with aluminous cement can be identified by:

(a) insufficient support bed lengths;(b) chloride chemical attack, resulting in surface softening.

1.7.2 REMEDIAL

Where (a) applies, stainless steel or equivalent brackets should be fixed tothe columns to extend the bedding lengths to an acceptable figure. Shimsof stainless steel or similar would probably need to be placed in the gapsand grouted up to give continuity. Any shims used should not give rise to

bimetallic corrosion reaction with the brackets.Where (b) applies, carry out remedial work to inhibit or preferably stop

condensation and/or leakage. Units may have their degraded surfacematerial removed if required and refaced with a polymer mortar.

1.7.3 AVOIDANCE

The best way of avoiding problem (a) is probably to design adequatetolerances and to ensure in both precast and in-situ concrete work that

these tolerances are achieved.Concerning (b), it is best to ensure that areas such as roof spaces,

kitchens, bathrooms and similar risk zones have their precast units in anadequately ventilated and leak-and condensation-free environment.

1.8 STEEL REINFORCEMENT: ADDITIONAL REQUIREMENTS

In addition to the normal structural need for reinforcing steel in concrete,two other problem areas were often met where steel was found to benecessary for other reasons. Although the two instances applied to precastconcrete, there could well be cases when a similar need could arise forinsitu concrete.



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The first additional requirement was for steel in the compression zonefor handling purposes. The problem concerned precast concrete step units,as illustrated in Fig. 1.5. Structural reinforcement had been placed in thetop of each step to counteract cantilever forces. What had not been

considered was that, although the units were delivered to site in goodcondition, each unit was lifted manually from the truck and taken to thestorage area. The two operatives involved in this lifting exercise causedthe units to behave as end-supported beams, and each unit cracked underthe bottom shoulder adjoining the boss.

Although this particular saga continued in a manner unrelated to thissection heading it shows how important it is to pay attention to handling.There were 14 of these units made for a spiral staircase to be prestressed toa ground anchor after installation. After making, scrapping and remaking

(with additional bottom of step steel), the units were put in place andprestressed without any packing mortar between the moulded andtrowelled faces. The spalling that occurred was the outcome. At theinstallation of the third remake units with 10mm thick mortar packingbetween the joints, only 13 could be installed up to the landing level,

Fig. 1.5Compression-induced spalling in step units.



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leaving a gap in the spiral. As far as is known these units were dismantledand a wooden staircase was installed.

The second problem was for a steel requirement in what was thought tobe the compression zone. This need was probably because of differential

moisture and thermal movement between the top and bottom of thesection. The units in question were architecturally faced duct covers, eachabout 1m square in plan by 80mm thick. The top of each unit was facedwith a cast stone finish of about 30mm nominal thickness, and theremainder was made up of an OPC concrete with 10mm maximum sizeaggregate.

Figure 1.6 illustrates the placement of each duct cover over anenvironment of high humidity and a reasonably constant temperature,with the visual face exposed to the weather. Shrinkage cracking occurred

on the top face of virtually all the units, and although these cracks did notapproach anywhere near the interface between the two mixes, let alonethe steel in the backing concrete, the units were rejected. They wereremanufactured with a light stainless steel mesh at about mid-depth in thefacing, and no further cracking was reported, as far as is known.


The symptom is cracking in the design compression zone. This cracking

tends to exhibit a rather random pattern, generally unrelated to the dryingshrinkage and thermal cracking discussed in sections 1.1and1.2.

1.8.2 REMEDIAL

If the crack apertures remain nominally static, and crack depths are notconsidered to be a corrosion risk, then consider a grinding to clean offdetritus at the crack apertures, followed by a silicone treatment. If there arestructural implications because of the cracking, seek the advice of a

chartered engineer concerning such actions as replacement, stitching inextra steel, or other solutions.

Fig. 1.6 Visual-faced concrete duct cover unit (not to scale).



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1.8.3 AVOIDANCE

When detailing reinforcement requirements, consider the need foradditional steel to withstand either handling needs and/or differentialthermal/moisture movements.

1.9 EXCESS STEEL REINFORCEMENT

Reinforcement is generally put into concrete to cater for its relativeweakness in tension compared with compression. The term cross-sectionalarea (CSA) is used to refer to the area of the section under consideration,both for the concrete and for the steel. The ratio of the area of the steel tothat of the concrete is the percentage of reinforcement, which for concretesections such as slabs, beams or columns could typically be 35%.

Many cases have been encountered where percentages of reinforcementof up to 25% have been used. These have led to problems on site, in theprecast concrete factory and in mix design at the preliminary stage.

One problem that was examined concerned precast columns in abuilding (Fig. 1.7). The columns were about 3.5m tall and 0.3m squaresection in plan. Four 40mm diameter bars, one at each corner with 40mmnominal cover, made up the main reinforcement. The main rebars were lappedby 10mm diameter stirrups at approximately 0.3m centres. The mix was a20mm/10mm/5mm to dust limestone aggregate with a 450kg/m3whitePortland cement content and a total water/cement ratio of about 0.5 with a

Fig.1.7Uncorroded rebar-induced spalling in new precast concrete column (notscale).



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slump of 100mm. About three months after installation on site, severevertical cracking with no steel corrosion occurred in lines withaccompanying spalling in lengths up to 0.5m long. This was diagnosed asprobably being due to there being too much rebar restraining influence on

a concrete with a high initial hydration shrinkage potential.Another example where excess reinforcement affected mix designconcerned bifurcated in-situ white concrete columns, where congestion ofreinforcing bars at the crossover resulted in there being about 25% steel ofthe CSA in plan at the throat. The original mix design, using a 20mmaggregate with a 75mm slump, had to be changed to a 10mm aggregatewith collapse slump. Fortunately, a Portland limestone aggregate was used,and the 3060 minute aggregate suction effect on the excess water contentgave cube results that built up quite significantly after four days. The

specified cube strength was obtained about a week later.


The problems that occur from using excess reinforcement are numerous.The list below highlights those that have been commonly experienced:

(a) pieces of tie wire and detritus on the soffit;(b) cracking mirroring the main rebars, without steel corrosion;(c) shrinkage cracking due mainly to the use of too much water and/or

too dusty an aggregate in the mix;(d) honeycombing above the steel due to the close-packing of the rebars,

allowing fine material sole passage.

1.9.2 REMEDIAL

(a) Remove tie wires, detritus etc. from the face of the concrete as soon aspossible, and reface cut-out areas with mortar of the same mix as thefine material in the substrate concrete.

(b) On the assumption that by the time the problem is observed most ofthe hydration shrinkage considered responsible for the cracking willhave taken place, remove all unsound concrete and repair (ConcreteSociety, 1984).

(c) Repair shrinkage cracking, if considered necessary, as described insection 1.2.2(b).

(d) Cut out and replace honeycombed zones with concrete or mortar togive a weathering match.

1.9.3 AVOIDANCE

The general way to avoid most of the problems listed is either to ensurethat no more reinforcement is put in than is needed and/or to distribute



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the placement of the main bars so as to spread stress shrinkage effects ontothe steel more uniformly. In addition, allow as much room as possible forconcrete to flow through the reinforcement, and use workability-promotingadmixtures and/or additives where possible.

Where high-workability mixes are necessary, consider the use ofaggregates with a suction Vacuum concrete effect. Not only can this assistin extracting some of the excess water, but the increased wetness of theaggregate can also assist in the moisture curing of the concrete.

1.10GRC AND ALKALI-GLASS REACTION

Publications on this subject (Swamy and Barr, 1989; Majumdar and Laws,1991) have tended to concentrate on the design and use of GRC and

composites rather than on materials science considerations. The problemencountered on several sites has been slight surface softening to a depth ofless than a millimetre but with exposure of fibre ends, which exhibited abrittle nature. The detriment was solely aesthetic in nature.

Concerning the chemistry of cement and glass, this form of degradationwas predictable, but a little explanation at a basic level may be useful. Whencement, mortar or concrete is splashed or otherwise brought into contactwith window glass, etching occurs. This is because the alkali in cementattacks some of the silicates that are used in glass manufacture. The stock

used in making glass fibres has better alkali resistance than window glassbecause zirconia is used as one of the constituents.

In order to improve the alkali resistance further, a polymer coating isapplied to the fibre as it is drawn from the melt and before it is choppedinto strands or rolled into spools. The chopping process can take place atthe manufacturing stage, or in the works, or on site during sprayapplication from the spool of glass fibre. Whichever method of cutting isused, the cut ends as well as polymer-bruised fibre sides (bruised duringthe mixing and/or compaction stages) lose some of the coating

protection.There is also a thermodynamic process concerning heats of solution

of cement and glass. It means that more heat is required per unit weightto make cement than to make glass. The typical clinkering temperatureof cement is about 1200C, and the melt temperature for glass is about800C. When the two materials are put side by side, especially in anenvironment where chemical reaction can take place, there will be atransfer of heat energy from the cement to the glass in the form of achemical reaction.

It can therefore be predicted that unprotected GRC exposed to theweather, or to other conditions where water or other solutes can contactthe surface, will have the fibre ends and bruised areas attacked.



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The symptom is exposure of fibre ends on the surface, with accompanyingslight dusting to a fraction of a millimetre deep. With the assistance of amagnifying glass it might be also possible to observe some hollowing out

of the fibre ends. This would be the residual thin skin of the polymerappearing as a pipe.

1.10.2 REMEDIAL

Rub down with a fine-grade disc, carborundum stone or similar toremove all softened material. Apply a silicate-based or polymer-basedpaint system to give an approved finish. The silicate systems are preferredas they have a track record of good performance exceeding 100 years,and this thus avoids continuous maintenance apart, possibly, fromcleaning.

1.10.3 AVOIDANCE

If GRC is likely to be subject to the exposure risk mentioned above, the useof a protective silicate-based or polymer-based paint would greatlyimprove the performance. The alternative would be to apply a rich (e.g. 1/

1) fine mortar 12mm thick to the risk face before laying up the GRCsystem. A protective coating, as described above, could also be consideredas an additional precaution.

1.11FIBRE-REINFORCED SANDWICH PANELS

Although the problem met on several sites was with GRC sandwich panelsthe same situation would almost certainly have arisen if the fibres hadbeen of fibrillated polypropylene, steel or carbon or other fibre. The

sandwich panels in question were of half to full-storey height, and hadinner and outer fibre-reinforced sheets about 812mm thick as well as sidewalls of the same material. The sandwich consisted of an expanded plasticsor mineral wool with a thickness of about 100mm.

The problem that occurred was of cracks of up to 2mm aperture at thevisual face edges. Where moisture had gained ingress into the sandwichthis cracking was sometimes accompanied by a lime leachate exudation atthe crack apertures.

As this cracking had not been reported from the returns or corners for

cases of single-skin cladding, it seemed that temperature variations couldwell have been responsible. Not only was the outer skin exposed to theexternal temperature and general weathering conditions, but its responsetime was rapid compared with a monolithic solid typical concrete system.



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The webs and rear skins of the panel would have remained comparativelytemperature static at most times, and there would therefore have beenconsiderable thermal differential strain at the face-web junctions.


The symptom is cracking at the face-web return junctions, sometimesaccompanied by a lime or carbonated lime leachate from the cracks.Differential movement may also cause the panels to bow in a convexmanner. This could be assessed with a straightedge, but should not beconfused with any convexity that may have been there when the unitswere made.

1.11.2 REMEDIAL

Although no remedial work was undertaken as far as is known on the sitesvisited, it is difficult to recommend anything that can cater for suchcracking. If, as is likely, the cause is a thermal one, then the cracking islikely to be live. Therefore, if a crack-filling material is to be used, thisshould be a low-modulus material capable of moving with the changingcrack apertures. The repair should be effected when the cracking is at themaximum opening so that the sealant used will generally be under

compression.

1.11.3 AVOIDANCE

Wherever possible, avoid the use of sandwich panels and consider usingsingle-skin cladding. If sandwich panels have to be used they could bedesigned so that the web-face skin junction is hinged rather than rigid.

1.12DELAYED ETTRINGITE FORMATION

The problem encountered was more one of trying to find instances of DEFthan of DEF causing distress. Time will tell whether DEF is a real problem;at the time of writing this book there is little or no indication of this.Ettringite is calcium alumino-sulfate, and in cement it is formed by thereaction of sulfate with the calcium aluminate in the cement. Ettringite isdeliberately present as a reacted product in Portland cement concrete andmortar because gypsum (calcium sulfate) is added during the clinker-grinding process to control the setting and hardening properties. Withoutthis addition, the speed of hydration would be too great for practical use.This ettringite is harmless, because it is probably in the form of well-dispersed microcrystalline particles. For ettringite to cause distress it would



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have to be larger than microcrystalline in size, and would need to form inthe hardened concrete. If such a reaction occurred solely at the surface ofthe concrete the effect would probably be that of surface softening, asdiscussed in section 1.1.1 (a).

The only observed case that could possibly have been attributed to DBFwas in the exposure of sections of hydraulically pressed kerbs containinghigh proportions of high-sulfate PFAs. This observation referred to a long-term laboratory study of units made in precast concrete works. The PFAloadings that caused trouble were 1.52.0 times the cement contents, andwith sulfate/PFA levels up to 1.8% (Levitt, 1982, section 4.2).After about10 years weathering, with no vehicular access, two of these kerb sectionswere found to have split cleanly through their mid-sections with no sign ofimpact. However, white crystalline deposits of up to 10mm in diameter

were observed at the broken faces. These deposits were found to be rich insulfate.The kerbs manufactured by hydraulic pressing were partnered by kerbs

made by Kango hammer tamping. None of these hammer-compacted kerbsections exhibited distress on weathering. This could have been due to theirweaker and more elastic properties, coupled with the relatively largernumber of voids that could have accommodated expansion products.

Manufacturers of hydraulically pressed products would be unlikely touse such high PFA loadings, because this would necessitate an unacceptable

increase in the pressing time.


If this problem is ever found it is likely to be in the form of severe crackingand/or spalling, with visible ettringite-rich white crystalline deposits atthe broken face. This is a prediction based upon a single observation inlaboratory samples and not from a site.

1.12.2 REMEDIAL

Not enough experience is available to address this point. If damage wereto occur on site in similar vein to that observed on the hydraulically pressedkerb samples (assuming that it was due to DBF) then it is likely that majorremedial work or replacement would be necessary.

1.12.3 AVOIDANCE

Until actual site problems become documented and the mechanism isunderstood and accepted it is not possible to take avoiding action. As faras is known at present, there does not appear to be anything to avoid.

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Health and safety 2

INTRODUCTION

The three examples in this short chapter reflect personal experience in theindustry coupled with involvement in litigation work. Health and safetymatters such as protection from falling objects, safety clothing, and the useof mechanical and electrical machines are well documented in companyprocedures, and are not covered here. The three risk areas discussed heredo not seem to have attracted the attention that they should. They should

interest not only safety officers but also site managers, operatives andpersonnel officers.

Because the first two sections refer to cement chemistry, it may be helpfulto put cement into perspective by comparing it with another chemical thatis commonly found on site and in the precast works: concentratedhydrochloric acid.

Not only is hydrochloric acid a simple two-element chemical, but it isone of the few acids that is a reducing agent and not an oxidising agent likenitric or sulfuric acid. Hydrochloric acids fuming property, its pungent

smell and (usually) delivery in glass carboys, coupled with its ability toetch concrete with accompanying bubbling, cause it to be respected.However, because human skin is generally acidic (except for the eyes), andbecause that acid is dilute hydrochloric, then provided the skin has nolesions, spillage of fuming hydrochloric acid onto the skin causes littleharm. It is not a caustic oxidising acid: it does not attack flesh.

Compare cement, which arrives as a dry powder, and commands verylittle respect. However, it should be treated with a level of safetyconsciousness that makes hydrochloric acid pale into insignificance. There

are two reasons for this, and they are both based upon the fact that water isto be added to cement, which is a multicomponent chemical. First, one ofthese components is water-soluble hexavalent chromium. Second, causticalkali is released. These two risk areas are discussed in the first two sections.



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2.1CEMENT ECZEMA

Chromium (chemical symbol Cr) is a minor ingredient in OPC. It is presentat a typical concentration of about 6mg/kg or 6ppm (parts per million). Asfar as is known, this figure represents the total Cr and not the water-soluble

part. I have encountered only a few cases of skin problems, and so it isreasonable to assume that UK cements have low contents of solublechromate. Chromium does not occur in the form of a heavy metal but as achromate salt. When this salt is present in its hexavalent Cr form it is water-soluble, and it is in this form thatfor some personnel-eczema, dermatitisor, more rarely, skin cancer can occur.

There is nothing new in the knowledge of this risk factor; it was firstreported over 30 years ago (Burrows and Calnan, 1965). In Denmark,research has been undertaken more recently into ways of inhibiting this risk

(Aunstorp, 1989a, b, c). Ferrous sulfate was added to Danish cement, andthe effects on operatives before and after this addition were reported. Danishlegislation then followed, with a limit on the maximum amount of solublechromate salt permitted in Portland cements, expressed as ppm of Cr.

Aunstorp found that the allergic reaction to the chromate in the cementwas a much more significant factor than attrition brought about, forinstance, by fresh concrete or mortar being rubbed onto the skin. TheDanish research was based upon a real situation, and was not a laboratoryassessment. Interestingly, the study found inter aliathat the use of protective

creams before starting work with concrete did not ease the irritant effect ofcontact with wet cement. The wearing of protective gloves had only amarginal effect in counteracting any allergy or in inhibiting irritation.Irritation, in all cases, would be the first sign of a skin reaction. The lastexamination of the Danish cement specification revealed that themaximum limit for the soluble chromate salt, as Cr, was 2ppm.

Publications by the Health and Safety Commission/Executive (HSC,1988a, 1988b; HSE, 1988) drew, seemingly, rather marginal attention tothese risks. However, at the time of writing it seems that an updated

document is due for publication.


The symptoms of cement eczema are irritation of the contact area, probablyaccompanied by discoloration and a blotchy appearance. If the effect ofcement burns is occurring at the same time (seesection 2.2),skin irritationis unlikely to be felt because of the damage to nerve ends.

2.1.2 REMEDIAL

Wash affected areas immediately with copious quantities of water, andremove cement-stained clothing. Seek medical advice as soon as possible



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after this, and ensure that there will be no future work likely to result incontact with fresh concrete, mortar or grout.

2.1.3 AVOIDANCE

Most people are relatively insensitive to the risk of cement eczema, and soit is advisable to question all personnel at the pre-employment ordeployment stages. Has the person and/or any member of that personsfamily ever been prone to allergic skin reactions or complaints? The 1965study reported earlier indicated that hereditary factors can play a role in apersons proneness to a reaction. For adequate documentation, the posingof these questions and the answers received need to be accurately recorded.

As far as protective handwear is concerned, only waterproof gloves

known to be unreactive to chromates should be used. Handcreams onlyprevent the skin from drying out; they do not offer resistance to thechromate salt.

2.2CEMENT BURNS

Throughout this section the term cement burns has been used. In USAlitigation reports and published articles the term concrete burns isgenerally used. It is the chemistry of the wet cement that causes burns, and

so cement burns is probably the more explicit term. Abrasion by theaggregate and/or cement on the skin exacerbates the caustic chemicalmechanism involving necrosis that is considered to be responsible.

The UK problems I have encountered have generally been whereoperatives have been kneeling and carrying out floor-topping work. Othercases, such as concrete getting inside a Wellington boot, have beenillustrated in the UK press, and these are identified later. Severe injury wassuffered in all cases, resulting in the inability of the persons concerned tocarry out further manual work. Continuous, permanent pain and unsightly

skin grafts were also common.The incidence of cement burns was recorded nearly six decades ago in

an American medical journal (Meherin and Schomaker, 1939), but thereappears to have been a dearth of reports in the literature for more than twodecades thereafter. Rowe (1962) described cement burns as unusual. Manycases of litigation took place in the USA, with Erlin, Hime Associates, aconsultancy practice in Illinois, not only acting for claimants but alsobuilding up a useful dossier of case histories and references to relevant USlegislation. At the beginning of my involvement as an expert witness acting

for claimants in a county court case, a close liaison was set up with Erlin,Hime Associates. Their earlier 1980s publication (Hime and Erlin, 1982)had a significant impact upon both US and UK litigation. This US



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publication did not pursue a detailed mechanism of the form that isproposed later. It will be argued that there is a simple theory capable ofexplaining the cement burn mechanism and of showing why the risk is farmore serious than previously considered.

Althoughsection 2.1refers to the possible issue of an updated Healthand Safety Executive publication, the two information sheets publishednearly a decade ago appear at present to be the sole UK guidance (HSC,1988a, 1988b). The references therein, together with warnings in documentssuch as delivery notes and safety instructions, will be highlighted later inthe proposed theory as not being detailed enough.

One of these publications contains a photograph of a cement burn to theknee of an operative. Three further publications appeared a few years later(Anon, 1993a, b, c). No reference can be made to personal case history

experience here as one of the cases was settled out of court and anotherone is current as at early 1997.It is not known how many cases of cement burns have occurred in the

UK without being reported in the press. The pressures being placed uponcontractors and subcontractors could have resulted in precautions beingsacrificed for the sake of speed and/or economy. However, the apparentlylow incidence of cement burns in the UK implies that a high quality of careis being exercised on site, in the works and plants.

In 1988 Hime and Erlin reported a study where research by others was

referenced, without much detail. This stated that trouser material acts as achemical buffer and increases the alkalinity at the skin, with an increase inpH from about 12.8 to nearly 14. It was not understood how material fabriccould cause this, and I have conducted laboratory tests using fresh cementmortar on one side of various fabrics such as worsted, linen, denim andnylon. In all cases the pH was found to be about 12.5 on both sides of allmaterials tested, with no significant gradients.

It is therefore logical to turn to what is known about personnel sufferingcement burns on site. Two factors seem to be necessary. First, the spillage

needs to be static and in contact with the skin for at least half an hour.Second, the skin where the spillage has taken place needs to be on arelatively warm part of the body. Cement burns do not appear to occurwhen the operatives hands are in and out of concrete.

These two conditions need to be correlated with the known cause of cementburns, which is necrosis, or in simple terms a caustic burning effect on skin,nerves and muscle. The most critical of these three effects is the destruction ofnerve ends, because this dulls any feeling of burning or irritation. This isprobably why operatives carry on working after spillage, not knowing that

flesh burning is continuing. The 1993 Construction Newsarticle (Anon, 1993c)describing spillage into a Wellington boot is an example of this.

The presence of alkali in the forms of sodium and potassium hydroxideswas mentioned in section 1.5and expressed as Na

2Oequiv. A typical level



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for OPC would be about 0.6%. This is equivalent to approximately 0.4% assodium hydroxide. This is known as caustic (i.e. it burns flesh) soda. Boththe alkaline lime and the caustic soda contribute to the alkalinity of cementpaste. What is important is that lime is alkaline without being an alkali,

but caustic soda is both. Furthermore, lime is only slightly soluble in water,whereas the alkalis are very soluble.Consider now a realistic scenario: an operative kneeling on fresh

concrete so that the chemicals in 1kg of concrete can access the skin. If thecement content is say 350kg/m3, then that 1kg of concrete will containabout 150g of cement, assuming a fresh wet concrete density of 2350kg/m3. At a 0.4% equivalent caustic soda level the operative is in contact with0.6g of one of the most caustic chemicals known.

Its uniform distribution throughout the contact area explains why body

heat and time are jointly required. As body heat causes the water in theconcrete to evaporate, the effective concentrations of both lime and thealkalis increase. Lime, being only fractionally soluble in the moisture onthe skin, will not cause much distress. The approximate half gram of causticalkali would be responsible for the necrosis that occurs.

This explains why the combined conditions of both warmth and timeare necessary, and why a continuous replenishment and/or renewal offresh concrete with the skin does not apparently cause the same distress.The alkalis do not have the chance to become concentrated by evaporation

of the water. In addition, it may be predicted that the abrasion effect ofcontact with aggregate will have an exacerbating effect on flesh alreadyundergoing necrosis.


The symptoms are soreness experienced after some hours, followed,usually the following day, by severe pain and ulceration, with a flesh colourvarying from green to purple and, generally, an accompanying discharge.

2.2.2 REMEDIAL

It is possible to ameliorate the situation slightly by immediate washingof the affected area with copious quantities of water, coupled by theremoval of all affected clothing. This clothing should be washed ordiscarded. The US articles referred to earlier advise not to cover the areanor to apply any form of dressing. Immediate hospitalisation is advisedwith, preferably, a department qualified to deal with cement burns. The

patients contact with cement and caustic alkalis needs to be mentionedto the hospital staff.



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2.2.3 AVOIDANCE

Protective waterproof (the latter word being commonly omitted in safetyguidance) clothing should always be worn. If operatives have to kneel onfresh cement then special knee covers or string-held cut-outs of car tyres

should be used. Any cement ingress behind these should be treated as insection 2.2.2.

2.3PUMPING GROUT

Typical uses of grout pumps are for operations such as filling the annulibetween prestressing post-tensioning strands/wires and the duct tubes,and for filling gaps under large machine baseplates. It was in the formerapplication that an operative suffered injury to an eye that nearly resultedin blindness of that eye. A blockage occurred in the tube line to theprestressed unit, and a coupling was loosened to open up the tube.Admittedly the person concerned should have been wearing eyeprotection, but he assumed that because the machine had been turned intothe recirculation mode, pressure in the feed line had been relieved. He didnot appreciate that, with the particular machine in use, the pressure hadnot been taken off the feed line.

The pump supplier, when advised of this mishap, informed the projectmanager that there was a third position for the control tap. In addition tothe 12 oclock recirculation and the 3 oclock line pressure positions, therewas another one between 1 and 2 oclock that took the pressure off the feedline. It was admitted by the pump supplier that this omission in theinstructions for use was an error, and that it would be rectified.

Figure 2.1 shows a simple schematic outline of the system.

Fig. 2.1Grout pump system (not to scale).



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This potential problem can be identified as non-existent or inadequatesuppliers guidance on how to deal with blockages in the system.

2.3.2 REMEDIAL

Ask the supplier for written instructions on how to deal with pressure lineblockages. If a coupling in the pressure line has to be undone because of ablockage, and no instructions are available, run the pump with the tap inapproximately the midway position between the recirculation andpressurising positions. Ensure that the operatives wear full eye and handprotection at all times, and that there is a convenient optical douche nearby.

2.3.3 AVOIDANCE

Use grout pumps only where there are distinct instructions on how torelieve pressure in the feed line should a blockage occur. Wear onlyapproved eye and hand protection, and ensure that there is a convenientoptical douche station.



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Concrete on site 3

INTRODUCTION

Most of the main problems encountered with in-situ concrete are groupedin this chapter, but inevitably there is some overlap with Chapters 1 and 5.It will be seen in Chapter 5 that the many instances of troubleshootinginvolving precast concrete products could have also applied to in-situconcrete. However, materials faults were seldom involved; the problemsgenerally related to design and/or workmanship.

It is easy to blame the material, because it is usually covered by a

stringent specification. Design and workmanship are dealt with by codesof practice, which are state-of-the-art documents, and can be interpretedin different ways. They also relate to a service rather than a product ormaterial, and are therefore difficult to define. When using materials on sitethere is a resulting tendency to concentrate unfairly on the standardisedmaterial shall wording, and to use or sometimes misuse the guidanceCodes should clauses.

As referred to in the Preface, less than 18% of all troubleshooting casesencountered were attributable solely to the material. This is not only my

experience, but also applies to many decades of Laing library problemanalyses. It leads inevitably to the conclusion that there is a misdirection ofeffort in construction quality control. The main focus of testing should beon the construction rather than on the material or product. Why is there aconcentration on specifications to resolve less than 18% of the problems?Presumably because it is fairly easy to specify materials, but difficult tospecify or control design and workmanship. Whether this situation willchange in the future remains to be seen.

3.1 COVERCRETEdORk

Although covercrete dand kare defined in the Glossary, some qualificationmay be useful. Concentration on the depth of cover, d,is universal; there is



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little or no interest in the quality, k,of the concrete in that cover. Thesevariables are generally recorded as d,the depth of cover in mm, and thelesser-known k,the permeability, in cm/s.

The problem encountered on site was due to non-compliance with a d

specification. This had secondary repercussions. First, it was not clearwhether the specification requirements were for minimum, nominal oractual cover. Second, where cover to the main steel was of concern, littleaccount seemed to have been taken of cover to the stirrups. As may be seenin Fig. 3.1, this cover is reduced by at least that of the stirrup steel diameter.

In general, no consideration was given to the quality of the allegedlyreduced cover area(s). Where this quality was considered to be good,attempts were made (sometimes with success) to achieve a deemed-to-satisfy situation with no other work being carried out.

Let us consider each of these variables, dand k,separately.

3.1.1 COVERCRETE d

Why have a din the specification? The simplest answer might be to protectthe steel from corrosion. Unfortunately, this reasoning seems tacitly toadmit an inability to qualify the answer. What is not apparently designedis whether dmaintains its protective property over the planned lifetime, orwhether it gradually loses it.

From site experience, it seems that the underlying philosophy of a dspecification is defer the fatal date. In effect, we could well assume that dshould be maximised, because the cover will lose its protectiveness withtime. However, there are drawbacks to having too much cover: fewer cracks, but each with an en

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Concrete Materials Problems