CBDG Fast Construction of Concrete Bridges

5/27/2018 CBDG Fast Construction of Concrete Bridges

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Published for and on behalf of the Concrete Bridge Development Group byThe Concrete SocietyRiverside House4Meadows Business ParkStation ApproachBlackwaterCamberleySurreyCU17 ABTef: +44 0)1276 607140Fax:+44 0)1276 60741E-ma;[email protected] Wehi2e:www.concrete.org.ukFirst published 2005

Concrete Bridge Development Group 2005ISBN 1904482 171Order reference: CBDC/ TG5All r ights reserved. Except as pe rmitted under current legislation no part of this work may be photocopied, stored in a retrievalsystem, published, perfo rmed n public, adapted, broadcast, transm itted , recorded or reproduced n any form or by any means,without the pr ior permission of the copyright owner. Enquiries should be addressed to the Concrete Society.Although th e Concrete Bridge Development Group (limited by guarantee) endeavours to ensure that any advice,recommendations or inform ation it may give either i n this publication or elsewhere is accurate, no lia bility o r responsibility of anykind (including iability for negligence) howsoever and from whatsoever cause arising, is accepted in this respect by the Group, i tsservants or agents.


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. . ..

Fast Construction of Concrete Bridges

Con entsMembers o f th e Task Group 3Figures 4

1. In t roduct ion 51.1 Background to this report 51.2 Scope 51.3 Objectives 61.4 Summary 6

7. Gener al issues relat ing t o the pro jec t2.1 Introduc tion 72.2 Initial considerations 72.3 Planning 82.4 Risk management 82.5 Land acqu isition 82.6 Utilities 9

' 2.7 Traffic management 92.8 Hea lth and Safety 92.9 Off-site fabrication 10

3. General issues relat ing t o t he struc ture 113.1 Repet ition 113.2 Simp licity 123.3 Top-down construction 123.4 Permanent formwork 123.5 Maintenance and durab ility 163.6 Quality and aesthetics 163.7 Concrete 173.8 Reinforcement 183.9 Adhesive connections 193.10 Travelling falsework 20


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4. Substructures 214.1 General 214.2 Foundations 224.3 End supports 234.4 Othe r buried structures 27

5. Superstructures 31~5.1 Precast concrete units 31

5.2 Precast segm ental bridges 32~5.3 In situ balanced cantilever bridges 33

5.4 Incrementally launched bridges 345.5 Transversely slid or rolled bridges 355.6 Jacked boxes 36

6. Other s t ruc tures 396.1 Gantries 396.2 Footbridges 39

7. Case Studies 418. References 55

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AcknowledgementsThanks are due to the members of the Task Group who contributed t e x t and illustrationsfor this Guide.

Members of the Task GroupRod McClellandBrian BellSimon BourneJohn ClarkeTony NorfolkAlan PhillipsPat Stan eyGeorge Toote llCiles WaleyDavid Williams

Alfred McA lpine Capital Projects (Chairman)Network RailRobert Benaim AssociatesThe Concrete SocietyKent County CouncilMay GurneyMott MacDonaldC V Buchan LtdEdmund NuttallJacobs Babtie

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I . Introduction

1.1 Background to thisrepor t

Society and the construction industry are continually setting targets where speed is ofthe essence. It is necessary for concrete constru ction t o rem ain innova tive andcompetitive in this en vironment. Adequate pre-planning, precasting of elements and theuse of appropriate technology in design and construction can make concrete thecheapest and fastest material for bridge construction, with out sacrificing quality anddurability.This report has been produced by a task group set up by the Concrete BridgeDevelopment Group, t o bring together cons truction methods and details that arerecognised as contributing to speeding up the co nstruction of concrete bridges. It setsou t t o help clients, developers, designers and contracto rs to understand better thefactors that contribute to fast construction and t o appreciate some essentialrequirem ents and consequences of ach ieving a short delivery period.Fast bridge construction in this context means any bridgework tha t improves upontraditional performance, whether this is a faster overall construction period or a speedyinstallation process. However, it is essential that quality should not be sacrificed for thesake of speed; any decision regarding the construction process mus t no t compromise thelong-term performance of the structure.Speed of installation on si te is particularly relevant where the cost of trafficmanagement, road congestion or possession of a railway is found t o be significant.Efficiency in cons truction tha t results in a reduced overall time for fabrication andinstallation almost invariably results in reduced costs. The effects of cons truction workon othe r facilities and services can of ten result in very sign ificant additional costelements. Reduced disruption, whether t o a motorway or to a railway, often becomes amajor objective. However, there will be times when a shorter period of major disruptionwil l be more acceptable and get a job done quicker than a longer period of continualcontrolled interruption.

1.2 Scope There is no unique approach t o achieving fast construction . This report reviews andconsiders the main elements of a wide range of bridge types by identifying appropriatesubstructures and a variety of superstructures. Consideration is given no t only t o theconstruction and erec tion methods bu t also to the p lanning and procu rement processesthat w ill complement the design and construction details. Simple and radical designs cansupplant a clients more traditional or preliminary expectations. The use of major itemsof plant can revo lutionise both design and construction, making prefabrication of largerand heavier items acceptable and reducing on-site tim e to a minimum. Prefabrication ina protected factory environment can increase production by improving wo rkingcond itions while enhancing quality and assuring delivery on time.

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FiguresFigure 1 Parapet on Channel Tunnel Rail Link.Figure 2 Purpose-made lifti ng frame for lattice girder units.Figure 3 Lattice girder units in place.Figure 4 CRP panels used with steel beams.Figure 5 Use of expanded me tal t o form stop end a t edge beam.Figure 6 Use of expanded metal to fo rm stop end wi th large pipe passing through.Figure 7 Self-compacting concrete being used for the reconstruction of a bridge pier.Figure 8 Prefabricated reinforcement for pier cross-head.Figure 9 Traveller being used for the cons truction of a deck cantilever and stringcourse.Figure 10 Traveller being used for construction of soff it of underpass.Figure 11 Example of permanent mesh used to form box-out a t abutment.Figure 12 Underpass formed from open cell units.Figure 13 Pedestrian and cycle underpass.Figure 14 Installa tion of precast concrete beam.Figure 15 Parapet clad with masonry.Figure 16 End of two-span railway bridge constructed off-line and slid into position.Figure 17 Concrete box jacked under the M I.Figure 18 Construction of Top Lane Underpass.Figure 19 Top-down construction of aqueduct.Figure 20 Completed canal aqueduct.Figure 21 Construction of Doncaster No rth Bridge.Figure 22 Impression of completed Doncaster North Bridge.Figure 23 C onstruction of new overbridge.Figure 24 Construction of Foundry Brook Bridge.Figure 25 Completed Foundry Brook Bridge.Figure 26 Lagavooren Bridge over railw ay.Figure 27 Cons truction of New Cowdens Railway Bridge.Figure 28 Overview of New Cowdens Railway Bridge.Figure 29 General view of const ruction of Creagan Bridge.Figure 30 Installing precast units on Creagan Bridge.Figure 31 Overview of Roundhill Viaduct.Figure 32 Construction of Roundhill Viaduct.Figure 33 Overview of Cross Harbour Link.Figure 34 Construc tion of Cross Harbour Link.Figure 35 Completed River Dee Viaduct.Figure 36 Construction of River Dee Viaduct.Figure 37 Construction of Ceiriog Viaduct.Figure 38 Overview of Ceiriog Viaduct.Figure 39 Installation of gantry.Figure 40 Com pleted gantry.

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Standardisation of design details and material specification wil l con tribute t o ease andspeed of construction. Small changes in site practices can lead to significantimprovements in programme time. The integration of the design process with theconstruction methods and availability of plant and equipment will also be major issues.Minimising the variation in elements, such as the choice of a single size of pile or a singletype and strength of concrete for different elements, wi ll a l l contribute to speed ofconstruction. S implification of the e rection and installation process can be greatlyassisted by the choice of tole rances; sometimes a realistic approach to tolerances tha tare fit for purpose can meet the requirements and even produce a superior product.The report includes chapters on:

H general issues affect ing speed of cons tructionW identification of appropriate substructuresH review of superstructure alternativesH consideration of design details and suitable products.The points discussed are illus trated by a variety of case studies.

1.3Objectives It is hoped that the report wi ll encourage early participation betweencontractors/designers and clients/developers intending to construct bridges wi thin alimited time frame. A range of options w ill be available for the construction o f a givenbridge; there wil l no t be a unique solution. The report is intended to provide generalguidance and advice, while including specific examples and wider references. It shouldbecome a handbook that summarises the processes and prov ides good ideas for fastconstruction in concrete and thus promotes the use of concrete in bridges. It shouldprove to be a reminder of good practice to those experienced in bridge engineering,taking in to account durability, maintainab ility and aesthetics while a t the same time,providing less experienced professionals with details and illustrations from successfulschemes which can be used directly or developed furthe r.

1.4 Summary Concrete can continue to be recommended for construction because it is durable andboth a renewable and a re-useable material. When carefully specified and used concre teprovides a high quality and aesthetically pleasing natural appearance. With properplanning none of these attributes need reduce the speed of e rection or construction.Although precasting s often the preferred method of achieving in advance the struc turalstrength and precise physical appearance required by even the most testing ofspecifications, in situ concrete can be simplified in design and be constructed quickly.Appropriate plant and equipment can install and erect major elements or wholestructures in very limited windows of time and in a wide range of environmentalconditions.

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, 2. General issues re lating t o the project

2.1 Int roduct ion Fast construction may be demanded for many different reasons and the approach tospeeding the construction process will be influenced by those requirements.Quicker construction can lead to cost savings simply through reduc tion in the durationof overheads associated wi th the construction process; thus any steps to improveefficiency may realise far greater cost saving than the immediate operational benefit.On the other hand, the main driver for fast construction may be an external demand,for instance minimisation of disruption, and in this instance the benefits of a shorton-site construction period may dictate the ado ption of a more expensive constructionmethod.Subsequent sections of this docum ent w ill consider in detail the various techniques thatcan reduce overall bridge construction time or facilitate construction with minimu mdisruption.Where a structure is replacing an existing structure, if site constraints and/or alignmentallow, off-line co nstruction for the new work is generally a speedier way to proceed thanon-line replacement with its attendant difficulties of accommodating existing use.

2.2 Ini t ial co nsiderations If the time it takes t o construct concrete bridges is to be reduced, a rigorous approach t odesign, construction planning and management must be taken.Design decisions must be made with buildability in mind and with recognition of theconstraints on cons truction imposed by the site. The involvement of mem bers of theconstruction team during the design process can yield significant benefits in improvingunderstanding. The resulting design details will re flect the construc tor s requirements,cons truction safety issues can be bette r recognised and the cost imp licatio n of decisionscan be readily assessed.This i s not to imply that a design -and-bu ild approach is necessarily the best means ofproject delivery. The recent move towards working in partnership offers the opp ortunityto involve al l the team members a t an early stage while s till maintaining the traditionalseparation of design and construc tion functions. The key fac tor in achieving success isthe involvement of the whole team so that decisions are made with a clearunderstanding of al l the issues. It s therefore equally importan t th at the client s prioritiesare fully understood.Subcontracto rs and suppliers, through their knowledge and expertise, can also make animp ortan t contrib ution to the early design and plann ing process in order to speed upconstruction.

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Finally, whatever decisions are made on the use of methods and strategies to speed upthe construc tion process, their adoption must n ot compromise safety.

2.3 Planning Planningof any construction project is of paramount importance. Issues regardingpurpose, style, m agnitude, environment and p rogramme a l l need careful consideration,as choices to suit one purpose may no t be appropriate to o thers.Programme issues are of major importance for fast construction. Detailed planning andcareful management will be needed to make the most appropriate use of resources andto minimise the effects of construc tion on o thers around the site, e.g. rail or road users.Programming must be given priority, for lack of it could negate any endeavours tocomplete a project quickly. Fas t construction may involve off-site fabrication which cantake place over a relatively long period while a very short period, sometimes hours, isrequired for erection on site.Sufficient time must be allowed for designers and contractors to adequately plan theconstruction sequence wi th due regard to any imposed restr ictions . The client must beconsulted and have an active part in the discussions to resolve any difficulties wi th o therparties. Indeed it may not be others, but the client who imposes the restrictions.

2 4 Risk management The decision to adopt f a s t construction techn iques can increase risks; this refers to al lproject risks and no t simply safety issues. The adoption of a formalised approach to riskassessment and management is a valuable to ol in the management of any constructionproject and especially if speed is the prime cons ideration. For example, the commercialrisks involved in the overrun of a possession on a rail bridge construction project are suchthat risk management techniques are essential.Guidance on risk management has been produced by a number of organisations over thepast few years and it is not intended to deal further with this topic here.

2.5 Land acqu isitio n Availability of sufficient working access and space can help t o s implify and speed upconstruction. Speed may be achieved through the use of large lifting equipment locatedalongside a bridge or perhaps by allowing a traffic diversion while construction proceeds.Often, once a construction contract i s awarded, there is insufficient time to acquireadditional land or the configuration of the bridge precludes this approach. Considerationof these issues a t the design stage can often elimina te the restrictions tha t slow downconstruction. Here again close co-operation between the client, designer and contractoris often a key factor.

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2.6 Uti l i t ies The presence of utilities will usually have a significant influence on the constructionprogramme, but their impact can be minimised by avoiding the need for diversion.Therefore, it is-worth considering whether diversions can be avoided by redesign, forexample by changing bridge spans to move piers away from existing apparatus orwhether a l ess intrusive piling m ethod such as Continuous Flight Auger (CFA) mightallow apparatus to remain unaffected. Where diversions are unavoidable, they should,wherever possible, be undertaken as advance works prior t o the start of the mainconstruction programme.The following issues need to be addressed from the outset t o avoid time-consum ingprob lems arising during the design and construction phases:

W Appropria te provis ion must be made for apparatus crossing the bridge, e.g.a troughwithin the structural deck, ducts in the verge above the deck, hangers below the deck.Establish wh,ether cable chambers wi ll be required with in the deck span; the maximumacceptab le spacing between characters for cable pulling is typically 2 5 m.W Consider any additional loadings that need to be taken account of in the design.

Consider any special measures necessary where apparatus crosses expansion joints orW Make provision for installation of future apparatus.W Consider the effects of ancillary apparatus; for example, water main air valve could

Consider the need for future maintenance of the apparatus and space requirementso he foundations need to accommodate additional apparatus in the future?

the e ffects of expansion of the apparatus itself (e.g. pipe loops).

obstruct the footpath.for access.

2.7 Traff ic management Road closures, lane closures, rail or waterway possessions can affect the speed of bridgeconstruction. Rail and waterway are usually cases where closure occurs for a period oftime to complete phases of the works. In the case of highway bridges, if sufficient roomis available it is often preferable to divert traffic around the construction site. Closures orpossessions have to be arranged many weeks in advance. Typical periods are 36 weeksfor rail and 8 weeks for roads, although they can often be far longer; disruptive railpossessions require a minimum of 52 weeks notice and may need far in excess of this.Traffic management often plays a major part in planning for the construction of concretebridges. The phasing of works w ill be dependent upon restrictions on the number o f lanesrequired to cope with peak traffic flows. Certain traffic management layou ts can becarried out wit h a restriction in the number of traffic lanes tha t can be occupiedor byusing lanes that are narrower than standard. Motorway works can have extremely tig htschedules for traffic management wit h lane occupation changing wit hin a few hours.

2.8 Health and safety When devising methods of fast construction speed must not comprom ise safety. Therequirements of Health and Safety planning and management are of even greatersignificance when speed of construction is paramount. In addition to the normal Health

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and Safety considerations associated wit h concrete construction, other factors oftenarise when working next t o live traffic or adjacent to a railway. These might includeheavy liftin g operations, working a t night or in adverse weather conditions; a possessiondoes not no rmally stop for the weather.Multip le shift working poses further problems. It is important that each new shift isbriefed on H ealth and Safety issues and that a formal handover system is establishedIt should be noted that prefabrication and the use of precast units can reducesignificantly the need for work ing a t height.Frequently, quick-se tting adhesives (see Section 3.9) and chem ical agents are used tospeed construction. It is essential that operatives using these m aterials are adequatelytrained, are aware of the appropriate COSHH Regulations and are provided wi th thecorrect personal protective equipment.

2.9 Off-Site fabrication Erection tim e on s ite can often be reduced through the p refabrication of elements in aworkshop or alongside the bridge. Examples of this include precasting of structuralelements or prefabrication of reinfo rcement cages (see Section 3.8). Over or under raillines where possession times are very restricted , complete deck elements can beprefabricated and slid, lifted or rolled in to place (see Chapter 5). An example is theHawkes Road Bridge replacement where a 17m span 1,700-tonne unit was constructedoff-line and slid into position during a single possession. The designer wi ll play animpo rtant role in the developm ent of such methods .The use of more off-site fabrication is being encouraged by various governmentinitiatives, such as the Egan Report and the general move towards modern methods ofconstruction. The improved working conditions wil l remove the risk of time being lostdue to bad weather and should improve the overa ll quality of the work.


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Genera l issues relating to th e structure 3

3.1 Repetition

Figure 1Parapet on Channel Tunnel Rail Link

3.2 Simplicity

3.Genera l issues re lat ing t o th e structure

Standardisation of details and dimensions within and between different bridges w illreduce construction tim e. Where mu ltip le uses of formwork can be achieved, time formaking and reconfigura tion s saved. In addition, the more uses that can be made offormwork and reinforcement designs, the more efficient production will become throughfamiliarisation.A secondary benefit is that other structures on the same project willappear as par t of the same family . For example, on the Channel Tunnel Rail Link (CTRL)project, while divided into several different contracts, a l l bridge parapets were designedto look similar when viewed from the outside (see Figure 1). This applied to both roadand footbridges.

This is a key element for efficiency. The more complex a component is, the more timewill be needed for production or fixing. A careful balance is needed to achieve efficiencywithout creating a bland or boring structure.Methods of handling precast units should be engineered to be simple while safe. Thereare several patented systems available that wi ll speed up the operation .

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3 General issues re lat ing to th e structureSupports that are inclined invariably complicate construction as they may need to besupported unti l the propping action of the deck is introduced. Consequently they shouldbe avoided if possible.Slip-forming, particularly or barriers, is a fast construction process and there are manyorganisations offering well-tried and tested systems. Some offer systems that wil laccommodate varying sections.

3.3 Top-down COnStrUCtiOn Top-down construction s the process by which the abutments of a bridge are formed bypiling (see Section 4.2). The superstructure is constructed in its finished position a tground level, and the soil beneath the deck excavated and the road below constructed.Clearly this technique is appropriate to situations in which the new road wil l pass underthe existing one. Examples include the Top Lane Underpass on the A64 [see Case Study11where the existing road was on an embankment and the construction of a new roadunder an existing canal [see Case Study 21. For the M8/M9 Newbridge Interchange,top-down construction was used to build two motorway overbridges. Contiguous boredpiles formed the abutments and wing walls, with the bridge superstructures being castin situ. Top-down construction was also used for reconstructionof the A34/M4interchange3.One of the major advantages of top-down construction s that it greatly simplifies trafficmanagement. One disadvantage is that piling adjacent to live carriageway requirescareful planning to satisfy the Health and Safety regulations. In addition, groundconditions for piling can be more cri tical than w ith conventional excavation, dependingon the method used. If steel sheet piles are used the bridge span may be lim ited by thebending capacity of the sheet piles and the moment connection between the pile topand bridge deck.

3.4 Permanent form workGenera1

Precast concrete

Permanent formwork4 s used to speed construction by removing he need for extensiveformwork and falsework. Economies can be achieved where the formwork becomes anintegral part of the completed structure. Permanent formwork is used in localities whereit would be difficu lt or impossible to remove conventional ormwork. It may beparticularly beneficial in situations where additional possession time would be requiredto remove formwork, such as when working close to live roads or over railways.

Precast concrete sections are often used as permanent formwork.A common applicationis the use of lattice girder units forming the soffits of composite slabs, in which theprecast units wil l contain a proportion of the reinforcement required for the finished deckslab. Figure 2 shows a purpose-made liftin g frame used to speed up the placing ofprecast lattice girder units on a bridge deck and Figure 3 shows the units in place prior tothe casting of the in situ concrete topping.

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Figure2Purpose-made ifting rame for latti ce girder units

Figure 3bttkc girder units n p L m

General issues rela ting to the structure 3

The Doncaster North Bridge, South Yorkshire [see also Case Study 31, made extensive useof lattice girder units.

F1An alternative approach is to use a precast unit to form the exposed face ofa wall orcolumn. Inthis instance, high quality concrete (for example using white cement orpigments in the mix) can be used to give the required appearance w ith Lower qualityconcrete for the infill material. An exampleof this approach was the white concrete shellunits for the tower o f the Flintshire Bridge .

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3 CeneraI i ssues relating t o the structure

Glass fibre rein forced cement CRC (glass fibre reinforced cement) is widely used as permanent soffit formwork forbridge decks, spanning between closely-spaced precast concrete beams. The CRC is non-partic ipating , prov iding no strength to the finished structure. Various flat or corrugatedprofiles are available, suitable for a range of spans. However, it should be noted that notall are currently perm itted by BA 36/906.RC panels were used for the Junction8 WestOverbridge on the M62 [see Case Study 41 to enable deck construction o con tinue whilethe m otorw ay was running below, hence minim ising he number of lane closuresrequired. .-

Glass fibre reinforced plastics CRP (glass fibre reinforced plastics) are widely used in a range of constructionapplications. For bridges,a variety o f CRP panels are available that can be used aspermanent formwork, for the decks of bridges, spanning between the main precastconcrete beams. However, it is essential that the material is correctly specified as manyof the commonly used resins are attacked by the alkalis in concrete. Generally CRPpermanent formwork w ill be non-pa rticipating and n ot contribute anything to the load-carrying capacity of the concrete. Figure 4 shows CRP panels, spanning between steelmembers, prior to the concrete being cast.

Figure 4CRP panels used with steelbeams

Expanded me ta l Expanded metal can be used as permanent formwork, generally in vertica l applications.The expanded meta l retains the concrete, though there w ill be some grout loss. If thesurface is to be exposed in the completed structure it wi ll require rendering or theapplication of some other surface layer. An additiona l benefit is that some water passesthrough the mesh, lowering he pressure on the formwork and hence reducing thesupport needed. Expanded metal can be a very effective method of form ing a stop endfor a construction oint as the rough surface requires no further preparation prior to thesubsequent phase of casting. Continuity reinforcement can be easily installed throughthe expanded metal. Figure 5 shows the application of expanded metal to form a stopend a t the junction of a slab with an edge beam. Figure 6 shows a stop end with a largepipe passing through it .

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General issues relating to the structure 3

igure 5Use of expanded metal o form s top end a t edge

beam

Figure6pipcpassing WLhUse of expanded metal o f m top endwith large

Expanded polystyrene

EL. .-

.--

Mesh with larger openings, covered wi th a layer of polyethylene sheeting on b oth faces,can be used in foundation works such as ground beams. The material is rather flexibleand is generally supported by the backfill. The reinforcem ent cage must be sufficientlyrobust to support the pressures from the backfill un til the concrete has gained sufficientstrength.

Expanded polystyrene can be used as permanent formwork to form voids in concretemembers where the stripping of formwork would be difficu lt. The materia l can easily becut or trim med t o the required shape. One practical consideration is tha t ties are

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3

3.5 Maintenance anddurabi l i ty

3.6 Qual i ty and aesthet ics

3.7 ConcreteConcrete specification

required to prevent void formers from floating in the wet concrete. (WARNING:Expanded polystyrene is combustible and care should be taken in i t s storage and use,particularly f welding or burning operations are taking place in close proxim ity.)

When fast construction s to be achieved, the design and construction sequence mustno t compromise the durability or quality of the finished structure. Adequate andappropriate supervision must be provided a t all times, particularly when novel echniquesand/or materials are being used. The finished structure should not require additional orearlier maintenance, nor should i ts design be such that it is more difficult to maintainthan one constructed conventionally.Some of the off-site techniques described later are likely to improve durability, e.g. thecon trol of the cover and concrete quality for precast concrete beams, which w illgenerally be better than for in situ construction. Other techniques have the potentia l toadversely affec t durability. Consideration should be given to concrete cracking due toearly-age therm al effects; some admixtures can offer advantages in this area. In addition,the construction process may mean that high loads are applied at an early age, with theresultant risk of cracking. The expression get it right first time is very relevant to fastconstruction, as remedial work may seriously disrupt the construction programme.

Achievement of an acceptable quali ty standard depends largely upon the abilit ies andattitude of the workforce.A degree of involvement will help if targets are explained.However, supervision will always be needed, particularly or fast construction.The aesthetics of the overall structura l appearance should not be compromised by fastconstruction but, depending upon use, certain finishes may be required which createeither longer finishing imes or a delay in striking formwork. I t is important that theselected finishes are appropriate. For example, a bridge only seen by train passengersneed not have special finishes, bu t a footbridge, where the users will be in contact, willrequire generally a very high quality or textured finish.

Various options are available when specifying concrete in accordance wi th BS 85007 ndBS EN 2068 o meet a given environmental condition. These will include different typesof cement and different concrete strengths. The selection of the cement type w ill havean impact on the speed of construction, because of the early temperature rise and theassociated rate of gain of strength. Various parts of the structure wil l be subjected todifferent environments, each requiring a different concrete specification. S imilarlydifferent concrete strengths may be specified. It may be possible to select one concretethat will be satisfactory for a number of locations, thus simplifying the supply. This mayspeed the construction by allowing for greater flexibility, with the option t o switchdelivery from one part of the structure to another.

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Self-corn pact ng concrete Self-compacting concrete can improve the speed of placing in situ concrete, particularlywhen the reinforcement is congestedg.As the material wi ll flow from one end of a pourto the other, the need for temporary access for operatives and plant will be significantlyreduced. Because compaction s not required, the ra te of placing can be increased.However, this may lead to increased formwork pressures, particularly or high lifts.Research has recently been carried out at Dundee University.When construction s being carried out in an urban area there may be restrictionsonthe hours of working because of the level of noise. It has been found that the use ofself-compacting concrete, because of the absence of vibrators, can overcome thisrestriction.A further Health and Safety benefit s the avoidance of white finger andother vibration-induced njuries to operatives.Figure7 shows the formwork in position for the replacement of an integral bridge pier9m high. Because of the d ifficulty of placing the concrete at the top of the shutter andsubsequent compaction of the concrete, self-compacting concrete was used. Theconcrete was pumped through ports in the side of the formwork. This method requiredan allowance to be made for an increase of concrete pressure on the formwork.

Figure7Self-compactingConcrete being or the

reconstructionof a bridge pier , ; .

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3 General issues relating to the struct

High strength concrete

Concrete surfaceimpregnation

Corrosion inhibitors

3.8 ReinforcementRationalisation

Prefabrication

High strength concrete can offer advantages for bridgesO. The achievement of high eadyage strengths can obviously speed the construction process, for example by allowing earlierremoval of formwork or application of prestressing. However, high early strength may resultin a high temperature rise due to the heat of hydration. This needs to be taken into accountand sufficient reinforcement provided to control any cracking that may occur.

Hydrophobic pore lin ing impregnants are required to be applied to specified areas ofconcrete structures t o help prevent and control chloride-induced reinforcementcorrosion from de-icing salts. Currently monomeric a lkyl (isobutyl) trialkoxy silane isrecommended in BD 43/03 but there is the opportun ity to use alternative products.Although silane is very effective i ts application is slow and consequently several newproducts, which will speed up the process, are being trialled. It is also claimed that thesenew materials have improved health, safety and environmenta l benefits.

Another method of providing protection o the concrete is the use of corrosion inhibitorswhich are included in the concrete mix. These inhibitors are expensive and are onlyrequired in the surface area of the structure; therefore, in thick sections most of theadditive is unnecessary. There is, however, the opportunity to use them in thin precastsections, e.g. precast concrete permanent formwork for bridge decks. This type ofapplication s also being trialled.

Rationalisation may be defined as the process of e liminating unnecessary varia tion bysimplifying, reducing complexity and taking advantage of opportunities provided bymanufacturingand prefabrication approaches. For a complex structure, rather thandesigning each element separately, the designeddetailer should try to identify typica lreinforcement arrangements that will be suitable for the majority of elements. Thoughthis may result in some elements being over-designed, there w il l be subsequenteconomies in terms of the time taken to fix the reinforcement on site, with resultingsavings. In slabs and other flat structures welded fabrics can replace loose bars. This mayincrease the weight of reinforcement required but w ill significantly reduce the time forfixing. Similarly loose shear reinforcement may be replaced by proprietary systems*.

Prefabricated reinforcement is available from some suppliers or can be assembledadjacent to the construction site. This may be standard cages, for example for columnsor bases, or may be made to order. Alternatively, reinforcement cages can beprefabricated on the ground on site and lifted into position. Figure 8 shows thereinforcement for a pier crosshead, which was fabricated on site. Some additiona lreinforcement wil l generally be required to avoid distortion of the cages duringtransportation, lifting etc.

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Figure 8Prefabricated reinforcement for pier crosshead

Couplers

3.9Adhesive connect ions

As an alternative to weldedsteel fabric reinforcement, which is available in sheets oflimited size, some suppliers offer reinforcing bars in the form ofa carpet . The system issuitable for large areas of deck. The required arrangement of reinforcing bars isrationalised to reduce variation in barsizes and spacing. Bars of the required diameter arewelded at the necessary pitch to flexible strips ina special machine, the outpu t beingaro ll of reinforcement. This is transported to the site and lifted into position by crane,being placed a t one end of the required area. It is then rolled out like a carpet; generallyonly two operatives are required for a 10-tonne ro ll. Obviously this approach providesreinforcement in only one direction;a second roll of reinforcement is required for theorthogonalsteel. One limitation with this method is the high local loads imposedon theformwork and falsework by the roll before it is spread out.

Connecting precast units to in situ concrete, or consecutive in situ pours, may requirerelatively ong lengths of reinforcing bar projec ting rom the elements, which wi ll belapped. Connections can be simplified, and the construction process speeded up, byusing couplers, wh ich provide a direc t connection between the ends of the bars. This isparticula rly economic for large bar diameters. Various types of coupler are available;some carry both compression and tension while others are designed to work incompression only. The most com mon type are threaded, requiring the ends of both barsto be prepared. Others are swaged or clamped over the bars.

The major advantage of using adhesives (suchas epoxies) is the ir rapid gain in strength.Suitably formu lated epoxy adhesives gain their fu ll strength ina matter of hours whilecementttious materials w ill take several days or weeks. The main use of adhesives inconcrete bridges is in glued segmental construction. However, the epoxy mortar placedbetween the match-cast units is primarily to seal the joint and to provide a uniformbearing area. Adhesives have been used in a number of building applications to form theconnections between precast units. For the roofof the Sydney Opera House adhesiveswere chosen in preference to conventionalmortar because of the very thin, watertight

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3.10 Travelling falsework

Figure 9Traveller being used for the construction of a deck

cantilever and stringcourse

Figure 10Traveller being used for construction of sof f i t of

underpass

bond lines that could be obtained and the improved rate of erection because of the rapidgain in strength. It was estimated that about 6 months of construction ime was saved inall3. There may be opportunities to benefit from the technology in bridge construction.

Travelling falsework can be used to speed up the construction where significant repetitionis required. The units can be built from proprietary equipment or purpose-madesteelframes. Figure 9 shows a traveller being used for the construction o f a deck cantilever andstringcourse. Figure 10 shows the use of a heavy-duty scaffold traveller to support thesoffit of a large underpass. The traveller was constructed in two sections across the wid thto allow clearance during moving; this was carried out by removing the central bracingframes between the sections and attaching wheels to the base of the support legs.

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4. Substructures

4.1 General

Location

Access

Foundations and substructures are invariably concrete and a wide range of options canbe adopted to achieve speed:

The use of fewer larger or longer piles might rule out the need for the testing normallyAn increase in site investiga tion can not only reduce the risk of unforeseen conditionsConcrete can be placed directly in to excavations and onto the ground, removing theSpecial constituents can contribute t o the wo rkability, density, durability and stability

required.but suggest or confirm a quicker option .need for fo rmwork, which saves time as well as expense.of a concrete rendering it easy and quick to place in large quantities and ina minimumof time.

For the sake of c larity and simplicity this section on substructures has been split betweenfoundations and abutments/walls/piers.

The Location of substructures can significantly a ffec t their speed of construction.Foundations should be kept ou t of water wherever possible to avoid the need forcofferdams and the restrictive effects of navigation, floods or tidal variations.Substructures should be kept back from riverbanks and railwaysso that they can be builtand maintained without the need for costly access arrangements and avoid the need fortrack possessions, which inevitably have a delaying and cost effect on a job. It is best toavoid situations that necessitate piling through existing abutments especially if thereis apossibility hat they may themselves be piled and there is uncertainty about the locationof the piles. Also, avoid services unless diversions can be achieved in advance of andwithout hindrance to the progress of the work.I t should be noted that columns adjacent to highways have to be designed to resist highimpact forces if they are within 4.5 m of the carriageway. Avoiding this demandingrequirement can achieve columns of significantly smaller section, which are easier t oconstruct and offer less complicated connections to foundation and superstructure.Several of the above may result in longer-span bridges. This may not, however, haveadetrimental effect on cost or speed of constructionas the init ial savings may cover theinvestment in the extra span length.

Access for construction plant is an important issue for bo th the buildability offoundations and the speed with which they can be constructed. Foundations inevitablytake up more ground space than is required for the next stage of construc tion and thismust be taken into account early in the planning of the job.

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4 Substructures

Kickers

Secondary finishes

4.2 Foundations

While there are differing views on the use of kickers they do speed up the const ruction ofthe wa ll by affordinga good line and secure support for the temporary formwork. It isimportant to note that they must be formed in the same quality concreteas the mainwall stem and this will affect either the quality of concrete in the whole of thefoundation or the strength of concrete used in the design of the abutm ent/wall/pier.

There are three types o f finish t o concrete substructures, namely plain, featured or facedwith a cladding. For fast constructiona plain finish is best but with the attendant lack ofaesthetic appeal. Ifa featured finish is required reuse of what w ill be more expensiveformwork is desirable and should be de tailed to avoid remake between pours. Suchform work can of course be prepared in advance of the job andso i ts fabrication shouldnot be critical to the programme.

.

Cladding allows the main concrete work to be done more quicklyas there s not theattendant concern with quality of finish. The cladding itself,at least in bridge works, canbe done outside of the more critica l works window and without delaying the opera tionaluse of the structure.

There are two broad types of foundation, spread foo ting or p iled. The former wil lgenerally be used unless ground conditions or other issues dictate otherwise. There aresituations where deeper excavations and replacement f ill undera spread footing willoffer a cheaper, quicker solut ion than piling. Mass concrete offers the simplest, speediestform of foundation wherever there is the space to use the solu tion. This is probably amuch-underrated form of construction.

Spread fOOtingS Time spent in getting the blinding concrete layer right will improve both th e speed ofconstruction and economy of spread footings. Where possible self-levelling concreteshould be used for the blind ing layer. The concrete of the foo ting should be placed to theedges of excavation rather than using side formwork. The extent of over-d ig wil ldetermine this. (It should be noted that concretingdirectly against the ground offersbet ter passive resistance than compacted fill.) The excavation must be kept freeof water,a suitable drainage sump should be provided.Simplicity of detailing s of paramount importance. Examples include making bases deepenough to avoid the need for shear links, though some links will be necessary to supportthe top reinforcement mat. Where possible prefabricate the reinforcementcage;additiona l reinforcement may be required to make the cage sufficiently rigid forit to betransported.

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P i ed foundations

Intermediate supports

4.3 End supportsIntroduction

The conventional type of piled foundation uses piles with a foundation slab on top . Tospeed up and simplify construction, consider the use of driven precast concrete piles,although these will be of lim ited capacity due tosize ava ilability . For augered or boredpiles use the largest size that access for plant and ground conditions will allow, to reducethe number required. If acceptable, avoid the need for a pile test by designing empiricallyand adopting a more conservative factor of safety.With regard to the differen t typesof piling available some offer a faster rate of build thanothers, particularly n poor ground conditions. For example, current developments inCFApiling offer significant benefits especially where precasting or the driv ing of pilesisunsuitable.

For the foundation slab similar points as for spread footings apply.Spread footings have been eliminated by the use of single large diameter piles(1.5-2.5 m), for example on the Doncaster North Bridge in South Yorkshire[see CaseStudy 31 This method has been used to support both road and railway bridges and isparticularly suitable for twin bea dg irde r decks.

Where columns are designed to resist impact forces they are of ten detailedso that athicker upstand projects from the foundation to reduce the effective length. Thiscomplicates the construction of the foundation and for speed and simplicity could beavoided.A bigger but single section of co lumn may also provide aesthetic and costbenefits.To speed construction, emporary works should wherever possible be designedasintegral with the permanent work or sacrificial. In the Lattercase they should offermin imal obstruc tion to the permanent works.Hinged connections to columns, piles or abutments should be avoided for fastconstruction.

End supports is the general term coveringall forms of construction used to support theends of a bridge deck, and includes the follow ing:W full-he ight abutmentsW integral abutmentsW bank seats abutmentsW cantilever wallsW reinforced earth wallsW ancho r-tied walls.The use of precast units wherever possible will generally speed up construction and this ishighlighted where appropriate in the follow ing sections.


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4 Substructures

Full-height abutments

In egraIabutmen s

Bank seat abutments

24

Reinforced earth techniques also offer the benefit of speed but there are somelimitations on their use, which are highlighted.

A full-height abutment is a solid wall of concrete or masonry emanating from road ororig inal ground level and supported on a foundation. Splayed or in-line wing wallsretaining fill beyond the approach to the deck are either free-standing or attached to theabutment. It is a robust element of the bridge where, with few exceptions, aesthetics aresecondary to overall func tion. Unless there are over-riding ssues, the sheer size of anabutment for a standard road bridge makes precasting impractical and therefore in situreinforced concrete construction s standard. Access is required for inspection andmaintenance unless the structure s integral.The fo llow ing measures may be taken to speed the construct ion ofa free-standingabutment:

The overall shape of the wall and reinforcement detailing should be made as simple aspossible.

H Rectangular sections should be used wherever possible, which simplifies construc tionUse preformed reinforcement cages.Design to avoid shear reinforcement wherever possible, which again aids fixing.Consider using precast bearing plinths.Where separate access chambers to galleries are needed, consider the use of precastunits.

BA 42/9614requires that bridges up to 60 m in length and no t exceeding30skew shouldnormally be continuous over interm ediate supports and the decks should be integral withthe abutments. By definit ion, in tegral abutments do no t have bearings and hence do notrequire galleries and bearing shelves. This w il l considerably speed up the constructionprocess.The detail at the connection between the deck and the abutment requiresa considerablequantity of reinforcement.To simplify the erection of precast deck beamsit is preferableto fix this afterwards. Toassist with this a box-out can be formed a t the rear of theabutm ent wall using a permanent mesh. An example is shown in Figure 11.

Bank seat abutments are small height abutments generally usedas end supports insituations where existing ground is at or slightly below the finished level of the roadcarried by the new structure. Their design is usually simple with the wa ll thickness beingdictated by the need to accommodate bearing plinths and drainage channel.As with fullheight abutments, access for maintenance is required. Wing walls are generally short andshare the same foundationas the wall. Ballast walls t o allow deck movement and retainthe fill immediately behind the deck complete the structure.


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Substructures 4

Figure 11Exampleof permanent mesh used to form box-out at

abutment

Cantilever walls

EY

1Although much smaller than a full-height abutment their size would sti ll be to o great tomake precast construction economic unless other over-riding circumstances prevail.Bank seat abutments can also be used in integral bridges, but the connection iseffectively rigid with limited rotation capacity. On longer bridges, where temperaturemovement must be considered, the foundations wi ll need to be detailed toaccommodate the longitudinal movement range. This may involve the use of flexiblepiles, typically in steeL, or a sliding interface with the blinding concrete if the foo ting is apad foundation.The aids to fast construction w ill be the same as those outlined above for full-heightabutments. In addition, the re inforcement wil l be light and uncomplicated. Considerationshould be given to the use of unreinforced concrete and to pre-consolidation techniquesfor embankment construction.

Used in top-dow n construction, these are formed either in contiguous/secant pileconstruction or diaphragm walling of various configurations. They are expensive whencompared with conventional construction bu t become economic when the use of openexcavation is limited by site constraints, e.g. the proxim ity of existing property. Theappearance of the ensuing wall when excavated is generally unacceptable as a long- termfinish and requires treatment either by facing with in situ concrete or cladding.For a simply supported design, the wa ll would be topped wi th an in situ concrete cappingbeam and would require all of the features for maintenance associated wi th aconventional abu tment. For integral designs the capping beam becomes the enddiaphragm for the deck.

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4 Substructures

Anchor-t ed wa ls

Reinforced earth walls(hard faced)

These factors will contribu te to the fast construction of piled walls:H Provide adequate access and working area; pil ing plant requires a lot of room andH Avoid pile tests; obtain sufficient soils data to establish a safe pile length.H Prefabricate reinforcement cages; avoid lapping reinforcement and use couplers ifH Avoid a sophisticated drainage system.H Avoid box-outs for secondary fixings for cladding.

there wi ll be an attendant crane for the reinforcement.

necessary.

For diaphragm walls, in addition to the above points:H Avoid curves or sudden changes in direction.Points to consider for the capping beam include the following:H The overall shape of the wall and reinforcement detailing should be made as simple asH Use rectangular sections wherever possible; this simplifies construction.H Use simple reinforcement details to aid fixing and allow the use of preformed cages.H Design to avoid shear reinforcement wherever possible; again this aids fixing.H Consider using precast bearing plinths.H Where separate access chambers to galleries are needed, consider the use of precast

possible.

units.

Anchor-tied walls are used in top-down construction n conjunction with a piled ordiaphragm wall t o limi t the size and depth of wall construction.They should beconsidered only if circumstances rule out the use of other more simple methods ofconstruction.They are not suited to integral bridge design.

As an aid to fast construction :H Install fewer, large-capacity anchors.H Avoid where possible multip le rows of anchors.H Use precast wall ing beams.

Reinforced earth offers a quick solution to constructing retaining walls. An example is thewidening between Junctions8 and 9 of the M62, where the embankments could be builtentirely from the rear, thereby e liminating the need for adjacent lane closures [see CaseStudy 41. The system comes in many guises bu t the ones of interest here are those thatuse reinforced concrete panels t o support and pro tect the soil fill. Steel or geotextilestraps anchor the panels to the f ill materia l and provide the fric tional strength of thewall. Facing panels are typically of cruciform shape but may be the ful l height rectangular

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4.4 Oth er buried structuresSubways and culverts

type. Because these walls rely upon friction being developed between the fi ll mater ialand the straps, water mains should no t be allowed in embankments formed this way. Ifthey are unavoidable, extra precautionary measures must be taken such as throughducting. Also vehicular parapets must be mounted on an independent ground beam.Reinforced earth has been used to support bankseat abutments, generally for simplysupported structures with short-span, low-skew decks, where differential settlem entisnot an issue. However, there is stil l some concern among practitioners about usingreinforced earth in such applications.It is impo rtant to highlight some limita tions and concerns relating o use of the systemto ensure maximum serviceability:W Reinforcing straps must extend well back from the face and under the carriageway,W Top layers could be disturbed by the insta llation of ut ilities so must be suitablyW In the event of a soil failure, problems arise, especiallya t an abutment, as to how toH The standard parapet anchor slab does not meetstatic design criteria. I t may alsoH For reasons of aesthetics, preformed facing panels should be used. Special at tent ion

railway etc.protected.deal wi th the support of the deck and how to carry out repairs.extend under the runn ing carriageway.

should be paid to the long-term serviceability of the connecting anchorages.Reinforced earth is a fast construction echnique. It does not require a foundation butthe existing ground must meet limiting se ttlement criteria. However,a small base isrequired for the facing panels. Various decorative finishes are available for the panels,which wi ll avoid the need for secondary treatments.

Subways are short-span structures intended for use by pedestrians and/or cyclists. Theyare designed to provide a safe passage beneath a highway or railway and must beattractive and safe t o the user. Surface finishes, lighting, good alignment and thedisposal of surface and ground water are essential requirements fora good design.Culverts, however, are used to carry waterways under highways and railways.Subways and culverts can be constructed using in situ concrete or precast concretefactory-produced units, which are transported to thesite and installed on preparedfootings.Precast concrete box sectionsRanges of sizes are available, with spans from 1.2m t o 6 m and internal heights varyingbetween 600mm and 3 m. Usually spigot and socket joints are formeda t the ends ofthe units. Although the joints between abut ting units can be rebated andleft openwithin culvert construction, it is common practice to fill the joints w ith a hydrophilicsealant and/or a compression seal. The face of joints can be finished using polysulphidesealant.

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4 Substructures

Figure 12Underpass ormed fro m open cellunits

Precast concre te arches

28

Precast concrete open cell por tal sectionsUsing a configuration of tw o inverted T wall units anda roof unit comprising aninverted U section, spans from 4 m to 12 m w ith clearance heights of up to 6 m can bemanufactured. The horizontal and longitud inal o ints formed by abutting un its can betrea ted sim ilarly to those between box sections. Figure12 shows an underpass formedfrom open cell units.Precast concrete closed cell portal sectionsAn inverted U may be mounted on another Uo form a box, with a horizontal ball andsocket joint in the walls. Spans up to 12m can be factory produced and internal clearanceheights of 6m can be achieved.C ne ralThe above precast options offera rapid means of cons truction and can be designed toaccommodate the range of highway and railway loadings, in addition to the dead loadsfrom embankments and the pavement or track construction.The larger sections aresuitable for the construction of vehicle underpasses. Specially designed box sections canalso be made available for installation by thrust bore methods.To simplify construc tion,wing walls perpendicular to the st ructure can be avoided byallowing the structure to project beyond the embankment side slope. Alternatively,special splay-end units can be used.Considerations for fast construc tion nclude the fo llowing:

Precast concrete box or portal frame units are quick to insta ll but the joints have to beadequately sealed. In situ const ruct ion,while slower, does not have this attendantproblem.avoid wing walls i f possible.Precast concrete reta ining wa ll units should be considered for the w ing wallsbut

for in situ construction:The overall shape of the wall and reinforcement detailing should be madeas simple asUse rectangular sections wherever possible, this simplifies construction .W Simple details aid fixing and a llow the use of preformed cages.

Design to avoid shear reinforcement wherever possible, also to aid fixing

possible.

reinforcement.

Arches offer an alternative profile to the more conventional slab deck designs. Structuralefficiency can be achieved while p rovidinga smooth, pleasing, aesthetic profile. Figure 13shows a pedestrian and cycle underpass in the Netherlands. Due to the arrangementofthe structura l elementsa wide range of geometric configurations can be achieved toprovide a varied selection of clearance envelopes usinga limited stock of casting moulds.


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Substructures 4

Depending on location , use, ground conditions and depthof cover, a structure comprising two, three or fourelements can be used t o provide econom ically viablespans of between 2.5 m and about 20 m, with internalheights up to 8 m. The simple structu ral form means thatstructures can be installed in a comparatively short time.Quality can be maintained and easily controlled, husensuring the long-term durab ilityof the structure.Units w ill usually be 2 m t o 4 m long, between 180 mmand 300 mm thick and weigh up to 34 tonnes. In the

Figure 13Pedestrian and cycle underpass

arrangement in which two units are used with the joint a t the crown o f the arch, theunits can be erected in a staggered fashion, thus avoiding temporary propping. In thecase where three un its form the arch, the wa ll units are free standing and support thecurved roof units, again avoiding the need for temporary works [see Case Study 51.Usingthese installation techniques, 30-40 units can be installed duringa working day.This form of construction s typically suited t o a project where cut-and-cover tunnel,bridge or subway construc tion s to be used, or for bridging over highways, railways[seeCase Study 61 or watercourse^'^. Most comm only the single-span arch is used. Howevermulti-arch designs are available, and have been installed widely in Spain, Portugal andJapan.Because of the speed of erection, the route below can be returned to use quickly with themin imum of disruption . An example is the New Cowdens Railway Bridge on the M74 inScotland [see Case Study 71, where the precast arch units to form the190 m tunnel wereerected in a single weekend possession of the railway. Spandrel or wing walls can be insitu or reinforced earth panels tied back to the arch fill.F i l l depths over the crown of thearch can be as l i ttle as 400-500 mm.Considerations for fast construction nclude the fo llowing:

Precast units are quick to erect and can accommodate medium length spans.Effective long-te rm waterproofing and join t sealingis required.Although curves in the hor izontal alignment can be accommodated, sharp changes inA limited amount of skew can be accommodated.

pandrel walls can be avoided by extending the walls of the structure using adaptedSteep gradients in the vertical alignment should be avoided.

direction should be avoided.

tapering wall un its to suit the slope of the approach embankments.

An alternative approach is to use a number of arched ribs to form the bridge, suchas thethree-span Sir Thomas Fairfax bridge recently constructed across the River Weaver inCheshire16.A total of 17 arch units with a span of 16 m were used for the main spanacross the river; this fo rm of const ruction was selected so tha t the river would notbeobstructed at any time. The brick-faced precast spandrel units incorporated mocksandstone voussoirs. Foamed lightwe ight concrete was used for the fi ll over the arches.

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Superstructures 5 ,

5.1 Precast conc rete un itsPrecast con crete beams

Figure 14Installationo f precastconcrttcbeam

5. Superstructures

Precast pre-tensioned concrete beams are a form of construction hat has been used formany years. Various shapes and depths of cross-section are produced, suitable for arange of spans and applications. This form of construction s economic for a wide rangeof spans, but the span may be limited by the length of beam that is permissible fortransport. In the UK this is normally 27.4m, though lengths up to 4 0m may betransported via the motorways with special permits. Figure 14 shows the installation of aprecast beam duringa night-time possession.Inverted Tee beams are placed adjacent to each other, transverse reinforcement passedthrough preformed holes and the space between the units filled to form a solid slab.Typical spans, depending on the depth of the unit, vary from about 6 m to 18m. Widebox beams are used in the same way as inverted Tee beams, but require significantly lessin-f ill concrete, giving a lighter cross-section.M beams, U beams, Y beams and super-Y beams are spaced apart, permanent formworkis placed between the top flanges and an in situ deck slab cast over the top. Special unitsare used for the edge beams, which are considerably heavier and more expensive.Consequently, on some structures, internal beams have been used for the edge beams.The top surface of the beams is suitably prepared and has projecting reinforcement sotha t the slab and beams act compositely. Typical spans, depending on the type anddepth of the unit, vary from about 16m to 40m, the longest spans being with super-Ybeams.


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Figure 15Parapetclad with rnrs~nry

Precast concrete deck panels

4

Figure 15 shows a P6 parapet prior to erection on a river bridge, which has been clad withmasonry in the precasting yard to eliminate the need for access over the river for themasons.

An alternative to longitudinal precast units is precast concrete deck panels that spantransversely. These can be the fu ll width of the carriageway, including the edge beam. Anexample is the A828 Creagan Bridge [see Case Study 81where the units were installed ontwin steel plate girders, with which they were made composite with in situ concrete stitches.Although steel main beams were used here, and in other applications, the approach wouldbe equally suitable for concrete main beams if sufficient span could be achieved.

5.2 PreCaSt Segmentalbridges

The f irs t use of precast segmental construction was over the River Seine a t Choisy-le-Roiin 1962. A further example was on the Water O rton section of the M42 where thecontractor changed the in situ post-tensioned concrete box interchange bridges toprecast segmental. Because they were able to construct th e segments in advance in thewinter months they were able to save several months of construction ime. Thetechnique i s now commonly used in Europe, South East Asia and America. Examples inthe UK include the Roundhill Viaduct, Folkestone [see Case Study 91, the Cross HarbourRoad and Rai l Links, Belfast17 see Case Study 101 and the 1.75km long A13 Viaduct forthe Channel Tunnel Rail Link18.Segmental cons truction requires a considerable capital outlay for moulds, casting shedsand erection equipment. Thus economics dictate that the project should be of asufficient scale to warrant these costs; typically the to ta l deck area should be more than10,000 mz.The methods particularly end themselves to construction over difficu lt orobstructed ground, roads, railways or rivers- a l l locations where the cost of conventional

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..-,..

. .. . ures 5

falsework can become prohib itive. The m inim um span length of 25-30 m is determinedby the need to keep box depths above a practical min imum of around 1.4 m, whereasmaximum spans can be up to 200 m.The easiest form to create is that of a single-cell box which can be poured as a completeunit on a daily cycle. Lengths can vary between 2 m and5 m, though a 3 m length susual. Typical weights can be between25 and 50 tonnes, wi th units closer to the pierspossibly being up to 100 tonnes. The casting area should generally be located on thesitein a purpose-built shed. The whole factory process is then akin to that of a productionline, with as much as possible being taken off the critical path.It is the programme advantages of segmental construc tion that generate significantbenefits, in that the deck casting can start at the same time as the substructure works.Erection of the superstructure canstart as soon as a sufficient stockpile of units has beencast. The erection methods dependon the available access to areas of the site. The twomain types of construction most widely used are balanced cantilever and span-by-span.In balanced cantilever construction, units should be erected sequentially either side ofapier unit so that the overall cantilever is never more than one un it out of balance.Temporary props should be used a t the pier to provide stability un til further continuity sachieved and the props should sit on the permanent works foundations. If the groundconditions permit, then the simplest solution should be to erectall the units usingamobile crane. Sophisticated gantries should become economical for larger structureshaving a to ta l deck area of 20,000 mz or more. In span-by-span construction, the units ofone span should all be erected onto a gantry beam spanning between pier locations.

5.3 In Situ balanced Balanced cantilever const ruction s an economical me thod when access from below iscanti lever bridges expensive or practically mpossible. An example was the A483 River Dee Viaduct inClwydg [see Case Study 111, which was constructed on piers up to 50 m high overa

steeply-sided valley.This type of construction s generally used for spans over 50 m, typically 70-100 m, andworldwide up to the biggest beam span of 301 m. The methods are particularly suited toconstruc tion high over difficult ground, suchas valleys, rivers or estuaries, which arelocations where the cost of conventional alsework is proh ibitive. Spans up t o about60m should be built with a constant depth, but a variable depth is required for longerspans, using either linear or parabolic haunches.The cross-section is best developed as a single-cell box, poured in one operation. Unitlengths can vary between 3 m and 5 m, typicallycast as a balanced pair on a weekly cycle.The unit formwork is generally supported from an overhead travelling rame tha t isattached to the end of the last segment cast. Under-slung frames, which keep the top ofthe deck clear, are becoming more popular as they allow the segment reinforcement cageto be prefabricated and lifted inas a whole piece. This allows the casting cycle to bereduced to less than a week. The new segments are cast and once the concrete hasreached i ts required min imum strength, the section is post-tensioned t o i ts balanced pair.

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Construction starts from the top of a pier with a hammerhead unit that forms the init ialplatfo rm for the travelling forms. Temporary props, sat on the permanent foundations orsupported off the piers, should be used a t the pier to provide stab ility to the balancedcantilever un til further continuity o f the spans is achieved. Alternatively, thehammerhead units are often cast monolithically with the piers.The main programming and economic advantages of in situ balanced cantileverconstructionare the use of a bespoke travell ing formwork system that is used manytimes on a regular production-line cycle, obviating the need for falsework from theground to support the deck. Construction a t several piers can also be started a t the sametime, thus reducing the programme further.

5.4 lncrementally launchedbridges

Examples of incrementally launched bridges in the UK include the seven-span CeiriogViaduct on the A5 Chirk Bypass [see Case Study 121,he 1,025 m long Thurrock Viaductfor the CTRL (Channel Tunnel Rail Link), the Brides Glen Bridge in IrelandZo nd the twin320 m long footbridges attached t o Hungerford Bridge, London*.The economics of incremental aunching, with the capital outlay for moulds, casting areas,launching noses and jacking equipment, dictate that the project should be of a sufficientscale to usti fy these costs. Typically the total deck length should be more than about200m. The methods again particularly lend themselves to construction that is high andover difficu lt or obstructed ground, such as roads, railways or rivers. These are all locationswhere the cost of conventional falsework would be prohibitive. The typical span forlaunching should be around 40-50m, with a minimum span of 25-30 m for a boxstructure. Maximum spans can be around 70-80m, but such spans will invariably needtemporary intermediate props during the launch. The main series of spans should, whereverpossible, be of equal, or very similar, length. Bridges must always be of constant depth, andmust have an alignment that is of constant curvature, in both plan and elevation. Smallerspan to depth ratios of around 15:l tend to be used to control the amount of prestressingneeded. The methods are therefore particularly suited to long, straight bridges.

Single-cell boxes are preferred with units varying in length between 15m and 25m,which are cast on a typical weekly cycle. Generally, the bo ttom slab and webs are castfirs t followed by the top slab. The length should be chosen to su it the particular spanarrangement, i.e. by using either half-span or third-span lengths.In order to limit the stresses in the deck as it progresses out over the piers, a launchingnose is attached to the front of the bridge. This generally consists of a braced twin-plategirder with a length equal to about 2/3 of a typical construction span. The to ta l weightbeing launched can vary from 5,000 onnes up to 30,000 tonnes, which should then bepulled forward using prestressing strands or pushed forward wi th jacks.The programme advantages of incremental aunching are significant, in that the deck castingand launching can be completed efficiently and quickly on a regular cycle. Ideally, the wholebridge should be cast from a single area located behind one of the abutments. The factoryprocess for casting and launching s then comparable to that of a production line.

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Superstructures 5

5.5 Transversely slid orro lled bridges

Where an existing bridge needs to be replaced, it is increasingly desirable to lim it thedisruption to the existing road or railway by building the new structure alongside.A shortbut large disrupt ion can often be more preferable thana long series of minor disruptions.The traf fic management in the area can be greatly improved by avoiding the phased orpiecemeal construction of traditiona l replacement operations. The temporary support ofservices during the sliding operation also becomes more feasible, w ith a potentialreduction in the need to divert or move services. Though these options may appear tomore expensive, the greater degree of programme certainty and the reducedlevel of risk,as well as the unhindered deck construction, w ill o ften make these solutions faster, andtherefore more economic. They may be the only possible solutions in the railwayenvironment.

The new bridge deck can be built on an adjacent temporary platform. Runway beamsand temporary supports should be installed below the existing bridge to carry thetransverse slide tracks. In a single road or rail closure, over a weekend for example, theexisting bridge can be demolished, or disconnected and slid away on toa furthertemporary p latform . Demolition of the existing bridge can then occur off the criticalpath. The new deck should then be slid across and connected t o i ts piers and abutments.Existing piers can be re-used or new piers can be installed, under, or adjacent to theexisting locations.

The new deck can be either pulled into place using strand jacks, or pushed into i ts finalposition with ram acks. These techniques are frequently used in the railway environmentwhere possession time management is vita l to the feasibility of the projec t. Railwayunderbridges within embankments can also be bu ilt alongside the track and slid intoposition. Concrete porta l structures can be bu ilt in an adjacent casting area and slid induringa long possession on pre-bored slide tracks, with the embankment having beenpartially removed.

An example is the new underline bridge a t Bridgend, where the 4,000-tonne reinforcedconcrete two-span railway bridge was constructed off- line and slid into i ts permanentposition during a 171-hr possession. This method reduced the project dura tion from 123weeks to 85 weeks. Figure 16 gives a view of part of the structure.Another example is Hawkes Road Bridge, which replaced an underbridge beneath theNewcastle-Sunderland railway line. New foundations were constructed in small diametertunnels bored th rough the embankment during normal railway operations. The 17 m span1,700-tonne po rta l was constructed in i ts entirety on slide tracks adjacent to the tunnelsand slid into position, complete with ballast and railway tracks duringa single closure ofthe railway.

Complete structures can be lifted using heavy-dutyjacks and moved using multi-axlerubber-ty red ransporters. Redundant bridge spans can be lifted off t heir supports andtransported to a remote site for demolition. Replacement spans, or complete porta l-typestructures, can be constructed off -site and moved into position. In thecase of railbridges, they can be complete with ballast and track.

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End of -span railway bridgeconstructedoff-linaand slid in to posi t ion

Examples include the dua lling of the A90 Brechin Bypass, where a precast concreteporta l overbridge was moved in to place usinga heavy lifting vehicle. In addition asteel-concretecomposite bridge was constructed adjacent to the main road, turnedthrough 900 and landed on reinforced earth abutments, using two heavy lif ting vehicles.For replacing small-span bridges, it may be possible to construct the complete deck inone loca tion and lif t it into place using a crane. Clearly this approach will be limited bythe economics of providinga crane of sufficientsize and reach.Rapid construction can also be achieved by the casting and erection of full-span units, orfull-bridge length units. Heavy spans or decks, weighing up t o several thousand tonnes,can be cast away from the site, transported and then installed in very large pieces.Typical examples would include the use of massive prefabricated pieces being slid ontomarine barges, for transport by water to river crossing locations. Tidal or ballastingactions can then lower the un its down onto prepared pier and abutm ent locations. Otherexamples would be the s imilar use on land of large-wheeled transporters, to movecomplete spans or com plete bridges from adjacent casting areas to preparedsubstructure locations.Al l these techniques use the advantage of unhindered deckcasting away from the critica l locations, followed bya concentrated burst of erectionactivi ty. These solutions would all be developed to speed erection and to minimisedisruptionat the bridge site.

5.6 Jacked boxes Precast concrete box culverts (see Section4.4 and pipes can be jacked beneath existingembankments, obv iating the need to close the road or railway above. Larger concretebox structures, suitable for vehicular traff ic, can also be jacked through embankments.These boxes are formed in adjacent casting areas and then pushed in to the embankmentusing suitable jacking points in the casting area.A steel or concrete shield is used tosupport the advancing front face beneath the embankment. The fric tiona l load on top o f


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Superstructures 5ithe box can be limited by either the use of ant i-drag systems, or by the prior installationof a steelwork grillage that supports the road or railway. Major projects include thetunnels of the Central Artery in Boston .The first application of the technique in the UK for a major road was on the tunnel underthe M1 in Northamptonshire o provide a second carriageway for the A43 betweenTowcester and No rtham ptonz3. he 3,500-tonne, 45 m long box was fitted withatunnelling shield and jacked under the motorway ina continuous process lasting threeweeks, with no disruption to the live motorway,see Figure 17.

Figure 17Concrete box acked under the M1

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6.1 Gantries

6.2 Footbridges

6.Other structures

Sign gantries, consisting of prestressed beams and reinforced columns, are being used onmotorways and major roads. Depending on the cross-section of the beam used, spans upto 40m can be achieved. Generally they are designed to span one carriageway thoughthey can be used to span both carriageways, thus elim inating the support in the centralreservation [see Case Study 131. The units offer fast, economic erection, which is aparticular advantage when they are being installed on a live road. The columns areplaced in preformed sockets in the foundations; if required these can be designed to bedemountable so that the complete gantry can be removed and reused elsewhere. Thebeams are provided with the necessary fixings for the signage alreadycast in.

A number of manufacturers offer precast footbridge deck units, usually prestressedbeams with pre-tensioned strands. The deck beams usuallytake the form of box,Tee ordouble-Tee sections. Fixings for handrails are cast in or the units can be supplied withprecast concrete parapets where additional protection s required, e.g. over railway lines.The beams are best suited to rectangular, straight layouts. The dimensions and weight ofunits are lim ited by transport and the availablesize of crane for erection. Typical spansare 10-40 m.

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Case Study 1A64 lop Lane Underpass


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Case Studies 7

rn Early commencement of the Aqueduct rrn Disruption to canal use was avoided.rn TemDorarv works were elim inated therefore reducinn costs and i m w w i m Health and

I

ssto he structure.


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L -.


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Case Studies 7

M62 New ]unctionwidening Junction8

overbridge


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Studies

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No in situ works requiring craMinimal comptetion WaFkS all took

railway.d e nvironment.


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Case Studies 7

Case Study 8 DescriptionA828 Creagan Bridge A828 Trunk Road l i d he t m f C W X ~ nU alMUtlshn thewestCOMfScatband The contt cteliminateda 9krn longourney round the s k m f Loch C r m

by constructing a 1.2krn long divaion on he lineof the abandonedWest HighlandRailway.New win Steaplate girden were installed on the existing granite piers of thed d 150m ong Creagan railway bridge,which were strengthened to meet tncdernSt andah A totel of 43 full width pfecast ccmcretedeck units, complete with sideparapet upstands,were installed usinga craneon he bank. In s i tum e t e titches anda 100mm thick upper layer weye used to maketheunitsact compositetywith thegirdus.

tags of approach adoptedPrefabrication of deck units in contRapid erectionof precast units over theW fkwirtg tidal loch.c l o ~ r e f t . ZB

G r r m i v k wa f o o r w r u l o n d c r u g u t ~ eckun rapet upstands, dirnilzatedtheSignificant reduction ncontract period.Be lwnqh t 3~ ~ ~ ~ n ~ w g ~

r

c

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case sm* 9A20 RoundhillViaductFolkestone, Kent

DescriptionAn attemative design of two iaducts built wing the @ segmmtdW m anc 63m. Thenfoundationwas a b n

a n t i k rup af spans typicatty off340rniWd38rlrn

redesign af the s u p p d k q c a ~ umm ndn taking nto conSjd%ration he ChannetTumd, w h i i

Advantages of approach adoptedm Precastsegmental deck construction shortened construction the.factay conditions for casting of qrnmts itnprmdq u a l i .Erection by wane provided flexibility in cmtruct ionprogmmm Minimised falsework.Efficient and economic design achiewd.AestheticaHy pleasing appearwlce.

Repetition in construction sa t ime a d osts.


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Case Study 10Cross Harbour Roadand Rail

Links, Belfast

Case Studies 3

Advantagesof approach adwedH Casting in s h i t a r sqnmts in factayc a n d i i h errsured

W U p t o s i x s e g m e m e t . e a e d o n ~ b w rSimple falsework prop off (and ies to) thepile caps provided stability.W Reduced risk and disruption taad- Udfk.H Minimal in situ Hwrks to complete he stmctw.H Abii ty towmk on seviwal fronts at ancetospeedereaibn

the critical path.

L1


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ACase Study 11 D,483River Dee Viaduct, k

3lwyd~tn situ m t e eck, re?sign hat was kdopediOm iom prestressedCOtrries the roadmrternative was promoted pwtiatlarty o reduce

~ n W c l s h o f R c c*iDwrTravCnMUgaIlandRobertnb.ctor Edmund NuttallIkw f4Mrmplr dtm

Advantages of approach adFully prestressedstructure eplaced part-pratmsed,part- nfcmd s u m.A m e fficient structure.a Castingsimilar segments in controlled cord- ensuredgreaterRepetitiveweddy castingcyde

Below left Figure 35- Rim--

Below nght Figure 36 at m e o spited casting.c o n r t r u c t k n o f ~ i v a ~ n v b d ~ t ~ N o c o m ~ w o r k s r r e c ~ s s a r y .

c '


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Case Studies 7

Case Study 12 DescriptionCeiriog Viaduct, A Chirk A @h level, incrementallybunched concrete, boxgirder, seven viaduct. siiuatedin an area of scenic beauty, the design was subject to the approvalof the Royal FineArtCommission. The methodof construction, while requiring he contractor to work to very

lose olerances, resulted inconstruction and he abilitycasting area.

Bypass, North Walesits induding ease of supervision, all-weatherrate safety rdts and finishes in the concrete

Omnr h e m umce W ~ K MD d p u ony Geeand PartnersCm@wctwChristiani N e i hWm f5.25McoqfbmllQ9~Advantages of approach adoptedW Minimised alsework needed.

8 w[efr 37 W Minimised disruption to valley and surroundings.W Factoryconditions for casting concrete.W Repetition in constructionsaved time and costs.

thmtmdm WVWuct8aowdg,,L Flrurr38

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Motorways Stage 4

DescriptionConventional motorway sign gathii contract enabled Alfred McAlpine to devetw an alternativec o w m ebeam designtuspan both Camageways, thus eliminating the need for support legs in the ccrttral

s span one carriageway.The designm4mtumd

resewation.The beams were 40.4m long prestressed

Documents

CBDG Fast Construction of Concrete Bridges