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August 2001

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 1 HIGHWAY STRUCTURES:APPROVAL PROCEDURESAND GENERAL DESIGN

SECTION 3 GENERAL DESIGN

PART 8

BA 57/01

DESIGN FOR DURABILITY

SUMMARY

The existing Standard and Advice Note (BD 57 andBA 57) have been updated to include:

a) Lifting of the moratorium on internal groutedpost-tensioned construction (excluding internalgrouted post-tensioned segmental structures).

b) Improvements to durability that can be made bythe use of controlled permeability formwork,dense near surface concrete, corrosion inhibitorsand other materials such as lightweight aggregateconcrete, and stainless steel reinforcement.

c) To include references to thaumasite sulfateattack.

d) To rationalise references and terminology.

INSTRUCTIONS FOR USE

This revised Advice Note is to be incorporated in theManual.

1. This document supersedes BA 57/95, which isnow withdrawn.

2. Remove BA 57/95, which is superseded byBA 57/01, and archive as appropriate.

3. Insert BA 57/01 in Volume 1, Section 3, Part 7.

4. Archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

BA 57/01

Design for Durability

Summary: The existing Standard and Advice Note (BD 57 and BA 57) have been updatedto include:

a) Lifting of the moratorium on internal grouted post-tensioned construction(excluding internal grouted post-tensioned segmental structures).

b) Improvements to durability that can be made by the use of controlledpermeability formwork, dense near surface concrete, corrosion inhibitorsand other materials such as lightweight aggregate concrete, and stainlesssteel reinforcement.

c) To include references to thaumasite sulfate attack.



THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT

THE NATIONAL ASSEMBLY FOR WALESCYNULLIAD CENEDLAETHOL CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 1 Section 3Part 8 BA 57/01

August 2001

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 1 HIGHWAY STRUCTURES:APPROVAL PROCEDURESAND GENERAL DESIGN

SECTION 3 GENERAL DESIGN

PART 8

BA 57/01

DESIGN FOR DURABILITY

Contents

Chapter

1. Introduction

2. Factors Affecting Durability

3. Improved Durability - The Conceptual Stage

4. Improved Durability - Problem Areas

5. Improved Durability - Detailed Requirements

6. Detailed Requirements - Steel Bridges

7. References

8. Enquiries


August 2001


Chapter 1Introduction

1. INTRODUCTION

Background

1.1 Feedback from the inspection and maintenanceprogramme of highway structures has highlighteddurability problems even where materials, specificationand construction practices have been satisfactory. Theseproblems can often be linked to a design philosophy inwhich minimising the initial cost was paramount.Inadequate consideration may have been given to thelong-term performance of the structure either in thechoice of structural form or in the design ofconstruction details. This has, in too many cases,resulted in maintenance problems requiring costlyrepair. Consequently the Overseeing Organisations arekeen to promote the concept of design for durability,thereby shifting the emphasis to a lowest whole life costdesign philosophy.

Feedback from the assessment and strengtheningprogramme has shown that some structures particularlydating from the 1960s and 1970s were substandard. Inmany cases the assessed capacity was not compromisedby any deterioration in condition, but was mainlyinfluenced by the introduction of more onerous designrequirements in the period since their construction.However considerations of future changes to designstandards are outside the scope of this Advice Note, andare matters for evaluation as part of the design processand technical approval procedures.

The existing Standard and Advice Note (BD 57 andBA 57) published in 1995 have been updated toinclude:

a) Lifting of the moratorium on internal groutedpost-tensioned construction (except for segmentalconstruction).

b) Improvements to durability that can be made bythe appropriate use of controlled permeabilityformwork, dense near surface concrete, corrosioninhibitors and other materials such as lightweightaggregate concrete and stainless steel rebar.

c) To include references to thaumasite sulfateattack.


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Definition of Serviceability, Durability

1.2 Serviceability is the ability of structures to fulfil,without restriction, all the needs which they aredesigned to satisfy. In the design of a highway structure,these needs include:

i) the ability to carry without restriction all normaltraffic permitted to use the structure;

ii) maintenance of user safety by provision ofadequate containment, separation of classes ofusers, effective evacuation of surface water etc;

iii) maintenance of user comfort by avoidingexcessive deflections, vibrations, uneven runningsurfaces etc;

iv) avoidance of public concern caused by excessivedeflections, vibrations, cracking of structuralelements etc;

v) maintenance of acceptable appearance byavoiding unsightly cracking, staining, deflectionsetc.

1.3 In the design of structures, however, the first ofthe above needs is supplemented by a separate check onthe maximum load carrying capacity, known as theultimate limit state. The ability to carry abnormalvehicles is also a need which the OverseeingOrganisations’ new structures must satisfy, but theoccurrence of such loading is deemed to be infrequentand not relevant to the maintenance of the structure’sserviceability.

1.4 Durability is the ability of materials or structuresto resist, for a certain period of time and with regularmaintenance, all the effects to which they are subjected,so that no significant change occurs in theirserviceability. In the design of highway structures thetarget period during which structures must remaindurable, corresponds to the design life as defined inBS 5400: Part 1.

1.5 Durability is influenced by the following factors:

i) design and detailing;

ii) specification of materials used in construction;

iii) quality of construction.

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Chapter 1Introduction

1.6 The control of items (ii) and (iii) is achievedthrough the use of accepted standards and procedures.However the design of structures is not so readilyassociated with the achievement of durability, beyondsuch considerations as cover to reinforcement, crackwidth limitation or minimum steel plate thicknesses.This lack of attention to the durability aspect of designhas resulted in a premature loss of serviceability inmany highway structures.

Objective of Advice Note

1.7 The objective of this Advice Note is to improvethe durability of highway structures by drawing to theattention of designers aspects of design which arerelevant to the durability of structures, but not coveredadequately in the existing requirements for the designof these structures.

Scope

1.8 The advice contained in this document, whichelaborates and supplements the requirements of BD 57(DMRB 1.3.8), covers areas of design and detailingwhich are relevant to design for durability. The AdviceNote considers various ways in which the design cancontribute to the durability of a structure and identifiesaspects of structural form and detail which requirespecial attention. Many items covered in this documentare acknowledged by designers as being good practicebut their use has not been as widespread as would bedesirable. Certain aspects of inspection, maintenance,specification of materials and construction practicesrelating to durability, which are dealt with in moredetail in the Specification for Highway Works(MCHW 1) and the Notes for Guidance (MCHW 2), arealso briefly mentioned.

1.9 The main points of this Document concerningimproved durability are included in BD 57 (DMRB1.3.8). It should be emphasised that this Advice Note isnot comprehensive and designers should use theirjudgement and experience to ensure that durabilityaspects are catered for adequately in new structures.

1.10 The figures incorporated in this Advice Note areonly indicative. Designers should satisfy themselves asto the suitability of the suggested details to specificdesigns.

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Implementation

1.11 This Advice Note is to be implementedforthwith for all schemes currently being preparedprovided that, in the opinion of the OverseeingOrganisation, this would not result in significantadditional expense or delay progress. Designorganisations should confirm its application toparticular schemes with the Overseeing Organisation.

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Chapter 2Factors Affecting Durability

URABILITY
2. FACTORS AFFECTING D
General

2.1 A survey of 200 highway concrete bridges,commissioned by the Department of Transport, TheMaunsell Report (reference 1), identified a number offactors which contributed to the inadequate durabilityof many of the Department’s structures. Most of themwere in areas where amendments to existingspecification requirements, or to inspection andmaintenance procedures, should provide improveddurability of structures in the future. The mostimportant of these are briefly discussed below.However, there are a number of important aspectsrelating to durability which need to be addressed byimprovements in conceptual design or in designdetailing; these topics are often not adequately dealtwith in BS 5400, and are discussed further in thisdocument.

Drainage, Joints and Waterproofing

2.2 By far the most serious source of damage is saltywater leaking through joints in the deck or serviceducts, and poor, faulty or badly maintained drainagesystems. Of crucial importance is the provision of apositive, well designed, detailed and constructeddrainage system for managing water from the deck, andinto a drainage system. Particular attention should begiven to detailing through deck drainage, and to ensurethat all systems can be maintained. Work undertaken byHighways Agency and the County Surveyor’s Societyand published by the Transport Research Laboratory(reference 12) provides detailed guidance on watermanagement, and designers are strongly advised toconsult this document. Advice on the design ofexpansion joints is given in Chapter 5 and methods ofeliminating deck joints are suggested in Chapter 3.

2.3 Also of crucial importance is the provision of aneffective waterproofing system on the bridge deck. Themost important properties of an effective waterproofingsystem are its waterproofing ability and its bond to thedeck. It should be noted that if bonding is effective overthe whole deck area, then any local lack ofwatertightness in the waterproofing layer is incapableof causing significant damage to the deck. Furtheradvice is given in Chapter 5. Reference should be madeto BD 47/BA 47 ‘Waterproofing and surfacing ofconcrete bridge decks’.

August 2001

2.4 An observed source of damage in highwaystructures is the splashing or spraying of salty waterfrom de-icing salts on to bridge abutments, piers,parapet edge beams and deck soffits. Advice is given inparagraph 5.18 on the provision of additional concretecover to reinforcement and impregnation to waterproofthese areas.

Workmanship

2.5 A number of aspects of poor workmanship inconcrete bridges were highlighted in the MaunsellReport. The most critical of these, from the point ofview of durability, was the failure to achieve thespecified concrete cover to steel reinforcement. Thiswas found to be an extremely frequent problem, andwas the cause of a great deal of deterioration, especiallywhen it occurred in association with joint leakage etc.For further advice on concrete cover see paragraph 5.2.

2.6 Curing of concrete is probably the second mostcritical aspect of workmanship revealed by the survey.The vital role of curing in providing a dense concretecover to the steel reinforcement cannot be emphasisedtoo strongly. Problems of poor compaction,honeycombing etc, were in themselves less significantalthough they might compound the effects of otherinadequacies. Compliance with the Specification forHighway Works (MCHW 1) would eliminate theseproblems in future.

Cracking

2.7 It was found that cracking due to early thermaleffects was a widespread problem. For advice on thissee paragraph 5.3.

2.8 Cracking and damage due to Alkali SilicaReaction (ASR) was found to be rare.

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Chapter 3Improved Durability - The Conceptual Stage

- THE CONCEPTUAL
3. IMPROVED DURABIITYSTAGE
General

3.1 The type of structure selected for a particularlocation can have an important bearing on its durability.This section looks at certain types of constructionwhich have performed well and considers theirsignificance from the point of view of durability.

Continuous bridge decks and Integral Abutments

3.2 Continuous structures have been more durablethan structures with simply supported decks, primarilybecause deck joints have allowed salty water to leakthrough to piers and abutments. In principle, continuousbridge decks should therefore be used whereverpossible.

3.3 Traditionally, simply supported bridge deckshave been used in areas where large settlements, suchas that due to compressible soil strata or mining, waslikely to be a problem. In view of the durabilityproblems associated with deck expansion joints,consideration should be given to the use of continuousstructures even where large differential settlements areanticipated. Due allowance should be made for thepredicted movement, including hogging off bearings, inthe design of deck elements. Wherever possible theseeffects should be ‘designed out’ utilising methods suchas kentledge or ‘tie’down’ arrangements. The degree ofsettlement which can be accommodated in continuousstructures must be individually evaluated. Where theseeffects cannot be catered for using full continuity,partial continuity as described in paragraph 3.7 shouldbe considered. The ability of continuous bridge decks toaccommodate differential settlements is enhanced bythe use of increased span/depth ratios, but care shouldbe taken to avoid excessive liveliness, which may beinduced by the use of very slender decks.

Continuous decks using precast prestressed beams

3.4 There are two ways in which multi-span deckscan be made continuous, thereby reducing deck joints:incorporating either full or partial continuity atintermediate supports. Partial continuity is achieved byproviding continuity to the deck slab only, whereas fullcontinuity involves the provision of fully continuousmain beams or girders. In the case of reinforced

August 2001

concrete structures, post-tensioned prestressedstructures and structural steel members, this poses noparticular problem of design or detailing. In the case ofcomposite bridge decks using precast prestressed beamsthe achievement of full continuity involves providingin-situ concrete over supports to the full depth of thebeam and slab. Partial continuity is generally preferredto full continuity in such structures because of thedifficulty in assessing the long-term effects of prestress-induced deflection in full-continuity construction.

3.5 Figures 3.1 to 3.5 show five types of continuityconstruction which have been used in the UK and haveperformed satisfactorily. These details may be modifiedfor use with structural steelwork. Continuity detailsother than those shown may also be used providing thedesigners are satisfied with their past performance.Designers must carefully assess the structural designimplications of use of the different types of continuityjoints, in terms of the joint itself, and imposed effectson bearings. There are also implications formaintenance operations such as bearing replacement.

3.6 Types 1, 2 and 3 have in-situ integral crossheadswhich may be designed to develop full continuitymoments. Type 2 has been extensively used in NorthAmerica and details of this method of construction canbe found in reference 2.

3.7 Types 4 and 5 provide partial continuity throughthe deck slabs only. They are not designed to developthe full live load continuity moment but rather toeliminate expansion joints between each span. In theType 4 detail, the various relative rotations anddeflections at the support positions are accommodatedwithin the connecting slab elements. This approachretains the simplicity and economy of simply supportedconstruction whilst obtaining the various advantages ofdeck slab continuity. The Type 5 detail, on the otherhand, does not accommodate support rotations andcould be susceptible to cracking. These methods can bemodified for use in composite bridge decks with steelbeams. A joint detail similar to that shown in Figure 3.4has been promoted in the UK by Dr A Kumar; moredetails can be found in references 3 and 4. Whenassessing the suitability of arrangements such as Types4 and 5 designers should carefully consider designissues such as tension/compression effects in theconnecting slab and bearing translation due to in-span

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live load deflections and tension/compression effectsand bending moments generated in the top slab due totemperature variation and the effects of end restraint atthe abutments. Designers should also consider anddevelop method statements (based on calculation) forinclusion in Maintenance Manuals for replacement oradjustment of bearings taking due account of jacking/top slab effects and road traffic on the deck.

Integral abutments

3.8 As an extension to the concept of deckcontinuity, bridges can be designed with abutmentsconnected to the bridge deck without movement jointsfor expansion or contraction of the deck. The form ofconstruction known as integral construction, should beadopted in all cases where predicted relative settlementsare sufficiently small to allow it, and where bridgespans are not too long to incur unacceptable problemsin the design of the structure for thermal effects. Itshould be noted that the Overseeing Organisations’present bridge stock contains bridges of this typehaving overall lengths of up to 60m. In these situationsboth bearings and expansion joints can be eliminatedand maintenance requirements reduced.

3.9 In designing a bridge with integral abutmentwalls, the load effects due to temperature changes,shrinkage and creep should be considered inconjunction with soil/structure interaction.

3.10 When using integral (portal type) abutments atthe ends of long, including multi-span, bridges, thermaland other movements may be large enough to inducepassive earth pressures behind the abutment walls,especially near the top. Although the design againstthese pressures may result in costly, heavily reinforcedsections, they are still preferable to the use ofconventional expansion joints, and give much lesstrouble in service. There are some benefits in usingslender abutment walls (“balanced” design), becauseflexure of the walls tends to relieve the earth pressurebehind them. Further guidance on the design of integralbridges is provided in BA 42, ‘The Design of IntegralBridges’. As a variation, so called ‘semi-integral’bridges have been built which have the advantages ofthe elimination of deck surface expansion joints, butmay retain bearings, and tend to minimise soil structureinteraction effects. They however require very carefuldetailing to overcome potential future maintenanceproblems. Run-on slabs have also been utilised in thepast, and have some advantages in spanning areas ofpotential settlement of structural backfill behind theabutment. However they have tended to produce

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ongoing maintenance problems when they havecracked, tilted or collapsed through loss of support onthe approach embankment. It has been found that‘making up’ road pavements has generally been easierand less expensive where bridges have not utilised run-on slabs. On balance run-on slabs are not generallyrecommended, although where it is essential to utilisethem, careful design and construction is necessary.

3.11 In North America, multi-span continuous bridgeswith integral bank seats or short abutment walls arefrequently used. A typical arrangement of this type ofintegral construction is shown in Figure 3.6 and moredetails can be found in reference 5.

Buried structures

3.12 Rigid buried concrete box construction, which isan extension of portal frame construction, may bepreferable to a simply supported or a portal frame typebridge structure for short span bridges. In somelocations flexible designs incorporating corrugated steelstructures may be suitable. In general, buried structureshave important maintenance and durability advantagesover conventional bridge structures. Being remote fromthe immediate road construction, they are less sensitiveto all road influences, including the effects of de-icingsalts. Maintenance of the highway is also easier becausethe structure imposes fewer restraints on highwaymaintenance operations. Where conditions are suitable,their use is recommended.

Box sections

3.13 The size of box sections in bridge decks,abutments and piers should be such that properinspection and maintenance can be carried out withinthe box. Statutory provisions for access are contained inthe relevant Health and Safety legislation (reference 6).This may dictate the minimum practical size of boxsections. The minimum sizes of access openingsrequired by the Act, or by other requirements, should betreated as absolute minima; wherever possiblesubstantially larger openings should be provided.

3.14 If voids are too small to afford reasonableaccess, exceptional care must be taken to ensure thatthey are adequately sealed to prevent water ingress andfree from other durability problems. Consideration maybe given to the use of foamed concrete, polystyrenevoid formers or other means to fill voids, subject todead load and other design constraints. Such voidsshould however be provided with adequate drainageholes.

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3.15 In catering for ventilation it is highly desirable,and often possible, to incorporate a level of naturalillumination within boxes so that inspection is nottotally reliant on artificial lighting.

Plain Concrete

3.16 As ferrous reinforcement is susceptible todurability problems, consideration should be given tothe use of masonry or plain concrete construction by thechoice of suitable types of structure.

3.17 Plain concrete or masonry arch structures maybe feasible in some locations. In plain concrete archstructures the need for reinforced cantilevered spandrelwalls may be avoided by using mass concrete infill overunreinforced arch vaults. Options open to designersinclude the use of precast unreinforced voussoirs (withor without natural stone facing), unreinforced concretearches incorporating shrinkage reducing additives andsimilar structures with proprietary or other crackinducers at quarter points. Some designers have alsoconstructed concrete arches utilising dispersed non-ferrous fibres. Abutments and retaining walls in massconcrete should also be considered.

3.18 External cladding may be necessary to mask anyunsightly cracking due to early thermal effects. Thefixing of such cladding should be done using corrosionresistant materials of proven durability, for instancestainless steel, bronze or fibre reinforced polymer(FRP) inserts.

3.19 Where possible, the detailing of claddingsystems should be such that cladding panels can beeasily removed for the purpose of Principal Inspectionof the structure, or for maintenance work.

Reinforcement

3.20 As an alternative to the above, the control ofearly thermal cracking in plain concrete sections maybe achieved by using corrosion resistant reinforcement.The stresses in such reinforcement may be calculatedusing short-term properties of the materials andignoring the phenomenon of long-term loss of strengththrough creep. Creep is often significant with suchreinforcement, but is not considered relevant to thecontrol of early thermal cracking which is reasonablyshort term.

3.21 For the design of primary structural members,the use of non-ferrous reinforcement such as dispersedglass or aramid fibres in a resin matrix may in due

August 2001

course provide a significant improvement in thedurability of reinforced and prestressed concretestructures. Non-ferrous rebar may also be suitable insome situations, particularly in vulnerable concretesections and inaccessible locations, which may be proneto unseen deterioration. However there is comparativelylittle published research currently available, althoughthere are standards being developed in the United Statesand elsewhere. Any proposed use would require carefuldesign consideration from first principles. It would beappropriate to consider these applications in whole lifecost terms.

3.22 Stainless steel reinforcement may also beconsidered for use. Austenitic and duplex stainlesssteels can prevent chloride induced corrosion ofreinforcement and therefore improve durability. Theadditional cost of using stainless steel may to somedegree be offset by other design changes that may saveon initial construction costs without affecting durability.Over the life of a structure the use of stainless steel canbe justified by a reduction in routine maintenance andrepair. Consideration should be given to the use ofstainless steel in particularly vulnerable areas, such asbelow expansion joints, parapet edge beams, splashzones and in substructures in marine environments,particularly on heavily trafficked roads that tend to beregularly salted during the Winter months. For a limitednumber of structures, more extensive use of stainlesssteel throughout all the structural elements may bejustified. Since this would mean that initial constructioncosts may be significantly greater, this approach mustbe supported by a detailed whole life costing, andrequiring the prior approval of the OverseeingOrganisation. An Advice Note dealing with the use ofstainless steel is in preparation, and will deal with theassessment of where to use the rebar, changes to normaldesign rules and the selection of the appropriate gradesof stainless steel.

3.23 Epoxy coated reinforcement is not currentlyadvocated for use in highway structures. Experiencefrom structures elsewhere and research evidencesuggest that there have been some durability problemsassociated with the use of epoxy-coated rebar. It isparticularly prone to coating damage, which may leadto pitting corrosion.

Inspection and Maintenance

3.24 When considering structural forms, details andany relevant aspects in the design procedure, designersshould ensure that the structure, as well as itscomponents, can be effectively inspected and

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maintained. Inspection and maintenance considerationsshould be assessed in the design and technical approvalprocedures. Early identification of durability problemsby inspection should prevent severe and costly damageto a structure. Areas which are likely to be affected byde-icing agents or other corrosive elements must beaccessible for inspection and, where necessary, bedesigned and detailed to allow for repair or possiblereplacement. Designers should refer to BA 35 (DMRB3.3) for further details.

3.25 It is often cost-effective to incorporate in astructure facilities for routine inspection andmaintenance. In providing access, the general objectiveshould be to give the inspector a dry, comfortable andpleasant environment in which to work. Experience hasshown that, where access is difficult and where workingspaces are cramped, badly lit and poorly ventilated,damp or otherwise uncomfortable to work in, inspectiontends to be less frequent and the inspector’sobservational efficiency may be significantly impaired.

3.26 The following provisions for access should alsobe made at the design stage:

a) Access for cleaning, maintenance and painting.

b) Access to parts that may require maintenance orreplacement during the life of the bridge, forinstance, bearings, joints, anchorage locations,drainage, pipes, manholes, lubrication of movingparts, lighting systems etc.

c) Access for jacking at bearings and for theirremoval and replacement.

d) Access to closed cells or box sections.

3.27 Access points should preferably be at each endof the structure at points which are safe and easilyaccessible and do not require traffic control. Means ofaccess could include gantries, walkways, scaffoldingladders, rails or ‘cherry pickers’. However permanent orsemi-permanent facilities such as gantries requirecareful consideration, and assessment in whole life costterms. They have considerable implications for Healthand Safety issues and require special testing facilitiesand trained staff to operate them.

3.28 Public use of any of these access facilities andcolonisation of the areas in question by plants, animalsand birds, should be prevented.

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Bridge abutment galleries

3.29 Abutment details such as that shown in Figure3.7 create inaccessible areas which are vulnerable toconcrete contamination by de-icing salts throughleakage at joints, and are difficult to inspect andmaintain. In paragraph 3.8 the use of integral abutmentsis recommended wherever possible, for new designs.However, there will still be some locations wherearticulation at the ends of bridge decks is necessary. Insuch cases abutment galleries should be provided tofacilitate inspection and maintenance of both rotationalas well as expansion joints, bearings, abutment curtainwalls and deck ends. A typical arrangement of anabutment gallery is shown in Figure 3.8. The width andheadroom clearance of such galleries should preferablybe at least 1000 x 1800mm respectively and never lessthan 800 x 1500mm.

3.30 Abutment galleries can be useful for thedischarge and maintenance of drainage pipes throughbridge decks and waterproofing to relieve waterpressure within surfacing at joints. They may also assistbridge maintenance by facilitating access for futuredeck jacking. In mining areas, ground movement canclose bridge expansion joint gaps and the provision ofabutment galleries should reduce the extent of anyremedial works which are necessary to free such joints.

3.31 Access to abutment galleries will be possible insome bridges between or alongside deck beams. Entrycan also be arranged in some cases via secure lockabledoors in abutment or wing wall faces. Access throughdecks should be avoided as it can create hazards andcause maintenance problems. Abutment galleries inmost bridges will be permanently ventilated betweenbearings. Where this is not the case, ventilation shouldbe provided, particularly if gas mains exist or are likelyto be present in the vicinity of the bridge.

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Chapter 4Improved Durability - Problem Areas

- PROBLEM AREAS
4. IMPROVED DURABILITY
General

4.1 It is apparent from recent surveys on bridges thatthere are some structural forms and elements which aremore susceptible to durability problems than others.This section gives advice on the use of these forms andconsiders other areas which require special attention.

Half-Joints and Concrete Hinges

4.2 Half-joints, both in steel and in concrete, usuallypresent severe maintenance problems. They are difficultto inspect and repair and should not be used for newdesigns unless there is absolutely no alternative. Wherehalf-joints are used, steel and concrete surfaces shouldbe given additional protection. Adequate provision mustbe made for drainage, inspection and maintenance.

4.3 Concrete hinges are highly stressed areas where,because of the amount of reinforcement present,compaction of concrete is difficult. The steel in thehinges is vulnerable to corrosion from the ingress ofsalty water. Concrete hinges should not be used for newdesigns unless there is absolutely no alternative. Whereconcrete hinges are used, they should be visible forinspection and maintenance. Deck hinge joints areparticularly vulnerable to corrosion and they should notbe used in new designs.

Pre-tensioned prestressed concrete construction

4.4 Precast pre-tensioned concrete members havegenerally proved to be durable. Apart from concernabout occasional problems, for example, horizontalcracking of the beam in the end zones, the poorperformance of some bridges constructed with thesemembers has been associated with the use of simplysupported spans. The remedies for that are discussed inChapter 3.

4.5 De-bonded tendons at the ends of precast beamsshould be adequately protected against corrosion.

Post-Tensioned Concrete Construction

4.6 Unlike precast pre-tensioned concrete,construction using post-tensioned members has notproved to be particularly satisfactory in terms of

August 2001

durability. Most post-tensioned bridges built to datehave been of the internally grouted duct type, andproblems have been encountered in a number of these,largely due to the greater vulnerability to corrosion oftendons as a result of inadequate grouting of the ducts.The reduced durability has caused particular concernsince the deterioration often cannot be identified in thecourse of regular bridge inspections; this means thatserious loss of carrying capacity may remainundetected, with consequent risk to public safety. Insome instances there may be little or no warning ofcollapse in post-tensioned bridges, and this makes therisk of undetected deterioration more serious.

4.7 However, provided suitable safeguards areadopted in the process of grouting, internal post-tensioned grouted construction in non-segmentalbridges can be durable. Concrete Society TechnicalReport TR47 ‘Durable Bonded Post-tensioned Bridges’,as outlined in Interim Advice Note 16 ‘Post-tensionedgrouted duct concrete bridges’, details best practice anda specification for grouting, and these recommendationsshould be adopted. The detailed guidance should ensurethat ducts are fully grouted and post-tensioned systemsare protected and will be durable.

Segmental construction

4.8 Insitu joints between precast concrete segmentsare the areas most at risk from penetration by water andde-icing salts. This may lead to severe local corrosionof pre-stressing strands. Although new systems arecurrently under development to ensure the continuityacross the joint, and to provide greater protection, forthe time being such forms of construction using internalgrouted tendons are not permitted. Precast concretesegmental construction utilising external post-tensionsystems are permitted.

4.9 Another problem with segmental constructionwhich has not been widely recognised by designers isthe additional prestress loss due to large elasticcompression and subsequent creep deformation of thejoint material and closure of cracks at interfaces. As aresult, the final level of prestress in segmentalconstruction may be somewhat less than normal post-tensioned members.

4.10 Shortening due to shrinkage may also occur atthe ends of each precast unit. This could cause

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additional opening at the joints prior to stressing andhence reduce the compressive stresses at the interfacesand encourage cracking.

External Post-tensioned Tendons

4.11 Post-tensioned tendons positioned outside theconcrete section have the advantage of being accessiblefor inspection and replacement, and can be designed tofacilitate restressing. This must be balanced againstsome concerns about increased exposure andvulnerability. Where external post-tensioned tendonsare used, they should be properly protected and haveadequate facilities and access for inspection,maintenance and replacement. The method andsequence of cable replacement should be allowed for atthe design stage, and where possible designed toeliminate the necessity for traffic restrictions. It shouldbe noted that the Concrete Society Technical ReportTR47 ‘Durable Bonded Post-tensioned Bridges’ iscurrently being updated and is due for republication,and it is intended that it will include recommendationsfor best practice for external post-tension systemswhich should be adopted. Further information on designissues is available in BD 58 and BA 58 (reference 8).

Voided slabs

4.12 The adoption of pseudo-slab and similarstructures using void formers to achieve the final cross-section has lead to some serious problems, usuallyrelated to the buoyancy of the formers duringconstruction and the difficulty of compaction under thevoids. Special precautions should be taken in the designand construction of this type of structure.

Foundations and Buried Concrete Structures

4.13 Foundations and other buried concrete structuresin certain aggressive ground conditions have beenfound to be susceptible to sulfate attack, leading toeventual deterioration of the concrete. Although thishas been judged to be a serviceability issue, rather thana short-term safety concern, it does have implicationsfor long term durability. Although buried concrete is notoften or routinely inspected, most structures would beexpected to exhibit above ground indications of belowground concrete deterioration, before safety wasimpaired.

4.14 A range of measures to minimise the risks ofsulfate attack are recommended in the DETRpublication ‘The thaumasite form of sulfate attack.

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isks, diagnosis, remedial works and guidance for newnstruction’. In England this has been implemented bye Highways Agency Interim Advice Note 25easures to minimise the risk of sulfate attack

ncluding thaumasite). New construction and structuresnder construction’. The documents include options forncrete mixes, and additional protective measuresch as coatings for buried concrete and subsurface

rainage where appropriate, which will minimise thesk of all forms of sulfate attack. It is particularlycommended that vulnerable design details such asncrete hinges, joints and slender concrete sections areoided by ‘designing out’ such features.

.15 Further research is underway at the Buildingesearch Establishment and elsewhere and it ispected that BRE Digest 363 ‘Sulfate and acidsistance of concrete in the ground’ will be updated orplaced and will incorporate the latest guidance to dealith aggressive ground conditions. The 2001 edition ofe Specification for Highway Works and the Notes foruidance include requirements to minimise the risks oflfate attack.

oncrete subject to freeze thaw and wetting andrying cycles

.16 Concrete elements such as parapet upstands arearticularly vulnerable to freeze thaw action andetting and drying cycles. They may also be vulnerable chloride ingress. In accordance with the Specificationr Highway Works Notes for Guidance clause 1703.3i), where concrete of Grade 40 or lower is being used,en air entrainment should be adopted to increase

urability to counteract freeze thaw action and wettingd drying cycles. Concrete impregnation should also

e used to minimise chloride ingress. In Scotland airtrainment is adopted more widely for all exposedncrete, including bridge decks, as a result of more

nerous environmental conditions encountered.

ervices and service bays

.17 One of the areas where there are often durabilityroblems is in service bays. They are not easy tospect, and are prone to leakage from ill fitting,correctly replaced or damaged cover slabs. Water canso enter the service bay via badly detailed ornstructed concrete through deck ends and ballastalls, at joints or via the service ducts themselves.ervice bays should be provided with drainage holes,d should have all exposed concrete surfaces carefullyaterproofed. In general it is not recommended to fillrvice bays with ‘lightweight fill’, but it is better to

August 2001



assume that they will leak and deal positively with thewater that enters. Service bays should also facilitateaccess for authorised service providers.

4.18 Where possible it is recommended that drainagepipes, ducts and sleeves penetrating through bridgedecks and ballast walls, should be provided with puddleflanges cast monolithically into the deck, rather than asa second operation with a concrete ‘box-out’. This willrequire extremely careful positioning of the pipe orduct, and in some cases will not be practical. Otherrelevant information and details are contained inreference 12.

August 2001 4/3


- DETAILED

Chapter 5Improved Durability - Detailed Requirements

General

5.1 The life of a bridge can be considerablyenhanced at little additional expense by sound detailingof structural elements. This section gives advice onaspects of detailed design which should enhancedurability.

Reinforced Concrete

5.2 BD 57 (DMRB 1.3.7), increases the concretecover to reinforcement specified in BS 5400: Part 4:Table 13. However, in sensitive or critical areas of thestructure such as in the region below expansion joints,or where the reinforced concrete is in contact withflowing water, serious consideration should be given tothe use of concrete covers greater than those specifiedin BD 57. It should be noted too that the requirementsof BS 5400: Part 4, do not penalise the designer forusing greater cover than the Table 13 values withrespect to crackwidth calculations; the definition of cnomin Clause 5.8.8.2 makes it clear that the designer mayignore extra cover in calculating crack widths. It shouldbe noted that as BS 5400: Part 4 already makesprovision for an additional 10 mm cover for lightweightaggregate concrete, the BD 57 requirement foradditional concrete cover does not apply.

5.3 The minimum areas of main and secondaryreinforcement given in BS 5400: Part 4 Clause 5.8.4are, in many instances, not adequate to limit thecracking of concrete caused by the dissipation of heatof hydration while the concrete is immature. Designersshould refer to the requirements given in BD 28(DMRB 1.3), Early Thermal Cracking of Concrete. Indesigning reinforcement for early thermal effects thedesigner should bear in mind that the strength andcement content of the as-built concrete may be a gooddeal higher than that specified in the contract drawings.As the cement content has a significant effect on theheat evolution during hydration, the temperature effectsdue to the likely maximum cement content should beused.

5.4 Cement replacements, such as pulverised fuelash and ground granulated blastfurnace slag, mayreduce early thermal effects and improve resistance tochloride ingress, sulfate attack and Alkali-Silica

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5. IMPROVED DURABILITYREQUIREMENTS

August 2001

eaction. Reference should be made to thepecification for Highway Works and the Notes foruidance for specific requirements in concrete mixesign.

.5 Where local cracking of the concrete may occurue to restraint from adjacent elements, eg at corners of

o-way slabs, reinforcement should be carefullyetailed to control such cracking. In some cases, aetailed investigation of the stresses in these areas maye necessary.

restressed Concrete

.6 In post-tensioned structures, one location which of particular concern is the anchorage of tendons.esigners should ensure that sufficient anti-burstinginforcement is provided and that the layout of thechorage zone reinforcement is not congested or likely cause difficulties in placing and compactingncrete. Increased concrete cover should be provided ensure effective protection to the steel.

.7 Externally post-tensioned structures should beetailed to facilitate replacement or re-stressing of andividual tendon, without restricting traffic flow acrosse bridge. The provision of special monitoring devices detect loss of pre-tensioning or corrosion should bensidered. External tendons should be positioned soat they can be easily inspected and maintained,

owever this should be balanced against increasedposure and vulnerability.

rainage and Waterproofing Systems

.8 Drainage and waterproofing play a vital role ine durability of structures. Designers should refer toD 47 (DMRB 2.3.4) and BA 47 (DMRB 2.3.5) whenesigning drainage and waterproofing of concreteridge decks, and to reference 13.

.9 Drainage systems should be designed toinimise the risk of blockage and be accessible foreaning. They should be robust enough to withstandamage during cleaning, as this has been an importantuse of problems on many existing bridges. Theyould also be resistant to damage from chemicalillage on the road surface. The drainage of water from

5/1



bridge decks and waterproofing layers should normallybe done using closed systems which lead the waterpositively to the main highway drainage system.Allowing water from deck drainage to fall freely fromopen ended downpipes should be avoided for thefollowing reasons:

i) In windy conditions such water may becomefinely atomised and spray onto the structure, evenwhen downpipes project well below the soffitline.

ii) Freely discharged water may contaminate rivercourses.

iii) Freely discharged water may cause local damageto the soil surface below the bridge.

iv) Water from open-ended downpipes may fall ontoa carriageway or footway beneath and freeze,causing a hazard to both pedestrians and vehicles.There is also a danger that icicles can form onopen-ended downpipes and fall onto vehicles andpedestrians.

5.10 Drainage systems integral with the structure, forinstance gulleys cast into beams and pipes cast intocolumns, should not be used. Essential drainage runsthrough deck slabs should be made as short as possible.On short span bridges it may be preferable to collectsurface water off the bridge deck, although this willrequire careful design of deck and carriageway falls anddetailing, to ensure that no ponding on or beneath thesurfacing occurs. On bridges with shallow falls kerbdrainage may be used to good effect.

5.11 Drainage systems should be provided withadequate facilities for rodding and cleaning operations.Rodding access should be provided so that roddinglengths are straight or virtually straight, and do notnormally exceed 45m on straight runs, and should beroddable from either end. Careful thought should begiven to the practical needs of cleaning andmaintenance operations, and full details providedaccordingly. They should be designed to minimise theneed for traffic management during cleaning operations.All gullies should be fully trapped.

5.12 Surface water drainage of bridge decks shouldnever be directed into the drainage layers in the vicinityof piers and abutments since salty water from the bridgedeck may cause corrosion of the reinforcement in thesubstructure. Moreover, accumulated road silts anddebris may eventually clog the drainage layers.

5/2

5.13 The durability of a bridge can be improved bytaking the following precautions:

a) The top surface of bridge decks should haveadequate falls to avoid ponding especially in thevicinity of deck joints. Drainage outlets shouldbe formed using adequately sized products, atregular intervals.

b) Additional measures, such as coating and extrawaterproofing layers etc, may be considerednecessary where a concentration of de-icingagents is likely to occur.

c) Areas around kerbs, parapets and service trapsare most vulnerable to water seepage and shouldbe carefully detailed.

d) Access holes should be located on the undersideof bridge decks to avoid water leakage into thedeck. When this is not possible, properly sealedor/and positively drained manholes may be used,but only with the agreement of the OverseeingOrganisation.

e) Drainage should be provided at piers andabutments including the back of abutments.

f) Holes should be provided to drain the voids ofbridge decks, such as box beams and cellular andvoided slabs, as water may find its way into thesevoids causing corrosion and deterioration.

g) Box members should be provided with sealedaccess hatches or manhole covers to preventleakage into the box. Adequate and effectiveventilation and drainage holes should also beprovided to reduce condensation and eliminateany ponding inside the box as a result of apossible ingress of water. Ventilation anddrainage holes should be detailed to preventaccess and colonisation by birds and animals.

5.14 The following concrete surfaces should bewaterproofed using tar, cut back bitumen or appropriateproprietary materials as allowed in the Specification forHighway Works:

i) Vertical faces at deck ends and abutment curtainwalls.

ii) Top faces of piers and abutment bearing shelves.

iii) Inaccessible areas which may be subject toleakage; for instance beam ends.

iv) Buried concrete surfaces.

August 2001



Where waterproofing membranes may be directlysubject to foot traffic, they must be sufficiently robustto withstand such use, and should not be slippery.Concrete surfaces in splash zones should beimpregnated as detailed in clauses 5.18 and 5.22.

Expansion Joints

5.15 Designers should refer to BD 33 (DMRB 2.3)and BA 26 (DMRB 3.3) when designing and detailingexpansion joints and drainage provisions in bridgedecks. Guidance is also given in TRL ApplicationGuide 29 (reference 13) and designers are stronglyadvised to consult this document.

5.16 To prevent salty water from penetratingdownward to the substructure, expansion joints shouldbe watertight. However, these joints will eventuallyleak and therefore designers should not only applyprotective coating to surfaces at risk, but also providedrainage under the joints in the form of abutmentgalleries as described in paragraph 3.28.

5.17 Careful detailing around expansion joints inbridge decks can make a major contribution to thedurability of a structure. Failure of deck expansionjoints often leads to severe corrosion of adjacent partsof the structure. The areas around a joint should bedetailed in such a way that they do not provide traps forwater and that an effective system is provided toremove the water quickly. All the elements should bedetailed so that they are accessible for inspection andmaintenance.

Splash zones

5.18 Designers should be aware that the splash zoneof river or road piers and abutments are particularlysusceptible to deterioration. In some situations saltywater may be splashed up to the soffit of overbridgescausing deterioration and corrosion. In addition thespray may result in a retention of salt in the soiladjacent to the carriageway thus causing severechloride attack to the concrete sub-structure. Specialprecautions should be taken in these areas by theapplication of protective coating, for instance chemicalimpregnation, and additional cover to steelreinforcement should be provided (see paragraph 5.2).

Other details

5.19 It is essential to provide drip checks at all edgebeams, deck ends over abutments and other locations

August 2001

such as copings to retaining walls, to prevent waterfrom running back along horizontal surfaces. Where,for reasons of concrete cover, the provision of groovetype drips is not practicable continuous unreinforcedconcrete downstands or continuous non-ferrous anglesections properly fixed to deck edges, may be used asdrippers. BA 33 (DMRB 2.4) shows a prefabricateddrip strip for use on existing structures.

5.20 Bridge decks should be designed to projectbeyond the substructure to prevent salty water fromrunning down columns and abutments.

5.21 The designer should always consider the ease ofconstruction and maintenance of the proposed details.For example, adequate provision should be made forcompacting concrete and painting of structural steel.

Impregnation of Concrete Surfaces

5.22 Impregnation of concrete surfaces provideseffective protection against the ingress of chlorides.Requirements for impregnation procedures are given inBD 43 (DMRB 2.4) and BA 33 (DMRB 2.4), and otheraspects are dealt with in the Specification for HighwayWorks and the Notes for Guidance. The materialspecified is monomeric alkyl (isobutyl) tri-alkoxysilane, although other materials are permitted providethey comply with the performance specification.

Other measures

5.23 Recent research carried out by the TransportResearch Laboratory for the Highways Agency hasindicated that there can be benefits to durability byproducing good quality near surface concrete. Althoughthe research looked at various materials and techniques,the clearest benefits came from the use of concreteswith lower water cement ratios (incorporating the use ofsuperplasticisers) and the adoption of controlledpermeability formwork (CPF).

Controlled Permeability Formwork

5.24 CIRIA have published a report CB511‘Controlled Permeability Formwork’, which hascomprehensively reviewed the technique and availablematerials. Whilst there are advantages in using CPF,this must be balanced against additional costs, andsome practical difficulties that may occur duringconstruction, particularly with complex shapes. Thecurrent position is that CPF may be used in specificnew construction situations where there are:

5/3



i) Concrete elements in close proximity tocarriageways, which are heavily salted on aregular basis each Winter.

ii) Concrete elements having simple geometricshapes and plain finishes.

CPF must also be justified in whole life cost terms.

Silane impregnation will still be required in accordancewith clause 1709 of the Specification for HighwayWorks. For the time being the use of CPF will beregarded as an aspect not covered by Standards, andOverseeing Organisation approval will be required, aspart of technical approval procedures.

Corrosion Inhibitors

5.25 Research is being undertaken at TRL andelsewhere, to assess the benefits of using corrosioninhibitors in concrete of different mixes, qualities andcondition. There are a number of corrosion inhibitorson the market today that claim to reduce chloridegenerated corrosion in rebars by forming a protectivelayer around, and operating on the surface chemistry ofthe metal. These materials are soluble salts that areadded to the concrete at the construction stage, to repairconcrete during refurbishment, or as surfaceapplications on mature concrete. The inhibitors areclassified as either cast-in or migrating types, with onesupplier having a pelleted delivery system. A literaturereview has shown that commercial materials sold undervarious brand names contain calcium nitrite, borax,zinc borate, sodium malonate, sodiummonofluorophosphate, amines or amino alcohol basedcompounds and other formulations. Although there aremany research papers examining the corrosioninhibition properties of a number of these compounds,their long term efficacy in real structures with varyingconcrete condition and subject to a range ofenvironmental conditions has yet to be fully proved.

5.26 The TRL research, which was conducted withreasonably good quality concrete, indicates positiveresults for the effectiveness of inhibitors in the form ofcast-in concrete admixtures based on calcium nitriteand amino alcohols, used in new construction. Theresults for the migrating surface applied and thepelleted delivery system corrosion inhibitors tested isless encouraging. However other researchers havefound in tests conducted in lower quality concrete thatthere may be some beneficial effects with thesemigrating inhibitors. They may be considered for usewhen applied to concrete of poor quality, where the

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hloride levels are low. However for the present theirse is not advocated on high quality relatively

permeable structural concrete.

.27 The benefits in using corrosion inhibitors asoncrete admixtures appears to lie in their use inoncrete elements which are in close proximity toarriageways, which are heavily salted on a regularasis each Winter. Any proposal to use a corrosionhibitor concrete admixture would need to be justified whole life cost terms. Silane impregnation will still

e required in accordance with clause 1709 of thepecification for Highway Works, for areas oftructures as detailed in BD 43 and BA 33. For the timeeing the use of cast-in corrosion inhibitors will beegarded as an aspect not covered by the Standards.

ther additives

.28 Research evidence and site experience indicatesat there may be benefits in using proprietary materialsat comprise both water reducing superplasticisers and

ore blockers to provide a dense concrete matrix withydrophobic properties. Although the capital costs ofuch materials are relatively high compared to normaloncrete, they may be justifiable in whole life costrms. Consideration may be given to their use in

xtremely aggressive environments and structurallements that are difficult to access for inspection andaintenance.

ightweight concrete

.29 Structural Lightweight Aggregate ConcreteLWAC) is generally accepted as being more durablean normal weight concrete with good resistance to

reeze-thaw cycles and corrosion of steel reinforcementue to the effects of de-icing salts. LWAC typically with strength of 40N/mm2 and a density of around 75%at of normal weight concrete, utilises aggregatesanufactured from the industrial by-products of

lectricity generation (pulverised fuel ash - Lytag) andteel manufacture (blast furnace slag -Pellite). It canlso be made from the processing of natural materials,or example expanded clay, but these manufacturedggregates are not currently available in the UK.

.30 TRL have carried out research into LWAC forse in bridges. The research concluded that, althoughWAC is more expensive than normal weight concrete, may result in overall savings in construction cost,ainly due to its reduced dead weight. LWAC bridge

ecks exhibit smaller thermal movements, and there areerefore additional benefits associated with abutments

August 2001



of integral bridges. Construction cost savings of about3% may be achievable, and can be higher where LWACfacilitates modifications to the conceptual design; forinstance, the elimination of a pier or expansion joint.There are however large regional variations in the costof LWAC, and some concrete production facilities maynot be able to supply LWAC. The results of the TRLresearch suggest that there are clear durability benefitsfrom using LWAC made from pulverised fuel ash,though the results are less encouraging for LWAC madefrom blast furnace slag.

5.31 LWAC will also reduce the impact of bridgeconstruction on the environment and the demand onfuture bridge maintenance. In view of its cost andenvironmental benefits it should be considered at thefeasibility stage as an option for most structures withspans of over 15m. If it is a viable option it will besubject to Overseeing Organisation approval, as part oftechnical approval procedures.

Electrochemical techniques

5.32 Electrochemical techniques such as cathodicprotection and electrochemical chloride removal(desalination) are generally outside the scope of thisAdvice Note, but can be considered as methods toenhance the durability of in service structures. Aseparate Advice Note dealing with cathodic protectionis in preparation.

August 2001 5/5


NTS - STEEL BRIDGES

Chapter 6Detailed Requirements - Steel Bridges

General

6.1 Where steels are welded in areas of highrestraint and where tensile stresses occur perpendicularto a plate surface, eg in cruciform joints, corners of boxsections and heavily welded sections, lamellar tearingcould occur. In such situations, designers should payproper attention to weld joint design and use steels withguaranteed through-thickness properties.

6.2 Welds for temporary attachments can act asstress raisers and increase the risk of fatigue. Suchwelding should not be allowed in critical areas.Temporary attachments should be removed and weldsground flush. (See 1800 NG 1801)

6.3 Transverse bracing members between parallelgirders are often subjected to stress reversal due to liveloads. Therefore the effects due to fatigue at theirconnections with main girders should be considered indesign.

6.4 Simple connections and weld details, which areeasier to inspect and maintain, should be used whereverpossible.

6.5 Intermittent fillet welds should not be used,except in situations where the welded connections arecompletely protected from the weather, for example,where they are wholly inside closed box structures; insuch cases appropriate fatigue checks should be carriedout. Intermittent welding, where one or both sides of theconnection are exposed to the outside atmosphere,cannot be properly protected against the ingress ofwater into the welded joint by capillary action orpenetration of water through the connection.

6.6 Steelwork should be detailed so that it is self-draining and prevents the accumulation of water. Areaswhere dirt and debris may collect should be avoided.Particular measures that can be adopted are theomission of stiffeners from the outer face steel girders,provision of drainage ‘mouseholes’ at stiffener/bottomflange connections and detailing for water runoff atpiers and end supports. Attention is also required wheresteel is used as packing material or as shims.

6. DETAILED REQUIREME

August 2001

Corrosion protection of steelwork

6.7 The most common method of corrosionprotection of steelwork is by painting. Designers shouldrefer to MCHW Volume 5 Section 2 for maintenancepainting and the Specification for Highway Works(MCHW 1) and the Notes for Guidance (MCHW 2) forthe Overseeing Organisations’ requirements on paintingof steelwork.

6.8 Designers should be aware that the success ofcorrosion protection depends not only on the protectivesystem specified but also on the surface preparation,quality control and the effectiveness of the paintingoperation. Steel components should therefore bedesigned and detailed with the recognition that theymust be capable of being effectively prepared, painted,inspected, cleaned and repainted. Particular attention isrequired at plate edges where corrosion may initiate,where packing and shims are used, and for metalliccomponents such as bearings.

Metal coating of steelwork

6.9 Galvanising and suitable sprayed metal coatingscan give effective corrosion protection to steelwork.Designers should refer to the Specification for HighwayWorks (MCHW 1) and the Notes for Guidance(MCHW 2) for their use. Care must be observed whendetailing steelwork for galvanising. Some details areunsuitable for dipping and advice should be soughtfrom the Galvanisers Association.

6.10 In specifying galvanising for high tensile steelsuch as bolts, post-tensioning bars and cables which aresubjected to high fluctuating stresses, designers shouldbe aware of the danger of hydrogen embrittlementassociated with galvanising.

Steel Box Sections

6.11 The recommendations of Section 3.13 applyequally to steel box sections.

6.12 The interior of steel box sections should bepainted a light colour to improve visibility.

6/1


Chapter 6Detailed Requirements - Steel Bridges

Other considerations

6.13 Bridge deck enclosures may be considered foruse in particularly aggressive environments. They offerthe benefits of reduced maintenance liabilities in termsof painting of steelwork, and may be appropriate toconsider where access to the superstructure is limitedeg major rail, river and road crossings. They must beevaluated and justified on whole life cost grounds.More detailed information and requirements arecontained in BD 67 and BA 67 (reference 8).

6.14 Weathering steel may be considered for use asan alternative to conventional steel deck construction,as it corrodes more slowly, and should minimisemaintenance liabilities. However there are somerestrictions on its application and these are detailed inBD 7 (reference 8).

August 20016/2


Chapter 7References

7. REFERENCES

(1) The Performance of Concrete Bridges:G Maunsell & Partners, HMSO. HighwayStructures.

(2) National Corporative Highway ResearchProgram Synthesis of Highway Practice 322,‘Design of Precast Prestressed Bridge GirdersMade Continuous’. Transportation ResearchBoard, National Research Council, BridgesWashington, DC 1990.

(3) Kumar, A. Detailed Design of CompositeConcrete Bridge Superstructures. British CementAssociation, 1988.

(4) Kumar, A. Composite Concrete Bridge Superstructures.British Cement Association, 1988.

(5) National Cooperative Highway ResearchProgram, Synthesis of Highway Practice 141,Bridge Deck Joints. Transportation ResearchBoard, National Research Council, Washington,DC, 1989.

(6) Health and Safety legislation relevant toOverseeing OrganisationFactories Act 1961Section 4 Approved Code of Practice (ACOP):Management of Health and Safety at Work.

Northern Ireland

Workplace (Health, Safety and Welfare)Regulations (NI) 1993Confined Spaces Regulations (NI) 1999

(7) BS 5400 Steel, Concrete and Composite Bridges:Parts 1 and 4.

(8) The Design Manual for Roads and Bridges

BD 7 (DMRB 2.3.7), Weathering Steel forHighway Structures.

August 2001

BD 28 (DMRB 1.3), Early Thermal Cracking ofConcrete.

BD 33 (DMRB 2.3.6), Expansion Joints for Useon Highway Bridge Decks.

BD 43 (DMRB 2.4), Criteria and Material for theImpregnation of Concrete Highway Structures.

BD 47 (DMRB 2.3.4), Waterproofing andSurfacing of Concrete Bridge Decks.

BD 57 (DMRB 1.3.7), Design for Durability.

BD 58 (DMRB 1.3.9), The Design of ConcreteHighway Bridges and Structures with Externaland Unbonded Prestressing.

BD 67 (DMRB 2.2.7), Enclosure of Bridges.

BA 26 (DMRB 2.3.7), Expansion Joints for Useon Highway Bridge Decks.

BA 33 (DMRB 2.4), Impregnation of Concrete

BA 35 (DMRB 3.3), Inspection and Repair ofConcrete Highway Structures.

BA 42 (DMRB 1.3.1) The Design of IntegralBridges

BA 47 (DMRB 2.3.5), Waterproofing andSurfacing of Concrete Bridge Decks.

BA 58 (DMRB 1.3.10), The Design of ConcreteHighway Bridges and Structures with Externaland Unbonded Prestressing.

BA67 (DMRB 2.2.8), Enclosure of Bridges.

Interim Advice Note 16 Post-tensioned groutedduct concrete bridges.

Interim Advice Note 25 Measures to minimisethe risk of sulfate attack (including thaumasite).

(9) The Manual of Contract Documents for HighwayWorks

Specification for Highway Works (MCHW.1)Notes for Guidance on the Specification for

7/1


Chapter 7References

Highway Works (MCHW.2)Maintenance Painting of Steel HighwayStructures (MCHW 5.2)

(10) DETR publication ‘The thaumasite form ofsulfate attack. Risks, diagnosis, remedial worksand guidance for new construction’.

(11) CIRIA report CB511 Controlled PermeabilityFormwork.

(12) Transport Research Laboratory ApplicationGuide 33 ‘Water management for durablebridges’ by S.Pearson and J.R.Cuninghame(funded by Highways Agency and CountySurveyor’s Society).

(13) Transport Research Laboratory ApplicationGuide 29 ‘Practical guide to the use of expansionjoints’ by C.P.Barnard and J.R.Cuninghame(funded by Highways Agency and CountySurveyor’s Society).

(14) Concrete Society Technical Report TR47‘Durable Bonded Post-tensioned Bridges’published in 1996 is currently being updated andis due for republication.

(15) BRE Digest 363 ‘Sulfate and acid resistance ofconcrete in the ground’ published in 1996 will bereplaced in 2001.

August 20017/2


August 2001 8/1

8. ENQUIRIES

All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:

Head of Civil EngineeringThe Highways AgencySt Christopher HouseSouthwark Street A J PICKETTLondon SE1 0TE Head of Civil Engineering

Chief Road EngineerScottish Executive Development DepartmentVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerThe National Assembly for WalesCynulliad Cenedlaethol CymruCrown BuildingsCathays Park J R REESCardiff CF10 3NQ Chief Highway Engineer

Assistant Director of EngineeringDepartment for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street D O’HAGANBelfast BT2 8GB Assistant Director of Engineering

Chapter 8Enquiries