Design of Steel Bridges for Durability: Technical Report

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TECHNICAL REPORT SCI PUBLICATION 154

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Design of Steel Bridges for Durability

C W BROWN MA(Cantab) CEng FICE D C ILES BSc MSc ACGI DIC CEng MICE

Published by: The Steel Construction Institute Silwood Park Ascot Berkshire SL5 7QN Tel: Fax:

01344 623345 01344 622944

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SCI Technical Reports Technical Reports are intended for the rapid dissemination of research results, as and when they become available, or as “specialist documents” for further discussion. They provide an opportunity for interested members to comment and offer constructive criticisms.

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Please forward your comments to Mr D C Iles, The Steel Construction Institute, Silwood Park, Ascot, Berkshire, SL5 7QN.

© 1995 The Steel Construction Institute Apart from any fair dealing for the purposes of research or private study or criticism or review, as permitted under the Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the UK Copyright Licensing Agency, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers, The Steel Construction Institute, at the address given on the title page. Although care has been taken to ensure, to the best of our knowledge, that all data and information contained herein are accurate to the extent that they relate to either matters of fact or accepted practice or matters of opinion at the time of publication, The Steel Construction Institute, the author and the reviewers assume no responsibility for any errors in or misinterpretations of such data and/or information or any loss or damage arising from or related to their use. Publications supplied to the Members of the Institute at a discount are not for resale by them. Technical Report Number:

SCI-P-154

ISBN 1 85942 028 1 British Library Cataloguing-in-Publication Data. A catalogue record for this book is available from the British Library.

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FOREWORD The purpose of this Technical Report is to present, for consideration by designers, the aspects of conceptual and detailed design which affect the durability of steel and composite bridges, and thus to assist designers to make an effective and economic use of steel. The protection of steelwork against corrosion is a known technology which, when properly applied, will ensure minimum maintenance in a predictable inspection and rehabilitation programme. Likewise, design against fatigue failure under a given loading regime uses well-developed techniques based on extensive research. Much of design for durability is common sense, although sometimes reminders are necessary that certain matters need to be considered.

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The work leading to this Technical Report was funded by British Steel, Sections, Plates & Commercial Steels. The authors are grateful to Mr W Ramsay (British Steel) for assistance during the preparation of the document.

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CONTENTS

Page No.

FOREWORD

iii

1

INTRODUCTION

1

2

GENERAL

2

3

DURABILITY OF PROTECTIVE COATINGS 3.1 General 3.2 Breakdown in protective coatings 3.3 General guide to the use of ‘problem-free’ details 3.4 Choosing and specifying coating systems 3.5 Surface preparation for painting 3.6 Initial painting 3.7 Maintenance painting 3.8 New or unconventional paint systems

3 3 3 4 5 5 6 7 7

4

ALTERNATIVES TO COATINGS 4.1 Weather resistant steel 4.2 Enclosure 4.3 Ventilation and air conditioning inside box girders

9 9 10 10

5

PARTICULAR ISSUES OF DURABILITY 5.1 Bearings and expansion joints 5.2 Parapets 5.3 Waterproof membranes and road surfacing 5.4 Concrete deck slabs 5.5 Fatigue-sensitive structural components 5.6 Stiffened steel deck plates

11 11 11 12 12 13 13

6

ACCESS 6.1 General 6.2 Mobile access platforms 6.3 Scaffolding from ground level 6.4 Suspended scaffolding 6.5 Maintenance cradles or gantries 6.6 Enclosure

15 15 15 15 16 16 16

7

CHECKLIST

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P154: Design of steel bridges for durability Discuss me ...

1

INTRODUCTION

Over the past 20 years, engineers have become increasingly aware of the importance of ensuring the durability of structures. Any structure, regardless of the material in which it is constructed, is likely to require periodic inspection, maintenance and, occasionally, repair, to ensure that it continues to function satisfactorily over its lifetime (commonly, for bridges, specified as 120 years). The objective of design for durability is to ensure that such inspection and maintenance is kept to a practical minimum and is as easy as possible to carry out when needed, so that the total ‘whole-life’ cost of the bridge is minimised. This can mean that the ‘first cost’ of the structure may have to be increased in order to reduce the subsequent cost of inspection, maintenance and repair. To have a basis for comparison, attention has to be given to methods of converting future costs into equivalent capital costs at current prices. In the UK, guidance on how to carry out the cost-discounting calculations is given in the Highways Agency (HA) Standard BD 36/92, together with the associated Advice Note BA 28/92. The sum of the initial cost plus the ‘discounted’ future cost of inspection, maintenance and repair gives the whole-life cost of the bridge.

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It is not in the scope of this document to give a detailed commentary on the HA Standard and Advice Note mentioned above. However, it is important to understand that maintenance costs are deemed by HA to include not only the relatively easily quantified direct costs, but also the more conjectural notional costs arising from traffic delay and disruption during maintenance operations. Frequently the notional costs exceed the direct costs by a significant amount. As the notional costs are directly related to the length of time that traffic restrictions apply, part of the effort in design for durability must be directed to ensuring that this time is minimised, even at the expense of higher direct costs. While design to avoid repair is an integral part of design for durability, it is much less easy to quantify the implications, since repair by its nature is unpredictable. Damage to bridges that requires repair may arise from many causes, such as vehicle impact, sabotage, fatigue, serious materials failure, etc. A designer must keep abreast with all reported incidents to avoid, as far as is possible, repeating any errors that led to them. Much of design for durability is common-sense. However, there are still too many designs being produced which are deficient in some respects, and the purpose of this short publication is to give guidance on how to improve the durability of steel bridges by appropriate design. It is not intended to be a specialist treatise on any aspects where detailed guidance is available elsewhere (for example, the detailed specification of protective systems). Nor is any guidance on costing included - figures vary widely and in any case soon go out of date. Hence particular projects must be carefully costed using the best estimates available at the time, with maintenance costs being discounted to present values, as in BD 36/92. To achieve long life with quantifiable maintenance costs requires that a bridge is designed with durability as a prime consideration. The following subjects will therefore be discussed in this publication: C

Durability of protective coatings (breakdown; problem-free details; specification; surface preparation for painting; initial painting; maintenance painting; new or unconventional paint systems)

C

Alternatives to coatings (weather resistant steel; enclosure systems; ventilation of box girders)

C

Particular problems (bearings and expansion joints; parapets; waterproof membranes; concrete decks; steel decks; fatigue)

C

Access (mobile access platforms; scaffolding; cradles and gantries; enclosure).

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2

GENERAL

The design of a new bridge can directly affect the durability and the maintenance costs, and hence the whole-life cost. Careful consideration should be given to whether an increase of initial capital cost may be warranted in order to reduce the subsequent maintenance costs. Such consideration would be particularly important in the case of a bridge where the notional costs of traffic delay and disruption during maintenance were disproportionately high - in such cases large increases in initial capital cost can be justified to reduce the duration of maintenance.

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Steel bridges have an unjustified reputation for high cost maintenance. For instance, to some designers, the one word “rust” is a serious disincentive to the use of a steel bridge; this is a shortsighted and unreasonable attitude, for several reasons. To a large extent this view has been based upon experience of older structures where the choice of steelwork details and protective coating had been based on lower relative labour costs than currently prevail. Furthermore, it cannot be emphasised too strongly that corrosion of structural steel is a surface phenomenon; it is readily detectable in its early stages and, provided remedial action is taken, will not affect the overall integrity of the structure in any significant way. Another perceived “problem” with steel bridges is that of fatigue. The resistance to fatigue of some early bridges in high strength structural steel was not adequate: some of these bridges are showing evidence of premature fatigue failure, although in most cases this can be repaired relatively easily. In the last twenty years a vast amount of research into fatigue has been carried out and this, together with extensive test programmes, has ensured that the subject of fatigue is much better understood now than it was in, say, 1950. Designers can now design bridges that have a high probability of lasting without serious fatigue problems for more than 100 years, using well-proven details. It is important not to become complacent, since there are unfortunately occasional examples of steel bridges where unexpected and unwelcome problems have occurred. However, it is pertinent to point out that repair or strengthening of deficient steel bridges can be a comparatively straightforward process, usually involving in-situ welding or bolting on additional steelwork without restricting traffic. To summarise, a steel bridge is durable, when properly designed and maintained, and its whole-life cost is competitive with that of a bridge in any other material. In particular, it should be noted that: C

Design for durability is a well known and proven technology

C

Steel bridges have more than 100 years proven record of durability

C

Corrosion of structural steel is a surface phenomenon; it is readily detectable in its early stages and, provided remedial action is taken, will not affect the overall integrity of the structure in any significant way

C

When deterioration is noted, it can readily be rectified

C

Reliable coating systems, both for initial painting and maintenance painting, are available

C

Modern steel bridges are designed to be easily inspectable and maintainable

C

Maintenance should be regular; it is a known technology with quantifiable costs, and is thus easy to discount to present values that can be included in the whole life cost of the bridge

C

Satisfactory design against fatigue has now become routine for steel bridge designers.

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3

DURABILITY OF PROTECTIVE COATINGS

3.1 General Modern protective coatings, properly specified and applied, should last for at least 15 years, and often last longer. In Sections 3.4 to 3.8 some general guidelines are given on the specification of coating systems; however the performance of any coating system is highly dependent on the detailing of the structural steelwork to which it is applied. Consideration is therefore first given in Sections 3.2 to 3.3 to the ways in which efficient and effective steelwork details in steel bridges can contribute to, and assist with, the prolonged protection of the steelwork. This helps to minimise any breakdown in the protective coating. Conversely, bad or inappropriate details can cause premature breakdown of even the best coatings.

3.2 Breakdown in protective coatings A designer must be aware of the basic reasons for breakdown of coatings in order to produce designs which will minimise it.

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The chemistry of corrosion of steel is the formation of hydrated ferric oxide (rust) as a result of an electrochemical reaction between the steel and the environment. For corrosion to occur requires the presence of oxygen and water. If the water is pure, the corrosion rate is low. If the water is impure (e.g. due to the presence of contaminants, such as sodium chloride in solution), it acts as an electrolyte and then the corrosion rate increases significantly. The ideal coating, were it to exist, would be tolerant of poor surface preparation and the presence of moisture. It would be effectively applied in thick coats, be fast drying, and have good chemical and corrosion resistance to abrasion and ultra violet light. It would be suitable for over-coating after a time interval without undue preparation. Finally, it would be durable and moderately priced. Very few paint formulations satisfy all or even most of these requirements. Hence, the design, detailing and manufacture (fabrication, assembly and erection) of the steelwork must take into account all the criteria which should be observed to achieve the maximum life of the protective coating. Recent experience in the maintenance of steel bridges has shown that the most common causes of breakdown are the following: C

Poor consideration of problems when detailing.

C

Damage caused or initiated during erection.

C

Poor workmanship when applying coatings.

C

Incorrect appreciation of local site environmental factors.

C

Poor consideration of future maintenance requirements.

The first of these is directly related to the design and detailing of the structure, whilst the others can be alleviated to some extent by good practice. In particular maintenance can be improved by provision of adequate access.

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3.3 General guide to the use of ‘problem-free’ details There is a wide selection of protective systems available, and that chosen in any situation should protect the steel adequately and effectively at the most economic cost. Detailing has an important influence on the life of a protective coating, since it can affect the quality of protection achieved locally, and poor detailing can modify the local environment (around the detail) detrimentally. In particular, details should avoid the entrapment of moisture and dirt between elements.

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Most coatings are applied by spray, and for high quality protection all surfaces need to be blast cleaned. Detailing of steelwork should recognise this and not create areas which cannot be blasted and sprayed effectively. Sharp edges can be sources of problems in exposed areas since the coatings will inevitably be thin over them. In some circumstances grinding a sharp edge to a smooth curve will be necessary. It is important to avoid obstructions on the bottom flanges of girders that might prevent the passage of water running along the surface. Fabrication of plate girders often causes some upward curvature of the bottom flange, and a water channel can form at the web. Where obstructions are unavoidable, such as at bearing stiffeners, preventive measures should be considered to allow water to drain from these areas, for example by providing notches or cope holes. These requirements have to be considered against the difficulty of achieving good surface preparation and applying the protective coating at notches, cope holes or the like. Indeed, modern fabrication techniques allow transverse stiffeners to be welded all round, without corner snipes. This has considerable advantages for surface preparation and application, but can lead to the problems of entrapment of moisture and dirt described earlier. Reservations have been expressed about the efficiency of cope holes. Some designers hold the view that a cope hole area is impossible to protect properly, and that the lesser evil is to weld all round a transverse stiffener and be certain that the protective treatment is carried out properly, rather than have water passing an inadequately protected area. Particular care is required at abutment diaphragms and in the design of the bridge deck drainage system. The possibility that any part of the steelwork may remain permanently damp or may collect debris must be avoided. Cases have occurred where drainage gullies with open-ended pipes which discharge 150 mm below the bottom flanges of the bridge girders have allowed the saltwater discharge to be blown back onto the bottom flanges causing localised corrosion damage. Drainage is prone to blockage from debris, and the design should prevent this as far as possible. With normal articulated construction, abutment galleries can facilitate inspection and maintenance of joints, bearings, abutment curtain walls and deck ends, and allow positive drainage arrangements to deal with the inevitable leakage of carriageway joints. For short spans, jointless bridges could be used more extensively in highway bridge construction, and this is discussed further in Section 5.1. As stated in Section 3.2, both oxygen and water need to be present for corrosion to occur. For example, the internal surfaces of hollow sections do not corrode, provided complete sealing is achieved to prevent the entry of moist air; this fact can be put to use in small box girders (less than 750 mm × 750 mm), which can be left unpainted if the seal is guaranteed (probably by air pressure testing). Enclosed spaces such as the inside of large steel box girders will contain moist air, which may result in condensation. Condensation may in turn lead to water ponding inside the girder. Significant corrosion can result where the water is standing, but corrosion of other parts of the box is very slow because of the absence of pollutants in the air. Condensation, and hence ponding, can be minimised by well detailed ventilation and/or drainage of the box. A reduced paint specification, and longer intervals between repainting, can therefore be used for the inside of steel box girders in these circumstances.

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3.4 Choosing and specifying coating systems

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The correct choice and specification of coating systems and surface preparation are an essential part of design for durability. Some general aspects are listed below: (a)

Compared with ‘paint-only’ systems, metal coatings such as galvanising and aluminium metal spray give a durable coating more resistant to site handling and abrasion, but they are generally more costly (see also Section 3.6.2). Metal coatings are usually over-coated, for added protection and appearance.

(b)

Steel-concrete interfaces are particularly vulnerable. Steel surfaces in contact with concrete should be free of loose scale and rust but may otherwise be untreated. The treatment on adjacent exposed areas should be extended at least 25 mm into the interface. Any metal spray coating on the interface must be over-coated.

(c)

Galvanising is not common for primary elements of bridge structures, apart from some small bridges fabricated from rolled sections. It is particularly suitable for piece-small fabrications that may be vulnerable to handling damage, particularly when despatched to site overseas. In all cases, attention to detailing is essential at an early stage. Galvanising is sometimes not suitable for welded members, especially if slender, asymmetric, or comprising plate thicknesses less than 5 mm, as they are liable to distortion due to release of residual stress and may require straightening. Care must be taken when doubler plates are used and cope holes provided to ensure drainage when members are extracted from galvanising baths. (Guidance is available from the Galvanisers Association on detailing for drainage during the dipping process.)

(d)

The time between surface preparation and application of the first coat of the protective system should be as short as possible. The exact time will be dependent on the environment and the protective system itself.

(e)

Where possible, lifting cleats should be provided for large steelwork members, to avoid damage to the coating during handling.

(f)

The maximum amount of protective treatment should be applied in a controlled environment. It is generally appropriate to apply at least the final coat at site after making good any erection damage, although there are good examples where all paint coats have been shopapplied, or the final coat has been applied on the ground at site prior to erection.

(g)

For High Strength Friction Grip bolted joints the interfaces should be grit blasted or metal sprayed only, without any paint treatment, to achieve friction. During painting in the workshop the interfaces are usually masked with tape that is removed before site assembly.

(h)

Galvanised or electro-plated bolts should normally be used, or blast cleaning will be necessary after installation. Site blast cleaning of the areas of bolted joints should be avoided if possible because of the damage it causes to other parts of the structure.

3.5 Surface preparation for painting The importance of surface preparation, both for initial painting and for maintenance painting, cannot be too strongly emphasised if maximum durability is to be obtained. Inadequate surface preparation is one of the major causes of premature failure of a coating. The cleanliness and roughness of the steel surface are both important, and the specification of surface preparation and paint system should be compatible - some systems are much more demanding than others on the quality of preparation. The detailed requirements should be made clear in project specifications. Surface preparation must also include such matters as removing sharp edges by grinding corners etc.

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Since the quality of surface preparation and the paint system chosen are interrelated, the choice of system for maintenance painting might well depend on the extent to which a clean bare surface can be achieved.

3.6 Initial painting 3.6.1 General The initial protective treatment to be applied to a bridge depends, of course, on the environment in which it is to be built. All bridge authorities have specifications for paint systems to be applied, and generally speaking all are aiming at a period of at least 15 years and sometimes as much as 30 years between major maintenance of the coating. This does not mean that no maintenance is expected earlier; minor touching up must always be expected and, if done well, can extend the period between major repainting. It is not the purpose of this publication to describe or specify standard paint systems (typical systems are given in many standards and specifications, for example by the Highways Agency, Railtrack, London Underground, etc.). However, two general questions are worth considering in principle: whether to specify a metal coating under the paint system, and whether to use zinc-rich primers.

3.6.2 Metal coating as a first coat

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Galvanising is an effective metal coating which can be used for small components, such as parapets, but it is not suitable for the main structural members, because they are too large. The question whether to specify metal spray, has in the past been debated at length in UK. Currently, for the main steelwork, the Highways Agency Specification (and associated Notes for Guidance), requires the specification of aluminium spray under a multi-coat system in any situation of “difficult” access. Difficult access, in HA's definition, refers not only to physical difficulty but also to situations where access would cause traffic disruptions and delays. Aluminium spray is currently specified for over 80% of bridges in UK. Initial painting systems using aluminium spray are expensive, and there is a belief by some engineers that the full benefits are seldom realised. Much has been said about the potential benefits of aluminium metal spray as a ‘fall-back’ system, as it will continue to protect the steel after substantial breakdown of the paint has occurred. When a structure is protected only by a paint system and the system is allowed to deteriorate for too long, additional cost, effort and time will be necessary to restore the steel substrate to a suitable standard to receive a high performance paint system, if subsequent maintenance cycles are to be maintained. Alternatively, the paint coating could be reinstated soon after first deterioration is noted, but this might preclude the adoption of the most effective overall maintenance strategy. The use of aluminium metal spray enables maintenance to be delayed as a result of the ‘fall-back’ nature of the coating, thus extending the intervals between repainting, or enabling it to be planned at a time that fits the maintenance strategy. This would be of maximum advantage in places where the physical access was very difficult, but the advantage should also be recognised by the designer when evaluating the traffic delay costs as part of his estimation of whole life costs. Because the aluminium metal spray will continue to protect the surface after deterioration of the paint coatings becomes apparent, the repainting can be arranged when the road is closed for other purposes (such as resurfacing). Then no additional traffic delay costs would be incurred at all. Provided advantage can be taken of its ‘fall-back’ protection, the use of aluminium metal spray will have clear benefits. If, however, it is just treated as “another coat of paint” it may just be an expensive luxury. It should also be recognised that if the metal coating itself is allowed to deteriorate, its removal and reinstatement as a maintenance operation can be very costly.

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3.6.3 Zinc pigmented primers The second question, whether zinc-rich or zinc silicate primers are suitable for bridges, also gives rise to much difference of opinion. These are successfully used in many industries around the world, including offshore, but are not generally used in UK for bridges. It seems that the reason for not using such primers is the belief that zinc rich primers are prone to failure due to a lack of cohesive strength within the coating. However, this is based on experience reported in the past by the Highways Agency, and the failures probably resulted from the high percentage of zinc pigment and low binder content in the dried film. This however, does not reflect either modern working practice or advances in paint technology; the use of zinc flake type coatings gives improved particleparticle contact, and binder levels can thus be increased with resulting better cohesive strength. Properly used, zinc-rich or zinc silicate primers are excellent galvanic coatings, provided they are in good contact with the clean steel; zinc silicates, in particular, require a very high standard of surface preparation, and it is possible that some reported failures were a result of not recognising this.

3.7 Maintenance painting

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3.7.1 General Systems to be used for maintenance painting must (unless the initial paint is completely removed) be compatible with what has been used initially. Sometimes this will mean reusing the initial system; more frequently it may mean finding an alternative which will adhere to the original coating. Solvent-based paints as initial coats will show to advantage, since over-coating softens the original paint thus aiding adhesion. Epoxy paints can be particularly difficult to over-coat subsequently because of adhesion problems (although ‘sweep cleaning’ can be used to improve adhesion).

3.7.2 Metal spray first coat Reference was made in Section 3.6.2 to the difficulty and cost of removal and reinstatement of a metal sprayed coating if it is allowed to deteriorate. For this reason, metal spray is unlikely to be used in maintenance painting.

3.7.3 Zinc pigmented primers Zinc pigmented primers can have a place in maintenance painting, particularly if an initial metal zinc coat (either zinc sprayed or galvanised) has been employed in the original system. In such circumstance the use of a zinc pigmented primer can prove an economic and effective way of repairing the metal coating.

3.7.4 Washing of paintwork It has been suggested by various authorities that the life of a paint coating can be extended by regular washing with clean water. Tests that have been carried out tend to support this view, and show that the repainting intervals might be extended by two to five years. However, washing is a costly operation in itself and has, to be effective, to be done about every two years. There is therefore considerable doubt over the cost-effectiveness of the proposal.

3.8 New or unconventional paint systems 3.8.1 General Engineers are always seeking new and more efficient paint systems. The search is for systems which are cheaper, easier to apply, more effective, require less maintenance, be more tolerant of damage, be non-toxic, be possible to apply at night (to reduce traffic delay and disruption), and be easier to over-coat. Many of the best of today's conventional systems are environmentally unfriendly or toxic, and are being banned by various local, national or international authorities. For

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example, chlorinated rubber paints, which have been used in preferred systems in the UK for many years, are being phased out because of the release of large quantities of carbon tetrachloride during solvent evaporation. Chlorinated rubbers are generally being replaced by acrylated rubber or vinyl, but there is a continuing search for better systems.

3.8.2 High build systems For some years standard practice in France has been the use of two coats of a moderately high build system. This idea has been taken further in the UK where a substantial number of bridges have been coated with a single coat of some 1000 µm dry film thickness, using a system based on an elastomeric urethane. Laboratory tests and general field experience indicate that a period of at least 20 years to first maintenance could be expected with a coating of this type. Originally the first cost of the high build elastomeric urethane system was some 30% higher than that of a conventional 6-coat system, but with increasing use and familiarity, this premium has now almost been eliminated. Other high build systems based on, for example, glass flake epoxy coatings can be applied in thicknesses up to 750 µm in a single coat as part of a two or three coat system. These systems produce extremely tough abrasion resistant and durable coatings which are being considered in UK as possible painting systems for new bridges.

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3.8.3 Moisture tolerant paints Where traffic delay and disruption is a significant problem, consequent effects can be much reduced by carrying out maintenance painting at night. The main problem is that of dew forming on the steelwork, and hence special moisture tolerant paints are being developed for this application. These still present some problems in application, and also from the point of view of health and safety.

3.8.4 Other systems Research has been commissioned by the Highways Agency into the use of a number of other coatings. These include the high build urethane and glass flake epoxy coatings mentioned above and the following systems: C

Moisture cured polyurethanes

C

High build epoxy

C

Urethane alkyd

C

Epoxy coal tar

C

Surface tolerant systems for hand prepared steelwork

C

MIO epoxy/urethane top coat

C

Glass flake/reinforced polyester

C

Zinc silicate/HB Vinyl intermediate and top coats

C

Fluoro polymer based top coats

It is hoped that the results of the research will be published in due course. Apart from any physical problems in use, a number of the more sophisticated alternative systems as well as existing systems may suffer from problems of toxicity and/or can cause environmental damage during curing (solvent evaporation, etc.). It seems possible that some will be banned, or at least made more difficult (and expensive) to use, as a result of increasingly stringent environmental legislation.

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4

ALTERNATIVES TO COATINGS

4.1 Weather resistant steel As an alternative to the provision of a protective coating, the use of weather resistant steel in bridges is now well established worldwide. Unpainted weather resistant steel offers large potential savings in whole-life costs through the elimination or reduction of maintenance painting requirements. These savings are enhanced where access for maintenance painting would be particularly difficult, such as over electrified railway lines.

4.1.1 Environment

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Where suitable atmospheric conditions prevail, weather resistant steel develops a tightly adhering oxide layer which reduces the rate of subsequent corrosion. Weather resistant steel is not, however, appropriate in all environments; the following environments are not suitable for weather resistant steel in highway bridges: C

in marine environments

C

where the structure may be subjected to road spray containing de-icing salts

C

in continuously wet or damp locations

C

when buried

C

in heavily polluted or corrosive industrial environments.

4.1.2 Detailing The successful use of weather resistant steel depends on good bridge detailing, and many of the points discussed in Section 3.3 for painted bridges are equally valid for bridges in weather resistant steel. In particular, since weather resistant steel behaves less well in confined spaces, there is a case for painting it in these areas (which, if properly detailed, should be readily accessible). Generally, experience has shown that the worst corrosion occurs near the expansion joints, emphasizing the need to presume that leakage will occur whatever form of road joint is specified, and to ensure that measures are taken to cater for this to avoid damage to the steel. A practice which has been adopted is to provide a full depth concrete trimmer beam at the ends to prevent contamination from the expansion joint area. If the deck edge cantilever is constructed at least as wide as the depth of the main beams, the likelihood of drips from the edge staining or corroding the webs and flanges of the beams is much reduced. An additional benefit of the shelter provided by the wide cantilever is that the beams are more likely to develop a uniform colour. With a narrow cantilever, the lower part of the beam is significantly less sheltered than the top and this can result in different corrosion behaviour over the depth of the beam. Drip plates have been used with some success on certain weather resistant steel bridges. These are small non-structural elements, typically welded across a flange to direct water (running on the surface during rain) off the surface, but these may require maintenance unless they are treated as sacrificial. The use of box girders may avoid horizontal surfaces on which water and debris can collect (such as the bottom flange of an I beam), but will attract a cost penalty in the fabrication rate.

It has been suggested that weather resistant steel is more prone to fatigue than painted structural steel, as a result of pitting. A desk study carried out some years ago by The Steel Construction Institute indicated that this is not true for welded structures in appropriate environments, since the welded details are generally of a lower fatigue class than the corrosion pits. It is true, however, that surface breaking fatigue cracks are harder to detect visually in the oxide layer of weather resistant steel than on a painted surface.

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P154: Design of steel bridges for durability Discuss me ...

4.2 Enclosure The use of a durable enclosure around the primary structural elements, such as the main girders of a composite bridge, offers a controlled and improved environment for the steelwork. It will increase the period between maintenance operations, and enable a reduced specification for the protective system to be used, thus reducing the whole-life cost of the bridge. Initial capital costs of the bridge steelwork are reduced, since a painting specification appropriate to the internal steelwork of a box girder can be used for enclosed steelwork and there is no need for a final site coat of paint. Indeed, some authorities believe that enclosed steelwork need not be coated at all. It is obviously important that the material chosen for the enclosure itself does not require maintenance. The main disadvantage of an enclosure is its high initial cost, although if maintenance of the bridge would otherwise give rise to high notional costs from traffic disruption throughout its life, an enclosure may prove to reduce the whole-life cost.

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4.3 Ventilation and air conditioning inside box girders The advantage of a controlled environment in reducing corrosion of steelwork has been mentioned earlier (e.g. inside enclosures, see Section 4.2). In some instances the designer has taken this to its logical extreme in the inside of box girders by installing air-conditioning plant to reduce the humidity (and hence condensation) and pollution. This certainly permits the surface protection of the internal steelwork to be much reduced - indeed it is unlikely that any significant corrosion would arise in such a controlled environment even in the absence of any painting. Air-conditioning plant, however, is extremely costly to install, and there are also operating and maintenance costs. It may be questioned, therefore, whether it is cost effective, despite its efficiency in protecting the steelwork. Observations inside large steel box girders without such plant show that, provided there is no ponding of water, corrosion is minimal despite relative humidity frequently over 90%. High humidity causes condensation, however, so if standing water is to be avoided, drainage and ventilation must be provided. Proposals made some years ago for sealing large box girders and putting desiccant in them are not practical for three reasons: (a)

If a large box girder is completely sealed, the differential between the internal and external pressures on the walls can cause unacceptable stress levels

(b)

If the seal proves incomplete (much more likely, because of the difficulties in ensuring complete sealing) moist air will enter and condense; since the box was intended to be completely sealed there will be no drainage or ventilation, hence water will collect and corrosion will occur.

(c)

Provision will be needed for internal inspection to check performance of the system and refresh the desiccant.

Determination of whether air conditioning, or drainage and ventilation, would provide the lower whole-life costs, would have to be checked for individual designs.

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P154: Design of steel bridges for durability Discuss me ...

5

PARTICULAR ISSUES OF DURABILITY

5.1 Bearings and expansion joints Bearings and expansion joints are regular sources of durability problems. Virtually all expansion joints leak, allowing salty water through in winter, and when this is not properly dealt with the leakage runs onto the structure beneath and onto the bearings. Corrosion causes bearings to seize and stop fulfilling their function (although it has to be admitted that this is seldom a cause for serious structural concern).

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To alleviate potential problems, a number of general guidelines for the design of bearings and expansion joints can be given: C

Minimise the number of expansion joints by using continuous spans where possible. (With steel or composite bridges continuous construction is almost always used.)

C

Ensure that the detailing of expansion joints and bearings makes them easy to maintain and replace

C

Ensure that the bridge has easily accessible jacking points to enable it to be raised for bearing replacement

C

Where joints must be provided, ensure that they are designed to be watertight and maintainable

C

Even if joints are designed to be watertight, provide drainage below them to carry away any water that leaks through

C

Provide drainage all round bearings

C

Provide adequate access galleries in abutments and clearances at piers to enable joints, bearings and drainage to be fully inspected and maintained

C

Use bearings of as simple a construction as possible, which will continue to function even if seriously corroded or full of debris.

Alternatively: C

Design the structure as an integral bridge. For shorter spans, expansion joints at the end of the structure can be eliminated by integrated design of substructure and superstructure. The Highways Agency is now requiring consideration of integral construction for all bridges up to 60 m length (except where there is a large skew).

Observance of many of the above guidelines will add only insignificantly to the capital cost of the bridge and some, such as continuity or integral construction, can actually reduce it.

5.2 Parapets In theory, a properly designed and fabricated parapet should be maintenance free or at most, if in steel, need only simple periodic repainting. In practice, every parapet is liable to suffer periodic damage from vehicle impact, vandalism, etc. Hence the primary durability requirement for the parapets themselves is easy replaceability. The fixings of parapets into concrete are particularly susceptible to deterioration and should therefore be protected since replacement is difficult. Note should also be taken of the tendency of a parapet to act compositely with the structure in resisting bending moments on a cross section. Some designers have tried to avoid this by making the parapet in short lengths with breaks between them. This seems a misconceived idea - analysis shows that the breaks have to be at very frequent intervals to achieve the objective, and there will then be durability problems at such points (for example, discontinuous parapet rails are undesirable, and hence there will be a multiplicity of sliding expansion joints). The correct solution appears to

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P154: Design of steel bridges for durability Discuss me ...

be to design the parapet to withstand the strains imposed on it by bridge flexure; the magnitude of these may be reduced by not making the parapet continuous until all the dead load is on the bridge.

5.3 Waterproof membranes and road surfacing 5.3.1 General There is no doubt that the presence of a sound waterproof membrane on a bridge deck is one of the most important items in promoting durability. If salt water is able to penetrate this membrane, serious damage will result, whether the deck is of concrete or steel. On a bridge with a stiffened steel plate deck the surfacing will also act to improve the fatigue resistance of the deck (see Section 5.6.1).

5.3.2 Materials

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Many materials which in previous years had been considered satisfactory for deck waterproofing have now been shown to cause problems. Until comparatively recently many concrete decks (either on concrete or steel beams) were surfaced with hot rolled asphalt, with no separate waterproofing membrane. This surfacing does not provide adequate waterproofing, and serious degradation of the concrete, mainly through salt contamination, has been the inevitable result. Most bridge authorities now require a membrane between the concrete and the asphalt. Mastic asphalt, on its own or with a membrane, has been used on both steel and concrete decks; even on its own it was a better waterproofing barrier than hot rolled asphalt. Use is now being made of epoxy or acrylic membranes, with apparent success.

5.4 Concrete deck slabs 5.4.1 General Concrete deck slabs on composite steel bridges are prone to deterioration from various causes as outlined below, and precautions have to be taken.

5.4.2 Road de-icing salts The primary cause of deterioration of a concrete deck slab is corrosion of the steel reinforcement. The initiation of corrosion is usually due to the ingress of chlorides from road de-icing salts, and this generally takes about 15 to 25 years. It can also arise from carbonation, although this can easily be avoided by the use of good quality concrete mixes and construction practice.

5.4.3 Inadequate cover to reinforcement A major factor in premature concrete deterioration is the use of inadequate cover, either because the specified minimum was too low or because of high variability of cover actually achieved on site. This leads to penetration of salt which attacks the steel reinforcement. The Highways Agency Standard on Durability (BA & BD 42/95, due to be issued in 1995) requires the use of increased cover distances, above those given in BS 5400: Part 4. A designer may be tempted to reduce cover to achieve weight saving in the decks of bridges. However, to achieve durability for 100 years or more, the cover to the reinforcement of a minimum of 40 mm must be achieved in practice; appropriate tolerances from the specified cover must be chosen and met. This is particularly important in bridge deck slabs where lower covers make them critically sensitive to a breakdown of the waterproofing.

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P154: Design of steel bridges for durability Discuss me ...

5.4.4 Repair Deterioration generally arises in the deck slab where the waterproofing has broken down and salty water penetrates to the slab. Once salt has been absorbed into the porous matrix of concrete to the depth of the reinforcement and corrosion has begun, the corrosion rate will be accelerated by a factor of about 10 relative to bare steel. Unfortunately, there are no cost effective remedies comparable to the cleaning down and repainting of steelwork. Indeed, there is little prospect of controlling the corrosion cost effectively without major disruption to the operation of the bridge. As a last resort, it may be possible to restrict the traffic without closing the bridge completely, and then to replace the complete deck slab on a lane-by-lane basis. This would be possible if, as is usually the case, a composite bridge has been designed with the steel beams alone carrying the dead load, without relying on composite action.

5.5 Fatigue-sensitive structural components

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Fatigue has long been seen as a major source of unplanned (and hence very expensive) maintenance and repair of steel bridges. Whilst this might have been true, up to a point, in the past, the enormous effort that has been put into research (both theoretical and practical) in the last 25 years or so means that the problems are now much more fully understood. Designers now have the information required to avoid future problems. Extensive design codes are available (e.g. BS 5400 Part 10), and Eurocode 3 Part 2 will contain rules that extend the applicability of the fatigue requirements in Part 1 to bridges. Few difficulties should arise in the general detailing of steel bridges to avoid fatigue failures; it is normally only necessary to relate details to the expected levels of variation of stress. The design codes referred to above make it comparatively easy for a designer to determine which details should be used in what circumstances, and which should generally be avoided. However, certain areas such as stiffened steel decks have proved difficult to design, and some early decks (e.g. Severn, and several bridges on the continent) have shown premature fatigue failures, as discussed in Section 5.6.1. Elements of bridges which cannot be guaranteed to last for the life of the bridge should be designed to be replaceable. The replacement cost can then be calculated and discounted in the same way as any other future expenditure, and added to the total whole-life cost of the bridge. An example that comes into this category is a cable, either as the stay of a cable stayed bridge or as the hanger of a suspension bridge. In modern designs these can always be designed as replaceable, usually without reducing the capacity of the bridge during the actual replacement operation. Provision should be made in the design for any jacking necessary.

5.6 Stiffened steel deck plates 5.6.1 Fatigue Stiffened steel deck plates are highly loaded by wheel loads from traffic. The details of the stiffeners and their connections are generally of a comparatively low fatigue category, and this combination can result in premature fatigue. In the past, when designers concentrated only on the static strength of the deck, fatigue problems arose. However, experience has now identified the particular trouble spots and enabled them to be overcome; modern designs of stiffened steel decks, properly fabricated, are expected to last for at least 100 years. The qualification “properly fabricated” has been shown to be extremely important. Many premature failures of decks have occurred because the welds connecting the longitudinal stiffeners (or “stringers”) to the plate are of poor quality, usually fillet welds with a gap between the plate and the stringers. The weld then flexes about a longitudinal axis as wheel loads pass close by on each

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P154: Design of steel bridges for durability Discuss me ...

side, and it is not surprising that early fatigue failure results. Today the welds would either be butt welds, or the stiffeners would be pressed into hard contact with the plate and deep penetration fillet welds used. Where the stringers were designed solely for static strength, fatigue problems occurred at the splice between contiguous lengths of stringer. These have been overcome by increasing the size of the stringers, and refining the details of the splices. It is interesting to note that modern UK practice (second Bosporus Bridge) and Danish practice (Storebælt East Bridge) have ended with virtually the same design of deck from quite independent calculations. It has been proposed seriously that a ‘European Standard’ stiffened steel bridge deck should be developed; the probability is that it would be very similar to those just mentioned.

5.6.2 Surfacing

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Another special consideration with a steel deck is the surfacing. Special types of surfacing are required to ensure adhesion and waterproofing, and also to act compositely with the steel deck plate, thus reducing the flexural stress induced by wheel loads and improving the fatigue resistance. Practice has differed in various countries - the UK practice of 30 years ago (3 mm of rubber bitumen waterproofing overlain with 38 mm of hand laid mastic asphalt) gave adequate service for many years (usually 15 or more if properly executed) but the problems of hand laying the asphalt (nobody will now do it) and the not entirely satisfactory performance of machine-laid mastic have forced a search for alternatives. The deck plate itself requires special surface treatment (usually metal spray and etch primer) before laying the surfacing. The importance of extensive tests and trials in the correct environment is emphasised; to take an extreme case, surfacing designed for a bridge in the tropics would be totally unsuitable for one in northern Europe, and vice versa.

5.6.3 Fatigue of surfacing Stiffened steel decks are flexible, and the continued flexing is likely to cause fatigue of the surfacing, because it tends to act compositely with the steel deck plate and hence develops fluctuating tensile strains in the top surface. This is particularly serious close to a hard line support such as a web of a main girder. Various expedients have been tried to avoid cracking, with its consequent ingress of water, in such areas. For example, it has been specified for some bridges that the surface should be cut by sawing a longitudinal groove about half the depth of the surfacing above such hard line supports, and the groove sealed with a suitable bitumastic sealer. Whilst this will prevent water ingress, structurally the plate will lose the benefit of most of the composite action with the surfacing which reduces plate fatigue, and a careful analysis will be needed to check acceptability. Another approach has been proposed for Eurocode 3: Part 2 (Design rules for steel bridges). In this approach (as presently drafted), the stringers would have to be designed to a minimum stiffness to reduce the flexing of the deck. The required stiffness will be higher for stringers adjacent to a web, to reduce the relative deflection and hence the flexural strains in the surfacing. Whilst superficially this sounds an attractive approach, a full study of the implications has still to be done, although preliminary checks suggest that the modern design of deck referred to above complies with the proposed requirements.

5.6.4 Further possibilities On many large bridges the saving of dead weight is very important for economy. Often, therefore, proposals have been made for using one of the modern thin epoxy or similar surfacings which have been used successfully on footways. Because of their thinness they offer no improvement from composite action with the deck plate, and hence make the problems of fatigue of the roadway deck under traffic loading more severe. To overcome this, the steel deck plate would have to be substantially thickened; an estimate made some years ago indicated that to replace the “fatiguebeneficial” effects of 40 mm of mastic asphalt, the deck plate would have to be thickened by 7 mm at least. At that time, the relative costs of materials made this uneconomic.

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P154: Design of steel bridges for durability Discuss me ...

6

ACCESS

6.1 General Since steel bridges will require regular inspection and maintenance (even if at long intervals), it is particularly important that the question of access is considered during design. Access provisions added as an afterthought are seldom entirely satisfactory. The need for provision of, for example, gantries for external inspection of major bridges is now fully appreciated. What is not so obvious is that inside a large steel box girder bridge, the deck soffit may be out of reach of an inspector standing on the bottom flange. Provision must therefore be made for some form of mobile gantry or tower to enable the inspector to get within touching distance of the whole area of the deck. For more modest structures, scaffolding is by far the most common method of access. However, even with scaffolding, adequate provision in design for anchorage and bracing points can make erection and dismantling of scaffolding much easier. At the other extreme, permanent enclosure of the superstructure could enable inspection and maintenance to be carried out at any time in a controlled environment. In practice, what is provided is usually between the two extremes.

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In general, apart from some very small bridges that can be reached without any special provisions for access, the options for maintenance access fall into five main categories: C

Mobile access platforms,

C

Scaffolding from ground level,

C

Suspended scaffolding,

C

Maintenance cradles or gantries,

C

Enclosure.

Each of these options has to be considered on the basis of cost, both initial and recurring, the effect that it has on bridge users, and its “flexibility” in terms of the extent of the structure that can be inspected or maintained at any one time.

6.2 Mobile access platforms Typical examples of this type of access are “underbridge” units and hydraulic platforms. For the bridge owner these systems will generally represent no initial cost as they can readily be hired whenever they are needed. Generally these types of access are better suited to inspection than to maintenance. In particular, hydraulic platforms and “cherry pickers” generally have little capacity for carrying maintenance equipment. Furthermore, the extent of the bridge that can be accessed is limited. Relocation may only be possible within the reach of the machine unless operations are temporarily suspended. Ground supported “underbridge” units can only be used where the ground is suitable. This automatically rules out their use for bridges over water or where they would block the service below the bridge.

6.3 Scaffolding from ground level For small overland bridges scaffolding from ground level can appear to be a cheap option in terms of the direct cost of access. However, for major bridges, particularly over water, it is not appropriate.

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P154: Design of steel bridges for durability Discuss me ...

6.4 Suspended scaffolding The use of suspended scaffolding can also be comparatively cheap in terms of direct cost and provided that sufficient headroom is available below the bridge, it can offer an efficient means of access for both inspection and maintenance. The costs of suspended scaffolding lie principally in erection and dismantling and it is therefore most cost effective, in terms of direct costs, for programmed inspection and maintenance. The disadvantages of suspended scaffolding are that local damage may be caused where the scaffold is supported from the bridge and remedial works to these areas can be difficult to carry out with the scaffold in place, and it may cause operational problems where headroom below the bridge is limited. The latter will not cause problems with high bridges over water.

6.5 Maintenance cradles or gantries Maintenance cradles or gantries which are particular to an individual bridge have a high initial cost, dependent on the degree of sophistication required of the system. The extent of access available at any one time is dependent on the size and number of cradles or gantries provided, and is therefore directly related to cost. Permanent gantries will usually only be economic or practicable on major bridges.

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The details of moving gantries and their whole system must be determined during design of the bridge. Decisions which will have to be made, and which will significantly affect the cost and ease of operation of the installation, include: C

What loading will the gantry be required to carry in terms of workforce, supplies, plant, etc?

C

Will the gantry be of steel or aluminium? the latter will be lighter but probably more expensive but may save on the cost of rails, etc.

C

Will the gantry run on a structural element (e.g. fascia beams) or will special rails have to be provided?

C

What facilities need to be provided to allow the gantry to pass bridge piers or towers, or will separate gantries be provided for each span?

C

Will the gantry be powered or hand-driven? If the former, what type of motors and what power will be required?

C

What control system for operating the gantry will be required?

Provided that all relevant questions are resolved during design, a purpose-made gantry should provide the most efficient access from which to inspect and maintain a high level bridge. Unfortunately there has been a history of unsatisfactory gantries (nearly always as a result of underestimating the performance requirements), and as a result the Institution of Structural Engineers in London has set up a working group specifically to consider the design and operation of such devices. The group has not, as yet, reported its findings.

6.6 Enclosure Enclosure (Section 4.2) is independent of obstructions under the structure, and with adequate clearance below the enclosure membrane offers no obstruction to traffic or shipping below. The total flexibility which an enclosure thus allows for access for the programming of inspection and maintenance operations, plus the improved environment for carrying out maintenance work, will reduce the overall length of time for maintenance by making operations independent of weather conditions.

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P154: Design of steel bridges for durability Discuss me ...

7

CHECKLIST

The following is a checklist of points that a designer should consider to achieve maximum durability of the steelwork in a bridge. In detailing: C

Ensure that all surfaces to be coated can be effectively blast cleaned.

C

Avoid sharp edges, particularly in exposed situations. If necessary, specify grinding to a smooth curve.

C

Avoid traps for water and debris on the bottom flange of girders.

C

Avoid small notches and cope holes which cannot be coated effectively.

C

Ensure that any drainage outlets are well away from steelwork, so that spray cannot be regularly blown back onto it.

C

Provide access for inspection and maintenance of steelwork at supports, particularly below expansion joints.

C

In box girders, avoid details which can trap any condensation or leakage.

C

Provide shop-fitted lifting cleats, to avoid damage to protective treatment.

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In specifying the protective coating: C

Recognise the full advantages of metal coating as a first coat, i.e. an increased life of the coating system and the ability of the metal to prevent deterioration of the steel surface long after the paint coating shows signs of breakdown.

C

Ensure that steel-concrete surfaces are properly sealed.

C

Apply as much of the coating system as possible in a controlled environment (limit site coats to splices and readily visible surfaces).

C

Specify galvanising or electroplating for bolts.

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