Austroads

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GUIDE TO BRIDGE TECHNOLOGY

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Guide to Bridge Technology Part 2: Materials Summary

The Guide to Bridge Technology, Part 2: Materials covers all aspects of the common building materials available to the engineer including concrete, metallic and non-metallic materials and timber. Part 2 of this guide discusses material characteristics, their properties, durability, construction issues when using such materials, and protection and preservation treatments. A detailed section on concrete reinforcing materials is also included.

Keywords

Bridge Materials, Concrete Materials, Concrete Characteristics, Concrete Durability, Concrete Steel Materials, Metallic Materials, Non-metallic Materials, Connections, Fibre Reinforced Polymers, Timber

First Published September 2009 © Austroads Inc. 2009

This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads.

ISBN 978-1-921551-46-8 Austroads Project No. TP1564

Austroads Publication No: AGBT02/09

Project Manager

Geoff Boully, VicRoads Prepared by

Don Carter Ray Wedgwood

Published by Austroads Incorporated Level 9, Robell House

287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email: [email protected] www.austroads.com.au

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Austroads profile

Austroads purpose is to contribute to improved Australian and New Zealand transport outcomes by:

 providing expert advice to SCOT and ATC on road and road transport issues

 facilitating collaboration between road agencies

 promoting harmonisation, consistency and uniformity in road and related operations

 undertaking strategic research on behalf of road agencies and communicating outcomes

 promoting improved and consistent practice by road agencies.

Austroads membership

Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government in Australia, the Australian Local Government Association, and New Zealand Transport Agency. Austroads is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its 11 member organisations:

 Roads and Traffic Authority New South Wales

 Roads Corporation Victoria

 Department of Transport and Main Roads Queensland

 Main Roads Western Australia

 Department for Transport, Energy and Infrastructure South Australia

 Department of Infrastructure, Energy and Resources Tasmania

 Department of Planning and Infrastructure Northern Territory

 Department of Territory and Municipal Services Australian Capital Territory

 Department of Infrastructure, Transport, Regional Development and Local Government

 Australian Local Government Association

 New Zealand Transport Agency.

The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.

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TABLES

Table 3.1: Strength and ductility of reinforcement ... 18

Table 6.1: Typical properties of cast iron ... 69

Table 6.2: Typical bridge material properties... 76

Table 6.3: Typical properties of aluminium used in bridge applications ... 80

Table 8.1: Bolting classification ... 93

Table 9.1: Comparison of material properties of FRP to steel, concrete and timber ... 106

Table 10.1: Strength properties of green timber ... 119

Table 10.2: Strength properties of seasoned timber... 119

Table 10.3: Minimum target values for visually graded timber ... 121

Table 10.4: Minimum target values for machine stress-graded timber ... 121

Table 10.5: Durability class and in-service life... 125

Table 10.6: Preservative treatment and classification ... 126

FIGURES

Figure 2.1: Plastic shrinkage cracking in deck ... 7

Figure 2.2: Pier headstock cracking caused by AAR ... 7

Figure 2.3: Cross-section of concrete core showing expansive gel around aggregate... 9

Figure 2.4: Vertical cracks below water in octagonal prestressed concrete pile... 9

Figure 3.1: Damage to reinforcement due to poor quality tack welding ... 19

Figure 3.2: Butt splice ... 20

Figure 3.3: Welded butt splices in column ... 20

Figure 3.4: Welded lap splice... 21

Figure 3.5: Macros of welded lap splice to check the penetration of weld ... 21

Figure 3.6: Tangent modulus ... 26

Figure 3.7: Secant modulus ... 26

Figure 3.8: Barrel and wedges and seven wire strand... 28

Figure 4.1: Effect of excess cover on cantilever ... 31

Figure 4.2: Air void as a result of pouring concrete both sides of void former ... 34

Figure 4.3: Vertical core through deck showing plastic cracking and voids – poor compaction ... 36

Figure 4.4: Top surface of the cored deck showing severe plastic shrinkage cracking .... 37

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Figure 5.5: Fire damage to Yowaka River bridge ... 59

Figure 6.1: Coalbrookdale cast iron bridge ... 67

Figure 6.2: Graphitisation of cast iron ... 70

Figure 6.3: Wrought iron caissons above ground – cast iron caissons below ground ... 71

Figure 6.4: Cast iron columns on timber bridge pier ... 71

Figure 6.5: Cast iron shoe at lower end of timber truss member ... 72

Figure 6.6: Wrought iron lattice truss ... 73

Figure 6.7: Wrought iron plate showing laminar structure ... 74

Figure 6.8: Increase in yield point by repetitive straining ... 77

Figure 6.9: Effect of hardening and tempering... 78

Figure 6.10: Tensile test of reinforcing bar – ductile failure ... 79

Figure 6.11: Fracture in aluminium weld ... 81

Figure 7.1: Corrosion due to accumulation of dirt in member ... 83

Figure 7.2: Crevice corrosion at steel/timber interface ... 83

Figure 7.3: Crevice corrosion at steel/steel interface ... 83

Figure 7.4: Schematic drawing of a standard impact testing apparatus ... 84

Figure 7.5: Brittle failure of King Street Bridge girder ... 85

Figure 7.6: Welding of hollow steel to base plate - full penetration weld compared to fillet weld ... 89

Figure 7.7: Base plate showing corrosion of fillet weld ... 90

Figure 8.1: Power riveting ... 92

Figure 8.2: Markings for high strength bolts... 93

Figure 8.3: Stud shear connectors used for composite action girder/slab ... 94

Figure 8.4: Stud Shear Connectors on top flange of a steel girder ... 95

Figure 8.5: Shielded manual metal-arc welding and submerged-arc welding... 97

Figure 8.6: Metal inert gas welding and flux-cored arc welding ... 98

Figure 8.7: Fillet weld terminology and dimensions ... 99

Figure 8.8: Butt weld terminology and dimensions ... 99

Figure 8.9: Partial penetration butt welds ... 100

Figure 8.10: Macro – full penetration fillet weld flange to web ... 102

Figure 8.11: Macro – butt weld (double sided)... 102

Figure 9.1: FRP span – bridge over Orara River at Coutts Crossing... 108

Figure 9.2: Proof loading of FRP span for Coutts Crossing ... 109

Figure 9.3: Trial FRP cross girder for timber truss bridge ... 110

Figure 9.4: Shear strengthening of a reinforced concrete T- beam bridge with CFRP strips ... 111

Figure 10.1: Softwood cell structure ... 116

Figure 10.2: Hardwood cell structure ... 117

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1 INTRODUCTION AND GUIDE STRUCTURE

1.1 Scope

The purpose of the Guide to Bridge Technology is to provide guidance to bridge owners and authorities on technology related issues relevant to bridge ownership, design procurement, vehicle and pedestrian accessibility and bridge maintenance and management practices, including the use and application of Australian and New Zealand bridge design standards. Bridge owners are a diverse group including state road authorities, toll road concessionaires, local governments, private landowners and businesses such as shopping centre owners. The guide has also been written with the young engineer in mind particularly those recently graduated, and looking at specialising in the design and construction of bridges.

The Guide to Bridge Technology, Part 2: Materials covers all aspects of the common building materials available to the engineer including concrete, metallic and non-metallic materials and timber. Part 2 of this guide discusses material characteristics, their properties, durability, construction issues when using such materials, and protection and preservation treatments. A detailed section on concrete reinforcing materials is also included.

1.2 Guide Structure

The Austroads Guide to Bridge Technology is published in seven parts and addresses a range of bridge technology issues, each of which is summarised below.

Part 1: Introduction and Bridge Performance

 This part covers the scope of the Guide to Bridge Technology, includes factors affecting bridge performance, the relationship to the bridge design standards, and an understanding of the evolution of bridges and bridge loadings. Technical and non-technical design influences are also discussed along with the evolution of bridge construction methods and equipment. Specifications and quality assurance in bridge construction are also included in this Part.

Part 2: Materials

 The full range of bridge building materials is discussed in Part 2 including concrete, steel, timber and non-metallic components. It also discusses the material characteristics including the individual stress mechanisms.

Part 3: Typical Bridge Superstructures, Substructures and Components

 Included in discussion in this part are superstructure and substructure components - namely timber, steel, wrought iron, reinforced and pre-stressed concrete. Typical bridge types such as suspension, cable stayed and arched types are discussed. Included in this part is a section on bridge foundations.

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Part 5: Structural Drafting

 This part covers the detailed drawing aspects required to clearly convey to the

consultant/construction contractor the specifics of the project. It discusses the various standards including details required for cost estimating and material quantities. Coverage also includes reinforcement identification details.

Part 6: Bridge Construction

 This part provides guidance to the bridge owner's representative on site and focuses on bridge technology, high-risk construction processes e.g. piling, pre-stressing, and the

relevant technical surveillance requirements during the construction phase. Bridge geometry, the management of existing road traffic and temporary works are also discussed in this part.

Part 7: Maintenance and Management of Existing Bridges

 Maintenance issues for timber, reinforced and pre-stressed concrete, steel, wrought and cast iron bridges are discussed in this part. Other bridge components including bridge bearings and deck joints are also referred to. This part also covers the monitoring, inspection and management of bridge conditions.

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2 CONCRETE MATERIALS

2.1 Cement

2.1.1 Source

To meet today’s demands for concrete supply, Portland cement is currently manufactured locally and imported into Australia and New Zealand from many sources. Major concrete suppliers usually purchase cement from the same suppliers to ensure they produce concrete with consistent properties. Some producers, however, may vary their cement source, and cement suppliers themselves may source their cement or raw materials from different sources. This situation means that the consistency of cement, and therefore concrete, cannot be guaranteed.

In Australia a government-based cement quality assurance scheme is being recreated. However, until it is implemented state authorities will need to have their own quality assurance schemes to cover cement supply. This problem should be managed centrally by a concrete expert in each road authority.

In New Zealand, cement and concrete quality is managed though the NZRMCA plant certification scheme. The concrete purchaser manages quality by specifying concrete supply in accordance with NZS 3104 (2003) and concrete construction in accordance with NZS 3109 (1997). These standards provide a minimum level of quality assurance. Extra quality control processes can be specified to address particular concerns for individual structures, such as durability requirements in aggressive environments.

The issue is one of having confidence in the quality of cement being used to construct bridges. Once bulk cement is placed in silos the traceability of the origin of the cement becomes

problematic.

For complex bridges the consistency of the cement properties and concrete mix characteristics becomes more critical.

2.1.2 Cement Reactivity – Setting Process

Historic perspective

The reactivity of cement is related to the fineness. The finer the cement, the more rapid the rate of hydration and the rate of strength gain.

Cements produced pre the 1960s were of a coarser grind compared to current materials. The changes in the cement manufacturing process, which began in the 1960s, resulted in increasing fineness. As a result, comparatively higher concrete strengths were achieved in shorter periods of time. However, the increase in the heat of hydration of the finer cement causes comparatively higher thermal shrinkage as the concrete cools.

In addition, the high early strength results in high early elastic modulus, lower creep and higher drying shrinkage compared with lower strength concrete unless water-reducing admixtures are

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Numerous papers and articles exist in the literature on concrete cracking and durability and provide an historic perspective on the issue. Some examples include Mehta and Burrow (2001) and Purvis et al (1995).

For an exhaustive list of references search the Internet under ‘concrete bridge deck cracking’. The scale of the problem of bridge deck crack is indicated by the number of research projects carried out, or currently in progress, into the problem worldwide.

2.1.3 Cement and Durability

The issue of concrete durability arose in the late 1970s when it was noted that bridges constructed in marine environments in the 1960s and 1970s were exhibiting premature deterioration because the cement content had been reduced due to the use of finer and therefore more reactive cements. The strength requirements could still be met even though less cement was used. Investigations carried out by researchers showed that chloride ions had passed through the cover concrete at a comparatively fast rate in the newer bridges, resulting in the loss of alkalinity of the concrete surrounding the reinforcement, thereby leading to its corrosion. The resulting corrosion product being of greater volume than the steel caused the spalling of concrete.

Bridges in marine environments constructed prior to the 1960s were in many cases performing satisfactorily. Investigations carried out showed that in many of these bridges the probability of corrosion of the reinforcement was low due to the higher durability of the concrete, because the mixes were designed by proportions of cement, sand and coarse aggregate. In addition, extra cement was added in bridge concretes, which enhanced the durability. As discussed above, the change in cement properties to achieve high early strengths is considered a major contributor to the durability issue.

Numerous papers and articles in concrete journals give a background to the concrete durability issue and these can be found listed separately at the end of the References section of this Part.

Blended cements

As an outcome of the durability issue blended cements consisting of OPC (ordinary Portland cements) from different sources and fly ash were developed to enhance the resistance to chloride and sulphate attack in marine environments. Products referred to as ‘marine blends’ emerged to address the durability deficiencies of the existing cements. More flexibility can be gained if cement suppliers can store cement etc. in separate silos so that blends can be varied to suit particular requirements.

Durability measures

Numerous research projects were implemented worldwide to address concrete durability. Outcomes of the research pointed to a number of contributing factors including the following:

 Specifications that had previously specified a minimum cement content for various concrete grades had been amended to only require that the required 28-day strength be met.

 The introduction of fine grind cements had resulted in the situation where the specified 28-day strength could be achieved with reduced cement content compared to the past because of the higher reactivity of the cement. As a result, the water cement ratios had increased for the same strength resulting in less durable concrete (Hawkins 1987).

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As an outcome of the research, either minimum cement contents or significantly higher concrete strengths were specified e.g. basic concrete 20  40 MPa, prestressed concrete 45  55 MPa. In addition, supplementary siliceous materials such as fly ash and silica fume were used.

2.2 Supplementary Cementitious Materials (SCMs)

SCMs are often incorporated into modern concretes to improve durability. The SCMs currently used include fly ash, granulated blast-furnace slag and silica fume which are industrial by-products. In New Zealand a proprietary natural geothermal silica is currently used instead of silica fume. The blending of ordinary Portland cement with fly ashes with specific properties results in concrete with increased durability by enhancing its chemical resistance in terms of chloride ion ingress and alkali aggregate reaction (AAR). It should be noted that not all fly ashes have the chemical properties that result in the enhancement of concrete durability. Hence the need for testing of proposed materials for compliance to specifications.

An additional advantage of fly ash is that it reduces the heat of hydration, strength and elastic modulus at an early age resulting in a reduction of concrete prone to cracking.

In marine environments the increase in chemical resistance from the use of fly ash results from the fact that the concrete is able to chemically bind free chloride ions that have the potential to cause corrosion of the reinforcement in time.

The use of SCMs in the appropriate quantities enhances the resistance to alkali aggregate reaction (AAR) by reducing the alkalis in the concrete and preventing the reaction with the aggregates.

2.3 Aggregates

The requirements for aggregates for concrete are set out in AS 2758.1 (1998) and NZS 3121 (1986).

2.3.1 Coarse Aggregate

Issues associated with coarse aggregates include:

 source

 shape

 degradation

 strength.

Source

The need for ongoing testing of aggregates from quarries needs to be highlighted. Within any quarry the possibility exists for changes in the petrology of the rock as different areas are mined as a result of encountering dykes, intrusions, etc. The fact that aggregate from a particular quarry was tested and found to be acceptable does not mean that all material in the future will be of the

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Large aggregate provides better interlock with resulting higher shear strength. Aggregates larger than 20 mm may be used in special circumstances with wider spaces.

Smaller aggregates of 10–14 mm may be used in congested areas such as prestressing

anchorages. However, the concrete will have a comparatively lower shear strength compared to concrete with larger aggregate.

2.3.2 Fine Aggregates

The requirements for fine aggregates for concrete are also set out in AS 2758.1 (1998) and NZS 3121 (1986).

The type of fine aggregates used in concrete has traditionally been river sand and crushed

sandstone. However, there is an increasing use of manufactured sand as the availability of natural sands diminishes.

The use of manufactured sand (quarrying by-product – crusher dust) introduces a number of potential problems for the placement, compaction and finishing of concrete particularly in bridge decks with a large surface area per volume compared to other members.

Manufactured sands have a comparatively high surface porosity and surface absorption and as a result have a high and sometimes variable water demand. Therefore the control of the moisture content of the manufactured sand in the batching process is critical. The workability of the concrete is extremely sensitive to variations in the moisture content of the fine aggregate.

Instances have occurred where concrete placed at 150 mm slump has become unworkable before it has been fully compacted and finished.

In one bridge project the plastic cracking that occurred due to poor compaction as a result of loss of workability, required the complete removal of the deck. The cracks were the full depth of the slab and 1 mm wide. An investigation revealed that in that instance half of the free water was assumed to be in the fine aggregate when in fact that was not the case. The moisture content of the manufactured sand was an assumed figure rather than that determined by testing (Figure 2.1).

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Source: RTA NSW

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2.3.3 Alkali Aggregate Reaction (AAR)

The reactivity of both coarse and fine aggregates in the concrete matrix has become an increasingly critical issue in concrete bridge construction.

AAR is a chemical reaction that occurs between the aggregates and the alkali hydroxides in the pore solution of concrete forming an expansive gel. The gel expands on absorbing water and this can lead to extensive cracking of the concrete with potentially significant effects on the

serviceability and capacity of a structure (Figure 2.2, Figure 2.3 and Figure 2.4).

The alkali hydroxides most commonly associated with AAR are sodium hydroxides and potassium hydroxides. These may be present initially in the cement, admixtures or the mixing water.

For deleterious AAR to occur in a structure the concrete must contain sufficient amounts of reactive aggregates, alkali and moisture. The absence of one or more of these will inhibit the reaction. Using low-alkali cements that limit sodium and potassium content is one approach to reducing the incidence of AAR damage that may result with some potentially reactive aggregates. On the other hand, additional moisture entering the concrete via cracks caused by AAR can accelerate the process.

The period for significant AAR damage to occur can be as short as five years and as long as 30 years or more.

Three types of AAR have been identified:

 alkali-silica reaction

 alkali-silicate reaction

 alkali-carbonate reaction.

Alkali-silica reaction (ASR) occurs between the alkali hydroxides and various forms of silica with a more disordered crystalline structure including chalcedony, flint, chert, opal, strained quartz and quartz cement.

Alkali-silicate reaction has not been well defined and is considered to occur with aggregates of complex mineralogy such as greywacke, phyllite and argillite. Silicate minerals such as micas and clays have also been reported as AAR susceptible.

It appears that alkali-silicate reaction is basically similar to alkali-silica reaction as far as the reaction products are concerned, but the rate of reaction is lower. In general, no distinction is made between these two types of reaction. In both, an expansive gel is formed which produces large swelling pressures on absorbing water, and this may crack the affected concrete. After cracking, the gel penetrates some of the cracks and some of the pressure is relieved.

Alkali-carbonate reaction occurs between the alkali hydroxides of the pore solution of concrete and certain dolomitic carbonate rocks, but this is far less common than ASR, and has not been

reported in Australia.

Specifications for the supply of concrete for bridge works now include requirements for all proposed aggregates to be assessed for AAR reactivity.

Typically, a petrographic examination is carried out according to ASTM C295 (2008). Aggregates containing opaline material, unstable silica materials or sheared rock containing moderate amounts of strained quartz and microcrystalline quartz may be eliminated without further testing.

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Aggregates not eliminated by petrographic assessment are then assessed for potential AAR using an accelerated mortar bar or a concrete prism test method.

Aggregates classified reactive using the accelerated mortar bar test may be deemed satisfactory for use up to a specified limit of reactivity, subject to:

 the use of a blended cement in the concrete containing supplementary cementitious materials (SCM) such as fly ash, slags and silica fume

 retesting using a concrete prism test.

Aggregates classified as reactive according to a concrete prism test must not be used.

Source: RTA NSW

Figure 2.3: Cross-section of concrete core showing expansive gel around aggregate

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2.3.4 Lightweight Aggregates

Where lightweight concrete is required lightweight aggregates are used. Applications may include drop in spans where craneage requirements may limit the mass of members. The current type of lightweight aggregate available is a volcanic material called scoria. In New Zealand pumice is also used. In the past coke breeze was used as a lightweight aggregate. Man-made lightweight

aggregates are also available.

It should be noted that excess vibration during placement may cause lightweight aggregates to ‘float’.

Lightweight aggregates should be used with caution, as they are susceptible to excessive creep and shrinkage. Additional testing is required to determine the concrete characteristics.

2.4 Admixtures

The Cement and Concrete Association of New Zealand, www.cca.org.nz, and Cement and Concrete and Aggregates Australia, www.concrete.net.au, websites provide information on concrete admixtures and their applications.

Concrete admixtures provide a means to enhance the characteristics of concrete in the fluid, plastic and solid states. The admixtures available for use include:

 Water reducing agents to produce high slump, flowable concrete, while lowering the water/cement ratio to increase strength and improve durability. Available as high range (superplasticers) for high slumps – 150 to 200 mm and normal water reducers to produce the specified slump with 10-15% less water. Superplasticers are usually used together with normal water reducers for maximum efficiency. The superplasticers work by coating the cement particles, which reduces friction, increases the slump, and retards the hydration. Hence when the superplasticer evaporates rapid stiffening (reversion) occurs.

 Air entraining agents must be used in concrete subject to freeze-thaw to reduce the risk of damage as a result of the freezing. The air entraining agent produces micro air bubbles in the concrete matrix, which results in discontinuous pores. They are also used in warm climates to enhance durability. Air entrainment is also used to improve workability by making the concrete flow due to the presence of the air bubbles. The presence of the discrete air bubbles reduces the ingress of moisture that under freezing conditions will expand and damage the concrete. The addition of air entraining agents to a mix results in some loss of compressive strength. Consequently, excessive use of air entraining agents by incorrect dosing may be detrimental to the concrete.

 Accelerators used to promote early setting, particularly in cold weather conditions. The use of accelerators needs to be treated with caution as a number of the products contain calcium chloride that are a source of harmful chloride ions that can lead to corrosion of reinforcing steel and metal fitments. Only chloride free accelerators should be used in reinforced concrete.

 Retarders to delay initial set of the concrete to allow time for placement, compaction and finishing particularly in hot weather.

 Shrinkage reducers to reduce the drying shrinkage and consequential cracking.

 Corrosion inhibitors for use in concrete in marine environments to maintain the passive environment of reinforcing steel and thereby prevent corrosion. These materials require specialised knowledge and should be thoroughly tested and used with great care.

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There are three classes of corrosion inhibitors – anodic, cathodic and mixed.

Anodic inhibiters act to suppress the anodic reaction. The most common of these is calcium nitrite. Cathodic inhibitors act to suppress the cathodic reaction. Mixed inhibitors act to suppress both the anodic and the cathodic reactions.

In simple terms, by suppressing the anodic reaction the onset of corrosion will be delayed, but the rate of corrosion will be unaffected or, under certain circumstances, increased. Suppressing the cathodic reaction, the onset of corrosion will not change, but as the reaction rate is governed by the cathode and the availability of oxygen, the reaction rate will be reduced. Mixed inhibitors will both delay the onset of corrosion and reduce the reaction rate.

Where inhibitors are used in a concrete element then the inhibitor must be used in all concrete elements electrochemically connected to that element. Failure to do so may result in increased corrosion. The dosage should not be varied within the concrete or unusual and unsafe corrosion conditions may occur. Inhibitors are quite expensive, costing up to $100/cubic metre of concrete, (at the time of writing) and may cause changes in the plastic properties of the concrete.

Cautionary Note: It should be noted that while the use of admixtures can improve the characteristics of concrete there are a number of issues to consider in their use:

 There is potential for the use of admixtures to mask clues to problems with the mix design.

 Admixtures are not designed to correct deficiencies in the mix design.

 As more admixtures are used the interaction between them may produce adverse affects. Multiple admixtures should not be combined in one concrete mix without the approval of the admixture manufacturer and after thorough testing.

 As more admixtures are added the level of control in the batching process becomes more critical as the mix may become more sensitive to minor changes in the constituents.

 Mix designs that are highly refined in terms of cement content and maximum packing density may also be very sensitive to changes in the constituents including admixtures.

 The possibility exists for concrete with superplasticers to undergo premature ‘reversion’ or ‘slump loss’ i.e. revert from 150 mm to 75 mm slump before the time expected. Placement and compaction of the concrete in the event of this occurring becomes problematic. Some superplasticers are more prone to reversion than others. During the construction process planning, there needs to be a nominated site person who will be responsible to determine if reversion has occurred. The available working time needs to be known for specific site conditions for the actual mix.

 Incorrect dosages of admixtures may be detrimental to the performance of the concrete. Dosage rate should be determined by trial mix in consultation with the admixture

manufacturer.

 The lack of adequate mixing will result in uneven distribution of admixtures resulting in differential characteristics of the concrete. This will offset any potential benefits of using the

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 The order in which constituents, including admixtures, are added to the mixer will have a significant effect on the efficiency and therefore the amount of admixture needed and its effectiveness. Admixture manufacturers will be able to advise on the appropriate order for particular combinations of admixtures. For example, it may be necessary to add water reducer before superplasticer to ensure the superplasticer is uniformly distributed through the concrete.

 Admixtures can add significant amounts of alkali to the concrete. Where AAR is managed by controlling concrete alkali content the alkalis contributed by admixtures must be included in calculations of concrete alkali content.

2.5 Grouts and Mortars

2.5.1 Grouts

Grouts are used in bridgeworks in a number of applications including:

 The grouting of ducts in post-tensioned prestressed concrete members after the stressing operation to provide corrosion protection for the prestressing steel. The prestressing steel will consist of either bar, strand or wire.

 The grouting of permanent rock anchors to provide bond to develop the anchor capacity and provide corrosion protection for the tendon.

 The grouting of the ducts of tie-backs that are used to provide stability to retaining walls and abutments. The grout provides corrosion protection for the steel tendon resisting the forces involved.

 As a surface primer on the hardened concrete at construction joints.

Grout consists of neat cement and water mixed to the specified water/cement ratio to provide the required performance requirements. In some instances fine aggregate may be used but its nominal maximum aggregate size is limited to 1 mm.

The performance requirements are:

 Strength – which is controlled by cement properties and water/cement ratio.

 Fluidity (ability of a batch of grout to be pumped and to flow into voids for the duration of the grouting operation), which is controlled by the cement particle characteristics and

water/cement ratio.

 Early expansion (to counter shrinkage), which is controlled by the addition of expansive admixtures to counter early expansion and prevent segmentation of the grout.

 Bleed characteristics – to ensure excess water does not remain after hydration is completed. Any excess water will collect in high points of ducts in prestressed concrete girders and in the ducts of vertically prestressed concrete columns and this is unacceptable from a corrosion protection point of view.

The specification requirements for cement and fine aggregate for grout are the same as those required for concrete.

Standard specifications for grout specify standard test methods and acceptance criteria for performance requirements. The use of iron or aluminium powders as expanding admixtures is precluded by standard specifications.

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The grout mix design for a project may be developed through a testing program. Alternatively, a proprietary grout may be used provided testing is carried out to confirm the product complies with all requirements of the specification. This includes performance requirements and material properties and the use of approved admixtures.

Some state road authorities preclude the use of premixed grouts for mortar pads (Section 2.5.2). Grouting is a critical activity and needs to be strictly controlled. The fact that it is extremely difficult to assess the quality of grouting after completion makes the need for strict supervision of the process imperative.

2.5.2 Mortars

Cement mortar

Cement mortar is used in bridgeworks in a number of applications, including:

 support of bearings

 support of traffic barrier posts

 support of lighting standards

 support of fixtures including noise barriers

 infill at prestressing anchorage recesses

 as a bedding layer for bridge deck joints

 minor concrete repairs.

Mortar consists of cement, fine aggregate and water mixed to a specified water/cement ratio to provide the required performance requirements. The mix proportions of cement/sand ratio will vary depending of the performance requirements but will generally range from 1:1 to 1:3.

The performance requirements will be determined by:

 specified strength – determined by cement properties and water/cement ratio

 control of shrinkage characteristics

 friction requirements (particularly for elastomeric bearings)

 time to initial set

 accessibility of area

 specified thickness

 plan area

 method of installation of the member - e.g. mortar pad installed prior to or after member is temporarily supported in position.

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Dry pack mortar

In some instances it may be advantageous to use what is referred to as ‘dry pack mortar’. For a dry pack cement mortar the amount of water used in the mix is only that sufficient to ensure

hydration of the cement. The cement mortar is compacted in place by hammering in vertical layers under a horizontal base plate of a component. This results in a high strength, high density mortar which does not slump ensuring uniform bearing over the base plate being supported. The use of dry pack mortar will require the use of packing, usually in the form of steel wedges, to support the component until the cement mortar has cured. When the cement mortar has cured the wedges are removed and additional dry pack mortar installed in the gap remaining.

Curing

It should be noted that cement mortar requires the same curing regime as concrete to ensure the required strength is obtained and to prevent drying shrinkage cracking. The general tendency on construction projects is for a lack of attention to the curing of mortar.

Surface preparation

The concrete surface on which the mortar is to be placed should be scabbled to remove surface laitance. It should also be saturated to prevent the hardened concrete absorbing moisture from the wet mortar resulting in a loss of strength and increased drying shrinkage. The surface should be primed immediately prior to installation of the mortar with a cement grout to enhance adhesion.

Polymer mortar

In some instances, bridge designers may specify a polymer mortar rather than a cement mortar. In a polymer mortar the cement binder is replaced with a polymer such as epoxy resin.

Polymer binders are available in a range of types and characteristics depending on the application. For example, thixotropic binder polymers are available that will produce a mortar than will not flow under gravity.

For detailed information on polymer binders and mortars consult the manufacturers such as Epirez, Sika and Vivacity Engineering.

Cautionary note: (1) The use of proprietary premixed grouting compounds for bridge bearing mortar pads is precluded by some state road authority standard specifications. (2) The use of polymer mortar for elastomeric bearings is problematic because of the lack of friction on the top surface of the mortar. The potential exists for the bearings to ‘walk out’ if the friction allows the bearing to slide rather than shear under horizontal loads.

2.5.3 Mortar Pad Set Up

It is imperative that mortar pads conform to the drawing requirements in terms of position,

dimensions and reduced level. Instances have occurred where the top surface of mortar pads for bearings has cast out of level and as a consequence has used part of the rotational capacity of the bearing in compensating for the error.

2.5.4 Mortar Pad Problems Mortar pad problems include:

 cracking – lack of curing, lack of thickness, lack of strength

 drumming, edge lifting – lack of curing, lack of bond, concrete substrate dry when mortar placed, expansive mortar used with no constraints.

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3 CONCRETE REINFORCING MATERIALS

The material characteristics of the steels (reinforcing, prestressing and structural) depend on a number of variables, including:

 manufacturer

 country of origin

 strength grade

 method of manufacture

 year of manufacture (of particular importance for assessment of existing bridges)

 standard to which it was produced

 chemical composition.

3.1 Material Certification

In view of the situation regarding identification of reinforcing, prestressing and structural steel it is imperative that a certificate be obtained from the supplier/contractor that identifies the material and certifies its mechanical properties.

Random testing of samples of imported steel is required to verify the veracity of the certification provided. Instances have occurred where test results did not correlate with the information provided by the certification.

In addition to obtaining a genuine material certificate, it is also important that the heat and batch number can be traced to the material delivered to site.

For a report on the performance of seismic grade steel from various suppliers refer to the New Zealand Department of Building and Housing website:

http://www.dbh.govt.nz/blc-product-certification

3.2 Heat Numbers

The heat number of a piece of steel is the identifier that relates the product to a particular batch of steel produced in the steel making process. The manufacturer of the steel carries out metallurgical testing from each batch of steel to determine its properties to ensure compliance with the required standard.

If the heat number of the steel is known it can be related back to the manufacturer’s records at any time.

3.3 Country of Origin

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One of the issues with imported reinforcing, prestressing and structural steel is the traceability once it is put into storage.

It is imperative that the country of origin of the steel used in bridge construction is determined. For example reinforcing and structural steel cannot be reliably welded without knowing the country of origin and hence the chemical composition.

The standards under which steels are produced overseas do not necessarily comply with ANZ standards. For example, USA and EU requirements for elongation for reinforcing steel do not comply with ANZ standards.

For further information refer to the Pacific Steel New Zealand website www.pacificsteel.co.nz particularly the paper by Allington, C and Bull, D Influences of Locally Produced and Imported

Reinforcing Steel on the Behaviour of Reinforced Concrete Members.

Reinforcing steel is required to have raised marks that identify the manufacturer and steel grade. However, caution should be exercised with overseas products as there have been instances of fraudulent identification marks being used.

In Australia, a non-profit organisation ACRS (Australian Certification Authority for Reinforcing Steels Ltd) has been set up to administer a third party product certification scheme for steel

reinforcement and prestressing strand. The organisation is supported by key construction industry bodies, including Austroads. The reinforcing standards AS/NZS 4671 and the prestressing

standard AS/NZS 4672 allow for voluntary third party product certification as one of the methods to prove compliance.

3.4 Carbon Steel Reinforcement

3.4.1 Material Characteristics

Information on the characteristics and issues of reinforcing steel is available on the websites of steel manufacturers in Australia and New Zealand.

The OneSteel website www.reinforcing.com under Publications and Design Tools provides information on material issues including:

 frequently asked questions on reinforcement

 ductility of reinforcing steel

 history of reinforcing steels

 Australian standards update for reinforced concrete

 application of 500 plus reinforcing bars

 technical references on steel reinforcement.

The Pacific Steel website www.pacificsteel.co.nz under Product Information and Product Technical

Data and Essential Information about Seismic Reinforcing Steel also provides information on

material issues, including:

 reinforcing bar and coil

 reinforcing bar welding and bending guidelines

 seismic QT and MA bars

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 seismic reinforcing bar welding and bending guidelines

 technical references on steel reinforcement. 3.4.2 Method of Manufacture

Australian and New Zealand steel reinforcing manufacturers produce a range of reinforcing bars.

Australia

OneSteel produces three types of reinforcing steel, namely:

 500PLUS Tempcore – straight bar 12-40 mm diameter

 500PLUS Microalloy – straightened from coil 10, 12, and 16 mm diameter

 500PLUS Reidbar – continuously threaded Tempcore bar.

New Zealand

Pacific Steel produces a range of reinforcing steel products, including:

 500 grade QT (Quench and Temper)

 500 grade MA (Micro-alloyed)

 500 grade Reidbar.

The Tempcore and QT bars are quenched, when red hot, in water. The resulting bar has a hard strong casing and a softer more ductile inner core. Microalloying is the more expensive process as it involves the addition of alloys, such as vanadium, at the steel making stage, therefore there is no quenching required. The MA bar has the same hardness and strength and ductility across the full cross-section of the bar.

It is important that the Tempcore and QT bars are not heated above the tempering temperature as this will cause normalising of the outer casing to the properties of the core of the bar resulting in a loss of strength. The processes that cause normalising are welding and hot bending.

Specific requirements/issues apply to the bending, rebending, welding, and temperature effects of these materials. Refer to specifications, standards and manufacturer’s recommendations.

Tempered and quenched bars should not be welded, re-bent or threaded. 3.4.3 Old Reinforcing Steels

Over time various types of reinforcing steel have been used in bridge construction. The material properties, anchorage development lengths and weldability have changed. When assessing the load capacity or rehabilitating an existing bridge it is important to know details of the material used in the bridge.

The older types of reinforcing steel include:

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The assessment of the load capacity of an existing bridge will require confirmation of the material properties of the reinforcing steel. This may necessitate the removal of samples of the reinforcing steel for metallurgical investigation.

For further information on the history of Australian and New Zealand reinforcing steel refer to the OneSteel website www.reinforcing.com (Publications and design tools – articles and papers) and the Pacific Steel website www.pacificsteel.co.nz.

3.4.4 Packaging and Handling

The microalloyed reinforcing steel with diameters of 12 mm and 16 mm is produced in coils. An issue arises when the ends of the coils are straightened. This straightening results in cold working of the bars which results in decreased ductility. The straightened material should be discarded. 3.4.5 Ductility

The ductility of reinforcing steel has come under focus following the introduction of low and normal ductility classes in AS 4671-2001.

Ductility class

Ductility classes comprise:

 low ductility, Class L, applies to cold drawn wire used in reinforcing mesh. Elongation < 5%

 normal ductility, Class N, applies to reinforcing steel. Elongation > 5%

 earthquake ductility, Class E, applies to reinforcing steel. Elongation > 10%. In New Zealand a specific ductility Class E was developed to use in seismic design. The introduction of ductility classes raises issues in terms of the need for awareness of construction staff and of the identification of the different types of material. It is important that construction staff be trained in the identification of the grades of steel used on a specific site. It is recommended that the use of different grades of reinforcing steel on any one site be avoided to mitigate the risk of the incorrect grade of reinforcing steel being placed in a member (Table 3.1). Information on the research history of the ductility of reinforcing steel is available on the OneSteel website under ‘Ductility of Reinforcement – Research History’.

Table 3.1: Strength and ductility of reinforcement

Type Designation grade Yield strength

MPa

Ductility class Clause 6.2.1 AS5100

Bar: plain(fitments only) R250N 250 N

Bar: deformed D500N 500 N

Bar: plain deformed and indented (fitments only)

500L 500 L

Bar: deformed 500E 500 E

Welded mesh: plain, deformed and indented 500L 500N

500 500

L N

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3.4.6 Weldability

The weldability of reinforcing steel cannot be determined without knowing the material properties and chemical composition.

If the source of the material is unknown then welding should not be carried out without material testing. The properties and chemical composition of the weld metal used needs to be compatible with the parent material.

In addition, the welding procedure, which includes preparation, consumables (stick electrodes or continuous wire), preheat requirements, temperature limits, weld runs and weld machine settings can only be determined once the material properties and chemical composition are known. Standard welding procedures for reinforcing steel manufactured in Australia are published by the respective companies.

ACRS certification of the material will ensure that the weldability issue is addressed. 3.4.7 Tack Welding

Tack welding of reinforcing steel is widely used to enable the prefabrication of cages or to fix reinforcing steel placed in situ.

However, the higher strength steels currently being used require a greater degree of control and expertise to ensure a satisfactory result compared to older materials. There is greater propensity for tack welding to reduce the strength of the steel.

Tack welding must be viewed as a welding process and therefore needs to be done in a controlled manner. The heat input has the potential to adversely affect the properties of the steel in terms of strength and fatigue. In addition, inappropriate weld settings can result in loss of section, resulting in a loss of strength (Figure 3.1). Tack welding should be carried out by qualified welders.

AS/NZS 1554.3 (2008) includes procedures to be adopted for the tack welding of reinforcing steel. The standard also includes a procedure to have a non-standard weld procedure tested for

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3.4.8 Welded Splices

Butt splice

A butt splice involves welding two pieces of reinforcing steel end-to-end. The process requires an approved weld procedure (Figure 3.2).

For bar diameters > 20 mm the weld procedure becomes more restrictive as the heat input has to be controlled to ensure the properties of the steel are not changed. The weld procedure may require partially welding a series of bars to reduce the heat input to avoid normalising the steel. The butt welding of all bars in column on the one plane is considered poor practice and should be avoided. Welding of half the bars at two levels is better practice (Figure 3.3).

Source: D Carter

Figure 3.2: Butt splice

Alternate column bars spliced at

different levels

Alternate column bars spliced at

different levels

Source: D Carter

Figure 3.3: Welded butt splices in column Welded lap splice

A welded lap splice involves welding two pieces of reinforcing steel by overlapping them and then running a weld down one or both sides of the splice.

The main problem with this type of splice is the difficulty in obtaining good fusion of the weld at the point of contact of the two bars (Figure 3.4 and Figure 3.5).

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Source: RTA.NSW

Figure 3.4: Welded lap splice Macro test

A macro test involves setting up the proposed weld on sample material and carrying out the weld to the approved weld procedure. The welded sample is then cut into sections, polished and visually checked to assess the quality of the weld.

This process can be carried out at any time in the process of the work to assess the quality of the work. It should also be used when a new welder is proposed to check competency of the person to carry out the weld to the approved procedure.

Source: RTA NSW

Figure 3.5: Macros of welded lap splice to check the penetration of weld

3.4.9 Mechanical Splices

Mechanical splices are an alternative method to welding to join lengths of reinforcing steel. The use of mechanical splices obviates the need for strict control of the welding process which if not complied with will affect the strength of the bars.

In congested areas, particularly in splices in columns, the use of mechanical splices is a more practical alternative and avoids potential problems of congested reinforcement.

3.4.10 Mechanical Couplers

Reinforcing steel can be spliced using proprietary threaded mechanical couplers supplied by a number of companies. It imperative that suppliers be required to provide documentary evidence of testing carried out to demonstrate that the coupler has a tensile capacity equal to or greater than

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3.4.11 Rebending

It is not permissible to re-bend 500 grade bars that are manufactured by tempering and quenching as the ductility of the bar is reduced by cold working and cracking is likely to occur.

3.4.12 Protective Treatments

In an attempt to improve concrete durability two protective treatments have been used on reinforcing steel.

Galvanising

Hot dip galvanising of reinforcing steel has been used in a number of bridges in aggressive environments to extend their service life. Opinions vary on the cost/benefit of galvanising

reinforcing steel. It is considered by some authorities that the thickness of zinc achievable on a bar does not give long-term protection in the situation where chloride ions diffuse through the concrete to the reinforcement.

Galvanised reinforcing steel is not used by all Australian state road authorities or in New Zealand. It has been more popular in pre-cast concrete building panels.

Epoxy coated bar (ECB)

Epoxy coated reinforcing steel has been adopted by a number jurisdictions in the USA, particularly in bridge decks, to attempt to mitigate the effects of chloride ion diffusion from de-icing salts. There are a number of issues that arise in using ECB:

 the risk of damage to the coating during construction

 the risk of pin holes (holidays) in the surface of the coating which would allow moisture penetration and result in possible loss of adhesion

 the presence of pinholes may also be the location of potential corrosion cells when chloride ions reached the bar

 epoxies are not waterproof and moisture may permeate through the coating

 cost.

A report by the US Department of Transport (Lee & Krauss, 2004) generally concluded that there was benefit in using ECB. The testing program was carried out on a simulated deck section. However, Pyc et al. (2000) gives a less favourable report of the effectiveness of ECB. The report states that loss of adhesion of the coating occurred before the chloride ions reached the bars. The report does not recommend the use of ECB.

The recommendations of the latter report were based on field trials and are considered more indicative of in-service performance. ECBs are not used by Australian and New Zealand road authorities. They are not recommended by Austroads for use in bridges.

3.4.13 Fire Damage to Steel

The implication of a fire on reinforcing steel is potentially serious if the cover concrete is lost. A loss of structural capacity will occur as the temperature increases and the material begins to flow plastically. In the case of high tensile steel annealing of the steel will occur at temperatures in excess of 400 °C.

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3.5 Stainless Steel Reinforcement

The selected use of stainless steel reinforcing in substructures of bridges in marine environments offers a means of ensuring that the 100-year design service life of a bridge can be achieved

without expensive concrete repairs after 30-50 years. It also obviates the need for other measures to protect the reinforcing steel, such as the use of corrosion inhibitors in the concrete, surface treatments or cathodic protection.

A number of Australian state road authorities have recently constructed bridges with stainless steel reinforcing used selectively in the substructures. The stainless steel reinforcing has been used in the outer reinforcement of piles, pile caps and in columns. In columns it is recommended that the stainless steel reinforcing be used at least within the splash zone. However, in relatively short columns and high exposure sites consideration should be given to using stainless steel reinforcing over the full height of the columns.

Based on current costs, the selective use of stainless reinforcing steel in the substructure of a bridge increases the cost by approximately 8% compared to using 100% carbon steel reinforcing (at the time of writing). Net present worth calculations indicate the economic benefits of its use taking into account projected maintenance costs over 50 years.

Economic considerations indicate that the use of stainless steel reinforcement will be limited to selected use in bridge piers in marine conditions with a maximum bar diameter of approximately 30 mm and small tonnages. For major bridges with high piers the use of cathodic prevention (CP) is considered more economical.

The suitable grades of stainless steel reinforcing are:

 304

 316

 duplex.

For additional information on material properties see the websites provided in Section 3.5.1. Some producers are attempting to reduce the cost of stainless steel reinforcement by the use of cladding. Whilst the cladding is very tough, the method of producing a clad bar introduces some potential weaknesses into the system. Specific areas of concern are:

 cracking under bending – especially for stirrup bends

 the need to cap the ends of the bar in the factory – so no field cutting is permissible

 welding will damage the cladding, so no welding is permissible

 there is no satisfactory repair for a damaged bar

 corrosion is anoxic, so there will be no expansive rust and the bar may be lost with no visible deterioration of the member.

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Storage

Stainless steel reinforcing must be separated from carbon steel reinforcing.

Bending and cutting

Dedicated equipment must be used for the material. Equipment used to process carbon steel will result in pitting of the stainless steel.

Handling

To avoid pitting stainless steel reinforcing must not be dragged over carbon steel reinforcing.

Re-bending

Re-bending of stainless steel reinforcing is not permitted. The passive layer on the bars is only 1-2 mm thick. Re-bending may result in cracking of the passive layer.

Welding

Welding of stainless steel reinforcing is possible but is not recommended, as there is a risk of affecting the mechanical material properties of the material.

Splicing

Splicing of bars is to be achieved by laps or mechanical couplers.

Splicing with carbon steel

The splicing of stainless steel reinforcing with carbon steel reinforcing has been investigated by a number of researchers. Research has shown that there is no issue in terms of galvanic corrosion. Highways Agency UK (2002) provides detailed information on the use of stainless steel

reinforcement in bridges.

Ontario Ministry of Transport (2002) provides useful information for site staff.

The Australian Stainless Steel Development Association at its website www.assda.asn.au and websites www.arminox.co.au, www.ssina.com and www.stainles-rebar.org, also provide information on stainless steel reinforcement.

3.5.2 Material Characteristics Refer to 0 for discussion of this subject. 3.5.3 Supply

The stainless steel bar should originate from a UK Certification Authority for Reinforcing Steels (CARES) registered manufacturer. CARES is the British equivalent of Australian Certification Authority for Reinforcing Steels Ltd (ACRS). A special audit of the supplier should be carried out prior to commencing supply and the manufacturer should be required to guarantee the supply of the reinforcement for the project.

Austroads - Guide to Bridge Technology Part 2 - Materials

Austroads

GUIDE TO BRIDGE TECHNOLOGY

Austroads profile

Austroads membership

CONTENTS

TABLES

FIGURES

1

INTRODUCTION AND GUIDE STRUCTURE

1.1 Scope

1.2 Guide Structure

2

CONCRETE MATERIALS

2.1 Cement

2.2 Supplementary Cementitious Materials (SCMs)

2.3 Aggregates

2.4 Admixtures

2.5 Grouts and Mortars

3

CONCRETE REINFORCING MATERIALS

3.1 Material Certification

3.2 Heat Numbers

3.3 Country of Origin

3.4 Carbon Steel Reinforcement

Alternate column bars spliced at

different levels

Alternate column bars spliced at

different levels

3.5 Stainless Steel Reinforcement