guide to pavement technology part 4c - austroads

Guide to Pavement Technology Part 4C: Materials for Concrete Road Pavements

Sydney 2017


Second edition prepared by: George Vorobieff and Michael Moffatt Publisher

Austroads Ltd. Level 9, 287 Elizabeth Street Sydney NSW 2000 Australia

Phone: +61 2 8265 3300

[email protected] www.austroads.com.au

Second edition project manager: Andrew Papacostas

Abstract

Part 4C of the Guide to Pavement Technology summarises aspects of Australian and New Zealand practice in materials for use in concrete road pavements including:

• base concrete and lean-mix concrete subbase

• concrete curing compounds

• steel reinforcement including tie bars and dowel bars

• joint sealants and fillers.

About Austroads

Austroads is the peak organisation of Australasian road transport and traffic agencies.

Austroads’ purpose is to support our member organisations to deliver an improved Australasian road transport network. To succeed in this task, we undertake leading-edge road and transport research which underpins our input to policy development and published guidance on the design, construction and management of the road network and its associated infrastructure.

Austroads provides a collective approach that delivers value for money, encourages shared knowledge and drives consistency for road users.

Austroads is governed by a Board consisting of senior executive representatives from each of its eleven member organisations:

• Roads and Maritime Services New South Wales

• Roads Corporation Victoria

• Queensland Department of Transport and Main Roads

• Main Roads Western Australia

• Department of Planning, Transport and Infrastructure South Australia

• Department of State Growth Tasmania

• Department of Infrastructure, Planning and Logistics Northern Territory

• Transport Canberra and City Services Directorate, Australian Capital Territory

• Australian Government Department of Infrastructure and Regional Development

• Australian Local Government Association

• New Zealand Transport Agency.

Keywords

Concrete, base concrete, lean mix concrete subbase, pavement, concrete mix design, curing, reinforcement, reinforced concrete, tie bars, dowel, joint, joint sealant, filler

Second edition published December 2017

First edition published July 2009

Editorial changes and minor technical changes throughout; updated references to relevant standards; changes to Section 3.5, Section 4.4, Section 5.2.

ISBN 978-1-925671-17-9

Austroads Project No. APT1816

Austroads Publication No. AGPT04C-17

Pages 57

© Austroads Ltd 2017

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.

Acknowledgements

First edition prepared by Justin Moss, Kieran Sharp, David Dash, John Hodgkinson, Gary Boon, David Svolos, John Nichols, John Keith and project managed by Ken Porter.

This Guide is produced by Austroads as a general guide. Its application is discretionary. Road authorities may vary their practice according to local circumstances and policies. Austroads believes this publication to be correct at the time of printing and does not accept responsibility for any consequences arising from the use of information herein. Readers should rely on their own skill and judgement to apply information to particular issues.

mailto:[email protected]

http://www.austroads.com.au/


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Contents

1. Introduction ............................................................................................................................................. 1 1.1 Guide to Pavement Technology ............................................................................................................... 1

2. Concrete Pavement Types .................................................................................................................... 2 2.1 Base Types ............................................................................................................................................... 2

2.1.1 PCP ............................................................................................................................................ 2 2.1.2 JRCP .......................................................................................................................................... 2 2.1.3 CRCP .......................................................................................................................................... 2 2.1.4 SFCP .......................................................................................................................................... 2 2.1.5 General Notes............................................................................................................................. 2

2.2 Subbase Types ......................................................................................................................................... 3

3. Selection of Concrete Component Materials ....................................................................................... 4 3.1 Introduction ............................................................................................................................................... 4 3.2 Sources of Aggregates ............................................................................................................................. 5 3.3 Aggregate Production ............................................................................................................................... 6 3.4 Aggregate Properties ............................................................................................................................... 6

3.4.1 General ....................................................................................................................................... 6 3.4.2 Durability and Soundness ........................................................................................................... 7 3.4.3 Density ........................................................................................................................................ 9 3.4.4 Water Absorption/Porosity .......................................................................................................... 9 3.4.5 Surface Microtexture ................................................................................................................ 10 3.4.6 Deleterious Chemical Properties .............................................................................................. 10 3.4.7 Thermal Expansion ................................................................................................................... 11 3.4.8 Particle Shape .......................................................................................................................... 11 3.4.9 Particle Size Distribution ........................................................................................................... 12 3.4.10 Cleanliness ............................................................................................................................... 13

3.5 Cementitious Binders ............................................................................................................................. 14 3.5.1 Cement ..................................................................................................................................... 14 3.5.2 Fly Ash ...................................................................................................................................... 15 3.5.3 Ground Granulated Blast Furnace Slag ................................................................................... 15 3.5.4 Geopolymer Binders ................................................................................................................. 16

3.6 Water ...................................................................................................................................................... 16 3.7 Admixtures .............................................................................................................................................. 16

3.7.1 General ..................................................................................................................................... 16 3.7.2 Set-acceleration ........................................................................................................................ 17 3.7.3 Set-retardation .......................................................................................................................... 18 3.7.4 Water-reducing Admixtures ...................................................................................................... 20 3.7.5 Air-entraining Admixtures ......................................................................................................... 21 3.7.6 The Effect of Admixtures on Drying Shrinkage ........................................................................ 22

4. Concrete Mix Design ............................................................................................................................ 24 4.1 General ................................................................................................................................................... 24 4.2 Requirements ......................................................................................................................................... 24

4.2.1 General ..................................................................................................................................... 24 4.2.2 Fresh Concrete ......................................................................................................................... 24 4.2.3 Hardened Concrete .................................................................................................................. 28

4.3 Mix Design Process ................................................................................................................................ 37 4.3.1 Techniques ............................................................................................................................... 37 4.3.2 Shilstone Method ...................................................................................................................... 38 4.3.3 Optimum Sand Method ............................................................................................................. 39 4.3.4 American Concrete Institute (ACI) Method ............................................................................... 40 4.3.5 Texas DOT Method .................................................................................................................. 41


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4.3.6 NZ Flow Cone ........................................................................................................................... 43 4.4 Trial Mix Process .................................................................................................................................... 44

5. Steel Reinforcing Materials ................................................................................................................. 45 5.1 Introduction ............................................................................................................................................. 45 5.2 Reinforcement ........................................................................................................................................ 45

5.2.1 Role of Reinforcement .............................................................................................................. 45 5.2.2 Steel Types ............................................................................................................................... 45 5.2.3 Material Requirements ............................................................................................................. 46 5.2.4 Surface Coatings and Conditions ............................................................................................. 46

5.3 Steel Fibres ............................................................................................................................................ 47 5.4 Tiebars .................................................................................................................................................... 48 5.5 Dowel Bars ............................................................................................................................................. 48

6. Curing and Debonding Treatments .................................................................................................... 50 6.1 Introduction ............................................................................................................................................. 50 6.2 Curing Materials ..................................................................................................................................... 51 6.3 Debonding Treatments ........................................................................................................................... 51

7. Joint Sealants and Fillers .................................................................................................................... 52 7.1 Role of Joints .......................................................................................................................................... 52 7.2 Role of Joint Sealants ............................................................................................................................ 52 7.3 Sealing Contraction and Construction Joints ......................................................................................... 53 7.4 Sealing Isolation and Expansion Joints .................................................................................................. 54

References ...................................................................................................................................................... 55

Tables

Table 3.1: Typical proportion of mix constituents used in Australasian mixes (by mass) ............................... 4 Table 3.2: Typical aggregates for concrete ..................................................................................................... 5 Table 3.3: Relative properties of rocks for concrete ....................................................................................... 7 Table 3.4: Example durability requirements of coarse aggregates for concrete ............................................. 8 Table 3.5: Coefficient of thermal expansion of aggregates ........................................................................... 11 Table 3.6: Coarse aggregate particle shape ................................................................................................. 11 Table 3.7: Typical base concrete grading envelope ..................................................................................... 13 Table 3.8: Range of fly ash and GGBFS limits permitted in base concrete in Roads and

Maritime specification 3211 .......................................................................................................... 16 Table 3.9: Setting time of Portland cement pastes containing set-retarding admixtures .............................. 19 Table 3.10: Effect of admixtures on concrete characteristics ......................................................................... 20 Table 3.11: Effect of overdose (x3) of retarder on compressive strength ....................................................... 21 Table 3.12: Effect on shrinkage of admixture containing calcium chloride and triethanolamine

(control results shown in brackets) .............................................................................................. 23 Table 4.1: Minimum concrete strengths specified in Roads and Maritime specification R83 ....................... 44 Table 5.1: Standard grades of reinforcing steels .......................................................................................... 46 Table 5.2: Cross-sectional areas of plain hard-drawn steel mesh sizes ....................................................... 47 Table 6.1: Curing and debonding treatments ................................................................................................ 51

Figures

Figure 4.1: Loss of strength through incomplete compaction ........................................................................ 26 Figure 4.2: Effect of duration of water curing on strength of concrete ........................................................... 27 Figure 4.3: Effect of duration of water curing on the permeability of cement paste ....................................... 28 Figure 4.4: Relationship between modulus of rupture of concrete at 28 days and water/cement ratio ......... 29 Figure 4.5: Effect of air voids on concrete strength ........................................................................................ 30 Figure 4.6: Effect of curing temperature on strength ...................................................................................... 30


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Figure 4.7: Typical development of concrete strength with age ..................................................................... 31 Figure 4.8: Effect of water/cement ratio on mass loss of concrete subject to abrasion ................................. 32 Figure 4.9: Effect of cement content on abrasion resistance of concrete made of different aggregates ....... 32 Figure 4.10: Effect of cement content and curing period on abrasion resistance of concrete ......................... 33 Figure 4.11: Effect of water/cement ratio on the permeability of cement paste ............................................... 34 Figure 4.12: Effect of fog curing on concrete permeability ............................................................................... 34 Figure 4.13: Rate of evaporation from concrete freshly placed on site ............................................................ 36 Figure 4.14: Shilstone coarseness factor chart ................................................................................................ 39 Figure 4.15: Combined aggregate grading curve ............................................................................................. 40 Figure 4.16: Coarseness factor chart ............................................................................................................... 41 Figure 4.17: 0.45 power chart .......................................................................................................................... 42 Figure 4.18: Flow cone apparatus .................................................................................................................... 44 Figure 5.1: Steel fibre shapes permitted in agency specifications ................................................................. 47 Figure 5.2: The location of tiebars must be placed at the centre of the slab ................................................. 48 Figure 7.1: A sealant either fails from adhesion (left) or cohesion (right) ...................................................... 53 Figure 7.2: Typical details of field-moulded joint sealants .............................................................................. 54 Figure 7.3: Typical details of full-depth isolation joint ..................................................................................... 54


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1. Introduction

1.1 Guide to Pavement Technology

Part 4C of the Guide to Pavement Technology summarises aspects of Australian practice in materials for use in concrete road pavements including:

• base concrete and lean-mix concrete subbase

• concrete curing compounds

• steel reinforcement including tiebars and dowel bars

• joint sealants and fillers.

Unbound granular materials, cemented materials and asphalt may also be associated with concrete road pavements. The requirements for these materials are discussed in other parts of the Guide.

This Guide needs to be read in conjunction with the other parts of the Guide to Pavement Technology.

The Guide to Pavement Technology consists of the following 10 parts:

• Part 1: Introduction to Pavement Technology

• Part 2: Pavement Structural Design

• Part 3: Pavement Surfacings

• Part 4: Pavement Materials:

– Part 4A: Granular Base and Subbase Materials

– Part 4B: Asphalt

– Part 4C: Materials for Concrete Road Pavements

– Part 4D: Stabilised Materials

– Part 4E: Recycled Materials

– Part 4F: Bituminous Binders

– Part 4G: Geotextiles and Geogrids

– Part 4H: Test Methods

– Part 4I: Earthworks Materials

– Part 4J: Aggregates and Source Rock

– Part 4K: Seals

– Part 4L: Stabilising Binders

• Part 5: Pavement Evaluation and Treatment Design

• Part 6: Unsealed Pavements

• Part 7: Pavement Maintenance

• Part 8: Pavement Construction

• Part 9: Pavement Work Practices

• Part 10: Subsurface Drainage.



2. Concrete Pavement Types

2.1 Base Types

The principal types of cementitious concrete pavements are:

• jointed plain concrete (unreinforced) (PCP)

• jointed reinforced concrete (JRCP)

• continuously reinforced concrete (CRCP)

• jointed steel fibre reinforced concrete pavements (SFCP).

The following notes are indicative of current Australian (state road agency) and New Zealand practice.

2.1.1 PCP

A slab is a portion of concrete base that is bounded by joints and/or edges. There are two types of PCP:

• Slabs with undowelled transverse contraction joints having a maximum spacing of 4.2 metres. In rural areas the joints are typically skewed at 1:10, whilst, in urban areas, the joints are typically normal to the centreline to facilitate joint layout design.

• Slabs with dowelled transverse contraction joints having a maximum spacing of 4.5 metres. These joints are normal to the centreline to facilitate joint layout design.

2.1.2 JRCP

Slabs with dowelled transverse contraction joints at a maximum spacing of 8 metres. These joints are normal to the centreline. Welded wire fabric (‘mesh’) reinforced is provided in the slabs but not continued through the joints.

2.1.3 CRCP

There are no transverse contraction joints in CRCP. Sufficient reinforcement, substantially greater than in JRCP, is provided to induce transverse cracking at spacings in the intended range 0.5–2.5 metres. Transverse reinforcement is provided principally to support longitudinal reinforcement.

In both JRCP and CRCP the role of the reinforcement is not to stop cracks occurring but rather to retain structural integrity by limiting the opening of these cracks.

2.1.4 SFCP

In irregular shaped, i.e. non-rectangular slabs, cracking is likely to develop in PCP, whilst, in JRCP, a substantial amount of reinforcement cutting is required. SFCP is resistant to micro-cracking and is suited to irregular slab layouts. SFCP has particular application to joint layouts required in roundabout pavements.

2.1.5 General Notes

Longitudinal joints: For all pavement base types, tied longitudinal joints are provided at a maximum spacing of 4.3 metres for trafficked slabs and up to 4.5 metres for un-trafficked slabs. This is to either define construction runs or control curling/warping stresses. The total tied width is typically limited to 16 metres, i.e. a three-lane carriageway.



Structural concrete shoulders with the same base thickness are typically provided for all concrete pavement types.

Information on pavement base design is available in Chapter 9 of Part 2 of the Austroads Guide to Pavement Technology: Pavement Structural Design (Austroads 2017).

2.2 Subbase Types

The principal role of the subbase is to provide long-term continuous uniform support to the base and to limit joint deflections and faulting. The subbase also has the following supplementary roles:

• reduce deflections at joints and cracks

• assist in the transfer of loads between joints and cracks

• assist in the control of volume changes in subgrade soils arising from wetting or drying.

The principal subbase material property is erosion resistance to avoid long-term ejection of fines (‘pumping’) through joints or cracks arising from the pressure generated by numerous repetitions of heavy vehicles.

In order to achieve erosion resistance a minimum of 5% cement by mass of untreated granular aggregate material is incorporated. When used as a cement bound dense-graded base or as ‘dry rolled’ concrete, only sufficient water to achieve compaction is added. Dense-graded asphalt may also be used.

The most common form of bound subbase used in practice is lean-mix concrete, which is placed as mass concrete. The preferred pattern of cracking that will develop is a series of relatively closely spaced fine cracks using a low shrinkage concrete mix. To assist this crack development, the lean-mix concrete has the following desirable properties:

• a seven-day compressive strength not exceeding 5 to 7 MPa and a 28-day compressive strength of about 15 MPa

• low drying shrinkage, which, when tested using standard laboratory methods, is typically in the range 450 to 500 microstrain at 21 days.

To make a ’paveable’ lean-mix concrete, a cement content of 250 kg/m3 is targeted, typically 90 kg/m3 of cement and 160 kg/m3 of fly ash.

Typically the subbase layer is supported by a defined ‘selected material zone’. The selected material zone in turn overlays embankment or cutting floor constructed to specified uniform standards for compaction, deflection and drainage practice. The selected material zone for highway-standard performance usually comprises two layers, each of 150 mm compacted thickness. The upper layer consists of a material with a California bearing ration (CBR) value (based on 10-day lab-soaked CBR and standard compactive effort) exceeding 30%. The lower layer is acceptable where the CBR value exceeds 15%. The upper layer may achieve the required CBR value after modification by lime or other cementitious binder, but the material must be of a quality to achieve a CBR value of 15% prior to modification.

A fine aggregate seal is normally applied to the top of the selected material zone (typically 7 mm nominal size) in order to provide protection and control moisture migration during the construction period.

No-fines concrete is used as a subbase in many of Sydney’s road tunnels and typically the base layer is CRCP. A 30 mm thick asphalt layer (size 10 mm) between the base and subbase concrete allows the CRCP to debond from the no-fines concrete. The purpose of the no-fines concrete is to allow the layer to act as a drainage layer while providing a strong and durable support for the CRCP base layer.



3. Selection of Concrete Component Materials

3.1 Introduction

This section provides information on the material constituents, and their characteristics, required for the manufacture and placement of concrete base and lean-mix concrete subbase for concrete roads.

Concrete comprises coarse and fine aggregates (sand) bound together with a paste of cementitious material and water. Coarse aggregate is that portion retained on the 4.75 mm sieve, whilst fine aggregate is that portion passing the 4.75 mm sieve. Admixtures are commonly added to provide additional beneficial properties.

Table 3.1 lists typical proportions of mix constituents in lean-mix concrete subbase and base concrete. These proportions will be different for steel fibre concrete.

The minimum property requirements for the mix concrete materials are defined in the road agency specification usually in conjunction with the appropriate Australia Standard. Each of the materials for concrete is discussed below.

Table 3.1: Typical proportion of mix constituents used in Australasian mixes (by mass)

Constituent Percentage by mass

Lean-mix concrete subbase Base concrete

Coarse aggregate 48 48

Fine aggregate (sand) 33 32

Cement 4 12

Fly ash 6 3

Water 6 6

Water/binder ratio 0.55–0.60 0.40–0.45

Aggregates constitute about 80% of the volume of concrete, and their characteristics greatly influence the workability, strength, durability, wear resistance and economy of the concrete. The concrete must be resistant to abrasion, withstand weathering, be strong in flexure and compression, and resist impact and wear. It must also be inert to chemical reaction with alkalis in cement. Aggregates containing weak, friable or laminated particles are therefore undesirable.

The shape, surface texture and proportion (as it relates to interlock) of the aggregate influence the strength of the concrete, especially the flexural strength, on which the design of concrete pavements is commonly based. A rough surface, such as that typical of crushed particles, assists in the creation of a better bond because of the interlock between the matrix and the aggregate surface. Although rounded aggregates generally produce more workable concrete mixes, their use will result in a concrete weaker in flexure than a concrete mix composed of crushed aggregate at the same water-cement ratio.

A minimum of approximately 40% of the total aggregate content should be fine aggregate to enhance the friction characteristics of the surface (the ‘sand-paper’ effect). Approximately one-third of the fine aggregate should contain quartz, or a blend of quartz and chert, to ensure durability under traffic. When it is planned to place an asphalt overlay over the concrete, this proportion may be reduced to about one-quarter. In some areas, the use of manufactured sand or blends is increasing due to environmental pressures and material shortages. The specification of manufactured sands, including quartz and chert requirements, are therefore currently under review by industry and several road agencies.



3.2 Sources of Aggregates

Aggregate may be produced from:

• crushed and screened quarry products

• natural sands and gravels

• manufactured or recycled products.

Igneous rocks, including basalt, dolerite, andesite, granite, porphyry, rhyolite, diorite, etc. are the most common source of processed quarry aggregate. Metamorphic rocks such as hornfels, schists, gneisses and quartzites are also used as concrete aggregates. Sedimentary rocks (especially bedded) and low-grade (foliated) metamorphic rocks may be precluded if they possess planes of weakness or are of inadequate strength.

Natural sands and gravels may be crushed and screened, washed and screened, or obtained as untreated bank run or pit sand. Where an asphalt surfacing is not required or the application is not a subbase layer, at least 50% of sand content should be derived from natural sources, of which 70% should be quartz or chert particles to ensure suitable skid resistance.

A petrological examination of coarse aggregates is necessary to identify the rock types and deleterious material or the presence of reactive minerals. The group classification does not imply suitability of any aggregate for concrete making; unsuitable material can be found in any rock type, although some types tend to have a better record than others. Certain specified test limits are set by known characteristics of rock types and are given in AS 2758.1. Where possible, materials from a source with a proven history should be used in concrete applications. It is recommended that the ‘proven’ sources be verified by petrological examination and that the material properties are consistent with agency specifications.

Table 3.2 provides broad guidelines for rock types commonly used as concrete aggregates. Further details regarding source rock requirements are presented in Part 4J: Aggregates and Source Rock of the Guide to Pavement Technology (Austroads 2008).

Table 3.2: Typical aggregates for concrete

Rock group Rock type used most commonly as aggregate

Other types having similar properties

Fine/medium-grained basic igneous rock Basalt Dolerites

Fine/medium-grained intermediate igneous rock Andesite Microdiorite, microsyenite, trachyte

Fine/medium-grained acid igneous rock Rhyolite, dacite Microgranite, aplite, obsidian

Coarse-grained igneous rock Granite Granodiorite, diorite

Regional metamorphic rock Slate Schist, gneiss

Contact metamorphic rock Hornfels Certain quartzites, contact-altered volcanic breccia

Sedimentary and low-grade metamorphic rock Quartzite, limestone Certain sandstones

Sedimentary rock of volcanic association Breccia Agglomerate

Arenaceous sediments River gravel Conglomerate



3.3 Aggregate Production

The production of aggregate for concrete manufacture must be adequately controlled to produce a final product that has the properties required. Aggregate production from hard rock quarries generally involves crushing followed by screening, as summarised in Austroads (2008).

The range and distribution of particle sizes, and shape of aggregate from crushing, are largely determined by the relationship between the rock type, crusher types and crusher settings. Hard rock quarries are the predominant source of concrete aggregates in Australia. Fine aggregates (sands) are increasingly sourced from off-stream land-bound deposits as well as manufactured sand operations.

With an increase in mobile quarrying/processing operations and the limited use or availability of water, quality monitoring, particularly with respect to shape and cleanliness, is of increasing importance. The frequency of testing of aggregates specified by road agencies and Australian Standards must be strictly followed to ensure close monitoring of the uniformity of the aggregate. Variations in fines particularly can impose dramatic changes on concrete rheology and plastic shrinkage (leading to full depth cracking).

3.4 Aggregate Properties

3.4.1 General

The properties of aggregates may be considered in the following two groups:

• properties primarily dependent on the type of material used

– toughness (strength, hardness and resistance to wear)

– durability and soundness

– density

– water absorption/porosity

– surface microtexture

– chemical properties, including alkali reactivity

– thermal expansion

• properties that can be partly controlled (as a direct result of processing)

– particle shape

– particle size distribution (grading)

– cleanliness (silt, clay and organic matter content).

The suitability of aggregates for use in concrete mixes is evaluated by a series of tests that are described in the AS 1141 (series). Typical limits for the range of tests are given in road agency specifications and in AS 2758.1. Typical properties that are characteristic of aggregates obtained from various rock sources are summarised in Table 3.3.



Table 3.3: Relative properties of rocks for concrete

Rock type Mechanical strength Durability Chemical

stability Surface

characteristics Hardness toughness

Surface texture

Crushed shape

Igneous Granite

Good

Good Good

Good

Fair Fair Fair Syenite Good Good Good Fair Fair Dolerite Good Good Good Fair Good Basalt Good Good Good Good Good Diabase Fair Questionable Good Good Good Periodite Fair Questionable Good Good Good Metamorphic Gneiss, schist Good Good

Good

Good Fair Good Good Quartzite Good Good Good Good Good Fair Marble Fair Good Good Poor Fair Fair Serpentine Fair Fair Fair to poor Good Good Good Hornfels Good Good Good Good Good Good Amphibolite Good Good Good Fair Fair Slate Good Good Poor Good Fair Fair Sedimentary Limestone/dolomite Good Fair Good Good Poor Good Poor Sandstone Fair Fair Good Good Fair Good Good Chert Good Poor Poor Fair Good Poor Good Conglomerate/breccia Fair Fair Good Good Shale Poor Poor Poor Good Poor Fair Fair

3.4.2 Durability and Soundness

The aggregate should be sound (not susceptible to degradation) and free from planes of weakness (normally associated with sedimentary and foliated metamorphic rocks).

An indication of the soundness of aggregates may be obtained from a petrological examination of the aggregate. Such examination is necessary to identify rock types, as the limits in certain tests are dependent on the type of material being tested. The other major requirement for petrological examination is to supplement acceptance testing by examination of the composition and physical and chemical constituents of the materials. From this information and probable response of the aggregates to such phenomena as the presence of cement alkalis, wetting/drying and heat/cooling may be estimated. Petrological examination is a rapid means of assessing potential alkali reactivity of an aggregate. It is especially valuable because of the long-time commonly required for the potential alkali reactivity tests.

The specific procedures employed in the petrological examination of any sample will depend largely on the purpose of the examination and the nature of the sample. Hence, detailed test procedures are not specified; however, American Society for Testing and Materials (2003) provides a standard recommended practice for the petrographic examination of aggregates for concrete.

In Australia, tests available for assessing soundness include:

• Aggregate Soundness: Evaluation by Exposure to Sodium Sulfate Solution (AS 1141.24)

• Iron Unsoundness (AS 1141.37)

• Degradation Factor (AS 1141.25.1, AS 1141.25.2 & AS 1141.25.3)

• Wet/Dry Strength Variation (AS 1141.22)

• Secondary Minerals Content in Igneous Rocks (AS 1141.26).



The sodium sulfate soundness test subjects an aggregate to alternative wetting and drying cycles with a solution of sodium sulfate. The process causes the growth of sulfate crystals in the pores of aggregate particles. This is intended to reproduce the destructive forces of freezing water. It is commonly used to assess the soundness of concrete aggregates. Sulfate soundness value limits for coarse aggregates are generally between 9 and 12%, whilst fine aggregates are in the range of 12 to 15%, depending on the exposure classification of the pavement.

The degradation factor test is performed on samples of rock taken from quarry spalls or core samples and crushed and screened to specific sizes. It is used to categorise the fines produced by self-abrasion in the presence of water. Most of the experience with the test has been with basic igneous rocks, although it may also be applied to other rock types.

In the wet/dry strength variation test, the forces required to produce 10% fines for an aggregate in both a dry and a saturated surface dry condition are measured. The variation is the decrease in force required to produce 10% fines for wet aggregate expressed as a percentage of the force required for dry aggregate. Concrete aggregates are typically required to have a maximum wet/dry strength variation of 35% (Table 3.4).

Table 3.4: Example durability requirements of coarse aggregates for concrete

Mix type Minimum wet strength (kN)

Maximum wet/dry strength variation

Base concrete 80 35%

Lean-mix concrete 50 35%

The secondary mineral content determination involves petrological examination of a thin section of rock and the use of a point-counting procedure of the identified secondary minerals.

Aggregate should also be free of contamination with unsound particles. This can be evaluated by testing for unsound particles (AS 1141.30.1, AS 1141.37) or weak particles (AS 1141.32). The unsound particles, e.g. clay lumps and weak particles, are those that, when wet, will deform under finger pressure. These particles are removed and the loss of such particles is reported as a percentage by mass of the original test portion. The test portion is the aggregate fraction retained on the 4.75 mm sieve.

Unsound particles, present either as natural components in the aggregate source or contaminant particles are those that, under hot/cold or wet/dry cyclic conditions, cause disruptive expansion and/or lead to aggregate breakdown. Such particles produce problems ranging from cosmetic staining or reduction in durability. Shale is often regarded as unsound as are other soft inclusions such as coal and coal wash reject. If present in quantities over 5%, these particles often adversely affect the durability and strength; they should not be used in concrete. In the case of the group of materials that lead to disruptive expansion, the absorption characteristics and pore size distribution are often critical.

Amongst those considered unsound are certain shaley and cherty rocks, and some picritic and other basic materials containing olivines that may weather rapidly (to form expansive montmorillonite-type clays).



3.4.3 Density

Accurate determination of aggregate particle density is an important element of mix design, as small variations have a significant effect on calculation of volumetric properties. It is also useful in classification of aggregates and in the calculation of yield in terms of volume per mass.

The density of coarse and fine aggregates is determined according to AS 1141.6.1 and AS 1141.6.2 for coarse aggregates, and AS 1141.5 for fine aggregates. The test methods provide for three measures of particle density:

• Particle density on a dry basis is the mass per unit volume of particles where the volume includes both the permeable and impermeable voids inherent in the particles. It is determined from the ratio of mass of an oven-dry sample to the volume of water displaced by the sample mass in a saturated surface-dry condition.

• Apparent particle density is the mass per unit volume of the impermeable portion of the aggregate particles (inaccessible to water by 24-hour soaking). It is determined from the ratio of the mass of an oven-dried sample to the volume of water displaced by that sample after 24-hour soaking.

• Particle density on a saturated surface dry basis is the mass per unit volume including both the permeable and impermeable voids. It is determined from the ratio of the mass of a saturated surface dry sample to the volume of water displaced by that saturated surface dry sample.

Particle density on a dry basis is generally used for the determination of the volumetric properties of concrete. The particle density of aggregates used in concrete is typically required to exceed 2.1 t/m3.

Bulk density (previously known as unit mass), determined using AS 1141.4, is a measure of the amount of compacted or loose material for a unit volume. It is a useful measure for determining yield for transportation purposes and is an indication of packing efficiency.

3.4.4 Water Absorption/Porosity

The porosity of an aggregate, measured by the amount of water it absorbs, has an impact on mix batching, placement operations and durability. The determination of absorption is useful in the determination of net (effective) water/cement ratio.

Where supplementary cementitious materials are present in a mix, references to Water/Cement (w/c) Ratio within this document should be interpreted as the ratio of water to all cementitious materials – that is, the effective Water/Cement Ratio of a concrete containing supplementary cementitious materials is the Water/Cementitious Materials ratio.

Aggregates with low water absorption will usually demonstrate good resistance to freeze/thaw activity and dimensional stability during wetting/dry cycles. Research has highlighted trends that indicate that aggregates with higher absorption values are associated with higher concrete drying shrinkage.

Where severe exposure conditions exist, high-porosity aggregates will contribute to the overall porosity of the concrete, allowing for easier ingress of sulfates and chlorides, leading to the corrosion of embedded reinforcement.

Water absorption is determined by testing in accordance with AS 1141.6.1 and AS 1141.6.2. It is the ratio of mass of water held in the permeable voids of the aggregate particles brought to the saturated surface dry condition, to the oven dry mass of material. Maximum water absorption values of 2.5% and 5% are typically specified for base concrete and lean-mix subbase concrete respectively.



3.4.5 Surface Microtexture

The Transport and Road Research Laboratory (Franklin 1988) carried out accelerated wear tests on a wide range of concrete and mortar mixes to establish the influence of materials, mix design and type of texture on the skid resistance values and texture depths of concrete road surfaces. It was concluded that the most important constituent in the mix was the fine aggregate. The use of high-silica content natural sands always yielded higher skid resistance values than did relatively soft sands or crushed fine materials. The incorporation of calcined bauxite fines in the mix yielded the highest skid-resistance values. The polishing characteristics of the coarse aggregate had only a very slight effect on the skid-resistance value of concrete. Hence, in Australia, it is unusual for frictional characteristics to be included in specifications for concrete aggregates.

3.4.6 Deleterious Chemical Properties

Chemical substances in aggregates that are likely to affect its properties are:

• Water-soluble substances, such as common salt, which may cause efflorescence or reinforcement corrosion. Sands won from sea shores or river estuaries may be high in salt content. If so, they will require washing prior to use.

• Set or hydration retarding substances, e.g. gypsum and certain organic compounds. Gypsum may be found in aggregates won from river beds in the drier parts of Australia. It can cause difficulties such as premature set or may render the concrete susceptible to sulphate attack. Suspect organic levels are indicated by a colourimetric test; however, the test is variably used as an indicator of potential ‘suspect’ organic levels. Comparisons of concrete strength with and without a sand with ‘suspect’ organic levels can be used to assess whether the presence of organic materials is, in fact, detrimental or not.

• Problems associated with the presence of wood and charcoal are prevalent in Australian sands and river gravels. Although not deleterious in the sense of reacting with the cement paste, they can lead to unsightly staining and small pop-outs at the surface of concrete products.

• Problems associated with alkali aggregate reactions (AAR) between reactive forms of silica in aggregates and alkalis from cement forming an alkali-silica gel and leading to expansive stresses within concrete (and eventual disintegration of concrete) have long been recognised (Standards Australia SA HB 79-2015). Reactive materials are divided into two types

– highly siliceous materials such as opal, chalcedony and tridamite, and

– dolomitic limestones.

Two physical tests, intended to gauge this reaction are the accelerated mortar bar and concrete prism methods. A petrological examination, whilst not a substitute for a physical test, is intended to screen out those aggregates that can readily be identified (by a petrographer) as reactive from mineral composition or in service performance. Aggregates that show mild reactivity to the alkalis in cement are not necessarily deemed inappropriate; mild AAR can be managed effectively by minor process and constituent changes, such as the inclusion of fly ash.

• Ore minerals containing sulphides may be deleterious. Chalcopyrite and marcasite are examples of these. The maximum desirable limit of total sulphur is considered to be 2%. Slag aggregates with high concentrations of lead, zinc and copper can lead to retardation of cements.



3.4.7 Thermal Expansion

Generally the problem of compatibility between the thermal expansion of aggregate and matrix has not attracted much attention and it is not likely to cause problems in pavement construction. Aggregate type can, however, have an effect on the magnitude of thermal expansion. Typical values are provided in Table 3.5, but no guidelines are available on acceptable limits. Whilst differential thermal expansion has an impact on internal stresses, overall concrete thermal expansion can impart a significant effect on concrete durability by way of diurnal hogging and curling of slabs.

Table 3.5: Coefficient of thermal expansion of aggregates

Aggregate (geological group)

Coefficient of thermal expansion x 10-6 mm (per ºC)

Range Mean

Chert 11.4–12.2 11.8

Quartzite 11.7–14.6 13.2

Quartz 9.0–13.2 11.1

Sandstone 9.2–13.3 11.3

Marble 4.1–7.4 5.8

Siliceous limestone 8.1–11.0 9.6

Granite 8.1–10.3 9.2

Basalt 7.9–10.4 9.2

Limestone 4.3–10.3 7.3

Mean 8.9

3.4.8 Particle Shape

Particle shape of processed aggregates is dependent on the nature of the rock and the crushing process. The shape, angularity and flakiness of aggregate particles can influence the strength of a concrete mix by virtue of the amount of mechanical interlock. Best interlock is generally obtained with well-shaped (cubic) crushed angular particles. Particles with rounded faces may not provide as efficient mechanical interlock and thus reduced deformation resistance.

Shape, angularity and flakiness are also important factors in the endurance of joints that function by aggregate interlock. Those aggregates most prone to failure are gneisses and schists. Fine-grained rocks generally crush to more flaky particles than rocks with a large crystal structure; these flaky particles are more pronounced for size 10 mm and fines fractions.

A description of coarse particle shape characteristics is provided in Table 3.6.

Table 3.6: Coarse aggregate particle shape

Classification Description

Rounded Fully water-worn (completely shaped by attrition)

Irregular Naturally irregular, or partly shaped by attrition and having rounded edges

Angular Possessing well-defined edges formed at the intersection of roughly planar faces

Flaky Materials in which the thickness is small relative to the other two dimensions

Elongated Materials, usually angular, in which the length is considerably larger than the other two dimensions

Flaky and elongated

Materials having the length considerably larger than the width, and the width considerably larger than the thickness



Particle shape also influences workability, or ability to compact a concrete mix. Rounded particles, particularly the incorporation of natural sand in the fine aggregates, provide more workable and more readily compacted mixes.

Long, flat particles, and those that are very angular, require a higher percentage of fine material and more cement or more water to produce workable concrete than aggregates that are round or nearly cubicle. Concrete made with an excess of misshapen particles may be less economical because of the increased cement required for workability, durability and strength to meet service conditions.

Particle shape of coarse aggregates may be evaluated using the flakiness index test (AS 1141.15) or proportional caliper test (AS 1141.14).

Where shape is specified in terms of the proportional caliper test, typical maximum values for the proportion of flat or misshapen particles retained on a 9.50 mm AS sieve are 10% using a 3:1 ratio, and 25% for a 2:1 ratio.

A further factor in the shape of aggregates for base concrete is the presence of fractured faces. This is particularly relevant to natural gravels that may contain a proportion of rounded particles. Generally, at least 80% of particles retained on a 4.75 mm AS sieve should have at least two fractured faces. The area of the fractured face(s) should also be a significant proportion of the total surface area of the particles.

The angularity value of an aggregate gives an indication of the workability of concrete made with that particular aggregate. The workability of concrete mixes is typically assessed using the Vebe test (AS 1012.3.3) in the laboratory during trial mixing and assessed for uniformity in the field using the slump test (AS 1012.3.1).

3.4.9 Particle Size Distribution

The achievement of consistent aggregate particle size distribution (grading) is essential for quality control of concrete production. The grading of coarse aggregate particles is determined by dry-sieving using the method described in AS 1141.11.1.

The particle size distribution of the coarse and fine aggregates should be such that, when combined, the resultant particle size distribution will meet the combined aggregate particle size requirements for the mix. These requirements are usually initially specified with broad limits but are narrowed once a design mix has been determined based on a combination of submitted aggregates.

Several aggregates may have to be combined to produce the specified particle size distribution. Generally, aggregates that will combine to give a smooth grading curve produce the best results. Very fine sands require more cement paste and hence are uneconomical, while very coarse sands produce harsh, unworkable mixes. However, mixtures of very coarse and fine sands have been used successfully. For a given workability, the coarser the mix, the less will be the total specific surface area of the aggregates, and hence the less the demand for cement paste, i.e. a more economical mix.

General particle size distribution guidelines are provided in AS 2758.1, including the permissible deviation on each grading limit.

A typical base concrete grading envelope for combined aggregates is given in Table 3.7. Grading limits are not commonly specified for lean-mix concrete subbase.

Manufactured sands are typically much harsher than natural sands. The use of manufactured sands in a paving mix will require special allowances; for example, extra cement paste or added fines to ensure cohesiveness.



Table 3.7: Typical base concrete grading envelope

AS sieve Percentage passing by mass

19.0 mm 95–100

13.2 mm 75–90

9.5 mm 55–75

4.75 mm 38–48

2.36 mm 30–42

1.18 mm 22v34

600 µm 16–27

300 µm 5–12

150 µm 0–3

75 µm 0–2

3.4.10 Cleanliness

Materials finer than 75 µm, depending on their parent rock type, may or may not be clay. Crusher dust occurs as fine flour adhering to the surface of crushed stone or crushed sand. Such material must be washed from the aggregate; however, well-bonded coatings are sometimes not removed.

There is no objection to the presence of a limited amount of such material provided the coatings are not detrimental in terms of chemical reactivity or bond. An excess of very fine material is likely to cause an increase in water demand.

The ‘settling method’ test AS 1141.33 provides a guide to the amount of silt and clay in fine aggregates. It provides an indication of the amount of possibly harmful materials in the fine aggregate and, in some instances, is used as an indicator of the effectiveness of sand washing plants. It is rarely used as an acceptance test by itself.

Sugar acts as a set retarder and its presence should be avoided. It is typically limited to less than 1 part in 10 000 (Weller & Maynard 1970) when tested in accordance with AS 1141.35.

The AS 1141.34 test for organic impurities other than sugar in aggregates is rather subjective. Failure to pass the test should not be the basis of rejection for an aggregate, but rather an indication for undertaking additional testing. Additional testing could include comparing the setting time and mortar strength results with those from a control sand.

The presence of harmful materials such as coal, lignite or chert may be detected by flotation in heavy liquids in accordance with AS 1141.31. The proportion of light particles should not exceed 1% by mass.



3.5 Cementitious Binders

3.5.1 Cement

Cement is a generic term used to describe a wide variety of organic and inorganic binding agents. The most widely used binding agents are those known as hydraulic cements. They are finely ground inorganic materials that possess a strong hydraulic binding action, i.e. when mixed with water they harden to give a stable, durable product.

Calcium silicate cement, also known as Portland cement, is basically a calcined (burnt) mixture of limestone (the major component) and clay. The mixture is subsequently ground to a fine powder and mixed with gypsum to retard an initial rapid reaction in the hydration process. Portland cement is defined in AS 3972 as a hydraulic cement that is manufactured as an homogeneous product by grinding together Portland cement clinker and calcium sulphate and which, at the discretion of the manufacturer, may contain up to 5% of mineral additions. Mineral additions are defined as being selected fly ash, slag and limestone containing more than 80% calcium carbonate, or combinations of these materials.

There are a variety of cement types and blends commercially produced, and each has different properties and characteristics. They are defined as:

• general purpose cement (Type GP)

• blended cement (Type GB)

• general purpose limestone cement (Type GL)

• high early strength cement (Type HE)

• low heat cement (Type LH)

• shrinkage limited cement (Type SL).

Type SL type cement, usually with a proportion of Supplementary Cementitious Materials (SCM), is used for base and subbase concrete. SCM include fly ash, ground granulated blast furnace slag and amorphous silica.

Type SL is a special-purpose cement with a drying shrinkage limit of 750 microstrain at 28 days with a mean limit of 600 microstrain1. It is the preferred binder for base and subbase concrete. Note that the use of Type SL cement may not ensure low drying shrinkage in concrete, as other significant factors, including aggregate type, water content and admixtures, should be considered.

In some cases, blended cements, such as Type GB, are used. These are defined as blends of Portland cement and a quantity comprised of one or both of the following:

• greater than 5% of fly ash or ground granulated iron blast furnace slag, or both

• up to 10% silica fume.

For base concrete, a minimum cement content is specified to enhance the durability of the surface and to improve slip forming characteristics. Commonly a minimum cement content of 280 to 300 kg/m³ (of concrete) is specified.

For subbase concrete, a minimum cement content of 90 kg/m³ (of concrete) is specified to enhance early strength gain. This is usually mixed with up to 160 kg/m³ of fly ash to ensure suitable plastic workability and to enhance the concrete’s rate of strength development.

1 Mean value is based on not more than 180 days or 30 samples, as per specification SP-43 (ATIC 2014).



Cements used by agencies are normally required to be pre-registered with the Australian Technical Infrastructure Committee (ATIC). This pre-registration scheme, together with required testing frequencies, is given in ATIC Specification No 43 (ATIC 2014).

3.5.2 Fly Ash

Coal fly ash is produced from the burning of coal in coal-fired boilers in electricity generating stations. Fly ash is a fine-grained powdery particulate material that is carried off in the flue gas and collected by electrostatic precipitators, bag-houses or cyclones. The type of coal used and the mode of operation of the plant determine the chemical composition and particle size distribution. Consequently, not all fly ashes are suitable for use in concrete. Generally, fly ash derived from burning black coal is high in silica and alumina and low in calcium and carbon, and, as such, is well suited for use in concrete. Fly ash derived from the burning of brown coal contains large percentages of calcium and magnesium sulfates and chlorides and other salts, and, as a result, it is unsuitable for use in concrete.

As unburned organic carbon breaks the continuity of contact in the cementitious reactions, its proportions are limited to less than 10%. However, unburned carbon at variable levels, even at low regimes, can have a significant impact on the effectiveness, and consistency, of air entraining admixtures.

Fly ash should conform to AS/NZS 3582.1 and be a ‘fine grade type’, i.e. solid material extracted from the flue gases of a pulverised coal fed boiler that has at least 75% passing the 45 micron sieve and also a maximum 4% loss on ignition.

For base concrete, a limited addition of fly ash is allowed to compensate for aggregate grading deficiencies, reduce shrinkage, improve workability and offset cement use to reduce costs. Quantities vary from nil to about 70 kg/m³ of fly ash. A minimum total binder content (fly ash plus cement) of 300 to 330 kg/m3 is typically specified for durability reasons.

To reduce stripping times in structural applications, modern cements are usually ground finer than in the past. The finer grinds increase the likelihood of increased drying shrinkage and aggregate reactivity. As a result, the use of about 20% fly ash has become routine in base paving and about 70% fly ash in the lower strength lean-mix subbase. The use of at least 20% fly ash also provides good protection against alkali aggregate reaction.

For subbase concrete, a minimum fly ash content of 100 kg/m³ is specified to enhance the plastic properties of the mix and the rate of strength development as it affects cracking behaviour. A minimum total binder content (fly ash plus cement) of 250 kg/m3 is typically specified for durability reasons.

Pozzolans such as fly ash are discussed further in Section 3.7.6. Further details of fly ash requirements are given in Part 4L of the Guide to Pavement Technology (Austroads 2009).

3.5.3 Ground Granulated Blast Furnace Slag

Ground granulated iron blast-furnace slag (GGBFS) is a waste product from the steel making industry. Its properties must conform to AS 3582.2 when used in concrete. Very little blast furnace slag is sourced locally due to the low production of steel making in Australia and much of the granulated iron blast-furnace slag is imported into Australia and New Zealand.

The Roads and Maritime Services (Roads and Maritime) specification for base concrete permits the use of fly ash and GGBFS in the proportions listed in Table 3.8.



Table 3.8: Range of fly ash and GGBFS limits permitted in base concrete in Roads and Maritime specification 3211

Binder type AAR class Limits(1), (2)

Minimum (%) Maximum (%)

Fly ash Non-reactive 15 – (0.5 x GGBFS%)

40 – (0.5 x GGBFS%) Reactive 20 – (0.5 x GGBFS%)

GGBFS Non-reactive 10 – (2.0 x FA%)

65 – (2.0 x FA%) Reactive 40 – (2.0 x FA%)

1 Where FA% = Percentage of fly ash by mass of total cementitious material and GGBFS% = Percentage of GGBFS by mass of total cementitious material.

2 By mass, relative to total cementitious material.

3.5.4 Geopolymer Binders

The term geopolymer describes a wide range of synthetic aluminosilicate materials that have the potential to replace Portland cement. The composition of some geopolymers is dominated by silicon (Si) and aluminium (Al), whereas calcium (Ca) dominates the composition of Portland cement. Austroads (2016) conducted research into the application of geopolymer concrete and noted that the manufacture of geopolymer concretes and their formulations are not open to external review and are marketed as proprietary products. At this stage, geopolymer concrete has not been used on highway pavements and further trials are required to comprehensively assess the performance of this special concrete.

3.6 Water

Water used in the production of concrete must be free from materials harmful to concrete and reinforcement and be neither salty nor brackish. The water must conform to Clause 2.4 of AS 1379, except for:

• chloride ion – maximum 300 parts per million

• sulfate ion – maximum 400 parts per million.

Chloride and sulfate ion contents may be determined by testing individual constituents (including aggregates, water and/or cementitious materials) and then calculating a total content and proportion, or by testing the hardened concrete providing the water used is from the same source.

3.7 Admixtures

3.7.1 General

Admixtures for base and subbase concrete should comply with AS 1478.1. Accelerators are not permitted because of concerns about their possible effects on shrinkage, reinforcement corrosion, durability, etc. Methods of sampling and testing admixtures for concrete, mortar and grout are contained in AS 1478.2.

For base concrete, the use of an air-entraining agent is mandatory as an aid to slip-forming and also to reduce potential bleeding. In summer, a set retarding admixture (Type Re or WR Re) is mandatory to control slump. This admixture may also be a water-reducing agent (Type WR Re).

A similar retarding admixture is required for subbase concrete. An air entraining agent is not mandatory but will often be nominated by the contractor. Where used, its content must be closely monitored and controlled because of the potentially high variations in air content as a result of variability in the carbon content of fly ash. Losses on ignition results are usually an indicator of unburned carbon levels.



The effect of an admixture in concrete is influenced by the:

• time when it is introduced into the mixing or batching cycle

• extent of mixing thereafter

• presence of other admixtures

• sequence of batching of all other ingredients.

As a result, careful note should be made of the admixture dosage and batching/metering details when carrying out mixer efficiency tests.

Admixtures are an increasingly important part of concrete technology. In some applications they are considered essential (e.g. mixing and placing under hot climatic conditions).

The most common types of admixtures are:

• set accelerating admixtures such as calcium chloride, calcium formate, triethanolamine and salicylic acid

• set-retarding admixtures such as carbohydrates, including sugars, starches, and methyl celluloses; salts or hydroxy carboxylic acids such as citric, tartaric, gluconic and mucic acids; lead, zinc and copper salts, and soluble borates

• water-reducing and set retarding admixtures including salts of lignosulphonic acids and hydroxy carboxylic acids

• water-reducing admixtures such as salts of lignosulphonic acids and salts or hydroxy carboxylic acids containing small additions of either calcium chloride or triethanolamine to counteract the set component The amount of calcium chloride or triethanolamine may be varied to produce water-reducing admixtures that give normal or accelerated setting with cement.

• air-entraining admixtures, including salts or resin acids (abietic and pimaric acids), oxidised resin ('vinsol' resin), and tall oil (a mixture of resin acids and fatty acids); and salts of sulfated and sulfonated hydro-carbons, such as sodium lauryl, and sodium lauryl benzene sulfonate

• pozzolans, which are finely divided materials such as crushed rock powder, bentonite, diatomaceous earth, fly ash, and ground granulated blast furnace slag. They can, on occasion, be incorporated in concrete with beneficial effects. For example, lean concrete of high workability can give rise to excessive bleeding. This can be overcome by the admixture without any increase in the water content of the mix because the concrete is made more cohesive and can be handled more easily with less risk of excessive segregation. However, if incorporated into a relatively rich mix these powders are liable to increase water demand, resulting in lower strength and possibly higher drying shrinkage.

3.7.2 Set-acceleration

Ingredients such as calcium chloride, calcium formate, triethanolamine or other accelerator are discouraged because of concerns over properties such as shrinkage (particularly if over-dosed), and corrosion potential on reinforcement such as tie bars.

Where accelerators must be used (such as high early strength repair slabs), calcium nitrate and calcium formate are now used in preference to calcium chloride for set acceleration and early strength development. However, because of their potentially harmful effects, they are commonly recommended for use at rates not more than 1% for most applications.

Disadvantages associated with the use of calcium chloride are increased drying shrinkage, increased risk of corrosion of embedded metals, increased risk of sulphate attack (in situations where sulphate attack is possible), and, in warmer climates, an increased rate of hydration can lead to cracking of concrete upon subsequent cooling.



Calcium chloride can also react with other admixtures, and, if added simultaneously to the mixer, care is required in the batching operations. It is preferable that it be dispensed in solution rather than in solid or flake form.

Calcium chloride increases the compressive strength of concrete made with all types of Portland cement. Strength increases are produced at all ages, but the greatest effects are obtained at early ages and at temperatures below 20 °C. By contrast, however, calcium chloride generally increases flexural strength only during the first few days and either has no effect on, or decreases, flexural strength at later ages.

Amounts of calcium chloride greater than 2% by mass of cement increase early strengths but often decrease strength at later ages.

Triethanolamine and other organic set-accelerating agents are seldom used alone; they are used mainly as constituents of water-reducing admixtures.

As an alternative to the use of accelerating admixtures, it is possible in many applications to accelerate strength development by the judicious use of water-reducing admixtures in conjunction with super-plasticisers. Briefly, the ‘package' is as follows:

• design the mix with a water-reducing admixture to minimise the water/cement ratio (this will enhance potential ultimate strength)

• batch the mix to a nominal slump of, say, 20 to 25 mm (a minimum water/cement ratio of about 0.4 is required to ensure full hydration under potentially dry site conditions)

• adjust at site to the required consistency with a super-plasticiser.

For paving applications, the following measures may also be feasible:

• Adopt a 42 MPa mix in lieu of the nominal 35 MPa mix. (Note that this may not be appropriate for a CRCP where closer control is required on upper strength).

• Maintain higher curing temperatures by covering the slab with black sheeting. This will be particularly effective in the early stages if the heat of hydration can be retained. For full effectiveness it would be necessary to fully contain the edges of the wrapping.

• To avoid thermal cracking, special care needs to be taken to ensure large temperature differentials are not created between the top, the base and the outside of the slab.

3.7.3 Set-retardation

The degree of set-retardation obtained in concrete depends on a number of factors:

Type and concentration of the admixture

Typical data on the effects of different set-retarding agents on cement setting times is listed in Table 3.9 and the values show that the degree of set-retardation depends on the type and amount of the agent used. It can also be seen that the possibility of excessive set-retardation, due to an accidental overdose of agent, is much greater with sucrose and hydroxy carboxylates than with lignosulphonates. Large overdoses may affect the cement hydration and future hardened properties of the concrete. Even minor overdoses can lead to set time delays and a potential increase in plastic shrinkage cracking resulting from an extended period in the plastic state.



Table 3.9: Setting time of Portland cement pastes containing set-retarding admixtures

Admixture Concentration (% by mass of cement) Initial setting time (h) Final setting

time (h)

None 4 6.5

Calcium 0.10 6.5 10

Lignosulphonate 0.25 11 15

0.50 22.5 29

1.00 76 120

Sucrose 0.05 9 15

0.10 17 28

0.25 140–200 400

Sodium gluconate 0.05 19 22

0.10 38 42

Calcium 0.05 11 12.5

0.10 18 21

0.25 65 S0

Note: Temperature 21 °C. Values shown are typical only and will be influenced by water/cement ratio.

Source: RTA NSW (1991).

Sequence of addition of admixtures

Bruere (1963) and Dodson and Farkas (1964) demonstrated that the addition of a given amount of a lignosulphonate a few minutes after the cement has been in contact with water results in longer setting times than when the lignosulphonate is added with the mixing water directly to the dry cement. This effect occurs with all classes of set-retarding agents and tends to be more pronounced with cements with high rather than low tricalcium aluminate contents. Specifications should require that the sequence of addition of admixtures be established and followed both in trial mixing and in on-site operations.

Temperature

A greater amount of set-retarding admixture is required to produce a given delay in setting at high temperatures than at lower temperatures. The controlled use of approved set-retarding admixtures is to be favoured in preference to the adjustment of water content for the control of consistency in conditions of varying temperature and length of haul.

Type of cement

The effectiveness of set-retarding admixtures varies according to the type and source of the cement. They are usually more effective in cements with low alkali and tricalcium aluminate contents.

Concrete retarded for perhaps four hours will probably attain similar strengths at 24 hours to concretes without an admixture and therefore form striking times are often not affected by the use of a retarder.

Admixtures such as sucrose and hydroxyl carboxylates, when used without water adjustments, usually increase compressive strength by significant amounts (10 to 15%) at all ages.



3.7.4 Water-reducing Admixtures

Water-reducing admixtures such as lignosulphonates act to disperse the cement more readily throughout the water and to eliminate 'clumping' of the cement. In so doing, they decrease the yield stress of the cement paste and hence also increase the slump of freshly mixed concrete at a given water/cement ratio. The extent to which the amount of mixing water can be reduced depends on many variables but is typically 5 to 10%.

One of the difficulties associated with assessing the effectiveness of water-reducing admixtures and several other types is associated with the measurement of workability. This is because the term workability covers a number of factors (fluidity, mobility, stability), which are measured to varying extents by each of the workability tests. As a result, an admixture concrete might have the same slump as a plain concrete but the general workability and handling characteristics might be better, i.e. the yield stress remains the same but the plasticised concrete has a lower plastic viscosity.

Admixtures of the lignin group are likely to produce a more cohesive concrete with a reduced tendency to bleed, whereas those of the hydroxy-acid group are likely to have a lesser influence on cohesiveness and may tend to increase bleeding. This effect on bleeding is confirmed in results of a laboratory investigation shown in Table 3.10. The tendency for the lignin-type water-reducer to have a greater effect on cohesiveness is probably due, at least in part, to the fact that it is likely to entrain 1.5 to 2% of air into the mix.

The lignosulphonates permit greater water reductions than the hydroxy carboxylates, which do not entrain air. However, the greater effectiveness of lignosulphonates is not entirely due to the entrained air.

Table 3.10: Effect of admixtures on concrete characteristics

Concrete mix Control (no additive)

Hydroxy-acid type (266 mL/100 kg)

Lignin type (866 mL/100 kg)

Compressive strength (MPa)

@ 7 days 38.0 43.1 39.4

@ 28 days 48.1 52.2 48.0

Water/cement ratio 0.45 0.4 0.45

Slump (mm) 50 75 75

Bleeding

mL/cm2 of surface 0.081 0.130 0.039

% of mixing water 1.56 2.58 0.80


The dosage of admixture will depend on many factors and can be correctly judged on the basis of trial mixes using the particular cement and aggregates to be used for the job. The dosage rate for the hydroxy carboxylic acid type is generally lower and certainly more sensitive than with the lignin-type 'water-reducer’. However, hydroxy-acid-type admixture finds application in 50 MPa concrete, where the cohesiveness associated with the use of a 'lignin-based admixture may show up disadvantageously as a form of stickiness, making the mix too difficult to work.

Water-reducing admixtures are effective with all types of Portland cement and Portland cement/pozzolan blends. When used with Portland cements, water-reducing admixtures are generally most effective with cements containing low alkali and low tricalcium aluminate contents, this aspect being further emphasised by the adverse effects in relation to drying shrinkage for 'high-alkali' cements.



Reductions in water contents of concretes, at equal cement contents, increase strength in accordance with the water/cement ratio law, providing the concretes can be fully compacted. However, the use of water-reducing and set-retarding admixtures such as the lignosulphonates increases compressive strengths at all ages by greater amounts than would be predicted from the reduction in water/cement ratio alone. At equal water/cement ratios, pastes containing 0.25% of lignosulphonate by mass of cement have 7 and 28 days compressive strengths 15% higher than plain pastes.

The small amount of air that is usually entrained by lignosulphonates tends to reduce the strength. However, the combined strength-increasing effects of water-reduction and the presence of the admixture itself overcome the influence of air and cause substantial increases in strength providing air contents are kept below about 4%.

The effects of water-reducing and set-retarding admixtures on early strengths of concrete depend on the degree of set-retardation produced. Normal doses of these admixtures do not reduce one-day strengths, but overdoses will, of course, reduce early strength.

Such effects are not permanent, and over-retarded concrete ultimately will be as strong as, or stronger than, equivalent plain concretes. However, if a significant over-dosage occurs, the effect on strength may prove quite dramatic, particularly with the hydroxy carboxylic acid type. This is illustrated by results shown in Table 3.11, which were obtained in an investigation conducted in the laboratory for over-dosage of the order of three times (Wallace & Ore 1960).

Lignosulphonates containing small additions of a set-accelerating agent (to produce normal or accelerated setting behaviours) increase strength at all ages. However, at early ages (one to seven days), these materials produce greater strength increases than lignosulphonates alone.

Table 3.11: Effect of overdose (x3) of retarder on compressive strength

(a) with Hydroxy carboxylic acid type

Compressive strength (MPa) Dosage 800 ml/100 g Control

@ 14 days 2.8 42.3

@ 28 days 5.9 46.7

@ 90 days 53.9 53.8

Slump (mm) 75 75

(b) with Lignosulphonate type

Compressive strength (MPa) Dosage 2800 ml/100 g Control

@ 14 days 37.7 41.0

@ 28 days 39.9 45.4

@ 90 days 52.5 55.7

Slump (mm) 75 75


3.7.5 Air-entraining Admixtures

Air-entraining admixtures are organic substances that promote the formation of discrete minute air bubbles in concrete during the mixing operation.

The division between entrapped and entrained air is normally considered to be between 1 mm and 2 mm. Entrained air bubbles less than, say, 2 mm, are typically spherical and, to ensure an optimum bubble spacing factor for freeze/thaw resistance, have a mean size of 0.2 mm. Above 2 mm, entrapped air voids are not typically spherical, are randomly distributed and can usually be expelled with normal vibration effort.



Air-entraining agents provide the following benefits for paving concretes:

• reduction in segregation and bleeding

• increase in homogeneity and cohesiveness

• improved workability and slipformability.

Air-entraining agents absorb strongly on cement grains with the polar groups attached to the cement surface and the non-polar groups pointing into the water. This absorption process confers a hydrophobic nature to the cement surface so that air tends to displace water from it.

This process is the key to the action of air-entraining agents in cement paste and concrete. Thus, when a paste containing an air-entraining agent is mixed vigorously, air bubbles are generated, and since the cement grains are in a hydrophobic condition, the bubbles adhere to them.

At equal water/cement ratios, air-entrained cement pastes are more viscous and cohesive than plain pastes and have lower bleeding capacities.

The entrainment of air increases both the volume and viscosity of the cement paste fraction of the concrete. The increase in paste volume reduces the interference and friction between the aggregate particles and this increases the slump of the concrete while the increase in paste viscosity increases the cohesiveness of the concrete. Air-entrainment will modify the yield and design of a given mix. Increases in workability make it possible to re-design a mix such that strength and durability reductions are offset.

Concrete suppliers might not always use air-entraining agents, despite their contribution to workability, because their use requires increased production control without positive strength gain or cement savings. However, they do provide concrete with a greater homogeneity and cohesiveness and a reduced tendency to bleed when at a similar workability to a comparable 'non-air' concrete. They are extremely useful in this regard in wet lean-mix concretes that are typically susceptible to bleeding.

In freeze-thaw situations the specification of an air-entraining agent is virtually automatic (at a dose rate of 5–6%), whilst this admixture lends itself to serious consideration in other durability situations requiring impermeable concrete.

Care is required in the selection and control of air-entraining agents with concrete containing fly ash. Vinsol resin compounds require a relatively small dosage but are more sensitive to overdoses, whereas hydrocarbon compounds require a larger dose but are much less sensitive to overdoses.

In concrete in which the water/cement ratio is held constant, each per cent of entrained air reduces the 28-day compressive strength by 5 to 10%. This relationship remains fairly constant over the range of mixes used in practice where water/cement ratios vary from 0.45 to 0.70 by mass, and cement contents vary from 380 to 220 kg/m3. These strength reductions would normally prohibit the use of air-entrained concrete. However, when re-designed with reduced water and sand contents, concretes can be made with a strength reduction of only 3% for each per cent of entrained air.

In view of the consequences of high air contents, it is imperative that regular checks be carried out at the site on the concrete mix as supplied. AS 1379 details requirements for determination of air content.

3.7.6 The Effect of Admixtures on Drying Shrinkage

Albeit not recommended for use, calcium chloride is potentially the most deleterious of the commonly used admixtures. Its addition increases the drying shrinkage in proportion to the dose rate, particularly at early ages. Increases of about 70% at 14 days and 15% after three months are not uncommon for a 2% anhydrous addition by mass of cement (representing a 5% increase in solution added to the mix).



Other types of admixture are also liable to increase the drying shrinkage. This results from either the use of a higher water/cement ratio or because they contain a quantity of accelerator (calcium chloride or triethanolamine).

Water reducing and/or retarding admixtures based on lignosulphonates may increase drying shrinkage even where the water/cement ratio has been decreased. Certainly, the simple addition of a lignosulphonate admixture to concrete without changing mix proportions results in a significant increase in early (14 to 91 days) drying shrinkage irrespective of the mix. The effect, however, reduces with drying time, to the extent that, at an age of about seven months, the influence is no longer significant (Morgan 1974).

Where an accelerator is incorporated with the Iignosulphonate, the extent of drying shrinkage is further aggravated. The addition of the accelerating admixtures calcium chloride or triethanolamine results in additional significant increases in the early drying shrinkage strains. While this effect also reduces with time, there is a tendency for these accelerators, particularly triethanolamine to cause permanent increases in long-term drying shrinkage, even with relatively small additions. Thus, the relative order of influence of the admixtures on drying shrinkage (at equivalent recommended dosages) is:

lignosulphonate lignosulphonate

plain < lignosulphonate < + +

calcium chloride triethanolamine

The effect on shrinkage of a commonly used admixture containing both calcium chloride and triethanolamine is shown in Table 3.12.

Table 3.12: Effect on shrinkage of admixture containing calcium chloride and triethanolamine (control results shown in brackets)

Property 20 MPa mix 25 MPa mix

Dosage (mL/m3) 1350 (nil) 1500 (nil)

Cement (kg/m3) 270 (285) 300 (315)

Strength 7 days 20 (16) (22) (22)

28 days 27 (24) (33) (33)

Shrinkage 3 weeks 683 (427) 763 (432)

8 weeks 837 (675) 920 (661)


Shrinkage reducing admixtures are sometimes used to assist the mix designer with lowering the concrete shrinkage of concrete to within the specified limits. Addition of admixtures are usually only considered after changes to material components have been considered and evaluated.



4. Concrete Mix Design

4.1 General

This section provides information on the required mix characteristics of base concrete and lean-mix concrete subbase. The objective of the mix design is to obtain a concrete that will have the desired or specified properties in the fresh and hardened states.

The required attributes of concrete mixes are described in Section 4.2, while the processes used to combine the mix components (Section 3) to obtain these attributes are described in Section 4.3.

4.2 Requirements

4.2.1 General

Of the many factors that affect the performance of concrete structures, those most commonly measured or monitored for control purposes are:

• in fresh concrete

– workability

– compaction

– curing

• in hardened concrete

– strength (compressive and flexural)

– durability

– shrinkage

– skid resistance.

These factors are discussed in detail in Sections 4.2.2 and 4.2.3.

4.2.2 Fresh Concrete

Workability

The strength of concrete of given mix proportions is very seriously affected by the degree of its compaction. It is vital, therefore, that the consistency of the mix be such that the concrete can be transported, placed and finished sufficiently easily and without segregation.

Workability is best defined as the amount of useful internal work necessary to produce full compaction. It is affected by many properties including:

• water content, and water/cement ratio

• aggregate size, grading, shape and texture aggregate/cement ratio

• degree of air entrainment

• admixtures.



Unfortunately there is no test that will measure directly the workability. Useful indicators are the slump test, the Vebe test and the compacting factor test (although this test measures the inverse of the definition, i.e. the degree of compaction achieved by a standard amount of work). Another term commonly used to describe fresh concrete is 'consistency', which refers to the firmness of form or to the ease with which it will flow. Concretes of the same consistency may vary in workability.

Workability is an important consideration in that it is of utmost importance to the standard of the finished pavement that a mix be used which provides adequate workability whilst satisfying all other criteria as discussed in Sections 3.1 to Section 3.3. Trial mixing is very important in this regard; it is unlikely, for example, that a mix suitable for hand placement will also be suitable for slipforming.

The Vebe test is used in conjunction with trial mixing to assess the workability of particular mixes. It is especially applicable to low slump mixes. The slump test is not an absolute measure of workability but rather a means of quality control for a specific mix where it can be used to signal changes in the character of the materials, the mix proportions or the water content.

For slip-form construction, stiff concrete is required with typical slump values in the range 20 to 40 mm because side forms are not used. This type of paving is used in road pavements but is not common in airport construction.

For mechanised fixed-form paving, the recommended slump values will vary according to the characteristics of individual plant items. However, under normal conditions, a slump value in the range 35 to 50 mm will be suitable.

For manually operated fixed-form paving equipment (and provided that the power of the vibratory unit is compatible with the depth and width being paved) slump values in the range 55 to 65 mm are recommended. Studies have shown that for this method of paving, low-density concretes were obtained when slumps were either too high or too low.

Excessive bleeding will not normally occur in concrete used for road pavements where slump values are in the above ranges.

Compaction

Compaction of concrete is the process of achieving maximum density, without segregation of the mix, by the systematic removal of air voids entrapped in the plastic concrete during the mixing and placing operations. Voids in this context refers to entrapped but not entrained air. Long-term pavement performance is dependent upon the concrete fatigue performance under repeated loading, and crack initiators formed by entrapped air are to be avoided.

Compaction is usually carried out by subjecting the plastic concrete to mechanical vibration. If properly applied, it allows the use of stiffer consistency mixes resulting in higher-quality concrete for given mix proportions.

It is important that the principles of compaction be understood by all supervisory personnel because the consequences of improper compaction are very significant. Concrete placing is an everyday event in the construction industry and it could reasonably be expected that compaction would be a routine and well-refined process that, subject to regular surveillance and supervision, can safely be left to the judgement of an experienced operator.

Road agency paving specifications include a requirement that the concrete in the pavement be compacted to a minimum specified standard. This requirement recognises the vital importance of proper compaction to pavement performance.

Compaction is assessed by extracting cores from the hardened pavement and determining their unit mass. This is then compared with the unit mass of the cylinder specimens cast for strength testing during paving.



The ‘relative compaction’ is defined as the ratio of unit mass of the core to that of the cylinder, expressed as a percentage. The core and the cylinders are extracted from the same ‘lot’ of concrete to minimise material variability and changes in entrained air content, etc. A typical specification requirement is that the minimum conforming relative compaction is 98%.

To reduce test variability, moulding of the concrete cylinder needs to be in accordance with Roads and Maritime Test Method T304, which sets standards for internal vibration of cylinders (Roads and Maritime 2014a).

It has been found necessary to dress cores (after weighing) that have voids with a dimension greater than 5 mm before measuring their volume by water displacement. It is also necessary to adjust the unit mass of cores for the presence of embedded reinforcing steel. Roads and Maritime Test Method T368 provides a procedure (Roads and Maritime 2014b).

Compaction significantly affects the critical concrete properties of strength, shrinkage, creep, permeability, abrasion resistance and durability.

Compaction versus strength

The need for adequate compaction is readily apparent from a study of the relationship between density and compressive strength as shown Figure 4.1.

Figure 4.1: Loss of strength through incomplete compaction

Source: Cement Concrete & Aggregates Australia (2006a).

It can be seen that a density deficiency of 1% causes a strength deficiency of 6% at the upper end of the scale. The effect of incomplete compaction on flexural strength is similar.

Whilst both entrapped air and entrained air have a similar influence on strength, the nature of entrained air is such that it forms an integral part of the mix design and its influence is offset by a corresponding reduction in water content. There is no inherent difference between entrained and entrapped air except that entrained air is less than 1 mm in size and has a generally spherical shape.



It has been observed that elongated aggregate is vertical in cylinders and horizontal in cores, often with entrapped air underneath, and considerable compaction energy must be expended by vibrators in realigning this aggregate.

Compaction versus drying shrinkage

Data presented by Heaton (1966) indicates that incomplete compaction results in increased shrinkage. This is apparently caused by the relative movement of absorbed gel water and by the degree of restraint offered by the ‘tightness' of aggregate interlock.

Compaction versus permeability, durability and abrasion resistance

These properties are all detrimentally affected by lack of compaction. These effects are treated in more detail in Section 3, but it is worth noting here that research (Base & Murray 1982; Beeby 1983) indicates that permeability presents much greater corrosion hazard in reinforced structures than does cracking.

Curing

Curing is described as the maintenance of an environment favourable to the setting and hardening of concrete. A suitable environment is one that promotes cement hydration, and ideally consists of a control of temperature and of moisture movement into and out of the concrete, both in the absence of premature stressing or disturbance.

Curing not only affects concrete strength (Figure 4.2), but also durability, abrasion resistance, permeability (Figure 4.3), frost resistance, modulus of elasticity and shrinkage.

Curing of road pavements is now almost universally achieved by the use of sprayed impermeable membranes, the properties of which are described in Section 6.

Figure 4.2: Effect of duration of water curing on strength of concrete

Source: Cement Concrete & Aggregates Australia (2006b).



Figure 4.3: Effect of duration of water curing on the permeability of cement paste

Source: Cement Concrete & Aggregates Australia (2006b).

4.2.3 Hardened Concrete

Strength

The 28-day concrete flexural strength is a key design parameter in pavement performance and is used in the determination of base concrete thickness. The 28-day design flexural strength of base concrete suitable for road pavement construction is typically in the range 4.0 to 5.0 MPa, with 4.5 MPa being the minimum strength for moderate to heavily trafficked roads (> 106 HVAG). Steel-fibre reinforced base concrete should have a 28-day flexural strength in the range 5.0 to 5.5 MPa.

Usually, however, it is more convenient and economical to adopt the compressive strength AS 1012.8.1) for acceptance testing. The durability of the base concrete wearing surface requires a 28-day characteristic compressive strength of not less than 32 MPa (AS 3600). For moderately- to heavily-trafficked roads, the minimum 4.5 MPa flexural strength requirement typically requires a minimum 28-day compressive strength of 32 MPa.

An empirical correlation between flexural and compressive strength is set out in Part 2 of the Guide to Pavement Technology (Austroads 2017); this is commonly used for the purpose of pavement thickness design.

To provide adequate support to the base concrete, and to resist erosion due to traffic induced movement of water at the base/subbase interface, lean-mix concrete subbase is required to have a minimum 42 day compressive strength of 5 MPa.

Strength levels are important not only for their structural significance but also because they provide an indicator of related properties such as impact and abrasion resistance. It is important to differentiate between the two most common forms of compressive strength testing:

• cylinder testing, which is used as a measure of the quality of the concrete as supplied; it can also be taken as an indicator of the potential strength of the mix

• core testing, which not only reflects the quality of the concrete supplied, but also the quality of handling, placement, compaction and finishing.



It needs to be stressed that strength testing alone is not always an adequate indicator of the quality of a concrete. Related properties such as density and porosity may also need to be assessed, particularly in reinforced concrete. Adequate compressive strength, of itself, may not therefore provide a total measure of the acceptability, or otherwise, of a specimen.

The factors which have the greatest effect on strength are:

• cementitious material content

• water/cement ratio (Figure 4.4)

• degree of compaction (Figure 4.5)

• degree of hydration (Figure 4.2)

• compaction and curing temperature (Figure 4.6).

Figure 4.4: Relationship between modulus of rupture of concrete at 28 days and water/cement ratio

Source: Cement and Concrete Association of Australia (1982b).



Figure 4.5: Effect of air voids on concrete strength


Figure 4.6: Effect of curing temperature on strength


Note that the concrete strength development with age varies with type of cement used (Figure 4.7). Type GB is slower setting than Type GP and if the same 28-day strength is specified, mixes with Type GB may have higher long-term strength than mix with Type GP. The use of fly ash added in excess of GB cement percentages is encouraged, not only to address aggregate-alkali reactivity but also to gain the benefit of long term strength gain.



Figure 4.7: Typical development of concrete strength with age

Source: Cement and Concrete Association of Australia (1998).

Durability

Concrete must have a high level of durability to resist deterioration and wear under service conditions. To achieve this durability, the concrete should be dense, resistant to the abrasive action of traffic and should have a high level of impermeability.

In road pavements the concrete surface is subjected to wear from vehicular traffic. Therefore, the resistance of concrete to abrasion becomes an important factor in maintaining pavement serviceability. The abrasion resistance of concrete is a direct function of its strength. Hence all measures taken to increase the strength of the concrete will enhance its abrasion resistance. These measures are summarised in the following points:

• high cement content

• low water/cement ratio

• proper placing and compaction of the concrete

• proper curing starting immediately after the concrete has been finished. The longer the curing period, the more resistant the concrete will be to abrasion.

Figure 4.8 and Figure 4.9 show the effects of water/cement ratio and cement content on the abrasion resistance of concrete. The effect of cement content and length of curing period is shown in Figure 4.10. It should be noted that standard finishing techniques in concrete paving (the tamper bar that pushes down coarse aggregate in front of the conforming plate in slipform paving and the mortar roll in front of the vibrating screed in fixed form paving) means that the top approximately 6 mm is mortar rich.



Figure 4.8: Effect of water/cement ratio on mass loss of concrete subject to abrasion


Figure 4.9: Effect of cement content on abrasion resistance of concrete made of different aggregates




Figure 4.10: Effect of cement content and curing period on abrasion resistance of concrete


The permeability of hardened concrete to air and moisture also has a significant influence on durability, particularly reinforced pavements. The less permeable the concrete, the less deterioration there will be. Permeability depends on the pore size and distribution, and on the continuity of the pore system and not simply on the porosity as such. Voids in the concrete are formed in one or more of the following ways:

• excess water in the mix

• incomplete compaction

• incomplete curing, which allows the concrete to dry out prematurely.

Therefore, in order to reduce the permeability of concrete, the water/cement ratio should be kept at the minimum compatible with sufficient workability for placing and effective compacting. The use of air-entraining agents is encouraged in this regard because of the reduction in water/cement ratio afforded by its inclusion. Special attention to curing can further reduce the permeability of concrete. The effect of water/cement ratio, curing and cement content on permeability is shown in Figure 4.11 and Figure 4.12.



Figure 4.11: Effect of water/cement ratio on the permeability of cement paste


Figure 4.12: Effect of fog curing on concrete permeability




Shrinkage

Knowledge of the drying shrinkage characteristics of concrete is a necessary starting point in the design of pavement structures for crack control. The drying shrinkage of concrete is a complex property that can be affected significantly by any of the following factors:

• cement type and content

• total water content

• water/cement ratio

• admixtures

• void content

• degree of hydration

• aggregate characteristics.

An upper limit of shrinkage of 450 microstrain at 21 days or 580 microstrain at 56 days is typically specified for both base and subbase because the amount of opening at control joints and cracks is critical to their load-transfer capacity.

Cracking that sometimes occurs in the surface of fresh concrete in pavements soon after it has been placed and while it is still plastic is called ‘plastic shrinkage cracking’ and should be distinguished from drying shrinkage. Plastic cracking appears mostly on the surface as short cracks often varying in width across their length. This form of cracking can be caused by high water/cement ratio, low sand content, excessive bleeding or, most frequently, hot or windy conditions at the construction site. Most agency specifications do not permit concrete to be placed on hot or warm windy days when the rate of evaporation, exceeds 1 kg/m2/h (see Figure 4.13).



Figure 4.13: Rate of evaporation from concrete freshly placed on site

Source: Roads and Maritime (2013).

Skid resistance

The fine texture on the road surface is formed by the sand in the cement-mortar layer, which imparts the adhesion component in the tyre/pavement interaction.

For high-speed roads, a coarse texture is also required and is formed by the ridges of mortar left by the method of finishing. A primary function of these ridges is to provide for the escape of water from under tyres, giving a more intimate contact, and hence, greater friction between tyre and pavement, and thus reducing the possibility of the vehicle aquaplaning. Coarse texturing provided for increased skid resistance also contributes to significant reduction in water sprayed from beneath vehicles, particularly on high-speed roads, and also diffuses night-time glare from oncoming vehicles.

A variety of skid-resistant texture patterns can be applied to concrete surfaces using brooming, burlap dragging, or other methods. The selected texturing methods should be compatible with the environment, speed and density of traffic, topography and geometry of the pavement.



4.3 Mix Design Process

4.3.1 Techniques

Within the paving industry, mix design is largely an empirical process based on experiences with past mixes, knowledge of locally-available aggregates and cement technology. The concrete mix design should also reflect the delivery method of the concrete, such as hand placing versus slipforming. A lower-slump concrete is needed for slipformed construction compared with a hand-placed concrete. In some situations a paving mix may be designed to allow the concrete to be pumped to the forms if access is restricted for transit mixers or tippers.

New mixes are usually modified for the available materials, especially the sands. An increasingly limited choice of sands means that blending natural sands with manufactured sands is a popular option. A method of checking the workability of blended sands is described in Section 4.3.6.

The inclusion of manufactured sands demands closer scrutiny of the fines introduced by the materials (for its effect on water demand) and the nature of the particles larger than 1 mm (which, if angular as a result of crushing, may cause workability and finishing complications).

There are several concrete mix design processes and three of the more common are briefly described in Sections 4.3.3 to 4.3.5. Two of the methods use a combined aggregate grading basis as the division between coarse and fine aggregates at the 4.75 mm AS sieve (or 5 mm).

In theory, a concrete mix could be idealised by packing small spheres to:

• maximise aggregate content

• minimise cement/mortar

• minimise shrinkage

• optimise workability (to maximise compaction)

• minimise (moderate) water/cement ratio.

Assuming small spheres were used as aggregate, in order to achieve maximum volume stability (i.e. minimum drying shrinkage) a container would be filled with 20 mm spheres, followed by the next largest remaining voids with spheres of 50% size (10 mm), then the next largest remaining voids with 50% size (5 mm) and so on, until the smallest aggregate is reached. Then just enough mortar would be added to fill all of the remaining voids. This is the ‘Fuller’ curve, where the grading is obtained from successive sieves, each approximately half the size of the previous sieve (Equation 1).

% passing =100 �𝑑𝑑𝐷𝐷�0.5

1

where

𝑑𝑑 = the sieve size being considered

𝐷𝐷 = the nominal maximum sieve size

A grading of this type would yield optimum results in two key aspects:

• It requires an absolute minimum of cement to fill the voids (which is good, because cement is the most expensive part, and also the part that causes shrinkage).

• The mix will have optimum volumetric stability because it has maximum possible aggregate packing (mechanical interlock); it cannot physically shrink further even if the paste would normally shrink.



Real aggregate is not spherical and, for maximum strength, it is desirable that cubic aggregate be produced. This upsets the workability and the grading needs to be modified. This is achieved by reducing the aggregate component and increasing the mortar component of the concrete mix without going too far and producing a fatty mix. This is the reverse situation to heavy-duty asphalt, where the ideal grading is made coarser to provide voids for the binder and some remaining air voids at refusal compaction for stability.

The benefits of good grading are:

• good workability at lower slump

• less susceptibility to segregation during transport and vibration

• less susceptibility to bleeding

• lower shrinkage.

The typical methods to determine the optimum particle size distribution for concrete mixes are:

• ‘Shilstone’ Coarseness workability chart (refer to Section 4.3.2)

• Optimum sand method (refer to Section 4.3.3)

• American Concrete Institute (refer to Section 4.3.4)

• Texas DoT (refer to Section 4.3.5)

• Haystack Mix maximum density

• Optimised graded concrete pavement (NCPTC 2012).

4.3.2 Shilstone Method

The Shilstone coarseness factor chart (Shilstone & Shilstone 2002) is used by mix designers as one of many tools to finalise or review a concrete mix design. Shown in Figure 4.14, the Shilstone chart plots the Coarseness Factor (x-axis) against the Workability Factor (y-axis). These factors are defined in Equation 2.

Coarseness Factor =100×% retained above 9.5 mm sieve

% retained above 2.36 mm sieve

Workability Factor =% passing 2.36 mm sieve+2.5(𝐶𝐶 − 335)

56

2

where

𝐶𝐶 = Cementitious material content (kg/m³)

As shown in Equation 2, the Workability Factor is:

• increased by 2.5% for every 56 kg/m3 of cementitious material used in excess of 335 kg/m3

• reduced by 2.5% for every 56 kg/m3 of cementitious material used less than 335 kg/m3.

This chart gives a simple visual guide as to whether the proposed aggregate grading will result in:

• Zone I – Coarse gap graded

• Zone II – Well graded and the preferred Zone on the chart

• Zone III – Finer mixes that may cause edge drop-off in slipforming

• Zone IV – Sandy mixes leading to segregation of the concrete

• Zone V – Rocky gradation.



The Texas DOT approach (refer to Section 4.3.5) is a modification to the Shilstone method.

Figure 4.14: Shilstone coarseness factor chart

4.3.3 Optimum Sand Method

Roads and Maritime NSW specifies a grading envelope for concrete paving that is finer than higher-slump structural concretes but has a tight requirement for clean aggregates in order to provide workability for slipforming at low slumps.

The Vebe test is used as an additional safeguard to ensure well-shaped aggregates. In the Vebe test a cone of concrete, moulded using a slump cone at the nominated slump, is placed on a vibrating table and the time to collapse the cone when the vibrating table is activated measured. A maximum period of three seconds is usually specified for slipform paving with a maximum of 2.5 seconds desirable for CRCP slipform mixes.

The grading curve specified is centred on Curve 3 with tolerances between Curves 2 and 4 as shown in Figure 4.15 for 20 mm aggregate (DMR NSW 1976).

From the target strength, the water/cement (w/c) ratio is chosen from charts and, after selection of the grading curve for the maximum aggregate size, the aggregate/cement (a/c) ratio is selected from tables.

Five trial mixes are then prepared at a/c ratios just above and just below that chosen, with the fine aggregate/total aggregate proportions changed slightly for each trial (refer to Section 4.4). The compacting factor is plotted against the fine aggregate percentage for each mix and an optimum sand line established by connecting the maximum compacting factor for each of the two mixes. Using the actual a/c ratio chosen, the fine aggregate percentage is then established.

The compacting factor is a measure of workability, which is more sensitive than the slump test for low slumps. It involves dropping concrete from one cone to the next and then into a cylindrical mould, with the compacting factor being the ratio of the density in the cylindrical mould to the density at full compaction.

20

25

30

35

40

45

3035404550556065707580

Wor

kabi

lity

fact

or (%

)

Coarseness Factor (%)

I

II

III

IV

V



Figure 4.15: Combined aggregate grading curve

Source: DMR NSW (1976).

4.3.4 American Concrete Institute (ACI) Method

The ACI mix design method, updated by Committee 211 in 2002, is the most common method used in North America. It involves nine steps to establish a trial batch. Mix design software is available at http://concrete.union.edu/design.htm.

The method is based on the relationship between the w/c ratio and target strength that provides the cement content. Coarse aggregate content is established before the remaining volume is used to establish the quantity of fine aggregate. Separate tables are used for air-entrained and non-air-entrained mixes. The nine steps are:

1. Assemble required material information (sieve analyses, unit weights, specific gravities, water absorption).

2. Select slump from table.

3. Select maximum aggregate size.

4. Select quantity of mix water for slump and maximum aggregate size from tables for air-entrained and non-air-entrained mixes.

5. Select w/c ratio from target strength table.

6. Calculate cement content from w/c ratio.

7. Estimate coarse aggregate content. ACI provides tables of the percentage (by unit volume) of coarse aggregate based on maximum aggregate size and fine aggregate fineness modulus (add % retained on each sieve /100).

8. Calculate fine aggregate volume as the remaining volume.

9. Adjust for moisture content in aggregates.



4.3.5 Texas DOT Method

The Texas Department of Transportation (2006) concrete mix design method is based on work by Shilstone (2007).

The procedure uses three criteria as follows:

• Coarseness factor

• 0.45 power Fuller curve

• percent retained.

Coarseness factor

Coarseness and Workability Factors are calculated as per the Shilstone method (Section 4.3.2).

Both the Coarseness Factor and Workability Factor must plot within the workability box in Figure 4.16. This is box is defined by:

• The Coarseness Factor must not be greater than 68 or less than 52.

• The Workability Factor must not be greater than 38 or less than 34 when the Coarseness Factor is 52.

• The Workability Factor must not be greater than 36 or less than 32 when the Coarseness Factor is 68.

Figure 4.16: Coarseness factor chart

Values that fall within Zone II (represented by the larger box in Figure 4.16) are stated by Shilstone to generally perform well. Caution is needed towards the left-hand boundary (too coarse-segregation) and the upper boundary (high fines – cracking, spalling and scaling).



0.45 power fuller curve

The maximum density line is taken as a straight line calculated using the Equation 3.

% passing =100 �𝑑𝑑𝐷𝐷�0.45

3

where

𝑑𝑑 = the sieve size being considered

𝐷𝐷 = the nominal maximum sieve size

The nominal maximum sieve size is one sieve larger than the first sieve to have ≤ 90% passing (usually 19 mm for concrete paving specifications).

The tolerance lines are straight lines drawn on either side of the maximum density line. They are drawn from the origin of the chart to 100% of the next sieve size smaller and larger than the maximum density sieve size (26.5 mm and 13.2 mm for concrete paving specifications).

The cumulative per cent passing should generally follow the maximum density line and should not deviate beyond the maximum and minimum tolerance lines. However, there will be a ‘hump’, possibly beyond the tolerance line above the maximum density line around the 1.18 mm sieve. There will always be a dip below the maximum density line around the 0.600 mm sieve. These deviations are typical and should not be a cause for rejection of a gradation unless results from trial batches indicate workability problems.

In creating a straight line for the maximum density grading, the usual log scale for sieve sizes needs to be changed to a 0.45 power base as shown in Figure 4.17. The Roads and Maritime concrete paving specification envelope is also plotted on this chart.

It can be seen from Figure 4.17 that the Roads and Maritime mix design specified for concrete paving can be coarser than the lower control limit.

Figure 4.17: 0.45 power chart


0

10

20

30

40

50

60

70

80

90

100

Sieve Size (mm)

% p

assi

ng 0.45 powerLower RTA Upper RTA

0 0.075 0.600 1.18 2.36 4.75 9.5 13.2 19.0 26.5 0.150



Per cent retained

A percentage retained chart is obtained by plotting the per cent retained (Y-axis) against the sieve size (X-axis). The sum of the per cent retained on any adjacent sieves, excluding the first and last sieve that retains material, must not be less than 13%. This is a check against gap grading and a check that normal grading limits in concrete paving specification are met.

4.3.6 NZ Flow Cone

Natural sands improve the workability of concrete and increased use of manufactured sands, which are crushed from hard rock, has resulted in the need to gauge fine aggregate workability.

The flow cone allows the assessment of blends of natural and manufactured sand at the material sourcing stage of mix design. Manufactured sands fill the grading gap between the 2.36 and 4.75 mm sieves. This occurs with most coarse aggregate and natural sand mixes, but they require processing to meet minus 75 micron combined aggregate grading limit.

Flow cones test the time for passage of a fixed volume or mass of the fine aggregate under consideration through a funnel or cone with a 12.7 mm diameter orifice into a receiving can. A resulting plot of flow time through the cone against uncompacted voids content in the receiving can gives a measure of angularity and texture of the fine aggregate.

The flow cone test developed under the US Strategic Highway Research Program program to test angularity in fine aggregate for asphalt (AASHTO 2008) removes the effect of particle size distribution by having fixed masses of each sieve size but uses a small total sample of 190 g.

For concrete paving blends, the flow cone shown in Figure 4.18, and detailed in standard NZS 3111 – Method 19, has been adapted to use a 1 kg sample in Cement Concrete and Aggregates Australia (CCAA) (2007) and Roads and Maritime research. The 1 kg sample is similar to the volume specified in the NZ Standard for specific gravities between 2.5 to 2.8. A flow time of less than 27 seconds, without tapping, indicates a workable blend.

Skid resistance and durability of surface mortar in concrete pavement surfacing has relied on the quartz content in natural sand. Where manufactured sands predominate, similar criteria as are used for asphalt for polishing, such as minimum source rock polished aggregate friction value (PAFV), and durability, such as the methylene blue test, are required.



Figure 4.18: Flow cone apparatus

Source: Clelland (1968).

4.4 Trial Mix Process

Most agency specifications require the contractor, in conjunction with the concrete supplier, to conduct one or more trial concrete mixes. The trial mix process should be used by the contractor to evaluate both the fresh and hardened properties of the proposed subbase and base concrete mixes. The contractor should also focus on compactability, workability, slipformability and bleed potential to provide the best paving conditions.

Over the last decade, Roads and Maritime introduced higher strength limits for the trial concrete mix compared to the hardened properties likely to be achieved on site under more adverse working and climatic conditions than a laboratory environment. Table 4.1 lists the compressive and flexural strength requirements for the trial mix and on site (works) test specimens. Finally, it is important that the contractor certifies that each nominated mix proposed, and its constituents for the construction of the concrete pavement layers, meet the requirements of the specification. If changes with the supply of one aggregate or fly ash source occur during construction, then it is imperative that trial mixes be repeated to verify that the specified requirements will be achieved with the revised concrete mix.

Table 4.1: Minimum concrete strengths specified in Roads and Maritime specification R83

Description Compressive strength Flexural strength

Non-SCM mixes(1) Trial mix 45.0 MPa 5.0 MPa

Works 40.0 MPa 4.8 MPa

SCM mixes Trial mix 40.0 MPa 4.8 MPa

Works 35.0 MPa 4.5 MPa

1 SCM refers to supplementary cementitious materials.

Source: Roads and Maritime (2013).



5. Steel Reinforcing Materials

5.1 Introduction

This section describes the requirements for reinforcing steel, tiebars and dowels for use in concrete pavements.

5.2 Reinforcement

5.2.1 Role of Reinforcement

In concrete pavements with slab lengths greater than those suited to plain concrete pavements (PCP), the design assumption is that joints alone will be insufficient to control transverse cracking and that one or more cracks may occur, depending on the slab length. The role of reinforcement in jointed reinforced concrete pavements (JRCP) is not to prevent such cracks but to restrain them from opening. Transverse cracks in JRCP can therefore be expected to occur and do not constitute a failure. This type of cracking is not load-related (it will frequently occur prior to trafficking) but comes as a result of combined contraction and curling/warping stresses and is therefore initiated at the top of the pavement base. Steel reinforcement, in the form of welded wire fabric (mesh), is located typically in the upper one-third of the base. In this location, the reinforcement does not contribute to the flexural strength of the base.

Current practice is for slab lengths to be limited to a preferred maximum of 8 metres for JCRP. This provides a balance between joint performance and the economics of longer slabs. An important criterion is that, in order to permit movement of transverse contraction joints, the reinforcement is not continued through joints but stopped short from them.

In continuously reinforced concrete pavements (CRCP), transverse contraction joints are eliminated altogether. As in JRCP, the role of the reinforcement is not to prevent cracking but rather to control transverse cracking. The design of reinforcement in CRCP is more complex than in JRCP. Longitudinal reinforcement is designed to induce transverse cracks in the range 0.5 to 2.5 metres centres, with individual crack widths not exceeding about 0.5 mm when measured at the surface. As with JRCP, cracks initiate at the top of the base and the longitudinal reinforcement is located in the upper one-third of the base. It does not contribute to the flexural strength of the base. Reinforcement is in the form of deformed bars.

The role of transverse reinforcement in CRCP is to support the longitudinal reinforcement and to prevent any unplanned longitudinal cracks opening.

The typical cross-sectional area of longitudinal steel in CRCP approximates that of dowels at transverse contraction joints in JRCP.

For either JRCP or CRCP, there would be no structural or economic advantage in providing a layer of reinforcement in the lower half of the base, unlike bridge approach slabs where two layers of reinforcement are used as the slab is design to be nominally suspended between the abutment and the formation.

Guidance on the amount of steel required is provided in Austroads (2017), whilst guidance on the detailing of reinforcement is provided in Roads and Maritime Pavement Standard Drawings (2016).

5.2.2 Steel Types

The principal types of steel reinforcement are deformed bars and/or welded wire fabric (hard-drawn steel wires welded together to form a mesh) and/or steel fibre reinforcement. Note that deformed bars have been rolled to give the surface a pattern of ribs or deformations. If there are no surface ribs, the bar is called a ‘plain bar’. Steel fibres are discussed in Section 5.3.



The standard grades of reinforcing steels according to their strength grade and relative ductility class are listed in Table 5.1 from AS/NZS 4671.

Table 5.1: Standard grades of reinforcing steels

Grade Minimum yield stress (MPa) Ductility class

250N 250 Normal

300E 300 Seismic

500L 500 Low

500N 500 Normal

500E 500 Seismic

5.2.3 Material Requirements

The required properties of reinforcement, including minimum yield strength, modulus of elasticity, stress-strain relationship and coefficient of thermal expansion are addressed in AS 3600. These are summarised below.

Strength

The strength of steel reinforcement for use in concrete pavements is characterised by its yield strength. It is important that the in-service or ‘working’ stress in the steel does not exceed the yield strength; otherwise permanent extension of the reinforcement could occur. The performance of the reinforcing steel nominated in a design is dependent on its yield strength, which is usually specified as a minimum value.

Ductility classes N and L typically apply to reinforcing bars and mesh respectively in Australia. The minimum yield strength of reinforcing bars and wire mesh fabric is 500 MPa.

Modulus of elasticity

The modulus of elasticity of steel may be assumed to be 2 x 105 MPa.

Coefficient of thermal expansion

The coefficient of thermal expansion may be assumed to be 12 x 10-6/ºC.

Areas of steel

Guidance on procedures to calculate the required amount of steel reinforcement is provided in Austroads (2017). To assist in the selection of appropriate mesh, cross-sectional areas of various steel meshes are listed in Table 5.2. Further requirements for mesh and bar cross-sectional areas can be found in AS/NZS 4671.

5.2.4 Surface Coatings and Conditions

Coatings applied to reinforcement must not reduce the performance of the reinforcement assumed in the design; the expected bond between the reinforcement and the concrete must be fully developed.

Galvanising of steel reinforcement is not recommended. The high alkali content of pore water in concrete will, in time, dissolve the zinc coating to form zinc hydroxide, which is an electrolyte. Where conditions permit corrosion (movement of chlorides in water in the presence of oxygen), the corrosion process is facilitated by the presence of zinc hydroxide.



Table 5.2: Cross-sectional areas of plain hard-drawn steel mesh sizes

Type Reference number

Pitch (mm) Area (mm2/m)

Longitudinal bars Cross-bars Longitudinal

bars Cross-bars

Rectangular mesh

RL1218 100 200 1112 227

RL1018 100 200 709 227

RL818 100 200 454 227

Square mesh

SL102 200 200 354 354

SL92 200 200 290 290

SL82 200 200 227 227

SL72 200 200 179 179

SL62 200 200 141 141

Source: AS/NZ 4671, Table 6A.

The surface of steel reinforcement should be free from any material that may impair its bond with the concrete. Examples of some contaminants are loose rust, grease, tar, paint, oil, mud, millscale and mortar. Where superficial rusting has progressed to produce loose rust, brushing with a wire-brush or pressure water cleaning or jetting or sufficient vibration may be sufficient to remove loose material. It is not intended that reinforcement should be brought to a smooth, polished condition.

5.3 Steel Fibres

Irregular or odd-shaped slabs, such as in roundabouts, are not suited to PCP. The cutting, placing and fitting of mesh reinforcement in roundabout slabs which have odd shapes can lead to wastage of mesh. Furthermore, bar and/or mesh reinforcement is unlikely to be effective in controlling the types of cracking (such as corner cracking) that are likely to occur in such odd-shaped slabs. By contrast, the addition of steel fibres is suited to such slabs and provides resistance to the growth of micro-cracking (if it occurs) and have proven very effective in containing corner cracking.

Steel fibres are typically 15 to 50 mm long. Some have enlarged ends to enhance anchorage within the concrete mortar during the initial setting/hardening/ drying shrinkage phase (see Figure 5.1). A typical amount of steel fibre is 75 kg/m3 of concrete.

Figure 5.1: Steel fibre shapes permitted in agency specifications

Steel fibres must be uniformly distributed throughout the concrete during batching, mixing, transport, placing and paving. To be effective, they must also develop adequate pull-out strength. This results in a very cohesive mix and may be difficult to place by slipforming.

A high pull-out strength (with a cement content typically 20 to 25% higher than normal base concrete is required) allows workability for both fixed form and slipform paving at lower concrete slump than normal base concrete.

The sequence of adding materials, including the fibres, into the mixing plant is important to produce uniform distribution and avoid segregation of the fibres.



5.4 Tiebars

Tiebars, in the form of deformed reinforcing bars, are used across longitudinal joints in multiple-lane paving and in longitudinal/transverse construction joints. The aim is to prevent the joint from opening but still allow some rotation arising from curling/warping movements.

Tiebars are typically 12 mm in diameter and about 1 metre long and are grade 500N bars. They are located at mid-base depth and need to be carefully aligned normal to the joint (see Figure 5.2). The design process is similar to the design of reinforcement in JRCP, but in a transverse orientation. Tiebars must be kept away from locations where they could inhibit the movement of contraction or isolation joints.

Figure 5.2: The location of tiebars must be placed at the centre of the slab

5.5 Dowel Bars

Dowels are used to provide a capacity to transfer loads across joints where the load transfer via aggregate interlock is inadequate. Dowels are used in PCP and JRCP. In providing the load transfer they must also allow free joint movement, both opening and closing, without restraint or ‘locking’ of the joint. For this reason, they are smooth bars.

For the jointed base thicknesses required for heavy-duty pavements, i.e. approximately 250 mm, typical dowel details are:

• 28 to 32 mm diameter smooth steel bars without burred ends

• 450 mm length centred on the joint at 300 mm centres

• effectively debonded from concrete over more than half their length.

In continuous paving they are assembled in support cages. The features of these cages are:

• the dowels must be held in their required arrangement with very accurate alignment with respect to each other, the pavement surface and the road centreline

• no steel (apart from the dowels) must cross the line of the joint

• one half of the dowel is coated with a debonding coating that will not be removed during paving, and one end of the dowel must be fixed to the cage

• the cage should be fixed to the supporting surface in a way that it will not be disturbed from its position during paving operations.

LCS

TiebarSawcut & sealant

Base

dep

thD

PCPPCP



In addition to transverse weakened plane contraction joints (sawn after paving), dowels are also used in locations such as:

• a construction joint that will also act as a contraction joint

• transverse contraction joints within 45 metres of the end of a pavement that will not have terminal anchors (because joint opening will progressively increase with time).

The term ‘dowel’ is frequently misused as a global term and can be inaccurately used to refer to a tiebar. It is important to understand the different roles of dowels and tiebars and their detailing.



6. Curing and Debonding Treatments

6.1 Introduction

Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. Curing methods can be divided into three broad categories as follows:

• the provision of surplus water to prevent or counteract evaporation

• the use of impermeable media to minimise the loss of pore water

• the application of artificial heat whilst the concrete is maintained in a moist condition.

Curing with artificial heat, also known as ‘accelerated curing’, is used mainly in the manufacture of precast concrete products, and there are practical limitations to its general use for paving work. Low-pressure steam curing accelerates strength development in the first 24 hours, and the hydrate products are essentially the same as those produced by normal curing. Creep and shrinkage are generally lower than for concrete cured at ambient temperature. However, ultimate strength may be lower because of the change in structure of the hydrate products caused by the rapid initial hydration.

Specialised field applications may exist for the use of this method as an alternative to the use of high early strength concretes. Since curing with artificial heat is not normally used for new concrete pavements, no further treatment is discussed here.

Water curing is still considered to be the most effective method available in terms of the quality of concrete achieved. However, for full hydration, the surface must be kept continually moist and there are therefore practical limitations to the use of this method for large surface areas such as in high-output paving work. In addition, wet curing must be applied to the sides of slipformed concrete to minimise moisture loss.

Several materials are commonly used to reduce the rate of evaporation from the surface; these include sand, hessian and burlap. Impermeable sheets can also be used, either singly or in conjunction with these materials. To be effective the sheets must be fully sealed against moisture loss, and this can be a difficult task with large areas of pavement and if the site is windy.

When using sheets, interim spraying is desirable until the surface has hardened sufficiently to support the sheets since even in moderately drying conditions up to 5% of moisture can be lost in three hours. If wet sand, hessian or burlap are subsequently used, this moisture may be regained, but if curing consists only of impermeable sheets then this moisture obviously will not be recovered.

Unless circumstances dictate otherwise, the quality of curing water should be potable, particularly where concrete appearance is important.

Because of the various difficulties just discussed, curing of pavements is now almost universally achieved by the use of sprayed impermeable membranes. This method is therefore treated here in detail.

A balance is required in the degree of bonding between the concrete base and subbase. Too low a friction level will encourage non-uniform openings at transverse contraction joints. However, where a lean-mix concrete subbase is provided, too high a friction level can result in reflection cracking from the subbase. Accordingly, a debonding treatment is required for the top of concrete pavement subbases in conjunction with, or additional to, curing treatments.



6.2 Curing Materials

Liquid membrane curing compounds have found wide acceptance in paving work because of their relative simplicity and economy. As long as they are applied correctly they require no further follow-up work and hence, in a contract situation, no further supervision. Another advantage is that, being applied in a light spray, they can be used much earlier than alternative methods, and therefore interim curing is not required and evaporation losses are kept to a minimum.

These materials act by forming a scuff-resistant coating that effectively limits the evaporation of moisture from the surface. They most commonly have a base of wax or hydrocarbon resin. They are commonly manufactured with either a pigment or a fugitive dye that provides a means of visually checking that the whole surface has been thoroughly covered. They are required to maintain their effectiveness for not less than 14 days after application.

Pigmented compounds should not be used on wearing course concrete. While titanium dioxide pigment possesses excellent durability, its use on a wearing surface is likely to lead to an unsightly appearance adjacent to the wheel-tracks.

Table 6.1 lists the curing treatments commonly used in Australia for various base and subbase types. Note that a bitumen sprayed seal is generally preferred for rolled concrete subbases. This heavier application curing treatment seals depressions and smooths out the coarser, more open-textured surface of rolled concrete. Liquid curing compounds must meet the requirements of AS 3799 and relevant road agency requirements or qualifications.

Table 6.1: Curing and debonding treatments

Subbase type Base type Recommended treatments

Lean-mix concrete curing Debonding treatment

Lean-mix concrete PCP & CRCP Wax emulsion Bitumen sprayed seal with 7 mm aggregate

JRCP (i) Wax emulsion, or (ii) Hydrocarbon resin

Bitumen sprayed seal with 7 mm aggregate, or bitumen emulsion

SFCP Wax emulsion Wax emulsion

RLC & CSCR(1) All Bitumen sprayed seal with 7 mm aggregate

1 RLC = rolled lean concrete; CSCR = cement stabilised crushed rock.

Note: Where subsequent application of a thin course (or courses) of asphalt wearing course is to be applied over any concrete bases, a bitumen emulsion or bitumen emulsion hydrocarbon blend curing compound is recommended for its affinity with subsequent bituminous treatments.

6.3 Debonding Treatments

Table 6.1 shows that for most concrete pavements, wax emulsion is provided to debond the base and subbase, whilst a sprayed seal is subsequently placed over the wax to protect it from damage during construction.

Under SFCP, the recommended treatment is to cure the subbase with wax emulsion but not to apply protective treatments such as bituminous seals.



7. Joint Sealants and Fillers

7.1 Role of Joints

Jointing systems in concrete road pavements are designed to ensure both the structural integrity and ride quality of the pavement. They are provided in Cement and Concrete Association of Australia (1982a):

• control transverse and longitudinal cracking due to restrained contraction as a result of concrete shrinkage, and subsequent changes in ambient temperature or moisture

• control the combined effects of restrained warping due to temperature or moisture gradients through the concrete and the action of traffic loads acting together

• divide the pavement into suitable lengths and widths for construction purposes

• control the movements of slabs at bridges and structures.

Concrete pavement joints fall within two general categories: transverse and longitudinal. They can further be divided into contraction, construction, isolation and expansion joints. A slab is a portion of concrete base bounded by joints and/or edges. The size of the slab will have an influence on the width of the joint and subsequently the type of sealant used.

7.2 Role of Joint Sealants

Joints are designed to include a reservoir for a joint sealant. The reservoir is typically formed by sawing a groove in the base surface. The primary role of a joint sealant is to prevent the ingress of fine incompressible material. The intent is to maintain the very small space that will result from drying shrinkage and cooling so that the pavement can expand in hot weather and avoid significant compression forces at the joint. A secondary role is to minimise water entry through the base to the subbase.

A sealant should have the following material properties:

• recover its installed dimensions after many thousands of cyclic joint openings and closings without internal rupture

• be able to bond to the faces of the sealant groove

• have long-term resistance to hardening embrittlement after many years including both hot and cold weather, and UV radiation

• a high level of water resistance

• a similar colour to concrete be easy to install and free from substances harmful to installers.

A sealant typically fails due to poor adhesion of the concrete or by cohesion as shown in Figure 7.1.



Figure 7.1: A sealant either fails from adhesion (left) or cohesion (right)

7.3 Sealing Contraction and Construction Joints

It is common practice to seal both contraction joints that are designed to open/close and construction joints that are not designed to open/close.

The type of sealant that has given good performance under operating conditions since the mid-1980s is a silicone formulated for highway usage. A schematic view of a silicone sealed joint is shown in Figure 7.2.

The groove is formed by sawing either following paving, in the case of transverse contraction joints, or weakened plane longitudinal construction joints in multiple-lane paving, or by sawing along the line of a construction joint after the second pour has been completed.

The depth of sawcuts in transverse and longitudinal joints using conventional saws are typically one-quarter and one-third the base depth respectively. The sealant groove is typically about 25 to 30 mm deep. The widths of the groove in contraction and longitudinal joints are typically 6 mm and 3 mm respectively.

Key features of the groove and sealant installation are as follows:

• Some time may elapse between sawing and sealant installation to allow the concrete surface to become dry. A thin-splined rubber strip with a free width slightly larger than the sawcut is installed at the bottom of the sawcut after washing slurry from the sawn groove to temporarily prevent ingress of solid material.

• A backer rod, typically a closed cell polyethylene strip, is installed at the bottom of the sealant groove. This is to stop the sealant bonding to the concrete on three sides, which would inhibit uniform compression/expansion of the sealant and to form the lower part of a ‘waisted’ shape in the sealant when installed.

• To permit bonding of the silicone to the side groove faces, those faces must be thoroughly cleaned and dry at the time of installation. This is typically done by dry compressed air using a narrow hose that can fit into the groove. The joints must not be cleaned using grit blasting. Care must be taken to clean the groove if early entry saws are used as the powdered residue and curing compound that may have entered the groove will reduce the capacity for the sealant to adhere to the concrete faces.



Figure 7.2: Typical details of field-moulded joint sealants

7.4 Sealing Isolation and Expansion Joints

Isolation joints are typically used at points of differential conflict in concrete pavements. Examples include:

• to isolate the legs of intersecting roads or pavements where the inherent longitudinal contraction/expansion directions are normal to each other and in conflict

• slabs containing pits in one corner: the intent is to isolate the pit from the slab to avoid differential stresses and movements

• bridge approach slabs or abutments, to avoid the effects of differential movement between the pavement and the bridge.

These joints are typically full depth through the base to provide effective isolation between the conflicting elements. They are typically formed joints and not sawn weakened plane joints. The joint width needs to be only sufficient to ensure full isolation. The typical width need not exceed 10 mm.

The sealant is detailed as shown in Figure 7.3. A preformed joint filler needs to be installed from the bottom of the joint to the underside of the backer rod or debonding strip. This can be a material such as closed cell foamed board, cork or bitumen-impregnated fibre board. The main criterion is that the free width of the filler is slighter more than the joint groove to provide some compression to ensure that the backer rod is fully supported across its width.

Figure 7.3: Typical details of full-depth isolation joint

Silicone sealantBacker rod

Temporary seal

Bottom of initial sawcut

Bottom of second sawcut



References

AASHTO 2008, T304 Standard method of test for uncompacted void content of fine aggregate, American Association of State Highway and Transportation Officials, Washington, USA.

American Society for Testing and Materials 2003, Standard guide for petrographic examination of aggregates for concrete, ASTM C295-03, ASTM, West Conshohocken, Philadelphia, USA.

ATIC 2014, Cementitious materials for concrete, specification SP-43, Australian Technical Infrastructure Committee, Sydney, NSW.

Austroads 2008, Guide to pavement technology part 4J: aggregates and source rock, AGPT04J-08, Austroads, Sydney, NSW.

Austroads 2009, Guide to pavement technology part 4L: stabilising binders, AGPT4L-09, Austroads, Sydney, NSW.

Austroads 2016, Specification of geopolymer concrete: general guide, AP-R531-16 Austroads, Sydney, NSW.

Austroads 2017, Guide to pavement technology part 2: pavement structural design, 4th edn, AGPT02-17, Austroads, Sydney, NSW.

Base, GD & Murray, MH 1982, ‘A new look at shrinkage cracking’, Civil Engineering Transactions, vol. CE 24, no. 2, pp.171-6.

Beeby, AW 1983, ‘Cracking, cover and corrosion of reinforcement’, Concrete International Design and Construction, vol. 5, no. 2, pp. 34-40.

Bruere, GM, 1963, ‘Importance of mixing sequence when using set-retarding agents with Portland cement’, Nature, vol. 199, no. 4888, pp. 32-3.

Cement and Concrete Association of Australia 1982a, Joints in concrete road pavements, Technical Note, TN47, Cement and Concrete Association of Australia, Sydney, NSW.

Cement and Concrete Association of Australia 1982b, Concrete for road pavements, Technical Note, TN44, Cement and Concrete Association of Australia, Sydney, NSW.

Cement and Concrete Association of Australia 1998, Hydraulic cements: properties and characteristics, Technical Note, TN59, Cement and Concrete Association of Australia, Sydney, NSW.

Cement Concrete & Aggregates Australia 2006a, Compaction of concrete, data sheet, Cement Concrete & Aggregates Australia, Sydney, NSW.

Cement Concrete & Aggregates Australia 2006b, Curing of concrete, data sheet, Cement Concrete & Aggregates Australia, Sydney, NSW.

Cement Concrete & Aggregates Australia 2007, Manufactured sand: national test methods and specification values, research report, Cement Concrete & Aggregates Australia, Sydney, NSW.

Clelland, J 1968, ‘Sand for concrete: a new test method’, New Zealand Standards Bulletin, no 14, October, pp. 22-6.

DMR NSW 1976, Procedure for design of concrete mixes by optimum sand method, MR form no 602, Department of Main Roads, Sydney, NSW.

Dodson, VH & Farkas, E 1964, ‘Delayed addition of set retarding admixtures to Portland cement concrete’, Proceedings American Society for Testing and Materials, vol. 64, pp. 816-26.

Franklin, RE 1988, The skidding resistance of concrete: performance of limestone aggregate experiment after 10 years, Research Report 144, Transport and Road Research Laboratory, Crowthorne, UK.

Heaton, BS 1966, ‘Strength, durability and shrinkage of incompletely compacted concrete’, Journal of the American Concrete Institute, Proceedings, vol. 65, no. 10, pp. 846-50.



Morgan, J 1974, ‘The effects of lignosulphonate-based admixtures on drying shrinkage of cement paste and concrete’, First Australian conference on engineering materials, Sydney, 26th to 28th August 1974, University of New South Wales, pp. 97-108.

NCPTC 2012, Concrete pavement mixture design and analysis (MDA): effect of aggregate systems on concrete mixture properties, technical report, National Concrete Pavement Technology Center, Iowa, USA.

Roads and Maritime Services 2013, Concrete pavement base, QA specification R83, 3rd edition, Roads and Maritime Services, Sydney, NSW.

Roads and Maritime Services 2014a, Moulding of concrete specimens for testing in compression, indirect tension and flexure, test method T304, Roads and Maritime Services, Sydney, NSW.

Roads and Maritime Services 2014b, Dressing of voids in concrete specimens and unit mass adjustment for embedded steel, test method T368, Roads and Maritime Services, Sydney, NSW.

Roads and Maritime Services 2016, Pavement standard drawings: rigid pavement: volume CP: plain concrete pavement, edn.4, rev.1, Roads and Maritime Services, Sydney, NSW.

RTA 1991, Concrete pavement manual: design and construction, RTA, Materials Services Branch, Roads and Traffic Authority, Sydney, NSW.

Shilstone, JM 2007, TxDOT combined aggregate grading: technology, applications, and results for paving and structures, presentation to the Texas section ASCE 2007, viewed 12 May 2017, <http://www.shilstone.com/library/Texas-ASCE-April-07.pdf>.

Shilstone, J & Shilstone, J 2002, ‘Performance-based concrete mixtures and specifications for today’, Concrete International, vol. 24, no. 2, pp. 80-3.

Texas Department of Transportation 2006, Optimized aggregate gradation for hydraulic cement concrete mix designs, test procedure Tex-470-A, Texas Department of Transportation, Austin, Texas, USA.

Wallace, GB & Ore, EL 1960, ‘Structural and lean mass concrete as affected by water-reducing, set-retarding agents’, Symposium on effect of water-reducing admixtures and set-retarding admixtures on properties of concrete, ASTM special technical publication 266, ASTM, West Conshohocken, Philadelphia, USA, pp. 38-94.

Weller, DE & Maynard, CP 1970, The evaluation of grain shapes in silica sands from a simple flow test, laboratory report LR334, Road Research Laboratory, Crowthorne, UK.

Australian/New Zealand Standards

AS 1012.3.1-2014, Methods of testing concrete: determination of properties related to the consistency of concrete: slump test.

AS 1012.3.3-1998 (R2014), Methods of testing concrete: determination of properties related to the consistency of concrete: Vebe test.

AS 1012.8.1-2014, Methods of testing concrete: method for making and curing concrete: compression and indirect tensile test specimens.

AS 1141.0-1999, Methods for sampling and testing aggregates: list of methods.

AS 1141.4-2000 (R2013), Methods for sampling and testing aggregates: bulk density of aggregate.

AS 1141.5-2000 (R2016), Methods for sampling and testing aggregates: particle density and water absorption of fine aggregate.

AS 1141.6.1-2000 (R2016), Methods for sampling and testing aggregates: particle density and water absorption of coarse aggregate: weighing-in-water method.

AS 1141.6.2-1996 (R2016), Methods for sampling and testing of aggregates: particle density and water absorption of coarse aggregate: Pycnometer method.

AS 1141.11.1-2009, Methods for sampling and testing aggregates: particle size distribution: sieving method.

AS 1141.14-2007, Methods for sampling and testing aggregates: particle shape, by proportional calliper.



AS 1141.15-1999, Methods for sampling and testing aggregates: flakiness index.

AS 1141.22-2008, Methods for sampling and testing aggregates: wet/dry strength variation.

AS 1141.24-2013, Methods for sampling and testing aggregates: aggregate soundness: evaluation by exposure to sodium sulphate solution.

AS 1141.25.1-2003 (R2013), Methods for sampling and testing aggregates: degradation factor: source rock.

AS 1141.25.2-2003 (R2013), Methods for sampling and testing aggregates: degradation factor: coarse aggregate.

AS 1141.25.3-2003 (R2013), Methods for sampling and testing aggregates: degradation factor: fine aggregate.

AS 1141.26-2008, Methods for sampling and testing aggregates: secondary minerals content in igneous rocks.

AS 1141.30.1-2009, Methods for sampling and testing of aggregates: coarse aggregate quality by visual comparison.

AS 1141.31-2015, Methods for sampling and testing aggregates: light particles.

AS 1141.32-2008, Methods for sampling and testing aggregates: weak particles (including clay lumps, soft and friable particles) in coarse aggregates.

AS 1141.33-2015, Methods for sampling and testing aggregates: clay and fine silt (settling method).

AS 1141.34-2007, Methods for sampling and testing aggregates: organic impurities other than sugar.

AS 1141.35-2007, Methods for sampling and testing aggregates: sugar.

AS 1141.37-2007, Methods for sampling and testing aggregates: iron unsoundness.

AS 1379-2007, Specification and supply of concrete.

AS 1478.1-2000, Chemical admixtures for concrete, mortar and grout: admixtures for concrete.

AS 1478.2-2005, Chemical admixtures for concrete, mortar and grout: methods of sampling and testing admixtures for concrete, mortar and grout.

AS 2758.1-2014, Aggregates and rock for engineering purposes: concrete aggregates.

AS 3582.2-2016, Supplementary cementitious materials: slag - ground granulated iron blast-furnace.

AS 3600-2009, Concrete structures.

AS 3799-1998, Liquid membrane-forming curing compounds for concrete.

AS 3972-2010, General purpose and blended cements.

AS/NZS 3582.1-2016, Supplementary cementitious materials: fly ash.

AS/NZS 4671-2001, Steel reinforcing materials.

NZS 3111-1986, Methods of test for water and aggregate for concrete.

SA HB 79-2015, Alkali aggregate reaction – guidelines on minimising the risk of damage to concrete structures in Australia.

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