GUIDE TO PAVEMENT TECHNOLOGY
Part 4A: Granular Base and Subbase Materials
Guide to Pavement Technology Part 4A: Granular Base and Subbase Materials
Guide to Pavement Technology Part 4A: Granular Base and Subbase Materials Summary This Guide contains advice on the selection, testing and specification of crushed rock and naturally occurring granular materials for use in pavement base and subbase construction. Keywords Specification, gravels, unbound granular material, aggregate, crushed rock, characterisation, material testing, laboratory testing methods, field performance, flexible pavements First Published September 2008 © Austroads Inc. 2008 This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without the prior written permission of Austroads. ISBN 978-1-921329-84-5 Austroads Project No. TP1159 Austroads Publication No. AGPT04A/08
Project Manager David Hubner Prepared by Binh Vuong, Geoff Jameson and Kieran Sharp ARRB Group, Barry Fielding VicRoads Published by Austroads Incorporated Level 9, Robell House 287 Elizabeth Street Sydney NSW 2000 Australia Phone: +61 2 9264 7088 Fax: +61 2 9264 1657 Email:
[email protected] www.austroads.com.au 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.
Guide to Pavement Technology Part 4A: Granular Base and Subbase Materials
Sydney 2008
Austroads profile Austroads’ purpose is to contribute to improved Australian and New Zealand transport outcomes by:
providing expert advice to SCOT and ATC on road and road transport issues
facilitating collaboration between road agencies
promoting harmonisation, consistency and uniformity in road and related operations
undertaking strategic research on behalf of road agencies and communicating outcomes
promoting improved and consistent practice by road agencies.
Austroads membership Austroads membership comprises the six state and two territory road transport and traffic authorities, the Commonwealth Department of Infrastructure, Transport, Regional Development and Local Government in Australia, the Australian Local Government Association, and New Zealand Transport Agency. It is governed by a council consisting of the chief executive officer (or an alternative senior executive officer) of each of its eleven member organisations:
Roads and Traffic Authority New South Wales Roads Corporation Victoria Department of Main Roads Queensland Main Roads Western Australia Department for Transport, Energy and Infrastructure South Australia Department of Infrastructure, Energy and Resources Tasmania Department of Planning and Infrastructure Northern Territory Department of Territory and Municipal Services Australian Capital Territory Department of Infrastructure, Transport, Regional Development and Local Government Australian Local Government Association New Zealand Transport Agency
The success of Austroads is derived from the collaboration of member organisations and others in the road industry. It aims to be the Australasian leader in providing high quality information, advice and fostering research in the road sector.
GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
CONTENTS 1
INTRODUCTION ............................................................................................................ 1
1.1 1.2
Scope.............................................................................................................................. 1 Background..................................................................................................................... 2 1.2.1 Improved Pavement Design Procedures .......................................................... 2 1.2.2 Improved Understanding of Durability .............................................................. 3 1.2.3 Increased Heavy Vehicles ................................................................................ 3 1.2.4 Improved Quarry Plant...................................................................................... 3 1.2.5 Improved Construction Plant ............................................................................ 3 1.2.6 Uniformity of Practice in Specification .............................................................. 3
2
ROLE AND FUNCTION OF GRANULAR MATERIALS IN A PAVEMENT................... 4
2.1 2.2 2.3 2.4
Function of a Road Pavement ........................................................................................ 4 Requirements of a Pavement Material ........................................................................... 5 Function of Granular Base and Subbase Materials ........................................................ 6 Operating Environment................................................................................................... 7
3
BEHAVIOUR OF GRANULAR PAVEMENT MATERIALS IN SERVICE ...................... 8
3.1 3.2
Introduction ..................................................................................................................... 8 Shear Strength................................................................................................................ 9 3.2.1 Particle Shape and Surface Texture............................................................... 10 3.2.2 Percentage Fines and Fines Plasticity............................................................ 10 3.2.3 Particle Size Distribution................................................................................. 10 3.2.4 Density and Moisture Content ........................................................................ 11 Modulus ........................................................................................................................ 11 3.3.1 Particle Size and Shape ................................................................................. 11 3.3.2 Particle Roughness and Shape ...................................................................... 11 3.3.3 Fines Content and Fines Plasticity ................................................................. 12 3.3.4 Particle Size Distribution................................................................................. 12 3.3.5 Density and Moisture Content ........................................................................ 13 Permanent Deformation................................................................................................ 13 3.4.1 Particle Shape ................................................................................................ 14 3.4.2 Particle Size Distribution................................................................................. 14 3.4.3 Fines Content ................................................................................................. 14 3.4.4 Density and Moisture Content ........................................................................ 14 Durability....................................................................................................................... 14 Permeability .................................................................................................................. 15 3.6.1 Particle Size Distribution................................................................................. 16 3.6.2 Fines Content and Fines Plasticity ................................................................. 16 3.6.3 Density and Moisture Content ........................................................................ 16 Compaction (Density and Moisture) ............................................................................. 16 3.7.1 Particle Size Distribution................................................................................. 17 3.7.2 Maximum Size ................................................................................................ 18 3.7.3 Particle Shape ................................................................................................ 18 3.7.4 Fines Content ................................................................................................. 18
3.3
3.4
3.5 3.6
3.7
4
PRODUCTION OF GRANULAR MATERIALS ............................................................ 19
4.1
Methods of Production.................................................................................................. 19 4.1.1 Crushing Plant ................................................................................................ 19 4.1.2 Additives ......................................................................................................... 24
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
4.2 4.3
Production of Naturally Occurring Granular Materials .................................................. 24 4.2.1 Methods of Production.................................................................................... 24 4.2.2 Assessment .................................................................................................... 26 Production of Crushed Rock......................................................................................... 28 4.3.1 Methods of Crushed Rock Production ............................................................ 28
5
CRUSHED ROCK PROPERTIES REQUIRING SPECIFICATION .............................. 31
5.1 5.2 5.3 5.4
General ......................................................................................................................... 31 Specification Types....................................................................................................... 31 Source Rock ................................................................................................................. 32 Product Requirements .................................................................................................. 32 5.4.1 Introduction ..................................................................................................... 32 5.4.2 Maximum Size ................................................................................................ 33 5.4.3 Particle Size Distribution................................................................................. 33 5.4.4 Particle Shape ................................................................................................ 35 5.4.5 Nature of Fines ............................................................................................... 36 5.4.6 Example Test Limits for Crushed Rock .......................................................... 36
6
PROPERTIES OF NATURALLY OCCURRING GRANULAR MATERIALS REQUIRING SPECIFICATION..................................................................................... 37
6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8
Introduction ................................................................................................................... 37 Particle Size Distribution............................................................................................... 37 Clay and Silt Content .................................................................................................... 37 Particle Shape and Texture .......................................................................................... 38 Maximum Particle Size ................................................................................................. 38 Particle Strength and Durability .................................................................................... 38 Compacted Density and Moisture Content ................................................................... 38 Example Test Limits for Natural Gravels ...................................................................... 39
7
TESTS FOR QUALITY................................................................................................. 40
7.1 7.2 7.3
General ......................................................................................................................... 40 Source Rock Tests for Crushed Rock .......................................................................... 43 Product Tests................................................................................................................ 43 7.3.1 Particle Size Distribution................................................................................. 43 7.3.2 Particle Shape ................................................................................................ 44 7.3.3 Particle Density and Absorption...................................................................... 44 7.3.4 Consistency Limits .......................................................................................... 44 7.3.5 Soil Fines ........................................................................................................ 45 7.3.6 Contaminants.................................................................................................. 46 7.3.7 Unsound Stone Content ................................................................................. 47 7.3.8 California Bearing Ratio.................................................................................. 47 7.3.9 Repeated Load Triaxial Test........................................................................... 49 7.3.10 Permeability .................................................................................................... 50
APPENDIX A
UNIFIED SOIL CLASSIFICATION SYSTEM ................................... 53
APPENDIX B
EXAMPLE TEST LIMITS FOR CRUSHED ROCK........................... 55
APPENDIX C
EXAMPLE TEST LIMITS FOR NATURAL GRAVELS .................... 61
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
TABLES Table 2.1: Table 3.1: Table 7.1: Table 7.2:
Pavement material requirements (Lay 1981).................................................... 5 Relationships between granular material properties and requirements of unbound layers ................................................................................................. 9 Australian Standard Test Methods for Product Assessment .......................... 41 New Zealand Standard Test Methods for Product Assessment ..................... 42
FIGURES Figure 2.1: Figure 2.2: Figure 2.3: Figure 3.1: Figure 3.2: Figure 3.3: Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Figure 4.6: Figure 4.7: Figure 4.8:
Components of a road pavement ..................................................................... 4 Rutting of granular base induced by trafficking................................................. 6 Stress distribution within a granular pavement ................................................. 7 Shape of aggregate particles (VicRoads 1998, Technical Bulletin 39) ........... 10 Effects of material grading on modulus (Andrews, 1996) .............................. 12 Typical crushed rock particle size distributions............................................... 18 Jaw crusher .................................................................................................... 20 Gyratory crusher ............................................................................................. 20 Cone crusher .................................................................................................. 21 Impact crusher ................................................................................................ 22 Vertical shaft impact crusher (courtesy Boral ACM) ....................................... 22 Some natural granular pavement materials .................................................... 24 Plant for mixing and breaking down oversize materials.................................. 25 Production of crushed rock ............................................................................. 28
Figure 5.1: Figure 5.2: Figure 5.3: Figure 7.1: Figure 7.2: Figure 7.3: Figure 7.4:
Compacted crushed rock with n ≈ 0.45 .......................................................... 34 Particle size distribution and workability (Ingles and Metcalf 1972)................ 35 Compacted crushed rock with deficiencies of some sizes.............................. 35 Assessment of unsound stone content using reference samples................... 47 Laboratory measurement of California Bearing Ratio (CBR).......................... 48 In situ measurement of California Bearing Ratio ............................................ 49 Repeated load triaxial test equipment ............................................................ 50
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
1
INTRODUCTION
1.1
Scope
Part 4A of the Guide to Pavement Technology presents Australasian practice in the selection and testing of unbound granular materials for base and subbase pavement construction. This includes the following generic material types:
Naturally occurring granular materials (natural gravels/sand-clay/soft and fissile rock), which do not require costly extraction or crushing processes. They are an important source of material used in the pavement (base and subbase) and shoulder construction of flexible pavements in Australia.
Crushed rock, which is produced by the crushing and screening of hard source rock (igneous, metamorphic or sedimentary rock), which would typically need to be excavated by the use of explosives, and river gravels. It is used in the pavement (base and subbase) and shoulder construction of flexible pavements.
Recycled materials are discussed in Part 4E: Recycled Materials of the Guide to Pavement Technology. Part 4A of the Guide to Pavement Technology supersedes two of the five parts of the NAASRA publication series ‘Pavement Materials’ which was published during the 1980s: Part 2 – Natural Gravel, Sand-Clay and Soft and Fissile Rock Part 3 – Crushed Rock. Relevant sections of Part 4 of the NAASRA publication series Pavement Materials – Aggregates will be incorporated into Part 4J: Aggregate and Source Rock of the Guide to Pavement Technology. Relevant sections of Part 5 of the NAASRA publication series – Quality Description and Assurance – will be incorporated into Part 8: Pavement Construction Assurance of the Guide to Pavement Technology. This Part introduces the basic properties of quality assessment and discusses the significance of variability and sampling risks to the specifications and assessment of quality. The Guide addresses the factors which lead to the appropriate selection and specification of unbound granular materials by reference to:
the role and function of granular materials in a pavement, including factors such as the intrinsic or manufactured properties and their relationship to in-service behaviour and performance
the physical properties affecting material requirements, including the properties that affect structural adequacy, serviceability, durability, volume stability, permeability, compaction, and workability
the production of naturally occurring granular materials, crushed rock and recycled materials
the different methods of specification, quality management, attaining required performance characteristics, and quality control and assurance (refer Part 8: Pavement Construction of the Guide).
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
This Guide should be read in conjunction with the other parts of the Pavement Technology series: 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 Aggregate 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. Further details on all available Austroads documents can be found at www.austroads.com.au
1.2
Background
Since NAASRA published Pavement Materials – Part 3 (Crushed Rock) in 1976, a much better understanding of the performance and characterisation of granular materials has been developed. Factors that have influenced these developments include the following. 1.2.1
Improved Pavement Design Procedures
The introduction of mechanistic pavement design procedures in 1987 has resulted in an improved understanding of how unbound pavements perform under load which led to improvements in the tools for the structural design of new pavements (Part 2 of the Guide to Pavement Technology). Recent research into the performance of unbound materials has also assisted in the development of performance based material characteristics based upon dynamic load laboratory testing and the development of standardised equipment and test procedures for the characterisation of these materials.
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1.2.2
Improved Understanding of Durability
As a result of a number of significant pavement failures due to the use of non-durable source rock for the production of crushed rock base, national and international research has resulted in the development of new or modified test procedures for the characterisation and specification of source rock and crushed rock product. As a result, and without underestimating the value of testing the final product, a better understanding of the part played by the inherent mineralogy of the source rock in the long-term durability of the manufactured product has been obtained. 1.2.3
Increased Heavy Vehicles
The gross vehicle mass, axle loads, tyre pressures and number of heavy vehicles have been steadily increasing. High pavement loading together with diminishing reserves of economically available naturally occurring granular materials of appropriate quality, combined with the environmental damage associated with the winning of natural gravels, have resulted in the need for increased use of crushed rock products in pavement construction, particularly in the rural areas of Australia. In some instances however, local deposits of quarry material may be suitable for road base application and may be more economical and cost effective both financially and environmentally. 1.2.4
Improved Quarry Plant
Improvements in the design and operation of quarry plant have led to better consistency in production of crushed rocks. This is commonly achieved by blending a number of crushed components and, in some cases, a fine additive or filler to provide a material that has the desired characteristics of strength, workability, cohesion and permeability. 1.2.5
Improved Construction Plant
There has been steady and significant improvement in the plant available for the placement, spreading and compaction of pavement materials. This, together with tight quality control testing, has enabled materials to be placed with much higher compaction levels, greater uniformity in density and to finer tolerances in finished level. 1.2.6
Uniformity of Practice in Specification
One of the strategic goals of Austroads is to work toward national uniformity of practice with respect to the specification of road construction materials and to encourage the use of appropriate National Standards. A number of Australian Standards have recently been published which provide a basis for the preparation of a works specification for aggregates and rock for engineering purposes (Australian Standard AS 2758 series). Test Methods for the sampling and testing of aggregates have also been developed (Australian Standard AS 1141 series) which are referenced in AS2758. These Standards are being progressively adopted by road agencies and industry, supplemented, where appropriate, by modification or by methods developed to address specific local requirements. The test methods are discussed in more detail in Section 7.
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2
ROLE AND FUNCTION OF GRANULAR MATERIALS IN A PAVEMENT
2.1
Function of a Road Pavement
The basic function of a road pavement is to support the traffic loading with acceptable ride quality and without undue deterioration over the period for which it is designed. To do this, the pavement must attenuate the traffic-induced stresses in all pavement layers and the subgrade sufficiently to prevent significant pavement distress. This is normally achieved by a structure consisting of several layers of differing quality material, with the highest quality materials in the upper portion of the pavement where load induced stresses are higher, and lesser quality materials in lower layers where stresses have reduced. The terms used to describe the various components of a road pavement are shown in Figure 2.1. Further details are provided in Part 1: Introduction to Pavement Technology of the Guide to Pavement Technology. L C Impervious surfacing
Roadbase Shoulder surface Shoulder material Drainage layer Subbase
Subgrade
Figure 2.1:
Components of a road pavement
The wearing surface is the top layer which covers all structural elements of a pavement. The base consists of one or more layers of material on which the surfacing is placed. It may be composed of fine crushed rock, natural gravel, broken stone, stabilised material, asphalt or Portland cement concrete. The subbase is laid on the subgrade below the base either for the purpose of making up additional pavement thickness, or to provide a working platform. The purpose of the shoulder is to provide:
lateral support for the pavement layers
an impermeable barrier to protect the base and subbase against the lateral infiltration of moisture
a trafficable surface for non-motorised traffic or occasional traffic (errant or stopping vehicles) and/or maintenance vehicles.
General pavement configurations that incorporate granular pavement layers include:
full depth granular with sprayed seal surfacing
full depth granular with thin asphalt surfacing
bound or unbound granular subbase with asphalt base and surfacing
thin asphalt or sprayed seal surfacing with unbound granular base and bound subbase.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
This Guide addresses issues associated with unbound granular materials used in the base and subbase of a pavement. Bound materials are addressed in Part 4B: Asphalt and Part 4D: Stabilised Materials of the Guide to Pavement Technology whilst concrete is addressed in Part 4C: Materials for Concrete Pavements of the Guide to Pavement Technology. Requirements for source rock to produce granular materials are described in Part 4J: Aggregate and Source Rock. The required thickness of granular material to support the design traffic loading can be determined either empirically or mechanistically using the procedures detailed in Part 2: Pavement Structural Design of the Guide to Pavement Technology.
2.2
Requirements of a Pavement Material
The requirements of a pavement material are generally as follows:
sufficient strength to withstand the applied traffic and environmental stresses
sufficient hardness to withstand applied loads without inducing particle breakdown
ability to be placed and compacted to meet specification requirements
durable and not degrade or disintegrate significantly over the life of the pavement
quality that is fit-for-purpose.
The basic properties that satisfy these requirements and their operative range are listed in Table 2.1 and each will be further discussed in this Guide. Figure 2.2 shows an example of traffic induced rutting of a granular base layer. Table 2.1: Pavement material requirements (Lay 1981) Property workability
Definition the ability to be placed, compacted and formed to the required condition and shape
Range construction
economy
the material must be available and workable at an acceptable cost
strength/stiffness
the ability to resist loads without unacceptable deformation or induce tensile fatigue in surfacings
in-service
hardness
the ability to withstand load without fracture and particle breakdown
in-service
durability
the ability to maintain its characteristics with time
in-service
volume stability
the ability to resist significant changes in volume as conditions, such as moisture content, change
in-service
wear resistance
the ability to resist erosion, abrasion and polishing
surface* course
in-service
surface finish
the ability to accept and maintain a bituminous surfacing
surface* course
in-service
impermeability
the ability to resist moisture penetration and resultant loss of load bearing capacity and stiffness
surface* course
in-service
There are some other cases where impermeability is needed, e.g. basecourse layers.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
Figure 2.2:
2.3
Rutting of granular base induced by trafficking
Function of Granular Base and Subbase Materials
The functions of a granular base layer in a pavement are to:
Provide sufficient stiffness to reduce stresses in the subbase and subgrade so that the pavement surface does not deform excessively.
For unsurfaced pavements, provide a layer which has high durability (wear resistance) under tyre/surface contact stresses.
Provide sufficient stiffness to support a bituminous surface without its cracking due to tensile fatigue or the sealing aggregate penetration under heavy wheel loads.
Provide a layer which will not excessively deform under repeated loading.
In most locations, provide a layer with sufficiently low permeability to inhibit the ingress of water into the underlying subbase and subgrade layers. However in some circumstance base material with high permeability may be considered. Environments where water entry into the base is inevitable may be more suited to using a permeable material. In such case the permeable layer will allow for more rapid drainage. In addition, locations where freeze/thaw is expected may be more suited to highly permeable material which allows for expansion of the water on freezing without causing damage. Regardless of which approach is used, careful consideration of sub-surface drainage design and permeability of underlying and adjacent materials is necessary.
The functions of a granular subbase layer in a pavement are as follows:
provide sufficient stiffness to distribute traffic loads transmitted through the pavement base, reducing their intensity to a level which will not cause excessive permanent deformation of the subgrade
provide a working platform on which base materials can be transported, placed and compacted to the required standards
depending on the pavement design requirements, drain the base and/or protect the subgrade from moisture infiltration, e.g. the lower subbase may be relatively impermeable whilst the upper subbase may be more permeable provided it is constructed in conjunction with appropriate sub-surface drainage.
The stress distribution within a granular pavement is illustrated in Figure 2.3. In all cases, the selection of the most appropriate granular material can have a profound affect on the structural and functional performance of the pavement.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
Figure 2.3:
2.4
Stress distribution within a granular pavement
Operating Environment
The operating environment of the pavement includes loading, moisture and temperature. The traffic loading environment, in terms of the spectrum of vehicle classifications and volumes, will normally influence the choice of materials used. For example, heavy traffic situations may preclude the use of an unbound granular pavement layer. The moisture environment under which a granular pavement material must operate will have a major impact on its performance. All granular materials lose strength, to a greater or lesser extent, with increasing moisture content and this needs to be taken into account in their selection. In particularly cold environments, temperature may also be a consideration. Where freeze/thaw occurs, damage to the pavement structure may arise through cycles of expansion and contraction (freeze/thaw) of water within the void space. Details regarding the selection of the most appropriate granular material are presented in Part 4: Pavement Materials of the Guide to Pavement Technology.
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3
BEHAVIOUR OF GRANULAR PAVEMENT MATERIALS IN SERVICE
3.1
Introduction
This section of the Guide discusses some of the factors which influence the performance of granular base and subbase materials in service, and the reasons for the selection of tests which are indicative of the ability of a material to perform satisfactorily. The physical properties of bound materials which affect material requirements are discussed in Part 4B: Asphalt, Part 4C: Materials for Concrete Pavements, Part 4D: Stabilised Materials, of the Guide to Pavement Technology. The behaviour of granular materials in service is governed by many factors which are related to the following:
the intrinsic properties of coarse particles, including hardness, surface friction and contamination, and the geological origin and history of the source rock from which the material is derived
manufactured aggregate properties such as particle shape, size and surface texture, particle size distribution, fractured faces, nature and quantity of fine particles, and fillers – these factors are related to processes used during manufacture to produce the final product
compacted layer properties such as density, moisture content and particle orientation, which are in turn related to the construction and compaction processes
boundary conditions such as in situ moisture and temperature regimes, and the stresses applied at the boundaries of the constructed pavement – these are external influences that will influence both short and long term behaviour.
These factors can be highly variable because of their random nature within space and over time. For practical reasons, the physical properties (intrinsic, manufactured and compacted properties) are often described by simple index tests and test result limits so that:
material attributes associated with structural adequacy (such as stiffness, shear strength, and permanent deformation), serviceability adequacy (such as micro texture), durability (hardness and wear resistance), volume change, permeability and workability can be quickly assessed
material requirements can be readily amendable to acceptance testing at the time of work.
Standards of compliance assessment based on these physical properties are often applied to material manufacture and construction in a quality control environment, which essentially includes a set of activities performed by a supplier. Table 3.1 summarises the significant physical properties of granular materials for pavement layers that can influence the material requirements. The potential suitability of a granular material for an unbound pavement layer is inferred from the physical properties of both the source rock and the end-product. However, requirements differ depending on the application. Basic properties currently adopted in specifications for the production and supply of granular materials for pavement construction and quality assessment are further discussed in Sections 5 and 6.
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Table 3.1: Relationships between granular material properties and requirements of unbound layers Physical property
Requirement of material physical property to produce the specified layer property requirement Structural adequacy requirements
Long term performance requirements
Construction requirements
High strength
High stiffness
Low Permanent Deformation
High durability or volume stability
Low permeability
Good compaction
Good workability
particle hardness and crushing strength
–
–
–
tough
–
tough
tough
particle surface texture
rough
–
rough
–
–
smooth
rough
particle shape
angular
rounded to angular
angular
–
–
rounded to angular
rounded to angular
particle size distribution
well-graded
well-graded
well graded
well-graded
-
well-graded
well-graded
particle size
–
large nominal size
–
-
large d10 size
–
small nominal size (10% retained on the 9.5 mm sieve. Thus the shape of material < 9.5 mm should be considered by adopting other means, possibly Average Least Dimension. 3.2.2
Percentage Fines and Fines Plasticity
The percentage fines passing the 0.425 mm sieve (this is the fraction from which Consistency Limits are determined (refer Section 7.3. 4)) and fines-plasticity have a marked affect on shear strength. Generally, the presence of too many fines prevents interlock between larger particles whilst too few fines reduces the compacted density. The presence of highly plastic fines results in a loss of shear strength of compacted material because of the reduction in friction between interlocking particles. For crushed rock (20 mm maximum size), the materials are most stable below a critical fines content between 8% and 12% depending on the material type. The effect of fines plasticity on shear strength at fines contents below the optimum level (say below 8%), is relatively small. However, for higher fines contents, high fines plasticity has a greater influence. In some instances, plasticity is controlled or reduced by blending. Notwithstanding the above, materials with a wide range of plasticities have been successfully used in the past. The success of one material over another is a case of the environmental conditions, especially moisture, material and pavement design. 3.2.3
Particle Size Distribution
The particle size distribution or ‘grading’ of a granular material that, for a given compactive effort, achieves the highest density, is often specified i.e. ‘maximum density principle’, viz Fuller’s principle (refer Section 3.7). At high density, mechanical interlock (and hence shear strength) is at its strongest and permeability is at its lowest, thereby reducing moisture sensitivity to shear strength.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
3.2.4
Density and Moisture Content
The effects of compacted layer properties, i.e. density, moisture content and particle orientation on shear strength, are very significant. Generally, material strength will increase as a result of increased density and reduced moisture content. At a higher density (or a lower voids content), more energy is required to overcome frictional resistance between particles and to densify the granular material against the confining stress. The presence of a small amount of water can slightly reduce the inter-particle friction, but also introduces an apparent cohesion between particles by capillary attraction. However, a high degree of saturation may produce high pore pressure (or low effective stress) and, consequently, low shear strength.
3.3
Modulus
Modulus is the elastic response of a material to imposed loading at loads below that which would cause shear failure. It is calculated from the measured strain in material under an applied load. It can be determined either under static or dynamic loading conditions. Static loading tests include triaxial shear and Texas triaxial tests and field plate bearing tests. In these tests the static modulus (Young’s modulus) is determined. Dynamic loading tests are limited to the repeated load triaxial test. In this test the resilient modulus is determined as the ratio of dynamic (resilient) stress/dynamic (resilient) strain. The repeated load triaxial test (refer Figure 7.4) has been accepted as the standard laboratory method for determining the resilient modulus of unbound granular materials. It is important to realise that the moduli derived from the static and dynamic loading tests may differ. Laboratory studies have indicated that the overall modulus of granular materials will be partly the result of the deformation of individual particles and partly the result of relative sliding and rolling between particles (dilation). Generally, individual particles are very rigid, with Young’s moduli exceeding 104 to 105 MPa. However, it is the inter-particle movement and resultant rearrangement that causes granular material to have an overall modulus of well below 1,000 MPa and a non-linear stress-strain behaviour accompanied by permanent deformation. No study has quantified all the effects of the intrinsic and manufactured crushed rock properties, and compacted layer properties, on the modulus of granular materials and further work is required. Based on limited studies of the modulus of crushed aggregates in Australia and overseas, the following significant properties that may affect the modulus of granular materials have been identified. Aggregate interlock that produces high shear strength should also produce high to moderate stiffness (modulus) if it has been placed and compacted adequately to a required in situ density. 3.3.1
Particle Size and Shape
The effect of these two intrinsic properties on modulus is similar to those described for shear strength. 3.3.2
Particle Roughness and Shape
The effects of particle roughness (as defined in AS1726) and shape on stiffness are not quite clear. Rough particle surfaces provide higher inter-particle friction which, when combined with angular and sub-angular aggregate shapes, provides high aggregate interlock resulting in higher stiffness. Conversely, smooth rounded particles have low inter-particle friction and poor interlocking properties, resulting in lower stiffness.
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3.3.3
Fines Content and Fines Plasticity
As plasticity and fines content influence soil suction, it is reasonable to assume that stiffness is also affected by these factors. This is particularly so when suction is the primary stress that binds particles together (e.g. at unconfined or low confining stress conditions and/or in a dry condition). In this case, higher fines content and higher cohesive fines will result in higher suction and, hence, higher stiffness. However, when suction is very small compared to confining stresses (e.g. as in the saturated condition), higher fines content and higher cohesive fines may result in lower stiffness due to the effects of particle size and lubrication as discussed above. It has been reported in a number of studies that, for a fines-content between 2-10%, the influence of finescontent on the stiffness was not well defined and can be dependent on aggregate type. 3.3.4
Particle Size Distribution
For a given confining area and for similar effective grain size, there is no marked difference in stiffness between uniform-graded and well-graded materials. It has been shown in a number of studies that the particle size distribution, or grading, of granular materials seems to have some influence on granular modulus, although it is generally considered to be of minor significance. For crushed limestone with angular shape, a uniform-graded material was only slightly stiffer than a well-graded material. For slag, however, the results were the opposite and the denser grading tended to give a higher modulus. When a crushed rock has less fines (finer than 0.425 mm) it is generally defined as ‘boney’ and the load transfer is through point-to-point contact of aggregate fractions, resulting in high modulus and low moisture sensitivity. In contrast, a material with high fines content relies more on the strength of the fine matrix material surrounding the aggregate fractions to achieve modulus. Where aggregate particles are flat or flaky, mechanical interlock is reduced.
800
MEAN
700
COARSE GAP
600
FINE
500 400 300 200 100
t ic
e al
ni
C
gn
ei
cr et
ss
ne to lt s Si
ol
om
iti
c
li m
es
G
to
ra
ne
/s
Q
il t
ua
st
rt
zi
on
e
te
0
D
Resilient Modulus (MPa)
The relationship between the resilient modulus of five source products with four different gradings is shown in Figure 3.2 (Andrews, 1996).
Figure 3.2:
Effects of material grading on modulus (Andrews, 1996)
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3.3.5
Density and Moisture Content
The effects of compacted layer properties (i.e. density, moisture content and particle orientation or degree of anisotropy) on granular modulus are very significant. For low plasticity crushed rocks, moduli increase significantly with density up to 100% of Modified Maximum Dry Density (MDD) (Vuong, 1992). However, at higher densities, there is little change in modulus, particularly at low moisture contents. At high degrees of saturation (say above 80%), the combination of a high degree of saturation, poor drainage and low permeability could produce high pore pressure (or low effective stress) and, consequently, low modulus. It should be noted that these effects may vary according to material type and further studies on the effects of manufactured layer properties for different materials are required, particularly at different stress levels. In the design of a flexible pavement it is imperative that the layers have adequate modulus to spread (reduce) the applied stresses and strains to the subgrade without unacceptable permanent surface deformation. Part 2: Pavement Structural Design of the Guide to Pavement Technology includes a relationship to estimate the allowable loading in terms of total permanent deformation of unbound granular materials and subgrade.
3.4
Permanent Deformation
Permanent deformation is the irrecoverable deformation of a compacted granular material upon unloading of applied stresses. Laboratory testing methods which are used to study the permanent deformation of pavement materials include the triaxial test, hollow cylinder test and the simple shear test. These tests involve the application of different stress conditions which simulate actual stress conditions likely to occur in a pavement layer under a rolling wheel load. The Repeated Load Triaxial (RLT) test has been accepted as the standard laboratory method for determining the permanent deformation of granular materials. Permanent deformation of the granular materials under applied loads results in rutting of the pavement surface. This may be accompanied by an increase or reduction in base modulus, depending on whether the material becomes denser and stronger, or more unstable and weaker. The former may lead to a more stable pavement surface condition; whereas the latter can lead to shear failure in the base layer or increased rutting in the subgrade with increasing loading cycles as discussed previously. Permanent deformation results from densification, local shear deformation and rearrangement of particles. Densification is the process of volumetric decrease through reduction of pore spaces. Shear deformation and dilation is the process of volumetric expansion through shear failure and rearrangement of particles. As in the case of stiffness, the permanent deformation of granular materials can be affected by many factors related to manufactured crushed rock and layer properties, stress level, uniformity of construction and loading history. As axle loads, load repetitions and tyre pressures increase, the potential for excessive rutting in the granular base of unbound pavements with thin bituminous surfacings becomes a major concern. As the thickness of the granular layer increases base rutting becomes the primary deterioration mode. There is little information available on the effects of intrinsic and manufactured crushed rock properties and compacted layer properties on permanent deformation. Because of the difficulties in conducting long term laboratory permanent strain testing, only limited studies on permanent deformation have been conducted. Some significant properties which may affect the permanent deformation of compacted granular materials, are as follows.
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3.4.1
Particle Shape
When different materials are compacted to the same density, angular materials such as crushed rock produce lower deformation compared to gravel with rounded and elongated and/or flat shaped particles. 3.4.2
Particle Size Distribution
The effects of particle size distribution or grading on permanent deformation depend on the level of compaction. When uncompacted, specimens with uniform grading have the least permanent strain. The permanent strain induced in heavily compacted specimens is similar for all gradings. For a given compactive effort, materials having a grading producing the highest density will exhibit the lowest permanent deformation. 3.4.3
Fines Content
Permanent deformation increases when the fines content is above a critical fines-content (about 812% depending on material type). The effects of fines content on permanent deformation are insignificant in the elastic zone (i.e. stress is much lower than shear strength), but it become more significant near the failure zone, indicating that excess fines prevent interlock between the larger particles. 3.4.4
Density and Moisture Content
The effects of manufactured layer properties (i.e. density, moisture content and particle orientation or degree of anisotropy) on permanent deformation can be very significant. As in the case of resilient modulus, resistance to permanent deformation in these materials under repetitive loading appears to be greatly improved as a result of increased density. However, at densities above an optimum dry density (say 100% Modified MDD), there is little change in permanent deformation with increasing density. At low moisture contents (or low degrees of saturation), the rate of permanent deformation is relatively small and governed by the lubricating effect of water in a granular assembly. At a high degree of saturation, the rate of deformation increases and is governed by the combination of a high degree of saturation and low permeability, which induces high pore pressure (or low effective stress) and, consequently, low deformation resistance.
3.5
Durability
Durability is the abrasion and weathering resistance of a material. It is related to changes in the performance of a material under repeated loading and long-term weathering. Durability is often measured by physical test procedures (e.g. wet/dry strength variation, Los Angeles value and degradation factor tests) of the final crushed or blended products. The purpose of specifying durability limits is to ensure that materials will not significantly break down, resulting in a change to the particle size and shape, and increases in the fines-content and fines-plasticity during construction or during the life of the pavement. As discussed previously, these factors strongly affect the engineering properties of unbound materials (shear strength, stiffness and permanent deformation) and, hence their long term performance. Durability requirements comprise:
Abrasion and crushing resistance: Aggregates must be able to withstand abrasion and crushing under traffic. Specifications may take into account traffic loading in determining an appropriate test limit.
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Soundness and durability: The durability of rock depends upon its ability to resist weathering agents. Physical and chemical changes in rocks, produced at or near the surface by atmospheric agents, result in disintegration and decomposition and are commonly grouped under the general name of weathering. The action of physical agents is called disintegration and results in the rock breaking into smaller particles without destroying its identity. The process by which mineral particles are changed into new compounds of less desirable characteristics is known as decomposition. Disintegration and decomposition usually occur together but one process is generally dominant. The incidence of decomposition is higher in humid and warm areas, while disintegration is more likely in regions of large temperature range. The rock-forming minerals can be classified as either primary or secondary. The alteration or reconstitution of primary minerals produces secondary minerals, which can be considerably varied by a range of geological conditions including deuteric alteration, hydro-thermal alteration, low grade metamorphism, action of groundwater and weathering. The greater the proportion of clay or clay-like secondary minerals, the more the internal bond between the minerals of the rock is weakened. Major studies of the effects of secondary minerals on the performance of crushed rock products used as pavement materials have been carried out overseas and in Australia (Minty 1960, Nyoeger 1964, Scott 1955, Weinert 1960). Igneous and metamorphic rocks derive their hardness and strength from the tough constituent minerals and the strong interlock between multitudes of small, angular crystals. Even a small amount of decomposition affecting only the margins of the crystals can seriously weaken some of these rocks. Some rocks, even though strong and tough when freshly quarried, degrade rapidly after exposure to air and water. Microscopic examination generally reveals that these rocks are deeply weathered. Another feature that is often associated with degradation of weathered rock is increasing permeability.
Generally, durability requirements for aggregates obtained from uniform rock sources can be inferred from mineralogy tests that describe the geological origin and history of the source rock. However, uniform rock sources may produce a range of products, which have different durability properties depending on the manufactory processes used to control durability. Part 4J of the Guide to Pavement Technology discusses source rocks in detail.
3.6
Permeability
Permeability is a measure of the amount of water which flows through a mass of soil for a given pressure. Standard methods used in the laboratory to determine the permeability of granular materials include the use of falling-head and constant-head permeameters (refer Figure 7.5). These standard permeability tests usually consider permeability in a saturated condition. However, trapped air has a significant effect on unsaturated permeability in an in-service condition. Permeability is specified by some authorities to ensure that the permeability gradients required in the pavement structure are met and to ensure that base layer material directly under a surface seal is of low permeability. As discussed previously, the engineering properties of unbound materials (e.g. modulus, shear strength, permanent deformation) are very dependent upon moisture content, and the permeability may influence the moisture regime in which a material operates. Other factors that can affect the moisture condition of a pavement material include sources of water entry, sub-surface drainage design, type of surface seal, pavement shape and the presence of impermeable layers in the pavement structure.
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It is not uncommon in Australasia for thin surface seals and unsealed shoulders to be quite permeable. Past experience indicates that, for this pavement type, it is preferable for the granular base layer to have lower permeability than the lower subbase or subgrade capping layers. This promotes the drainage of moisture away from the higher stressed areas near the surface. It is for this reason that, for base material under a surface seal or a thin asphalt layer, a minimum Plasticity Index (PI) value of 2 is specified by some road authorities. This results in the presence of cohesive fines within the base material and hence, provided there is an adequate fines content as discussed below, generally a less permeable material. Various drainage design and stage construction options have been applied for different environmental conditions to reduce the risks of early pavement failure due to water penetrating into the pavement layers. In these cases, the function of the unbound granular layer will determine the suitable range of permeability for the layer concerned. For example, if the material is to be used in a drainage layer, higher permeability is required, viz. open-graded. However, this may contradict the requirements for higher strength or modulus. It is well known that the permeability of a granular material is most influenced by the size, shape and connectivity of the water passages of the material and its degree of compaction. Some significant intrinsic and manufactured properties that may affect the permeability of compacted granular materials are briefly described below. 3.6.1
Particle Size Distribution
Permeability varies as the square of the effective particle size. For a given maximum grain size, a uniform-graded (or single size) material will produce higher effective grain size, and higher permeability, than a well-graded (or broader range of grain size) material. 3.6.2
Fines Content and Fines Plasticity
Permeability is sensitive to the fine components of a well-graded material. As a first approximation, permeability is inversely proportional to the square of the D10 particle size (i.e. size such that 10%, by mass, of the sample consists of particles having a smaller size). An increase in plasticity will similarly reduce permeability. More specifically, a well-graded material with less than 5% fines (i.e. passing the 0.075 mm sieve) will be relatively permeable. As the percentage of fines increases, the permeability decreases until 20% fines, after which no further effects occur. However, as discussed previously, the use of more than 10% fines creates other plasticity-related performance problems. 3.6.3
Density and Moisture Content
Permeability also varies as the cube of the void ratio (volume of voids/volume of solid). Therefore, permeability can also be reduced by heavy compaction to reduce voids. The degree of saturation also has an important influence on permeability, i.e. the higher the degree of saturation, the higher the permeability.
3.7
Compaction (Density and Moisture)
Compaction is the process by which the air void ratio of the granular material is reduced. There is an optimum moisture content (OMC) at which the maximum dry density (MDD) will be achieved for a particular compactive effort. It is usual to specify the density required for a material as a percentage of that achieved in a laboratory compaction test. The Standard Compaction test produces MDD and OMC values equivalent to field values produced with a medium mass-vibrating roller, and hence some road agencies restrict its use to subgrade. The Modified (heavy) Compaction test produces densities equivalent to those achieved with the use of heavy rollers, and hence some road agencies consider it more appropriate for granular bases and subbases than the
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Standard Compaction test. Note that may be difficult to achieve modified compaction of finegrained base and subbase material which are commonly used on low volume roads in arid areas. Compaction is achieved in the field through application of a static or vibratory roller of sufficient mass and energy for the particular material. As discussed previously, it is clear that the compacted layer properties (density, moisture content and degree of anisotropy) have a very high impact on material performance. A high level of compaction will result in a material having high strength, high modulus and low deformation under imposed traffic loading. However, caution must be exercised not to over-compact some materials as high levels of compaction can induce high particle breakdown which can lead to a reduction in strength and an increase in moisture sensitivity. A high compactive effort is required if the required dense packed structure is to be achieved. Generally, the resistance of a material to compaction depends on internal friction, cohesion, and permeability. As both internal friction and cohesion increase with density, the necessary compactive effort increases as density increases until no further compaction is possible. Permeability is also a factor as air and water can be trapped within the granular mass, and this can prevent the achievement of a higher density with additional rolling. A material exhibiting the required properties with respect to strength, modulus, resistance to deformation and permeability can be difficult to compact and a compromise is sometimes needed between satisfactory materials properties and compactability. For low-permeability, high-plasticity unbound materials, from which water is not displaced during construction, it is critical that the moisture content at placement is as close as possible to the OMC value if the MDD value is to be achieved. For highly permeable and non-plastic materials, from which water can be readily displaced during construction, the moisture content at placement is not so critical. In both cases, a dry-back period is required to reduce the base moisture content to an acceptable level before sealing. Different specifications of moisture condition at sealing are adopted for different material types to maximise the performance of both the seal and the base. Other intrinsic and manufactured properties that need to be controlled in a compaction process include the following. 3.7.1
Particle Size Distribution
The performance of an unbound granular layer is enhanced by low permeability and high loadbearing characteristics. The particle size distribution (grading) achieves these desirable attributes through the provision of maximum density (minimum voids) and mechanical interlock. A grading with the exponent n = 0.5 (refer Equation 3-1) is historically known as Fuller’s maximum density curve. However, for most crushed rocks the value of n varies between 0.3 and 0.45. P = (d/D)n x 100 where P = d
=
percentage passing sieve size d nominal sieve size (mm)
D =
nominal maximum particle size (mm)
n
the exponent (n = 0.5 for maximum density)
=
(3-1)
The above relationship is a useful approximation to the maximum density (i.e. minimum porosity) grading for a large range of materials. However, others have advocated that an exponent as low as 0.4 be used to produce a maximum density because of the shape of the particles. Figure 3.3 shows two typical crushed rock particle size distributions, the size 20 mm material has an exponent n = 0.43 and the size 40 mm material has an exponent n = 0.46.
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Figure 3.3:
3.7.2
Typical crushed rock particle size distributions
Maximum Size
It is usual to limit the maximum particle size so that the material can be laid by machine and a smooth finish, suitable for traffic or for sealing, achieved. For high quality crushed rock base materials, a maximum nominal size of 20 mm is considered suitable to provide for ease of compaction, minimisation of particle segregation and a smooth surface finish. However, larger maximum size (e.g. 30 mm and 40 mm) natural materials can be handled satisfactorily. 3.7.3
Particle Shape
The particle shape of material will affect compaction as particles tend to pack more efficiently when better shaped. Long, thin particles may fracture during placement and compaction and also affect the workability of the material, making it difficult to achieve satisfactory compacted density. Some micaceous fines components may meet normal grading requirements but, because of their platelike and flake-like shape and elastic properties, interfere with the compaction process. 3.7.4
Fines Content
The proportion of fines in a pavement base is a major factor affecting stability. Granular materials with little or no fines (and equally those with excess fines) may compact poorly, and be difficult to handle during construction.
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4
PRODUCTION OF GRANULAR MATERIALS
This section contains advice on the production and supply of granular materials from various material sources for pavement construction including the factors that lead to the appropriate selection and specification of natural granular materials, crushed rocks and recycled materials.
4.1
Methods of Production
The common methods used to produce a granular material to meet specified requirements are:
crushing and screening by means of a crushing plant
the use of additives to improve their characteristics.
4.1.1
Crushing Plant
A crushing plant controls particle size distribution. Different crusher types and crusher settings are used depending on the characteristics of the parent rock. The particle size distribution of the endproduct is mainly controlled by screens. Screen variables that can be controlled include aperture size and shape, screen angle, vibrating speed and direction. Once the system is set up and is stable, the plant generally only requires resetting daily. Crushing Massive hard rock deposits must be broken by blasting or ripping to reduce the rock to a size that can be fed to the crusher. Any new crushing operation requires consideration of the size of rock likely to be won initially by the blasting or ripping, as secondary blasting, if required, is costly. The range and distribution of the particle sizes and shape of aggregate are largely determined by the relationship between the rock type and the crusher types and settings. Therefore, it is necessary to select a crusher suitable for the particular application. The South Australian Department of Mines and Energy, Handbook on Quarrying and relevant publications of the Institute of Quarrying are considered appropriate references for those wishing to specify the types and number of crushers to be used to produce a nominated aggregate from a particular source rock. In terms of crushing, there are six principal types of rock crushers used in the manufacture of road construction materials viz:
jaw crushers
gyratory crushers
cone crushers
impact crushers
hammer mill crushers
vertical shaft Impact crushers.
All crushing relies on either compressing rock particles between two metal surfaces or by the high speed impact on or by rock particles against hard surfaces. Rock crushing can also be achieved by rock on rock contact to improve shape. Rock on steel contact promotes fracture whereas, rock on rock contact tends to promotes shape. Depending on the type of material being crushed, there are some characteristics, like particle shape and Atterberg Limits, which can be influenced by the inherent nature of the rock. The selection of the appropriate crusher type etc can modify these characteristics to some extent in the final product. Quarry plants are designed and established to provide efficient and cost-effective extraction, processing and sale from a given deposit and are usually built as a one off.
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Jaw Crusher The Jaw crusher (see Figure 4.1), which is the basic style of crusher has 2 hardened metal plates with a tapering gap between. One metal plate is fixed (fixed jaw) and the other (swing jaw) oscillates causing the taper to alternately open and close. In simple terms, the feed particles fall into the taper to the point where the open jaw separation matches their size; as the taper then closes, the particle is compressed and fractures. The broken particles then drop further down the taper either to be caught again or eventually fall through the gap at the bottom of the taper. The eventual maximum size of the material is controlled by the gap. Some particle on particle crushing occurs in the process; this is more likely to occur if the crushing chamber is kept full.
Figure 4.1:
Jaw crusher
Gyratory Crusher The gyratory crusher (see Figure 4.2) uses an eccentrically mounted tapered spindle rotating within an inverted static cone; the rotary oscillation of the spindle causes a progressive rotary closure of the gap between the cone and the spindle. The profile between the crushing surfaces is similar to that of the jaw crusher and the crushing process is likewise similar.
Figure 4.2:
Gyratory crusher
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Cone Crusher Cone crushers (see Figure 4.3) operate in a somewhat similar fashion to the gyratory crusher but with a significant difference in the shape of the crushing surfaces (cone and mantle) and the crushing chamber. The longer chamber shape and flatter lying orientation causes a higher degree of stone on stone contact which results in the production of finer particles by grinding actions rather than breakage by direct particle compression. This type of compression crusher is considered as more suited for the production of more material in the fine particle range as well as more equant shaped particles. A variety of cone and mantle profiles are available to suit the properties of various rock types and perhaps modify their inherent crushing characteristics according to product requirements.
Figure 4.3:
Cone crusher
Impact Crushers Impact crushers rely on the high speed impact of rock particles against a hardened metal surface. This can be either as a hammer (or bar) striking the rock particle or the particle having been accelerated striking a static anvil. According to the strength and structure of the particles, the impact causes fracture or (partial) pulverization of the particle. Pulverization tends to cause the rounding of particles with finer sized material being the result of the breakdown. Impact crushers are particularly susceptible to abrasive material and can suffer high wear rates.
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Figure 4.4:
Impact crusher
Hammer Mill Crushers. The hammer mill has a series of hammers or bars attached to a rapidly rotating horizontal shaft. Particles fed into the crusher are struck by the hammers and, consequently accelerated by the impact, will strike a static anvil. The impacts can cause breakage or pulverization of the particles; particle size control can be adjusted by controlling the size of a discharge aperture. Vertical Shaft Impact (VSI) Crushers. The VSI crusher has a rapidly rotating vertically mounted rotor into which rock material is fed; the rotation accelerates the particles horizontally through discharge ports in the rotor to impact against an anvil surface. With high speed rotation, the rock discharge from one port will strike rock that has been discharged from a previous port causing a high level of rock on rock impact. A modified version of the VSI can have feed rock cascading through the impact zone and will achieve a similar end result.
Figure 4.5:
Vertical shaft impact crusher (courtesy Boral ACM)
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Improving particle shape There are a number of ways that particle shape can be improved by the operation of the crushing plant. 1.
Choke fed crusher: If the crusher is kept full then, due to the interlocking of particles, all dimensions of the rock have the opportunity to be stressed and broken, thereby improving the shape. Choke feeding of crushers is important to provide consistent sized feed product to the next crusher in the plant. Commonly the Jaw is used as the Primary (1st) crusher, Gyratory the Secondary (2nd), Cones as Tertiary (3rd) and VSI the Quaternary (4th) stages. However, plants vary in design.
2.
Mixed feed: The crusher should be fed with different sizes mixed together; for similar reasons as choke feeding, the shape is improved.
3.
Multiple stages: The more crushing stages that are used, the better the shape of the stone because reduction ratios (size of stone in, to size of stone out) can be kept lower.
4.
Orientation of stone: If possible, the stone should be fed into the crusher in such a way that the particles are not orientated on their smallest dimension. Rather than feeding directly off a belt, it is better that a delivery chute is used which will mix the orientation of the stone when it is introduced into the crushing chamber.
Screening The purposes of screening are to:
grade the product, from crusher or source, into the required ranges and distribution of sizes
remove deleterious material from feed to a primary crusher (primary scalping)
separate material between steps in the crushing phase, in order to: — — —
return material for re-crushing, if over-size, in closed-circuit crushing discard, short-circuit or stockpile small sizes feed the next crusher-stage.
The types of equipment commonly used for screening are as follows. Grizzlies Grizzlies are normally made from bars or rails set longitudinally without cross-bars. They are used to remove material that is too large for any particular crushing stage or to eliminate contaminating overburden and fines from run-of-quarry stone. Screens Screens are used to separate particles into sizes between specified limits or for scalping fines to prevent them unnecessarily passing through a crusher. Rotary screens or trommels are normally on an inclined axis, and consist of one or more rotating cylinders with apertures of various sizes. Material is fed in at the upper end where the smallest apertures are positioned. Trommels are used for 50 mm and larger sizes. Vibrating or oscillating screens handle small-size material. When arranged in banks, they produce a number of sizes. Over-loading is a common cause of faulty product grading. Washing of the product is sometimes specified to remove deleterious materials, e.g. fines and salts; it is usually done as part of the screening process. There are a number of factors which affect production gradings, such as the correct orientation of screens, material bed depth, screen angle, correct aperture etc as they all have a bearing on the final product.
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4.1.2
Additives
Depending on the characteristics of the source rock, fines generated through normal crushing processes may contain excessive amounts of deleterious material which require removal through a scalping process, or may lack the required cohesive properties, or particular size fractions. These deficiencies may influence the structural performance, durability, workability and permeability of crushed rock materials. The controlled blending of suitable additive sands or sand-clays during the crushing process can be used to overcome these deficiencies. Additive holding bins equipped with calibrated discharge gates or computer-controlled load cells and conveyor systems are used to accurately control the proportion of additive in the product.
4.2
Production of Naturally Occurring Granular Materials
Naturally occurring granular materials, which do not require costly extraction or crushing processes, are an important source of material used in the construction of flexible pavements in Australia. These materials include fine grained materials such as sand-clays, medium and coarse grained materials such as gravel, and materials produced by ripping or rolling soft or fissile rocks (Figure 4.6). An understanding of the factors controlling the performance of these materials, when used for pavement construction, is a necessary preliminary to their evaluation and to the consequent determination of selection criteria appropriate to their intended application e.g. subbase in a wet environment. Particular attention must be given to the sampling of natural deposits to ensure that representative material is tested and the natural variability of the material properties established so that appropriate selection limits can be defined.
RED SAND CLAY
RIVER SHINGLE Figure 4.6:
4.2.1
LIMESTONE GRAVEL
Some natural granular pavement materials
Methods of Production
Generally, natural granular materials do not require costly extraction or crushing processes. Earthmoving equipment, including bulldozers and hydraulic excavators, can be used to rip, raise and mix naturally-occurring granular materials without additional processing. However, granular materials which are unsuitable for use as pavement materials in their natural condition may be improved to meet specified requirements by crushing and screening, and/or using additives to improve their characteristics.
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Crushing Where a material contains oversize particles, they may be crushed until reduced to the desired maximum size then recombined with the material (Figure 4.7). Alternatively, the oversize material may be broken down on the road by mobile crushing plant or by rolling, provided the rock is not too tough. Generally, a considerable improvement in the particle size distribution and thus in the stability of the material will result.
Figure 4.7:
Plant for mixing and breaking down oversize materials
Screening Where pit conditions are suitable it may be practical to remove a portion of the coarse material by screening. It may also be practical to remove excess fines by screening. Stabilisation Depending on the deficiencies of the original material, improvements may be achieved by modification or stabilisation. In general this consists of the controlled addition of one or more materials to the gravel. The principal types of stabilisation are:
granular
cementitious (Portland cement, lime, lime/fly ash/slag blends, etc.)
bituminous
chemical (polymer, lignin or ionic based products).
The selection of the most appropriate method of stabilisation requires consideration of the deficiency of the material to be improved, the effectiveness of the different methods of stabilisation in achieving an improvement, plant availability, and relative costs. All methods require adequate laboratory investigations to determine the proportions of materials to be mixed, and strict quality control throughout construction. Thorough dispersion of the stabilising binder is required, and in the case of clayey materials, pulverisation will be required to achieve this.
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(a)
Granular stabilisation Commonly termed mechanical stabilisation, this is the process of improving the particle size distribution and/or the plastic properties of a material by blending with it one or more selected materials. A common example is the addition of fine material of low plasticity to a coarse granular material deficient in fines. The addition of material can usually be carried out at the pit or on the road and this often provides an economic method of obtaining suitable pavement material.
(b)
Cementitious and bituminous stabilisation Portland and blended cement or lime or slow-setting blends of finely ground lime/fly ash/slag or bitumen may be added to a granular material to improve its tensile and compressive strength, and to reduce the permeability of a material and its sensitivity to changes in moisture content. A more detailed description of these methods is contained in Part 4D: Stabilised Materials in the Guide to Pavement Technology. It should be noted that materials are considered to have been modified if sufficient amount of stabilising binders have been added so as to improve the performance of the materials without causing significant increase in tensile capacity (i.e. producing a bound material). There are no firmly established criteria to differentiate between modified and bound materials. However, Part 2 of the Guide considers modified materials to have a 28 day Unconfined Compressive Strength greater then 0.7 MPa and less than 1.5 MPa.
(c)
Chemical stabilisation Chemical stabilisation using polymer, lignin or ionic-based proprietary products is an emerging development in material stabilisation. These products act to improve material properties through waterproofing, binding of fines or by flocculating clay particles. Use of these products may be appropriate for improving the in-service strength of moisture sensitive materials or for reducing dust emissions and gravel loss on unsealed roads. However, with all proposed applications of chemical stabilisation, it is necessary to carry out investigations for each pavement material type proposed to be stabilised to ensure that the dosage rates applied are optimal and will result in the design aim of the treatment.
4.2.2
Assessment
The suitability of a natural granular soil for use as a pavement material is usually assessed using a series of relatively simple tests. Attributes such as stability, wear resistance, permeability and workability may be inferred from the results of these routine tests, which include the following:
particle size distribution
plastic limit
liquid limit
plasticity index
linear shrinkage
maximum dry compressive strength
ball mill value
static triaxial shear tests (including Texas triaxial test)
repeated load triaxial (RLT) test
California Bearing Ratio (CBR).
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In addition to these tests, the rapid methods of field assessment and classification outlined below are also used. It is not usually necessary to use all the tests listed. Engineering judgement should be used to determine which tests should be employed, according to circumstances. The tests themselves are only discussed briefly in this Part, and standard test procedures should be consulted. Pre-treatment The application and interpretation of test results is based on an assumption that the samples are tested under similar condition to that those likely to be applicable in the field. As some materials may require mechanical breakdown before use, or be prone to excessive chemical or physical degradation, either during handling or subsequently, pre-treatment of material samples may be necessary before testing. Generally pre-treatment is not required if the particles are rounded, hard and weather resistant, or if the amount of fines exceeds that required to fill the voids in the coarse particles. Soft rock that requires crushing, and other materials prone to mechanical breakdown or weathering, should be pre-treated. The laboratory pre-treatment procedures available generally simulate mechanical breakdown, weathering or a combination of both. The form of pre-treatment depends on the nature of the particular materials. For example, materials such as conglomerates, concretionary laterites and limestones may be broken with a hammer, or subjected to a compactive effort equivalent to the crushing action expected from field construction plant. Materials such as shales, on the other hand, may be subjected to cycles of wetting and drying in an attempt to simulate any breakdown likely to occur under field weathering conditions. Field assessment and classification A preliminary assessment of the need for comprehensive testing of a material can be conducted using field procedures. The techniques available generally rely on visual observations of the material and its reaction to a variety of simple treatments. Experience is the most important guide. A simple assessment of a material may be gained by wetting the material and squeezing it in the hand. For example, with well-graded materials, if a portion with particles smaller than about 5 mm is wet and then squeezed in the hand, the following characteristics may be observed:
the material feels extremely gritty
the material can be formed into definite shapes that retain their form even when dried
the hands may be slightly discoloured because of the adherence of clay; if more than enough material to slightly discolour the hands adheres, examination should show that it consists of both sand and clay and not clay alone
when the wet sample is patted into the palm of the hand it will compact into a dense cake that cannot be penetrated readily with the blunt end of a lead pencil.
The grittiness of the sample indicates the presence of sufficient granular material. Development of some strength on drying indicates the presence of a sufficient amount of binder material. Resistance to the penetration of the pencil-size stick, even when the sample is thoroughly wetted, indicates interlocking of the grains and the presence of sufficient internal friction. The presence of too much sand will cause the sample to fall apart when dried whilst the presence of too much clay will leave the hand muddy after the wet sample is squeezed. As a result, the wet sample, after being patted, will offer little resistance to the penetration of the stick.
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The Unified Soil Classification System (Appendix A) provides a more formal method of assessing a material with simple field tests that may be confirmed by subsequent laboratory procedures. This system of classification is useful because the properties of the various material groups are known, and have been tabulated in engineering use charts. This system divides materials into two major divisions: coarse-grained and fine-grained. Highly organic materials are also described. Identification in the field is accomplished by visual assessment of the coarse grains, and by a few simple hand tests of the fine grained materials or fractions.
4.3
Production of Crushed Rock
4.3.1
Methods of Crushed Rock Production
General Crushed rock may be produced in a single crushing operation (crusher run) or its production may involve a number of crushing and/or screening processes. In addition, some separation and recombination of different aggregate sizes is required to produce a material to tight tolerances. Commercial hard rock quarrying operations can generally result in a variety of products, ranging from base crushed rock, produced to tightly controlled tolerances, to by-products such as overburden, ripped rock and scalpings, which may have a use as a lower subbase or selected fill material used to enhance the bearing capacity of the subgrade. A general view of the production of crushed rock is shown in Figure 4.8.
Figure 4.8:
Production of crushed rock
Crushing and screening The crushing plant controls particle size distribution through adjustments to the crushers and screens. Different crusher types and crusher settings are used depending on the characteristics of the parent rock. The particle size distribution of the end product is mainly controlled by the screens. Screen variables that can be controlled include aperture size and shape, screen angle, vibrating speed and direction. Once the system is set up and is stable, the plant generally only requires resetting daily.
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A crushing plant generally consists of a primary jaw crusher and up to four or more secondary crushers, which are usually gyratory (cone) crushers. These types of crushers work by applying force to the rock and crush in one direction only, which tends to produce elongated and flaky particles of stone. Impact crushers, which have the capacity to break the rock in all directions, produce much better shaped particles because the rock is broken on impact and breaks randomly rather than along cleavage planes producing a more cubical stone. However, impact crushers are very expensive to operate so their use is limited. Through suitable adjustment to the crushing plant it is often possible to produce ‘crusher-run’ crushed rock which has a particle size distribution within the specified tolerances. However, with some types of rock and some types of crushing plant, and for situations where only a narrow range of tolerances can be allowed, it is necessary to screen or separate the ‘crusher-run’ crushed rock into various fractions and then to recombine calculated portions of these to produce the specified product. Additives to crushed rock Some source rocks, due to hardness or mineralogical texture, do not produce sufficient fine and cohesive particles during normal crushing processes to make graded crushed rocks which have good handling properties. To improve the characteristics of the product, a fine plastic component may be added to the crushed rock to provide a more suitable grading in the fine sizes, to impart greater cohesion and workability to the material, and to reduce the permeability of the compacted product. Crusher fines from sources other than that from which the product is being crushed, and finegrained and plastic sands (e.g. granitic-derived sand) are commonly used as an additive to crushed rock. More recently, clayey fillers have been produced by drying, crushing or grinding the filler into a fine free-flowing powder which, when added to a hard crushed rock, produces a material with satisfactory cohesive properties. This is particularly important for granular pavement bases surfaced with a sprayed bituminous seal. It is important when selecting suitable additives to ensure that the principal components are composed of non-cementitious, durable material that will not significantly change in particle size or form during the design life of the pavement. To enable the additive to be effectively blended with the crushed rock it should be dry, friable and free flowing, free of vegetable matter, lumps and balls of clay and over-size particles of rock, and be sized such that it can be uniformly distributed throughout. If the additive material at the source is variable then it is important that the material is carefully selected from the pit or mixed to provide a product of consistent grading and plasticity. Typically, fine plastic sands (say Plasticity Index in the range 8-15) may be added to the crushed rock in proportions of 5-15% by mass. The success of such a material derives from the way in which the inert sand and silt-sized quartz particles act as a carrier to the relatively plastic clay fines, thus enabling the additive to be uniformly blended with the crushed rock. Some industrial waste products, such as cement works flue dust, are inherently cementitious and should not normally be used as a fines additive component to crushed rock. If such cementitious additives are used to modify the grading of crushed rock then an excessive cementing action may occur, resulting in poor compaction characteristics and excessive deformation and shrinkage cracking of the pavement. Cementitious additives should be avoided in crushed rock products used for pavements where cracking of the pavement cannot be tolerated.
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Other means of achieving desirable handling characteristics where hard, tough rocks are used and the gradings are on the coarser side of the grading envelope is by the addition of cohesive fines and fine aggregates derived from a quarry source operating in a ‘softer’ (but durable) rock type. In this case, up to 30% by mass of imported crusher fines may be required to be added to produce a material which conforms to the crushed rock base specification and has desirable handling characteristics. In some cases, weathered rock and quarry overburden material is added to a crushed rock product to impart specified characteristics of plasticity, grading and permeability. This practice should not be condoned for quality crushed rock production as there is a probability that degradation and disintegration of particles will occur during handling and compaction and during the service life of the pavement.
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5
CRUSHED ROCK PROPERTIES REQUIRING SPECIFICATION
5.1
General
In order to adequately specify a product it is necessary to have a very clear understanding of its functions and required performance characteristics. While notionally the desire may be to specify that the product is durable, workable, etc. this in itself is not sufficient. The required performance characteristic has to be defined by a test method or set of methods. Each test method has its own limitations, ranging from its application, repeatability, reproducibility, sample size, particle size, time to complete test, etc. In the end, products are judged not so much by their desired performance characteristics but by their compliance with a specified test method limit. This in turn requires a sound understanding of the available test methods and their application and limitations. It should be noted that the divergence in the specification requirements between the various road agencies, especially with respect to durability requirements, is related to their experience and practice. It is in part associated with the types of rock used in each State, or region within that State, and in part with the tests adopted to solve rock durability problems. For the most part, all of the tests now specified in road agency specifications are incorporated into Australian Standards. Specifications have to be practical: the selection of the appropriate test method is central to practical outcomes (timeliness, sample size, frequency, etc.). The specification has to work for all the major parties involved (viz. the customer, the road contractor and the product supplier). The principal interests of these parties will commonly be as follows:
the customer wants to be assured that the completed product performs as required and fully complies with the specification and provides value for money
the road contractor wants to be assured of the supply of a uniform, workable and costeffective pavement material that will produce the specified product
the crushed rock supplier wants to ensure cost effective production (appropriate use of rock source and plant) without rejection of the product at its point of delivery.
5.2
Specification Types
It is possible to specify requirements in three distinctly different ways, by defining either:
processes and products to be used, e.g. roller passes to compact base (method based specification)
the performance/functional characteristics of a pavement as a whole without reference to the properties of the constituent part (performance specification)
the properties of each and every constituent part ‘used’ and the required end-product state of each part (end-product, attribute specification).
Each method has the following advantages and disadvantages.
The method based specification method allows better control (lower risk) on the final outcome, but may prevent innovation and not encourage commitment to the best outcome.
The performance specification method has much in its favour because it encourages innovation and commitment, but there are currently considerable risks and difficulties in the definition of short-term and long-term performance characteristics of the pavement as a whole.
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The end-product specification method is the most common approach as it places no restriction on the processes which can be adopted by the road contractor or product supplier and hence provides more opportunities for innovation. However, it does place restriction on end-product requirements of the constituent parts, which can be in terms of either: — —
composition (e.g. particle shape and texture, particle size distribution, Atterberg limits, etc.) behavioural or performance-based characteristics (e.g. skid resistance, strength, stiffness, permeability, etc.).
Many current specifications are hybrids of all three specification types, but the future is seen to be in the increased specification of performance characteristics. The following sections describe specifications for typical granular products currently used in road construction in Australia.
5.3
Source Rock
The source rocks used to produce crushed rock pavement materials must possess characteristics which will ensure that the product will have the necessary strength and durability, both immediately and in the long-term, to withstand handling during construction, weathering agents and traffic stresses. Knowledge of the geological origin and history of rocks used in the production of crushed rock may give some indication of the likely quality of the product. The engineering properties of a rock are influenced by a number of factors, principal amongst these are:
primary mineralogy
grain size
rock structure
quantity and type of secondary mineralisation
degree of weathering.
The classification of rock used in the production of crushed rock is based upon the mineralogy and petrographic description of the rock. It is important for the source to be correctly identified and classified in accordance with a recognised classification system. Correct identification of rock type is a task that requires geological training and experience. Part 4J: Aggregate and Source Rock of Guide to Pavement Technology details the classification and description of source rocks and properties that require specification.
5.4
Product Requirements
5.4.1
Introduction
A product specification should relate to those properties of the crushed rock material which have a bearing upon its performance (Section 3). It should include requirements for the following:
maximum size
particle size distribution
particle shape
nature of fines (cohesive fines content and fines plasticity).
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5.4.2
Maximum Size
As the maximum size increases, so do the problems associated with segregation, and achieving the surface finish necessary for the application of the surfacing and good riding quality. Often 40 mm nominal size is specified for subbase and lower base layers and 20 mm for the upper base layer. A maximum size of 20 mm is commonly specified for all base layers and even upper subbase layers because such material is less prone to segregation and is easier to place and hence achieve compacted density. 5.4.3
Particle Size Distribution
It is impractical to determine the size of every particle in the crushed rock product. In practice, determinations are made of the quantities of particles, the sizes of which lie between sets of defined size limits, usually related to a fixed series of sieves. The material within a size range defined by any two limits is referred to as a fraction of the crushed rock. Often all the material retained above the 4.75 mm sieve is termed the coarse fraction, while that passing the same sieve is termed the fine fraction. Material passing the 75 μm sieve is referred to as fines. Ideally, for high quality crushed rock used as a base material in road construction, the value of the exponent n in Equation 3.1 (Section 3.7) should be between 0.45−0.50, as this generally produces a maximum density grading (i.e. minimum porosity), and provides a material of maximum stability and strength. The characteristics and performance of a crushed rock are influenced by the proportions of fine and coarse material in the particle size distribution as follows:
If the crushed rock contains an excess of coarse particles it will be difficult to handle during construction and will require lateral confinement to maintain strength. When compacted it will have a relatively low density and be highly permeable to water. Because of stone-tostone contact, the material will be less likely to suffer a loss of strength or stability in the presence of moisture.
If the crushed rock contains an excess of cohesive fines it will not be difficult to handle during construction, provided it is not over-wet, and also not require lateral confinement. When compacted it will have a relatively low density and low permeability. It is likely that the material will be very susceptible to loss of strength and stability with low resistance to deformation at higher moisture contents.
Between those extremes, a crushed rock with a particle size distribution represented by Equation 3.1, with a value of n near 0.45 will be workable and will not require lateral confinement to maintain strength (refer Figure 5.1). When compacted, it will have relatively high density and low permeability. Depending upon the nature of the fines it will be less susceptible to loss of strength and stability due to the presence of excessive moisture.
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Figure 5.1:
Compacted crushed rock with n ≈ 0.45
The particle size distributions specified should provide for the requirements of specific situations (refer Figure 5.2). For example, relatively high permeability may be required in the subbase, low permeability in the base, and very low permeability in unsealed shoulders. Laboratory or field testing methods can be used to determine permeability. To achieve the desired particle size distribution after compaction it is necessary to choose an initial particle size distribution which takes account of any particle breakdown resulting from compaction. If a standard specification is being prepared to cover a wide range of source rock then it may be possible to specify a number of initial particle size distributions to cover rocks classified into groups according to, say, the Los Angeles Abrasion value. Some indication of the probable extent of breakdown can be obtained from a laboratory test on a sample of material involving pre-treatment using repeated cycles of compaction. As well as specifying the initial target particle size distribution, practical upper and lower tolerances should also be fixed to allow for normal production fluctuations. Whilst it is possible for the particle size distribution of a crushed rock to be within the tolerances, or grading envelope as it is commonly termed, this alone does not ensure that the product is acceptable. For example, when the particle size distribution moves from the coarse limit to the fine limit, or visa versa, as it moves through the sieve sizes, the result is a material that s deficient in some fractions, i.e. it has ‘gaps’ in those size ranges, and has excess amounts of other fractions (refer Figure 5.3). To avoid this problem the specification could set limits on the percentage of material to be retained between each consecutive pair of sieves.
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Figure 5.2:
Particle size distribution and workability (Ingles and Metcalf 1972)
Figure 5.3:
5.4.4
Compacted crushed rock with deficiencies of some sizes
Particle Shape
It is generally accepted that it is desirable for aggregate particles in the coarse fraction to be cubical and angular in shape. Flaky stone will not pack properly, resulting in a matrix with low density, high voids and low strength. Cubical particles withstand higher levels of loading than flaky particles. Angular particles are preferred to rounded particles because of their better mechanical interlock and, therefore, superior stability and strength. The specification can set maximum allowable percentages for flaky and elongated particles. Tests are available to assess the percentages of such particles. Angularity is frequently assessed by visual inspection only.
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When using rounded river gravel or spalls as the source rock, it is common practice to specify that a proportion of particles in the crushed product should have a minimum number of fractured faces as determined by AS 1141.18 – 1996, Method 18: Crushed particles in coarse aggregates derived from gravel. The shape of the particles in crushed rock depends on the crushing methods used and on the characteristics of the source rock. Some source rocks have a tendency to produce flaky particles and this requires that more effort be put into the design of the plant to ensure that the aggregate particle shape is satisfactory. Generally a maximum Flakiness Index of 35 should be specified for base material. 5.4.5
Nature of Fines
The stability of the product will be influenced by the nature of the fines. Fines consist of materials crushed from the source rock, which will have the inherent properties of that rock, together with any overburden contamination, the properties of which may be very different from those of the rock fines. Generally, contamination can be reduced to an acceptable level by proper production practices. If the fines, whether from the source rock or from overburden, have a high clay content, problems are likely to arise with the field performance of the crushed rock (e.g. instability and rutting), especially in situations where excess moisture exists. Fines containing a significant proportion of clay are said to be plastic. The most common method of specifying requirements relating to clay content is the Plasticity Index (PI). Most specifications for crushed rock set maximum allowable values of PI and liquid limit (LL) values although, in some situations where the crushed rock must be able to withstand ravelling and erosion, e.g. unsealed pavement surface and unsealed shoulders, both a maximum and a minimum value of PI are specified. 5.4.6
Example Test Limits for Crushed Rock
Examples of test limits for the properties described in the previous sections are presented in Appendix B. It lists requirements about which there is general agreement and for which there is general application throughout Australia. However, there will be occasions and locations for which none of the materials specified in this Guide is applicable, either due to technical suitability, availability, cost or some other reason. Test limits listed in Appendix B are provided to illustrate the relationships between material characteristics. These limits are not intended or recommended for adoption nationally. Test methods are described in Section 8. The specification sets out material requirements for base and subbase materials manufactured from sources of hard rock and river gravel intended for use in the construction of pavements. It is not applicable to naturally-occurring pavement materials or to soft materials that are altered significantly during construction, or to bound or stabilised materials. The use of uncrushed fine material as part of the base and subbase materials is permitted.
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6
PROPERTIES OF NATURALLY OCCURRING GRANULAR MATERIALS REQUIRING SPECIFICATION
6.1
Introduction
The majority of the sealed roads in Australia and New Zealand were constructed in the 1950s and 1960s as light-traffic rural and urban roads (with a design traffic loadings 50
The 75 μm sieve size is about the smallest particle visible to the naked eye
Fine-grained soils More than half of material is smaller than 75μm sieve size.
Gravels More than half of coarse fraction is > 4.75 mm sieve size
2
Coarse-grained soils More than half of material is larger than 75 μm sieve size
1
Typical Names
Identification procedures on fractions smaller than 425 µm sieve size Dry Strength (crushing characteristics
Dilatancy (reaction to shaking)
Toughness (consistency near PL)
ML
Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity.
None to slight
Quick to slow
None
Inorganic clays of low to medium plasticity, gravely clays, sandy clays, silty clays, lean clays
Medium to high
None to very slow
Medium
CL OL
Organic silts and organic silty clays of low plasticity
Slight to medium
Slow
Slight
MH
Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts.
Slight to medium
Slow to none
Slight to medium
CH
Inorganic clays of high plasticity, fat clays.
High to very high
None
High
OH
Organic clays of medium to high plasticity, organic silts
Medium to high
None to very slow
Slight to medium
Peat and other highly organic soils
Readily identified by colour, odour, spongy feel and frequently by fibrous texture.
Pt
Boundary Classification: Soils possessing characteristics of two groups are designated by combinations of group symbols. For example GW-GC, well graded gravel sand mixture with clay binder.
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Information Required for Describing Soils
Laboratory Classification Criteria
6
7
Give typical name, indicate degree and character of plasticity, amount and maximum size of coarse grains, colour in wet condition, odour, if any local or geologic name, and other pertinent descriptive information; and symbol in parentheses.
Cu =
Determine percentage of gravel and sand from grain size curve. Depending on percentage of fines (fraction smaller than 75 µm sieve size) coarse grained soils are classified as follows 12% GM, GC, SM, SC 5% to 12% use of dual symbols
EXAMPLE: Silty sand, gravelly, about 20% hard, angular gravel particles 13.2mm maximum size, rounded and subangular sand grains coarse to fine, about 15% non-plastic fines with low dry strength well compacted and moist in place; alluvial sand (SM).
Use grain size curve in identifying the fractions as given under field identification
Give typical name; indicate approximate percentages of sand and gravel, max. size, angularity, surface condition, and hardness of the coarse grains, local or geologic name and other pertinent descriptive information, and symbol in parentheses.
Cc =
(D30 )2 D10 xD60
≥1≤ 3
Not meeting all grading requirements for GW Atterberg limits below the A Line or PI < 4
Above the A line With PI between 4 and 7 are borderline cases requiring use of dual symbols.
Atterberg limits below the A Line with PI < 7
Cu =
Cc =
D60 〉4 D10
(D30 )2 D10 xD60
≥1≤ 3
Not Meeting all grading requirements for SW Atterberg limits below the A line or PI < 4 Atterberg limits below the A line with PI < 7
Above the A line With PI between 4 and 7 are borderline cases requiring use of dual symbols.
A - line
40
CH
CI MH
20
For undisturbed soils add information, on structure, stratification, consistency in undisturbed and remoulded states, moisture and drainage conditions.
OH
CL SF
MI
S C
0
EXAMPLE: Clayey silt, brown, slightly plastic, small percentage of fine sand, numerous vertical root holes, firm and dry in place, loess. (ML).
D60 〉 6 D10
60
Platicity Index
For undisturbed soils add information on stratification, degree of compactness, cementation, moisture conditions and drainage characteristics.
ML
O L
OI
50
20
Liquid Limit (LL)
PLASTICITY CHART for laboratory classification of the fine-grained soils
2.
All sieve sizes of this chart are AS standard.
3.
Cu = Coefficient uniformity. Cc = Coefficient of curvature. D60 = Grain diameter at 60% passing. D30 = Grain diameter of 30% passing. D10 = Grain diameter at 10% passing.
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APPENDIX B
EXAMPLE TEST LIMITS FOR CRUSHED ROCK
Disclaimer Test limits listed in this Appendix are provided to illustrate the relationships between material characteristics. Although the limits outlined have been used in some specifications, they are intended as a guide only and are not intended to be adopted nationally.
B.1
Quality System
The supplier shall establish implement and maintain a Quality System in accordance with the requirements of AS/NZS ISO 9001 or a recognised equivalent.
B.2
Test Methods
All testing required by the specification shall be performed in a laboratory endorsed by the National Association of Testing Authorities (NATA). Tests shall be performed in accordance with the relevant Australian Standard testing procedures.
B.3
Materials
Crushed rock fragments shall consist of clean, hard, durable, angular rock fragments of uniform quality. The use of sands and/or filler and crushed fines from a source other than the source of the coarse aggregate is permitted, but shall be subject to approval of the purchaser. For the purpose of this specification crushed rock is to be supplied in various in classes broadly defined as follows:
Class 1 is normally specified as a premium cohesive pavement base material for unbound pavements where a very high standard of surface preparation for a sprayed sealed or thin asphalt surfacing is required. It has a minimum plasticity index requirement and may have additional requirement for maximum permeability when used for heavy duty unbound pavements.
Class 2 is normally specified as a high quality pavement base material for unbound flexible pavements in locations where a very high standard of surface preparation may not be required. Class 2 crushed rock does not have a minimum plasticity index or a maximum permeability requirement.
Class 3 is normally specified as a high quality upper subbase material for heavy duty unbound flexible pavements. It may have a minimum permeability requirement to provide positive drainage to the sub-surface drains.
Class 4 is normally specified as a lower subbase material for heavy duty pavements or a subbase material for most other types of pavements. It may have a maximum permeability requirement if used as a capping material.
B.4
Source Rock Specifications
The supplier shall supply details of the source and geological description of the rock to be used for production of the crushed rock.
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B.4.1
Coarse Aggregate Fraction (Portion Retained on a 4.75 mm AS Sieve)
Tables B.1, B.2 and B.3 provide three alternative combinations of durability and hardness requirements. Only one combination shall be applied and this must be selected by the purchaser. This should be the set that has been shown by local experience to be valid for the rock sources likely to be used. Materials from sources not complying with the given values may perform satisfactorily and may be used provided that there is local evidence of proven performance. B.4.2
Wet Strength and Wet/Dry Strength Variation
Base and subbase materials having wet strength and wet/dry strength variation values that comply with the limits given in Table B.2 have performed successfully in practice. B.4.3
Los Angeles Value and Unsound and Marginal Stone Content
Base and subbase materials having a Los Angeles Value not greater than the relevant limits given in Table B.3, when tested in accordance with AS 1141.23, have performed successfully in practice. In addition, base and subbase materials having unsound and marginal stone content not greater than the relevant limits given in Table B.3, when determined in accordance with AS 1141.30, have also been used successfully. B.4.4
Los Angeles Value and Sodium Sulphate Soundness Loss
Base and subbase materials having a Los Angeles Value not greater than the relevant limits given in Table B.4, when tested in accordance with AS 1141.23, have performed successfully in practice. In addition, base and subbase materials having a sodium sulphate soundness loss not greater than the relevant limits given in Table B.4, when determined in accordance with AS 1141.24, have also been used successfully. Table B.1: Wet strength and wet/dry strength variation requirements for hardness and durability Test value Test Property
Base Class 1
Subbase Class 2
Class 3
Class 4 (lower)
10% fines value (wet) (kN) (min)
100
80
50
–
wet/dry strength variation (%) (max)
35
35
45
–
Table B.2: Los Angeles value and unsound and marginal stone content requirements for hardness and durability Test value Test
Rock type
Base
Subbase
Class 1
Class 2
Class 3
Class 4
coarse grained
35
35
40
–
fine grained
30
30
35
–
total of marginal and unsound stone (%) (max)
all rock types
10
10
20
–
unsound stone (%) (max)
all rock types
5
5
10
–
Los Angeles value (%) (max)
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Table B.3: Los Angeles value and aggregate soundness requirements for hardness and durability Test value Test
Rock type
Los Angeles value (%) (max) Sodium sulphate soundness (maximum weighted average loss) (%)
B.4.5
Base
Subbase
Class 1
Class 2
Class 3
Class 4
coarse grained
35
35
40
–
fine grained
30
30
35
–
all rock types
6
9
12
–
Fine Fraction (Portion Passing a 4.75 mm AS Sieve)
The use of crushed fines produced from a quarry, or a location within a quarry, different from that used for the production of that fraction of the crushed rock retained on the 4.75 mm AS sieve is permitted subject to the approval of the purchaser. Such crusher fines produced from any igneous or metamorphic rock when tested in accordance with AS 1141.25.3 shall have a Degradation Factor-Fine Aggregate value not less than 60. The use of sands and/or filler is permitted subject to approval in writing by the purchaser as to the proposed source and nature of such materials, the proposed amounts to be added and the proposed method of incorporating such materials in the product. Where the supplier elects to use an additive component with the crushed rock, the additive shall:
be derived from sound and durable material
not be cementitious in nature
be free of vegetable matter, lumps and balls of clay and oversize particles of rock
be sized such that it can be effectively and uniformly distributed throughout the crushed rock
be kept dry to facilitate incorporation into the mixture
be blended in the base and subbase finished products and shall not be greater than 15% by mass.
B.5
Product Specifications
The crushed rock shall be free from vegetable matter and lumps or balls of clay and shall comply with the relevant requirements of Table B.4. Table B.4: Product requirements Test
Test values Base Class 1
Subbase Class 2
Class 3
Class 4
Liquid Limit % (max)
30
30
35
40
Plasticity Index (max or range) (1)
2-6
0-6
12
20
California Bearing Ratio % (min) (2)
–
-
30
15
PI x % passing 0.425 AS sieve (max)
–
-
–
600
Flakiness index % (max)
35
35
–
–
crushed particles % (max) (3)
60
60
50
–
(1) Minimum value for PI specified to ensure the material is sufficiently impermeable to resist penetration of water through thin surfacing layers. (2) Value applicable to material passing 19.0 mm sieve: compacted at Optimum Moisture Content to 98% of Modified Maximum Dry Density as determined by test; then soaked for 4 days prior to CBR testing. (3) Applicable to crushed river gravels.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
B.5.1
Sulphide Mineralisation
Unless otherwise approved by the purchaser, crushed rock produced from any source shall not be permitted if that fraction of the crushed rock product passing the 2.36 mm AS sieve, when prepared according to AS 1289.1 Clause 4.6, fails to comply with the requirements of Table B.5. Table B.5: Requirements for sulphide mineralisation Test pH (units) conductivity (mS/cm)
B.5.2
Test value
Soil to water ratio
6.0 (min)
1:2.5
1500 (max)
1:1
Grading of Crushed Rock Base and Subbase
The grading of the crushed rock after compaction shall comply with the relevant requirements of Tables B.6, B.7, B.8 and B.9. Table B.6: Grading requirements for Class 1 & 2, 20 mm base AS sieve size (mm)
Target grading (% Passing)
26.5 19.0 13.2 9.5 4.75 2.36 0.425 0.075
100 100 85 73 54 39 18 9
Test value Limits of grading (% Passing)
Retained between Sieves %
100 95–100 78–92 63–83 44–64 30–48 14–22 7–11
0–5 7–18 10–16 14–24 10–20 14–28 6–13
Table B.7: Grading requirements for Class 3, 20 mm subbase AS sieve size (mm)
Target grading (% Passing)
Test value limits of grading (% Passing)
26.5
100
100
19.0
100
95–100
13.2
85
75–95
9.5
75
60–90
4.75
59
42–76
2.36
44
28–60
0.425
21
14–28
0.075
10
6–13
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
Table B.8: Grading requirements for Class 3, 40 mm subbase AS sieve size (mm)
Target grading (% Passing)
Test value limits of grading (% Passing)
53.0
100
100
37.5
100
95-100
26.5
85
75-95
19.0
77
64-90
9.5
60
42-78
4.75
46
27-64
2.36
35
20-50
0.425
17
10-23
0.075
9
6-12
Class 4 crushed rock subbase shall comply with the relevant nominal size grading requirements given in Table B.9. The crushed rock shall not be graded from near the coarse limit on one sieve to near the fine limit on the following sieve or vice versa. Table B.9: Grading requirements for Class 4 subbase Test Value Limits of Grading (% Passing)
AS Sieve Size (mm)
Nominal Size (mm) 50
75.0
40
30
25
100
100
14
10
100
100
42-76
54-75
64-84
100
53.0
100
37.5 26.5 19.0
20
100 54-75
64-90
9.5
48-70
54-75
4.75 2.36
B.6
0.425
7-21
7-23
9-24
10-26
10-28
15-32
18-35
0.075
2-10
2-12
2-12
2-13
2-14
6-17
7-18
Moisture Content
Where payment is to be made on a mass basis, the average moisture content of the crushed rock at the plant shall not exceed 3.5% by mass unless otherwise specified or unless the contractor has, at the time of tendering, nominated an upper limit of average moisture content greater than 3.5%. In the latter case the difference between the nominated value and the specified value will be taken into account when tenders are being considered. The average moisture content of crushed rock supplied on any one day will be determined from three samples taken at random from that day’s supply. If the average moisture content is greater than that specified or nominated, the material may be rejected. If, at the discretion of the purchaser, the material is accepted, then payment will be made for the mass determined by deducting the calculated mass of excess moisture from the net mass shown on the delivery dockets.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
B.7
Stockpiling Prior to Delivery
Material may be stockpiled prior to delivery provided the following requirements are fulfilled:
the product, after recovery from the stockpile, complies with this specification
the stockpile site is clean, adequately paved, and well drained
if a stockpile is constructed in more than one layer, then each layer is fully contained within the area occupied by the upper surface of the preceding layer
the stockpiled material shall have a minimum moisture content of 3.5% to prevent segregation and excessive dust.
B.8
Handling of Materials
Handling of materials, including the loading of trucks and stockpiling, shall be effected in such a manner as to minimise segregation.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
APPENDIX C
EXAMPLE TEST LIMITS FOR NATURAL GRAVELS
Disclaimer Test limits listed in this Appendix are provided to illustrate the relationships between material characteristics. Although the limits outlined have been used in some specifications, they are intended as a guide only and are not intended to be adopted nationally.
C.1
Particle Size Distribution
C.1.1
Gravel and soft rock
The suggested particle size distribution for gravel and soft and fissile rock is given in Figure C.1. C.1.2
Sand-clays
Specific test limits for sand-clays are not well established. Where possible, materials with maximum density particle size distributions should be used. However, other information, such as modulus, resistance to permanent deformation and CBR strength test results, will normally be more important as selection criteria. The grading may be used to control quality once its correlation with strength is established.
C.2
Plastic Limit
A value in excess of 20% may indicate the presence of undesirable components.
C.3
Liquid Limit
The following values have been used in specifications:
unsealed base or shoulders 35% maximum
sealed base or shoulders 25% maximum.
Slightly higher values may be acceptable for some limestone rubbles or similar absorptive materials.
C.4
Plasticity Index
C.4.1
Gravel and soft rock
The following values have been used in specifications for gravel and soft rock: Material use unsealed base / shoulder unsealed subbase sealed base / shoulder sealed subbase
Annual Rainfall < 400 mm
> 400 mm
4% min – 15% max 18% max
4% min – 9% max 12% max
10% max 12% max
6% max 8% max
Note: For materials which contain natural cementing agents, higher values than those suggested have been used with satisfactory results.
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
C.4.2
Sand-clays
The following values are suggested for sand-clays: Material use
Annual Rainfall (mm) < 400
unsealed base / shoulder unsealed subbase
5% min – 15% max 3% min – 20% max
sealed base / shoulder sealed subbase
5% min – 12% max 3% min –15%max
Note: Sand-clay does not perform well when wet or subjected to abrasion or erosion.
Figure C.1: Suggested particle size distribution for fixed maximum size
It is suggested that the use of sand-clays in unsealed roads should be limited to those subjected to low traffic intensities. It is further recommended that sand-clay shoulders on a sealed road be sealed or stabilised with bitumen. C.4.3
Weighed plasticity index
As a guide, the product of the PI and the percentage passing the 425 μm sieve (relative to the whole material), commonly called the Weighted Plasticity Index (WPI) should not exceed 200 for a material which is to be used with a bituminous surfacing. A value of 400 has been used satisfactorily in arid areas ( 400
unsealed base / shoulder unsealed subbase
6% max 8% max
3% max 2% max
sealed base / shoulder sealed subbase
4% max 5% max
2% max 3% max
Maximum Dry Compressive Strength
The following values have been used in specifications: Material use
C.7
Maximum Dry Compressive Strength
unsealed base / shoulder unsealed subbase
2.8 MPa min 1.0 MPa min
sealed base / shoulder sealed subbase
1.7 MPa min 1.0 MPa min
Miniature Abrasion Loss Test
A maximum value of 15% loss has been specified for base and subbase materials.
C.8
Ball Mill Value
The following values have been used in specifications:
base 40 maximum
subbase 55 maximum.
Note that these values would exclude many of the ripped rocks used in rural areas. The values relate more to a relatively high-strength material that would probably require blasting to extract and crush.
C.9
Texas Triaxial Test
The following values are suggested at the expected in situ moisture density conditions: Material use
Class Number
Compressive Modulus (at zero lateral pressure)
unsealed base
2.3 max.
–
sealed base
2.0 max.
28 MPa min
subbase
3.0 max.
–
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GUIDE TO PAVEMENT TECHNOLOGY PART 4A: GRANULAR BASE AND SUBBASE MATERIALS
As a guide, modified Texas triaxial requirements specified by the RTA NSW are as follows: Design traffic loading (ESA) ≥ 107 4x
106
