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Technical basis of Austroads Guide toPavement Technology Part 2:

Research Report ARR 384

Pavement Structural DesignPrepared by G. Jameson


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Technical Basis ofAustroads Guide to Pavement Technology

Part 2: Pavement Structural Design

Prepared by

Geoff Jameson

ARRB Group LtdResearch Report ARR 384

November 2013


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ARR 384 Technical Basis of Austroads Guide to Pavement Technology Part 2 Pavement Structural Design: Chapter 1

A R R B G r o u p L t d w w w . a r r b . c o m . a u

Information Retrieval

JAMESON,Geoff (2013)

Technical Basis of Austroads Guide to Pavement Technology Part 2: Pavement Structural Design, ARRB Group Ltd,

Vermont South, Victoria, Australia.

Research Report ARR 313 pages, including figures and tables.

Abstract: The Austroads publication Guide to Pavement Technology, Part 2: Pavement Structural Designis intended to assist

those required to plan and design new pavements. It was originally produced in 1987 as a result of review of the NAASRA

Interim Guide to Pavement Thickness Design (1979). In 1992, the Austroads Pavement Design Guide was revised to include an

updated procedure for the design of rigid pavements and also relevant portions of Chapter 6 (Pavement Materials) and Chapter

7 (Design Traffic).

An essential element in the use of the Guide is a thorough understanding of the origins of the design procedures, their scope

and limitations. Accordingly, this report contains the following five technical reports detailing the technical basis of:

Chapter 1: 1992 Guide procedures for the design of flexible pavements

Chapter 2: 1992 Guide procedures for design of rigid pavements

Chapter 3: Guide to Pavement Technology Part 2 Design of flexible pavements

Chapter 4: Guide to Pavement Technology Part 2 Design of rigid pavements

Chapter 5: Guide to Pavement Technology Part 2: Development of design charts for lightly trafficked roads

ARR 384

November 2013

ISBN 1 876592 74 5

ISSN 0158-0728

ARRB Group Ltd.

All rights reserved. Except for fair copying, no

part of this publication may be reproduced,

stored in a retrieval system or transmitted in any

form or by any means, electronic, mechanical,

photocopying or otherwise, without the prior

written permission of ARRB Group Ltd.

Although the Report is believed to be correct at

the time of publication, ARRB Group Ltd, to the

extent lawful, excludes all liability for loss

(whether arising under contract, tort, statute or

otherwise) arising from the contents of the

Report or from its use. Where such liability

cannot be excluded, it is reduced to the full

extent lawful. Without limiting the foregoing,

respondents should apply their own skill andjudgement when using the information contained

in the Report.

Wholly prepared by

ARRB Group Ltd

500 Burwood Highway

Vermont South Vic 3133

Australia

Contact: Publication Sales

ARRB Group Ltd

Phone 61 3 9881 1561

Fax 61 3 9887 8144

E-mail [email protected]

About the author

Geoff Jameson is a graduate from the University of Melbourne with a

Bachelor of Science (Hons) degree. Prior to joining ARRB Group, he

worked for VicRoads on various aspects of road construction materials,

including eight years as the manager of the Pavement Design Section,

which was responsible for standards of pavement design and rehabilitation

of all freeways, highways, tourist and main roads in the State of Victoria. In

1992/1993 on secondment from VicRoads, Geoff was involved with the

development of Pavement Management Systems in Hong Kong and the

Philippines.

In 1994 he joined the Pavement Technology area at ARRB Group. He has

worked on a variety of projects addressing issues associated with pavement

design and performance, specifications and materials characterisation,

including Accelerated Loading Facility (ALF) trials of unbound granular

materials, asphalt, cemented materials, recycled materials and rigid

pavements. He is a member of the Austroads Pavement Technology Task

Force, Austroads Pavements Structures Working Group and Austroads

Foamed Stabilisation Working Group.

In 2011-12 he was appointed as the inaugural Director of the Western

Australian Pavement Asset Research Centre..


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A R R B G r o u p L t d

w w w . a r r b . c o m . a u

FOREWORD

The Austroads publication Guide to Pavement Technology, Part 2: Pavement Structural Design isintended to assist those required to plan and design new pavements. It was originally produced in1987 as a result of review of the NAASRA Interim Guide to Pavement Thickness Design(1979). In1992, the Austroads Pavement Design Guide was revised to include an updated procedure for the

design of rigid pavements and also relevant portions of Chapter 6 (Pavement Materials) andChapter 7 (Design Traffic).

An essential element in the use of the Guide is a thorough understanding of the origins of thedesign procedures, their scope and limitations. Accordingly, this report contains the following fivetechnical chapters which detail the technical basis of various editions of the Guide:

Chapter 1: 1992 Guide procedures for design of flexible pavements, David Potter (1999)

Chapter 2: 1992 Guide procedures for design of rigid pavements, George Vorobieff andJohn Hodgkinson (2001)

Chapter 3: Guide to Pavement Technology Part 2: Design of flexible pavements,Geoff Jameson (2012)

Chapter 4: Guide to Pavement Technology Part 2: Design of rigid pavements,George Vorobieff (2003)

Chapter 5: Guide to Pavement Technology Part 2: Development of design charts for lightlytrafficked roads, Zahid Hoque and Geoff Jameson (2007).

The five chapters are augmented by several Appendices which explain, in greater details, some ofthe background to the material presented in the Guide. A comprehensive list of References alsoaccompanies each chapter.

The material presented here represents over 40 years of work conducted in Australia andoverseas. A large number of people representing Austroads member authorities, ARRB localgovernment, industry and consultants have input into the development of the various editions ofthe Guide and their contributions are gratefully acknowledged.

Chris HarrisonRoads and Maritime Services, New South WalesProject Manager


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CONTENTS CHAPTER 1

1.......

INTRODUCTION ............................................................................................................................. 1

2.......SETTING THE SCENE ................................................................................................................... 2

2.1....The Interim Guide to Pavement Thickness Design ......................................................................... 2

2.1.1 Format of the IGPTD ........................................................................................................ 3

2.1.2 Release of the IGPTD ...................................................................................................... 8

2.2....Establishment of a Working Group to Revise the IGPTD ............................................................... 8

2.2.1 A Guide not a Manual ................................................................................................... 10

3.......GRANULAR PAVEMENTS WITH THIN BITUMINOUS SURFACINGS ........................................ 11

3.1....Origins of the CBR-Thickness-Traffic Chart .................................................................................... 11

3.1.1

Quality of Pavement Material and its Cover Requirements ............................................. 14

3.1.2

Concluding Comments ..................................................................................................... 153.2....Terminal Condition .......................................................................................................................... 16

3.2.1 Modification of Terminal Condition .................................................................................. 16

3.2.2 Implicit Model for Roughness Progression ...................................................................... 18

4.......DEVELOPMENT OF THE MECHANISTIC PROCEDURE............................................................. 19

4.1....Broad Issues.................................................................................................................................... 19

4.2....Elastic Characterisation ................................................................................................................... 22

4.2.1 Isotropic or Anisotropic Characterisation? ....................................................................... 22

4.2.2 Values for Poissons Ratios ............................................................................................. 24

4.2.3 Relationship between Subgrade Modulus and CBR ....................................................... 24

4.2.4 Typical Modulus Values ................................................................................................... 25

4.2.5

Stress Regimes for Triaxial Testing of Granular Materials .............................................. 25

4.2.6

Sublayering of Granular Materials and Assignment of Moduli ......................................... 25

4.2.7 Modulus of Top Sublayer of Granular Material ................................................................ 26

4.2.8 Modulus of Granular Material Overlying a Cemented Layer ........................................... 27

4.2.9 Relationships between Modulus and UCS for Cemented Materials. ............................... 27

4.2.10 Characterisation of Cracked Cemented Materials ........................................................... 28

4.3....Performance Relationships ............................................................................................................. 28

4.3.1 Subgrade Strain Criterion ................................................................................................ 28

4.3.2 Fatigue Cracking of Cemented Material .......................................................................... 31

4.3.3 Fatigue Relationship for Asphalt ...................................................................................... 33

4.4....Design Traffic .................................................................................................................................. 34

4.4.1 Axle Loads Which Cause Equal Damage ........................................................................ 34

4.4.2 Derivation of Standard Axle (or Traffic) Factors ............................................................. 35

4.4.3

Cumulative Growth Factor for Estimation of Design Traffic ............................................. 39

4.5....Incorporation of Location-Specific Temperature Regime ................................................................ 40


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5.......DEVELOPMENT OF OVERLAY DESIGN PROCEDURE ............................................................. 41

5.1....Design Deflection Curves ................................................................................................................ 41

5.1.1 Adoption of IGPTD Curve 1 ............................................................................................. 42

5.1.2 Adoption of IGPTD Curve 4 ............................................................................................. 43

5.2....The Curvature Function ................................................................................................................... 43

5.3....

Temperature Correction for Deflection and Curvature .................................................................... 44

5.4....Reduction in Deflection Parameters due to Overlay Placement ..................................................... 46

5.4.1

Reduction in Maximum Deflection due to a Granular Overlay ......................................... 46

5.4.2 Reduction in Maximum Deflection (at 25C) due to an Asphalt Overlay ......................... 47

5.4.3 Reduction in Curvature Function due to an Asphalt Overlay ........................................... 47

5.5....Adjustment of Asphalt Overlay Thickness to Allow for Locality Temperature ................................ 47

REFERENCES ......................................................................................................................................... 48

APPENDIX A ORIGINS OF UNBOUND GRANULAR THICKNESS CHART ....................................... 55

APPENDIX B DMR NSW PROCEDURE FOR PAVEMENT THICKNESS DESIGN IN 1947

EXCERPT FROM BRITTON (1947) ................................................................................ 64

APPENDIX C

THE ORIGINS OF DESIGN DEFLECTION CURVES EXCERPT FROMJAMESON (1996) ............................................................................................................ 68


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TABLES CHAPTER 1

Table 2.1: Methodology and rationale for procedures in IGPTD .................................................... 5

Table 2.2: Revision of IGPTD initial proposal .............................................................................. 9

Table 4.1: Distribution of axle group type by state/territory .......................................................... 36

Table 4.2: RoRVL load distributions on axle groups according to axle group type ...................... 37

Table 4.3: Number of standard axles for same distress as axle groups with (non-rounded)

load distributions .......................................................................................................... 39

FIGURES CHAPTER 1

Figure 3.1: Presumed CRB, Victoria thickness design chart ......................................................... 13

Figure 3.2: Plot of roughness/initial roughness against cumulative ESAs for standard

design ESAs of 105, 10

6and 10

7(based on procedure F1 thickness correction

factors) ......................................................................................................................... 18

Figure 5.1: Predictive ability of deflection parameters ................................................................... 45


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Chapter 1: Technical Basis of the1992 Guide Procedures for Design of Flexible Pavements

David Potter

June 1999

SUMMARY

This chapter records the work undertaken in the development ofPavement Design A Guide tothe Structural Design of Road Pavements, initially published by the National Association ofAustralian State Road Authorities (NAASRA) in 1987 and subsequently revised and re-issued byAustroads in 1992. It briefly describes the predecessor and progenitor the Interim Guide toPavement Thickness Design(NAASRA 1979) and then proceeds to review the technical issuesencountered, and the solutions adopted, in the formulation of the Guide.

This material presented in the Guide represented many years of development in Australia andoverseas in design procedures for flexible pavements for highway traffic. The Guide wasdeveloped by a series of (then) NAASRA Working Groups representing both the members ofNAASRA and industry. Note that the names of the various road authorities relevant at the time(rather than the current names) are used throughout this report.

This report does not address the origins of Chapter 9 of the 1992 Guide the Design of RigidPavements which is the subject of another chapter in this document (Chapter 2).


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ARR 384 Technical Basis of Austroads Guide to Pavement Technology Part 2 Pavement Structural Design: Chapter 1 1


1 INTRODUCTION

This chapter records the work undertaken in the development ofPavement Design A Guide tothe Structural Design of Road Pavements, initially published by the National Association ofAustralian State Road Authorities (NAASRA) in 1987 and subsequently revised and re-issued by

Austroads in 1992. It briefly describes the predecessor and progenitor the Interim Guide toPavement Thickness Design (NAASRA 1979) and then proceeds to review the technical issuesencountered and the solutions adopted in the formulation of the Guide.

This report does not address the origins of Chapter 9 of the 1992 Guide the Design of RigidPavements which is be the subject of a separate chapter in this document.


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2 SETTING THE SCENE

2.1 The Interim Guide to Pavement Thickness Design

The first document relating to pavement design to be produced conjointly by Australian road

authorities was the Interim Guide to Pavement Thickness Design known by its acronym IGPTD.The document was produced by the National Association of Australian State Road Authorities(NAASRA), the (then) umbrella organisation of the Australian State Road Authorities (SRAs). Itwas drafted by a sub-committee of NAASRAs Materials Engineering Committee (MEC) and vettedprior to publication by its Principal Technical Committee (PTC).

In 1975, a recommendation by the NAASRA Materials Engineering Committee (MEC) resulted in adirection from the Principal Technical Committee (PTC), at its 45th (September 1975) meeting, thata manual on pavement thickness design be prepared and that a Sub-Committee of MEC beformed to pursue that task, with the manual to be vetted by PTC prior to its publication. Themembers of the Sub-Committee were:

Alan Leask Department of Main Roads, NSW (DMR, NSW), Convenor

Peter Lowe Country Roads Board, Victoria (CRB, Vic.)

Ray Elliott Department of Construction (DoC)

Zandor (Vlas) Vlasic Main Roads Department, Qld (MRD, Qld)

Lester Goodram Main Roads Department, WA (MRD, WA)

John Scala Australian Road Research Board (ARRB)

The initiation of the project was possibly stimulated by the attendance of John Scala, Vlas Vlasic,Len Chester (Highways Department, SA (HD, SA)) and Alan Leask at the Fourth InternationalConference on the Structural Design of Asphalt Pavements, which was held in London in 1972. In

addition, there was an increasing desire by the MEC members for increased national cooperationand the achievement of a uniform national approach on the subject.

As instigator and Chairman of the project, Alan Leask was considered to be the driving forcebehind the development of the IGPTD. However, all MEC members provided full support.

In formulating the sections on flexible and semi-rigid pavements, the Sub-Committee membersdrew particularly on the research conducted by John Scala, while Ray Elliott contributedsubstantially to the section on rigid pavements. Meetings were frequent, usually lasting about fourdays at a time, and, as the Sub-Committee reported to MEC, input from other members of MECwas also significant. Moreover, as time went by there were changes in the composition of MECand the Sub-Committee. Other participants who made valuable contributions to the refinement of

the document included Len Chester, Eric Brown (DoC), Ralph Rallings (Department of MainRoads, Tasmania (DMR, Tas)), Rod Payze (HD, SA), Peter Rufford (DMR, NSW) and Ed Haber(DMR, NSW).

In principle it was postulated that an ideal pavement design procedure should be one which wouldpredict a thickness and composition which, without being conservative, would ensure that thepavement would not deteriorate beyond a tolerable level of serviceability in less than the chosendesign period. In practice, this would facilitate the planning of a maintenance and rehabilitationregime commensurate with the selected design period, and thus would permit the comparison ofdesigns comprised of different compositions on the basis of total whole-of-life costs, as distinctfrom initial cost only.


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The August 1978 draft was considered by the 33rd(September 1978) MEC, which agreed that thedocument would be ready for publication after some editorial and technical amendments, whichhad been agreed to at the meeting, were made. The document was then forwarded to PTCseeking their approval to publish.

Although the document records 1979 as its publication date (the Foreword is dated April 1979), itwas mid-1980 before it was released and then only on a very restricted basis. Individual copieswere numbered and it was not available for sale. The reason for this approach may be found inthe following sentence in the Foreword:

It is stressed that the document is interim in nature and that it is proposed that it be

reviewed after about two years, in the light of comments received, experience

obtained and further research.

Prior to its release, NAASRA PTC had taken a formal decision to review the status of thedocument before 1982. Had the decision to publish, albeit with this proviso, not been taken, then itis doubtful whether another attempt would have been made for a considerable period of time. Inthis regard, the attitude was taken that the document, though admittedly lacking in many respects,

had to be exposed in order to ensure that feedback was obtained and that further research wasconducted to ensure that the necessary knowledge was acquired to allow these refinements totake place.

The design systems included in the IGPTD were initially based on the approaches adopted by thevarious SRAs. However, the compilation process involved a considerable amount of definition,interpretation, rationalisation, compromise and innovation. Because the document was intended tobe applicable over the whole of Australia, involving diverse materials and environments, andbecause it might be used by other organisations having varying degrees of expertise andresources, some parts were deliberately broadly based in order to allow a hierarchical approach tobe taken to the evaluation of the input parameters and the design procedures.

The task of compiling the IGPTD was enlightening in that it emphasised the inadequacies of thetraditional systems, particularly with respect to pavements incorporating bound layers. Of specialconcern was the paucity of performance-related data substantiating the criteria used, and the lackof guidance regarding the evaluation of the parameters required in the thickness design process.By the same token, it was emphasised that there was little alternative than to implement newermethods, some of which were innovative and which would need refinement when compatibleperformance data became available.

Identification of the deficiencies which inhibited the unqualified implementation of the IGPTD didstimulate awareness of the need for more precise criteria and data, and resulted in a number ofresearch projects.

2.1.1 Format of the IGPTD

The format of the IGPTD was as follows:

Part 1 Information on factors affecting pavement design and performance

Part 2 Flexible pavement design procedures

Part 3 Rigid pavement design procedures

Part 4 Summary providing all the necessary information to carry out a design


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Whilst the detailing of the procedures for the design of the various types of pavement describedwas a primary objective of the Guide, another significant feature of the document was related tothe definition of the factors affecting the design procedures, especially the elaboration, inconsiderable detail, on the appraisal, in quantitative terms, of the evaluation of subgrade strength,traffic loading, etc.

For the design of new flexible pavements, the following four procedures were developed:

F1: Granular pavement with thin (< 25 mm) bituminous surfacing

F2: Granular pavement with asphalt surfacing up to 100 mm

F3A: Pavement containing a bound layer or with asphalt > 100 mm (see below)

F3B: Pavement containing a bound layer or with asphalt > 100 mm (see below)

In addition, the following procedures were developed for the design of overlays and rigid

pavements:

F4: Overlay design

R1: Rigid pavement design

R2: Rigid pavement design

Table 2.1 (after Potter 1981) lists, for procedures F1 to F4, the essential steps, together with a noteon the rationale behind each step.

The core of procedure F1 was the (now) familiar CBR-thickness-traffic chart. Its development is

discussed in detail later.

The basis of procedure F2 was estimation of the candidate pavements maximum deflection (froma quasi-elastic analysis), this being checked against a specified tolerable value which was afunction of both asphalt thickness and design traffic.

In the context of this brief overview of the design procedures, discussion of procedure F3B logicallyprecedes discussion of procedure F3A. Procedure F3B involved determining, firstly, the requiredthickness of granular cover (from procedure F1) and then substituting asphalt and/or cementedmaterial for the granular material on the basis of a table of layer equivalencies (thicknesses ofgranular material equivalent to unit thickness of the bound material). For asphalt, the layerequivalence depended on the climatic zone, design traffic, and depth below the surface. For

cemented material, it depended only on depth below the surface1

.

Procedure F3A contained elements of both F1 and F2. The first step was to determine (usingprocedure F1) the total (granular) cover requirement. Tentative thicknesses were then assigned toeach layer in the desired configuration. A check was then made, for each granular layer in thistentative structure, to ensure that there was sufficient Thickness of cover over it (regardless of typeof cover) the requirement being that specified in F1. Layer thicknesses were adjusted (ifnecessary) to satisfy this requirement. A full elastic layer analysis was then carried out on thepavement to estimate its maximum deflection under a Standard Axle.

1 With the load equivalence value being dependent on the depth below the surface, the issue that has never

been clear to the author is does this depth below the surface refer to the depth within the granular pavement

where the granular material is being replaced, or does it refer to the depth at which the substituted materialfinds itself in the final pavement configuration? The different interpretations result, in some situations, in non-trivial differences in final configurations (e.g. 265 mm cf. 300 mm for full-depth asphalt enough to besignificant in alternate tender situations).


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Table 2.1: Methodology and rationale for procedures in IGPTD

Procedure Method Rationale

F1 Determine subgrade design CBR

Estimate cumulative traffic over design life in terms of ESAs AASHO Road Test (relative damage)ERVL Study (Stevenson 1976)

(axle vehicle loads)

Determine thickness of cover (T) required for subgrade condition andestimated traffic

California Highways Department (Porter 1938)TRRL, SRA experience

Modify T for desired terminal roughness Scala (1977); (AASHO Road Test data)

Check minimum base requirement SRA experience

F2 Determine thickness of cover using F1

Determine pavement stiffness factor Limited modular ratio of adjacent layers(Heukelom and Klomp 1962)

Determine deflection under a Standard Axle Elastic layer theory

Reduce deflection by 5% for each 25 mm of AC surfacing SRA experience, Scala (1973)

Check reduced deflection against tolerable deflection SRA experience; Scala (1973)


F3A Determine thickness of cover (T) using F1 See procedure F1

Select a pavement composition having total thickness T

Adjust pavement layer thicknesses to satisfy cover requirements (from F1)for each unbound layer

Assign values to elastic parameters (Youngs modulus, Poissons Ratio)for each layer

Laboratory, field investigations

Estimate deflection under a Standard Axle Elastic layer theory

Check estimated deflection against tolerable deflection and, if less, reducethickness of bound layers

SRA experience; Scala (1973)


F3B Determine thickness of cover (T) using F1 or F2 (as appropriate) See procedures F1, F2

Determine individual layer thicknesses (ti) from suggested layer

equivalencies (ai) and relationship: T = aiti


F4 Determine representative deflection under a Standard Axle

Estimate traffic over design life in ESAs AASHO Road Test (relative damage)ERVL Study (Stevenson 1976)(axle vehicle loads)

Check representative deflection, dREP, against tolerable deflection, dTOL SRA experience; Scala (1973)

If dREP< dTOL, adopt nominal overlay

If dREP> dTOL, design structural overlay

For granular overlay, determine thickness on the basis of a 10% deflectionreduction for each 45 mm of overlay material

SRA experience

For AC overlay, determine thickness using recommended percentagereduction in deflection per 25 mm for the specific climate zone

SRA experience

The use of CIRCLY (Wardle 1977) was recommended for this analysis, provided adequatecomputer facilities were available. Otherwise, the use of the 2-layer or 3-layer tables developed byCSIRO (Gerrard 1969; Gerrard and Wardle 1976) was recommended. The estimated deflectionwas assessed against a specified maximum tolerable value which was a function of pavementcomposition and design traffic. The thickness of the bound layer was then adjusted (down) and themaximum deflection re-calculated until the requirement was (just) satisfied.


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The interim nature of the IGPTD is well attested by the qualifying comments within the documentregarding the use of procedure F3A. In introducing the alternative design procedures F3A andF3B, the document includes the following text (under the heading Qualification):

Procedure F3A is suggested as a method which can be developed to satisfy the

need for a completely satisfactory means of selecting the thickness of a flexible

pavement, the composition of which includes one or more layers of boundmaterials. The basis of the procedure is a comparison between the surface

deflection of a proposed pavement with the deflection that is assumed to be

indicative of the pavement capacity to produce the design performance. This

tolerable deflection is recommended on the basis of the recorded performance of

similar pavements. The deflection of the proposed pavement is estimated by an

elastic analysis of its behaviour under the action of a standard wheel load.

The present recommended values of tolerable deflection are based on the best

information currently available for Australian conditions. However, whilst they are

considered adequate as a secondary control over a primary design criterion, e.g.

subgrade CBR, they cannot be regarded as sufficiently well established to warrant

their acceptance as a single or primary basis for design. Thus, Procedure F3A, in

its present state of development, is recommended only as a conservative control

over those methods currently used, i.e. the substitution of bound for unbound

materials on the basis of empirically established equivalency factors. Such a

comparative use will encourage the accumulation of performance data which can

be expected to improve the accuracy of the method to a stage at which it provides

an acceptable degree of confidence in the ability of the selected pavement to

perform as designed.

The specific introduction to procedure F3B includes the following text:

The use of Procedure F3A is inhibited at the present time principally by the

difficulties of determining the appropriate values of moduli and also, by the current

lack of verification of the proposed tolerable deflection criteria. As an interim

measure, these restrictions are avoided in practice by the use of empirical

equivalency factors.

Procedure F3A was, in essence, (what is now called) a mechanistic design procedure. It involvedthe use of a Response Model (CIRCLY or tables) to determine a single critical response (maximumsurface deflection) of the candidate pavement to Standard Axle loading. The performance of thecandidate pavement was estimated from the (configuration-dependent) plot of tolerable maximumdeflection versus cumulative traffic (ESAs). The IGPTD recognised the relevance of the maximumvalues of both vertical strain at the top of the subgrade and horizontal strain at the bottom of boundlayers. However, it stated that:

limiting values for these critical design criteria are not well established for

Australian conditions, and it would be inappropriate to adopt overseas criteriawithout verification. Moreover, these particular criteria are difficult to measure and

it is unlikely that they would be monitored against pavement performance in order

to establish such relationships.

In support of maximum surface deflection as an estimator of horizontal tensile strain at the bottomof a bound layer, it made the following statement:

limited performance data from the ARRB and some state road authorities has

indicated that the vertical surface deflection provides a reasonable premise for

design in most practical situations. It is considered that a limiting vertical surface

deflection criterion does, for all practical purposes, control tensile strain at the

bottom of a layer. This is because the tensile strain is governed by the radius ofcurvature of the deflection bowl on loading which has been shown, for a given

pavement material and thickness, to correlate reasonably well with the maximum

surface deflection.


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It also stated:

The further advantage of the vertical surface deflection criterion is that it is easily

measured in the field and is, therefore, more suitable for verification of the design

procedure than would be other criteria.

With respect to the vertical strain on the top of the subgrade, the document states that, to inhibitloss of surface shape and ancillary surface cracking:

It is, therefore, essential to limit the subgrade deformation and this is achieved by

limiting the vertical compressive strain at the top of the subgrade. It should be

noted in this regard that such an approach supports the rationale of the CBR

method of design, which, by requiring a minimum thickness of cover over any

material (characterised by its CBR value) at a given level in the pavement, directly

ensures that the vertical compressive strain at the level does not exceed an

implicitly defined acceptable value.

It further states that:

...as discussed above, if the design procedure also satisfies CBR design thicknessrequirements, it provides a basis for controlling vertical compressive strain in the

subgrade.

The overlay design procedures (F4) involved calculating the thickness of overlay required toreduce the representative deflection of the section to a tolerable level. In the case of a granularoverlay (unbound), the thickness required was based on each 45 mm thickness of overlay reducingthe representative deflection by 10%. The reduction in surface deflection for a given thickness ofasphalt overlay is dependent, among other things, on the operating temperature of the material inplace. As this varies according to climatic conditions, five separate deflection reduction factorswere presented, ranging from 12% in the coldest climatic zone to 6% in the warmest. Since thetolerable deflection also decreases as the thickness of asphalt overlay increases, an iterative

process was used to estimate the thickness required to reduce the representative deflection to asufficient extent.

The procedures for rigid pavement design, R1 and R2, were virtually the same as those adoptedby the US Portland Cement Association (PCA). The R1 procedure was appropriate when thequality of the traffic data was such that both axle type and load distribution, and the number ofrepetitions of each axle/load combination, could be predicted. The procedure was based on thefatigue concept, in that it was assumed that, as the flexural stress ratio (ratio of flexural stresscaused by the wheel load to the concrete design flexural strength) decreases, then the number ofload repetitions to failure increases. When the stress ratio was less than 0.5 it was assumed thatthe concrete could sustain unlimited stress repetitions without loss of load-carrying ability.Conversely, at high stress ratios, only a limited number of the heavier loads could be sustained

before the concrete failed. Therefore it was essential that projected traffic estimates, especiallywith respect to the mass and numbers of heavier axle loads, were reliable, since these virtuallycontrolled the pavement thickness design.

The more common rigid pavement design procedure, R2, was used when the total trafficcomposition could not be predicted and the loading had to be estimated based on the number ofcommercial vehicles. The procedure was derived from procedure R1 by utilising data from theNAASRA (1976) Economics of Road Vehicle Limits (ERVL) study. The effect of traffic wasaccounted for by calculating the thickness of concrete required for unlimited stress applications ofthe most critical axle type at its design load, and then reducing that thickness to account for theexpected number of repetitions of the design axle load, which was assumed to be a fixedproportion about one design axle per 1000 commercial vehicles of the total commercial vehicle

traffic. The effect of concrete strength and subgrade support on pavement thickness wasaccounted for by applying suggested percentage variations in pavement thickness.


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Details of reinforcing and jointing techniques applicable to both procedures were also included inthe IGPTD.

2.1.2 Release of the IGPTD

The IGPTD was launched at a two-day NAASRA/ARRB Seminar on Heavily-Trafficked Flexible

Pavements, held at ARRB in June 1980. (The document was actually released three weeks afterthe launch). Attendance was strictly by invitation only.

Of the 54 delegates, the only non-SRA/ARRB attendees were as follows:

R. Brewis, K. Kiesel Australian Asphalt Pavement Association (AAPA)H. Luckhurst-Smith, J. Sutton Shell Company of Australia LtdR. Elliott Cement & Concrete Association of AustraliaB. Heaton University of NewcastleI Lee University of New South WalesJ Morgan Golder AssociatesR Sharp University of Sydney

K. Wallace James Cook University of North Queensland

It is of interest to note (in light of subsequent developments) that the following all participatedactively in the seminar (presentation of papers, chairing of syndicate sessions, panel discussion,etc.): Alan Leask, Peter Lowe, John Bethune, David Anderson, Ron Gordon, David Potter andGeoff Youdale. Jack Morgan, in his concluding remarks, made the following statement:

I believe this Guide is a document that we can justifiably be proud of as an

Australian product. There are many sound and even innovative approaches

described, and when we recognise that this forms a consensus of SRA thought on

this topic there must be high hopes that the standard of pavement design will

continue to improve. There is obviously work to be done in filling in the blanks, and

even deleting some material I believe to be now of only historical interest.However the Guide provides a framework for collecting information and highlighting

significant areas of needed research.

In commenting on Geoff Youdales excellent paper summarising the (then) state-of-the-art ofrepeated load testing of pavement materials (Youdale 1981), Morgan stated:

Some 15 years ago academics were trying to promote repeated load testing to the

SRAs. Now at least one SRA has taken it up and who knows but in another 15

years the results may even be used!

Very prescient of him!

The Seminar Proceedings were printed some 10 months later (Sparks 1981), with availability beingrestricted to SRAs and Seminar attendees. The delay in printing the Proceedings was probablyattributable to the usual delay in collating written reports from syndicate Chairmen, etc., coupledwith concerns about the sensitive nature of some of the data presented presumably an allusionto the (then) recent significant pavement failures. The reasons behind the delay in releasing theIGPTD are not fully clear. However, it is understood that AAPA (represented by Ken McKenzie,Ollie OFlynn, Ron Ekberg, etc.) had serious concerns about the layer equivalency values forasphalt incorporated in procedure F3B, and expressed their concerns to NAASRA.

2.2 Establishment of a Working Group to Revise the IGPTD

The initial steps taken to institute a revision of the IGPTD were as follows:

In March 1981, NAASRA MEC decided to submit to PTC a detailed proposal recommendingthat revision of the IGPTD commence in 1982.


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In August 1981, a Sub-Committee of MEC met and formulated the detailed proposal whichwas subsequently submitted to PTC. The author attended the meeting by invitation. It is ofinterest to note MECs initial intentions. Table 2.2 presents the proposed layout of therevised IGPTD, together with the (proposed) responsible individual(s).

The Working Group which was proposed to undertake the components requiring major

revision comprised:

David Potter ARRB (Convenor)David Anderson CRB, VictoriaRon Gordon MRD, QueenslandGeoff Youdale DMR, NSW

Gavin Donald (DMR, NSW) was to undertake overall editing.

The September 1981 meeting of the PTC approved the proposal.

The Working Group met for the first time in December 1981.

Table 2.2: Revision of IGPTD initial proposal

Proposed Format Extent of Revision Person(s) Responsible

1. Introduction Total re-write R Payze

2. Scope Total re-write R Payze

3. Terminology Total re-write A Leask

4. Choice of pavement type Total re-write L Goodram

5. Basis for design Total re-write P Lowe

6. Evaluation of design parameters(a) moisture(b) subgrade CBR

(c) subgrade modulus(d) pavement materials(e) traffic(f) evaluation for rehabilitation

Total re-writeTotal re-write

Total re-writeMajor revisionTotal re-writeTotal re-write

R PayzeR Payze

R PayzeWorking GroupR PayzeR Payze

7. Design procedures for flexible pavements

F1F2F3

Total re-writeMajor revision

To Total re-write

Working GroupWorking GroupWorking Group

8. Design procedure for rigid pavements Total re-write A Leasksupported byE Haber, J Cruickshank, R Elliott

9. Design Procedures for Overlays major revision Working Group

10. Observations Total re-write P Lowe

In broad terms, the scope of the task assigned to the WG may be summarised as follows:

for the design of chip-sealed granular pavements, retain procedure F1 (in the IGPTD), i.e.the CBR Thickness Traffic of cover chart.

for the design of other flexible pavements, devise a mechanistic procedure consistent withthe F1 procedure

for the design of overlays, devise a procedure consistent with the above two procedures fornew pavements.

Alan Leask was retained as Convenor of the review Steering Committee and remained so until hisretirement from MEC in 1983. He was followed as Convenor by Peter Lowe and then by GavinDonald, who undertook the final editing.


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2.2.1 A Guide not a Manual

The decision to title the revised document a Guide (consistent with the original IGPTD) rather thana Manual reflected the policy of (the then) NAASRA to foster among its members harmonisation ofstandards, practices, etc. rather than foisting upon the members sets of mandatory rules. Itsreplacement, Austroads, pursues a similar policy. Further, the intent of the document was to

guide, educate, and provide assistance in deciding among alternative options, etc.


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3 GRANULAR PAVEMENTS WITH THIN BITUMINOUSSURFACINGS

3.1 Origins of the CBR-Thickness-Traffic Chart

The WG was directed to retain the CBR-Thickness-Traffic chart for design of chip-sealed granularpavements because it was the consensus view of pavement designers at that time (early 1980s)that pavements designed in accordance with it provided field performance consistent with designexpectations. Because it was retained in the 1992 edition of the Guide (as Figure 8.4) andbecause it constitutes a foundation stone for the mechanistic procedure in the Guide, a review ofits origins is considered appropriate.

Within Australia, the (then) Country Roads Board of Victoria (CRB) led the way in the developmentof thickness design for flexible pavements. Hence, following the developments within the CRBprovides an appreciation of the Australian scene.

Jameson (1996) has produced a most adequate review based predominantly on materialassembled by the author. The following comments are supplementary to Jamesons review andneed to be read in conjunction with it. It is attached as Appendix A. The interested reader is alsoreferred to an earlier review by Anderson of the evolution of pavement design within the CRB(Anderson 1981).

The appropriate starting point can be considered to be Porters development of the (laboratory)CBR test (Porter 1938). Using this test to characterise subgrades, the California State HighwayDepartment, in reviewing the performance of its roads over the period 1929-1938, found that soilhaving a certain CBR always required the same thickness of flexible macadam construction on topof it in order to prevent plastic deformation of the soil (Davis 1949). The curve relating the requiredthickness of flexible macadam to subgrade CBR is that labelled 7000-lb. wheel load (Light Traffic)

in Jamesons Figure 2 (the curve was originally unlabelled). Davis goes on to state that:The curve for the wheel load of 12,000 lb. was added later as the result of further

experience in California of heavier traffic conditions subsequent to 1938. The

curve for the wheel load of 9,000 lb. has been obtained by interpolation between

the curves for the wheel loads of 7000 and 12,000 lb. It is an implied assumption

of these curves that all kinds of flexible construction of the macadam type spread

the load to approximately the same extent.

Hence, the first CBR design chart was the single (uppermost) curve in Jamesons Figure 2 (seeAppendix A). It is to be noted that the CBR value refers to the subgrade only and that the materialto provide the thickness of cover is a flexible macadam. The Figure covers subgrade strengths upto CBR 80, requiring approximately 3 inches (75 mm) of cover on the CBR 80 subgrade.

Having monitored the rapid developments in California post 1942, the CRB released JamesonsFigure 3 (CRB 1945). The most spectacular progression was in traffic characterisation. From the1942 characterisation of traffic in terms of a maximum wheel load with unspecified repetitions, by1945 it was characterised by the two-way cumulative number of repetitions over the design periodof equivalent 5000 lb wheel loads2.

2For a wheel load of L lb, one repetition was equivalent to 2

(L5000)repetitions of a 5000 lb wheel load. This

corresponds to a Power Law exponent in the range 3.8-4.7.


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The factors taken into account in estimating cumulative traffic were:

day(s) of the week on which the relevant survey was conducted (every day had a differentweighting)

pavement width (to account for transverse distribution),

growth rate

design period

climatic factor (a multiplier whose value was average rainfall (inches) x average wet days per

year 10,000).

The Figure covers subgrade strengths up to CBR 80 (as previously), with no cover required on theCBR 80 subgrade (cf. 3 inches (75 mm) previously).3

However, by 1949, with the release of its Technical Bulletin No. 4 (CRB 1949), characterisation oftraffic for conventional design situations had reverted to 1.5 times the average number of trucks

and buses (in both directions) in a 12-hour day count essentially average daily commercialvehicles. For unusual traffic situations the 1945 approach was retained. Technical Bulletin No. 4also introduced estimation of (laboratory soaked) CBR from gradings, Atterberg Limits and LinearShrinkage. The design chart provided for subgrade strengths up to CBR 20 (c.f. 80 previously),with a minimum cover requirement of (approx.) 3 inches (75 mm).

Ten years after the release of Technical Bulletin No. 4, H.P. George (CRB) and C.A. Gittoes (DMR,NSW) included in their report to the 1959 PIARC Congress (George and Gittoes 1959) a thicknessdesign chart in a format very similar to the one currently in the Guide except that design trafficwas expressed as repetitions of a 5,000 lb wheel load, as shown in Figure 3.1. Hence, despite theissue of Technical Bulletin No. 4, some interest remained in the characterisation of traffic in termsof cumulative repetitions of a standard loading.

At this same PIARC Congress, MacLean (1959) presented Jamesons Figure 5 as the then statuswithin the UK. The UK development work behind this chart is well described by Jameson.Subsequent to the Congress, CRB rapidly embraced the thickness design chart presented byMacLean, issuing it in Technical Bulletin No. 21 in the following year (CRB 1960).

The chart is a series of curves providing required depth of construction according to subgrade CBRfor seven ranges of daily traffic daily traffic being determined as per Technical Bulletin No. 4except that the vehicles to be counted changed from trucks and buses to vehicles exceeding 3tons loaded weight. As Jameson notes, the three curves for mid-range traffic align well withCalifornias original three curves (for wheel loads of 7000, 9000 and 12,000 lb). The chart coverssubgrade strengths up to CBR 150 (c.f. 20 previously) and indicates a minimum cover requirement

of 2 inches (50 mm). However, in the text the minimum thickness requirement (of CBR > 80material) is stipulated to be from 3 inches to 8 inches (75 mm to 200 mm), depending on the trafficclassification (no minimum thickness requirement was specified for the highest traffic classification presumably an oversight). Technical Bulletin No. 21 also saw the introduction of the static anddynamic cone as a basis for estimating subgrade CBR.

3For the reader who is interested in where the (then) DMR, NSW stood in relation to pavement thickness

design at about this time, its procedure current in 1947 is attached as Appendix B. It is an extract from a

paper by A.T. (Sandy) Britton to the 1947 Meeting of the Highway Research Board (HRB) (Britton 1947).The pavement design was primarily based on classification testing of the subgrade and pavement materials.This was supplemented by utilising in situ CBR tests and CBR tests on samples conditioned to predictedmoisture conditions in the 1960s.


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In 1969, CRB issued Technical Bulletin No. 26, which superseded Technical Bulletin No. 21. Thecurves were re-drawn to reflect these minimum thickness requirements, while retaining coverage ofsubgrade strength up to CBR 150. An additional curve was added for unsealed shoulders. Fortraffic determination, the multiplier applied to the 12-hour count data increased from 1.5 (TechnicalBulletins Nos. 4 and 21) to 3, i.e. design traffic was doubled for the same project traffic.

Source: (George and Gittoes 1959)

Figure 3.1: Presumed CRB, Victoria thickness design chart

The CBR-thickness-traffic chart for granular pavements with chip seals in Technical Bulletin No. 26(Jamesons Figure 6), together with similar charts then in use in other SRAs, provided the basis forthe chart presented (as Figure 2.2) in the IGPTD (Jamesons Figure 7). According to Black (1977),the traffic classifications in Technical Bulletin No.26 (daily two-way volumes of vehicles exceeding3 tons loaded weight) were converted to cumulative one-way ESAs over the design period on thefollowing basis:

.....the following assumptions were made:

(i) One commercial vehicle equalled one equivalent standard axle.

(ii) The traffic was equally divided between the two directions.

(iii) A design period of 20 years was adopted.

(iv) The commercial vehicle traffic category was characterised by the average

commercial traffic volume in the category and this was taken as the average value

over the design period.

Jamesons interpretation that the daily traffic volumes were considered to be end-of-life volumesafter 3% per annum growth appears to be an over-complication of what actually transpired.

Table 2.11 of the IGPTD lists the number of ESAs per commercial vehicle (according to State andRoad Functional Class) which was recommended for use at the time. The Table entries werederived from the (1974) ERVL Survey data. For Victoria, the values were 1.4, 0.8, 1.2, 0.7 and 0.8for Functional Classes 1, 2, 3, 6 and 7 respectively. Taken across all States, the Table suggeststhat (rough) average values for rural and urban roads are 1.2 and 0.8 respectively. Hence, thevalue of 1 adopted for the translation is a good average value.


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It is the authors recollection that the formula attached to the IGPTD chart is not in exact agreementwith the chart, i.e. the chart was established prior to, and independent of, the formula.

3.1.1 Quality of Pavement Material and its Cover Requirements

During the course of evolution, later versions of the thickness design chart included cover

requirements, not only over the subgrade, but also over any placed material (the cover requirementbeing determined inter alia by the CBR of the placed material).

With regard to the evolution of quality requirements for cover material, Davis comment on the earlyCalifornia curves that It is an implied assumption of these curves that all kinds of flexibleconstruction of the macadam type spread the load to approximately the same extent has alreadybeen noted.

The Road Research Laboratory (1955) Road Note 20 characterises both subbase and basematerial by their CBR values, with base material having a CBR > 80 and the maximum contributionof a subbase to the subgrade cover requirement being the subgrade cover requirement minus thesubbase cover requirement. Hence, the Leigh and Croney (1972) statement quoted by Jamesonthat the design curves

....provided a means for estimating the total thickness of construction necessary for

various traffic and foundation conditions, but gave no guidance on the relative

thicknesses of surfacing, base and subbase.

is somewhat unfair to Road Note 20.

In Technical Bulletin No. 21, materials were allotted a Design CBR value. With the exception offine crushed rock and macadam (both allotted CBR values of 100), a material was allotted on thebasis of its grading and PI a CBR value (in the range 3-50) and also a minimum coverrequirement over itself (independent of design traffic).

The minimum thickness of CBR > 80 material has already been noted. The Bulletin states that theDesign CBR values should be used in the pavement design, presumably in the (now) conventionalmanner.

Technical Bulletin No. 26 states that:

The pavement itself should be made up of materials increasing in strength and

stability towards the top. Where the California Bearing Ratio of the pavement

material is known the chart provides a guide to the depth at which the material may

be used but other factors such as grading and plasticity index may be more

important than the actual California Bearing Ratio.

However, further on it states that:

Where the local materials are cheap but of poor quality they should be used in

substantial thicknesses since:- (i) While the CBR design method makes no specific

allowance for the quality of the pavement material, the design curves may be

considered as based on average quality materials. The ability of the poorer

materials to spread the load onto the subgrade will be less than for good materials.


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The IGPTD is most specific and unambiguous. It states in Section 2.1.4 that:

The total thickness of a granular pavement may be made up of a base and any

number of subbases. The composition of the pavement is made up by providing

sufficient cover over the subgrade and each successive subbase. The thickness of

cover required over a subbase is determined from its design CBR. If this CBR

value is less than 30, the cover required is determined as for a subgrade material,namely from Fig. 2.2 or Equation 2.1. For a subbase with a design CBR greater

than 30, it is necessary to provide a minimum thickness of a base material with a

CBR of 80 or above.

3.1.2 Concluding Comments

It is worthy of note that, during the development described above, essentially the only finding fromthe (1959-61) AASHO Road Test which was picked up was the adoption (in 1979 by the IGPTD)of the ESA as a basis for quantification of traffic loading. This provided no conceptual advanceover Californias in-place use of the 5000 lb wheel load coupled with its basis for determiningequivalent repetitions for different load magnitudes. The (probable) refinement was the adoptionof a Damage Exponent of 4 derived by Irick and Hudson (1964) subsequent to the AASHO RoadTest4.

The main advantage of a review of this nature is that it assists in getting things into perspective.As with any investigation relating to the performance of roads, one comes away feelingdisappointed at the abysmal lack of performance data (except at the macroscopic level).

Although the CBR design method has been in use for over 50 years, there are still misconceptionsof the philosophy and intent of the procedure. For example, Rodway (1997), in an overview ofmechanistic pavement design, makes the following statements in relation to the method:

The CBR design method is based on a failure mode that involves loss of shape of thepavement surface caused by overstressing the subgrade.

The empirical CBR method implies that loss of surface shape is primarily caused byoverstressing the subgrade. Deformation within the pavement layers is not directlyaddressed by the method.

The CBR design procedure involves increasing pavement life by increasing pavementthickness to further protect the subgrade, not by improving the pavement materials.

It needs to be clearly understood that the CBR design procedure requires inter alia that:

the uppermost material be of (relatively) high quality and of thickness in excess of a specifiedminimum

a minimum thickness of cover be provided over each of the other materials comprising the

pavement, with this minimum thickness increasing as material quality decreases

the minimum thickness of uppermost material and minimum thicknesses of cover over thepavement materials be increased as design traffic increases.

The intents of these requirements are that:

the stresses imposed on each and every material comprising the pavement be contained inorder to limit the development of permanent deformation within the material

the stresses imposed on each material be concomitant with design life requirements.

4 While granular pavements with a chip-seal surfacing were incorporated in the design of the AASHO RoadTest and were constructed and trafficked, because of their very rapid failure (attributed to poor constructioncoupled with the effects of cyclical freezing and thawing), their performance (to the authors knowledge) wasnot reported.


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Hence, it is contended that the above statements do not do sufficient justice to the CBR designprocedure because they do not acknowledge the provisions within the procedure for limiting thestresses imposed on (and hence the deformations developed within) the constituent pavementmaterials.

However, it needs to be borne in mind that, at the time of the development of the procedures, trucktyre pressures were below 600 kPa. In recent times, pressures of 900 kPa are not uncommon,leading to considerably increased stress levels within the pavement near the surface. Beingempirically based, the CBR procedure does not have a mechanism for incorporating tyre pressureas a design variable.

It should also be noted that the CBR test was initially developed as a tool for pavement thicknessdesign, with a CBR of 100 indicating that a material did not require further cover. The CBR valuehas since evolved into many material specifications where it provides an indicator of shear/bearingstrength. As the test involves pushing a plunger into a rigidly-constrained sample, its use forestimating the shear strength of base materials, particularly those with large particle sizes, is oflimited value.

3.2 Terminal Condition

Implicit in the design procedure for these pavements (Section 8.3 and, specifically, Figure 8.4 ofthe Guide) is a terminal condition which is considered to be unacceptable and, hence, signifies theend of life for the pavement. Quantification of this condition could be expected to be in terms oflevel of roughness and severity and extent of rutting.

As noted above, this design procedure was taken across (essentially unaltered) from the IGPTD.Unfortunately, no statement of terminal condition accompanied it. The view of the MEC ReviewCommittee at the time was that, in terms of rutting, it probably represented an average rut depth ofabout 20 mm. In terms of roughness, the procedure in Section 7.8 of the Guide (which provides a

basis for altering design traffic to achieve a specific terminal roughness) provides some guidance.This procedure (which was also taken across from the IGPTD) is based on the premise that theterminal roughness of pavements designed in accordance with Figure 8.4 is three times the initialroughness. This premise was formulated by John Scala (ARRB) based on inter alia analysis ofperformance data from the AAHSO Road Test. There appears to be no record of the developmentof it.

3.2.1 Modification of Terminal Condition

For the design of flexible pavements whose critical distress mode is permanent deformation of thesubgrade and granular layers, the use of Figure 7.2 allows the designer to select a pavementdesign which has a designer-specified terminal roughness (relative to its initial roughness). To

achieve this, the designer simply enters Figure 7.2 with a value for the Design Traffic and also avalue for the desired ratio of terminal to initial roughness and reads from the Figure a value forModified Design Traffic. This value is then used in the design procedure in lieu of the DesignTraffic.

The relationship plotted in Figure 7.2 was derived in the following manner.

The IGPTD (Section 2.1.3) provides for granular pavements with thin bituminous surfacing abasis for altering the thickness of cover over the subgrade to achieve a designer-specified terminalroughness value.

The thickness modification is expressed in the following relationship:


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T = t. [2R1/(R2 - R 1)]0.25 1

Where

R1 is the expected initial roughness

R2 is the desired terminal roughness

t is the thickness of cover determined from IGPTD ProcedureF1 (Figure 8.4 in the 1992 Guide)

T is the modified thickness of cove

This relationship is the foundation for the relationship in Figure 7.2.

One further piece of information from the IGPTD (Section 2.1.2) which is relevant here is that theCBR-thickness-traffic relationship in Figure 8.4 of the 1992 Guide is as follows:

T = [219 211 (log10CBR) + 58(log10CBR)2]. log10(N/120)

Wheret (mm) is the thickness cover required over a material of given

CBR when the design traffic is NESAs.

This equation can be rewritten more succinctly as:

t/f(CBR) = log10( N/120) 2

Where

f(CBR) = 219 211(log10CBR) + 58(log10CBR)2

The Modified Design Traffic denoted by NM is simply the Design Traffic associated with themodified thickness of cover T.

Hence, from Equation 2:

T/f(CBR) = log10( NM/120)

i.e.

log10( NM/120) = T/f(CBR)

Now, from Equation 1:

T = t.[2R1/(R2- R 1)]0.25


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Therefore:

= log10( NM/120) = [t/f(CBR)] * [2R1/(R2- R 1)]0.25

= [t/f(CBR)] * [2/(R2/R1-1)]0.25

Substituting from Equation 2:

log10( NM/120) = log10( N/120) * [2/(R2/R1-1)]0.25 3

i.e. log10 NM = [2/(R2/R1-1)]0.25log 10N + [1 - [2/(R2/R1-1)]

0.25] * log10120

This is the relationship which is plotted in Figure 7.2 of the 1992 Guide.

3.2.2 Implicit Model for Roughness Progression

It is of interest to note the model for the development of roughness with traffic that is implicit inFigure 7.2 of the Guide. From Equation 3 above, we have:

log10( NM/120) = log10( N/120) * [2/(R2/R1-1)]0.25

Rearranging these terms gives:

(R2/R1-1)/2 = [(log10( N/120))/ log10( NM/120))]4

i.e.

R2/R1 = 1 + 2 * [(log10( N/120))/ log10( NM/120))]4

This relationship indicates, for a given value of unmodified design traffic (N), how the ratioroughness/initial roughness increases as cumulative traffic NMincreases. Hence, it is a statementof the roughness progression model implicit in Figure 7.2. Representative plots of the model areshown in Figure 3.2.

0

1

2

3

4

5

6

7

8

9

10

1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Cumulative (ESA)

Ratio of Roughness

to Initial Roughness

105

107

106

Figure 3.2: Plot of roughness/initial roughness against cumulative ESAs for standard designESAs of 105, 106and 10 7(based on procedure F1 thickness correction factors)


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4 DEVELOPMENT OF THE MECHANISTIC PROCEDURE

4.1 Broad Issues

A major attribute sought in a pavement design procedure is versatility the ability to assess the

likely performance of a broad range of pavement configurations in a broad range of field conditions(traffic, environment, etc.).

Its versatility may be conveniently assessed by determining where it lies between two idealisedextremes. These extremes are the fully empirical procedure and the fully mechanistic procedure.A fully empirical procedure relies entirely on past observations of field performance. It allows noextrapolation beyond the range of these observations. At the other extreme, the fully mechanisticprocedure allows unbounded extrapolation beyond past observations. It has this capabilitybecause, intrinsic to it, is a fundamental understanding, for any pavement configuration, of:

the effect on each component material in the configuration of any wheel loading applied tothe pavement surface (such effects are changes to the stress-strain state within the material)

the performance of each of the component materials when subjected to the sequence ofchanges in its stress-strain state caused by the traffic loading.

The fully empirical procedure is idealised because it is unworkable no pavement about to be builtand trafficked will correspond exactly with one previously observed. The fully mechanisticprocedure is idealised because it is unattainable due to the level of complexity involved. Hence,all pavement design procedures fall between these two idealisations. Between them thecomplexity increases along with versatility.

The WG was directed to develop a mechanistic procedure for pavements containing boundmaterials because of the lack of versatility of the relevant procedures in the IGPTD.

Bearing in mind that the end users of the (yet to be developed) procedure were pavementdesigners out in the real world, a major decision confronting the WG was the appropriatecompromise between increasing versatility and increasing complexity (the latter translatingultimately into decreasing likelihood of use of the procedure).

The starting point for the WG was an assessment of the then state-of-the-art in mechanisticanalysis and design. The basis for mechanistic analysis may be summarised as follows.

The response within a pavement to the passage of a wheel load over it takes the form of changesin the levels of stress and strain within it. These changes are predominantly transient, i.e. after thepassage of the wheel load, the stress-strain state within the pavement is very close to its state priorto the passage of the wheel load. Although this residual change is small, it is nevertheless

important because it reflects the distress caused to the pavement by the passage of the wheelload.

This residual change in the stress-strain state may be quite large at some specific locations withinthe pavement (locations of localised shear, crack initiation, crack propagation). However, when itis averaged over the volume of material affected by the passage of the wheel load, the residualchange in the stress-strain state is very small compared to the peak change that occurs during thepassage of the wheel load. Such localised occurrences attributable to localised in homogeneitiesconstitute the essence of distress within the pavement. It is the accumulation of these occurrenceswith the passage of traffic loads which eventually becomes manifest as observable and readilyquantifiable distress.

Modelling of these localised inhomogeneities and their effects (by the use of statistical mechanics)was in its infancy at the time of the review.


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Models which assume homogeneity of materials and (iteratively) determine the stress-strain stateafter the passage of each wheel load in the sequence of traffic loading (and, hence, thedevelopment of distress) were beginning to appear. A leading example of this genre is YandellsMechano-lattice model (e.g. Yandell 1981).

The vast majority of modelling work being undertaken and implemented at the time was based onthe following premises:

The distress within a pavement which is attributable to a single passage of a specific load ona specific axle configuration can be assessed from the peak levels of the pavementstransient response (stresses and strains) to the passage of the axle load the peakresponse levels being determined in the early-life (undistressed) pavement.

The distress caused bynpassages of the axle group:

is proportional to n for fatigue cracking

for permanent deformation, either increases exponentially with n(conventional models)or asymptotes to a plateau value (shakedown model).

For mixed traffic loading:

fatigue damage is determined from Miners hypotheses

for permanent deformation damage, there are more than one alternative summation modelsin use.

Miners hypothesis states that, for mixed traffic loading, fatigue failure will occur when:

Nn ii

i/

reaches a value of 1, where:

ni = the number of passages (within the mixed traffic loading) of loading type i

Ni = the number of passages of loading type i which will cause fatigue failure when loadingtype i is the ONLY loading applied.

The summation is taken over all loading types present in the mixed traffic.

For the determination of peak transient response, the following two types of model were in use:

Linear elastic layer models, in which the pavement is assumed to be of infinite extentlongitudinally and transversely and downwards and the materials to be linear elastic withmoduli independent of applied stresses.

Finite element models, in which the pavement dimensions are finite and material moduli maybe stress-dependent.

While the above modelling work was, at the time, undergoing a vigorous phase of development, allmechanistically-based design procedures either in use, or assembled but yet to be evaluated,opted for:

a linear elastic layer model for determination of peak transient response

the peak level of tensile strain at the bottom of an asphalt layer as a predictor of fatigue life ofthe asphalt

the peak level of either tensile strain or tensile stress at the bottom of a cemented layer as apredictor of fatigue life of the cemented material.


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Procedures which incorporated the capability of predicting the development of permanentdeformation with the passage of traffic used the peak levels of compressive strain at locationsthroughout the relevant materials. None of these models enjoyed broad-based acceptance.

Procedures with the lesser capability of predicting cumulative traffic to produce a pre-assigned

level of surface rutting used the peak level of compressive strain at the top of the subgrade as thepredictor.

After due deliberation, the WG opted for a mechanistic analysis model comprising the following:

a linear elastic layer model to estimate peak levels of transient responses specifically, theCIRCLY model

peak tensile strain at the bottom of the layer as the appropriate response element forestimating the fatigue lives of both asphalt and cemented materials

peak compressive strain at the top of the subgrade as the appropriate response element forestimating permanent deformation both within the granular material and the subgrade.

The specific sub-models adopted to predict performance from these critical responses arediscussed below.

The model adopted incorporates major simplifications of the complex behaviour that actuallyoccurs. The WG was very aware of the nature and extent of the inherent simplifications at the timethis model was adopted. Its adoption was based on the following rationale.

The utility of an estimation model depends on:

the accuracy of its estimate when the inputs are known

the accuracy of its inputs

the likely extent of its use.

The more complex a model is, the better it scores on the first point and the worse it scores on theother two points. Hence, the choice of level of model complexity involves compromise and is, inthe final analysis, subjectivity based.

In this context, it is of interest to note the outcome of a recent critical review of the mechanisticprocedure in the Guide (Rallings 1997). The review was quite detailed and encapsulated the viewsof Australias leading proponents of pavement design and performance prediction. While theshortcomings inherent in the (in-place) estimation procedures were duly noted along with possiblefruitful areas for improvement, the alternatives offered to replace part or all of the model were:

the shakedown model

the mechano-lattice elasto-plastic model (Yandell 1981)

the Vesys model (Kenis, Sherwood and McMahon 1981)

an adaptation of the Vesys rut depth prediction model (Vuong 1994).

The WG, at the time of formulation of the mechanistic procedure, was cognisant of the Mechano-lattice and Vesys models and the early stages of the development of the shakedown model. TheWG was of the view that, while all models offered most desirable advances in the area ofsimulation of actual behaviour, the input data requirements could not be reasonably expected to beavailable to the routine pavement designer, nor could the designer be reasonably expected toachieve and retain both an understanding of the models and competency in their use. Hence, the

result of introducing such a level of analysis complexity would be to frighten the horses.


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In the view of this author, the situation has changed little since that time5. Rut depth predictionwithin Vesys forms part of its performance model. In common with the Guide, Vesys uses a linearelastic response model to determine the values of critical responses for use in the performancemodel (even though the word Vesys is an acronym for viscoelastic system). Hence, if one choosesto describe the mechanistic procedure in the Guide as an elastic design system (Rallings 1997),

then such a descriptor is equally applicable to Vesys and, further, to all mechanistic designprocedures which enjoy routine use.

4.2 Elastic Characterisation

4.2.1 Isotropic or Anisotropic Characterisation?

This issue relates to the elastic characterisation within CIRCLY of pavement and subgradematerials. Isotropic materials have the same properties in all directions, whereas anisotropicmaterials do not. In terms of the stress-strain behaviour of the material, the difference is illustrated

by considering the simple stress state of equal principal stresses (1 = 2 = 3). For an isotropicmaterial, the resultant strain state is 1 = 2 = 3; ij = 0. For an anisotropic material, theserelationships do not hold. Further, the values of i, ijdepend on the orientations of the symbol iwith respect to the material.

Isotropic materials are characterised by two parameters, usually Youngs Modulus (E) and

Poissons Ratio (). For comparison, the different classes of anisotropy require the followingnumber of parameters to characterise the material:

general case 36

strain energy conserved 21

orthorhombic 9

cross-anisotropic 5

The orthorhombic case is where there are three principal axes of anisotropy (conceptuallyappropriate for material which is mixed, placed and roller-compacted in one direction the axesbeing vertical, in the direction of rolling, and transverse to the direction of rolling).

The cross-anisotropic case is where there is an axis of symmetry of rotation, with the propertiesbeing equal in all directions perpendicular to this axis (but different to those in the direction of theaxis). This is the case modelled in CIRCLY.

This case is considered to be appropriate for naturally deposited material. Assuming the axis of

symmetry to be vertical, the five parameters required are (Wardle 1977):E, Eh, F, h, h. TheWG, having adopted CIRCLY as the response model for the mechanistic procedure, was facedwith the decision as to whether cross-anisotropic characterisation was appropriate for (any or all of)

pavement and subgrade materials.

An additional consideration was the observation that measured deflection bowls were narrowerthan those estimated from elastic layer analysis when isotropic characterisation was adopted. Theview was taken by the WG that, even if no evidence of anisotropic behaviour was discovered,anisotropic characterisation may be appropriate within the constraints of the CIRCLY model to act in the case of granular and subgrade materials as a surrogate for the (real) stress-dependenceof modulus.

5For example, the author is (and has been for 20 years) enamoured by the treatment within Vesys of the

effect of material variability on pavement performance. However, the input data requirements are

demanding to the extent that, to the authors knowledge, only one such data set has ever been assembled!Again, with regard to the Vesys rut depth prediction model, its formulation has considerable intuitive appeal.However, in application its predictive capability has been, at best, fair. The detailed level of its inputrequirements is well illustrated by Table IV of Vuong (1994).


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The issue was approached in two ways. Firstly, a literature review was carried out to determine(for each material type) evidence of anisotropic behaviour in laboratory stress-strainmeasurements (appropriate field measurements were non-existent). Approximately 30 referenceswere located and reviewed, leading to the conclusion that there was little evidence of anisotropicbehaviour in the cases of asphalt and cemented materials, while there was definite evidence in the

cases of granular and subgrade materials. However, there was considerable variability in thevalues of modular ratio (E/Eh) reported. For granular materials, they were predominantly greaterthan 1 (ranging up to values as high as 4), while for fine-grained subgrade materials they rangedfrom less than 1 to greater than 1.

The second approach involved response-to-load analyses (Youdale 1984a) to determine:

the effect on CIRCLY-estimated responses of different modular ratios for granular material

the effect on Finite Element Model (FEM)-estimated responses of incorporating (bothindependently and conjointly) the stress-dependency of modulus of granular material and amodular ratio of 2.

It was found that similar effects on FEM-estimated responses were obtained by: modelling the granular material as isotropic and incorporating stress-dependency

modelling the granular material as anisotropic (E/Eh > 1) without stress-dependency.

In both cases, there was (cf. isotropic, no stress-dependency) a narrowing of the deflection bowl,an increase in maximum deflection, and an increase in vertical compressive strain on top of thesubgrade.

The WG deduced from this that, regardless of the degree of anisotropy pertaining to granular

materials, adoption of anisotropic characterisation of granular materials (with E/Eh > 1) within theCIRCLY model would be a step in the right direction towards encompassing their known stress-

dependency of modulus together with their reported anisotropy. Further, to obtain a closer fitbetween observed and CIRCLY-estimated deflection bowls, it was decided to adopt anisotropic

characterisation (with E/Eh > 1) for subgrades.

A value of 2 for the modular ratio (E/Eh) was adopted for both granular and subgrade materials asa best estimate compromise.

The value of E (for use within CIRCLY) was taken to be the value of E determined from triaxial

test results under the assumption of isotropy. The values of h, h were taken to be the value of(isotropic) . The remaining cross-anisotropic parameter F was set equal to E /(1+ ).

With these additional assumptions, the cross-aniso

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