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AP-T188/11

AUSTROADS TECHNICAL REPORT

Review of Structural Design Procedures forFoamed Bitumen Pavements

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Review of Structural Design Procedures for Foamed Bitumen Pavements

Published August 2011

Austroads Ltd 2011

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 978-1-921709-89-0

Austroads Project No. TT1358

Austroads Publication No. APT188 /11

Project Manager

Allan Jones (DTMR Qld)

Prepared by

Alvaro Gonzales (ARRB)

Published by Austroads LtdLevel 9, Robell House287 Elizabeth Street

Sydney NSW 2000 Australia

Phone: +61 2 9264 7088Fax: +61 2 9264 1657

Email: [email protected]

Austroads believes this publication to be correct at the time of printing and does not acceptresponsibility 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.

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Sydney 2011

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About Austroads

Austroads purpose is to:

promote improved Australian and New Zealand transport outcomes

provide expert technical input to national policy development on road and road transport

issues

promote improved practice and capability by road agencies.

promote consistency in road and road agency operations.

Austroads membership comprises the six state and two territory road transport and trafficauthorities, the Commonwealth Department of Infrastructure and Transport, the Australian LocalGovernment Association, and NZ Transport Agency. Austroads is governed by a Board consistingof the chief executive officer (or an alternative senior executive officer) of each of its elevenmember organisations:

Roads and Traffic Authority New South Wales

Roads Corporation Victoria

Department of Transport and Main Roads Queensland

Main Roads Western Australia

Department for Transport, Energy and Infrastructure South Australia

Department of Infrastructure, Energy and Resources Tasmania

Department of Lands and Planning Northern Territory

Department of Territory and Municipal Services Australian Capital Territory

Commonwealth Department of Infrastructure and Transport

Australian Local Government Association

New Zealand Transport Agency.

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

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CONTENTS

1 INTRODUCTION ......................................................................................................................11.1 Project Background ..................................................................................................................11.2 Overview of the Report and Design Procedures ......................................................................12 SOUTH AFRICAN TG2 2002 DESIGN METHOD ....................................................................32.1 Background............................................................................................................................... 32.2 Observed Behaviour of Foamed Bitumen Pavements ..............................................................3

2.2.1 Field Observations from Accelerated Pavement Testing ........................................... 32.2.2 Laboratory Observations ........................................................................................... 6

2.3 TG2 2002 Thickness Design Method .......................................................................................62.3.1 Effective Fatigue Phase ............................................................................................. 62.3.2 Equivalent Granular Phase ........................................................................................ 7

2.4 Classification of the Foamed Bitumen Mixes ..........................................................................103 METHODS BASED ON AUSTROADS DESIGN GUIDELINE ...............................................113.1 Background............................................................................................................................. 113.2 Department of Transport and Main Roads, Queensland ........................................................11

3.2.1 Design Equation ...................................................................................................... 113.2.2 Other Pavement Design Considerations ................................................................. 123.2.3 Minimum Surface Requirements ............................................................................. 13

3.3 City of Canning .......................................................................................................................133.3.1 Background .............................................................................................................. 133.3.2 Development of Fatigue Equation ........................................................................... 143.3.3 Other Design Considerations ................................................................................... 153.3.4 Minimum Surface Requirements ............................................................................. 16

3.4 NZ Transport Agency Method .................................................................................................163.4.1 Background .............................................................................................................. 163.4.2 Design Inputs and Distress Models ......................................................................... 16

4 KNOWLEDGE-BASED TG2 2009 STRUCTURAL DESIGN METHOD ................................184.1 Background............................................................................................................................. 184.2 Concepts in the Development of the Pavement Number ........................................................18

4.2.1 The Effective Long-term Stiffness (ELTS) ............................................................... 184.2.2 Characterisation of Subgrade Materials .................................................................. 194.2.3 The Modular Ratio Limit Concept ............................................................................ 194.2.4 Assumed Behaviour of Bitumen Stabilised Layers .................................................. 194.2.5 Base Confidence Factor .......................................................................................... 19

4.3 Calculation of PN ....................................................................................................................204.4 Design Criteria PN Model ....................................................................................................... 22

4.4.1 Allowed Capacity ..................................................................................................... 224.4.2 Minimum Surface Requirements ............................................................................. 23

5 TRANSPORTATION RESEARCH LABORATORY ...............................................................255.1 Background............................................................................................................................. 255.2 Design Procedure ...................................................................................................................25

5.2.1 Road Type Categories ............................................................................................. 255.2.2 Foundation Class ..................................................................................................... 265.2.3

Classification of Foamed Bitumen Mixes and Design Chart .................................... 26

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5.2.4 Minimum Surface Requirements ............................................................................. 286 COMPARISON OF PAVEMENT DESIGNS ...........................................................................296.1 Summary of Design Methods .................................................................................................296.2 Case Study .............................................................................................................................31

6.2.1 Description of the Example Project .......................................................................... 316.2.2 TG2 2002 Design ..................................................................................................... 316.2.3 Department of Transport and Main Roads, Queensland Design ............................. 326.2.4 City of Canning Design ............................................................................................ 336.2.5 NZ Transport Agency Design .................................................................................. 346.2.6 Transportation Research Laboratory (TRL) Design ................................................. 346.2.7 Knowledge-based TG2 2009 Design ....................................................................... 356.2.8 Summary of Designs and Discussion ...................................................................... 36

7 INTERIM PROCEDURE FOR THE THICKNESS DESIGN OF FOAMED BITUMENPAVEMENTS .........................................................................................................................38

7.1

Introduction .............................................................................................................................38

7.2 Minimum Stiffness Requirements ...........................................................................................387.3 Temperature Adjustment ........................................................................................................ 397.4 Rate of Loading Adjustment ...................................................................................................437.5 Fatigue Criteria .......................................................................................................................447.6 Minimum Surface Requirements ............................................................................................448 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .................................................458.1 Review of Design Methods .....................................................................................................458.2 Comparison of Pavement Designs .........................................................................................468.3 Recommendations ..................................................................................................................47REFERENCES ................................................................................................................................48APPENDIX A ADJUSTMENT FACTORS FOR THE TG2 2009 THICKNESS

DESIGN METHOD...................................................................................... 50APPENDIX B DETAILS OF THE THICKNESS DESIGN OF FOAMED BITUMEN

PAVEMENTS ..............................................................................................52APPENDIX C INTERIM DESIGN PROCEDURE FOR FOAMED BITUMEN

PAVEMENTS ..............................................................................................60APPENDIX D DESIGN EXAMPLE USING INTERIM DESIGN PROCEDURE .................66

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TABLES

Table 2.1: Reliability factors based on road categories ...............................................................9Table 2.2: Foamed bitumen treated material classification .......................................................10Table 2.3: Foamed bitumen treated material properties ............................................................10Table 3.1: TMR modulus requirements for foamed bitumen materials for high

trafficked roads .........................................................................................................12Table 3.2: Temperature correction factor for TMR method .......................................................13Table 3.3: Adopted modulus values for crushed granular pavements in City of

Canning ....................................................................................................................15Table 5.1: Road type category for TRL method .........................................................................26Table 5.2: Foundation classes for TRL method .........................................................................26Table 5.3: Bitumen bound cold recycled material classification for TRL method .......................27Table 5.4: Thickness design of pavements up to 5 x 106 ESA ..................................................28Table 5.5: Requirements for surfacing thickness for TRL method .............................................28Table 6.1: Summary of design methods ....................................................................................30

Table 6.2: Elastic characterisation using TMR method .............................................................33Table 6.3: Elastic charactersiation for use in the City of Canning method ................................33Table 6.4: NZ Transport Agency elastic characterisation ..........................................................34Table 6.5: Rehabilitation treatment using the TRL method .......................................................35Table 6.6: Solutions using the knowledge-based TG2 2009 method ........................................35Table 7.1: Minimum mix design limits for initial modulus ...........................................................38Table 7.2: Minimum mix design limits for dry modulus for foamed bitumen base ..................... 39Table 7.3: Minimum mix design limits for dry modulus for foamed bitumen subbase................ 39Table 7.4: Effect of temperature on indirect tensile resilient modulus test ................................40

FIGURES

Figure 2.1: Foamed bitumen pavement tested in the HVS sections; multi-depthdeflectometer ..............................................................................................................4

Figure 2.2: Calculated elastic modulus of foamed bitumen layers versus loadrepetitions for P243/1 test, section 411A4 ..................................................................5

Figure 2.3: In-depth permanent deformation measured with MDD 8 for section4114A .........................................................................................................................5

Figure 2.4: Location of the critical design parameters for the TG2 2002 Guidelinesdesign method ............................................................................................................8

Figure 3.1: Graphical representation of test results ....................................................................15Figure 4.1: Steps in the knowledge-based structural design method for pavements ................. 20Figure 4.2: Example of pavement number determination ...........................................................21Figure 4.3: Criteria for determining allowed capacity based on PN ............................................23Figure 4.4: Recommended layer thicknesses versus structural capacity ...................................24Figure 5.1: Identification of material families ...............................................................................25Figure 5.2: Design curves for bitumen bound cold recycled material, Foundation

Class 1 ......................................................................................................................27Figure 6.1: Pavement rehabilitation case study ..........................................................................31Figure 6.2: Solution for different relative densities using the South African TG2 2002

design method ..........................................................................................................32Figure 6.3: Summary of pavement thicknesses (millimetres) for all design methods ................. 36

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Figure 7.1: Relation between subsurface temperature (at 150 mm deep) and back-calculated foamed bitumen mix (with 2.5% foamed bitumen and 1%cement) resilient modulus: (a) for 2003 data (b) for 2005 data ................................. 41

Figure 7.2: Variation of modulus at weighted mean annual pavement temperature(WMAPT) from Leek (2001) laboratory tests and Fu and Harvey (2007)FWD data ..................................................................................................................42

Figure 7.3: Variation of ratio of modulus at vehicle speed V to modulus fromstandard indirect tensile test (40 ms rise time) with design speed ...........................43

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SUMMARY

Pavement designers in Australia and New Zealand trying to use alternative treatments such asfoamed bitumen in rehabilitation projects are severely constrained by a lack of data on theperformance of this type of stabilised material.

Structural thickness design methods have been developed for the design of foamed bitumenpavements, most of them based on assumptions that do not necessarily represent theperformance of foamed bitumen pavements under Australia and New Zealand conditions. Thisreport presents a review of the following methods:

TG2 2002 guidelines (South Africa)

This method, published in 2002, was developed using testing data from a full-scale acceleratedtesting of foamed bitumen pavements and extensive laboratory work. The TG2 2002 Guidelinesmethod suggests that foamed bitumen pavements behave in two separate phases. The first phase

starts after construction, when the layer is in an intact, undamaged condition and provides fatigueresistance. This phase is called effective fatigue phase and ends when, due to the appliedloading, the layer reduces its stiffness. The second phase is called equivalent granular state,because the stiffness of the foamed bitumen layer is similar to that of a good quality granular base.The assumed distress modes of the first and second phase are fatigue and permanentdeformation, respectively.

Department of Transport and Main Roads, Queensland (Australia)

TMR adopted the Austroads asphalt fatigue relationship for the foamed bitumen layer. The asphaltfatigue relationship relates the admissible number of load cycles with the volumetric properties ofthe mix, the stiffness of the mix and the tensile strain at the bottom of the foamed bitumen layer.

The method assumes that fatigue is the primary distress mode.

City of Canning (Australia)

The City of Canning developed a fatigue relationship for foamed bitumen layers using data fromflexural beams prepared and compacted in the field and tested in the laboratory. It was found thatthe fatigue relationship is independent of the stiffness of the mixes. The method is different to theTMR method, but results similar pavement life predictions at typical strain levels.

NZ Transport Agency (New Zealand)

The NZ Transport Agency (NZTA) design procedure suggests that fatigue relationships are tooconservative and do not represent the observed behaviour in New Zealand foamed bitumen

pavements. Pavement designers in New Zealand normally consider the foamed bitumen layer asan unbound granular layer. The method recommends an elastic modulus of 800 MPa (anisotropic,no sub-layering) for the modelling of the elastic properties of the foamed bitumen layer. Thepavement thickness is calculated by reducing the vertical compressive strain at the top of thesubgrade to the value obtained by the Austroads subgrade strain criteria.

Transportation Research Laboratory (United Kingdom)

The TRL method assumes that foamed bitumen mixes behave similarly to hot mix asphalt mixes,fatigue being the dominant distress mode of these mixes. The method is based on tables andcharts that classify subgrade, traffic and foamed bitumen type. These assumptions are mainlybased on engineering judgment.

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Knowledge-based TG2 2009 Second Edition (South Africa)

A different approach known as the knowledge-based method was developed by South Africanresearchers. The knowledge-based method is described in the second edition of the TG2guidelines, published in 2009. The second edition of the TG2, TG2 2009, superseded the previous

TG2 2002 design method. The knowledge-based method is based on a pavement number (PN)that is calculated by multiplying the expected long-term stiffness of each layer and the respectivethickness of the layer, similar in principle to the AASHTO structural number approach to flexiblepavement design. The stiffness of the layers is corrected by other design factors such as the totalpavement thickness above the subgrade, the position of the layer within the pavement structure,and the stiffness of the underlying layer. The knowledge-based method also incorporates somepractical aspects that do not exist in other mechanistic methods, such as the recommendedsurface thickness and the minimum and maximum thickness for the foamed bitumen layer.

In order to compare the six design methods from a more practical, objective point of view, ahypothetical case study was conducted as the last part of the review, which involved the design ofa foamed bitumen pavement using the six design methods. The case study consisted in the

rehabilitation of an existing unbound granular pavement using an in situ foamed bitumenstabilisation technique. The pavement design carried out using the six design methods showedthat:

The TG2 2002 Guidelines method was found to be very sensitive to one of the inputs of theequivalent granular state distress model (i.e. relative density). In addition, it was found thatthis distress model provides unexpected outputs that contradict observed behaviour inrecently completed full-scale accelerated testing of foamed bitumen pavements.

The pavement methods that assume behaviour of foamed bitumen mixes to be similar to thatof hot mix asphalt mixes (TMR and TRL) yield similar foamed bitumen layer thicknesses(between 290 mm to 310 mm), indicating consistency in the outputs.

The City of Canning design procedure yields similar thickness (288 mm) to those given by

the TMR method, since the fatigue relationship developed by the City of Canning is similar tothat used by the TMR.

The less conservative pavement thickness was given by the NZTA design procedure(220 mm), because fatigue of foamed bitumen is ignored in the pavement design process.Pavements are designed only to inhibit rutting and shape loss.

The knowledge-based TG2 2009 method provided the most conservative solution, in which athick asphalt layer (90 mm) was required in addition to the foamed bitumen layer. However,this design method is currently under development, and only a limited number of foamedbitumen sections have been incorporated into the knowledge-based data set.

Finally, an interim pavement design method for foamed bitumen pavements in Australia isproposed, based on the Austroads asphalt fatigue relationship. The interim thickness designmethod is applicable to pavements with foamed bitumen contents representative of Australianmixes (normally about 3.5%). For the elastic characterisation of the foamed bitumen layer, it isrecommended the indirect tensile resilient modulus with temperature and vehicle speedadjustments.

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1 INTRODUCTION

1.1 Project Background

Over 90% of the Australian sealed road network consists of sprayed seal granular pavements.Growing traffic loadings are placing increasing pressure on these pavements, with somenon-standard materials no longer being fit-for-purpose. In many rural areas, the use of high qualitycrushed rock is not a cost-effective treatment to improve the structure of these pavements.Consequently there is increasing use of treatments that enhance the existing non-standardmaterials by the addition of cementitious and bituminous binders to allow recycling of our scarceresources.

The Austroads Guide (Austroads 2009) interim procedures for the thickness design of alternativestructural treatments such as in situ recycling with bituminous or cementitious binders are not aswell founded as those for conventional treatments due to lack of information about the performanceof these alternative treatments. In particular, there is not established procedure in the Austroads

Guide for the design of foamed bitumen pavements, leading pavement designers to moreconservative design approaches such as modelling the foamed bitumen layer as an unboundgranular material instead of a stabilised material. Therefore, improved design procedures arerequired that better reflect the structural contribution of stabilisation treatments as this will lead tomore cost-effective rural road rehabilitation treatments.

This report presents a review of the available structural design methods for foamed bitumenpavements, as a first step in the development of thickness design procedures. Six designprocedures were found in the literature, most of them based on well known pavement designmethods (e.g. South African Mechanistic Design Method, Austroads, United KingdomTransportation Research Laboratory) with some variations to accommodate the observed orassumed behaviour of foamed bitumen pavements into the pavement design. The last part of the

report summarizes the design methods an presents a case study in which the rehabilitation of onegranular pavement was designed using the six design methods.

1.2 Overview of the Report and Design Procedures

The report is divided into seven sections. Section 2 presents the design models published in theTechnical Guidelines (TG2 2002) for the design of foamed bitumen pavements (South Africa). Themethod proposes distress models, which were developed using data from a full-scale acceleratedpavement test and laboratory tests performed on foamed bitumen pavements. The methodsuggests that foamed bitumen pavements behave in two separate phases, the first being a fatiguephase and the second an equivalent granular phase.

Section 3 describes three design procedures that use the design concepts and equationspublished in the Austroads Pavement Design Guide. These were developed by the Department ofTransport and Main Roads, Queensland (formally Queensland Department of Main Roads), theCity of Canning (Western Australia) and the New Zealand Transport Agency (formerly Transit NewZealand). The three procedures use the Austroads subgrade strain criterion but differ in the use ofthe fatigue relationship for the foamed bitumen layer.

Section 4 presents the knowledge-based method, a new empirical pavement design procedurerecently developed in South Africa, which includes the design of foamed bitumen pavements. Theknowledge-based method, published in 2009, superseded the TG2 2002 South African method.The basis of the method is a pavement index (called pavement number), which is calculated usingthe assumed long-term properties of the pavement layers and the thickness of each layer. The

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pavement number was related with the allowable traffic loading by using an extensive set of datacollected from South African pavements. Section 5 describes the design methodology proposedby the Transportation Research Laboratory (TRL), which adopted a procedure similar to that forthe design of bitumen stabilised pavements.

Section 6 provides a summary and comparison of the six design methodologies previouslypresented in the report, using a hypothetical pavement rehabilitation design case. Results showedthat the NZ Transport Agency design procedure gives the lowest thickness for the foamed bitumenlayer, while the new South African knowledge-based yields the most conservative pavementthicknesses. The Department of Transport and Main Roads, Queensland (TMR), City of Canningand TRL methods give similar pavement thicknesses.

Section 7 presents an interim thickness design procedure for foamed bitumen pavements to beused in Australia. The procedure adopted the Austroads asphalt fatigue relationship for theestimation of the fatigue life of the foamed bitumen layer. For the elastic characterisation, it isrecommended the indirect tensile resilient modulus with temperature and vehicle speed

adjustments. Section 8 presents the summary, conclusions and recommendations.

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2 SOUTH AFRICAN TG2 2002 DESIGN METHOD

2.1 Background

One of the most widely accepted methods for the structural design of foamed bitumen pavementsis the South African Interim Technical Guidelines (TG2) (Asphalt Academy 2002). The primaryobjectives of the guidelines (referred to as TG2 2002 Guidelines) were to assist road authorities inthe adjudication of alternative designs for pavement rehabilitation projects and to assistpractitioners in the design and construction requirements of foamed bitumen pavements (Jenkinset al. 2008).

The models for the structural design of foamed bitumen pavements published in the TG2 2002Guidelines were developed using concepts and material behaviours that were part of the SouthAfrican Mechanistic Design Method (SAMDM) (Theyse & Rust 1996). The models were one of theoutcomes of a large research project that involved the full-scale testing of foamed bitumenpavements and extensive laboratory work.

The TG2 2002 Guidelines provide several important contributions to the structural design offoamed bitumen pavements were:

a description of the behaviour of foamed bitumen pavements (behaviour in two phases)

relationships between the elastic responses in the foamed bitumen layer (strains, stresses)with the number of load repetitions (for each of the two phases)

a material classification system for foamed bitumen mixes based on simple laboratory tests(unconfined compressive strength and indirect tensile strength).

2.2 Observed Behaviour of Foamed Bitumen Pavements

2.2.1 Field Observations from Accelerated Pavement Testing

The development of the design models for foamed bitumen layers published in the TG2 2002Guidelines was based on data from full-scale accelerated pavement testing (APT) and laboratorytesting. The APT was conducted using a Heavy Vehicle Simulator (HVS), a linear type of APTfacility used in South Africa.

The HVS tests consisted of two foamed bitumen treated test sections of 8.0 m each (namedsection 409A4/B4 and section 411A4). The construction of the sections was conducted using deepin situ recycling of a cement treated1 ferricrete base with old multi-seal surfacings and part of theuntreated ferricrete subbase. This material was treated with 2% cement and 1.8% foamed bitumen(80/100 penetration grade)2. The rehabilitated pavement consisted of a 25 mm asphalt surfacing,

250 mm of foamed bitumen treated base, 250 mm of untreated ferricrete subbase and the in situsubgrade (Figure 2.1 a). The performance of this pavement was investigated using 40 kN and80 kN dual wheel loading. The 40 kN dual wheel load is the same wheel load as an 80 kNstandard axle. The deflections, surface deformation, moisture conditions and pavement verticalstrains were continuously monitored during the application of the loads (Long 2001).

1The original cement content of the existing treated ferricrete base is not detailed in the South African report used in this

review (Jooste & Long 2007).2

It is important to note that these binder types and content are very different from those currently used in Australia

(normally 3%-4% foamed bitumen and 1%-2% of lime).

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The elastic and plastic vertical strains of the foamed bitumen layer were measured using a multidepth deflectometer (MDD), a device that measures strains at different depths of the pavement(Figure 2.1 b). The elastic vertical strains were used to back-calculate the elastic modulus of thefoamed bitumen at different stages of the test. In addition, the plastic vertical strains were used toestimate plastic deformation of the foamed bitumen layer.

Foamed bitumentreated base(250 mm)

Untreated granular

subbase (250 mm)

Asphalt surface

(25 mm)

In situ soil

Strain

Gauges

elastic,

permanent

MDD

Strain

Gauges

elastic,

permanent

MDD

(a) (b)

Figure 2.1: Foamed bitumen pavement tested in the HVS sections; mult i-depth deflectometer

The rehabilitation was conducted between January to October 2000, and the pavements weretested during 2001. During the initial loading of the pavements a reduction of the elastic modulusof the foamed bitumen basecourse was observed. The back-calculated modulus for section 411A4

is presented in Figure 2.2, where the initial modulus of 1000-3500 MPa reduced to 600-1000 MPaunder 40 kN loading. The elastic modulus was calculated using data from three MDD installed in411A4 section (MDD4, MDD8 and MDD12, depicted in Figure 2.2). This reduction in elasticmodulus was again observed when the load was increased to 80 kN, after the application of958 714 load cycles of 40 kN (Figure 2.2).

The increase of permanent deformation of the foamed bitumen layer with load repetitions and atvarious depth below the surface was also measured with the MDD (Figure 2.3). In section 411A4,2 mm of deformation were measured after 1x106 cycles of the 40 kN load, and after the applicationof the 80 kN load this value increased approximately up to 3 mm, 66% of the total surfacedeformation (4.5 mm). The measured rutting was unexpectedly low (Long 2001), and hence waterwas introduced into the pavement through surface cuts to induce further damage. Althoughsignificant rutting was only experienced after the addition of water, there was still relatively littlepermanent surface deformation at the end of the test (5.5 mm in section 409A4 and 5.4 mm insection 411A4). Very little cracking was observed in sections 409A4/B4 and 411A4 before theaddition of water. The cracking was observed in the asphalt surface only.

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ReductioninStiffness

ReductioninStiffness

1000340

0MPa

600 1000 MPa

, , , , , , , , , ,

Source: Long (2001).

Figure 2.2: Calculated elastic modulus of foamed bitumen layers versus load repetitions for P243/1 test, section 411A4

Depth below the

surface:

, , , , , , , , , ,

Source: Long (2001).

Figure 2.3: In-depth permanent deformation measured with MDD 8 for section 4114A

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2.2.2 Laboratory Observations

A laboratory testing program, using the same materials tested in the full-scale experiment, wasconducted in conjunction with the accelerated pavement test. Flexural beam tests, compressive

monotonic load triaxial tests (MLT) and compressive repeat load triaxial (RLT) were also used todevelop the structural design models for foamed bitumen mixes. The tests were conducted mixingthe untreated milled material (collected in the field during construction) with different contents ofbitumen and cement in the laboratory. The specimens were prepared with various moisturecontents and compacted at different bulk densities. The shear strength of the mixes (angle ofinternal friction and cohesion) was estimated using the peak stress measured in MLT tests. RLTtests were used to assess the permanent deformation resistance and resilient modulus of themixes.

Details of the laboratory study are summarised in Long and Theyse (2002). The results of thelaboratory study showed that cement and foamed bitumen had an important effect on flexibility andstrength of the mixes as follows:

Flexibility: The addition of foamed bitumen, or a decrease in the cement to foamed bitumencontent ratio (cem/bit), increased the flexibility of the flexural beams. The strain-at-breakvalue from the flexural beam tests showed that the higher the bitumen content (or the lowerthe cement to bitumen content ratio) the higher the strain-at-break value.

Compressive and flexural strength: The addition of cement or an increase in the cement tofoamed bitumen content ratio (cem/bit) increases both the compressive and flexural strength(defined as the peak stress attained during the flexural beam test) of the foamed bitumentreated materials. An increase in the foamed bitumen content or a decrease in the cement tobitumen content ratio decreased the compressive and flexural strength of cement treatedmaterials. An increase in the compressive strength results in an increased permanent

deformation resistance.

2.3 TG2 2002 Thickness Design Method

The development of the mechanistic-empirical structural design procedure was based on thematerial behaviour and distress mechanisms observed in full-scale testing of pavements, inconjunction with the observed strength and deformational behaviour of foamed bitumen mixestested in the laboratory.

The observations from the HVS test suggested that foamed bitumen pavements behave in twoseparate phases. The first phase starts after construction, when the layer is in an intact,undamaged condition and provides fatigue resistance. This phase is called effective fatiguephase and ends when, due to the applied loading, the layer reduces its stiffness. The secondphase is called equivalent granular state, because the stiffness of the foamed bitumen layer issimilar to that of a good quality granular base. The assumed distress modes of the first andsecond phase are fatigue and permanent deformation, respectively.

2.3.1 Effective Fatigue Phase

A structural design model for the effective fatigue phase was developed using the elastic stiffnessdata measured in the HVS experiment with the MDD. The model determines the number ofrepetitions to the equivalent granular state as a function of the strain ratio. The strain ratio is theratio of the maximum horizontal tensile strain calculated at the bottom of the foamed bitumen layerto the strain-at-break from the flexural beam test. The tensile strain in the pavement under an

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80 kN standard axle load is calculated using a software based on multi-layer linear elastic theorysuch as mePads (CSIR 2001).

The effective fatigue phase equation has the following form (Equation 1):

0.708

,10 b

A

F FBN

1

where

NF,FB = number of load cycles during the effective fatigue life of the foamedbitumen layer

A = coefficient related to the category of the road or reliability (risk) of thedesign

= calculated horizontal tensile strain under standard axle load at the bottomof the layer (see h in Figure 2.4)

b = strain-at-break measured in the flexural beam test.

The terminal distress condition for the first phase is a loss of stiffness (i.e. from bound material toan equivalent granular state) and a 2 mm permanent deformation of the foamed bitumen layer.

2.3.2 Equivalent Granular Phase

The structural design model for permanent deformation of the foamed bitumen treated material

after fatigue was developed using the MDD measurements and the permanent deformationmeasured in RLT laboratory tests. It was found that the permanent deformation increases with theratio between the applied stress in the RLT test and the peak stress attained in a MLT test (calledstress ratio), using specimens prepared with identical materials. Therefore it was decided todevelop the permanent deformation equation as a function of the stress ratio.

In the equivalent granular phase, the major and minor principal stresses are determined at fourlocations in the pavement (Figure 2.4):

one quarter below the top of the layer and, below and between a 40 kN dual wheel load

one quarter above the bottom of the layer, below and between a 40 kN dual wheel load.

These locations were suggested by Long (2001) as the critical locations in the foamed bitumenlayer, after the analysis of a pavement structure similar to that of the HVS test sections, using alinear elastic model.

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t / 4

3t / 4

Surfacing

Foamed

bitumen

basecourse

Subbase

t

Subgrade

Wheel

Loads

1

3

h

CentreofL

oads

20 kN 20 kN

Figure 2.4: Location of the critical design parameters for the TG2 2002 Guidelines design method

The allowable loading in the equivalent granular phase is (Equation 2 to Equation 4):

( 11.938 0.0726 1.628 0.691 ( / ))

,

110

30

A RD PS SR cem bit

PD FBN

2

where

NPD,FB = number of load repetitions

A = coefficient related to the category of the road or reliability (risk) of thedesign

RD = relative density of the foamed bitumen mix, determined using the formula:

100mix

mix w

DDRD

ARD D

3

where

DDmix = dry density of the mix (kg/m3)

Dw = density of water (kg/m3)

ARDmix = apparent relative density, determined using the individual solid density (SD)values for the aggregate, cement, bitumen and water.

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PS = allowed permanent strain of the foamed bitumen layer expressed as apercentage. For instance, if the allowed permanent deformation in thesecond equivalent granular phase for the foamed bitumen layer is 18 mm,and the foamed bitumen layer thickness is 180 mm, PS=18/180=0.1=10%.The terminal distress condition normally used for the second equivalentgranular phase is 18 mm of permanent deformation in the foamed bitumenlayer

(cem/bit) = cement to bitumen ratio of the foamed bitumen mix

SR = stress ratio3 calculated at one quarter below the top of the layer and onequarter above the bottom of the foamed bitumen layer, using the followingformula:

1 3

23 tan 45 1 2 tan 45

2 2

a a

a

SR

C

4

where

1a,3

a = major and minor principal stresses, expressed in kPa, calculated from theresponse model (Figure 2.4)

= angle of internal friction (expressed in degrees)

c = cohesion (in kPa) of the foamed bitumen material studied, determined frommonotonic load triaxial (MLT) tests.

For the calculation of NPD,FB the most critical (highest) stress ratio is used in Equation 2. Themodels (both effective fatigue and equivalent granular) were adapted to account for reliability levelsof the different traffic categories (95%, 90%, 80% and 50%), listed in Table 2.1.

Table 2.1: Reliabi lity factors based on road categories

Road category

Name A B C D

Reliability (%) 95 90 80 50

Description E.g. interurban freeways,

major interurban roads

E.g. interurban collectors,

major rural roads, major

industrial roads

Lightly trafficked rural

roads, strategic roads

Rural access roads

Importance Very important Important Less important Less important

Service level Very high High Moderate Moderate to low

Total equivalent traffic (ESA)

over structural design period

3-100x106 over 20 years 0.3-10x106 depending on

design strategy

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2.4 Classification of the Foamed Bitumen Mixes

The design method presented above requires a large amount of laboratory data to be used withaccuracy (shear strength values, flexural strength). This type of data normally is not readilyavailable in practice, hence an interim material classification system, based on simple laboratory

tests, was included in the TG2 2002 Guidelines.

The classification system consisted of four categories (FB1, FB2, FB3 and FB4) depending on theunconfined compressive strength (UCS) and indirect tensile strength (ITS) measured on thefoamed bitumen mix (Table 2.2). The UCS and ITS specimens are sealed after compaction andde-moulded after 24 hours. Then the specimens are tested after 72 hours of accelerated air-driedat 40 C, sealed in individual loose plastic bags with a sealed volume at least twice that of thespecimen. The testing temperature is 25 C.

The TG2 2002 Guidelines recommends values of stiffness for the first effective fatigue phase,

Poissons ratio, strain-at-break (b), cohesion and angle of internal friction for each mix category.The guideline developed material properties for materials FB2 and FB3 only (listed in Table 2.3),because only these two materials had been extensively tested in the laboratory when theguidelines were published.

Table 2.2: Foamed bitumen treated material classification

Material classi fication UCS, after 3 days ofaccelerated air-dried (kPa)

ITS, after 3 days ofaccelerated air-dried (kPa)

FB1 14002000 300500

FB2 14002000 100300

FB3 7001400 300500

FB4 7001400 100300

Source: Asphalt Academy (2002).

Table 2.3: Foamed bitumen treated material properties

Material classi fication FB2 FB3

Range Recommendedvalue

Range Recommendedvalue

Cem/bit 1.11 0.33

Stiffness in effective fatigue phase (MPa) 12402075 1600 6801625 1100

Poisson ratio 0.35 0.35

Strain-at-break 120225 172 390590 490

Cohesion (kPa) 110425 210 110210 120

Friction angle () 2755 49 3453 45

Source: Asphalt Academy (2002).

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3 METHODS BASED ON AUSTROADS DESIGN GUIDELINE

3.1 Background

Three pavement methodologies based on the design principles in Austroads (2004) were found inthe literature. The distress mechanisms for flexible pavements, assumed by the Austroads Guideinclude:

Fatigue of asphalt and cemented materials due to repetition of horizontal tensile strains atthe bottom of such layers.

Permanent deformation of unbound materials and subgrade which is related to verticalcompressive strain on top of subgrade.

The Austroads Guide does not include specific procedures for the thickness design of foamedbitumen pavements. However, the Austroads procedures have been adapted for foamed bitumenpavements by three organisations: the Queensland Department of Main Roads, the City of

Canning (Western Australia) and NZ Transport Agency.

The three methods adopted at least one of the failure mechanisms mentioned above; however,they differ in the specific fatigue model of the foamed bitumen layer, or simply do not use a fatiguerelationship.

It is important to emphasize that foamed bitumen contents currently used in Australia are normallyhigher than foamed bitumen contents adopted in some other countries (e.g. South Africa, NewZealand).

3.2 Department of Transport and Main Roads, Queensland

3.2.1 Design EquationSince 1997, pavement recycling using foamed bitumen has become an attractive alternative totraditional overlays for rehabilitation of existing pavements in Queensland. The experience gainedthrough pavement construction, laboratory testing, and monitoring of foamed bitumen pavementshas contributed to the development of a design equation for foamed bitumen pavements in thisstate.

Although field data in Queensland was limited when the design equation was suggested, there wasenough data to indicate that the primary distress mechanism of foamed bitumen stabilisedpavements was fatigue failure of the stabilised layer (Jones & Ramanujam 2004). Therefore, theDepartment of Transport and Main Roads, Queensland (TMR) adopted the Austroads asphaltfatigue criterion used for asphalt design for a reliability of 95% to estimate the fatigue performanceof foamed bitumen stabilised material. This relationship was recommended provided the assumedvolumetric percentage of binder does not exceed 8% and the laboratory soaked modulus does notexceed 2500 MPa. The design modulus (Smix) is based on the soaked indirect tensile resilientmodulus results at the nominated design binder content for the rehabilitation project (Equation 5).Therefore:

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5

36.0'

856.008.16918

mix

b

S

VN

5

where

N = allowable number of load repetitions to fatigue of the foamed bitumen layer

Vb = percentage by volume of bitumen in the foamed bitumen layer (normallybetween 6% and 8%)

Smix = foamed bitumen mix modulus, measured using the indirect tensile resilientmodulus test (MATTA testing) on soak specimens and corrected bytemperature (Section 3.2.2)

= horizontal tensile strain at bottom of foamed bitumen layer produced by theload (microstrain).

3.2.2 Other Pavement Design Considerations

As part of the mix design process, TMR normally conducts indirect tensile resilient modulus tests(Australian Standard AS2891.13.1) on 150 mm diameter foamed bitumen specimens compactedusing 50 blows Marshall Compaction. The specimens have to meet a minimum resilient modulusvalue depending on the design traffic loading. The TMR normal procedure is to test nominallyidentical specimens prepared in the laboratory, after:

3 hours of air-dried and curing at 25 C

3 days of accelerated air-dried and curing at 40 C

3 days of accelerated air-dried and curing at 40 C, followed by soaking in water undervacuum for 10 minutes.

TMR adopts the soaked resilient modulus (Smix) for the fatigue equation and also recommendsminimum Smix values for soaked-to-dry resilient modulus ratio for a range of design traffic as shownin Table 3.1. The Smix should not exceed 2500 MPa. If Smix is not known, the assumed valueshould correspond to the minimum soaked modulus (Table 3.1).

Table 3.1: TMR modulus requirements for foamed bitumen materials for high trafficked roads

Average daily ESA in

design year of opening

Minimum dry modulus

(MPa)

Minimum soaked

modulus (MPa)

Maximum soaked

modulus (MPa)

Minimum retained

modulus ratio< 100 2500 1500 2500 40%

1001000 3000 1800 2500 45%

> 1000 4000 2000 2500 50%

Source: Jones and Ramanujam (2004).

The stiffness of the foamed bitumen mix (Smix) is determined in the laboratory at 25 C. Therefore,this value is corrected by temperature to reflect the actual weighted mean annual pavementtemperature (WMAPT) of the site (Jones & Ramanujam 2008). The temperature correction factorto be applied to Smix is listed in Table 3.2.

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Table 3.2: Temperature correction factor for TMR method

Weighted mean annual pavementtemperature

Temperature correction factor

25 C 1.0

30 C 0.9

35 C 0.8

40 C 0.7

Source: Jones and Ramanujam (2004).

TMR recommends a minimum subgrade support of CBR 5%, based on observations made onearly failures of foamed bitumen pavements constructed on weak subgrades.

The TMR method may include a second post-cracking phase, similar to the TG2 2002 Guidelinemethod, where:

the average daily ESA in the design lane is less than 1000

not less than 175 mm of dense graded asphalt cover is provided over the foamed bitumenstabilised material.

In the post-cracking phase, the foamed bitumen stabilised material is considered to becross-anisotropic (degree of anisotropy n=2) with a presumptive modulus of 500 MPa, Poissonsratio of 0.35 and no sublayering.

3.2.3 Minimum Surface Requirements

TMR current practice is to use a sprayed seal surfacing for traffic below 1 x 107 ESA and a

minimum of 40 mm thickness of hot mix asphalt for higher traffic loadings. In these higher trafficareas, the actual thickness of asphalt placed above the foamed bitumen layer is governed by thepredicted fatigue life of the asphalt. The required thickness of asphalt may be much greater than40 mm, and are often in the order of 180 mm to 200 mm. The types of asphalt used and layeringare in accordance with the Queensland Pavement Design Manual (Queensland Department ofTransport 1990).

3.3 City of Canning

3.3.1 Background

The City of Canning in Western Australia conducted a research project (Leek 2009) to study themodulus and fatigue performance of in situ foamed bitumen pavement materials measured in the

laboratory. The study was conducted only to stabilisation of pavements containing crushedgravels, and hence the method is applicable only to this type of source aggregate.

Samples of foamed bitumen mixes were collected at various sites in the Cities of Canning andGosnells. The aim of the research was to determine if a design procedure could be developed topredict the fatigue life of in situ foamed bitumen stabilised pavements, and if the viscoelasticproperties of the bitumen binder were reflected in the stiffness and fatigue performance of foamedbitumen mixes.

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3.3.2 Development of Fatigue Equation

Slabs were cut from different foamed bitumen pavement sections located in the City of Canning.The slabs were then cut into beams for flexural beam testing, in order to measure the fatigueproperties of the foamed bitumen samples. It should be noted that the first beams were taken from

pavements in 1999, and have been taken from a number of subsequent foamed bitumen jobssince then. The results of the testing showed that the performance of individual beams variedwidely. While bitumen content and stiffness would be considered to influence fatigue life, due tothe scatter of test results no significant relationships were observed between modulus and fatiguelife or bitumen content and fatigue life (Leek 2009). Therefore a simplified equation was proposed,excluding specific reference to bitumen content and stiffness (Equation 6):

6

1558

N

6

where

N = allowable number of load repetitions to fatigue of the foamed bitumen layer

= horizontal tensile strain at bottom of foamed bitumen layer produced by theload (microstrain).

This equation is a best fit relationship to laboratory data, and predicts the mean fatigue life of theflexural beams Figure 3.1.

In terms of the use of this laboratory relationship to predict the performance of in-servicepavements, Leek (2009) argued that given the size of the aggregate in the fatigue test beams and

associated stress concentrations, the laboratory fatigue relationship would be sufficientlyconservative to use to predict in-service performance without the use of a laboratory-to-field shiftfactor.

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1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

0 50 100 150 200 250 300 350 400

Strain Level (e)

LoadRepititions

Test Beam Results

Predicted Life by Asphalt Model

MRDQ Predicted Life

Asphalt model

best fit curve

N = (1558.175/e)5.9369

Source: Leek (2009).

Figure 3.1: Graphical representation of test results

3.3.3 Other Design Considerations

In the absence of detailed site or laboratory testing, the presumptive design moduli in Table 3.3 areused by the City of Canning when stabilising existing pavements composed of crushed limestonesubbase, crushed granite basecourse and asphalt surfacing. These values were suggested afterthe analysis of a large data set of flexural beam modulus, indirect tensile resilient modulus andFalling Weight Deflectometer (FWD) measurements. It was found that modulus decreases withpavement depth and that the flexural modulus is approximately 60% of the resilient modulus.

Table 3.3: Adopted modulus values for crushed granular pavements in City of Canning

Depth below stabilised

surface (mm)

Design modulus

(MPa)

0100 4300

100200 3600

>200 2600

Source: Leek (2009).

A post-cracking phase may also be incorporated into this design method.

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In the City of Canning the standard practice is to apply a 30 mm thick asphalt layer over thefoamed bitumen stabilised layer. Dense grade and stone mastic asphalt has been placed onto thefoam surface. Local practitioners may recommend a sprayed seal if traffic is less than 106 ESA.

3.4 NZ Transport Agency Method

3.4.1 Background

To adapt the Austroads Pavement Design Guide(Austroads 2010) to New Zealand conditions, asupplement was published by NZ Transport Agency (2007). The supplement includes guidelinesfor engineering practitioners in applying Austroads design procedures resulting from researchresults and experience gained in New Zealand. It is important to notice that the normal bitumencontent used in New Zealand foamed bitumen pavements ranges from 2.7% to 3%, plus 1.0% to1.5% of cement or lime by weight. This is lower than the normal bitumen content used inAustralian foamed bitumen pavements (about 3.5%).

During the preparation of the supplement it was initially suggested to adopt the fatigue life of thefoamed bitumen layer using the Austroads asphalt fatigue equation, for a reliability of 95%,following the procedure adopted in the TMR. However, it was later argued (NZTA 2007) that theeffective fatigue behaviour was not observed in New Zealand foamed bitumen pavements, andthere was not enough evidence to justify the use of the Austroads fatigue equation for foamedbitumen mixtures. Therefore, the New Zealand supplement suggested that only the secondequivalent granular phase should be accounted for in the design:

While it is possible to analyse the seating-in (fatigue) phase using the Austroads

(Shell) hot mix asphalt performance criterion, it is unclear how appropriate the

criterion is for foamed bitumen stabilised materials. Given this uncertainty, it is

generally appropriate to design the foamed bitumen stabilised layer for the

steady-state condition only.

Finally, the New Zealand supplement gives the following advice to pavement designers:

At this stage, the majority of the expertise in the field of foamed bitumen

stabilisation is held by the contracting industry. Therefore, designers should seek

assistance from the industry regarding both the mix design and the layer thickness

analysis.

Normally, pavement designers in New Zealand design the pavement to inhibit rutting and shapeand do not consider the fatigue characteristics of foamed bitumen.

3.4.2 Design Inputs and Distress Models

The New Zealand supplement suggests the following general material properties for themechanical characterization of foamed bitumen layers:

vertical elastic modulus of the order of 800 MPa

anisotropic layer (Evertical=2Ehorizonal)

no sub-layering

Poissons ratio =0.3.

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The distress model adopted is the Austroads subgrade strain criterion, expressed in Equation 7:

7

9300

N

7

where

N = allowable number of standard axles before an unacceptable level ofpermanent deformation develops

= maximum vertical (compressive) strain (microstrains) at the top of thesubgrade.

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4 KNOWLEDGE-BASED TG2 2009 STRUCTURAL DESIGNMETHOD

4.1 BackgroundAfter the release of the interim South African TG2 2002 Guidelines (Section 2), it wasacknowledged that the guideline was not representative of current best practice, and would needupdating. An area of concern with the structural design models was the lack of validation usingfield performance data, and apparent differences between long-term field behaviour, behaviourunder accelerated testing and predicted performance from laboratory testing. South Africanpractitioners felt that the TG2 2002 Guidelines did not illustrate the benefits of foamed bitumentreatment and that mechanistic-empirical design methods could lead to the inappropriate design offoamed bitumen pavements.

Jooste and Long (2007) developed a new knowledge-based structural design method forpavements, which incorporated bituminous stabilised materials. The method is explained in thesecond edition of the TG2 guidelines, published in May 2009 (Asphalt Academy 2009). The TG2second edition, or TG2 2009, superseded the method previously published in the TG2 2002guidelines. The knowledge-based design method relies on an index, which quantifies theallowable traffic of a pavement. The index is called a pavement number (PN) and is used todetermine whether a pavement structure is appropriate for a given traffic intensity and confidencelevel.

The PN is similar to the structural number (SN) widely used in the AASHTO design method(AASHTO 1993). The PN is calculated using the layer thicknesses and assigned material classes(related to long-term stiffnesses). An empirical relationship between the PN and observedperformance of more than 80 pavement structures provides the basis for using the PN to assess

design capacity.

Jooste and Long (2007) suggest that the PN is more suitable than the AASHTO SN in determiningthe stiffness of each layer, because it couples the material class with the ratio between the layerstiffness and the stiffness of the supporting layer (using the modular ratio limit concept). Theother concepts and steps involved in this pavement design method are the effective long-termstiffness, the characterisation of subgrade materials and the base confidence factor, detailed in thefollowing section.

4.2 Concepts in the Development of the Pavement Number

4.2.1 The Effective Long-term Stiffness (ELTS)

The ELTS is a model parameter which serves as a relative indicator of the average long term insitu stiffness of a pavement layer. As such, the ELTS averages out the effects of the long-termdecrease of stiffness owing to traffic related deterioration, as well as seasonal variations instiffness. Thus the ELTS does not represent the stiffness of a material at any specific time.

It is also important to note that the ELTS, as defined for use in the PN, is not a stiffness value thatcan be determined by means of a laboratory or field test. Rather, it is a model parameter, whichwas calibrated for use in the PN-based design method and was developed as part of Jooste andLongs study. The ELTS values used in the calculation of the PN may therefore differ somewhatfrom the stiffness values that are conventionally adopted in the South African Mechanistic DesignMethod.

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4.2.2 Characterisation of Subgrade Materials

The first step in the calculation of the PN value is the determination of the subgrade class, whichrelates the equivalent material class for subgrade with stiffness. Then, the stiffness has to beadjusted by climate conditions (wet, moderate or dry) and by the placement of the subgrade within

the pavement structure (the thicker the pavement above the subgrade, the higher the equivalentstiffness).

4.2.3 The Modular Ratio Limit Concept

The modular ratio is a well known concept in flexible pavement engineering, and is defined as theratio of a layers stiffness relative to the stiffness of the layer below it. Thus, if the stiffness of abase layer is 400 MPa, and the stiffness of the support below it is 200 MPa, then the modular ratioof the base layer would be 2.0. An analysis of stress-sensitive material behaviour in finite elementmodels showed that, as a general rule of thumb, the modular ratio for unbound granular materialsis limited to less than 2.5, but in cases of more cohesive materials and weak support, modularratios as high as 5.0 may be possible (Jooste & Long 2007).

4.2.4 Assumed Behaviour of Bitumen Stabilised Layers

The behaviour of bitumen stabilised layers is assumed to be similar to that of unbound granularmaterials. However, it is also assumed in the method that bitumen stabilised materials are able todevelop significantly higher cohesive strength, and thus, compared to unbound granular materials,these materials are less dependent on the stiffness of the support layer. The assumption ofunbound material behaviour with a high cohesive strength places the behaviour of bitumenstabilised layers somewhere between that of a crushed stone and a cement stabilised material.For the purposes of determining the ELTS value, bitumen stabilised layers are modelled in asimilar manner to crushed rock materials, but a higher modular ratio limit was allowed compared tocrushed rock layers, to account for the higher cohesive strength.

4.2.5 Base Confidence Factor

The authors of the method argue that the required base quality is intimately linked to the intensityof the traffic loading, regardless of the overall pavement structure. South African experienceshows that there is a limit on the types of base materials that can be considered for a given trafficsituation. In particular, the suitable design options decrease significantly as the design trafficincreases.

This situation is specifically relevant in southern Africa, where thin pavements are the norm (similarto Australia and New Zealand). As the surfacing has a minor structural contribution, high stressesare applied to the underlying base. In such situations, the base is the main load-bearing elementin the pavement. Consequently, the method limits the types of base materials that can be used in

certain traffic situations.

In the proposed PN-based method, the appropriateness of the base material is controlled in twoways: (a) a base confidence factor (BCF) should be assigned to different material types; and (b)the design guidelines should include a checklist to ensure that practical considerations, such as theappropriateness of the base material, are taken into account.

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4.3 Calculation of PN

The steps involved for the calculation of the PN are presented in Figure 4.1, followed by anexample in Figure 4.2, which illustrates the calculation of the PN number for a pavement thatconsisted of a G8 subgrade material4, a G7 selected material (180 mm), a G6 subbase

(200 mm), a BSM2 bitumen stabilised layer (175 mm) and an AC asphalt surface (30 mm).

The first step of the calculation is the determination of the subgrade category and estimation of theinitial stiffness value (Eini). Then the initial stiffness (Eini) has to be adjusted by the climate zoneand the total pavement thickness. The initial stiffness (Eini) of the G8 subgrade material is 100 MPa(Table A 1 in Appendix A). Then Eini is adjusted for the climate zone (Fclimate = 0.9, Table A 2 inAppendix A) and pavement thickness cover above the subgrade (Fcover= -4 MPa, Table A 3 inAppendix A), which yields the ELTS of the subgrade layer, using Equation 8:

ELTS = (Eini x Fclimate) + Fcover 8

where

Eini = initial stiffness (MPa)

Fclimate = climate adjustment factor

Fcover = adjustment factor that varies with the thickness of the pavement layersabove the subgrade.

Figure 4.1: Steps in the knowledge-based structural design method for pavements

4G8, G7, G6, BSM2, etc. are material codes adopted by the material classification system used in the South African

Mechanistic Design Method.

Estimate subgradestiffness (Table A1)

For each pavement layer(i) determine: Emax andmodular ratio (Table A4)

Determine ELTS for layer i:ELTS = min [ Emax, modularratio(i) x ELTS(i-1) ]

Adjust by climate zone(Table A2)

Adjust by pavementthickness (Table A3)

PN(i)= ELTS(i) x thickness(i)

Adjust PN(base) byBCN (Table A4)

PN= PN(i)

Determine allowabletraffic (Figure 4.3)

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In Figure 4.2, this ELTS is 86 MPa (= 100 MPa x 0.9 - 4 MPa). The maximum stiffness (Emax) andmodular ratio has to be determined for each layer above the subgrade (a list of recommendedvalues for each material category is presented in Table A 4, Appendix A). Later, the ELTS isdetermined for each layer above the subgrade, starting with the bottom layer. The ELTS isdetermined as the minimum value between E

maxand the product between the modular ratio and

the ELTS from the layer located below the current layer.

Finally, the PN of each layer is calculated as the product of the layer thickness of each layer (inmetres) and the respective ELTS value in MPa, divided by 10. The PN of the basecourse has tobe adjusted by the base confidence factor (listed in Table A 4, Appendix A). The PN of thestructure is the sum of the individual PN for each layer, as shown in Figure 4.2.

1800 MPax

30 mm

AC

BSM2

G6

G7MR = 1.7

Emax = 140 MPa

MR = 1.8

Emax = 180 MPa

MR = 2.0

Emax= 450 MPa

MR = 5.0

Emax = 3500 MPa

Eini = 100 MPa

Fclimate = 0.9

Fcover= -4

30 mm

G8

Moderate

175 mm

200 mm

180 mm

1.7x86 = 146

Emax = 140

ELTS = 86 MPa

ELTS = 140 MPa

1.8x140 = 252

Emax = 180

ELTS = 180 MPa

5.0 x 360 = 1800

Emax = 3500 MPa

ELTS = 1800 MPa

PN = 2.5

PN = 3.6

PN = 4.4

PN = 5.4

Table A4

Table A1

Table A2

Table A3

Table A4

Table A4

Table A4

Table A4

140 MPa x

180 mm

180 MPa x

200 mm

0.7 x

360 MPa x

175 mm

PN = 16

2.0x180 = 360

Emax = 450

ELTS = 360 MPa

AC

Figure 4.2: Example of pavement number determination

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4.4 Design Criteria PN Model

4.4.1 Allowed Capacity

The relationship between PN and the allowed structural capacity (traffic) was developed by Jooste

and Long (2007) using three data sets, which together with the rules of pavement behaviourpresented above, formed the knowledge base. The three data sets are:

TRH4 Design Catalogue Set: this data set is comprised of structures extracted from theTRH4 design catalogue for Category A and B roads (reliability of 95% and 90%,respectively). The structures are those recommended for unbound base materials, to beused in wet and dry climates, and for traffic applications between 1 x 106 and 30 x 106 ESA.This data set was used as a foundation on which to calibrate the climate adjustment factors,as well as the material constants for unbound and cement stabilised layers.

The LTPP Data Set for Bitumen Stabilised Pavements: this data set comprises all theidentified in service pavements that incorporate bitumen stabilised layers, and for whichreliable historic pavement and traffic data could be obtained.

The HVS Data Set for Bitumen Stabilised Pavements: this data set comprises pavementsthat incorporate bitumen stabilised materials and which were tested using the heavy vehiclesimulator (HVS).

After an analysis of the trends exhibited by the combined data sets, a structural design criterion inthe form of a step function was developed (Figure 4.3). The criterion is not a traditional designequation, but rather represents a confidence frontier. This confidence frontier represents thestructural capacity for a given PN and assumed risk (road category), at which distress is unlikely tooccur.

The design method also provides recommendations for minimum and maximum foam bitumen

layer thicknesses of 100 mm and 350 mm respectively. The maximum thickness for the hot mixasphalt surface is 100 mm.

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Category A(95% Reliability)

Category B(90% Reliability)

Source: Jooste and Long (2007).

Figure 4.3: Criteria for determining allowed capacity based on PN


The authors of the method recommend that bituminous stabilised layers should be surfaced with asprayed seal if the traffic is less than 1 x 106 ESA. For traffic between 1 x 106 and 15 x 106 theyrecommend a hot mix asphalt layer. For traffic exceeding 15 x 106 ESA, a hot mix asphaltthickness of at least 50 mm is recommended. They support this recommendations by observingthe knowledge base (the three sets of data mentioned above) and also arguing that the risk ofearly failures caused by the in situ recycling process can be greatly alleviated by placing an asphaltlayer on top of the foamed bitumen layer. The asphalt surfacing requirements are summarised inFigure 4.4.

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.

Source: Jooste and Long (2007).

Figure 4.4: Recommended layer thicknesses versus structural capacity

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5 TRANSPORTATION RESEARCH LABORATORY

5.1 Background

A design guide was developed by the Transportation Research Laboratory (TRL) in the UnitedKingdom for the design and use of cold recycled materials for pavements (Merrill et al. 2004). Theguide presents design charts instead of design equations. The charts were developed usingasphalt fatigue relationships developed in previous research reports (Nunn 2004), assuming thatfoamed bitumen mixes behave similar to hot asphalt mixes.

The guide covers all material types that could be considered as cold recycled materials and both insitu and ex situ (recycled in plant) construction processes. The guide defines three families ofmaterials, presented in Figure 5.1. The apexes of this diagram correspond to fully hydraulicbound, fully visco-elastic bound and unbound material. Recycled materials using combinations ofbinder and curing behaviour can be characterised by areas within this chart. Four material typesthat fall into three material families are illustrated, in which foamed bitumen mixes are classified

within the visco-elastic hydraulic binders (Family 3). The four materials are defined as: quick hydraulic (QH) with hydraulic only binder(s) including cement

slow hydraulic (SH) with hydraulic only binder(s) excluding cement

quick visco-elastic (QVE) with bituminous and hydraulic binder(s) including cement

slow visco-elastic (SVE) with bituminous only or bituminous and hydraulic binder(s) excludingcement.

Fully visco-

elastically bound

Fully

hydraulically

bound

Unbound

Family 1 - Hydraulic

binders

Family 2 - Visco-

elastic binders

Family 3 - Visco-elastic/ Hydraulic binders

Source: Merrill et al. (2004).

Figure 5.1: Identification of material families

5.2 Design Procedure

5.2.1 Road Type Categories

The first two steps of the TRL design method are to classify the pavement by the expected trafficand the foundation class. Traffic is described in terms of a cumulative number of equivalent 80 kNstandard axles. These road categories are defined in terms of million standards axles (MSA) inTable 5.1. For each road type category, different levels of risk are assigned.

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Table 5.1: Road type category for TRL method

Road type category Traffic design standard (ESA x 106)

0 30 < Traffic < 80

1 10 < Traffic < 302 2.5 < Traffic < 10

3 0.5 < Traffic < 2.5

4 < 0.5


5.2.2 Foundation Class

The method proposes foundation stiffness classes, which represents the long term stiffness of thefoundation. The foundation stiffness classes are defined in terms of the equivalent half-spacestiffness of the composite foundations. The four foundation classes are listed in Table 5.2. A

Poissons ratio of 0.35 is proposed for the modelling of all foundation classes.

Table 5.2: Foundation classes for TRL method

Foundation class Assumed foundation support(MPa)

1 50

2 100

3 200

4 400

Source: Nunn (2004).

5.2.3 Classification of Foamed Bitumen Mixes and Design Chart

Bitumen bound materials are classified according to their stiffness measured in the indirect tensileresilient modulus test. The guide classifies the mixes into one of three zones labelled B1, B2 andB3.

The foamed bitumen specimens are compacted in the laboratory and later wrapped in cling-filmplastic. Once wrapped, the specimen is placed in a sealed plastic bag and then put in air or waterat 40 C for 28 days. Once the conditioning is finished the indirect tensile resilient modulus ismeasured and the mix is classified using Table 5.3. For instance, the assumed resilient modulus

for the Department of Transport and Main Roads, Queensland method for foamed bitumen mixes(2200 MPa) classifies between zone B1 and B2.

Once the foamed bitumen mix is classified, the designer selects the appropriate thickness designchart for the foundation class; the thickness for the design traffic can be determined for the curveassociated with the material zone. Table 5.3 shows a minimum long-term stiffness for each classwhich should be demonstrated in the specification using laboratory conditioning regimes defined inthe guide in the mix design process.

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Table 5.3: Bitumen bound cold recycled material classif ication for TRL method

Bitumen bound cold recycledzone

Minimum long-term stiffness(MPa) measured after 28 days

B1 1900

B2 2500

B3 3100


Figure 5.2 shows an example of the design curves used in the TRL guideline. The designer has tofirst decide which material zone is representative of the foamed bitumen mixes to be used in thefield, using laboratory resilient modulus measurements. Once the material zone is defined (Zones1, 2 or 3), the designer has to use the expected traffic input data (X-axis in Figure 5.2, expressed inESA x 106) and read the intersection of the traffic with the material zone, which gives the thicknessof the foamed bitumen layer.

T r a f f ic ( E S A x 1 06)

1 1 0 1 0 0

Totalthicknessofboundlayer(mm)

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

Z o n e B 1

Z o n e B 2

Z o n e B 3

T h e s e d e s i g n s in c lu d e u p t o 1 0 0 m ma s p h a l t s u r f a c in g

M a t e ria l Z o n e s( T a b l e 9 )


Figure 5.2: Design curves for bitumen bound cold recycled material, Foundation Class 1

For low volume roads, these minimum thickness requirements may give excess structural capacityand overly low risk of failure. Therefore, an alternative table with recommended thicknesses isprovided for roads with traffic less than 5 x 106 ESA (Table 5.4).

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Table 5.4: Thickness design of pavements up to 5 x 106 ESA

Type 2 road Type 3 road Type 4 road

Surfacing thickness (mm) 40 100 40 100 40 100

Subgrade design CBR 15 n/r 270 285 245 255 150



Table 5.5 shows the minimum thickness of surfacing, which relies on the traffic category.However, for Type 1 and Type 2 roads the thickness of the surfacing placed on top of the bitumenbound cold recycled material can be reduced to a minimum of 50 mm with a compensatingincrease in the thickness of the cold recycled structural course.

Table 5.5: Requirements for surfacing thickness for TRL method

Road type category Traffic design standard(ESA x 106)

Minimum thickness ofsurfacing (mm)

0 30 < Traffic < 80 100

1 10 < Traffic < 30 70

2 2.5 < Traffic < 10 50

3 0.5 < Traffic < 2.5 40

4 < 0.5 40


For Class B1 and B2 materials the compensation of the structural course can be determined usingthe equivalence relationship given in Equation 9:

SurfacingRBaseHH 3.1 9

where

HSurfacing = change in the thickness of bituminous surfacing

HRBase = change in the thickness of bitumen bound cold recycled base.

For example, a bitumen bound pavement design with a 100 mm surfacing could be reduced to a50 mm surfacing with a corresponding increase of 65 mm in the thickness of the cold recycledlayer.

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6 COMPARISON OF PAVEMENT DESIGNS

6.1 Summary of Design Methods

Six design methodologies for the design of foamed bitumen pavements have been presented inthe previous sections. A summary and comparison of the methods are presented in Table 6.1.The table shows that design methods have been developed using very different assumptions, dataand distress modes.

The TMR and TRL assumed that foamed bitumen mixes behave similarly to hot mix asphalt mixesand therefore fatigue relationships, with some modifications, were adopted for the thicknessdesign. However, this assumption is based on engineering judgment and there is no dataavailable to firmly support this approach. The fatigue distress mode proposed by TMR has beenobserved in Queensland foamed bitumen projects, but only in early failures, which could not betotally representative of the normal long-term deterioration on this type of pavement. The City ofCanning developed a fatigue relationship using data from several flexural fatigue tests. Although

this method is very different to the TMR method, they arrive to similar predictions of pavement lifeat typical strain levels. The City of Canning is the only method that sublayers the foamed bitumenlayer assuming different elastic modulus at different depths.

In the development of the TG2 2002 Guidelines method the foamed bitumen layer was treated as adifferent, new material and therefore more advanced testing was conducted to understand thefundamental performance of these mixes (i.e. accelerated full-scale testing). However, the distressequations were developed using a foamed bitumen layer stabilised with a high (2%) cementcontent and low bitumen content (1.8%), which is not the common practice in Australia.Furthermore, several limitations have been found by practitioners in the distress equations(Jenkins et al. 2008).

Due to the uncertainty of the performance of the foamed bitumen layers, NZ Transport Agencyadopted a slightly different procedure, which ignores the fatigue of the foamed bitumen layer.However, the method recommends an elastic modulus based on the observation of foamedbitumen pavements in New Zealand. Pavement thickness design assumes rutting and shape lossis the main distress mode.

A different approach was adopted by the knowledge-based South African method, which is theonly empirical method found in the literature. This method also incorporates some practicalaspects that are inexistent in the other mechanistic methods, such as the recommended surfacethickness and the minimum and maximum thickness for the foamed bitumen layer.

In order to compare the design methods from a more practical, objective point of view, a

hypothetical case study was conducted, which involves the design of a foamed bitumen pavementusing the six methods presented above. The study consists of the rehabilitation of an existingunbound granular pavement using the in situ foamed bitumen stabilisation technique, as describedin the following section.

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Table 6.1: Summary of design methods

TG2 2002 TMR City of Canning NZ Transport Agency

Country South Africa Australia Australia New Zealand

Design models used with South African Mechanistic

Design Method

Austroads design method Austroads design m

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