TRANSPORT RESEARCH LABORATORY Design of long-life flexible pavements for heavy traffic Prepared for Highways Agency, British Aggregate Construction Materials Industries* and the Refined Bitumen Association *British Aggregate Construction Materials Industries (BACMI) changed its name to Quarry Products Association on 24 June 1997 M E Nunn, A Brown, D Weston and J C Nicholls TRL REPORT 250 First Published 1997 ISSN 0968-4107 Copyright Transport Research Laboratory 1997. The information contained herein is the property of the Transport Research Laboratory and does not necessarily reflect the views or policies of the customer for whom this report was prepared. Whilst every effort has been made to ensure that the matter presented in this report is relevant, accurate and up-to-date at the time of publication, the Transport Research Laboratory cannot accept any liability for any error or omission. Transport Research Foundation Group of Companies Transport Research Foundation (a company limited by guarantee) trading as Transport Research Laboratory. Registered in England, Number 3011746. TRL Limited. Registered in England, Number 3142272. Registered Offices: Old Wokingham Road, Crowthorne, Berkshire, RG45 6AU. CONTENTS Page Foreword 1 Executive Summary 3 1 Introduction 5 2 Design considerations 6 2.1 Deterioration mechanisms 6 2.2 Design life 6 2.3 Traffic 7 2.4 Pavement construction 7 2.5 Summary 7 3 Pavement foundations 8 3.1 Foundation design 8 3.1.1 Thickness 8 3.2 End-product performance specification 9 3.2.1 Introduction 9 3.2.2 State of compaction 9 3.2.3 Elastic stiffness 9 3.2.4 Results of full scale trial 10 3.3 Construction practice 10 3.4 Summary 10 4 Structural layers 10 4.1 Design thicknesses 10 4.2 Design criteria 11 4.3 Summary 12 5 Wearing course 12 5.1 Surface requirements 12 5.2 Thick paver-laid wearing course 14 5.2.1 Hot rolled asphalt 14 5.2.2 Porous asphalt 15 5.2.3 Stone mastic asphalt 16 5.3 Thin surfacings 16 5.4 Further considerations 16 5.5 Summary 17 6 Modifications to standard designs 18 6.1 New or improved asphalt roadbase 18 6.2 Areas at high risk of surface rutting 18 6.3 Asphalt substitution 19 6.4 Summary 19 iii Page 7 Example of long-life design 20 7.1 Foundation 20 7.2 Structural layers 21 7.3 Wearing course 21 7.4 Summary 21 8 Condition assessment 22 8.1 Summary 22 9 Whole life costing 22 9.1 Summary 23 10 Risk analysis 23 10.1 Perceived risks 23 10.1.1 Design methodology 23 10.1.2 Construction risks 24 10.1.3 In-service risks 24 10.2 Observed pavement performance 25 10.3 Comparison of risks of conventional and long-life designs 26 10.4 Summary 26 11 Implementation 26 11.1 Long-life design 27 11.2 Innovative materials 27 11.3 Performance-based specifications 27 11.4 Pavement condition assessment strategy 28 12 Future design 28 13 Conclusions 28 14 Acknowledgements 28 15 Glossary 29 16 References 29 Appendix A: Pavement performance 32 A1 Introduction 32 A2 Performance 32 A2.1 General 32 A2.2 Rutting 32 A2.3 Fatigue 34 iv Page A2.3.1 Investigation of roadbase fatigue in UK motorways 35 A2.3.2 Further supporting evidence 36 A2.4 Surface cracking 36 A2.5 Curing of asphalt 40 A2.6 Long-term pavement strength 41 A3 Conclusions 42 A4 References 42 Appendix B: Pavement foundations 44 B1 Introduction 44 B2 Current UK practice 44 B3 European practice 44 B3.1 Germany 44 B3.2 France 45 B4 End-product testing 45 B4.1 State of compaction 46 B4.2 Elastic stiffness 46 B5 Pilot-scale trial 47 B5.1 Field testing methods and results 47 B5.1.1 CBR 48 B5.1.2 Density measurements 48 B5.1.3 Elastic stiffness 48 B5.2 Trafficking 49 B5.3 Conclusions from pilot-scale trial 52 B6 Full-scale trial 52 B6.1 Formation 52 B6.1.1 In-situ density 53 B6.1.2 Surface modulus 53 B6.2 Foundation 53 B6.2.1 In-situ density 53 B6.2.2 Surface modulus 54 B6.3 Conclusions from full-scale trial 54 B7 Implementation 54 B8 Practical implications 55 v Page B9 References 55 Appendix C: Design of long-life roads 56 C1 Factors influencing pavement performance 56 C1.1 Curing and traffic flow 56 C1.1.1 Threshold strength 57 C1.2 Surface Cracking 57 C1.3 Provision for increase in maximum legal axle load 57 C2 Risk of premature failure 57 C3 Design thicknesses 58 C4 Determination of the constant, Ko 58 C5 Input data for standard designs up to 80 msa 59 C6 References 60 Appendix D: Wearing course trials using modified binders 61 D1 Hot rolled asphalt 61 D1.1 M4, Avon 61 D1.2 A303, Wiltshire 61 D1.3 A38, Staffordshire 62 D1.4 M6, Staffordshire 62 D1.5 Assessment of other modifiers used in hot rolled asphalt 63 D1.5.1 Assessment Procedure 63 D1.5.2 Novophalt 63 D1.5.3 Evatech H 63 D1.5.4 Shell Multiphalte 63 D2 Porous asphalt 64 D3 Types of bitumen modifier and additive for asphalt surfacing 65 D4 References 66 Appendix E: Adapting designs for high modulus asphalt base (HMB) 68 E1 Design stiffness for HMB 68 E2 Early-life stiffness, curing and design 70 vi Page E3 References 71 Appendix F: Whole-life cost 72 F1 Introduction 72 F2 Pavement scenarios 72 F3 Whole-life costs 72 F4 Summary 76 F5 References 76 Appendix G: Summary of possible risks 80 Abstract 81 Related publications 81 vii Foreword The current Department of the Environment,Transport and maintained their strength or become stronger over time: the Regions pavement design standards are described in they were not gradually weakened with trafficking. The Volume 7 of the Design Manual for Roads and Bridges overall conclusion of this project is that well constructed (DMRB). For the particular case of fully flexible roads that are built above a threshold strength will have pavements, the Standard (DMRB 7.2.3) was established by long lives provided that distress, in the form of cracks and considering the performance of a wide range of ruts appearing at the surface, is treated before it begins to experimental pavements which formed part of the trunk affect the structural integrity. Such roads should be road network. These pavements were constructed over a 20 referred to as long-life roads. year period starting in the early 1950s. The interpretation This is a major finding of this project, commissioned by of the structural performance of these roads was based on the Highways Agency, British Aggregate Construction theoretical design concepts. This led to a design method, Materials Industries and the Refined Bitumen Association developed in 1984 and described in TRRL Laboratory and carried out by the Transport Research Laboratory. A Report 1132, that took full advantage of theoretical key element has been the contribution from other projects methods of analysis, whilst making due allowance for their commissioned by sponsors but particularly those from limitations. within the Highways Agency’s research programme. The A design life of 40 years was advocated in LR 1132 Agency has provided access to the findings of completed which was achieved for fully flexible pavements by projects and those emerging from current work with a total strengthening the road after about 20 years. A detailed value of over £5 million, without which the conclusions calculation of the cost of asphalt roads over 40 years, from the project would have been far less robust. The based on our understanding of pavement deterioration at detailed results of this project are contained in this report. that time, showed this to be the optimum economic solution. This calculation took into account variability of pavement performance, cost of traffic delays and other costs associated with maintenance and reconstruction. Since this method was introduced in the mid 1980s, traffic levels have continued to increase, with the consequent increase in user costs due to traffic disruption at roadworks. More recent work has indicated that it would be more cost effective to increase the design life of fully flexible roads for very heavily trafficked locations to at least 40 years, without the need for structural strengthening, to reduce future maintenance and the associated traffic delays. Coupled with this, more technical knowledge has become available in recent years on the performance of heavily trafficked, asphalt roads. In particular, LR 1132 introduced a criterion that ensured that future roads were at no greater risk of fatigue cracking than roads constructed in the past. This was a conservative measure based on knowledge at the time. Subsequent experience has not detected evidence of fatigue cracking or damage in the main structural layers of the thicker, more heavily trafficked pavements. This, and other information, has indicated that deterioration, as either cracking or deformation, is far more likely to be found in the surfacing than deeper in the pavement structure; this evidence is in conflict with conventional theory. Also, it was found that the great majority of the thick pavements examined have 1 2 Executive Summary This report describes research sponsored by the Highways presented and the implications of adopting an end-product Agency, British Aggregate Construction Materials performance specification for the road foundation are Industries and the Refined Bitumen Association. The outlined. The results of the foundation investigations are overall objective was to review current design practice and reported in more detail in Appendix B. information on flexible pavement performance that has The design of the structural layers, presented in Section accrued since the last revision of the design standards and 4, recognises that pavement behaviour is complex and that to develop an improved design method for heavily it is not possible to quantify precisely all factors affecting trafficked, flexible pavements.