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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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Suggested Citation:"Front Matter." National Academies of Sciences, Engineering, and Medicine. 2009. An Experimental Plan for Validation of an Endurance Limit for HMA Pavements. Washington, DC: The National Academies Press. doi: 10.17226/23277.
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ACKNOWLEDGMENT This work was sponsored by the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program (NCHRP), which is administered by the Transportation Research Board (TRB) of the National Academies. COPYRIGHT PERMISSION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FTA, Transit Development Corporation, or AOC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. DISCLAIMER The opinion and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the TRB, the National Research Council, AASHTO, or the U.S. Government. This report has not been edited by TRB.

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements TABLE OF CONTENTS List of Figures ................................................................................................................................ iv List of Tables ...................................................................................................................................v ACKNOWLEDGMENTS ............................................................................................................. vi ABSTRACT.................................................................................................................................. vii SUMMARY................................................................................................................................. viii 1. INTRODUCTION AND RESEARCH APPROACH .................................................................1 1.1 Introduction.......................................................................................................................... 1 1.2 Problem Statement and Research Objective ........................................................................ 1 1.3 Research Approach .............................................................................................................. 2 2. FINDINGS................................................................................................................................13 2.1 HMA Endurance Limit ...................................................................................................... 13 2.2 Framework for Incorporating Endurance Limit Behavior in Flexible Pavement Design.. 15 3. INTERPRETATION, APPRAISAL AND APPLICATIONS..................................................30 4. CONCLUSIONS AND RECOMMENDATIONS ...................................................................35 REFERENCES ..............................................................................................................................36 APPENDIX A. HMA ENDURANCE LIMIT WORKSHOP EXECUTIVE SUMMARY ........ A-i APPENDIX B. HMA ENDURANCE LIMIT VALIDATION STUDY RESEARCH PLAN ... B-i iii

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements LIST OF FIGURES Figure 1. HMA Endurance Limit Workshop Agenda.................................................................... 5 Figure 2. Results of Flexural Fatigue Tests by Carpenter et al., Including Extrapolated Results at Low Strain Levels . ...................................................................................... 14 Figure 3. Effect of Rest Periods on Plateau Value ...................................................................... 17 Figure 4. Effect of Rest Period on Fatigue Life........................................................................... 18 Figure 5. Plateau Value for Continuous Loading as a Function of Applied Strain Level. .......... 20 Figure 6. Application of Time-Temperature Superposition to Rest Periods. .............................. 23 Figure 7. Comparison of Applied and Allowable Strains............................................................ 27 Figure 8. Example of Minimum Asphalt Thicknesses to Resist Bottom Initiated Fatigue Cracking With Observed Performance of Four UK Pavement Sections. ..................... 28 Figure 9. Project Flow Chart........................................................................................................ 30 Figure 10. Overall Project Schedule. ........................................................................................... 32 iv

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements LIST OF TABLES Table 1. HMA Endurance Limit Workshop Invitees and Attendees. ............................................ 4 Table 2. Summary of Existing Pavement Analysis Approaches Considered. ............................... 7 Table 3. Summary of Proposed Laboratory Experiments.............................................................. 8 Table 4. Matrix for Field Calibration of the Allowable Strain Limit Design Procedure............. 11 Table 5. Effect of Rest Period on Fatigue Life. ........................................................................... 18 Table 6. Approximate Rest Periods for Various Design Traffic Levels...................................... 19 Table 7. Allowable Strains for Various Design Traffic Levels. .................................................. 22 Table 8. Computation of Allowable Strain Strains...................................................................... 25 Table 9. Applied Strains for Design Example. ............................................................................ 26 Table 10. Summary of Man-hour and Cost Estimates. ................................................................ 31 v

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 9-44 by Advanced Asphalt Technologies, LLC. Ramon Bonaquist, Chief Operating Officer for Advanced Asphalt Technologies, LLC, served as Principal Investigator for the project and authored this report and the research plan which was the primary product of NCHRP Project 9-44. Mr. Harold Von Quintus, Principal Engineer for Applied Research Associates, Inc. co-authored the research plan. NCHRP Project 9-44 included a facilitated workshop to discuss current asphalt concrete fatigue research and to identify alternatives for incorporating an endurance limit for asphalt concrete in mechanistic-empirical pavement design. Mr. Charles Markert, President of Dynamic Leadership Consulting Group, facilitated the workshop. Mrs. Rane Wagner, President of Rane Wagner and Associates, provided logistical support of the workshop. Special thanks are extended to the participants of the HMA Endurance Limit Workshop listed below. These professionals unselfishly presented ideas to the research team that helped shape the planned research. Name Affiliation Dr. David Anderson Consultant Dr. Samuel Carpenter University of Illinois Dr. Donald Christensen Advanced Asphalt Technologies, LLC Dr. Herve Di Benedetto Ecole Nat. des TPE Mr. Bruce Dietrich Florida Department of Transportation Mr. Kenneth Fults KWF Pavement Consulting Mr. Roger Green Ohio Department of Transportation Dr. Kevin Hall University of Arkansas Dr. Edward Harrigan National Cooperative Highway Research Program Dr. Richard Kim North Carolina State University Dr. Dallas Little Texas A&M University Dr. Leslie Ann McCarthy Federal Highway Administration Dr. Andre Molenaar Delft University Professor Carl Monismith University of California Berkeley Dr. David Newcomb National Asphalt Pavement Association Dr. Michael Nunn Lane One Limited Dr. Brian Prowell Advanced Material Services, LLC Dr. Rey Roque University of Florida Ms. Amy Schutzbach Illinois Department of Transportation Dr. Jacob Uzan Technion University Dr. Linbing Wang Virginia Polytechnic and State University Dr. Matthew Witczak Arizona State University vi

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements ABSTRACT This report documents the work completed in National Cooperative Highway Research Program (NCHRP) Project 9-44. The objective of NCHRP Project 9-44 was to prepare a research plan and associated cost estimate for a future study to validate the endurance limit for HMA and to improve mechanistic-empirical pavement design. The primary product of NCHRP Project 9-44 is the HMA Endurance Limit Validation Study Research Plan. The planned research is based on the hypothesis that the endurance limit for HMA is the result of a balance of damage caused by loading and healing or damage recovery that occurs during rest periods. Under this hypothesis the primary objective in designing a flexible pavement to resist bottom initiated fatigue cracking will be to make sure that the damage induced by loading remains small enough so that healing occurs and there is no accumulation of damage over the life of the pavement. This is a significant departure from current cumulative or incremental damage models, which assume that no healing occurs and that each load cycle uses up a portion of the finite fatigue life of the HMA. This research plan includes the framework for a design procedure that is based on layered elastic analysis and compatible with the Mechanistic-Empirical Pavement Design Guide (MEPDG). It uses allowable strains to identify satisfactory conditions for full healing. The allowable strains are a function of the properties of the HMA, the pavement temperature, and the duration of rest periods between traffic loads. Five laboratory experiments that are needed to fully develop the procedure are described. Studies using data from completed accelerated pavement tests and test roads are proposed to verify critical aspects of the design procedure. Finally, an experiment to calibrate the design procedure using selected test sections from the Long Term Pavement Performance Program is presented. vii

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements SUMMARY This report documents the work completed in National Cooperative Highway Research Program (NCHRP) Project 9-44. The objective of NCHRP Project 9-44 was to prepare a research plan and associated cost estimate for a future study to validate the endurance limit for HMA and to improve mechanistic-empirical pavement design. The primary product of NCHRP Project 9-44 is the HMA Endurance Limit Validation Study Research Plan. The research plan was formulated from a review of relevant research and the recommendations from a workshop that included participation by a number of international experts. Completed laboratory and field investigations clearly show that hot mix asphalt (HMA) exhibits endurance limit behavior. This endurance limit, however, does not reflect an absence of load induced damage in the HMA. It is the result of a balance of damage caused by loading and healing or damage recovery that occurs during rest periods. The endurance limit for HMA is, therefore, not a single value, but will change depending on the loading and environmental conditions applied to the HMA. To properly consider this form of an endurance limit in flexible pavement design requires consideration of the effects of loading, environment and material properties on both damage accumulation and healing. These findings concerning the endurance limit for HMA served as the research hypothesis upon which the HMA Endurance Limit Validation Study Research Plan was formulated. To effectively design laboratory and field experiments for the HMA Endurance Limit Validation Study, the framework for a pavement design procedure that incorporates healing and endurance limit behavior was conceived during NCHRP 9-44. The procedure is based on layered elastic analysis and is compatible with the Mechanistic-Empirical Pavement Design Guide (MEPDG). It uses allowable strains to identify satisfactory conditions for full healing. The allowable strains are a function of the properties of the HMA, the pavement temperature, and the duration of rest periods between traffic loads. The underlying principal of the design procedure is to make sure that the damage induced by loading remains small enough so that full healing occurs between traffic loads and there is no accumulation of damage over the life of the pavement. This is a significant departure from current cumulative or incremental damage viii

NCHRP Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements ix models, which assume that no healing occurs and that each load cycle uses up a portion of the finite fatigue life of the HMA. The HMA Endurance Limit Validation Study is included as a stand-alone appendix to this report. It is a comprehensive plan for research to rationally incorporate the concept of an endurance limit for HMA into a mechanistic-empirical algorithm for bottom initiated fatigue cracking in flexible pavements, and to validate the resulting procedure using performance data from full-scale pavement sections. The plan presents details of five laboratory experiments that are needed to fully develop the procedure. Studies using data from completed accelerated pavement tests and test roads are proposed to verify critical aspects of the design procedure. Finally, an experiment to calibrate the design procedure using selected test sections from the Long Term Pavement Performance Program is presented.

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TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 134: An Experimental Plan for Validation of an Endurance Limit for HMA Pavements explores a research plan and associated cost estimate for a future study to validate the endurance limit for hot-mix asphalt (HMA) and to improve mechanistic-empirical pavement design.

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