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NATIONAL NCHRP REPORT 646 COOPERATIVE HIGHWAY RESEARCH PROGRAM Validating the Fatigue Endurance Limit for Hot Mix Asphalt

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TRANSPORTATION RESEARCH BOARD 2010 EXECUTIVE COMMITTEE* OFFICERS CHAIR: Michael R. Morris, Director of Transportation, North Central Texas Council of Governments, Arlington VICE CHAIR: Neil J. Pedersen, Administrator, Maryland State Highway Administration, Baltimore EXECUTIVE DIRECTOR: Robert E. Skinner, Jr., Transportation Research Board MEMBERS J. Barry Barker, Executive Director, Transit Authority of River City, Louisville, KY Allen D. Biehler, Secretary, Pennsylvania DOT, Harrisburg Larry L. Brown, Sr., Executive Director, Mississippi DOT, Jackson Deborah H. Butler, Executive Vice President, Planning, and CIO, Norfolk Southern Corporation, Norfolk, VA William A.V. Clark, Professor, Department of Geography, University of California, Los Angeles Nicholas J. Garber, Henry L. Kinnier Professor, Department of Civil Engineering, and Director, Center for Transportation Studies, University of Virginia, Charlottesville Jeffrey W. Hamiel, Executive Director, Metropolitan Airports Commission, Minneapolis, MN Edward A. (Ned) Helme, President, Center for Clean Air Policy, Washington, DC Randell H. Iwasaki, Director, California DOT, Sacramento Adib K. Kanafani, Cahill Professor of Civil Engineering, University of California, Berkeley Susan Martinovich, Director, Nevada DOT, Carson City Debra L. Miller, Secretary, Kansas DOT, Topeka Pete K. Rahn, Director, Missouri DOT, Jefferson City Sandra Rosenbloom, Professor of Planning, University of Arizona, Tucson Tracy L. Rosser, Vice President, Corporate Traffic, Wal-Mart Stores, Inc., Mandeville, LA Steven T. Scalzo, Chief Operating Officer, Marine Resources Group, Seattle, WA Henry G. (Gerry) Schwartz, Jr., Chairman (retired), Jacobs/Sverdrup Civil, Inc., St. Louis, MO Beverly A. Scott, General Manager and Chief Executive Officer, Metropolitan Atlanta Rapid Transit Authority, Atlanta, GA David Seltzer, Principal, Mercator Advisors LLC, Philadelphia, PA Daniel Sperling, Professor of Civil Engineering and Environmental Science and Policy; Director, Institute of Transportation Studies; and Interim Director, Energy Efficiency Center, University of California, Davis Douglas W. Stotlar, President and CEO, Con-Way, Inc., Ann Arbor, MI C. Michael Walton, Ernest H. Cockrell Centennial Chair in Engineering, University of Texas, Austin EX OFFICIO MEMBERS Thad Allen (Adm., U.S. Coast Guard), Commandant, U.S. Coast Guard, U.S. Department of Homeland Security, Washington, DC Peter H. Appel, Administrator, Research and Innovative Technology Administration, U.S.DOT J. Randolph Babbitt, Administrator, Federal Aviation Administration, U.S.DOT Rebecca M. Brewster, President and COO, American Transportation Research Institute, Smyrna, GA George Bugliarello, President Emeritus and University Professor, Polytechnic Institute of New York University, Brooklyn; Foreign Secretary, National Academy of Engineering, Washington, DC Anne S. Ferro, Administrator, Federal Motor Carrier Safety Administration, U.S.DOT LeRoy Gishi, Chief, Division of Transportation, Bureau of Indian Affairs, U.S. Department of the Interior, Washington, DC Edward R. Hamberger, President and CEO, Association of American Railroads, Washington, DC John C. Horsley, Executive Director, American Association of State Highway and Transportation Officials, Washington, DC David T. Matsuda, Deputy Administrator, Maritime Administration, U.S.DOT Victor M. Mendez, Administrator, Federal Highway Administration, U.S.DOT William W. Millar, President, American Public Transportation Association, Washington, DC Cynthia L. Quarterman, Administrator, Pipeline and Hazardous Materials Safety Administration, U.S.DOT Peter M. Rogoff, Administrator, Federal Transit Administration, U.S.DOT David L. Strickland, Administrator, National Highway Traffic Safety Administration, U.S.DOT Joseph C. Szabo, Administrator, Federal Railroad Administration, U.S.DOT Polly Trottenberg, Assistant Secretary for Transportation Policy, U.S.DOT Robert L. Van Antwerp (Lt. Gen., U.S. Army), Chief of Engineers and Commanding General, U.S. Army Corps of Engineers, Washington, DC *Membership as of February 2010.

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NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP REPORT 646 Validating the Fatigue Endurance Limit for Hot Mix Asphalt Brian D. Prowell ADVANCED MATERIALS SERVICES, LLC1 Auburn, AL E. Ray Brown U.S. ARMY CORPS OF ENGINEERS1 Vicksburg, MS R. Michael Anderson ASPHALT INSTITUTE Lexington, KY Jo Sias Daniel Aravind Krishna Swamy UNIVERSITY OF NEW HAMPSHIRE Durham, NH Harold Von Quintus APPLIED RESEARCH ASSOCIATES, INC. Austin, TX Shihui Shen2 WASHINGTON STATE UNIVERSITY Pullman, WA Samuel H. Carpenter UNIVERSITY OF ILLINOIS Urbana-Champaign, IL Sudip Bhattacharjee3 ALABAMA A&M UNIVERSITY Huntsville, AL Saeed Maghsoodloo NATIONAL CENTER FOR ASPHALT TECHNOLOGY Auburn, AL Formerly 1National Center for Asphalt Technology, 2University of Illinois, and 3University of New Hampshire, respectively. Subscriber Categories Materials Pavements Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration TRANSPORTATION RESEARCH BOARD WASHINGTON, D.C. 2010 www.TRB.org

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NATIONAL COOPERATIVE HIGHWAY NCHRP REPORT 646 RESEARCH PROGRAM Systematic, well-designed research provides the most effective Project 09-38 approach to the solution of many problems facing highway ISSN 0077-5614 administrators and engineers. Often, highway problems are of local ISBN 978-0-309-11821-7 interest and can best be studied by highway departments individually Library of Congress Control Number 2010921174 or in cooperation with their state universities and others. However, the 2010 National Academy of Sciences. All rights reserved. accelerating growth of highway transportation develops increasingly complex problems of wide interest to highway authorities. These problems are best studied through a coordinated program of COPYRIGHT INFORMATION cooperative research. Authors herein are responsible for the authenticity of their materials and for obtaining In recognition of these needs, the highway administrators of the written permissions from publishers or persons who own the copyright to any previously American Association of State Highway and Transportation Officials published or copyrighted material used herein. initiated in 1962 an objective national highway research program Cooperative Research Programs (CRP) grants permission to reproduce material in this employing modern scientific techniques. This program is supported on 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, a continuing basis by funds from participating member states of the FMCSA, FTA, or Transit Development Corporation endorsement of a particular product, Association and it receives the full cooperation and support of the method, or practice. It is expected that those reproducing the material in this document for Federal Highway Administration, United States Department of 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 Transportation. from CRP. The Transportation Research Board of the National Academies was requested by the Association to administer the research program because of the Board's recognized objectivity and understanding of NOTICE modern research practices. The Board is uniquely suited for this purpose as it maintains an extensive committee structure from which The project that is the subject of this report was a part of the National Cooperative Highway Research Program conducted by the Transportation Research Board with the approval of authorities on any highway transportation subject may be drawn; it the Governing Board of the National Research Council. Such approval reflects the possesses avenues of communications and cooperation with federal, Governing Board's judgment that the program concerned is of national importance and state and local governmental agencies, universities, and industry; its appropriate with respect to both the purposes and resources of the National Research Council. relationship to the National Research Council is an insurance of The members of the technical committee selected to monitor this project and to review this objectivity; it maintains a full-time research correlation staff of report were chosen for recognized scholarly competence and with due consideration for the specialists in highway transportation matters to bring the findings of balance of disciplines appropriate to the project. The opinions and conclusions expressed research directly to those who are in a position to use them. or implied are those of the research agency that performed the research, and, while they have been accepted as appropriate by the technical committee, they are not necessarily those of The program is developed on the basis of research needs identified the Transportation Research Board, the National Research Council, the American by chief administrators of the highway and transportation departments Association of State Highway and Transportation Officials, or the Federal Highway and by committees of AASHTO. Each year, specific areas of research Administration, U.S. Department of Transportation. needs to be included in the program are proposed to the National Each report is reviewed and accepted for publication by the technical committee according Research Council and the Board by the American Association of State to procedures established and monitored by the Transportation Research Board Executive Committee and the Governing Board of the National Research Council. Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are The Transportation Research Board of the National Academies, the National Research Council, the Federal Highway Administration, the American Association of State Highway selected from those that have submitted proposals. Administration and and Transportation Officials, and the individual states participating in the National surveillance of research contracts are the responsibilities of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade Research Council and the Transportation Research Board. or manufacturers' names appear herein solely because they are considered essential to the object of this report. The needs for highway research are many, and the National Cooperative Highway Research Program can make significant contributions to the solution of highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement rather than to substitute for or duplicate other highway research programs. Published reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from: Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet at: http://www.national-academies.org/trb/bookstore Printed in the United States of America

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COOPERATIVE RESEARCH PROGRAMS CRP STAFF FOR NCHRP REPORT 646 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs Edward T. Harrigan, Senior Program Officer Melanie Adcock, Senior Program Assistant Eileen P. Delaney, Director of Publications Hilary Freer, Senior Editor NCHRP PROJECT 09-38 PANEL Field of Materials and Construction--Area of Bituminous Materials Linda M. Pierce, Applied Pavement Technology, Inc., Santa Fe, NM (Chair) Steven W. Krebs, Wisconsin DOT, Madison, WI Allen Cooley, Burns Cooley Dennis, Inc., Ridgeland, MS G. William Maupin, Jr., Virginia DOT, Charlottesville, VA Richard W. May, SemMaterials, LP, Tulsa, OK Leslie Ann McCarthy, Villanova University, Villanova, PA Carl L. Monismith, University of California - Berkeley, Berkeley, CA Amy M. Schutzbach, Illinois DOT, Springfield, IL Gregory A. Sholar, Florida DOT, Gainesville, FL Linbing Wang, Virginia Polytechnic Institute and State University, Blacksburg, VA Travis M. Wombwell, Missouri DOT, Macon, MO Wei-Shih "Wes" Yang, New York State DOT, Albany, NY Ernest Bastian, FHWA Liaison John "Jack" Youtcheff, FHWA Liaison Frederick Hejl, TRB Liaison

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FOREWORD By Edward T. Harrigan Staff Officer Transportation Research Board This report presents the findings of research performed to investigate the existence of a fatigue endurance limit for hot mix asphalt (HMA) mixtures, the effect of HMA mixture characteristics on the endurance limit, and the potential for the limit's incorporation in structural design methods for flexible pavements. The report describes the research per- formed and includes proposed standard practices using various experimental and analyti- cal procedures for determining the endurance limit of HMA mixtures. Thus, the report will be of immediate interest to materials and structural design engineers in state highway agen- cies and engineers in the HMA construction industry. Many well-constructed flexible pavements with a thick HMA structure have been in service for 40 or more years without any evidence of bottom-up fatigue cracking. This field experience suggests that an endurance limit, that is, a level of strain below which fatigue damage does not occur for any number of load repetitions, is a valid concept for HMA mixtures. The concept of an endurance limit is widely recognized in many areas of materials science, especially that of ferrous metals. The endurance limit is usually calculated from the relationship of strain to load repetitions to failure. Defining an endurance limit for HMA mixtures will result in more efficient structural design of flexible pavements built with mixtures of varying properties. For instance, small increases in the binder content of HMA mixtures provide longer fatigue lives (presumably because of a higher strain level for the endurance limit of these mixtures). Other factors likely to determine the value of the fatigue endurance limit for a given HMA mixture are the incorporation of a modifier in the asphalt binder, the aggregate type and gradation, the asphalt binder grade, and the mixture's volumetric properties. Previous research suggested that the fatigue behavior of flexible pavements is consistent with the existence of an endurance limit with an approximate value of 70 microstrains. However, few laboratory studies corroborate this value. Moreover, pavement design approaches, including the Mechanistic-Empirical Pavement Design Guide (MEPDG) developed in NCHRP Projects 1-37A and 1-40, do not fully incorporate the endurance limit concept. This is because, to date, research into the fatigue of HMA mixtures has been limited to strain levels well above the hypothesized value of 70 microstrains. The objectives of this research were to (1) test the hypothesis that there is an endurance limit in the fatigue behavior of HMA mixtures, (2) measure the value of the endurance limit for a representative range of HMA mixtures, and (3) recommend a procedure to incorporate the effects of the endurance limit into mechanistic pavement design methods. The research was performed by the National Center for Asphalt Technology, Auburn University, Auburn, AL, with the assistance of the following organizations: Applied Research Associates,

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Inc., Round Rock, TX; the Asphalt Institute, Lexington, KY; the University of Illinois, Urbana-Champaign, IL; and the University of New Hampshire, Durham, NH. The report fully documents the design and conduct of an extensive laboratory program of beam fatigue and uniaxial tension testing that experimentally confirmed the existence of an HMA fatigue endurance limit and quantified how the value of the limit is influenced by HMA mixture and binder properties. Based on these results, a practical definition of the endurance limit was developed, along with a methodology to estimate the endurance limit in the laboratory. Analysis of in-service pavements by the research team also demonstrated the existence of the endurance limit and indicated that polymer modification of asphalt binders improves the fatigue performance of HMA mixtures and flexible pavements. Finally, sensitivity analyses were conducted that indicated that the value of the endurance limit can affect the recommended thickness of perpetual pavements designed with the MEPDG and PerRoad methodologies significantly. This report includes six appendices as follows: Appendix A: A Proposed Standard Practice for Predicting the Endurance Limit of Hot Mix Asphalt (HMA) for Long-Life Pavement Design; Appendix B: A Proposed Standard Practice for Predicting the Endurance Limit of Hot Mix Asphalt (HMA) by Pseudo Strain Approach; Appendix C: A Proposed Standard Practice for Extrapolating Long-Life Beam Fatigue Tests Using the Ratio of Dissipated Energy Change (RDEC); Appendix D: A summary of results of the beam fatigue testing accomplished during the project; Appendix E: An analytical method for construction of a damage characteristic curve through calculation of pseudo strains; and Appendix F: A proposed design for an interlaboratory study to develop a precision statement for AASHTO T321, Standard Method of Test for Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending. The proposed standard practices are under consideration for possible adoption by the AASHTO Highway Subcommittee on Materials and the AASHTO Joint Technical Commit- tee on Pavements.

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CONTENTS 1 Summary 5 Chapter 1 Introduction and Research Approach 5 Introduction 6 Research Problem Statement 6 Objectives 6 Scope 7 Chapter 2 State of Practice 7 Arguments for the Existence of the Endurance Limit 9 Factors Affecting Fatigue Life 9 Strategies to Produce Long-Life Pavements 12 Laboratory Fatigue Tests and Analysis Methods 13 Laboratory Studies to Quantify the Endurance Limit 13 Modeling Fatigue and Relationship to Field Performance 15 Chapter 3 Research Plan 15 Introduction 16 Materials 17 Test Methods 20 Chapter 4 Beam Fatigue Test Results and Analyses 20 Extrapolation Methods to Predict Fatigue Life 32 Existence of the Endurance Limit 41 Estimate of Precision of Beam Fatigue Tests 42 Indirect Tensile Strength as a Surrogate for Endurance Limit Determination 44 Chapter 5 Uniaxial Tension Results and Analyses 44 Test Specimens 44 Dynamic Modulus and Phase Angle Master Curves 49 Damage Characteristic Curve 53 Evaluation of Endurance Limit 59 Chapter 6 Examination of LTPP Database for Indications of an Endurance Limit 59 Introduction 59 Including the Endurance Limit Design Premise into Mechanistic-Empirical-Based Pavement Design Procedures 62 Defining the Endurance Limit--A Survivability Analysis 66 Updated Survivability Analysis Using LTPP Data

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73 Chapter 7 Sensitivity of Pavement Thickness to the Endurance Limit 73 Summary of Predicted Endurance Limits 73 Estimate of Shift Factors between Laboratory Tests and Field Performance 83 Sensitivity of Mechanistic-Empirical Pavement Design Methods to the Endurance Limit 88 Considerations for Incorporating the Endurance Limit into M-E Design Procedures 91 Chapter 8 Conclusions and Recommendations 91 Conclusions 91 Recommendations 93 References 97 APPENDIX A Proposed Standard Practice for Predicting the Endurance Limit of Hot Mix Asphalt (HMA) for Long-Life Pavement Design 106 APPENDIX B Proposed Standard Practice for Predicting the Endurance Limit of Hot Mix Asphalt (HMA) by Pseudo Strain Approach 112 APPENDIX C Proposed Standard Practice for Extrapolating Long-Life Beam Fatigue Tests Using the Ratio of Dissipated Energy Change (RDEC) 117 APPENDIX D NCHRP 9-38 Beam Fatigue Data 122 APPENDIX E Construction of Characteristic Curve 127 APPENDIX F NCHRP 9-38 Beam Fatigue Round Robin

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AUTHOR ACKNOWLEDGMENTS The research described herein was performed under NCHRP Project 9-38 by the National Center for Asphalt Technology (NCAT) at Auburn University. Ray Brown served as the Principal Investigator and Brian Prowell and Michael Anderson served as Co-Principal Investigators. Brown and Prowell were employed by NCAT when this research began. The research team included the Asphalt Institute; the University of New Hampshire; Applied Research Associates, Inc.; the University of Illinois, and, later, Advanced Materials Services, LLC. The Asphalt Insti- tute assisted in beam fatigue testing in Phases I and II of the project. Jo Daniel oversaw uniaxial tension testing and analyses at the University of New Hampshire. Harold Von Quintus with Applied Research Associates, Inc., performed survival analyses of in-service pavements and provided guidance on the MEPDG. Samuel Carpenter oversaw beam fatigue testing in Phase II of the study with Shihui Shen at the University of Illinois and provided expertise on analyses using the ratio of dissipated energy. Brian Prowell was primarily responsible for the technical content of the remaining sections and for assembling the final report. Pamela Turner supervised laboratory testing at NCAT. The project team appreciates the support and technical assistance of Bor-Wen Tsai of the University of California, Berkeley; Rich May of SEM Materials; and Bill Maupin, Jr., of the Virginia Transportation Research Council. These three agencies volunteered to participate in the beam fatigue mini round-robin to help develop a better understanding of the variability of beam fatigue tests. They also assisted in the evaluation of the draft protocols for determining the endurance limit. Other individuals provided invaluable assistance to the project. Bor-Wen Tsai provided significant technical assistance with the single- and three-stage Weibull method. Chris Wagner (FHWA) assisted with the MEPDG. Buzz Powell provided technical data from the NCAT Test Track. David Timm and Richard Willis provided advice and guidance for calculating shift factors based on field strain measure- ments and incorporating the endurance limit into M-E pavement design.