Superpave Mix Design: Verifying Gyration Levels in the Ndesign Table (2007)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

TRANSPORTAT ION RESEARCH BOARD WASHINGTON, D.C. 2007 www.TRB.org N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP REPORT 573 Subject Areas Materials and Construction Superpave Mix Design: Verifying Gyration Levels in the Ndesign Table Brian D. Prowell E. Ray Brown NATIONAL CENTER FOR ASPHALT TECHNOLOGY Auburn, AL Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed research provides the most effective approach to the solution of many problems facing highway administrators and engineers. Often, highway problems are of local interest and can best be studied by highway departments individually or in cooperation with their state universities and others. However, the 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 cooperative research. In recognition of these needs, the highway administrators of the American Association of State Highway and Transportation Officials initiated in 1962 an objective national highway research program employing modern scientific techniques. This program is supported on a continuing basis by funds from participating member states of the Association and it receives the full cooperation and support of the Federal Highway Administration, United States Department of Transportation. 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 modern research practices. The Board is uniquely suited for this purpose as it maintains an extensive committee structure from which authorities on any highway transportation subject may be drawn; it possesses avenues of communications and cooperation with federal, state and local governmental agencies, universities, and industry; its relationship to the National Research Council is an insurance of objectivity; it maintains a full-time research correlation staff of specialists in highway transportation matters to bring the findings of research directly to those who are in a position to use them. The program is developed on the basis of research needs identified by chief administrators of the highway and transportation departments and by committees of AASHTO. Each year, specific areas of research needs to be included in the program are proposed to the National Research Council and the Board by the American Association of State Highway and Transportation Officials. Research projects to fulfill these needs are defined by the Board, and qualified research agencies are selected from those that have submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Research Council and the Transportation Research Board. 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 NCHRP REPORT 573 Project 9-9(1) ISSN 0077-5614 ISBN 978-0-309-09877-9 Library of Congress Control Number 2007901205 © 2007 Transportation Research Board 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, or Transit Development Corporation 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. NOTICE 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 the Governing Board of the National Research Council. Such approval reflects the Governing Board’s judgment that the program concerned is of national importance and appropriate with respect to both the purposes and resources of the National Research Council. The members of the technical committee selected to monitor this project and to review this report were chosen for recognized scholarly competence and with due consideration for the balance of disciplines appropriate to the project. The opinions and conclusions expressed 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 Transportation Research Board, the National Research Council, the American Association of State Highway and Transportation Officials, or the Federal Highway Administration, U.S. Department of Transportation. Each report is reviewed and accepted for publication by the technical committee according to procedures established and monitored by the Transportation Research Board Executive Committee and the Governing Board of the National Research Council. The Transportation Research Board of the National Academies, the National Research Council, the Federal Highway Administration, the American Association of State Highway and Transportation Officials, and the individual states participating in the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturers’ names appear herein solely because they are considered essential to the object of this report.

CRP STAFF FOR NCHRP REPORT 573 Christopher W. Jenks, Director, Cooperative Research Programs Crawford F. Jencks, Deputy Director, Cooperative Research Programs Edward T. Harrigan, Senior Program Officer Eileen P. Delaney, Director of Publications Beth Hatch, Editor NCHRP PROJECT 9-9(1) PANEL Field of Materials and Construction—Area of Bituminous Materials Maghsoud Tahmoressi, PaveTex Engineering and Testing, Inc., Dripping Springs, TX (Chair) Timothy B. Aschenbrener, Colorado DOT John Bukowski, Federal Highway Administration Cecil L. Jones, North Carolina DOT Rita B. Leahy, Nichols Consulting Engineers, Chtd, Sacramento, CA Richard C. Meininger, Federal Highway Administration Larry L. Michael, Hagerstown, MD Shakir R. Shatnawi, California DOT Ronald A. Sines, P.J. Keating Co., Lunenburg, MA Dale S. Decker, Other Liaison, Bailey, CO David E. Newcomb, National Asphalt Pavement Association Liaison, Lanham, MD Frederick Hejl, TRB Liaison C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 9-9(1) by the National Center for Asphalt Technology, Auburn University. E. Ray Brown, Director, was the principal investigator and Brian D. Prowell, Assistant Director, was the co-principal investigator, at the National Center for Asphalt Technology. The authors thank all of the state agencies and contractors who assisted with this project. The authors thank Shane Buchanan, Mike Huner, Graham Hurley, Robert James, Jason Moore, and all of the staff of the National Center for Asphalt Technology who began this project and assisted in the early field data collection. The authors thank Dr. Saeed Maghsoodloo for his assistance with the statistical analyses.

This report presents the findings of a research project to validate the gyration levels in the Ndesign table (Table 1) in AASHTO R 35 by following the behavior under traffic of a series of field projects. Its main finding is that, based on ultimate pavement densities achieved on 40 field projects in 16 states across the United States, modest reductions in Ndesign are possi- ble. Such reductions, if adopted, should lead to hot mix asphalt (HMA) mix designs that are more readily compacted in the field. The report will be of particular interest to materials engineers in state highway agencies, as well as to materials suppliers and paving contractor personnel responsible for the specification, design, and production of HMA. The original Ndesign table associated with the use of the Superpave gyratory compactor in HMA mix design was based on testing conducted on cores from 15 field projects that had been in service for at least 12 years when they were sampled. The asphalt binder was extracted from the cores and the recovered aggregate remixed with virgin AC-20 asphalt cement. The remixed samples were compacted in a gyratory compactor, and the numbers of gyrations necessary to match the core densities were determined. The density at the time of construction was assumed to be 92 percent of Gmm for all of the projects. The data from the 15 projects were extrapolated to produce the original Ndesign table, consisting of 28 lev- els representing four climatic regions and seven traffic levels. In 1999, this original Ndesign table was consolidated to four levels based on laboratory work conducted for NCHRP and FHWA that determined the sensitivity of mix volumetric properties and mix stiffness to Ndesign. However, these results were not verified for field conditions. Under NCHRP Project 9-9(1), “Verification of Gyration Levels in the Ndesign Table,” the National Center for Asphalt Technology at Auburn University was assigned the goal of ver- ifying through a series of field project evaluations that the gyration levels in the Ndesign table (Table 1) of AASHTO Standard Practice R 35, “Superpave Volumetric Design for Hot-Mix Asphalt (HMA),” are correct for the four stated 20-year design traffic levels (less than 0.3 million; 0.3 million to 3 million; 3 million to 30 million; and greater than 30 million ESALs). To accomplish this goal, the research team conducted an extensive field experiment that followed the in-situ pavement densification of 40 field projects from their time of con- struction to 4 years under traffic. The projects were located in 16 states in the Rocky Mountain, South Central, North Central, and Southeast regions of the United States, and they represented a wide range of traffic levels, asphalt binder performance grades, and aggregate types and gradations. At each project, gyratory compacted specimens were prepared from as-produced loose mix at the Ndesign level required for the project by Table 1 of AASHTO R 35, and the specimen den- sities were compared with those developed during the HMA mix design. The pavements F O R E W O R D By Edward T. Harrigan Staff Officer Transportation Research Board

were cored at the time of construction and at 3 months, 6 months, 1 year, 2 years, and 4 years after construction. Core densities were then measured and in-place air void contents were calculated. Thus, the densification of each project was followed over time until an ulti- mate in-place density was reached. These results were then compared with the original mix design data to assess how well the project Ndesign level matched the ultimate pavement den- sification achieved under traffic. Based on analyses of the experimental results, the research team concluded that the pres- ent Ndesign levels in AASHTO R 35 are higher than needed to match the ultimate in-place pavement density at design traffic levels greater than 0.3 million ESALs. The team recom- mended specific changes to the Ndesign values in Table 1 of AASHTO R 35 for consideration by the AASHTO Highway Subcommittee on Materials. In addition, the team proposed fur- ther reductions in Ndesign when the mix design is prepared with an asphalt binder with a per- formance grade of PG 76-XX or greater. Such binders are typically modified, and HMA pre- pared with them can provide superior performance. All 40 field projects proved to have excellent resistance to rutting. Based on this fact and on an evaluation of the original mix design data for each project, the team recommended removal of the Ninitial and Nmaximum requirements in Table 1 of AASHTO R 35 as superfluous to the design of well-performing HMA. This final report includes a detailed description of the experimental program, a discus- sion of the research results and their analysis, a summary of findings, and recommendations for implementation of key findings. These findings have been referred to the FHWA Asphalt Mixture Expert Task Group and the AASHTO Highway Subcommittee on Materials for review and revision of the applicable recommended practice.

C O N T E N T S 1 Summary 5 Chapter 1 Introduction and Research Approach 5 1.1 Background 5 1.2 Research Problem Statement 6 1.3 Objective 6 1.4 Scope 7 Chapter 2 Summary of Literature Review 7 2.1 Brief History of Mix Design 7 2.2 Field Densification of Asphalt Pavements 8 2.3 Gyratory Compaction 8 2.4 Ndesign 8 2.4.1 SHRP Ndesign Experiment 8 2.4.2 Validation of Ndesign After SHRP 10 2.5 Locking Point 11 2.6 Summary 12 Chapter 3 Research Test Plan 14 Chapter 4 Test Results and Analyses 14 4.1 Projects Selected 14 4.2 Test Results 16 4.2.1 Comparison of Mixture Data with Design Job Mix Formula 16 4.2.2 Estimation of Traffic 21 4.2.3 Pavement Densification 22 4.2.3.1 As-Constructed Density 24 4.2.3.2 Densification with Time 24 4.2.3.3 Determination of Ultimate Density 25 4.2.3.4 Factors Affecting Pavement Densification 28 4.2.3.5 Pavement Densification at the 2000 NCAT Test Track 30 4.2.4 Determination of Ndesign to Match Ultimate In-Place Density 30 4.2.4.1 Predicted Ndesign Versus 2-Year Traffic 32 4.2.4.2 Comparison of Laboratory Density at Ndesign and In-Place Density After 2 Years of Traffic 33 4.2.4.3 Predicted Ndesign Versus Accumulated Traffic for 2000 NCAT Test Track 35 4.2.4.4 Predicted Ndesign Versus 2-Year Traffic, Excluding Mixes Produced with PG 76-22 38 4.2.4.5 Predicted Ndesign Versus 2-Year Traffic for PG 76-22 Mixes 38 4.2.4.6 Predicted Ndesign Versus Traffic for All Sampling Intervals 38 4.2.4.7 Model Development to Account for Low As-Constructed Density

45 4.2.5 Evaluation of Locking Point 49 4.2.6 Pavement Condition After 4 Years 49 4.2.7 Evaluation of Ninitial 51 4.2.8 Evaluation of Nmaximum 51 4.2.9 Summary and Discussion of Test Results 56 Chapter 5 Conclusions and Recommendations 56 5.1 Conclusions 57 5.2 Recommendations 58 References 61 List of Acronyms 62 Appendixes