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3PROBLEM STATEMENT AND RESEARCH OBJECTIVE Low-temperature cracking, also called thermal cracking, is a serious type of pavement distress that occurs in temperate and sub-Arctic regions throughout North America. This type of pavement failure is the result of tensile stresses caused by sudden temperature drops in combination with embrittlement of the asphalt concrete at low temperatures. It is considered a more common problem in the northern United States and Canada, but has been observed in Texas and Florida. The indi- rect tensile (IDT) creep and strength tests were developed dur- ing the Strategic Highway Research Program (SHRP) to char- acterize the resistance of hot mix asphalt concrete (HMA) to low-temperature cracking (1,2). These methods have been standardized in AASHTO T322, Standard Method of Test for Determining the Creep Compliance and Strength of Hot-Mix Asphalt (HMA) Using the Indirect Tensile Test Device. Cur- rently, the IDT creep and strength tests are considered the most promising for predicting the low-temperature performance of asphalt concrete mixtures. During the mid-1990s, the FHWA procured six IDT test systems for use by the Turner Fairbank Highway Research Center (TFHRC) and five Regional Superpave Centers. These were unusual systems in that they were closed-loop electro- mechanical systems, rather than the more traditional servo- hydraulic loading systems. All of the users of these systems experienced a range of hardware and software problems, preventing a thorough evaluation of the device and procedure. Currently, there are about a dozen laboratories in the United States performing IDT creep and strength tests on a regular basis. Many contractors and state highway agencies are inter- ested in using these test procedures but feel that additional information is needed on the test device, procedures, and analysis. Therefore, the objective of Phase III of NCHRP Project 9-29, as stated in the expanded scope of work, is to âevaluate and refine the indirect tensile test (IDT) procedures proposed for use as the simple performance test for low-tem- perature cracking and as the materials characterization test for low-temperature cracking in the pavement design guide developed in NCHRP Project 1-37A.â SCOPE OF STUDY Given the limited budget, the scope of Phase III of NCHRP Project 9-29 primarily focused on the basic test methods, equipment, and analysis used in the IDT creep and strength tests. The general approach and several critical algorithms used in the computer program developed by Roque and his associates for analyzing IDT data and predicting thermal cracking are included in the scope of this study. However, an in-depth evaluation of this software was not possible given the available resources and was not contemplated in the research problem statement. Based upon the report generated by the NCHRP Project 9-19 team (5), this program appears to be functioning well, and an in-depth evaluation is probably not warranted at this time. RESEARCH APPROACH The NCHRP elected to have Advanced Asphalt Tech- nologies, LLC (AAT) perform the evaluation and refinement of the IDT creep and strength tests as Phase III of NCHRP Project 9-29. Phase III included eight tasks numbered Task 9 through Task 16: Task 9. Critically review AASHTO T322 and the changes proposed to the procedures in NCHRP Projects 1-37A and 9-19. Evaluate how these changes may affect the prediction of low-temperature cracking with the measured material properties. Task 10. Review the results of the ruggedness study con- ducted under NCHRP Project 90-06. Collect pub- lished and unpublished data obtained since 1993 with the various IDT equipment. Evaluate the data to determine whether there are significant differ- ences related to the equipment configuration, the details of the test method, or the method of data reduction and analysis. Task 11. Prepare a concise white paper discussing how equipment configuration (e.g., screw-type versus servo-hydraulic) might promote or hinder future adoption of the IDT method by the states for HMA mix and pavement structural design. Task 12. Critically examine the theoretical basis of the IDT test and data reduction methods recom- mended for structural and mix design in Projects 1-37A and 9-19. Insofar as possible, test their rigor through an independent analysis of test data obtained in Task 10. Task 13. Submit within 7 months of the initiation of Phase III an interim report presenting the findings of Tasks 9 through 12. Based on these findings, present recommended modifications to AASHTO CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH
T322 or the alternative approaches proposed in Projects 1-37A and 9-19. Propose a laboratory testing plan to verify the soundness of any recom- mended modifications. Task 14. Conduct the approved laboratory testing to verify the changes recommended to AASHTO T322 or the alternative procedures from Projects 1-37A and 9-19. Task 15. Develop an experimental plan for possible future ruggedness and precision/bias studies. Task 16. Prepare a final report for Phase III that (1) sum- marizes its findings, (2) documents the Task 15 experimental plan, and (3) presents any recom- mended modifications to AASHTO T322 or the alternative approaches from Projects 1-37A and 9-19 in AASHTO standard format for review and action by the TRB Superpave Mix and Aggre- gate Expert Task Group. An in-depth review of AASHTO T322 and related recent research is included in this report as Appendix A. Early work in Phase III included the collection of IDT creep data from six different laboratories throughout the country. These data were compiled and analyzed statistically; the findings are presented in Chapter 2. Appendix B is the white paper prepared for Task 11, in which various issues concerning IDT test equipment and procedures are discussed. One of the primary recommendations of this white paper was to explore in the laboratory testing part of Phase III the use of uniaxial tests for measuring low-temperature properties of HMA mixtures. Because uniaxial testing is being promoted for mix analysis in NCHRP Project 9-19 and for the con- struction of dynamic modulus master curves in NCHRP Project 1-37A, its use in low-temperature characterization should maximize equipment use and simplify implementa- tion and training activities. The theory underlying the IDT creep and strength tests is presented and discussed within Chapter 2 of this report. The results of a significant labora- tory test program comparing the IDT creep and strength tests to uniaxial test methods (both compression and ten- sion) are also included in Chapter 2. Chapters 3 and 4 are relatively brief, the former being a summary of findings and interpretation of those findings, the later being conclu- sions and recommendations. 4
