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