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29 CHAPTER 4 CONCLUSIONS AND RECOMMENDATIONS Based upon the literature review, review of AASHTO T322, XX-34 binder and softer. Reasonable adjustments should and the results of a substantial laboratory testing program per- be made for testing field cores, which may exhibit sub- formed during Phase III of NCHRP Project 9-29, the following stantial age hardening. conclusions and recommendations are made: A number of relatively minor revisions listed in this report should be made to AASHTO T322. These have The IDT creep and strength test should be retained as the been forwarded to the task force responsible for recom- standard method for determining the creep compliance mending revisions to this test method to AASHTO. and tensile strength of asphalt concrete mixtures at low The coefficient of variation of creep compliance values temperatures. measured using the IDT test was found to range from Asphalt concrete specimens compacted in the laboratory about 8 to 11 percent for tests performed at a number of exhibit substantial anisotropy in their creep compliance at different laboratories using several different test systems. low temperatures. The compliance measured across the This variability is probably somewhat high for a standard diameter of the specimen is greater than that measured in test method, but improved test procedures and equipment both compression and tension along the length of the will help to reduce this to an acceptable level. The preci- specimen. This anisotropy does not appear to be caused sion of the IDT strength test is probably acceptable in its by differences in air void content or air void distribution, current form. but is probably the result of preferential aggregate par- Ruggedness testing for the AASHTO T322 creep test ticle orientation that occurs during compaction. This procedure is the next logical step in the implementa- anisotropy is the primary reason for preferring the IDT tion of this procedure. An initial ruggedness testing creep and strength tests over other procedures. plan and estimated level of effort are provided in this Pavement engineers and researchers should be careful report. when using compliance and modulus values for asphalt Additional research is suggested in two areas. Further concrete in pavement design and analysis, because these testing and analysis is needed to refine the relationship and related properties are potentially anisotropic and their between uncorrected IDT strength and the actual strength values will depend upon the direction and sense of the as determined using the procedure currently given in applied stress with respect to the orientation relative to AASHTO T322 (using LVDTs to determine the point of the compaction process. failure). Research should also be undertaken to evaluate The IDT strength test should be performed without the accuracy of critical cracking temperatures determined LVDTs. The uncorrected IDT strength should be calcu- using the Hirsch model to estimate creep compliance lated based on maximum load, and the corrected or "true" values and empirical methods for determining approxi- strength estimated using the empirical equation devel- mate tensile strength. Such a procedure would be useful oped in this project. for general mixture selection, mixture design guidance, Test temperatures for the IDT creep and strength test quality control applications, and as a possible replace- should be linked to binder grade: -20, -10, and 0C for ment for the current Level 2 and 3 thermal cracking data PG XX-28 and PG XX-22 binders; -10, 0, and +10C for input for the pavement design guide developed in PG XX-16 and harder; and -30, -20, and -10C for PG NCHRP Project 1-37A.