Use of Nonuniform Stress-State Tests to Determine Fracture Energy of Asphalt Mixtures Accurately

Fracture energy of asphalt mixture is known to have a strong influence on the cracking performance of flexible pavement. Accurate determination of fracture energy of laboratory specimens requires accurate determination of strain through fracture, at the instant of fracture, on the plane where fracture occurs. Tests involving nonuniform stress states, such that the failure plane is known a priori, are suited for accurate and consistent determination of fracture energy because gauges from which strains can be determined can be placed directly on the failure plane. The Superpave^&regd; indirect tension (IDT) test and the recently developed dog-bone direct tension (DBDT) test fall into this category. They were used in this study to determine tensile failure limits and, more specifically, to test whether fracture energy determined from one of the tests agreed with that from the other test and thereby indicated that fracture energy can be accurately determined from these tests. Strength tests were performed at constant rate of load ram displacement on dense- and open-graded asphalt mixtures for a range of temperature and aging conditions. Results showed that although tensile strength and failure strain between Superpave IDT and DBDT tests were different, because of differences in loading rate on the failure plane, the resulting fracture energy from the two tests was the same. This supports earlier findings that fracture energy is a fundamental property that is independent of stress state or loading conditions and, perhaps more important, that either one of these tests can be used to determine fracture energy of asphalt mixture accurately.