Characterization of the Tensile Properties of Open Graded Friction Course Mixtures Based on Direct and Indirect Tension Tests

There are several issues of concern regarding indirect tension testing systems to evaluate and characterize tensile properties of open graded friction course (OGFC) mixture. It is necessary to evaluate the Superpave Indirect Tension Test (IDT) in terms of its ability to accurately determine tensile properties of OGFC mixture with an independent testing system. For this purpose, Superpave IDT and Dog-Bone Direct Tension Test (DBDT) were performed in resilient and creep loading modes to determine tensile properties of dense graded and OGFC mixture. It is also necessary to further verify with asphalt mixture, previously established correction factors based on three-dimensional finite element analysis to determine mixture properties accurately from the DBDT testing system. Prior verification was based only on polymer specimen of known properties. The tensile properties of dense graded and OGFC mixtures were successfully obtained with both testing systems employed in this study. Both Superpave IDT and DBDT provided reasonable and consistent test results with respect to test temperature, aging, and binder modification. Observed differences in creep response between the two were explained by well known phenomena involving effects of stress state on mixture properties (lower creep rate were obtained in Superpave IDT where the mixture is subjected to greater confinement). Thus, continued use of the Superpave IDT is recommended because it provides reasonable properties and is much more practical than DBDT. However, relationships to account for stress state effects need to be considered for mechanisms involving uniaxial stress states. DBDT creep compliance is more appropriate for uniaxial stress states (surface tension away from tires), while Superpave IDT creep compliance is more appropriate for stress states involving confinement (bottom or near-surface tension between tires).