Tensile properties of open graded friction course (OFGC) mixture to evaluate top-down cracking performance

Since its introduction, open graded friction course (OGFC) has provided unique functions as a surface layer for pavements. However, the tensile properties of OGFC, which influence the top-down cracking performance of asphalt pavement, have not been properly evaluated. Stress development at the top of the asphalt structural layer may be significantly affected by the material properties/characteristics of OGFC mixture. Therefore, it is necessary to accurately and reliably determine the fracture properties of OGFC mixture to determine its overall contribution to top-down cracking resistance of the pavement structure.A dog-bone direct tension test (DBDT) to accurately determine tensile properties of asphalt concrete, including OGFC, was conceived, developed and validated. Proper data reduction, analysis methods, and correction factors were developed based on three dimensional finite element analysis to account for non-uniform stress, strain, and rotation effects. The newly developed DBDT and existing Superpave IDT were used to perform resilient modulus, creep, and strength tests at multiple temperatures on dense graded and OGFC mixture. Tensile properties of dense graded and OGFC mixture were successfully obtained and both testing systems provided reasonable and consistent test results with respect to temperature and aging. Excellent correspondence was observed between properties determined from each type of test, indicating that fundamental properties can be accurately determined using either test. Differences in strain rate between the two tests resulted in expected differences in strength and failure strain. Creep compliance was highly correlated between the two tests but was lower for IDT than for DBDT, an effect that was attributed to the higher confinement in IDT. It was concluded that DBDT compliance is more appropriate for uniaxial stress states, while IDT compliance is more appropriate for biaxial stress states. This effect also helps to explain why conditions at the surface are more conducive to top-down cracking. Analysis with HMA fracture mechanics model indicated that OGFC mixture probably accelerates development of cracking relative to pavements with no OGFC. Continued use of Superpave IDT was recommended because it is much more practical than DBDT.