Low temperature fracture evaluation of asphalt mixtures using mechanical testing and acoustic emissions techniques

Low temperature or thermal cracking in asphalt concrete pavements is a major cause of pavement deterioration. Fundamental fracture evaluation of asphaltic materials is necessary in order to design pavements that are resistant to thermally induced cracking. At present, stress-strain response of asphalt binders within the linear material behavior range is commonly utilized in criteria for material acceptance. The fracturing of a material is a highly complicated phenomenon, and evaluation of the material beyond the linear response range will help close the gap between experimental results and actual field performance. In recent years it has been well established that at low temperatures asphalt mixtures behave in a quasi-brittle manner. For complete evaluation of asphalt mixture cracking performance, it is necessary to consider mixture response past the peak strength. In the present work, a set of nine mixtures, encompassing a variety of variables, including type of binder modification, presence of recycled asphalt pavement (RAP), and low temperature binder grade, are studied to assess their fracture behavior in light of two new fracture testing techniques. Testing was conducted in two stages; fracture energy measurements were obtained using the disk-shaped compact tension (ASTM D7313) test. Fracture energies were determined at two temperatures, two air void levels and two levels of age conditioning. Fracture energy testing was followed by acoustic emissions (AE) evaluation to characterize low temperature cracking behavior of the asphalt mixtures. The AE testing procedure, which has been demonstrated to successfully capture low temperature behavior of asphalt binders, was extended to include low temperature characterization of asphalt mixtures. Testing results also quantify the effects of RAP on low temperature fracture behavior and provide new insights on the effects of commonly used binder modifiers on mixture fracture behavior.