Evaluation of Low-Temperature Properties of Asphalt Binders and Mixtures

Because of the widespread use of modifiers and additives in asphalt bitumen, this study revisits and evaluates the adaptation of the time-temperature superposition principle that is incorporated into the low-temperature Superpave (R) binder specifications. The study builds on results of bending beam rheometer (BBR) tests conducted on samples of asphalt binders used in seven different surface layer mixtures. Both 240-s and 2-h BBR tests were conducted at three different temperatures: the low temperature of the asphalt performance grade (T-L), 10 degrees C higher than the low-temperature grade (TL+10), and at -5 degrees C to construct the flexural creep stiffness master curves. Additionally, indirect tensile tests were conducted on the surface layer mixtures to determine the low-temperature properties. The BBR test results show that the methodology adopted in using the equivalence principle to formulate the low-temperature Superpave binder specifications does not accurately apply for some of the binders used in this study. Thus, modifications to the specifications for loading times and test temperatures are suggested. Further, the time-temperature shift factors for the binders evaluated are compared with the universal shift factors used in the SHRP study. The observed mismatch in the shift factor sets is in disagreement with the implicit assumption that all asphalt binders can be characterized by similar shift factors at low temperatures. The shift factors of the binders and mixtures used in this study tend to match well, particularly at high testing temperatures.