Low-Temperature Physical Hardening of Hot-Mix Asphalt

Abstract

A study was conducted to determine if low-temperature physical hardening, which has been reported to be exhibited by asphalt binders, also affects hot-mix asphalt. Two asphalt binders, Designation AAM-1 and AAM-2 of the Strategic Highway Research Program, which are known to show the effects of physical hardening, were used to prepare asphalt mixtures with different mineral fillers. The asphalt mixtures were compacted into slabs from which cores were obtained and tested at low temperatures in the thermal stress restrained specimen test. Before testing, the cores were cooled unrestrained to –15°C and held isothermally for 1 h or 24 h. After conditioning, the specimens were restrained and held at a constant length while the temperature was dropped at a rate of 15°C per hour until the specimens fractured. The temperature at which the specimens fractured, the stress at the time of fracture, and the slope of the temperature-stress curve were measured and analyzed using several statistical techniques. The results showed that the mixture made with asphalt binder AAM-2 was affected by conditioning time. The mixture made with asphalt binder AAM-1 was not affected by conditioning time. Other factors, such as mineral fillers and air voids of the mixtures, had a greater influence on the results than did physical hardening.