Aging and low-temperature cracking of asphalt concrete mixture

Low-temperature cracking in asphalt concrete (AC) pavement is influenced by several factors broadly categorized as material, environmental, and pavement-structure geometry. Among the environmental factors affecting low-temperature cracking is pavement age. The older the pavement, the greater the incidence of thermal cracking. While some preliminary work has been done to investigate the significance of aging on the low-temperature cracking resistance of an AC mixture, a comprehensive research program has not been conducted. In recognition of this situation, a laboratory research program was undertaken to investigate the effect of aging temperature, duration of aging, and asphalt cement type on the thermal-cracking characteristics of AC mixtures. The thermal-stress-restrained specimen test (TSRST) was used to characterize the low-temperature cracking resistance of the asphalt aggregate mixtures used in the research program. Based on the results of this investigation, it was noted that the aging temperature strongly influences changes that occur in cold-temperature fracture properties of AC mixtures. Further, at low temperatures, say 50 degrees C, aging appears to ''quench'' at some point so that further aging time does not significantly change low-temperature cracking behavior. At high temperatures, say 85 degrees C, aging appears to continue so that further aging time significantly changes low-temperature cracking behavior. Asphalt cements with a high proportion of nonpolar components are not as susceptible to oxidative aging as other asphalts. However, a high proportion of nonpolar components makes asphalt cement relatively susceptible to thermal cracking because of the structuring that occurs at low temperatures. The air-voids content, within a reasonable range associated with in-service AC mixtures, does not influence the aging characteristics of AC mixtures. The Strategic Highway Research Program long-term aging procedure adequately reflects the field in service aging conditions, which pavements are subjected to in many cold regions.