The thermal stress restrained specimen test (TSRST) has been developed as an accelerated laboratory test to evaluate the thermal cracking resistance of asphalt concrete mixtures. This work was conducted at Oregon State University under a Strategic Highway Research Program contract. A statistical analysis of TSRST results indicated that asphalt type and degree of aging have a significant effect on fracture temperature. Air voids content and aggregate type have a significant effect on fracture strength. The fracture temperature of relaxed specimens was colder than that of nonrelaxed specimens. The decrease in fracture temperature because of stress relaxation was significant for stiffer asphalts and not significant for softer asphalts. Fracture strength was lower for re. laxed specimens. Fracture temperature was highly correlated with SHRP low-temperature asphalt cement index test results, namely, the limiting stiffness temperature and the ultimate strain at failure. A ranking of asphalt concrete mixtures based on fracture temperature from the TSRST compared favorably with a ranking based on fundamental properties of the asphalt cement.

Low-temperature cracking is attributed to tensile stresses induced in asphalt concrete pavement as the temperature drops to an extremely low temperature. If the pavement is cooled to a low temperature, tensile stresses develop as a result of the pavement's tendency to contract. The friction between the pavement and the base layer resists the contraction. If the tensile stress induced in the pavement equals the strength of the asphalt concrete mixture at that temperature, a microcrack develops at the edge and surface of the pavement. Under repeated temperature cycles the crack penetrates the full depth and across the asphalt concrete layer. Several factors reported to influence thermal cr'acking in asphalt concrete pavements may be broadly categorized under material, environmental, and pavement structure geometry. Specific factors under each of these categories are as follows (1):