Study of low temperature cracking in asphalt mixtures using mechanical testing and acoustic emission methods

This paper investigates the use of mechanical testing and acoustic emission to study microstructural phenomena and the corresponding effects on macroscopic fracture behavior of asphalt mixtures tested at low temperature. The fracture resistance of eight asphalt mixtures, which represent a combination of factors such as aggregate type, asphalt content and air voids, were evaluated by performing semi-circular bending fracture tests at three low temperatures. Two parameters, fracture energy and fracture toughness, were calculated from the experimental data. The statistical analysis indicated that fracture resistance is strongly dependent on temperature and significantly affected by type of aggregate and air void content. The results of the analysis showed that asphalt content was not a significant factor affecting fracture resistance. An acoustic emission system with eight channels of recording was used to monitor the failure process during testing. Algorithms to automatically detect the relative arrival time of the AE signal and to locate the AE source were developed. The analysis of the accumulated AE events illustrated the relationship between the micro damage and macroscopic behavior at different loading levels. Results showed that the load level corresponding to the localization played a critical role in the damage process of the material. The accumulative AE energy was found to be related to the fracture energy at the two lower temperatures. Fracture process zone (FPZ) was obtained for all mixtures by the event locations corresponding to 95% AE energy before peak load. Comparison results showed that the size of the FPZ significantly depends on air voids and aggregate type, but is less depend on the asphalt content.