Evaluation of the factors that affect the fracture resistance of asphalt mixtures at low temperatures using mechanical testing and acoustic emission methods

This dissertation 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 28 asphalt mixtures, which represent a combination of factors such as binder type, binder modifier, aggregate type, asphalt content and air voids, was 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 also confirmed the significance of binder grade and modifier type with relation to cracking resistance of asphalt mixtures. Asphalt content was also found to be significant for fracture energy but not a significant factor affecting fracture toughness. It was found that loading rate and initial notch length significantly affect the testing results at the highest test temperature, but the influence gets diminished with the decreasing of the test temperature.; An acoustic emission system with eight channels of recording was used to monitor the failure process during testing. 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. A method to obtain the fracture process zone was proposed by using the event locations that corresponds to 95% of total AE energy before peak. Comparison results showed that the size of the process zone significantly depends on air voids and aggregate type, but is less depend on the asphalt content. The length of the process zone is strongly affected by the initial notch length and temperature. No obvious difference was found with the size of fracture process zone for different loading rates. (Abstract shortened by UMI.)...