Characterization of asphalt mixture using X-ray computed tomography scan technique after freeze-thaw cycle and microwave heating

Microwave heating technology could heal micro-cracks caused by freeze-thaw (F-T) cyclic damage. The main objectives were to recommend the optimum heating moment and evaluate the difference between two typical mixtures during F-T cycle and microwave heating based on microstructure. To this end, cyclic F-T split tests and microwave heating tests were conducted to determine the best heating moment. In addition, the internal structure was scanned using X-ray computed tomography and evaluated using digital image processing. The results indicated that the optimum timing to heal two different asphalt mixtures (AC-13 and OGFC-13) occurred when the degrees of moisture damage were 39.8% and 42.4%, respectively. AC-13 exhibited better heating uniformity, and OGFC-13 exhibited better heating transferring. Moreover, F-T damage and microwave heating could change the internal structure: three types of moisture damage were seen in the voids: pore generation, connections, and expansion; pore number, porosity, pore area, and equivalent diameter first increased after the F-T cycles and then decreased after microwave heating, but the effect for OGFC-13 was more obvious; The F-T cycles mainly increased the number of valid pores, and microwave healing mainly increased the number of invalid pores; The damage and healing process of AC-13 and OGFC-13 mainly occurred on the 0.15-1.18 mm and 0.15-4.75 mm pores. This research could contribute to maintenance cost savings and revelation of F-T damage and microwave healing mechanism.