Microwave heating moment, thermal characteristics, and structural variations of different steel slag asphalt mixtures suffered from freeze-thaw damage

Microwave heating (MH) can effectively repair the cracks caused by low temperatures in asphalt pavements. This study aims to further recommend the moment of the implementation of MH and to investigate the thermal characteristics during the heating process. In particular, the pore changes resulting from cracking and healing will also be studied. For this purpose, freeze-thaw (F-T) cycle split tests and MH are conducted. An infrared camera is adopted to detect the temperature and CT scanning technology is used for investigating the structural variations during the cracking-healing process. Results indicate that the optimum moment of MH for AC-13 (Asphalt Concrete, nominal maximum size: 13 cm) and OGFC-13 (Open-graded Friction Courses, nominal maximum size: 13 cm) to heal low-temperature cracking is when the number of F-T cycles achieves 9 and 5. Thermal characteristics reveal that there is an uneven heating phenomenon of asphalt mixtures during the MH process, and the standard deviation basically linearly increases with heating time. In addition, AC-13 and OGFC-13 can achieve higher healing levels (98.8% and 90.7%) while having fewer overheating areas when they are microwaved for 120 s and 150 s, respectively. Furthermore, it can be concluded that the air void of asphalt mixtures will increase after F-T cycles and decrease after MH. This research can lead to further savings in pavement maintenance costs and provide solutions for sustainable road development and microwave self-healing technology.