Mechanical and Electromagnetic Induced Heating Self-Healing Properties of Asphalt Concrete with Hybrid Steel Fibers and Nano-CaCO3

Steel fibers with excellent electrical and thermal conductivity can convert electricity into heat by cutting magnetic inductances and then transferring heat in asphalt concrete, reducing viscosity of asphalt binder and allowing it to expand/flow and then fill cracks under loads. However, steel fibers only perform well at high aspect ratio and dosage, thus adversely influencing the degree of compaction and cost of asphalt concrete. This study innovatively proposes hybrid using of nano-CaCO3 and steel fibers to develop asphalt concrete with both high self-healing index and mechanical properties, especially under the selected mixing method and heating system where the average temperature of the self-healing region were controlled around 70°C. The self-healing index based on low-temperature semicircle bending (SCB) peak load and fracture energy of asphalt concrete with 13-mm steel fibers were 29.2% and 47.8% higher than that of asphalt concrete with 6-mm steel fibers, and this was consistent with the former reducing heating rate of asphalt concrete by 69.3%. However, peak load and cracking load of asphalt concrete with 13-mm steel fibers before self-healing were 19.5% and 27.2% lower than that with 6-mm steel fibers because of loose structure. Incorporating 5.0% by volume nano-CaCO3 by high-speed shearing method enhanced the self-healing index of asphalt concrete with 6-mm steel fibers based on peak load and fracture energy by 13.9% and 26.4%, respectively. Nano-CaCO3 can be absorbed on the surface of steel fibers to improve heat transfer ability to asphalt binder, which had been verified by the 4.0% increase of thermal conductivity and reduction of heating rate of asphalt concrete. Meanwhile, incorporating nano-CaCO3 can strengthen the bridging effect of steel fibers and lead to 32% extension of crack development duration time. Furthermore, nano-CaCO3 in the matrix made asphalt binder morphology changes from smooth to flaky, and enabled the flow of asphalt binder more easily at high temperature. Therefore, the self-healing index gap between asphalt concrete with 6- and 13-mm steel fibers was reduced by 14.3%; meanwhile, the fracture energy for asphalt concrete with hybrid 6-mm steel fibers and 5.0% by volume nano-CaCO3 after self-healing was 41.4% higher than that of composites without nano-CaCO3.