Self-healing analyses of steel slag asphalt mixture under microwave heating through multi-scale laboratory tests

Steel slag, an eco-friendly and low-carbon building material, is widely used in pavement construction. In this study, steel slag aggregate (SSA) was used to replace a portion of the basalt aggregate to enhance its microwave absorption ability. The self-healing ability of steel slag asphalt mixture under microwave heating was investigated through 16 orthogonal fatigue-healing-fatigue tests, considering factors of damage degree, healing temperature, and rest duration. Healing rate was assessed using the healing index (ratio of fatigue load cycles (N1) after healing, to fatigue load cycles (N0) before healing). Both range analysis and variance analysis revealed healing temperature as the primary factor influencing healing rate, followed by residual bending stiffness modulus, with rest duration having minimal impact. The healing rate decreased gradually with increasing residual bending stiffness modulus, initially increased and then decreased with healing temperature, and increased with rest duration. The optimal healing conditions were achieved at 80˚C with 60 % residual bending stiffness modulus and 8 h of rest duration. The results of 2D digital image processing and 3D reconstructed models obtained from CT scanning, demonstrated significant reductions of pore area percentage and crack width inside the healed specimen, indicating the fatigue cracks were effectively healed. The nanoindentation test revealed a 16.8 % increase in Young's modulus and a 22 % increase in hardness in the cracked area after healing, potentially due to asphalt binder aging under microwave heating. The healing performance of steel slag asphalt mixture can be effectively characterized through multi-scale laboratory tests including macro-scale fatigue-healing-fatigue test, meso-CT scanning, and nanoindentation. •16 orthogonal fatigue-healing-fatigue tests were performed to investigate the impact of damage, temperature, and rest period on healing effect.•Digital image processing was employed to analyze 2D images and 3D models of mixtures to evaluate changes in pores and cracks.•Nanoindentation tests were conducted to assess changes of properties of aggregates and asphalt mortar after healing.