Investigation on void connectivity characteristics of steel slag asphalt mixture subjected to dry–wet cycles and microwave heating utilizing computed tomography technology

The moisture infiltration and transmission processes within the pore structure of asphalt mixtures subjected to wet-dry cycling significantly impair their resistance to water damage, making it essential to investigate the effects on the connectivity characteristics of the pores and assess the reparative effects of microwave heating. Computed Tomography (CT) scanning technology was utilized alongside a void network model to investigate the impact of microwave heating on these mixtures under dry–wet cycles. Parameters such as coordination number, void length, and fractal dimension were analyzed to explore how microwave heating altered microscale void connectivity. It was found that the dense-graded asphalt mixture (AC-13) exhibited relatively weak connectivity, which was slightly enhanced in the stone mastic asphalt (SMA-13), while open graded friction course (OGFC-13) displayed the strongest connectivity, particularly in the intermediate layer, facilitating moisture and air flow. Significant increases in coordination numbers and fractal dimensions were observed during dry–wet cycles, with the largest changes noted for OGFC-13. Microwave heating resulted in a reduction of the total coordination number and average coordination number, with increases of 14.8%, 27.6%, and 43.4% over the initial values, respectively. The self-healing effect was significantly enhanced by microwave heating, especially pronounced in OGFC-13, where the standard deviation of void throat length decreased from 6.2961 to 5.2614 mm. The self-healing effect was manifested in the mixtures as a promotion of reduced void throat length and structural densification. Additionally, the characteristics exhibited following microwave heating, particularly the reduced pore throat length and fractal dimension, highlighted the necessity of selecting OGFC-13 type steel slag asphalt mixtures with enhanced microwave heating self-repair potential.