Mesoscopic Pore Characteristics of Steel Slag Ultra-Thin Wear Layer Asphalt Mixture and Their Impact on Performance

OGFC (open-graded friction course) steel slag ultra-thin wearing courses are a drainage-type layer used in preventive maintenance and have been successfully applied in road construction in China. However, research on the use of steel slag in ultra-thin wearing courses has mainly focused on macroscopic volumetric indicators and performance, often overlooking the impact of internal mesoscopic void characteristics. This study utilized X-ray CT to scan OGFC ultra-thin wearing course steel slag asphalt mixtures with varying void ratios. A custom digital image processing program was developed to comprehensively and quantitatively characterize the mesoscopic void features of the mixtures from multiple perspectives, analyzing their influence on macroscopic performance. The results show that the surface void ratio and void number exhibited opposite trends with respect to specimen height. Compared to conventional asphalt mixtures, the OGFC steel slag mixtures had a higher average surface void number; the maximum difference between the maximum and minimum surface voids rate reached up to 14.2%. As the equivalent void radius and fractal dimension increased, both the stability and dynamic stability of the mixtures decreased, and the maximum reduction in Marshall stability reached 32.4%. Previous macroscopic-scale studies have struggled to identify these internal mesoscopic void characteristics, and this research provides a deeper understanding of the mesoscopic void structure in OGFC ultra-thin wearing course steel slag asphalt mixtures.