Laboratory and Field Investigation on the Damp-Heat Aging Resistance of Steel Slag Asphalt Pavement

Damp-heat environments significantly accelerate the aging and performance deterioration of asphalt pavements. This study systematically evaluated the performance evolution of base and SBS-modified asphalt binders, as well as their corresponding steel slag and basalt asphalt mixtures, under diverse damp-heat aging conditions (60–80 °C, 90% RH, 0–8 days). Physical and rheological characterizations indicate that base asphalt exhibits a monotonic hardening law during damp-heat aging, whereas SBS-modified asphalt demonstrates a unique two-stage aging process characterized by initial softening followed by hardening. Chemical functional group analysis further confirms that damp-heat aging results from the synergistic effects of early-stage moisture ingress and long-term thermal-oxidative hardening—a process effectively delayed by the SBS polymer network. Furthermore, microscopic morphology and elemental analysis reveal that steel slag aggregates establish a robust dual mechanism to resist moisture erosion through the mechanical interlocking of surface micropores and the chemical anchoring of alkaline constituents. A comprehensive evaluation using the improved radar chart method (f-value) further substantiates that steel slag not only enhances the initial performance of the mixtures but also significantly reduces their time–temperature sensitivity to damp-heat environments. Finally, full-scale field applications in damp-heat regions demonstrate that steel slag asphalt pavements maintain a residual Marshall stability of 87.6% and a texture depth retention rate of 91.5% after service. These results indicate superior early-to-mid-term moisture damage resistance and safety compared to basalt-based pavements, providing critical theoretical and practical support for material design and performance improvement in extreme damp-heat climatic regions.