Microstructural and mechanical properties of the interfacial transition zone between asphalt and steel slag aggregate under light-heat-water-coupled aging

The interfacial transition zone (ITZ), as a weak region in steel slag asphalt mixtures, is significant for the service performance of asphalt pavements. However, accurately identifying and characterizing the microstructural and micromechanical properties of the ITZ between asphalt and steel slag aggregates under light-heat-water (LHW)-coupled aging remains a challenge. In this study, an LHW-coupled aging test is developed to simulate multifactored coupled aging effects. Three types of asphalt mixtures are prepared by mixing styrene-butadiene- styrene (SBS) asphalt with steel slag and basalt aggregates, and the corresponding ITZ samples are obtained. The changes in the chemical composition, microscopic morphology, elemental distribution, adhesion force, and micromechanical characteristics of the asphalt-steel slag ITZ are investigated under various aging conditions using Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, nanoindentation (NI), and other micro/nano testing techniques. Subsequently, the effects of ultraviolet radiation, temperature, water, and aggregate type on the microscopic characteristics of the ITZ are evaluated to further elucidate the interfacial interactions and coupled aging mechanisms. The results indicate that both the coupled aging and aggregate type exert a significant impact on the microstructural and micromechanical properties of the ITZ. With coupled aging, the thickness of the ITZ increases, whereas its roughness and interfacial adhesion forces decrease. The asphalt-steel slag ITZ exhibited superior aging resistance compared with the asphalt-basalt ITZ. The thickness range of the asphalt-steel slag ITZ after 120 h of coupled aging is determined to be 6–22 μm via two complementary characterization methods (i.e., element distribution curves and elastic modulus contour maps), and it exhibits a clear increasing trend the duration of the coupled aging. In addition, the NI technique enables a quantitative and precise characterization of the aging gradient in the ITZ of asphalt mixtures. The ITZ elastic modulus between the steel slag and asphalt mastic after 120 h of coupled aging is 1.48 times that of the ITZ between the basalt and asphalt mastic, and this difference tends to further increase with prolonged coupled aging. This study provides a comprehensive understanding of the coupled aging process in the ITZ between SBS asphalt and steel slag aggregates using multiscale characterization approaches.