Semi-flexible pavement (SFP) is a composite material comprising porous asphalt and cementitious grout. Its asphalt-grout interfacial transition zones (ITZ) are prone to cracking. While interfacial modifiers can improve ITZ crack resistance, their effects on microscopic morphology and chemical composition remain unclear, limiting the understanding of mechanisms influencing ITZ performance and the optimised use of modifiers. To achieve this challenge, the study characterised the microscopic morphology and chemical composition of the asphalt-grout ITZ in SFP, particularly those modified by immersion in a silane coupling agent solution. The asphalt-grout ITZ in SFP material is analyzed using a series of micromechanics techniques, including scanning electron microscopy (SEM) for microscopic morphology, energy dispersive spectrum analysis (EDS) for chemical components, X-ray diffraction (XRD) for phase composition, and Fourier-transform infrared spectroscopy (FTIR) for chemical bond information. The results indicate that an interfacial modifier can effectively enhance the microscopic bonding between asphalt and grout material, reducing the likelihood of tiny cracks between these two phases. The asphalt-grout ITZ exhibits a distinct double-layer structure, with altered morphology compared to the bulk material, including hexagonal calcium hydroxide crystals aggregation, which is more prone to cracking than hydrated calcium silicate (C–S-H) gel. Element distribution at the enhanced interface is more continuous, with synchronous enrichment. These findings provide insights into proactive strategies for improving crack resistance at asphalt-grout ITZ, thereby strengthening the road performance of SFP materials.

Semi-flexible pavement (SFP) is extensively used in airport and tunnel pavements due to its high strength and toughness. As a multiphase composite material, SFP contains widely distributed aggregate-asphalt interface transition zones (ITZ) and asphalt-grout ITZ. These ITZs are the weakest areas in SFP and are susceptible to cracking during operation. To enhance the crack resistance of SFP, this paper proposes an interface immersion method to immerse the porous asphalt mixtures in a silane coupling agent before grouting. However, the cracking mechanism and interface enhancement process of the ITZ in SFP are not clear. This paper employs atomic force microscopy (AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and nanoindentation (NI) to analyze the micro- and nanoscale properties of the aggregate-asphalt ITZ and asphalt-grout ITZ before and after interface reinforcement. Experiment results show that the aggregate-asphalt ITZ exhibits minimal interaction before and after enhancement, with the primary interaction occurring in the asphalt-grout ITZ, which has superior adhesion. Grout diffusion is more pronounced in the asphalt-grout ITZ, where cracks are significantly reduced after reinforcement. The element distribution in the aggregate-asphalt ITZ is uniform with a gradual transition, while the asphalt-grout ITZ shows a denser, more continuous distribution after surface immersion, enhancing mechanical properties. The width of the aggregate-asphalt ITZ remains constant (60–90 μm), while the asphalt-grout ITZ width increases notably after modification (from 60 to 90 μm to 90–120 μm). The research results can provide theoretical support for SFP design and maintenance.