Microstructural Analysis of Interface Modification by Silane Coupling Agents in Asphalt–Grout ITZ of Semiflexible Pavements

Semiflexible pavement (SFP) is a composite material composed of porous asphalt mixtures and cementitious grout substances. Numerous asphalt–grout interfacial transition zones (ITZ) exist within this material and present inherent susceptibility to cracking. However, the microstructural changes within these interfaces remain inadequately understood due to the material’s complex and multiphase nature. This study investigates the microstructural characteristics of the asphalt–grout ITZ and its relationship with SFP’s macroscale performance, focusing on silane coupling agent modification. Atomic force microscopy (AFM) was first employed to analyze the effects of curing age, grout strength, and interfacial modification. Then, scanning electron microscopy (SEM) analysis was used to explore the correlation between the micromorphology and macroscopic mechanical properties of the asphalt–grout ITZ. Finally, a semicircular bending test was applied to test the crack resistance of SFP after interface modification. The results show that immersion of the porous asphalt mixture specimens with the interface modifier can enhance the microscopic properties of the asphalt and cementitious grout materials. The ITZ between asphalt and grout forms a double-layer structure, with smoother interfaces observed after applying the interfacial modifier. The width of the asphalt–grout ITZ may exceed 30 μm after SFP formed for 28 days. Microcracks in the asphalt–grout ITZ were significantly reduced after interface modification. These findings provide insights into proactive strategies for reducing cracking at asphalt–grout interfaces, thereby enhancing the overall performance of SFP.