Study on the Shear Mechanical Behaviour of Asphalt Aggregate Interface Under the Combined Effect of Temperature and Asphalt Film Thickness

The interface between asphalt and aggregate acts as a key link in load transfer and damage development; its shear mechanical behaviour has a decisive influence on the fatigue cracking and durability of bituminous pavements. In this study, SBS-modified asphalt and limestone aggregate were selected. By precisely controlling the moulding load, asphalt film thicknesses ranging from 23 μm to 219 μm were achieved. Systematic shear tests were conducted within a temperature range of 10 °C to 40 °C to systematically investigate the evolution of interfacial shear strength, stress–displacement curve characteristics, shear fracture energy and failure modes. Two primary failure modes were identified: adhesive failure, defined as debonding at the asphalt–aggregate interface with minimal residual asphalt on the aggregate surface; and cohesive failure, defined as internal rupture within the asphalt film with asphalt residue covering both fracture surfaces. The results indicate that temperature has a significant deteriorating effect on interfacial shear strength; as temperature rises, shear strength decays rapidly at first and then more slowly before gradually approaching zero, whilst shear fracture energy decreases monotonically, with the rate of decay being more pronounced at lower temperatures; the thickness of the bituminous film exerts a monotonically decreasing influence on interfacial shear strength, whilst shear fracture energy remains at a relatively high level within the intermediate film thickness range; bituminous films that are either too thin or too thick are detrimental to the effective dissipation of interfacial energy. An analysis of interfacial failure modes reveals that, under low-temperature conditions, failure occurs primarily at the interface between the asphalt and the aggregate, manifesting as adhesion failure; under high-temperature conditions, the failure path shifts to within the asphalt film, manifesting as cohesion failure. An increase in asphalt film thickness promotes a shift towards cohesion failure, whilst a decrease in film thickness tends to favour adhesion failure; between these two extremes, a coupled and competitive relationship is observed. Based on interfacial thermodynamics and phase transition theory, this study provides an interpretation of how temperature governs the migration of failure modes through alterations in asphalt cohesion, intermolecular forces and interfacial surface tension, and reveals the pivotal role of film thickness in restructuring the distribution of interfacial weak points by regulating the ratio of structural asphalt to free asphalt.