In order to investigate the mechanical behavior of asphalt–aggregate systems subjected to direct shear loading and reveal the shear failure mechanism

This study focuses on stress–displacement curves and shear fracture energy of asphalt-aggregate systems subjected to direct shear loading for four different experimental conditions. The methodology for measuring shear fracture energy of asphalt-aggregate specimens is presented. Based on comparison of the experimental data, this paper indicates that the single-peak curves (SPCs) tend to envelop the double-peak curves (DPCs) and the values of shear fracture energy of the SPCs are much higher than those of the DPCs for each experimental condition. Using thermodynamic theory and slip theory, energy of interaction between the adhesion and the cohesion within the asphalt-aggregate systems is further analysed. The experimental results clearly show that the reduced fracture energy of the DPCs is related to the shear-dependent slip, including adhesive slip and cohesive slip that occur simultaneously and promote each other in the direct shear testing. This study also illustrates that fracture energy is capable of being a significant parameter in the failure and healing analysis of asphalt pavements.

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.