Characterisation of gradient variation in the structure and properties of the asphalt-aggregate interface

Abstract

The asphalt–aggregate interface plays a critical role in the mechanical integrity and durability of asphalt mixtures. Unlike conventional studies that treat the interface as a uniform bonding layer, this study demonstrates that it exhibits a distinct gradient transition behavior across multiple length scales. By combining molecular dynamics simulations with experimental characterizations, including SEM–EDS, AFM mapping, and contact angle measurements, a non-uniform interfacial structure with spatially varying molecular density, interfacial energy, and micromechanical properties is identified. Molecular simulations reveal a gradient distribution of asphalt components normal to the aggregate surface, forming an interfacial edge zone, a sub-edge transition zone, and an inner interface zone. These gradient features are consistently validated by experimental observations, including variations in elemental distribution, adhesion force, and surface energy. The proposed gradient change interface concept provides a unified multiscale interpretation of asphalt–aggregate interactions and offers new insights into interfacial design and long-term performance of asphalt mixtures.