Multiscale mechanics of concrete interfacial bonding: A critical review of mechanisms, characterization, and mechanical performance

Interfaces in concrete-to-concrete and concrete-to-rock systems are not passive geometric boundaries, but evolving mechanical zones that govern load transfer, damage localization, and structural reliability. This review critically examines concrete interfacial bonding from a multiscale mechanistic perspective, focusing on how interface formation and evolution govern load transfer, damage evolution, and eventual failure. Interfacial behavior arises from the coupled action of chemical adhesion, friction, and mechanical interlocking. Although governed by similar bonding mechanisms, the two interface systems exhibit distinct interfacial transition zone (ITZ) evolution driven by their different substrate characteristics. Their relative contributions evolve with hydration, confinement, and surface morphology. The inherent weakness of the interface is traced to the heterogeneous development of ITZ, where wall effects, ion migration, and differential deformation generate porous, calcium-rich, and microcracked regions that serve as preferential sites for damage initiation. Such heterogeneity provides the mechanistic link between microscale interface formation, mesoscale damage accumulation, and macroscale tensile-shear debonding, showing that bond strength alone is often insufficient to describe interface integrity. Accordingly, this review integrates interfacial mechanisms, multiscale characterization, mechanical testing, and numerical modeling into a unified framework for understanding interface-controlled mechanical performance. Future advances require explicitly bridging microscale ITZ evolution, mesoscale fracture process zone development, mixed-mode crack interaction, and macroscale thermo-hydro-mechanical coupled response. In particular, physics- and data-driven hybrid approaches offer a promising route for predictive design and interfacial performance optimization in concrete composite systems.