Interfacial Transition Zones in Concrete: A Critical Review of Characterization, Damage Evolution, and Mechanical Response

Local properties of the interfacial transition zone (ITZ) are closely related to damage evolution in concrete, but improvements in interfacial microstructure do not always translate into better overall mechanical performance. This review examines ITZ formation, multiscale characterization, damage evolution, and mechanical response through experimental, numerical, and interface-modification studies. ITZ boundaries and thicknesses depend on characterization parameters and identification criteria; interface quality therefore requires assessment of local properties and their spatial distributions. ITZ effects vary with damage stage and target response. Following preferential interfacial cracking, crack paths, localization, and macroscopic response depend on relative fracture resistance, weak-zone distribution, and loading conditions. The damage-controlling location can consequently shift among the ITZ, mortar, and aggregate, helping explain inconsistent improvements after interface strengthening. Links between measured ITZ properties, post-peak softening, and fracture-energy dissipation still require joint validation against local damage and macroscopic response. Priorities include identifying conditions for damage transfer, determining whether early interfacial cracks continue to contribute, and testing predictions across conditions using local parameters. This synthesis supports evaluation of ITZ contributions, model-parameter selection, and interpretation of interface modification, providing a basis for concrete design and optimization.