A review of the interfacial transition zones in concrete: Identification, physical characteristics, and mechanical properties

Interfacial Transition Zones (ITZ) with high porosity are easier pathways for damage initiation and propagation. However, their mechanical performance has hardly been examined owing to the length scale of the microregions, as well as the heterogeneous and opaque constituent properties, which have been increasingly studied in recent years. This study presents a comprehensive review of the ITZ, determining strategies at the meso-micro level, quantitative characterization, and physical features. It focuses on the effects of various casting and material composition factors on the microstructural and mechanical properties of the ITZ, including porosity, thickness, volume, and elastic modulus. The correlations between the microstructures and the essential mechanical properties are discussed in detail. The weak bonding properties of the ITZ can be tailored by the material composition, which changes its internal microstructure, enhances the adhesion of the paste to aggregates, and fills micropores in cement. Several approaches can be employed to achieve this, such as adding minerals, increasing the curing age, and adjusting the water -cement ratio and sand fineness. The content of the weak ITZ can be reduced by optimizing the size distribution and volume fraction of the aggregates. Furthermore, modelling the gradient distribution of the ITZ thickness and elastic modulus is crucial in numerical simulations to enable an accurate investigation of the damage accumulation process in concrete. This study also identifies the challenges in the mechanical behavior of ITZ and serves as an engineering design guide for more advanced fracture -resistant concrete structures.