AbstractThe interfacial transition zone (ITZ) in concrete is a layer of highly porous cement paste along the aggregate boundary and has a significant influence on the properties of concrete. Existing studies consider uniform thickness of the ITZ around the aggregate, which is not an actual representation of ITZ microstructure. Assuming uniform ITZ thickness around the aggregate leads to overestimating the volume fraction of the ITZ and, subsequently, its influence on the mechanical behavior of concrete. The local ITZ thickness depends on several factors such as water–cement (w/c) ratio, interaggregate distance, size, spatial distribution, and hydration of cement particles. The present work aimed to develop an analytical approach to incorporate differential ITZ thickness around the aggregate. The porosity profiles of cement paste between the two neighboring aggregates was investigated to predict ITZ thickness as a function of w/c ratio, interaggregate distance, spatial distribution, and hydration of cement particles. Voronoi tessellation was used to assign differential ITZ thickness around the aggregates. Furthermore, a formulation for predicting ITZ volume fraction that incorporated differential ITZ thickness was proposed. The variation in ITZ volume fraction with different parameters such as maximum and minimum size of aggregate, volume fraction of aggregate and w/c ratio was investigated. The elastic modulus of the concrete was calculated using a four-phase generalized self-consistent scheme to validate the proposed approach with the experimental results. The study found the ITZ thickness to be 7–21 μm and have a weak correlation with the spatial distribution of cement particles between the aggregates. The effect of the ITZ on the modulus of elasticity of concrete was negligible in concrete with low aggregate volume fractions.