The micro-structure of three-dimensional (3D) interfacial transition zone (ITZ) in concrete is usually investigated by one-dimensional (1D) and two-dimensional (2D) analyses. However, the information obtained in 1D and 2D cannot be directly used to characterize the actual 3D microstructure. If the two-dimensional section plane crosses through the ITZ region but not through the corresponding aggregate, this invalid region will be excluded from the analysis. Due to this exclusion, the ITZ thickness is underestimated and ultimately lead to a biased estimation of the contribution of the ITZ to the overall performance of the concrete. In this work, the underestimation of the ITZ thickness of concrete comprising mono- and multisized Platonic aggregates are investigated. By combining the underestimated ITZ thickness with a differential effective medium approximation, the bias on diffusivity of concrete induced by the areal analysis is evaluated. The results reveal that the bias increases with the increasing sphericity of aggregates.