Evaluation of the Spatial Distribution of Aggregate Particles in Concrete and its Implications for the Existence of Interfacial Transition Zones

The spatial distribution of aggregate particles was evaluated using conventional stereological procedures and point process statistical analysis for two-dimensional concrete sections. First, the influence of the image resolution on evaluation results was assessed, and the representative volume element and the image resolution necessary for accurate measurements were determined. Next, the interparticle spacing and paste–aggregate proximity were characterized using the mean free distance and spherical contact distribution function of the aggregate particles. These distances were discussed in relation to the likelihood of the presence and connectivity of interfacial transition zones (ITZs) formed in the vicinity of the aggregate particles. The size of the representative volume element used for evaluating the interaggregate distance was approximately comparable to the maximum aggregate size when the image file size was fixed. The common feature of the aggregate distribution was a random mixture of small aggregate particles and slightly smaller cement grains. The random mixture naturally produced clustered and sparse regions simultaneously comprising small aggregates and cement grains. The characteristic spatial structure derived from the actual distribution of aggregates differed considerably from that generally assumed for ITZs and their percolation. The method employed in this study provides the actual interaggregate distance and suggests the presence of porous regions throughout the cement paste matrix, and not just in the ITZs.