Statistical analysis of the critical percolation of ITZ around polygonal aggregates in three-phase concrete materials

Concrete is generally regarded as a kind of three-phase granular material composed of aggregates, interfacial transition zone (ITZ) and bulk cement paste, among which ITZ is an inescapable transport path of aggressive agents due to its high porosity. As the weakest part in concrete, the existence of ITZ usually triggers an unfavorable change of the holistic properties of concrete structures. In particular, the percolation behavior of ITZ is likely to be an important factor that accelerates the penetration of aggressive agents. Previous studies about the percolation of ITZ around aggregates are mainly carried out based on the models of circles, spheres and ellipsoids, which are very different from the real features of aggregates. In this work, the regular polygons which are more similar to the geometrical shapes of fine and coarse aggregates are applied to represent the aggregate particles. The ITZ around these aggregates is simplified to the soft interphase layer with identical thickness and the models of concrete structures can be modeled by the random sequential addtion (RSA) algorithm. By combining with the continuum percolation models of hard core/soft shell, the percolation properties of ITZ around different 2D multi-sized aggregates are studied in detail and the effect of both aggregates and ITZ on the percolation thresholds ϕagg,c of ITZ is quantified. Based on the obtained values of ϕagg,c here, an approximately numerical formula of ϕagg,c is proposed and its reliability is further verified by comparing with the results in the literature. The comparison confirms that the proposed model can be utilized to predict the critical thresholds of ITZ around various polygonal particles with the broad particle size range and ITZ thickness. The research findings may provide a helpful guidance for the design and optimization of concrete materials.