Modeling of Concrete Microstructure for the Assessment
of the Percolation Threshold of Interfacial Transition Zone

Daniel Rypl, Tomáš Bým

Department of Mechanics
Faculty of Civil Engineering
Czech Technical University in Prague
Thákurova 7, 166 29 Prague, Czech Republic


The present contribution deals with the modeling of the concrete microstructure in order to reliably assess the percolation threshold of the interfacial transition zone. The hard core - soft shell model is utilized. In this continuum model, each aggregate particle described in terms of the spherical harmonic expansion is surrounded by a shell of constant thickness representing the interfacial transition zone (ITZ). While the hard core aggregates may not overlap one another, the soft shell ITZ regions are free to overlap one another. The individual aggregate particles are randomly placed into the representative volume using a packing procedure. It starts with packing randomly generated aggregate particles, taken from a container with representative aggregate shapes, into the cubic box and then continues by expanding and shifting individual aggregate particles to make the ITZ propagate between some of the neighbouring particles. The percolation of the ITZ is verified if its propagation takes place from one side of the representative box to the opposite one at least in two directions. To obtain a reliable estimation of the ITZ percolation threshold for a particular type of aggregates and aggregates size grades, a large enough set of aggregates packs must be generated and investigated. The threshold is then calculated as the average of volume fractions from those realizations that are within the appropriately chosen tolerance from the largest volume fraction for which the percolation has not occurred and from the smallest volume fraction for which the percolation has occurred.