Keywords: percolation, concrete, interfacial transition zone, aggregates, hard core - soft shell, spherical harmonic analysis.

This paper deals with the modeling of the concrete microstructure in order to reliably assess the percolation of the interfacial transition zone. The main concept of percolation theory is the idea of connectivity. Percolation threshold denotes the volume fraction of a particular phase of a composite material at which that phase goes from being disconnected to connected (or vice versa) so that there is a change in topology of the microstructure. Percolation properties are now more or less commonly accepted as the critical geometrical and topological factors influencing the transport properties of multiphase materials [1].

In the case of mortar and concrete, the transport properties are strongly dependent on the thin region of cement paste close to aggregate surface. This region, known as interfacial transition zone (ITZ), exhibits higher capillary porosity and larger pores than the bulk cement paste matrix [2]. These features are commonly attributed to the cement particle packing effect and the one side growth effect. However, if the ITZs do not percolate, their effect on transport will be fairly small, as any transport path through concrete would have to go through the bulk cement paste. Transport properties would then be dominated by the bulk cement paste transport properties.

The problem of the percolation of the ITZs in concrete is computationally not simple, as the geometry and topology of this phase is complex. So far, the ITZ percolation problem has been studied for spherical and ellipsoidal aggregate particles [3]. In order to get realistic results of the ITZ percolation problem, it is highly desirable to be able to perform the simulation with realistic shapes of aggregates. Note also that micrometer and millimeter length scales have to be considered simultaneously in such a study. This makes the standard models based solely on digital image processing prohibitive in terms of memory requirements. Therefore 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 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 the randomly generated bounding spheres of aggregate particles. These spheres are then fitted by randomly chosen and randomly oriented aggregate particles. Finally the individual aggregate particles are expanded 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 volume to the opposite one.

1

J.D. Shane, T.O. Mason, H.M. Jennings, E.J. Garboczi, D.P. Bentz: "Effect of the Interfacial Transition Zone on the Conductivity of Portland Cement Mortars", Journal of the American Ceramic Society, 83, pp. 1137-1144, 2000.

2

E.J. Garboczi, D.P. Bentz: "Digital Simulation of the Aggregate-Cement Paste Interfacial Zone in Concrete", Journal of Materials Research, 6, pp. 196-201, 1991. doi:10.1557/JMR.1991.0196

3

D.P. Bentz, E.J. Garboczi, P.E. Stutzman: "Computer Modelling of the Interfacial Transition Zone in Concrete", in "Interfaces in Cementitious Composites", J.C. Maso et al. (eds), pp. 259-268, 1993.

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