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Concrete is a multi-length scale composite material. From the nanometer to the millimeter scale, it is a random composite, and a different random composite at each length scale. Percolation processes play a key role in the microstucture of concrete, and help to describe the overall dependence of transport properties like ionic diffusivity on the microstructure. Computer models have been developed to describe the microstructure and transport properties, as the randomness of the material precludes most (but not all) analytical formulations. The overall description of concrete, over six orders of magnitude of length scales, in terms of computer models, percolation theory, and composite ideas is of interest to those studying other random materials as well, like ceramics and rocks. This report is written to present the ideas for concrete in such a way so as to be acessible to the non-cement researcher. It is hoped that these ideas will prove to be useful in other materials.
Keywords: Building technology, cement paste, computer modelling, concrete, microstructure, mortar, percolation, transport properties.