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The effect of concrete's porosity on its resistance to degradation is quite complex. In determining the rate of ingress of an aggressive medium into concrete, the distribution of the pores in the cement paste portion of this composite has to be considered, especially their size and connectivity. In concrete and mortar this pore structure is affected by the presence of the aggregate. Using mercury intrusion porosimetry (MIP), Winslow and Liu [1] showed that the pore structure of paste developed in the presence of aggregate is quite different from that of neat cement paste. The aggregate-paste interface, or "transition zone", has a definite effect on the pore size distribution due to its considerably higher porosity and the larger pores that it contains. The effects of the transition zones on the transport properties should depend on the aggregate content. When the transition zones are isolated by a less porous bulk paste, the rate of transport should be significantly lower than if the transition zones overlap, which would create a continuous path of low resistance to penetration. This interconnection has been referred to as "percolation". As evidence of such percolation, Winslow et al. [2] observed that the MIP results for mortars depend on the sand content, with the intrusion curves for sand contents higher than some critical value showing a disproportionate increase in the volume of larger pores. It is the goal of the present research to assess the effects of these changes in pore structure on transport properties such as ionic diffusion and water permeability and to establish relationships amongst these transport coefficients and the critical pore sizes as measured by MIP.