and high capillary porosity. In consequence, concrete and mortar of the same age and water content should be less permeable than pure paste. Results of tests, however, indicate that the opposite is true. Data in Fig. 4.2 show the considerable increase in permeability when aggregates are added to a paste or mortar [4.2, 4.3]. The greater the size of the aggregates, the greater is the permeability. These results have been analysed by Mehta [4.1], who attributes the permeability increase to the presence of microcracks in the transitional zone at the interface between paste and aggregates [4.4]. In addition, two phenomena contribute to this permeability increase: