The classical measure of permeation capacity in concrete is permeability, i.e. the measure of the rate of mass transfer of water under a given pressure head, once steady-state flow has been established. The concept is straightforward, but experimental measurement is difficult for most concretes. The disparity of values found in replicate measurements is often appreciable, and the general scatter of the results is unfortunately very high.39 The usual range in values for reasonably mature archetypical concretes is of the order of 10^-12-10^-14 m/s; lower values are obtained for concretes with supplementary cementing components. Permeability values generally decrease with increasing hydration, as would be expected; they are also reported to show reductions with time during a particular test,39 which is somewhat disconcerting.
