Another important (albeit extremely complex) aspect of permeation capacity in concrete involves measurement of ion diffusivities. Diffusion coefficients of specific ions, principally Clÿ ions, have been studied for many years, primarily in connection with estimating the time that a given concrete cover will protect reinforcing steel against chloride-induced corrosion (as discussed in further detail in Chapter 5). Typical values quoted for Clÿ ion apparent diffusion coefficients in archetypical concretes are of the order of ca. 2 x 10^-12 to ca.
10 x 10^-12 m2/s.23 As might be expected, diffusion coefficients are lower for lower w:c ratio concretes, and again, still lower values are typically found for concretes with substantial contents of fly ash, silica fume, or slag.
As pointed out by Delagrave et al.,66 the specific ion diffusion coefficient values obtained depend very much on the particular method of measurement and calculation procedure used. However, each of the methods surveyed by these authors is sensitive to differences in concrete microstructure, and it was suggested that any of them could serve as a measure of permeation capacity in the sense used in this chapter. It was pointed out by these authors, however, that ions do not diffuse independently; interactions occur between different types of ions diffusing simultaneously, as well as with the microstructure. As might be expected, interactions with the microstructure are more severe at low w:c ratios, and when components such as silica fume are present. Much current effort is underway to mathematically describe ion migration as part of the more general processes of ionic transport. A general method for doing so for all ions, based on migration test results, was recently provided by Samson et al.67