An equation has been developed for predicting the chloride ion diffusivity of high performance concretes containing silica fume as a function of mixture proportions and expected degree of hydration. The equation can be used in an absolute sense to predict the diffusivity of a specific concrete, or in a relative sense to estimate the improvement in diffusivity that can be achieved by the addition of silica fume to a concrete of known diffusivity. The model-predicted relative improvements appear to be in good agreement with the experimental data generated in two recent studies. Silica fume is observed to be particularly efficient in lower w/c ratio systems; for w/c=0.3, a 10 % silica fume addition may reduce diffusivity by a factor of 15 or more. Thus, properly designed, placed, and cured silica fume concretes should provide a substantial increase in the service life of steel-reinforced concretes in severe corrosion environments.
The author would like to thank the Partnership for High Performance Concrete Technology (PHPCT) program at NIST for funding this research.