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Building and Fire Research Laboratory
National Institute of Standards and Technology
100 Bureau Drive Stop 8621
Gaithersburg, MD 20899-8621 USA
Based on a set of multi-scale computer models, an equation is developed for predicting the chloride ion diffusivity of concrete as a function of water-to-cement (w/c) ratio, silica fume addition, degree of hydration, and aggregate volume fraction. Silica fume influences concrete diffusivity in several ways: 1) densifying the microstructure of the interfacial transition zone (ITZ) regions, 2) reducing the overall (bulk and ITZ) capillary porosity for a fixed degree of cement hydration, and 3) producing a pozzolanic C-S-H gel with a relative diffusivity about 25 times less than that of the C-S-H gel produced from conventional cement hydration. According to the equation and in agreement with results from the literature, silica fume is most efficient for reducing diffusivity in lower w/c ratio concretes (w/c < 0.4). In these systems, for moderate additions of silica fume (e.g., 10 %), the reduction in concrete diffusivity may be a factor of fifteen or more, which may substantially increase the service life of steel-reinforced concrete exposed in a severe corrosion environment.
Keywords: Concrete(D), diffusion(C), hydration(A), modeling(E), silica fume(D).