Statistical approach to ingress of chloride ions in silica fume concrete for bridge decks

This paper presents the results of a study of chloride ion ingress into silica fume concretes designed for full-depth and overlay placements of highway bridge decks. Mixtures for each application were prepared over a range of silica fume contents and water-to-cementitious material ratios (w/cm). Concrete laboratory specimens were prepared for ponding of chloride solution over a 180 day period. Cores taken from the specimens were milled at 1 mm increments and powders analyzed for total chloride ion. Data were then fitted to Fick's second law of diffusion to obtain apparent chloride ion diffusion coefficients. Regression analyses were performed on data developed from the laboratory testing and response surface models were developed. Analyses of variance were carried out to compare behavior of Type K cement concreted and concretes prepared with silica fume in slurry form. For any given w/cm, as silica fume content was increased the apparent chloride ion diffusion coefficient changed at a faster rate as the silica fume content was increased. At silica fume contents over 4 to 6%, a much greater addition was needed to effect a large change in diffusivity. A point of diminishing returns may be reached at silica fume contents over about 6%. At the lower w/cm values, the apparent chloride ion diffusion coefficient is relatively insensitive to increases in the silica fume content above 6%. The effects of w/cm on apparent chloride ion diffusion coefficient were much less than that of silica fume content. Use of Type K cement or type of silica fume (slurried or dry-densified) did not have a significant effect on results.