Abstract
Corrosion of steel in concrete in concrete due to the ingress of chloride is a major factor in the premature failure of steel reinforced concretes. Thus changes in the concrete mixture to reduce permeability to chloride are highly desirable. Lowering the water-to-cementitious ratio (w/cm) is a very effective means of reducing permeability. Pozzolans such as fly ash and silica fume are very effective in reducing the apparent diffusion coefficient for chloride at a given w/cm. Typically, most of the reduction in diffusion with silica fume occurs within the first 28 days, whereas fly ash containing concretes see a more gradual reduction over time. Several life-cycle predictions models use a time dependent value of the apparent diffusion coefficient. In this paper, data are presented that can be used to estimate how the apparent diffusion coefficient and corrosion behavior changes in time, thus providing useful information for modeling. The laboratory results in experiments that have been in progress for over 10 years are in good agreement with results from larger specimens exposed to simulated tidal conditions using ocean water in an outdoor test facility in Daytona Beach Florida. The data indicate that the reductions in diffusion coefficients for fly ash concretes primarily occur in the first 2 to 3 years, and that the overall slope on a log-log scale for diffusion versus time is less than –0.35 and mostly under –0.2.
