Eric S. Lagergren
Statistical Engineering Division, ITL
Construction Technology Laboratories, Skokie, IL
The use of silica-fume concrete for bridge decks has become an accepted practice. This is primarily due to its favorable effects on permeability and compressive strengths. However, experience suggests that the use of silica fume in concrete contributes to high shrinkage levels that can cause deck cracking.
The primary goal of this project is to determine the effect that silica fume and other mix design parameters have on the properties of silica-fume concrete most pertinent to bridge decks. These properties include the cracking, shrinkage, diffusivity coefficient, compressive strength, and elastic modulus.
A central-composite response surface design was used to study the effect that silica fume and water-to-cement ratio have on these properties. Three independent batches of concrete were made at each of nine conditions of silica fume and water-to-cement ratio specified by the design. The design permits response surfaces to be fit for each property. The design also has the characteristic that the precision of the fitted values is independent of the direction from the center of the design.
The top figure gives the mean diffusivity coefficient, Dc, for each of the nine experimental conditions. The diffusivity coefficient measures the ability of the concrete to restrict the diffusion of chloride ions. Diffusion of chloride ions into the concrete damages the steel-bar reinforcements and ultimately destroys the concrete. The lower the Dc the better and so we see that increasing the amount of silica fume and reducing the water-to-cement ratio improves the concrete's ability to restrict the diffusion of chloride ions. The bottom figure gives a contour plot for the predicted Dcgenerated from fitting a second-order model to log(Dc). The plot shows that for any fixed water-to-cement ratio, increasing the amount of silica fume decreases the diffusivity coefficient. However, the decrease in Dc is much larger at lower amounts of silica fume than higher.
The investigators are currently collecting additional data. The statistical analysis of these data will provide guidance for selecting appropriate levels of silica fume for bridge decks.
Figure 21: The top figure shows the mean diffusivity coefficient, Dc (10-13), at each of the nine conditions of silica fume and water to cement ratio. The bottom figure is a contour plot for predicted Dc (10-13) as a function of silica fume and water to cement ratio. Contour levels increase in steps of four units.
Date created: 7/20/2001