Performance and Constructability of Silica Fume Bridge Deck Overlays

Abstract

The effects of construction practices and material properties on the performance of concrete bridge decks are evaluated. Emphasis is placed on comparing bridge decks with silica fume and conventional concrete overlays and determining if the silica fume overlays commonly used on bridges in Kansas are performing at a level that justifies the extra cost and construction precautions. Forty continuous steel girder bridges, 20 with silica fume overlays, 16 with conventional overlays and 4 with monolithic bridge decks are included in the study. Field surveys were conducted to document cracking patterns and crack density and to obtain samples for chloride content and rapid chloride permeability (RCPT) analysis. Construction data was collected from construction documents, field books, and weather data logs. Information from the current study is combined with data from a 1995 study by Schmitt and Darwin. Twenty-seven variables are considered, covering bridge age, material properties, site conditions, construction procedures, design specifications, and traffic volume. Comparisons are made based on the properties of the upper surface and on the properties of the subdeck for bridges with overlays. The study demonstrates that crack density increases with age for bridge decks with silica fume overlays. Younger decks with conventional overlays, however, exhibit increased cracking compared to older decks. The differences are attributed to differences in construction procedures. The limited number of silica fume and conventional overlay decks that are similar in age have similar crack densities, effective diffusion coefficient values, and chloride contents, both at and away from cracks. Chloride content increases with the age of the bridge deck, regardless of bridge deck type. Chloride content taken at crack locations at depths just above and below the transverse reinforcement exceeds the threshold level for corrosion in as little as 1000 days, regardless of bridge deck type. Increased paste contents in bridge subdecks result in cracking in decks with overlays, regardless of the quality of the overlay, and neither higher cement contents nor compressive strengths are beneficial iii to the cracking performance of the concrete. Both fogging immediately after finishing and the application of precure material should be specified for conventional overlay and monolithic bridge decks, as they are now for silica fume overlay decks. Because of the relatively high number of silica fume overlay decks with ages under two years at the time of the study, these decks should be reexamined when they reach the age of the conventional overlay decks in the study.