Cracking and Chloride Contents in Reinforced Concrete Bridge Decks

Abstract

The effects of material properties, design specifications, construction practices, and environmental site conditions on the performance of reinforced concrete bridge decks are evaluated. Field surveys were performed on 59 bridges to measure deck cracking, chloride ingress, and delaminated area. The surveys were limited to steel girder bridges – bridges that are generally agreed to exhibit the greatest amount of cracking in the concrete decks. The study includes two bridge deck types with silica fume overlays, one in which 5% of the cement is replaced by silica fume (19 bridges) and the other in which 7% of the cement is replaced by silica fume (11 bridges), plus decks with conventional overlays (16 bridges) and monolithic bridge decks (13 bridges). Information from the current study is combined with data from two earlier studies. In total, 27 variables are evaluated, covering bridge age, construction practices, material properties, site conditions, bridge design, and traffic volume. The performance of silica fume overlay decks relative to conventional overlay and monolithic decks is of particular interest due to the widespread use of silica fume overlays in the state of Kansas. The results of the study indicate that chloride contents increase with the age of the bridge deck, regardless of deck type. In addition, concrete for all bridge deck types sampled in the same age range exhibit similar chloride contents for samples taken both at and away from cracks, regardless of deck type. For bridges within the same age range, the average chloride concentration taken away from cracks at the level of the top transverse reinforcement rarely exceeds even the most conservative estimates of the corrosion threshold for conventional reinforcement. Chloride concentrations taken at crack locations, however, can exceed the corrosion threshold in as little as nine months. Based on these observations, it appears clear that attention iii should be focused on minimizing bridge deck cracking rather than concrete permeability. The study demonstrates that crack density increases with increases in the volume of cement paste and that neither higher compressive strengths nor higher concrete slumps are beneficial to bridge deck performance. In addition, crack density is higher in the end regions of decks that are integral with the abutments than decks with pin-ended girders. The results of the crack surveys indicate that cracking increases with age, although a large percentage of the cracking is established early in the life of the deck. Even with the increase in crack density over time, however, both monolithic and conventional overlay bridges cast in the 1980s exhibit less cracking than those cast in the 1990s. The differences are attributed to changes in material properties and construction procedures over the past 20 years. The trend in cracking for decks with silica fume overlays cast in the 1990s (containing 5% silica fume), however, is quite the opposite. A decrease in crack density is observed for 5% silica fume overlay decks, which appears to be the result of increased efforts to limit evaporation prior to the initiation of wet curing. Recently constructed 7% silica fume overlay decks, however, have not shown continued improvement. In light of the chloride and cracking observations, conventional high-density overlays are recommended in lieu of silica fume overlays, and full-depth monolithic decks are recommended for new deck construction.