Evaluating Corrosive Site Performance of Coastal Bridges
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Abstract
Corrosion of reinforcing steel in concrete structures, such as bridges, has historically been a concern in coastal environments. Such environments are chloride-rich due to proximity to the salt water found in oceans and brackish water further inland. When chlorides penetrate the concrete and become present in sufficient concentrations in the vicinity of the reinforcing steel, the chances of corrosion are increased. To combat this issue, the North Carolina Department of Transportation (NCDOT) implemented a construction policy in 1999 to mandate corrosion prevention measures in all new concrete structures built within environments believed to be conducive to corrosion. The policy provides guidelines to determine which of several approved corrosion prevention measures are required for structures in specific areas. Some such prevention measures include low water-to-cement ratios, pozzolans, epoxy coated reinforcement, and corrosion inhibiting admixtures. The goal of the policy is to increase the service life of these new structures and to decrease maintenance and repair costs. This research study aimed to assess the effectiveness of this construction policy. In order to do this, a series of field and laboratory tests were conducted on selected concrete bridge elements within corrosive coastal environments. These tests provided insight into the existing condition of these bridges as well as their future performance by using service life modeling. By utilizing the results of this study, deficiencies in the policy were identified and a revised policy was suggested.Eight bridges located in corrosive sites were selected based upon a specially developed set of criteria. Field evaluation of these bridges involved taking drilled powder samples, measuring the concretes’ surface resistivity, and determining the corrosion rate of embedded reinforcing steel. Laboratory testing included testing powder samples to determine chloride content (and subsequently diffusion coefficients) and quantifying the presence of corrosion inhibiting admixture. Modeling efforts utilized field measured concrete performance characteristics to predict future performance and produce an estimated maintenance free service life for each bridge.The findings of the study indicated that most of the concrete bridge elements observed showed little, if any, corrosion related distress. Field tests showed that the concrete had high surface resistivity, indicative of good resistance to chloride ingress and ultimately desirable long-term durability. Corrosion readings indicated that most concrete elements do not have high levels of, if any at all, corrosion. Laboratory tests showed that the chloride content of the concrete quickly diminished through the cover and typically only background chloride levels were detected at the depth of steel. This resulted in the calculated diffusion coefficients being low, which is indicative of concrete with a high resistance to chloride ingress. Corrosion inhibitor dosage rates were detected within range of the minimum specified by the policy and at consistent concentrations throughout the concrete elements. Synthesizing results of laboratory and field testing show little to no risk of corrosion-related distress for elements that are located outside of the tidal zone. Within the tidal zone, where tidal fluctuations regularly allow the concrete to become wet and then dry, some concrete elements observed showed high levels of chloride loading and increased corrosion rates while other similar elements were performing well. The conclusion was made that the policy is providing sufficient protection in its current form for the concrete bridges that have been constructed in accordance with it. The only recommendation made for the NCDOT is to potentially enhance acceptance standards for the construction process, particularly for concrete structural elements located within the tidal zone. With this exception of encouraging the addition of acceptance standards, no further recommendations to enhance the NCDOT corrosion policy were deemed necessary based upon the results of this research project.