Concrete bridge deck behavior under thermal loads
Johnson, Jeffrey Keith
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A major area of concern with concrete bridge decks is durability. The service life of bridge decks designed by traditional procedures is often shorter than desired. Typically, the decks crack under environmental loads, which lead to corrosion of the reinforcing steel and general deterioration of the concrete. In this investigation, the response of three different bridge decks was studied in the field under environmental loads. The decks, located within a mile of each other on the same road, differ in their steel reinforcement and the type of concrete used; otherwise, they are identical. Therefore, they offer an opportunity to observe how these factors affect their behavior under environmental loads. This investigation was divided into two tasks. In the first task, the basic response of the bridge deck concrete under changes in temperature was examined in the laboratory using specimens cast with the decks. The second task correlated the laboratory findings with measured responses in the bridges and examined the global thermal response of the decks. The laboratory investigation showed that concrete has a distinct nonlinear strain response to temperature changes. This response is related to the concrete's nanostructure, which affects the behavior of water within the paste during the heating and cooling process. The deck concretes' coefficient of thermal expansion was generally found to range from 6 to 14 æå/°C as the temperature changed from -35°C to 40°C, and distinct differences were seen in the relationship between the strain and temperature for high performance versus standard concrete. The bridge investigation showed that locally the thermal response of the bridges was similar to that measured in the laboratory. Globally, the transverse bridge response to changes in temperature was as expected (expansion and contraction with increasing and decreasing temperatures, respectively). Importantly, there were no significant differences in behavior between the three bridges. In reviewing the strain response to thermal events, it was discovered that anomalies in this response may be indicators of physical damage within the deck. Because of the Saco bridges relatively young age (two years), continued investigation will be necessary as they mature and environmental degradation more readily presents itself.