Condition assessment of existing concrete bridge pier caps using the general shear provisions of the AASHTO LRFD Bridge Design Specification has caused the Georgia Department of Transportation (GDOT) to post a large number of bridges in the State of Georgia. Posting of bridges disrupts the free flow of goods within the region served by the bridge and has a negative economic impact. To prevent structural deterioration, diagonal cracking or failure of concrete pier caps in shear, the GDOT employs an in-situ strengthening technique that utilizes an external vertical post-tensioning system. However, the fundamental mechanics of this system and its effectiveness under service load have not been examined previously. This research examines the behavior of reinforced concrete pier caps that utilize the above strengthening system in a combined analytical and experimental program. In the experimental part of the study, two groups of full-scale reinforced concrete deep beam specimens were tested. The first group consisted of six deep beams with shear span/depth ratios of approximately 1.0, which is typical of bridge pier caps; of these six, two included the external post-tensioning system. In the second group, nine deep beam specimens that included a segment of the column representing the pier were tested; four of those tests included the external post-tensioning system. The tests revealed that the shear capacity computed using the AASHTO LRFD Bridge Design Specifications provided a conservative estimate of the specimen capacity in all but one case when compared to the experimental results. However, the AASHTO strut and tie provisions were found to provide a much closer assessment of the load carrying mechanism in the pier cap than the general shear provisions, in that they were able to predict the load at which yielding of the tension reinforcement occurred as well as the angle of the compression strut. The presence of the column segment in the second group had a significant impact on the failure mechanism developed in the specimen near ultimate load. The stress concentration at the reentrant corner between the pier cap and column interface served as an attractor for the formation of diagonal shear cracks, a mechanism not observed in previous deep beam tests in shear. The research has led to recommendations for improving the design of pier caps and the external post-tensioning system, where required, based on mechanics which are consistent with the results of the experimental program.

The shear capacity of reinforced concrete pier caps in existing bridge support systems can be a factor which limits the capacity of an existing bridge. In their usual configuration, pier caps behave as deep beams and have the ability to carry load through tied arch action after the formation of diagonal cracks. Externally bonded fiber reinforced polymer (FRP) reinforcement has been shown to increase the shear capacity of reinforced concrete members which carry load through beam action. However, there is an insufficient amount of research to make it a viable strengthening system for beams which carry load through arch action, such as pier caps. Accordingly, this research was aimed at investigating the behavior of reinforced concrete pier caps through a coordinated experimental and analytical program and to recommend an external strengthening method for pier caps with perceived deficiencies in shear strength. The experimental study was performed on laboratory specimens based on an existing bridge in Georgia. A number of factors were examined, including size, percentage longitudinal reinforcement and crack control reinforcement. The results showed that increasing the longitudinal tension reinforcement increased the beam capacity by changing the shape of the tied arch. In contrast, the presence of crack control reinforcement did not change the point at which diagonal cracking occurred, but it did increase the ultimate capacity by reinforcing the concrete against splitting. The results of the experimental study were used in conjunction with a larger database to examine different analytical methods for estimating the ultimate capacity of deep beams, and a new method was developed for the design of external strengthening. Two specimens were tested with externally bonded FRP reinforcement applied longitudinally to increase the strength of the tension tie. The test results correlated well with the proposed method of analysis and showed that increasing the strength of the longitudinal tension tie is an effective way to increase the strength of a reinforced concrete deep beam.