Experimental testing, analysis, and strengthening of reinforced concrete pier caps by exterior post tensioning

Condition assessment of existing concrete bridge pier caps shear capacity using the simplified procedure for non-prestressed sections 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 beam tests with shear span/depth ratios of approximately 1.0, which is typical of bridge pier caps; of these six tests, two included the external post-tensioning system. In the second group, nine deep beam tests were performed that included a segment of the column representing the pier; 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 strut and tie provisions 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 is consistent with the results of the experimental program.;This research program demonstrated that the behavior of a composite pier cap and column differed significantly from the behavior of a deep beam or even a non-composite pier cap and column. This observation is significant since the majority of existing experimental data on the behavior of deep beams has been obtained from tests conducted on simply supported deep beams in either three or four point bending. In contrast, this program showed that failures of the pier cap specimens were governed by the interaction of the pier cap and column. In addition, the success of the external post tensioning system was due to its ability to shift the location of the diagonal compression strut away from the reentrant corner at the pier cap and column interface. It was concluded from the pier cap tests that for any rehabilitation or strengthening technique to be effective at increasing the strength and/or ductility of similar pier caps it must address the presence of the stress concentration at the reentrant corner where the pier cap and column meet. Any strengthening technique that is focused only on increasing the shear resistance of the pier cap would have a minimal effect on the ultimate capacity of the pier cap-column assembly.