External strengthening of reinforced concrete pier caps

The shear capacity of reinforced concrete pier caps in bridge support systems can be a factor which limits the capacity of an existing bridge. Pier caps are loaded over a short shear span making them behave as deep beams. Reinforced concrete deep beams 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 in flexure. Unfortunately, there is insufficient research on the effect of externally bonded FRP reinforcement on strength of deep beams to make it a viable strengthening system for pier caps.;This research was aimed at investigating the behavior of reinforced concrete pier caps through a coordinated experimental and analytical program and using the knowledge gained from that investigation to recommend an external strengthening scheme. The experimental study was performed on laboratory specimens based on an existing bridge in Georgia with perceived shear deficiencies in its pier caps. A novel part of modeling the behavior of a pier cap was to include the effects of the column supporting the pier cap. This was accomplished by including a stub column in each of the specimens. The stub column induced a stress concentration at the reentrant corner between the column and pier cap, which dictated the failure mode in some of the specimens. Two failure modes were observed: yielding of the longitudinal tension reinforcement, and splitting of the concrete in the arch. The effects of changes in longitudinal tension reinforcement ratio, beam depth, and crack control reinforcement were examined. The results showed that increasing the longitudinal tension reinforcement decreased the principal compression strut angle; this increased the capacity by changing the shape of the tied arch which forms, and reducing the stress concentration. The inclusion of crack control reinforcement did not change the point at which diagonal cracking occurred, but it did increase the ultimate capacity by reinforcing the splitting crack in the concrete. There was a significant size effect when splitting failure governed the ultimate limit state.;The results of the experimental study were used in conjunction with a larger database developed from the literature to examine different analytical methods for determining the ultimate capacity of reinforced concrete deep beams. A new method based on a modification of a previous approach suggested by Zararis was developed for use in the design of external strengthening schemes. 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.