An Investigation for Strengthening Existing Reinforced Concrete Beams in Shear Using a MF-FRP Retrofit System

Abstract

The purpose of this research was to determine if a Mechanically-Fastened Fiber Reinforced Polymer (MF-FRP) Retrofit System is viable for increasing shear strength in existing reinforced concrete beams. Due to the more rigorous, LRFR ratings standards for structures such as bridges, an increasing portion of in-service reinforced concrete beams are considered under-designed in shear. Consequences of under-designed beams in shear can result in shear cracking and sudden, catastrophic failure. Therefore, a method to increase the shear strength economically, for the short term could have a large impact on the safety on these in-service structures. A review of previous research indicated success with a MF-FRP system in flexure strengthening of reinforced concrete beams at UW-Madison, in addition to success with an Adhesively-Bonded FRP system in shear strengthening of reinforced concrete T-beams at the University of Alberta. The many benefits of Mechanically-Fastened Systems compared to Adhesively-Bonded Systems, which include ease of installation, installation time, and curing time, make this research a worthwhile process to determine if a MF-FRP system is viable in shear application. Therefore, a MF-FRP system was designed for use in shear application. The pre-emptive failure mode for such a system is FRP bearing, so the selection of fastener type, number of fasteners, and the FRP material properties was chosen to maximize load per FRP sheet at FRP bearing failure. Eight tests were conducted on large-scale reinforced-concrete beams with dimensions 8? wide by 12? deep by 10? long. Two controls were used to determine the concrete shear strength and steel shear strength, separately. The FRP configuration was varied by changing 3 the FRP orientation (vertical/angled), FRP spacing, and FRP location within the critical shear zone. Each test consisted of a point load located at L/3 of the effective span to cause failure in the smaller shear zone. Test results showed a shear strength increase between 8 and 30%. The upper limit of the shear strength increase range given was controlled by concrete crushing in failure, and no tests had FRP bearing as the failure mode. FRP strains confirmed visual observations as the largest FRP strains measured for any test of 512.7 ?? was much less than the average strain at FRP bearing failure of 956 ??. Since the FRP sheets did not fail in the expected failure mode of FRP bearing, the controlling failure mode of the FRP system was unconclusive. Due to an undesired high concrete strength, the design calculations could not be compared to the test results to validate the design procedure. However, calculations were compared to the test results which provided similar failure modes. Test results and calculations showed that a MF-FRP Retrofit System is viable for strengthening existing reinforced-concrete beams in shear. Issues that will need to be addressed in the future to accurately and reliably design for a MF-FRP System in shear while maintaining all the benefits over an Adhesively-Bonded FRP System include determination of the actual shear capacity of any MF-FRP system, tolerance requirements, fastener embedment depth requirements for effective bonding, and necessity of gap filler.