U-Jacketing Applications of Fiber-Reinforced Polymers in Reinforced Concrete T-Beams against Shear—Tests and Design

TLDR

Externally bonded fiber‑reinforced polymer (EB‑FRP) is a promising shear reinforcement for reinforced concrete beams, but its effectiveness in U‑jacketed T‑beams is limited by premature debonding that reduces predicted strength. This study constructs and tests five shear‑critical T‑beam RC specimens strengthened with epoxy‑bonded carbon FRP sheets, examines bolted steel laminate anchors, and evaluates the adequacy of Greek, Eurocode 8, and ACI 440 design provisions for U‑jacketed EB‑FRP retrofits. The beams were strengthened with C‑FRP sheets bonded on three sides and along the length, bolted steel laminates were used as anchors, and the resulting shear capacity was measured in four‑point bending and compared against 165 experimental data and code predictions. Results show that C‑FRP strengthening increases shear capacity but does not eliminate brittle failure due to FRP debonding, while the mechanical anchors delay debonding and improve performance.

Abstract

The application of externally bonded fiber-reinforced polymer (EB-FRP) as shear transverse reinforcement applied in vulnerable reinforced concrete (RC) beams has been proved to be a promising strengthening technique. However, past studies revealed that the effectiveness of this method depends on how well the reinforcement is bonded to the concrete surface. Thus, although the application of EB-FRP wrapping around the perimeter of rectangular cross-sections leads to outstanding results, U-jacketing in shear-critical T-beams seems to undergo premature debonding failures resulting in significant reductions of the predictable strength. In this work, five shear-critical RC beams with T-shaped cross-section were constructed, strengthened and tested in four-point bending. Epoxy bonded carbon FRP (C-FRP) sheets were applied on the three sides and along the entire length of the shear-strengthened T-beams as external transverse reinforcement. Furthermore, the potential enhancement of the C-FRP sheets anchorage using bolted steel laminates has been examined. Test results indicated that although the C-FRP strengthened beams exhibited increased shear capacity, the brittle failure mode was not prevented due to the debonding of the FRP from the concrete surface. Nevertheless, the applied mechanical anchor of the C-FRP sheets delayed the debonding. Moreover, the design provisions of three different code standards (Greek Code of Interventions, Eurocode 8 and ACI Committee 440) concerning the shear capacity of T-shaped RC beams retrofitted with EB-FRP jackets or strips in U-jacketing configuration are investigated. The ability of these code standards to predict safe design estimations is checked against 165 test data from the current experimental project and data available in the literature.