Experimental investigation of the strengthening of hollow reinforced concrete beams

During the design phase of reinforced concrete (RC) beams, openings may be intentionally provided to accommodate anticipated or unanticipated electrical and mechanical service requirements. Alternatively, circular or rectangular openings of various sizes may be introduced at later stages through mechanical drilling. The objective of this study is to experimentally investigate the strengthening behavior of RC beams with web openings using different fiber-reinforced polymer (FRP) strengthening techniques. All beam specimens were manufactured at full scale (1:1) using commonly adopted practical dimensions. To realistically simulate actual beam behavior, a four-point bending test configuration was employed under fixed-end support conditions. A total of six RC beam specimens with dimensions of 25 × 60 × 350 cm were cast using C25 concrete and identical reinforcement layouts consisting of B420C reinforcing steel. One specimen was produced as a solid reference beam without openings, while five specimens were rendered hollow by introducing a circular web opening with a diameter of 300 mm through core drilling after casting. Among the beams with web openings, one specimen was designated as the hollow reference beam, while another specimen was classified as a damaged beam by intentionally inducing sectional damage in the form of a flexural crack near the opening region. The damaged hollow beam and one additional hollow beam were strengthened using three-sided FRP fabric wrapping. The remaining three hollow RC beams were strengthened using different FRP application configurations and subsequently subjected to experimental testing. The experimental results demonstrated that strengthening the midspan web openings using various FRP techniques resulted in a significant recovery of both load-carrying capacity and ductility that had been lost under pure bending conditions.