Polyurethane-FRP External Strengthening of RC Beams with No Steel Stirrups

Abstract

Externally bonded (EB) fiber-reinforced polymer (FRP) composites are an effective means for the strengthening of shear-deficient reinforced concrete flexural members. Previous studies have predominantly employed epoxy-based wet layup systems. In this study, carbon FRP preimpregnated with polyurethane resin is utilized in strengthening shear-deficient RC beams and compared with an epoxy resin. Fourteen small-scale (2,438×152×305 mm) and five large-scale (3,353×305×432 mm) flexural specimens were tested, considering FRP type (polyurethane, epoxy), size effect, shear span-to-depth ratio, FRP configuration (U-wraps, side bonding), and FRP scheme (sheets, strips at 45° or 90°). Four-point loading tests demonstrated similar or enhanced shear capacity for the polyurethane-strengthened RC beams compared to the epoxy-strengthened beams. A mechanics-based analytical model, utilizing the principle of effective FRP stress and the upper-bound theorem of limit analysis, illustrated that the shear behavior and debonding mechanism were dependent on both FRP composite and bond characteristics.