Flexural Behavior of Concrete Beams Strengthened with Polyurethane-Matrix Carbon-Fiber Composites

Fiber-reinforced polymer (FRP) materials have been increasingly used for external strengthening of reinforced concrete (RC) infrastructure. However, the focus has been almost exclusively on epoxy-based composites, and other potential options such as polyurethane (PU) have not been fully investigated. This paper characterizes the flexural behavior of concrete strengthened with PU matrix-adhesive laminates using small-scale single lap shear specimens, unreinforced flexural specimens, and large-scale RC girders. Experimental results demonstrate that although the normal and shear strengths of PU-based adhesives are low, PU-strengthened beams show increased strength and deformability, owing to the load redistribution ability within the bond line. It is found that a linear brittle bond-slip model adequately predicts the interfacial shear behavior of the PU adhesive, exhibiting a lower stiffness and comparable cohesive energy to epoxy. Finite element (FE) analysis was performed to validate the effect of the bond-slip model relative to the experimental results.