Structural Behavior and Failure Modes of Strengthened Deep Beams Using Fiber-Reinforced Polymers: A Review

Deep beams (DBs) play a crucial role in structural systems due to the higher demand for flexural load-carrying capacity, but they also face challenges due to design complexities and stress patterns. This paper reviews the flexural performance of strengthened deep beams in near-surface mounted (NSM) and externally bonded reinforcement (EBR) techniques using steel, fiber-reinforced polymers (FRPs), and hybrid approaches. The analysis of compiled data from the literature focuses on strengthening materials and techniques, flexural behavior, the effect of reinforcement ratios, interfacial bonding, shear-span-to-depth ratios, stiffness, ductility, failure modes, cracking behavior, and analytical models. It was concluded that the hybrid strengthening technique and high-strength jacket techniques in the EBR method have proven to be highly effective for strengthening DBs for flexural behavior. Hybrid strengthening considering different grades of concrete and struts at a 45° angle showed 53–93% higher ultimate loads and 33–70% higher deflection compared to control solid deep beams. Carbon FRP (CFRP) strips and glass FRP (GFRP) fabrics in the hybrid technique NSM-EBR, combined for strengthening of DBs with openings, showed much better results; it increased the ultimate strength and deflections by about 52% compared to solid DBs without openings. Premature debonding and concrete-cover separation may prevent externally bonded reinforcement from achieving its maximum capacity. But an increase in the number of layers or the bonded length does not necessarily mean providing higher strength; the resistance of the surrounding concrete and adequate anchorage are equally crucial.