Flexural Behavior and Capacity Modeling of Damaged RC Beams Strengthened with CFRP Grid

This study investigates the strengthening mechanisms of a Carbon Fiber-Reinforced Polymer (CFRP) grid and Polymer-modified Cement Mortar (PCM) system for damaged reinforced concrete (RC) beams in flexure. Experimental tests were conducted on five short beams to systematically observe the failure modes, load-carrying capacity, strain development, and deflection evolution. A finite element model was established and validated against the experimental results to analyze the effects of key parameters, including the damage degree, number of grid layers, and grid spacing. Theoretical formulas for calculating the ultimate flexural capacity under different failure modes were also derived. The results demonstrate that strengthening undamaged beams yields a more significant improvement in ultimate and cracking loads than strengthening pre-damaged beams. The composite system effectively suppresses crack propagation by enhancing stiffness, albeit at the expense of reduced ductility. The theoretical predictions show good agreement with the experimental data. Parametric analysis reveals that lightly damaged beams exhibit a higher load-bearing potential, whereas severely damaged beams display more ductile behavior. The increase in load capacity converges when the number of grid layers exceeds three. In contrast, reducing the grid spacing significantly enhances flexural capacity due to improved meso-scale structural effects.