Mechanical Behavior and Failure Mechanism of Impact-Damaged RC Columns Strengthened with CFRP: A 3D Meso-Scale Numerical Study

Impact-damaged reinforced concrete (RC) columns often experience significant reductions in load-carrying capacity and ductility when subjected to subsequent axial loading. Carbon fiber-reinforced polymer (CFRP) sheets have been widely used to strengthen such damaged columns; however, the underlying strengthening mechanism remains insufficiently understood, largely due to the difficulty of experimentally capturing the evolution of internal damage. To address this issue, a three-dimensional (3D) meso-scale finite element (FE) model has been developed to investigate the mechanical behavior of CFRP-strengthened impact-damaged RC columns. The proposed model captures the evolution of micro-damage within concrete and provides a more realistic representation of impact-induced damage compared with conventional homogeneous models. The model was first validated against available experimental results, showing good agreement in both failure modes and responses. Based on the validated model, three typical strengthening schemes, including the longitudinally applied CFRP, U-shaped CFRP, and fully wrapped CFRP, are systematically examined in terms of failure patterns, load-carrying capacity, stiffness, ductility, and energy dissipation. The results indicate that the fully wrapped CFRP configuration most effectively mitigated damage in the impact-affected zone and increased the load-carrying capacity by up to 86%. Furthermore, a quantitative evaluation framework based on strengthening indices for axial capacity and energy dissipation is proposed, indicating that strengthening with two CFRP layers can lead to a desirable ductile failure mode within the scope of this numerical investigation. These findings provide useful mechanistic insights into the strengthening process and offer preliminary guidance for the rehabilitation of impact-damaged RC columns, though further validation is required before practical implementation.