Nonlinear finite element modeling of RC beams strengthened with NSM FRP rods

The introduction of Near Surface Mounted (NSM) strengthening technique to retrofit reinforced concrete (RC) members in flexure using Carbon Fiber-Reinforced Polymer (CFRP) rods is becoming more attractive to both researchers and designers in recent years. Although NSM strengthening systems has been used extensively, still further experimental, analytical, and numerical research is warranted to understand the effect of the several strengthening parameters on the flexural performance of RC members. This paper presents the development of a detailed 3D nonlinear finite element (FE) numerical model that can accurately predict the load-carrying capacity and response of RC beams strengthened with NSM FRP rods subjected to four-point bending loading. The developed FE model considers the nonlinear constitutive material properties of concrete, yielding of steel reinforcement, cracking of the filler bonding materials, bond slip of the steel and NSM reinforcements with the adjacent concrete surfaces, and bond at the interface between the filling materials and concrete. The numerical FE simulations were compared with experimental measurement tested by other investigators comprising of seven specimen strengthened with NSM CFRP rods in addition to one un-strengthened control specimen. Overall, the predicted FE mid-span deflection responses agreed very well with the corresponding measured experimental tested data at all stages of flexural loading. Furthermore, the developed models were also capable of predicting the failure mode of the strengthened tested specimen such as NSM rod debonding (peeling off) and concrete cover separation. The validated FE models are then used to study the effect of different NSM material bar types and sizes to provide further information over the limited available experimental data. It is concluded that the developed FE model is suitable as a practical and economical tool especially in design-oriented parametric studies for accurate modeling and analysis of flexural strengthening of RC members with NSM reinforcement.