Modified force method for the nonlinear analysis of FRP reinforced concrete beams

Among the different techniques adopted to retrofit structures that had become inadequate, the use of fiber reinforced composite materials (FRP) has gained more and more popularity for their relevant properties such as high strength to weight ratio and excellent corrosion resistance. However, debonding of FRP lamina from the substrate, made of either concrete or steel, may prevent the achievement of the full load bearing capacity of the reinforced element. In this paper a numerical method, able to follow the complete equilibrium path of a reinforced concrete beam externally strengthened by means of FRP sheets up to the complete detachment of the external reinforcement, is proposed. According to this method, the FRP sheets are separated from the reinforced concrete substructure by displaying the interface stresses that are taken into account by means of the corresponding interface slips (which are the primary unknowns). Compatibility equations are then imposed at the interface in order to restore the behavior of the externally reinforced beam. The method is first validated by benchmarking experimental results taken from literature and then exploited to perform a parametric analysis in order to point out the effect of the main design parameters on the response of the reinforced beam.