Evaluation of the effectiveness of current guidelines in determining the strength of RC beams retrofitted by means of NSM reinforcement

Near surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement represents a valid alternative to externally bonded (EB) FRP reinforcement for strengthening existing reinforced concrete (RC) elements. NSM composites are usually comprised of glass, carbon, and aramid fiber reinforcement with circular (bars) or prismatic (strips) cross-section. When the NSM technique is employed for flexural strengthening, FRP composites are embedded into grooves cut in the concrete cover and filled with an inorganic (e.g. cement grout) or organic (e.g. epoxy resin) binding agent. Although many studies on the bond behavior of NSM FRP composites can be found in the literature, limited work is available regarding analytical models for designing NSM strengthening of RC members. In this paper, a database comprised of 155 experimental tests on RC beams strengthened in flexure using NSM reinforcement is collected from the scientific literature. The experimental database is employed to assess the accuracy of the analytical provisions obtained following the American, English, Canadian, and Italian guidelines. The assessment shows that, although conservative, the analytical models considered are (relatively) poorly accurate and further studies are needed to provide a reliable tool for designing RC flexural strengthening with NSM reinforcement.