Near-surface mounted flexural strengthening of reinforced concrete beams with low concrete strength

Among the strengthening techniques based on Fibre Reinforced Polymer (FRP) composites, the use of near-surface mounted (NSM) FRP reinforcement has recently become an attractive option for increasing flexural and shear strength of deficient reinforced concrete (RC) structural elements. In order to widen the existing test database, which is currently more limited than that related to externally bonded FRP laminates, this project investigated flexural strengthening by testing six 4.3-m long RC beams with 200x400 mm rectangular cross-section, of which two control and four strengthened beams. In the latter, carbon FRP (CFRP) round bars were embedded in grooves cut onto the bottom concrete surface and filled with a two-component, high-viscosity epoxy paste. The concrete compressive strength was very low (cylinder strength equal to 15 MPa), and all the beams were designed to fail by concrete crushing after yielding of the steel reinforcement. The incidence of debonding failures under these conditions could then be evaluated. The investigated variables were the ratio of internal steel reinforcement and the ratio of FRP superficial reinforcement. Experimental results are discussed in this paper, and their comparison with predictions of a simple analytical model is also presented.