Near surface mounted CFRP laminates for shear strengthening of concrete beams

Abstract

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminate strips into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets; and two beams strengthened with NSM precured laminate strips of CFRP, one of them with laminates positioned at 90º and the other with laminates positioned at 45º in relation to the beam axis. Influences of the laminate strip inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminate strips at 45º was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. This technique was also faster and easier to apply than the externally bonded one. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.