Analysis of Bond-Slip Effects in RC Beams Strengthened with NSM CFRP Rods

The use of the near surface mounted (NSM) fiber reinforced polymer (FRP) technique for retrofitting RC beams is a mainstream. Nevertheless, the bond mechanisms of FRP rods between rods and adhesive resin and/or rod–resin with concrete surfaces are not known completely. This paper investigates the experimental bond-slip effects of circular and rectangular carbon-FRP (CFRP) rods inserted in grooves in RC elements through experimental pull-out and bending tests, and using theoretical models. Five RC beams were built and subjected to bending loading; three of the ones strengthened using NSM CFRP rods were subjected to bending until failure. The experimental results showed changes in the response of beams such as stiffness in the elastic phase, reduction of ultimate capacity and ductility due to bond-slip effects. On the other hand, strain measurements on CFRP rods recorded by pull-out tests theoretically valid the linear elastic analysis capable of defining the behavior of CFRP rods before loss of bond. Finally, the comparison between experimental results and theoretical data opens a discussion concerning bond-slip effects in cracked RC beams strengthened with NSM CFRP rods and leads to the definition of actual strain limits for FRP under service loads.