Internal and external transversal reinforcement interaction in RC beams strengthened in shear with externally bonded composites

The overall shear strength of Reinforced Concrete (RC) beams strengthened in shear with Fiber Reinforced Polymer (FRP) and Fiber Reinforced Cementitious Matrix (FRCM) composites is generally computed as the simple summation of the individual contributions of concrete and transversal internal (i.e. steel stirrups) and external (i.e. FRP or FRCM composites) reinforcement. However, the interaction between transversal internal and external shear reinforcement might cause an overestimation of the strength of the strengthened beams when this simplified approach is used for FRP strengthened elements. Although the experimental evidence is still limited, a similar interaction has been reported for beams strengthened in shear with FRCM composites. This paper presents the results of an experimental campaign carried out to investigate the aforementioned interaction. RC beams were strengthened in shear using FRP and FRCM composites and then tested using a traditional four point bending scheme. Interaction between the internal and external transversal reinforcement was observed, as strains measured in the stirrups of the strengthened beams were lower than those measured in the unstrengthened beams at peak load. Although this reduction was more evident for FRP strengthened beams, the presence of the external reinforcement precluded stirrups from yielding for either FRP or FRCM strengthened beams in this study.