Concrete, stirrup, and FRCM contributions to the shear strength of RC beams

Fiber-reinforced cementitious matrix (FRCM) composites have shown promising results as shear strengthening of reinforced concrete (RC) beams. However, due to the limited available experimental evidence, further research is needed to develop accurate and reliable design formulations. In this paper, the results of an experimental campaign previously carried out by the authors on RC beams strengthened in shear with FRCM composites are used to identify the shear strength contributions of the concrete, internal transverse reinforcement, i.e., stirrups, and external transverse reinforcement, i.e., FRCM jacket. Two approaches are used. In the first, the concrete contribution is calculated as the difference between the strengthened beam capacity and the internal and external reinforcement contributions, computed based on experimental strains. In the second, the concrete contribution is estimated from the control (unstrengthened) beam and then combined with the internal reinforcement contribution obtained from the experimental strains to estimate the FRCM contribution. Results show that the concrete and stirrup contributions to the shear strength of strengthened beams are lower than those of corresponding control beams. This conflicts with the assumptions of available design guidelines that compute the shear strength of FRCM-strengthened beams as the summation of the maximum contributions by concrete, internal reinforcement, and FRCM.