Effectiveness factors for bond strength in FRP shear-strengthened RC beams

A study to describe theoretically the shear interaction effect between internal and external shear reinforcements in FRP-shear strengthening RC beams is presented. Such interaction phenomenon is characterized by the premature and brittleness debonding failure of the FRP and by the under-mobilization of the internal steel stirrups. The average stress (strain) in steel stirrups when FRP debonding occurs may be below the yield value, so not all steel stirrups intercepted by the critical shear crack reach yielding at the shear failure of the beam. In practice, the contributions of both internal shear steel reinforcement and externally bonded FRP are interacting parameters. By means some rationale simplifications to describe the debonding process, the problem has been developed in a way that a closed-form solution can be obtained. The average stresses in the FRP and steel reinforcements intersected by the critical shear crack at the debonding failure have been defined as a function of their strength, using effectiveness factors that have been conveniently expressed for the common FRP shear strengthening configurations, such as side bonding, U-jacketing and complete wrapping. A parametric analysis was also carried out to evaluate the influence of some fundamental parameters on such effectiveness factors.