Effect of longitudinal CFRP strengthening on the shear resistance of reinforced concrete beams

The flexural strengthening by external bonding of fiber reinforced polymer (FRP) to the tension face of reinforced concrete (RC) beams has proven to play similar role as that of the internal longitudinal reinforcement in increasing flexural strength and stiffness of the beams. Internal longitudinal reinforcement is also known to influence the shear strength of RC beams particularly the concrete shear strength component Vc. However, the effect of longitudinal FRP strengthening on Vc of RC beams has not yet been investigated. It is well established that Vc of flexural members can be reliably quantified from shear tests on beams without shear reinforcement. Therefore, RC beams without stirrups were constructed and strengthened with carbon FRP (CFRP) reinforcement to assess the effect of longitudinal strengthening on the shear strength of the beams. The study included five strengthened beams and two unstrengthened beams; all were tested under three-point bending. The test results indicated that the shear strength of the strengthened beams increased over that of the unstrengthened beams. The strength increase was up to 35%. The beneficial effect of the longitudinal strengthening was accounted for by considering the FRP reinforcement ratio in addition to the steel reinforcement ratio. Both reinforcement ratios were combined together in an equivalent reinforcement ratio accounting for the difference in the modulus of elasticity of the two materials. The equivalent reinforcement ratio was introduced in the conventional design methods for steel-reinforced beams to assess the shear strength of the strengthened beams. The experimental shear strength of the test beams were compared with the predictions of the modified design methods and good agreement was found.