Effect of size on the failure of geometrically similar concrete beams strengthened in shear with FRP strips

Fiber reinforced plastics (FRP) are commonly used for the strengthening of concrete members. For shear strengthening of beams, FRP strips can be bonded to the sides of the member alone, to both the sides and the bottom (i.e., the U configuration), or wrapped around the whole beam. For the various strengthening configurations, empirical equations have been proposed for predicting the contribution of strips to the shear capacity of the member. However, for the same strengthened member, the equations recommended by different design guidelines (American Concrete Institute, International Federation for Structural Concrete, and Japan Society for Civil Engineers) predict different shear capacities. Moreover, as the equations were obtained through the fitting of laboratory data on relatively small beams, their applicability to beams of practical sizes have not really been assessed. In the present investigation, geometrically similar beams with depth of 180, 360, and 720 mm were retrofitted in shear with carbon FRP strips in both the U configuration and fully wrapped configuration. The retrofitted members were tested to failure to (1) provide data on beams of practical sizes for verification of design equations and (2) investigate if the strengthening effectiveness is similar for small and large beams. Measured FRP contribution to the shear capacity is also compared to predictions from equations in the various guidelines. Based on our findings, for beams retrofitted with strips in the U configuration, the strengthening effectiveness may significantly decrease with member size, and none of the available design equations can consistently provide conservative values for the shear capacity. For beams with fully wrapped strips, strengthening effectiveness is independent of member size, and the FIB equation appears to be most appropriate for practical design.