Tests and design equations for FRP-strengthening in shear

The experimental/analytical study presented herewith arrived at developing a mechanics-based (as opposed to regression-based) model of the shear capacity of reinforced concrete beams, strengthened with externally bonded fibre reinforced polymers (FRP). The model is obtained through the following steps, with due consideration of the underlying physical mechanisms: (a) the generalised constitutive law of an FRP layer bonded to concrete is defined first, then, (b) the compatibility imposed by the shear crack opening and the appropriate boundary conditions – which depend on the strengthening configuration (either side bonding, U-jacketing or wrapping) – are included in the formulation, and, finally, (c) analytical expressions of the stress field in the FRP strip/sheet crossing a shear crack are obtained. Through these expressions, closed-form equations for the effective debonding strength of FRP strips/sheets are defined as function of, both, the adopted strengthening configuration, and of some basic geometric and mechanical parameters. The so-obtained FRP contribution is then added to those of concrete and steel, which, for the sake of comparison, have been considered as given by different codes. The equations accuracy has been verified by predicting – a priori, with the developed equations – the shear strength of experimentally tested r.c. beams, both collected from the literature and obtained from purposely carried out tests on under-designed real-scale beam specimens, strengthened with different FRP schemes. No a posteriori calibration of the model was performed. The prediction capability of the developed equations has been finally compared to other approaches available in the literature.