Flexural Crack Pattern on R/C Beams Strengthened with FRP Plates

During the last five years, the application of fiber reinforced polymers (FRP) for the strengthening of existing R/C beams has rapidly increased due to the ease of execution, structural efficiency and economical reasons. Although more than 150 experimental programs reported in the literature have highlighted the full spectrum of failure modes that can be activated by a selected combination of the strengthening design parameters, some questions are still open. Concerning the ultimate limit state, even if the failure modes due to the end plate and midspan debonding of the reinforcing plate are quite common ultimate events, the mechanics of their activation is only roughly understood. It has been proved that, for the above mentioned failure modes as well as for the case of the concrete cover rip off, the fracture mechanics of the problem is dependent on the crack pattern of the beam; however, the effect of the reinforcing plate on the crack position and width has received only minor attention in the studies carried out in the past. Concerning the servicebility limit state, it is well known that a major role is played by the deflection verifications and crack width evaluations; however, the crack spacing and inclination are known with a very low confidence and even a first approximation model is hard o be set due to the deficiency of addressed experimental results. Actually, a clear and systematic analysis of the crack pattern of externally reinforced beams is still lacking being that, in the pioneer phase, the investigator’s interest was mainly focused on the overall mechanical behavior. In this framework, mainly four point bending tests were performed and published on FRP strengthened R/C beams while very few uniform load bending tests can be found in the literature. It is to recall that this type of load distribution is the most representative of the common applications and has to be considered for design purposes. In the present paper, the authors consider the crack pattern developed by 4 FRP strengthened R/C beams brought to failure for flexure under uniform load conditions. of the observed failure modes are due to concrete cover rip-off; the plate strain distribution and beam deflection for increasing load values are fully documented. Based on the available experimental results, a comparative study of different models for crack spacing evaluation is presented, accounting the influence of the FRP strengthening for flexure on crack development and stabilization. The proposed models are mechanically sound and derived by previous formulations developed for conventional R/C beams. Among the considered models, the authors select a simple and efficient expression suitable to be proposed as a design tool. Finally, thanks to the selected crack spacing model, the authors work out a simple model that can predict the rip-off failure load of R/C beams externally strengthened with FRP with an acceptable accuracy. The model is calibrated making use of the 4 available experimental beams and is validated accounting for 23 experimental beams derived from the literature. All of the considered experimental beams present concrete cover rip-off failure mode, despite the very different geometrical and mechanical characteristics.