Intermediate Debonding Failure of RC Beams Retrofitted in Flexure with FRP: Experimental Results vs Prediction of Codes of Practice

An experimental investigation has been conducted with the aim to investigate on the midspan debonding failure of reinforced concrete (RC) beams retrofitted in flexure by means of the application of a fiber-reinforced polymer (FRP) lamina externally applied to concrete substratum. Experimental tests on a series of RC beams with different geometries and type of internal steel reinforcing bars have been carried out in four-point bending up to failure to evaluate the influence of flexural/shear cracks on the debonding of FRP reinforcement from concrete substratum. It is widely known that the overall performance of a standard RC element is primarily influenced by the interaction between FRP and concrete substratum rather than by the strength of the FRP. The failure of such innovative retrofitting techniques may be attributed to an inadequate anchorage length; in other cases, when the FRP is correctly applied at the ends of beam, debonding starts in the vicinity of cracks and propagates toward the supports. The results of the experimental tests are used to investigate on the effectiveness of design procedures currently recommended for such debonding failure mode by the codes of practice. The results of the analytical calculations based on such models are presented in this paper and will provide an element of comparison and discussion with the experimental results. The codes of practice propose different analytical models based on the extensive research done in the last two decades, which involve different types of approximation and often yield quite different results.