Experimental Assessment of Predamage Effects and Analytical Prediction of Debonding Load in FRCM Shear-Strengthened RC Beams

Abstract This study investigates the shear behavior of reinforced concrete (RC) beams strengthened with unanchored polyparaphenylene benzobisoxazole fabric-reinforced cementitious matrix (PBO-FRCM) U-wrap systems, with particular emphasis on debonding-controlled failure mechanisms, adverse shear interaction with internal steel stirrups, and the influence of predamage. An experimental campaign was carried out on seven full-scale RC beams, including one unstrengthened control specimen and six beams strengthened with PBO-FRCM systems, tested in both undamaged and predamaged conditions. The predamage load level was defined as 0%, 60%, and 80% of the ultimate load of the control beam. The results show that FRCM shear strengthening is highly effective for undamaged beams. However, its efficiency progressively decreases as the predamage load level increases. This reduction is more evident in cases with higher FRCM strengthening ratios, in which premature debonding governs failure and limits the effective activation of the external strengthening. Furthermore, two experimental databases were considered: 75 RC beams shear-strengthened with unanchored FRCM U-wraps failing by debonding, collected from the literature, and 284 direct shear tests on FRCM-to-concrete joints, used to characterize bond-controlled stress transfer mechanisms. Based on the combined experimental evidence, a debonding-based analytical model is proposed for evaluating the shear contribution of FRCM systems. The formulation explicitly accounts for the mechanical role of the mortar and adverse shear interaction with internal steel reinforcement. The effective strain of the composite is derived from regression analysis of independent bond–test data, avoiding the use of arbitrary strain limits. The predictive performance of the proposed model is assessed through comparison with experimental results. The results show improved overall accuracy and reduced dispersion, highlighting the need to explicitly account for bond behavior and interaction effects in the prediction of FRCM shear contribution.