Assessment of fib Bulletin 90 Design Provisions for Intermediate Crack Debonding in Flexural Concrete Elements Strengthened with Externally Bonded FRP

With the assessment of intermediate crack debonding (ICD) being a subject of main importance in the design of reinforced concrete (RC) beams strengthened in flexure with externally bonded fibre-reinforced polymer (FRP), several approaches to predict the debonding loads have been developed in recent decades considering different models and strategies. This study presents an analysis of formulations with different levels of approximation collected in the fib Bulletin 90 regarding this failure mode, comparing the theoretical predictions with experimental results. The carried-out experiments consisted of three RC beams strengthened with carbon FRP (CFRP) tested under a four-point bending configuration with different concrete strengths and internal steel reinforcement ratios. With failure after steel yielding, higher concrete strength, as well as a higher reinforcement ratio, lead to a higher bending capacity. In addition, the performance of the models is assessed through the experimental-to-predicted failure load ratios from an experimental database of 65 RC beams strengthened with CFRP gathered from the literature. The results of the comparative study show that the intermediate crack debonding failure mode is well predicted by all models with a mean experimental-to-predicted failure load ratio between 0.96 and 1.10 in beams tested under three- or four-point bending configurations.