Flexural behaviour of concrete beams reinforced with different grade steel and strengthened by CFRP strips

Externally bonded (EB) carbon fibre-reinforced polymer (CFRP) strips can be used to increase load-bearing capacity of reinforced concrete (RC) elements or to improve serviceability conditions of the structures. Such a strengthening technique can be applied both in new buildings, as well as in existing ones, erected years or even decades ago. Therefore, the elements to be strengthened could differ considerably because of the physical parameters of the materials used during construction. The bulk of experimental studies on debonding phenomena of EB-strengthened RC structures focus on factors related to the concrete substrate and composite material. The assumptions presented in the existing international codes and guidelines, as well as in numerous theoretical bond models, neglect the influence of steel grade or yield strength on bond properties, and in turn, the load-bearing capacity of the connection between the composite and concrete substrate as well. Four RC beams were flexurally strengthened with CFRP laminates and tested. Low-strength concrete and four steel grades with different yield points were used to cast the test elements. The results indicate that CFRP laminates with the externally bonded reinforcement are an effective technology to increase cracking, service, and maximum loads of RC beams, even when made with poor-quality concrete. Therefore, it is noted that the yield point of steel, and therefore, the grade of steel is the next parameter influencing the bond efficiency of CFRP laminates in the RC structures to be studied.