Effect of the crater-height ratio on the shear behavior of deep concrete beams reinforced with carbon polymer fibers by the NSM-EBR method

Deep reinforced concrete beams are very important structural elements that are used in concrete structures such as high-rise buildings, marine structures, silos, and etc. Because of the importance of these elements, they sometimes need to be strengthened. One common method of strengthening reinforced concrete structures is the use of carbon polymer sheets (CFRP). In recent decades, little research has been done to investigate the effect of various variables affecting the behavior of concrete deep beam, strengthened using polymers reinforced with single or multi-directional fibers. The valuable features of composite materials such as simple and fast installation and corrosion resistance have led to the increasing use of these materials. It is possible to strengthen the reinforced concrete deep beams by gluing FRP plates or strips in a full-cross-section, gluing in a U shape or mounting on the sides of the beam at different angles to the longitudinal axis. Laboratory and analytical results prove that the external installation method, EBR, can be used as an acceptable solution in strengthening the shear of reinforced concrete members. Other methods such as installation Near the Surface Method (NSM) or methods that prevent them from separating from the concrete surface by inhibiting Composite tape can be used as an EBR alternative method that delays separation. Despite extensive research on strengthening and strengthening conventional reinforced concrete beams with FRP sheets, little research has been done on deep concrete beams. As we know, many parameters affect the NSM and EBR method. The lack of research in this field has caused these methods and details to be narrowly stated in the statutes.

One of the problems with using polymer sheets is their separation when loading a prototype. Therefore, this article attempts to use near-surface mounting methods (NSM) to strengthen such beams. This method plays a significant role in reducing the separation of reinforcement sheets. By reducing the effect of early detachment of polymer sheets, increasing the load capacity, increasing the formability and maximizing the use of the material capacity can be expected. Six deep reinforced concrete beams with a span-to-height ratio of 2, 3 and 4 are laboratory-loaded and reinforced by the NSM-EBR method using CFRP and their shear behavior is investigated. Given that the desired failure in deep beams is of the shear type, the NSM reinforcement grooves are installed perpendicular to the direction of the main diagonal cracks and reinforced with carbon polymer fibers. By strengthening the beam in shear using the EBR-NSM method, the type of beam failure did not change, but the number of cracks and tensile bands increased, which caused increase the load-bearing capacity of the beams. In the failure modes, it was observed that no delamination and slippage in the EBR-NSM strengthening system. Due to the presence of the adhesive at a greater depth of the beam, there is no slippage and delamination in the CFRP fibers within the tested specimen. This system has high reliability in terms of stability against slippage. The results showed the shear capacity of the deep beams increase by implementing this approach. Based on the results, it can be concluded that the amount of beam shear load capacity in samples with a span ratio of 2, 3 and 4 increased by 30%, 19% and 12.5% respectively