Effect of C-FRP (Carbon Fiber Reinforced Polymer) Rope and Sheet Strengthening on the Shear Behavior of RC Beam-Column Joints

This study focuses on the investigation of an innovative external strengthening technique for reinforced concrete (RC) beam–column joints, utilizing near-surface mounted (NSM) Carbon Fiber Reinforced Polymer (C-FRP (Carbon Fiber Reinforced Polymer)) ropes. The ropes are installed diagonally on both main faces of the joint, forming external X-shaped configurations. Additionally, vertical reinforcement is provided at the four corners of the column, with one rope placed per corner. Complementary strengthening is also applied to the beam’s plastic hinge region using externally bonded C-FRP (Carbon Fiber Reinforced Polymer) sheets. The proposed technique is based on the premise that the X-shaped diagonal ropes provide adequate confinement to the joint core and act effectively as shear reinforcement, thus mitigating the shear distortions typically developed in the joint area under seismic loading. At the same time, the use of C-FRP (Carbon Fiber Reinforced Polymer) sheets in the beam further enhances the load-bearing capacity of the joint, particularly in the region of plastic hinge formation. To evaluate the effectiveness of the technique, full-scale cyclic loading tests were conducted on three RC beam–column joint specimens. These specimens were specifically designed to concentrate damage within the joint core, simulating conditions of insufficient initial reinforcement. Comparative analysis between the strengthened and unstrengthened specimens clearly demonstrated that the external application of C-FRP (Carbon Fiber Reinforced Polymer) ropes significantly contributed to preserving the integrity of the joint region and reducing shear deformations, especially under large displacement demands. In particular, the specimen strengthened with both X-shaped C-FRP (Carbon Fiber Reinforced Polymer) ropes in the joint core and C-FRP (Carbon Fiber Reinforced Polymer) sheets on the beam (JB0VF2X2c+C-FRP (Carbon Fiber Reinforced Polymer)) exhibited a peak load increase of up to 65%, an enhancement in equivalent viscous damping of up to 55%, and a reduction in shear deformations exceeding 40%, compared to the unstrengthened specimen. Furthermore, improved energy dissipation capacity, better stiffness retention, and delayed damage progression were observed. Overall, the proposed strengthening approach using externally applied C-FRP (Carbon Fiber Reinforced Polymer) ropes and sheets emerges as an effective and practically applicable solution for retrofitting existing RC joints in seismic regions. It enhances both strength and ductility without altering the geometry or increasing the mass of the structural elements.