Experimental Investigation of Retrofitting and Structural Health Monitoring of Reinforced Concrete Beam–Column Joints Under Seismic Loading

Beam–column joints (BCJs) are widely recognized as critical structural components that govern the seismic performance of reinforced concrete (RC) frame structures. Their measurements captured changes associated with susceptibility to brittle joint shear failure under cyclic loading, which often results in rapid strength degradation, limited energy dissipation, and reduced structural resilience. Although various retrofitting techniques have been developed to enhance the seismic behavior of deficient BCJs, continuous assessment of their structural condition remains essential for ensuring their long-term reliability. This study experimentally investigates the seismic performance and structural health monitoring (SHM) of RC BCJs retrofitted using a hybrid strengthening scheme comprising Near-surface-mounted (NSM) carbon fiber reinforced polymer (C-FRP) ropes and externally bonded C-FRP sheets. Two full-scale exterior BCJ subassemblages were subjected to quasi-static cyclic loading. Their structural response was evaluated in terms of hysteretic behavior, load-carrying capacity, stiffness degradation, energy dissipation, and joint shear deformation. In parallel, the applicability of the electromechanical impedance (EMI)-based SHM technique was investigated using externally bonded piezoelectric lead zirconate titanate (PZT) transducers. The experimental results showed that, compared with the reference specimen, the retrofitted specimen exhibited a 17.9% higher positive peak lateral load and a 54.0% higher total cumulative dissipated energy, together with reduced joint shear deformation and delayed strength degradation. Rupture of the externally bonded C-FRP sheet occurred at 3% story drift (sixth loading step). Furthermore, the EMI measurements captured progressive changes in the monitored structural condition during cyclic loading, while also highlighting the influence of sensor location and sensor–structure coupling on damage detection, particularly after rupture of the externally bonded C-FRP sheets.