Considering the crucial role of columns in maintaining the stability of reinforced concrete (RC) structures an‎d their vulnerability to seismic loading, the strengthening of these elements has long been a major focus of research. Among various advanced strengthening strategies, fiber-reinforced polymer (FRP) composites have proven to be a proper alternative to conventional techniques such as concrete an‎d steel jacketing. While extensive research has been conducted on the performance of FRP-strengthened columns under monotonic loading, comparatively fewer studies have investigated their behavior under cyclic loading, which more accurately simulates seismic deman‎ds. In the present study, the cyclic loading apparatus at Isfahan University of Technology - capable of applying loads with bidirectional eccentricity - was employed to experimentally investigate the performance of FRP-strengthened RC columns subjected to reversed cyclic loading. In the present investigation, twelve reinforced concrete (RC) column specimens with a square cross-section measuring 125 × 125 mm an‎d 500 mm height were tested under three eccentricity levels of 0, 30, an‎d 90 mm. All specimens were cast using self-compacting concrete (SCC) with a target compressive strength of 35 MPa. For each eccentricity level, one unstrengthened control specimen (REF), one specimen strengthened by the intermittent wrapping (IW) technique, an‎d two specimens strengthened using grooving methods known as EBROG an‎d EBRIG were prepared an‎d tested. A novel approach was employed to compare the grooving methods by using a groove group with dimensions of 10 × 10 mm spaced at 25 mm center-to-center an‎d identical FRP strip widths in both methods; accordingly, the EBROG-strengthened specimens incorporated three longitudinal grooves, whereas the EBRIG-strengthened specimens incorporated two longitudinal grooves on the relevant faces. Finally, all specimens were subjected to reversed cyclic axial eccentric loading at the specified eccentricity levels. The results indicated an increase in load-carrying capacity for all strengthening methods compared to the unstrengthened specimen. Longitudinal strengthening with FRP composites using the EBROG an‎d EBRIG techniques demonstrated highly favorable performance in enhancing load carrying capacity, particularly at higher eccentricities; these methods increased the load capacity by 42% an‎d 50%, respectively, at an eccentricity of 90 mm. This phenomenon is attributed to the predominance of tensile stresses generated by bending at higher eccentricities, which leads to the effective performance of the mentioned strengthening methods in tension. Notably, at lower eccentricities, the IW strengthening method exhibited superior performance in improving ductility index an‎d energy dissipation compared to the grooving methods, owing to the confinement effect; however, with increasing eccentricity an‎d the consequent reduction in confinement, the effectiveness of the IW method diminished. For instance, strengthening specimens with the IW method resulted in a 59% increase in the ductility index an‎d a 144% increase in energy dissipation at zero eccentricity. Furthermore, the results revealed that at all eccentricities, the EBRIG method improved the load-carrying capacity compared to the EBROG method, despite having fewer grooves, reduced grooving operations, an‎d a 33% decrease in epoxy consumption. Key words: RC column, Reversed cyclic axial eccentric loading, Strengthening, Fiber-reinforced polymer (FRP) composites, Self-compacting concrete (SCC), EBROG grooving method, EBRIG grooving method, Intermittent wrapping (IW) technique.

داود مستوفي نژاد , عليرضا سلجوقيان

محمدرضا افتخار , ابوالفضل اسلامي حسن‌آبادي