Strengthening of concrete columns by near surface mounted method with different material bars and confined with CFRP sheet

In recent years, Fiber-reinforced polymer (FRP) materials are increasingly used for strengthening and retrofitting of reinforced concrete structures. Fiber-reinforced polymers in the form of jackets with the fibers typically in the columns’ circumferential direction, are quite effective in carrying shear and in providing confinement. In addition, the previous studies show that the FRP jacket can greatly enhance the dissipation capacity and ductility of reinforced concrete columns. However, FRP jackets are not effective for the strengthening of columns in flexure. To overcome difficulties associated with FRP jacketing, recent research efforts have focused on the use of near surface mounted (NSM) FRP or stainless steel reinforcement through a combination of externally bonded (EBR) FRP sheets and anchors for the flexural strengthening of columns. NSM strengthening technique consists of FRP rods embedded in grooves made on the surface of the concrete and bonded in place with epoxy. The NSM technique is not effective in terms of enhancing the energy dissipation capacity of RC columns. Therefore, by combining NSM technique with FRP confinement sheets, a high effective technique (hybrid FRP-based strengthening technique) can be obtained. FRP and steel bars have been used together for reinforcing the concrete beams in previous studies. However, the combination of both materials for strengthening of reinforced concrete columns is not common. The present paper experimentally investigates the effects of combined NSM-GFRP bars with the CFRP confinement sheets and also NSM-GFRP bars with NSM-Steel rods on strengthening of reinforced concrete columns. It should be noted that in combinatory method of NSM-GFRP with NSM-Steel, NSM-Steel to NSM-GFRP ratios is different.
The experimental program includes five square columns with a cross section of 250*250 mm and the length of 1200 mm. The columns were tested to failure by applying constant axial compressive and cyclic lateral loading. According to the results, it is indicated that using these two combinatory reinforcing methods cause an increase in load capacity, ductility, dissipated energy and initial stiffness of columns compared to the control column and also columns reinforced by using NSM-GFRP bars. Furthermore, NSM-GFRP technique increases the load capacity 43 percent in comparison with the control specimen. In the case of combined retrofitting methods, load capacity increase was up to 60 percent. Also, using NSM-GFRP reinforcement causes 7 percent increase in ductility and 6 percent decrease in dissipation capacity. In contrast, in the specimens reinforced by the combinatory methods, the ductility parameter and dissipation capacity increase 3 to 150 percent and 24 to 133 percent, respectively. In addition, hysteretic diagrams corresponding to specimens strengthened by combinatory methods has less pinching effect than those of the control specimen. Experimental results indicate that the reinforced concrete column strengthened by combinatory method of NSM-GFRP with NSM-Steel is a viable solution toward enhancing the behavior of RC columns subjected to simultaneous axial and seismic loads. This is especially the case when the retrofitting scheme contains higher NSM-Steel to NSM-GFRP ratio. Finally, due to corrosion of steel reinforcement, NSM-GFRP bars combined with CFRP confining sheets is selected as the best choice of strengthening.