Effect of inner steel tube on compression performance of GFRP-reinforced concrete columns

Concrete structures where a harsh environment might induce corrosion of steel rebars, FRP (fiber-reinforced polymer) rebars may be utilized as a feasible substitute for steel bars. The compression performance of reinforced concrete (RC) columns having a mix of GFRP (glass FRP) and steel reinforcement, on the other hand, is unclear. The present study focuses on incorporating an inner steel tube in RC columns having GFRP rebars. The employment of GFRP rebars as vertical reinforcement of high tensile strength will be effective in resisting flexure, whereas the inner steel tube will carry the axial load. In this research, a testing campaign was carried out on square columns having dimensions of 250 × 250 × 1000 mm, wherein the columns were experimentally examined in the event of concentric and eccentric compression. The variables studied include (i) two rebar types, namely steel and GFRP, (ii) two diameters of vertical GFRP bars having almost the same percentage of reinforcement (1%-1.1%), and (iii) unblended versus hybrid (i.e., having a steel tube) reinforcement. The axial compression response of RC columns having almost the same percentage of GFRP rebars and a steel tube was not affected by the rebar diameters. However, it affected the eccentric compression behavior due to the change in the layout of the rebars. The use of a steel tube as internal reinforcement introduced ductility in the eccentrically loaded columns. Based on the section analysis, analytical models were developed for creating the P-M graphs for RC columns having GFRP bars combined with a steel tube used as internal reinforcement. The equations consider the compression response of GFRP bars. Incorporating the compressive capacity of GFRP rebars results in the expansion of the P-M interaction plots, thereby bringing the experimental curve into closer alignment with the envelope.