Full-scale seismic testing of concrete building columns reinforced with both steel and CFRP bars

This paper presents an experimental study of an innovative concrete building column which is longitudinally reinforced with both steel bars and fiber-reinforced polymer (FRP) composite bars. The main objectives of this study include: (1) investigating the seismic failure mechanism of such FRP-steel reinforced concrete (FSRC) columns under relatively large gravity load and (2) analyzing the effects of the FRP bars on the seismic performance of the FSRC columns. To this end, a total of six full-scale cantilever columns with variable carbon FRP (CFRP) bar and steel reinforcement ratios were tested under combined constant axial load and lateral displacement reversals. The specimens had 400 mm square sections and effective heights of 1800 mm. Test results showed that adding additional CFRP bars into the conventional steel reinforced concrete (SRC) columns was efficient in improving the post-yield stiffness ratios and mitigating the residual displacements, while the hysteretic energy dissipation could be maintained. Failure modes of the tested FSRC specimens were characterized by crushing of FRP bars and buckling of steel bars at drift ratios larger than 2.4%. Furthermore, confining the FSRC column with external CFRP wraps was effective in delaying the crushing of internal CFRP bars and reducing the post-earthquake residual displacement.