Confinement of ultra-high-performance fiber reinforced concrete columns

This study investigates the axial load response of ultra-high-performance fiber-reinforced concrete (UHPFRC) columns with compressive strengths of 163 and 181 MPa (design strengths of 150 and 180 MPa). The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) and did not contain coarse aggregate. A total of nine UHPFRC columns confined by transverse reinforcement with volumetric ratios of 0.9-9.9% and two different configurations (Types A and C) were tested under pure axial load to investigate the influence of these variables. The overall behavior of the UHPFRC columns was compared with the response of similar strength ultra-high-strength concrete (UHSC) columns having coarse aggregate. Test results showed a pronounced effect of the volumetric ratio of the transverse reinforcement on the confinement. Hybrid micro-steel fibers controlled brittle cover spalling very well and assisted the transverse confinement reinforcement after the peak load. Applicability of the confinement reinforcement equations in the current seismic design provisions for developing ductile behavior of the UHPFRC columns was investigated. The analytical study examined the ability of the existing high-strength concrete (HSC) confinement models for predicting the axial load response of the UHPFRC columns, and a prediction model that accounts for the effects of steel fibers and the stress-strain relationship of UHPFRC is proposed.