Seismically Robust Ultra-High-Performance Fiber- Reinforced Concrete Columns

Ultra-high-performance fiber-reinforced concrete (UHP-FRC) has a high compressive strength of 22 to 30 ksi (152 to 210 MPa) and a substantial shear strength as well as exceptional compressive ductility and confinement characteristics due to the addition of high-strength steel microfibers, which alleviate the need for excessive transverse reinforcement in high-strength concrete. The application of UHP-FRC in seismic-resistant reinforced concrete (RC) columns was investigated in this study. Two full-scale columns, one with normal strength concrete and the other with UHP-FRC in the plastic hinge region, were tested under simulated earthquake loads to evaluate their damage-resistance ability, deformation capacity, and failure mechanism. Experimental results show that the use of UHP-FRC changes the failure mode of RC columns as it improves confinement and shear capacity, as well as prevents concrete from crushing. The UHP-FRC column exhibits a higher peak strength and a greater deformation capacity before succumbing to significant strength degradation compared to the normal-strength RC column. The lateral displacements of the ACI 318-19-compliant RC column mainly result from distributed reinforcing bar yielding. Conversely, displacements of the UHP-FRC column are dominated by the slip deformation at the column-footing interface due to the strain penetration of the longitudinal reinforcing bars into the footing. Unlike the RC column, the failure of the UHP-FRC column is controlled by the low-cycle fatigue life of its longitudinal reinforcing bars. Concrete crushing in the RC column started at 1% drift ratio and became nearly unrepairable beyond 2.75% drift ratio. On the other hand, the UHP-FRC column experienced limited damage even at large drift ratios. This will result in great post-earthquake functionality and considerable cost savings in repairs for structures with UHP-FRC columns. In addition, incremental dynamic analyses of a four-story prototype RC moment frame indicate that buildings with UHP-FRC columns can sustain earthquakes with 20% higher peak ground acceleration before collapsing due to the greater deformation capacity.