Axial compressive behavior and failure mechanism of CFRP partially confined ultra-high performance concrete (UHPC)

With comparable mechanical properties, higher cost-effectiveness and ease of column joints rehabilitation, partial fiber-reinforced polymer (FRP) confinement holds promise as an alternative to full FRP confinement in ultra-high performance concrete (UHPC). Nevertheless, the compressive behavior and underlying failure mechanism of FRP partially confined UHPC are not fully understood. To address this issue, axial compression tests on ultra-high strength concrete (UHSC, removing fibers in UHPC) and UHPC cylinders fully and partially confined by FRP are conducted in this study. The stress-strain relationship, strain distribution and crack development of specimens subjected to axial compression are profoundly analyzed using a combination of strain gauges and digital image correlation (DIC). Experimental results indicate that partial FRP confinement has a comparable effect on compressive performance compared to full FRP confinement. FRP confinement and fiber inclusion can effectively overcome the brittle nature of UHSC in compressive failure and consequently extend the transition stage, raising the stress/strain retention ratio from 0.91% to 26.45% and improving the enhancement ratio of ultimate strain from 1.97% to 34.4%. Furthermore, a more reliable and accurate prediction model is proposed in this study for evaluating the stress-strain relationship of partial FRP confinement under ultimate conditions.