Compressive behavior of FRP-confined ultra-high performance concrete (UHPC) in circular columns.

• Typical three-segment axial stress-strain curves of FRP-confined UHPC are defined. • Stress reduction and the actual confinement ratio threshold reduce with the content of steel fibers. • Addition of steel fibers compromises effects of specimen size. • Performances of existing axial stress-strain models for FRP-confined concrete are verified. Ultra-high performance concrete (UHPC) usually exhibits a weak ductility under axial compression. However, the ductility can be enhanced by implementing confining devices such as fiber-reinforced polymer (FRP) wraps. In order to gain in-depth understandings on the compressive behavior of FRP-confined UHPC, axial compression tests on 52 FRP-confined and 12 unconfined UHPC cylinders were carried out in this study. The failure modes, stress-strain behaviors, confinement efficiency, and the influences of crucial variables (steel fiber contents, specimen sizes, FRP fiber types, and FRP thickness) on the compressive behavior of FRP-confined UHPC were investigated and discussed. Results show that the increase in steel fiber content could increase the first peak stress and its corresponding strain, hoop strain efficiency, avoid the abrupt stress reduction after the first peak stress, as well as alleviating the effect of specimen sizes. Addition of steel fibers also alters the actual confinement ratio threshold for sufficient confinement. Comparisons suggest that FRP confinement fiber types have limited influence on the normalized stress-strain behavior of specimens with similar level of actual confinement ratio. Additionally, performances of four existing axial stress-strain models for FRP-confined concrete are verified and the results reveal that Teng et al.'s model performs the best although this model is incapable in capturing the strain softening behavior after the first peak stress. [ABSTRACT FROM AUTHOR]