Abstract
Based on the axial compression test of 5 ultra-high performance concrete (UHPC) columns confined by high-strength stirrups and 4 UHPC columns confined by normal-strength stirrups, the bearing capacity, failure mode, steel strain and stress-strain curve for the confined UHPC columns were studied, and the influences of volume stirrup ratio, strength, spacing and configuration of stirrup on the axial compression behavior of confined UHPC were analyzed by combining ductility and toughness index. The results show that all the specimens have ductility damages, and the damage degree of UHPC columns confined by high-strength stirrups is lighter. The confined UHPC columns with high volume stirrup ratio, closely spaced, high-strength and complex ties configuration have good confinement efficiency, bearing capacity and deformation performance, which also leads to an ideal axial compression behavior. The effect of volume stirrup ratio on the axial compression behavior of specimens is greater than that of stirrup strength. Among the three factors affecting the volume stirrup ratio, such as spacing, configuration and diameter of stirrup, the stirrup spacing contributes most to the improvement of confinement performance, followed by the configuration and diameter of stirrup. The UHPC confined by high-strength stirrups presents better axial compression performance and residual bearing capacity than the normal-stirrups. The micro-bending of the longitudinal bars accelerates the cover spalling, while the high-strength stirrups with close space effectively delays the buckling of longitudinal bars, and significantly improves the overall performance of the confined UHPC. The micro-bending of longitudinal bars weakens the confine effect of the high-strength stirrups on the core UHPC, and the combination of high-strength longitudinal bars and stirrups is suggested. Based on the test data, the formulas to determine the bearing capacity of UHPC columns confined by stirrups are drawn.
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