THE AXIAL COMPRESSION STRESS-STRAIN MODEL FOR ULTRA-HIGH PERFORMANCE CONCRETE COLUMNS CONFINED BY HIGH-STRENGTH STIRRUPS

The axial compression constitutive model of confined ultra-high performance concrete (UHPC) is the basis of structural design and nonlinear analysis of UHPC columns. The axial compression test of nine UHPC columns confined by stirrups were performed in this study. The high-strength stirrups showed advantages in improving the strength and deformation performance of confined UHPC over normal stirrups, and better compatibility with UHPC than normal stirrups. The confined UHPC columns with high volume stirrup ratio, closely spaced, high-strength and complex ties configuration may lead to a smooth stress-strain curve, and significantly improves the strength and deformation performance of confined UHPC. The high-strength stirrups may not yield completely at the peak strength, indicating that using the yield strength of stirrups to calculate the confining pressure at the peak may overestimate the peak strength of confined UHPC. The formula of peak strain and high-strength stirrup stress at the peak point, and the formula of peak stress based on the Ottosen's failure criterion were drawn for confined UHPC. A new confined UHPC constitutive model was presented. Compared with other typical models, the new model agreed with the test curve.