Study on the Constitutive Model of Concrete Confined by Multi-Spiral Composite Stirrups

Through axial compression tests, the influence of three stirrup indexes (space, form, and strength) on the confining performance of multi-spiral composite stirrups in square reinforced concrete (RC) columns were analyzed, and the square RC columns with traditional well-shaped composite stirrups were used as the reference group. The results show that the multi-spiral composite stirrups had a positive contribution to the important indexes (bearing capacity and ductility) of the square RC columns due to its multiple restraint mechanism on core concrete. In terms of constraint effect, the five-spiral composite stirrup is the best, followed by the four-spiral composite stirrup, and the last is the traditional well-shaped composite stirrup. The section of the concrete square column is divided into highly constrained, partially constrained and unconstrained regions and the constraint mechanism of multi-spiral composite stirrups is discussed. The formulas for calculating the peak stress, peak strain, and ultimate compressive strain of the constrained column are presented, and the relative error between the theoretical values and the tested values is small. The constitutive model of concrete constrained by multi-spiral composite stirrups is established and compared with other constitutive models. The results show that the proposed model fits well with the experimental curves.