Axial behavior of high-strength concrete confined with multiple spirals

This paper investigates the behavior of high-strength reinforced concrete columns confined using a new cross-spiral confinement technique and compares the behavior with three models for confined concrete subject to monotonic axial loads. This new confinement method uses two opposing spirals to confine the concrete core resulting in enhanced strength and ductility compared to traditional columns containing a single spiral. Twenty-one reduced-scale circular high-strength reinforced concrete columns with four different spiral spacings and four longitudinal reinforcing ratios were tested. The program included seven control columns containing single spirals and fourteen columns constructed using the cross-spiral confinement technique – seven with spacings equal to the control specimens and seven with double the spacing. The study demonstrated that the new cross-spiral technique with the same volumetric confinement ratio (and double the spacing) as the conventional single-spiral column obtained similar ultimate strengths with an increase in the ultimate displacement of about twenty percent. The cross-spiral confinement technique using twice the volumetric confinement ratio (and the same spiral spacing) greatly outperformed conventional single-spiral columns both in terms of strength and ductility. The models evaluated matched the experimental results for the cross-spiral columns on the ascending branch of the stress–strain curve until about 75% of the confined compressive strength was reached. The existing models did not capture the increase in strength and ductility provided by the new cross-spiral technique.