This study evaluated the pure confinement effect of steel spirals embedded in concrete with varying compressive strengths. The experimental variables in the study were the compressive strength of concrete and the yield strength and volumetric ratio of the steel spirals. To estimate the pure confinement effect of the steel spirals, all the specimens were designed to exclude concrete cover and longitudinal reinforcement. The experimental results showed that the confinement effect of spirals increased with yield strength and volumetric ratio of the spirals, but greatly decreased as the concrete compressive strength increased. Using the relationship between axial and lateral strains at peak stress of spirally confined concrete, an analytical model was developed to predict the stress–strain relationship of confined concrete. The proposed model can be successfully used to predict the structural behaviour of concrete confined by normal-, high- and ultra-high-strength steel spirals, regardless of the compressive strength of concrete.