Strength and Ductility of Confined Concrete

An analytical model is proposed to construct a stress‐strain relationship for confined concrete. The model consists of a parabolic ascending branch, followed by a linear descending segment. It is based on calculation of lateral confinement pressure generated by circular and rectilinear reinforcement, and the resulting improvements in strength and ductility of confined concrete. A large volume of test data, including poorly confined and well‐confined concrete was evaluated to establish the parameters of the analytical model. Confined concrete strength and corresponding strain are expressed in terms of equivalent uniform confinement pressure provided by the reinforcement cage. The equivalent uniform pressure is obtained from average lateral pressure computed from sectional and material properties. Confinement by a combination of different types of lateral reinforcement is evaluated through superposition of individual confinement effects. The descending branch is constructed by defining the strain corresponding to 85% of the peak stress. This strain level is expressed in terms of confinement parameters. A constant residual strength is assumed beyond the descending branch, at 20% strength level. The model is compared against a large number of column tests. Circular, square, and rectangular columns, with spiral and rectilinear reinforcements, as well as welded wire fabric, are used for comparison. Comparisons include concentric and eccentric loadings, as well as slow and fast strain rates. The results indicate good agreement.