Stress-strain characteristics of concrete confined in steel binders

Synopsis The ductility of concrete can be increased by confinement in steel binders but, for a proper analysis of the ultimate strength of structures utilizing this gain, an undertsanding of the stress-strain characteristics of confined concrete is necessary. The paper presents the results of axial compression tests on specimens in which the variables were: strength of concrete, size and shape of test specimen and diameter and type of spiral wire. A new factor called the ‘confinement index’ is introduced to define the confinement quantitatively. Ultimate strengths and strains increase with confinement and linearly with the confinement index. Confinement is found to be effective only when the pitch of binders is less than the least lateral dimension of the confined specimen. Of the three types of binder studied—circular spirals, square spirals and stirrups—the first was the most and the last the least effective. The mechanics of the action of steel binders and a similarity between the confining effects of steel binders and of hydrostatic pressure are discussed.