Compressive strength of circular hollow reinforced concrete confined by an internal steel tube

When concrete is confined, its strength is enhanced by confining stress. Therefore many researchers have studied the confinement and its effect in a reinforced concrete (RC) member. However, their research has focused on making confining pressure from the outside of the member. In a hollow RC member, it is impossible to confine its concrete triaxially because it has a hollow section. The concrete in a hollow RC member is usually in the state of biaxially confinement, and it shows lower strength than the concrete in a solid RC member, which is confined triaxially. In this study, the compressive strength of the concrete in a hollow RC column with an internal steel tube was investigated. The internal tube makes the concrete confined triaxially with internal confinement. To show the existence and the effect of the internal confinement, an experiment was performed. Fourteen specimens were tested. The test results showed that the internal confinement existed and made the concrete in the state of triaxial confinement.