Simulating the passive confinement of circular concrete cylinders allowing for size effect

ABSTRACT There are innumerable tests on small stocky circular cylinders with either internal passive encasement of the concrete with circular stirrups or spirals, or external passive encasement through fibre-reinforced polymer (FRP) wrapping or through the use of FRP tubes or steel tubes. These tests have shown that passive confinement can increase the strength but, in particular, can substantially increase the ductility of concrete cylinders. Because of these important benefits, the effect of passive confinement on the concrete stress/strain in a particular cylinder section is invariably derived from tests such that substantial member testing is required. In this paper, it is shown how the passive stress/strain of the concrete for a particular circular member can theoretically be determined directly from their partial-interaction shear-friction and partial-interaction bond-slip material properties for any reinforcement arrangement and geometry of the circular cylinder. This procedure provides tools for designing for the benefits of passive confinement directly without the need for member testing.