Behavior of circular RC columns with two layers of spirals

RC columns with two layers of hoops is a new kind of column recently proposed. The concrete of the column is under three different degrees of confinement, namely unconfined concrete cover, singly-confined concrete between the two layers of hoops and doubly-confined concrete inside the inner layer of hoops. The strength and ductility of the column can be greatly enhanced due to better confinement. In this study, the behaviour of circular RC columns confined using two layers of spirals were investigated through axial compression test on scaled-down column models. The responses and failure modes of the columns were examined. The average stress-strain curves of the singly-confined and doubly-confined concrete were derived, taking advantage of the concept of average effective confinement pressure. The performance of a stress-strain model previously proposed for square RC columns with two layers of hoops was also investigated.