This paper deals with the complete stress-strain behavior of confined concrete.It is well known that, by confining the concrete with lateral reinforcement, significant strength enhancement and more ductile behavior can be obtained, so that it is possible to offset the strength loss due to spalling of concrete cover and to increase the capacity of concrete to sustain large deformations without a dramatic loss in strength. Besides these are well known, the effects of several variables which determine the behavior of the confined concrete still need further research. The strength enhancement and especially more ductile behavior due to confinement are vitally important for reinforced concrete building columns, especially for the earthquake resistant structures since the inelastic deformability of reinforced concrete columns is essential for overall strength and stability of structures during a strong earthquake.

Experimental study was made on reinforced concrete columns with various amount of confinement reinforcement, since the complete stress-strain curve of the confined concrete is very important, to predict the behavior of concrete rationally. 12 columns with circular cross-section were tested under monotonic concentric compression. Since the number of specimens were limited, concrete strengths of all specimens were designed same, and the test variables about confinement were the volumetric ratio of confinement reinforcement, the diameter of the confinement bars,the spacing between confinement bars, and type of the confinement bars, like hoops or spirals.

The main purpose of this paper is to present the experimental data on complete stress-strain curves of reinforced concrete columns of circular cross-section, confined by circular hoops or spirals and to investigate the failure types of the columns which have different confinement reinforcement.

According to the test results, significantly different stress-strain behaviors were obtained even for the columns which contain same volumetric ratio of confinement reinforcement when the diameter and the spacing of confinement bars were changed.The effect of spacing between the confinement bars appeared to be very important on the behavior and failure type of reinforced concrete columns.