In order to evaluate ultimate strength and ductility of flexurally failing reinforced concrete members, the use of axial stress-strain relation models of concrete, which represent behaviors of concrete practically as far as in the range of descending part after maximum stress, will be necessary. In this study, two series of experiments were conducted in order to propose a more practical stress-strain relation model for square confined R/C columns. In series A, 15 square columns, each 150 mm square by 530 mm high, containing different arrangements of square steel hoops, were subjected to concentric axial force only. In series B, 26 square columns, each 150mm square by 530mm high, with different concrete strength (400, 600, 800 kg/cm^2) and different reinforcement strength (4000, 8000, 12 000 kg/cm^2), were subjected to concentric axial force only. The stress - strain relation model was proposed based on the results of experiment series A and B. The increments of maximum stress and axial strain at maximum stress by hoop confinement were considered. The effects of square confinement were also considered in this model. Also, the negative stiffness of the descending part after maximum stress could be evaluated using confining stress by hoop reinforcement at the axial stress peak point. The ultimate axial stress could be evaluated using maximum confining stress by hoop reinforcement.
