This paper presents a series of uniaxial compressive loading tests on concrete cylinder confined by tie reinforcements in order to develop an unloading and reloading stress-strain model. The specimens were 600mm high and 200mm in diameter with various volumetric ratios of tie reinforcement and cylinder strengths of concrete. The tie reinforcement ratio ρ_s and the cylinder strength of concrete σ_(c0) were varied from 0.67% to 2.67% and from 23.0MPa to 36.7MPa, respectively. The number of unloading from an unloading strain and reloading was studied as one of the parameters. It is found from the test results that unloading and reloading do not essentially change the shape of envelope curves of stress-strain relation, and that to predict an unloading and reloading paths, the plastic strain ε_(pl·n) and the stress at the unloading strain σ_(pl·n) are controlling parameters. These controlling parameters were analysed based on the test data to propose an empirical relation. Unloading path was idealized as a parabolic function, while reloading path was idealized as a combination of a parabolic curve and a straight line. The proposed unloading and reloading stress-strain model was compared to the test results. The comparison showed that the predicted unloading and reloading stress-strain relation provides good agreement with the test data.
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