Strain-Gradient-Dependent Stress-Strain Curve for Normal-Strength Concrete

Abstract

The stress-strain distribution of concrete in the compression zone of reinforced concrete (RC) flexural members is the most important parameter for assessing the ultimate flexural strength and ductility. Currently, the stress-strain curve of concrete developed in flexure is taken as the uni-axial compressive curve incorporating a scale-down factor k3, which is the ratio of the maximum concrete stress developed under flexure to the concrete cylinder strength. In current RC design codes, the ratio of the equivalent concrete stress to cylinder strength is taken as constant equal to 0.85 for normal-strength concrete but reduces as concrete strength increases. However, in a recent study carried out by the authors, it was found that the maximum concrete stress developed in the flexure increases significantly as the strain gradient (ratio of extreme concrete strain to neutral axis depth) increases, until reaching a maximum limit. Therefore, the value of k3 should not be taken as a constant for flexural RC members. In this study, the authors will adopt the results obtained in the previous experimental tests on concentrically, eccentrically and horizontally loaded RC columns to derive the stress-strain curve of concrete under different extents of strain gradient. The derived values of k3 are then correlated to the strain gradient using empirical equation. The applicability of the proposed equation is checked by comparing the flexural strengths of NSC beams and columns so calculated with those experimentally measured by different researchers, in which good agreement has been obtained.