Strain gradient effects on concrete stress–strain curve

Abstract

The stress–strain characteristic of concrete developed in flexure is one of the essential parameters for the ultimate flexural strength design of reinforced concrete (RC) members. Currently, the stress–strain curve of concrete developed in flexure is obtained by scaling down the uniaxial stress–strain curve. In current RC design codes, it is represented by an equivalent rectangular concrete stress block depending solely on the concrete strength. By comparing the theoretical strength evaluated for the stress block with the measured strength, the authors found that current codes underestimate the actual flexural strength of RC beams and columns by 9% and 19%, respectively. Since the underestimation is different for beams and columns, which are subjected to different strain gradients at ultimate, it is suggested that the maximum concrete stresses developed in flexure should depend also on strain gradient. The effects of strain gradient on the concrete stress developed in flexure were investigated in this work by testing RC columns under concentric and eccentric axial loads or horizontal loads. The concrete stress–strain curves of the eccentrically/horizontally loaded specimens were derived by modifying those of concentrically loaded counterparts based on axial force and moment equilibriums. The results indicate that the maximum concrete stress developed in flexure depends significantly on strain gradient. Formulas were developed to correlate the maximum and equivalent concrete stresses developed in flexure to the strain gradient. Their applicability was verified by comparing the results with measured flexural strengths of RC beams and columns.