Strain gradient effects on flexural strength design of normal-strength concrete columns

The equivalent rectangular concrete stress block has commonly been adopted for the flexural strength design of reinforced concrete (RC) members for decades. In this stress block, the equivalent concrete stress is expressed as α f c ′ ( f c ′ is the uni-axial concrete cylinder strength). Currently, the value of α adopted in various RC design codes is only concrete-strength dependent and equal to 0.85 for NSC. However, in an experimental study conducted previously by the authors on NSC columns subjected to concentric and eccentric axial loads, it was found that α increases significantly as the strain gradient increases. Therefore, the effect of the strain gradient should not be neglected. In this paper, a review on the previous test results on NSC columns is presented and a strain-gradient dependent equivalent rectangular concrete stress block is developed. Based on this proposed stress block, a new flexural strength design method for NSC columns that incorporates the effects of the strain gradient is proposed. A series of column interaction diagrams with various concrete strengths and steel ratios are derived for design purposes. Lastly, these interaction diagrams are compared with the existing column design charts provided in various RC design codes to verify their applicability.