Modification on equivalent stress block of normal-strength concrete by incorporating strain gradient effects

Abstract

The equivalent rectangular concrete stress block is widely adopted in flexural strength design of normal-strength reinforced concrete members. From the past research work reported by other researchers, the equivalent stress block parameters was assumed to be dependent only on concrete strength. Nonetheless, the theoretical flexural strengths predicted by the currently used codes were often significantly lower than the actual strengths. In this study, the authors studied the characteristics of equivalent concrete stress block by investigating other factors than concrete strength only. In total 14 inverted T-shaped specimens in 6 groups were fabricated and tested. Each group had identical cross section properties and contained one concentrically loaded specimen and one/several eccentrically loaded specimen(s). The equivalent concrete stress block parameters of the eccentrically loaded specimen were obtained by adopting the modified stress-strain curve of its counterpart concentrically loaded specimen using a numerical analysis method. Based on the experimental results, the authors found that the equivalent stress block parameters are dependent on strain gradient besides concrete strength. A new set of equivalent rectangular concrete stress block parameters incorporating strain gradient effects were proposed for design purpose, and their validities were verified by comparing with the measured flexural strengths of beams and columns tested by previous researchers.