An experimental study on strain gradient effect of equivalent rectangular stress block of normal-strength concrete

Abstract

In flexural strength design of normal-strength reinforced concrete (RC) members, the maximum concrete stress adopted by various design codes is originated from the scaled down uni-axial concrete stress-strain curve, in that the behaviour of concrete under flexure is wrongly considered to be more inferior than that under uni-axial compression. Therefore, from published literature, this scaled down concrete stress might be the reason for significant disparity between theoretical and actual flexural strengths. In this study, 12 test specimens, including plain concrete and RC, were fabricated to investigate the maximum concrete stress developed under flexure affected by strain gradient. These specimens were cast in several batches, each batch consisted of one concentrically loaded specimen and one or several eccentrically loaded specimen(s). The concrete stress-strain curve developed in each of the eccentrically loaded specimens was derived by modifying that of its counterpart concentrically loaded specimen based on axial force and moment equilibriums. From the obtained stress-strain curves, it was found that the maximum concrete stress developed In flexure Increased with strain gradient. A formula correlating the maximum concrete stress in flexure to strain gradient is thus developed. Lastly, the widely used equivalent rectangular concrete stress block parameters stipulated in the Hong Kong Concrete code are modified by considering the strain gradient effects.