Effect of specimen sizes on ACI rectangular stress block for concrete flexural members

It is important to consider the effect of concrete member sizes when estimating the ACI rectangular stress block of a concrete flexural member. The experimental data and analytical results, however, are still insufficient for a proper evaluation. For all types of loading conditions, the trend is that the size of the ACI rectangular stress block tends to change when the member size changes. In this paper, the size variations of strength coefficients for ACI rectangular stress block (α₁ and β₁) and the stress block parameters (k₃, k₁k₃, and k₂) have been studied. The results from a series of C-shaped specimens subjected to axial compressive load and bending moment were adopted from other references. The analysis results show the effect of specimen sizes on the strength coefficients for ACI rectangular stress block and the stress block parameters of a concrete member to be apparent. They also show that the current strength criteria-based design practice should be reviewed to include member size effect. More studies are needed in the future to verify that.