Ultimate concrete strain and equivalent rectangular stress block for design of high-strength concrete beams

Abstract

High- and normal-strength concretes have different stress-strain characteristics and therefore high-strength concrete beams should not be designed in the same way as normal-strength concrete beams. In this study, the non-linear flexural behaviour of high-strength concrete beams has been analysed and compared to that of normal-strength concrete beams. It was revealed that because of the gradual change in stress-strain characteristics of the concrete, the ultimate concrete strain varies significantly with the concrete grade. Moreover, the tension reinforcement does not always yield at the point of peak moment and thus the maximum compressive strain of the concrete when the tension reinforcement yields, which is needed for evaluation of the balanced steel ratio, is not the same as the ultimate concrete strain. Nevertheless, compromised values of ultimate concrete strain that can be used for evaluation of both flexural strength and balanced steel ratio without producing significant errors have been derived. Lastly, the existing concrete stress-strain curve given in the British Standard BS 8110 for beam design has been reviewed and modification to cater for high-strength concrete proposed.