Flexural design of reinforced concrete members in structures such as buildings, bridges, etc. presented in various standards are based on the equivalent rectangular stress block and stress-block parameters for high strength concrete. However, with design of high strength concrete mixes these existing stress block parameters for normal and high strength concrete have become redundant and does not give a true representation of the obtained stress-strain curves for ultra-high strength concrete. The primary stress-strain characteristics affecting the stress block parameters are strain at peak stress, the ultimate strain and the shape of the rising limb of the curves. Past studies have confirmed a varying behaviour of all these characteristics in the stress strain curves at higher strength. The present study first evaluated the stress strain curves for high strength concrete from 90 to 140 MPa. The characteristics of the curves were compared with those of normal and high strength concrete. Based on the observed stress strain characteristics, the modified stress block parameters for the ultra-high strength concrete are presented. The proposed stress block was compared with the Indian standard IS456 and European design standard EC: 02-2004. The proposed stress block will be useful for the required modification and updating of various standards pertaining to the flexural design of ultra-high strength concrete with cylindrical compressive strength in from 90 to 140MPa.
