A 2 PARAMETER STRESS BLOCK FOR HIGH-STRENGTH CONCRETE

The rectangular stress block parameters in the current ACI Code are li mited to concrete strengths in the range 20 to 50 MPa (2900 to 7250 ps i). This paper looks at the applicability of the ACI rectangular stres s block parameters to high-strength concretes. New rectangular stress block parameters are proposed that ale based on a probabilistic analys is using a stress-strain relationship for high-strength concrete and t hat include estimates of variability and distribution of the input pro perties. A sensitivity analysis is also carried out to ascertain the e ffect of parameter uncertainty. The probabilistic models proposed can be used in a code calibration of design formula for high-strength conc rete. It is shown that for a ductile singly-reinforced rectangular sec tion, the ultimate moment capacity is relatively insensitive to the st ress block model. Estimates of the ductility level at both ultimate an d column capacity in primary compression failure, however, are signifi cantly affected by the choice of the stress block model.