Q. What is Critical steel ratio?

Critical steel ratio - only consider 250mm of concrete from outer face

The purpose of critical steel ratio is to control cracking pattern by having concrete failing in tension first. If steel reinforcement yields first before limit of concrete tensile strength is reached then wide and few cracks will be formed. In calculation of critical steel ratio the thickness of whole concrete section is adopted for analysis. Though if the concrete section exceeds 500mm in thickness only outer 250mm concrete has to be considered in calculating minimum reinforcement to control shrinkage and thermal cracks. It is due to experimental works showed that for concrete section greater than 500mm, outer 250mm on every face could be regarded as surface zone whereas the remaining could be regarded as core. The minimum reinforcement to control cracking must therefore be calculated based on a total maximum thickness of 500mm.