How does one obtain design strength of concrete = 0.45fck as given in IS456:2000???
Here is the explanation:
In india, we test concrete cubes to determine 28-days characteristic strength of concrete, which is depicted by “fck”
However, we all know that, if we cast cylinder from the same concrete mix, we will get 28-days characteristic strength of concrete = 0.80 times the cube strength i.e. = 0.80fck
But during experimental work conducted on beam or column, it was found out that the strength of concrete in the compression zone at failure is = 0.85 times the strength of cylinders i.e. = 0.85 x (0.80fck) = 0.68fck
When you divide this final value with the partial FOS of 1.5, you will get the design strength of concrete = 0.68fck/1.5 = 0.45fck
and another different point is
In limit state method design of RCC we take the partial safety factor of concrete as 1.5. so for getting characteristic strength we divide the value of fck by 1.5, then we get 0.67 fck,, then again we divide the value by 1.5,,,then we get the value of 0.45 fck,,,,
We divide partial safety factor two times for concrete because comparing to steel( which is factory manufactured) concrete is a field method, and the level of accuracy of quality control achieved for concrete is less.