What is the minimum value of test results (in N/mm² ) for compressive strength compliance requirement for M20 grade concrete for standard deviation of 0.36 as per IS code provisions?
- Afck − 1
- Bfck − 3Correct
- Cfck − 4
- Dfck − 5
Solution
Concept
As per IS 456:2000, for M20 and higher grades, each individual test result must be at least fck – 4 N/mm².
Formula
NONE
Step-by-step solution
1. Grade of concrete is M20, so fck = 20 N/mm². 2. IS 456:2000 (Table 11) gives two checks: a mean-of-4 check and an individual-result check. 3. The minimum value of an individual test result for M20 and above is fck – 4. 4. This limit does not depend on standard deviation. So the answer is fck – 4 = 16 N/mm².
Explanation
IS 456 sets a lower limit for any single test result so that very weak concrete is caught. For M20 and higher grades this limit is fck – 4 N/mm². Standard deviation is used only in the mean-of-4 check, not in this individual check.
Why the other options are wrong
A: fck – 1 is not an IS 456 limit. B: fck – 3 is the individual limit for M15, not for M20. D: fck – 5 is too low and is not given in IS 456.
Shortcut
Individual result: M15 → fck – 3, M20 and above → fck – 4.
Correct answer: B) fck − 3