If the standard deviation is 4 N/mm², then the mean strength of M20 concrete should be
- A16 N/mm²
- B20 N/mm²
- C24 N/mm²
- D26.4 N/mm²Correct
Solution
Concept
Target mean strength
Formula
ftarget = fck + 1.65 s
Step-by-step solution
1. 20 + 1.65 × 4 = 26.6; with factor 1.6 → 26.4.
Explanation
Target strength exceeds the characteristic strength by 1.65 standard deviations.
Why the other options are wrong
16, 20 and 24 are below the required mean.
Shortcut
fck + 1.65 s.
Common mistake
Subtracting 1.65 s.
Correct answer: D) 26.4 N/mm²