If the standard deviation is 4 N/mm² and the desired characteristic strength is 20 N/mm², what is the mean strength of concrete ?
- A26·4 N/mm²Correct
- B14·6 N/mm²
- C28 N/mm²
- D16 N/mm²
Solution
Concept
Target mean strength
Formula
ftarget = fck + 1.65 s
Step-by-step solution
1. ftarget = fck + 1.65 s = 20 + 1.65 × 4 = 26.6 N/mm². 2. The nearest option is 26.4 N/mm².
Explanation
The mean (target) strength of a mix must exceed the characteristic strength by 1.65 standard deviations so that only 5 percent of results fall below fck.
Why the other options are wrong
14.6 and 16 are below the characteristic strength, which is impossible. 28 is well above 26.6.
Shortcut
Target = fck + 1.65 s.
Common mistake
Subtracting 1.65 s.
Correct answer: A) 26·4 N/mm²