In Concrete Mix Design, the Target Mean Strength fck' is calculated as fck + 1.65σ. The factor 1.65 ensures that
- AAt least 95% of the test results fall below the characteristic strength.
- BNot more than 5% of the test results are expected to fall below the characteristic strength.Correct
- CThe average strength is always double the characteristic strength.
- DThe standard deviation is always treated as zero.
Solution
Concept
Target Mean Strength and Characteristic Strength
Formula
f'ck = fck + 1.65σ
Step-by-step solution
1. Identify the given formula for Target Mean Strength: f'ck = fck + 1.65σ. 2. Recall that in a normal distribution, the multiplier 1.65 ensures a 95% confidence level above the characteristic strength (fck). 3. Conclude that not more than 5% of test results will fall below fck, matching Option (b).
Explanation
In concrete design, characteristic strength is defined as the strength level below which no more than 5% of test results are expected to fall. Adding 1.65 times the standard deviation to the target mean strength ensures that 95% of all concrete samples will meet or exceed this required strength.
Common mistake
A common mistake is selecting Option A by confusing the percentage threshold. Students often associate the factor 1.65 with 95% confidence and mistakenly choose the statement saying 95% of results fall below characteristic strength, whereas in reality, 95% of results fall above it (meaning no more than 5% fall below).
Correct answer: B) Not more than 5% of the test results are expected to fall below the characteristic strength.