Strength of Concrete MCQ with answer and solution

Concept

Characteristic strength 95% confidence definition

Formula

f_ck = f_m - 1.65 × s

Step-by-step solution

1. In accordance with IS 456 (Clause 6.1.1 and 36.1), concrete strength follows a normal Gaussian distribution. 2. Characteristic compressive strength (fck) is formally defined as that value of strength below which not more than 5 percent of the test results are expected to fall. 3. Mathematically, fck = fm - 1.65 × s, corresponding to a 95% statistical acceptance probability. 4. Thus, option C expresses the exact standard definition of characteristic strength. Answer: C

Explanation

Under limit state design principles defined in IS 456, characteristic compressive strength (fck) represents the statistical strength below which not more than 5 percent of the test results of 150 mm concrete cubes cured for 28 days are expected to fall. This guarantees a 95% statistical confidence level for structural safety calculations.

Why the other options are wrong

A) Maximum strength obtained does not serve as a design parameter in structural codes. B) Average strength of samples represents the target mean strength (fm), not the characteristic strength. D) Absolute minimum strength achieved is a sample extreme, not the statistically defined characteristic value.

Shortcut

Characteristic strength (fck) = 95% pass rate / not more than 5% fall below.

Common mistake

Confusing characteristic strength with target mean strength or absolute minimum test value.