Question

Concrete of characteristic strength 30 MPa is required. If 40 specimens of concrete cubes are to be tested, the minimum number of specimens having at least 30 MPa strength should be

• A. 35
• B. 37
• C. 38
• D. 39

Hint:

Characteristic strength is based on a 95\% confidence level, meaning at least 95\% of the tested specimens should meet or exceed the specified strength.

Answer 1

The Correct Option is C

Step 1: The characteristic strength of concrete is defined as the strength below which not more than 5\% of the specimens are expected to fail. In other words, 95\% of the specimens should have a strength of at least 30 MPa. Step 2: Given that a total of 40 specimens are tested, the minimum number of specimens that must have at least 30 MPa strength can be calculated as: \[ 0.95 \times 40 = 38 \] Step 3: Thus, at least 38 specimens should meet the required strength criteria to satisfy the definition of characteristic strength. Conclusion: The correct answer is option (C) 38.