The correct answer is: C. Directly proportional to square root of compressive strength.
According to IS : 456-1978, the flexural strength of concrete is directly proportional to the square root of its compressive strength. This means that if the compressive strength of concrete increases by 100%, the flexural strength will increase by 31.6%.
The reason for this relationship is that the flexural strength of concrete is determined by the tensile strength of the concrete. The tensile strength of concrete is much lower than its compressive strength, so the flexural strength is also much lower. However, the tensile strength of concrete is proportional to the square root of its compressive strength. This means that if the compressive strength of concrete increases, the tensile strength will also increase, and the flexural strength will increase as a result.
The following are the explanations for each option:
- Option A: Directly proportional to compressive strength. This is not the correct answer because the flexural strength of concrete is not directly proportional to its compressive strength.
- Option B: Inversely proportional to compressive strength. This is not the correct answer because the flexural strength of concrete is not inversely proportional to its compressive strength.
- Option C: Directly proportional to square root of compressive strength. This is the correct answer because the flexural strength of concrete is directly proportional to the square root of its compressive strength.
- Option D: Inversely proportional to square root of compressive strength. This is not the correct answer because the flexural strength of concrete is not inversely proportional to the square root of its compressive strength.