Question

M20 concrete as per IS 456: 2000 refers to concrete with a design mix having \underline{\hspace{4cm}.}

• A. an average cube strength of 20 MPa
• B. an average cylinder strength of 20 MPa
• C. a 5-percentile cube strength of 20 MPa
• D. a 5-percentile cylinder strength of 20 MPa

Hint:

Remember: In IS 456, the "M" value denotes the characteristic cube strength at 28 days, defined as the 5th percentile strength (not average). Cylinders are not used in IS code classification.

Answer 1

The Correct Option is C

Step 1: Understanding M20 concrete.
The designation M20 in IS 456:2000 indicates that the characteristic compressive strength of the concrete is 20 MPa.

Step 2: Definition of characteristic strength.
As per IS 456, the characteristic strength of concrete is defined as the strength of material below which not more than 5% of the test results are expected to fall.
This means the characteristic strength is a 5th percentile strength value, not an average.

Step 3: Specimen type.
In India, compressive strength is tested using 150 mm cubes, not cylinders (cylinders are more common in US/Europe). Hence the reference is to cube strength.

Step 4: Conclusion.
Therefore, M20 concrete refers to a mix having a 5-percentile cube strength of 20 MPa.
\[ \boxed{\text{M20 concrete has a 5-percentile cube strength of 20 MPa.}} \]