Which of the following is a commonly used nominal mix ratio for M20 concrete?
- A1:1.5:3Correct
- B1:2:4
- C1:3:6
- D1:4:8
Solution
Concept
Nominal mixes are prescribed standard mix proportions by volume for various grades of concrete (such as M5, M7.5, M10, M15, M20) as specified in codes like IS 456 when designed mixes are not used.
Step-by-step solution
1. Recall the standard nominal mix proportions as per IS 456:2000 Table 9 for different grades of concrete. 2. Identify the grade given in the question: M20. 3. Match the grade with its corresponding nominal mix ratio: M5 is 1:5:10, M7.5 is 1:4:8, M10 is 1:3:6, M15 is 1:2:4, and M20 is 1:1.5:3.
Explanation
According to IS 456 (Table 9), nominal mix concrete of grade M20 has a standard proportion of cement, fine aggregate, and coarse aggregate as 1:1.5:3 by volume. This means 1 part of cement, 1.5 parts of fine aggregate (sand), and 3 parts of coarse aggregate.
Why the other options are wrong
B: 1:2:4 is the nominal mix ratio for M15 concrete. C: 1:3:6 is the nominal mix ratio for M10 concrete. D: 1:4:8 is the nominal mix ratio for M7.5 concrete.
Shortcut
Remember the sequence of M10 to M20 nominal mixes: M10 is 1:3:6, M15 halves the sand and coarse aggregate to 1:2:4, and M20 further reduces them to 1:1.5:3.
Common mistake
Confusing the mix ratio of M20 (1:1.5:3) with M15 (1:2:4) or M10 (1:3:6).
Correct answer: A) 1:1.5:3