M20 grade of concrete has the following proportion of the ingredients of cement, sand and coarse aggregates.
- A1: 2 :4
- B1: 1.5: 3Correct
- C1: 4: 8
- D1:1.25:2.5
Solution
Concept
Nominal mix for M20 concrete is cement : sand : coarse aggregate = 1 : 1.5 : 3.
Step-by-step solution
1. Nominal mix proportions: M10 = 1:3:6, M15 = 1:2:4, M20 = 1:1.5:3, M25 = 1:1:2. 2. So M20 = 1 : 1.5 : 3. 3. The answer is option B.
Explanation
In nominal mixes, a richer mix (more cement) gives a higher grade. M20 uses 1 part cement, 1.5 parts sand and 3 parts coarse aggregate. Its strength is about 20 N/mm² at 28 days.
Why the other options are wrong
A: 1:2:4 is M15. C: 1:4:8 is M7.5, a lean mix. D: 1:1.25:2.5 is not a standard nominal mix and is richer than M20.
Shortcut
Remember the series 1:3:6 (M10), 1:2:4 (M15), 1:1.5:3 (M20), 1:1:2 (M25).
Common mistake
Choosing 1:2:4, which is M15, not M20.
Correct answer: B) 1: 1.5: 3