As per IS 456-2000, in the case of columns of minimum dimension of 200 mm or under, whose reinforcing bars do NOT exceed 12 mm, the nominal cover used is
- A25 mmCorrect
- B50 mm
- C20 mm
- D40 mm
Solution
Concept
IS 456:2000 allows a reduced nominal cover of 25 mm for small columns (min. dimension ≤ 200 mm) with bars not more than 12 mm.
Step-by-step solution
1. As per IS 456:2000 (Cl. 26.4.2.1), nominal cover for longitudinal bars in a column is not less than 40 mm, and not less than the bar diameter. 2. Exception: if the minimum column dimension is 200 mm or less and bar diameter does not exceed 12 mm, the cover can be reduced. 3. For this case the code allows a nominal cover of 25 mm. 4. So the answer is (A) 25 mm.
Explanation
Normally, column longitudinal bars need at least 40 mm nominal cover. For small columns (200 mm or less) with bars of 12 mm or less, IS 456 allows 25 mm cover. This keeps enough space for concrete in small sections.
Why the other options are wrong
(B) 50 mm is too high and is used for footings, not small columns. (C) 20 mm is the minimum cover for mild exposure in slabs/beams, not this column case. (D) 40 mm is the normal column cover, not the reduced value for small columns.
Shortcut
Column normal = 40 mm; small column (≤200 mm, bar ≤12 mm) = 25 mm.
Common mistake
Choosing 40 mm without reading the special condition for small columns.
Correct answer: A) 25 mm