IS 456 asks for at least 40 mm of cover to column bars, or the bar diameter if larger, and more in severe exposure (45 mm) or very severe exposure (50 mm). Columns 200 mm or smaller with bars up to 12 mm may use 25 mm. ACI 318 and NSCP 2015 ask for 1½ in (40 mm) inside buildings.
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Minimum cover
40mm
In inches
1.57 in
Governing rule
IS 456:2000
Cover is to the outermost steel, including stirrups, ties and links. Fire rating or the drawings may require more.
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cl. 26.4.2.1: columns 200 mm or smaller with bars up to 12 mm may use 25 mm.
Table 16 note 2: the actual cover may be up to 10 mm more than the nominal cover, never less.
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NSCP 2015 follows ACI 318-14: 1½ in (40 mm) for columns not exposed to weather, 2 in (50 mm) for #6 and larger bars exposed to weather, and 3 in (75 mm) cast against earth.
Is column cover measured to the ties?
Yes. Cover is to the outermost steel, which in a column is the ties or links. The vertical bars sit one tie diameter further in.
When can a column have 25 mm cover?
IS 456 cl. 26.4.2.1 allows 25 mm for columns of 200 mm or less in their smallest size whose bars do not exceed 12 mm.
References
IS 456:2000 Table 16 and cl. 26.4 (nominal cover); Table 3 (exposure conditions)
ACI 318-19 Table 20.5.1.3.1 (specified concrete cover); NSCP 2015 uses the same values
