Ask three engineers on a site what cover a slab needs and you will often get one answer: twenty millimetres. That is right only for mild exposure, and mild exposure is rarer than people assume.
IS 456:2000 sets nominal cover against exposure condition in Table 16. Mild takes 20 mm. Moderate takes 30 mm. Severe takes 45 mm. Very severe takes 50 mm. Extreme takes 75 mm. These are nominal covers to all steel including links, not to the main bar alone, which is the first place a detail quietly loses ten millimetres.
The exposure class is not a judgement call. A member sheltered from rain and not in aggressive soil is mild. Concrete exposed to condensation, rain, or buried in non-aggressive soil is moderate. Coastal work, alternate wetting and drying, and exposure to sea-water spray push into severe and beyond. A slab in a Mumbai high-rise facing the sea is not a 20 mm cover problem.
The second half of the requirement is the one that gets dropped. Table 5 fixes the minimum cement content and the maximum water-cement ratio for the same exposure classes: 300 kg per cubic metre and 0.55 for mild, 300 and 0.50 for moderate, 320 and 0.45 for severe, 340 and 0.45 for very severe, 360 and 0.40 for extreme. Cover and mix are one durability requirement expressed in two tables. Meeting the cover with a lean, wet mix satisfies neither.
Two clauses worth keeping to hand while detailing. Clause 26.5.2.1 sets the minimum reinforcement in either direction of a slab at 0.12 per cent of the gross cross-sectional area for high-strength deformed bars, and 0.15 per cent for mild steel. Clause 26.5.2.2 caps the bar diameter at one-eighth of the total slab thickness — which rules out the 16 mm bar somebody will suggest for a 100 mm slab.
None of this is obscure. It is four tables and two clauses, and the cost of getting it wrong is a structure that meets its strength check on paper and spalls in eight years.