Cover for concrete cast against formwork in the ground (BS 8500: 2015)
BS 8500 was revised in 2015 and the relevant advice for the cover to reinforcement of concrete in the ground clarified.
BS 8500-1: 2015 Concrete, Table A.10 note E) specifies the lowest nominal cover for concrete in the ground as 25mm plus a deviation Δc. For concrete cast against blinding or prepared ground Δc should be 15mm, for concrete cast directly against soil Δc should be at least 50mm.
For concrete cast against formwork before exposed to soil, Δc should be 5 to 15mm. Where the ground contains chlorides the nominal cover should comprise the minimum cover for the associated XD or XS class (BS 8500-1: 2015, Table A.1) plus the appropriated Δc. The more onerous cover should be used.
The National Annex to Eurocode 2, the structural design standard BS EN 1992-1-1 clause 126.96.36.199 Table NA.1, identifies nominal cover Cnom for concrete cast against uneven surfaces as;
against blinding: Cnom = k1 at least 40mm
directly against soil: Cnom = k2 at least 75mm