Cover for concrete cast against formwork in the ground (BS 8500:2006)
BS 8500-1: 2006 Concrete, Table A.9 specifies the lowest nominal cover for concrete cast against the soil as 75mm and for concrete cast against blinding as 50mm, in relation to aggressive ground conditions i.e. sulfate bearing soils. Where chlorides are present, the nominal cover should be as recommended for the appropriate XD or XS exposure class (BS 8500-1 Tables A.4 or A.5 ) plus an allowance Δc of 25mm for concrete cast against blinding and 50mm against soil.
For concrete cast against formwork in the ground it is considered reasonable to specify a cover of at least (25 + Δc) mm as the lowest nominal cover. Normally Δc is taken as 10mm but for groundwork a value for Δc of 15mm is considered appropriate, unless otherwise agreed. Thus the lowest nominal cover for concrete cast against formwork in the ground is (25 + 15) = 40mm. For concrete cast directly against soli Δc = 50mm. Where exposure class limitations (for XD or XS) dictate a higher minimum cover, for example where chlorides are present, the more onerous value should be used in conjunction with a Δc of 15mm.
Note that BS 8500 was revised in 2015 and the guidance clarified, now in table A.10.