Cover, formwork and curing
Nominal cover check (Eurocode 2 / BS 8500)
- where
- minimum cover (mm)
- bar diameter (mm)
- minimum cover for durability (mm)
- required nominal cover (mm)
- allowance for deviation (mm)
- minimum when cast against the ground: 0 for formwork, 40 for prepared ground or blinding, 75 directly against soil (mm)
- margin, provided minus required (mm)
- cover you are providing (mm)
Explain this equation
Checks that the spacers or chairs give enough concrete cover to the reinforcement.
Cover protects the steel from rusting and fire, and lets it bond. The minimum is the largest of the bar diameter (for bond), the durability cover for the exposure class from BS 8500, and 10 mm. A tolerance Δc is added because steel is never placed exactly, giving the nominal cover. Concrete cast against ground needs at least 40 or 75 mm whatever the other figures. The margin is what you are providing less what is needed.
The largest requirement governs, plus a fixing tolerance.
Sense check: 12 mm bars with 25 mm durability cover and 10 mm tolerance need 35 mm nominal; 40 mm spacers give 5 mm margin. Against soil you would need 75 mm.
Watch out: durability cover depends on the exposure class and concrete strength. Too much cover can also be a problem, causing wider cracks.
Checks whether the cover you are providing to the reinforcement meets the nominal cover from Eurocode 2, using the durability cover from BS 8500-1.
Enter your values to see the result.
Good to know
Minimum cover is the larger of the bar diameter (for bond), the durability cover for the exposure class, concrete quality and design life (read cmin,dur from the BS 8500-1 tables or the drawings), and 10 mm. A deviation allowance, usually 10 mm, is added to get the nominal cover that spacers are sized to. Eurocode 2 also asks for at least 40 mm nominal cover when casting on prepared ground or blinding and 75 mm when casting directly against soil.
Common mistake
Sitting bottom mesh directly on the sub-base or membrane, or on bits of brick, so the steel ends up with almost no cover and rusts.
Limits
Covers bond, durability and casting against ground. Fire resistance can need more cover to the bar centre (axis distance); aggregate larger than 32 mm adds 5 mm to the bond cover; bundled bars use an equivalent diameter. The cover on the engineer’s drawings governs.
Sources
- BS EN 1992-1-1:2004 Eurocode 2 (3.1.2 Eq. 3.1/3.2 strength with time; 4.4.1 cover Eq. 4.1/4.2 and 4.4.1.3(4) 40/75 mm against ground; 8.4 and 8.7 anchorage and laps; Annex B Eq. B.10 temperature-adjusted age)
- BS 8500-1:2023 Concrete. Complementary British Standard to BS EN 206. Method of specifying and guidance for the specifier
- The Concrete Centre, Eurocode 2 FAQs (deviation allowance usually 10 mm added to minimum cover)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.