Maximum cover to steel reinforcement
The position of steel reinforcement in relation to the concrete’s surface is important for durability and structural integrity.
The specified cover, as stated in the Standard for concrete EN 206 and its UK complementary standard BS 8500, which reflect the structural concrete design Eurocode 2 (BS EN 1992), applies a nominal value, Cnom.
Nominal cover Cnom is the required minimum cover Cmin with a permitted deviation ΔCdev, where ΔCdev is between 0 and 10mm depending on the quality control associated with the concrete construction (see National Annex to EN 1992-1-1). No reference to maximum reinforcement cover is given except in relation to fire resistance where covers greater than 70mm will require surface reinforcement.
BS EN 13670 Execution of concrete structures, has a maximum tolerance or ΔC(plus). The minimum deviation is termed ΔC(minus).
ΔC(plus) varies depending on the depth of the structure h and the Tolerance Class of the structure. The National Structural Concrete Specification (NSCS) 4th edition follows this standard, although
ΔC(plus) is slightly different as shown in the following table.
|+ 10 mm
||+ 5 mm
||+ 10 mm|
||+ 15 mm
||+ 10 mm
||+ 15 mm|
|+ 25 mm
||+ 20 mm
||+ 20 mm|
For foundations and members in foundations, the permitted plus-deviations may be increase by 15mm.
Where links are used (refer to Bar schedule dimensions: deductions for permitted deviations, p45 et seq The Structural Engineer, Institute of Structural Engineers, December 2013), it would appear to be unreasonable to expect full compliance with permitted deviations on maximum cover to reinforcement ΔC(plus), while complying with the safeguards for minimum cover on the opposite face. Where, in large sections, excess covers might be considered too great, consideration should be given to using two U-bars rather than one link or two L-bars rather than one U-bar.
One issue that is often raised is crack width. In terms of durability, the note to National Annex to EN 1992-1-1 (AMD 1) Table NA.4 suggests that crack width should be based on Cnom.