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Surface crack width depends on the percentage reinforcement and depth of cover. The depth of cover is usually dictated by the exposure conditions.
The nominal cover (Cnom) to reinforcement necessary for durability should be used in the design calculations to achieve the desired surface crack width. However where the actual cover is greater than the nominal cover, the actual surface crack width will be proportionally greater than the design value. This approach assumes the crack width varies linearly with zero width at the face of the bar.
For instance where the required cover for durability is, say, 35mm with a designed crack width of, say, 0.3mm but the cover is actually 70mm, then the actual surface crack width will be proportionally greater at 0.6mm. The durability is still maintained as the crack width 35mm from the reinforcement is still 0.3mm.
Where the surface crack width is limited for the purpose of appearance, the full cover should be assessed to calculate the area of reinforcement for the desired crack width.
See the National Annex to BS EN 1992-1 Table NA.4 Recommended values for ωmax and associated text.
Acknowledgement: Concrete Society
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