Work out nominal concrete cover and the minimum strength class for durability to EN 1992-1-1 Section 4.4 and Annex E. Duplicate the calc and set your cover.
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About this Nominal Cover and Minimum Strength Class for Durability Calculator
This calculator sets the nominal concrete cover to reinforcement or prestressing steel, and checks the indicative minimum strength class for durability, to EN 1992-1-1 Section 4.4 and Annex E. It works the structural class of Table 4.3N, the minimum cover for bond and for durability, the minimum and nominal cover, and checks the concrete class against Table E.1N.
- Structural designer set the cover on the drawings from the exposure class and design working life rather than a rule of thumb.
- Specification writer confirm that the specified concrete class meets the indicative minimum for both steel corrosion and attack on the concrete.
- Precast and post-tensioning engineer get the bond cover for strand, wire or ducts, and the durability cover from the prestressing table.
The cover tables are editable on the page, so national annex values can replace the recommended ones. It is an engineering-grade calculation on CalcTree that shows how each cover is built up.
More info on Nominal Cover and Durability Class
Bond cover
Table 4.2 sets the minimum cover for bond from the bar diameter, a multiple of the strand or wire diameter for pre-tensioned steel, or the duct size for post-tensioned tendons, using the diameter of a circular duct or the dimensions of a rectangular one. The bond cover rises where the nominal maximum aggregate size is large.
Structural class
Table 4.3N starts from the recommended structural class for a 50 year design working life and adjusts it: up for a 100 year life, and down for a strength class at or above the threshold for the exposure, for slab geometry where the steel position is not affected by construction, and for special quality control of the concrete production. The result stays between S1 and S6.
Durability, minimum and nominal cover
The durability cover is read for the structural class and exposure class from Table 4.4N for reinforcing steel or Table 4.5N for prestressing steel, then adjusted by the additive safety element and the reductions for stainless steel and additional protection. The minimum cover of Expression (4.2) is the greatest of the bond cover, the durability cover and the absolute minimum, and the nominal cover of Expression (4.1) adds the allowance for deviation. Textured surfaces and concrete cast on prepared ground or against soil take the increases of Clause 4.4.1.3(4).
Minimum strength class
Table E.1N gives an indicative minimum strength class for each exposure class, for corrosion of the steel and for attack on the concrete. The higher of the two governs, and the page checks your concrete strength against it.
Common Calculation Errors to Avoid
- Quoting minimum cover as nominal cover the drawings need the nominal cover, which adds the allowance for deviation to the minimum cover.
- Forgetting the ground condition concrete cast against soil or on prepared ground needs much more cover than formed concrete.
- Taking the structural class reduction without the concrete the reduction for strength class only applies when the concrete is at or above the threshold for that exposure.
- Using the reinforcement table for prestressing steel prestressing steel takes its durability cover from Table 4.5N, which is larger.
- Assuming the recommended values apply national annexes commonly change Tables 4.3N to 4.5N and E.1N, so check the annex for your country.
- Ignoring the freeze-thaw or chemical exposure the indicative strength class can be set by the attack on the concrete rather than by the corrosion exposure.
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FAQs
What is the difference between c_min and c_nom?
c_min is the minimum cover from Expression (4.2), the greatest of the bond cover, the durability cover with its allowances, and the absolute minimum. c_nom adds the allowance for deviation in design, Δc_dev, and is the value shown on the drawings.
How does design working life change the cover?
A 100 year design working life raises the structural class in Table 4.3N, which moves you down Tables 4.4N and 4.5N to larger covers.
Can I use national annex values?
Yes. The structural class thresholds, the cover tables and the indicative strength classes are all held in editable tables on the page, so they can be replaced with your national annex values.
Does the page cover fire resistance?
No. Cover for fire resistance is set by EN 1992-1-2 and should be checked separately; abrasion is also outside the scope of this page.
Why are there two exposure class selections?
Corrosion of the steel (X0, XC, XD and XS classes) sets the durability cover, while attack on the concrete (XF and XA classes) can set a higher indicative strength class. The page takes both into account.
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