An exposure class states which environmental actions a concrete element will face during its service life. From that class follow the requirements for the concrete mix and for the cover over the reinforcement. A foundation below the water table simply needs something different from a balcony edge by the sea.
The classes are set out in EN 206, the European concrete standard, together with the national application document — BS 8500 in the UK, NEN 8005 in the Netherlands, where the term used is milieuklasse.
The main classes
| Class | Deterioration mechanism |
|---|---|
| X0 | No risk of attack (unreinforced concrete, dry environment) |
| XC | Reinforcement corrosion caused by carbonation |
| XD | Reinforcement corrosion caused by chlorides other than seawater (de-icing salts) |
| XS | Reinforcement corrosion caused by chlorides from seawater |
| XF | Freeze-thaw attack, with or without de-icing agents |
| XA | Chemical attack from aggressive ground or water |
Each main class is subdivided by a number indicating severity: XC1 covers dry or permanently wet conditions, XC4 cyclic wet and dry — and that alternation is the most demanding of all. Together they make eighteen classes.
How it feeds into design
A single element can fall into several classes at once. A parking deck treated with de-icing salt is both XD3 and XF4. All applicable requirements then apply simultaneously, and for each parameter the most onerous one governs.
In practice the exposure class determines:
- The maximum water/binder ratio. Less water means a denser paste and therefore less ingress of water, COâ‚‚ and chlorides
- The minimum binder content and sometimes the permitted cement types
- The minimum strength class, for example C30/37 instead of C20/25
- The concrete cover. This is the single most important measure in practice: the more aggressive the environment, the thicker the concrete over the steel
- The air content where freeze-thaw applies, since entrained air bubbles give expanding ice somewhere to go
- The permissible crack width
When in doubt, choose the more severe class. Specifying a class too low is almost impossible to put right later without demolishing the element; a class too high only costs a little extra cement.
Related terms
- Concrete
- Concrete cover
- Strength class
- Carbonation
- Reinforcement
- Silica fume
- Durability
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