Home / Articles / Building Materials MLZ-02
Exposure Classes: What Is Concrete Designed Against?
A bridge pier is exposed to seawater, a car-park slab to de-icing salt, a basement wall to sulfate-bearing soil. TS EN 206 groups these environments into classes and sets separate requirements for each. This article covers the classes and the physics behind them — and the physics gives a surprisingly simple result: doubling the cover quadruples the time to corrosion initiation.
1. Why classes?
Most reinforced concrete structures end their lives not through structural collapse but through reinforcement corrosion: rusting steel expands, cracks the concrete and causes spalling. The structure reaches the end of its economic life long before it loses its load-carrying capacity. The process is driven by the environment the concrete faces. TS EN 206 therefore asks the designer to state, as classes, the environmental actions the concrete will be exposed to.
2. Six groups
| Class | Action | Mechanism |
|---|---|---|
| X0 | No risk | Plain concrete or very dry interior |
| XC1–XC4 | Carbonation | Atmospheric CO₂ lowers the alkalinity of concrete; the passive layer protecting the steel breaks down |
| XD1–XD3 | Chlorides other than seawater | De-icing salts or industrial chlorides break down the passive layer locally |
| XS1–XS3 | Chlorides from seawater | Airborne salt, permanent submersion, or the tidal and splash zone |
| XF1–XF4 | Freeze–thaw | Expansion of pore water on freezing; more severe with salts |
| XA1–XA3 | Chemical attack | Aggressive substances such as sulfates and acids in soil and groundwater |
The number within each group indicates severity. An element can be exposed to several classes at once; a pier on a coast that freezes is in both XS and XF, and the most severe requirement applies.
3. Chloride: Fick's law
Chloride ingress into concrete is described approximately by Fick's second law of diffusion. If the surface concentration is taken as constant, the concentration at depth is given by an error function (erf). Corrosion begins when the chloride at the depth of the steel reaches a critical threshold. Inverting the solution gives the initiation time:
where D is the chloride diffusion coefficient of the concrete. D depends strongly on w/c, binder type (pozzolans reduce it markedly) and curing.
4. The square of the cover
An example computed with the site's chloride engine:
| Cover (mm) | Initiation time, D = 1·10⁻¹² m²/s (years) | Initiation time, D halved (years) | Ratio to 25 mm |
|---|---|---|---|
| 25 | 3.3 | 6.6 | 1.00× |
| 40 | 8.5 | 16.9 | 2.56× |
| 50 | 13.2 | 26.4 | 4.00× |
| 60 | 19.0 | 38.0 | 5.76× |
But the table's real message is independent of the inputs: raising the cover from 25 mm to 50 mm makes the initiation time exactly 4 times longer; halving D makes it 2 times longer. Cover acts quadratically, the concrete's impermeability linearly. That is why cover is the most effective durability parameter a designer has. The analytical solution in the site's chloride engine is verified against an independent finite-difference solution.
5. What the classes decide
The exposure class enters design in two places:
- Concrete composition: Annex F of TS EN 206 gives recommended limiting values for each class: maximum w/c, minimum strength class and minimum cement content. In severe classes the maximum w/c falls, which means a lower D (see MLZ-01). For the actual values, consult the standard itself.
- Cover: the required minimum cover also increases with the exposure class. The quadratic relation above explains why this is so effective.
6. Uncertainty and limits
- The model is a simplification. Fick's law treats concrete as homogeneous and D as constant. In reality D decreases with time (hydration continues), cracks give chlorides a shortcut, and the surface concentration changes over time. The model is therefore reliable for comparing options, not for predicting an absolute service life.
- Workmanship can defeat the theory. If a cover specified as 50 mm is built as 35 mm, the quadratic relation halves the initiation time: (35/50)² ≈ 0.49. Cover spacers and checking bar placement matter as much as mix design.
References
- TS EN 206 — Concrete: Specification, performance, production and conformity (exposure classes; Annex F, recommended limiting values).
- Crank, J. (1975). The Mathematics of Diffusion, 2nd ed. Oxford University Press. Collepardi, M., Marcialis, A. and Turriziani, R. (1972). Penetration of chloride ions into cement pastes and concretes. Journal of the American Ceramic Society, 55(10), 534–535.
- Table values from the site engine engine/chloride-diffusion.js (example inputs: surface chloride 0.6%, critical threshold 0.05%).
The numbers in this article were produced by the site's engines; code values are given with their clause numbers.