Exposure classes

The environmental (exposure) classes according to NEN-EN 206-1/NEN 8005 are divided into 6 groups. The XO, XC, XD and XS groups are based on the reinforcement degradation due to corrosion, the XF and XA groups - on the concrete degradation itself.

The main groups are subdivided further on the basis of the moisture which the concrete with the (possible) existing reinforcement is exposed to. The total number of environmental classes is 18.

 

Exposure classes for reinforcement

Degradation mechanisms

Class

No. of subclasses

Description of environment

Informative examples where exposure classes may occur

X0 (0 = "zero risk")

No risk of corrosion or attack

X0

1

For concrete without reinforcement or embedded metal: all exposures except where there is freeze/

thaw, abrasion or chemical attack

For concrete with reinforcement or embedded metal: very dry

Concrete inside buildings with very low air humidity

XC (C = "carbonatation")

Corrosion induced by carbonatation (where concrete containing reinforcement or other embedded metal is exposed to air and moisture)

 

XC1

4

Dry or permanently wet

Concrete inside buildings with low humidity

Concrete permanently submerged in water

XC2

Wet, rarely dry

Concrete subjected to long-term water contact

Many foundations

XC3

Moderate humidity

Concrete inside buildings with moderate or high air humidity

External concrete sheltered from rain

XC4

Cyclic wet and dry

Concrete surfaces subject to water contact, not within exposure class XC2

XD (D = "deicing salts")

Corrosion induced by chlorides other than from sea water (where concrete containing reinforcement or other embedded metal is subject to contact with water containing chlorides, including deicing

salts from sources other than sea water)

XD1

3

Moderate humidity

Concrete surfaces exposed to direct spray containing chlorides

XD2

Wet, rarely dry

Concrete exposed to industrial waters containing chlorides

Swimming pools

XD3

Cyclic wet and dry

Parts of bridges exposed to spray containing chlorides

Pavements

Car park slabs

XS (S = "seawater")

Corrosion induced by chlorides from sea water (where concrete containing reinforcement or other embedded metal is subject to contact with chlorides from sea water or air carrying salt originating from sea water)

XS1

3

Exposed to airborne salt but not in direct contact with sea water

Structures near to on the coast

XS2

Permanently submerged

Parts of marine structures

XS3

Tidal, splash and spray zones

Parts of marine structures

 

Environmental classes for concrete

Degradation mechanisms

Class

No. of subclasses

Description of environment

Informative examples where exposure classes may occur

XF (F = "frost")

Freeze/thaw attack with or without de-icing salts (where concrete is exposed to significant attack from freeze-thaw cycles whilst wet)

XF1

4

Moderate water saturation, without de-icing agents

Vertical concrete surfaces exposed to rain and freezing

XF2

Moderate water saturation, with de-icing agents

Vertical concrete surfaces of road structures exposed to freezing and airborne de-icing agents

XF3

High water saturation, without de-icing agents

Horizontal concrete surfaces exposed to rain and freezing

XF4

High water saturation, with de-icing agent or sea water

Road and bridge decks exposed to de-icing agents

Concrete surfaces exposed to direct spray containing de-icing agents and freezing

Splash zones of marine structures exposed to freezing

XA (A = "aggressive")

Chemical attack

XA1

3

Slightly aggressive chemical environment

 

XA2

Moderately aggressive chemical environment

 

XA3

Highly aggressive environment

 

 

According to NEN-EN-206-1/NEN 8005 several environmental classes may be applied for a building component. There should be established at least one environmental class related to environmental degradation of the concrete (XF, XA) and then the corresponding classes related to damage to the reinforcement (XC, XD, XS): XA1(XC), XA1(XD), XA1(XS), XA2(XC), XA2(XD), XA2(XS), XA3(XC), XA3(XD), XA3(XS), XF1(XC), XF1(XD), XF1(XS), XF2(XC), XF2(XD), XF2(XS), XF3(XC), XF3(XD), XF3(XS), XF4(XC). There is always a 1-1 relation. For example, "XA1(XC)" means exposure class XA1 in combination with corresponding class XC1. So, "XF2(XD)" with corresponding class XD3 is not possible, it's always XD2 in such case. For XF4 there is no corresponding class XD or XS, because XD4 as well as XS4 doesn't exist, so only "XF4(XC)" could be used, because XC4 exists. The corresponding classes are designated without numbers of subclasses. Also X0 is not needed to combine.