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.