XO: No risk of corrosion or attack - For all the exposures of unreinforced concrete (except freeze-thaw, abrasion or chemical attack) and very dry for reinforced concrete.

XC: Corrosion induced by carbonation - For reinforced surfaces exposed to air and moisture

XC1: Dry or permanently wet - For reinforced surfaces inside dry enclosed structures or submerged to non-aggressive water.

XC2: Wet, rarely dry - For reinforced surface buried in soil classed as AC-1 and with a hydraulic gradient not greater than 5.

XC3 and XC4: Moderate humidity or cyclic wet and dry - For reinforced surface subject to high humidity or exposed to alternate wetting and drying.

XD: Corrosion induced by chlorides other than from sea water - For reinforced surface in contact with water containing chlorides or de-icing salt.

XD1: Moderate humidity - For surfaces exposed to airborne chlorides, reinforced surface more than 10m away from a carriageway and parts of structures exposed to occasional or slight chloride conditions.

XD2: Wet, rarely dry - For reinforced surfaces totally immersed in water containing chlorides.

XD3: Cyclic wet and dry - For reinforced surface within 10m from a carriageway or buried in less than 1m below carriageway level and reinforced pavements and car park slabs.

XS: Corrosion induced by chlorides from sea water - For reinforced surfaces subject to contact with chlorides from sea water or air carrying salt originating from sea water.

XS1: Exposed to airborne salt but not in direct contact with sea water - For reinforced surfaces in coastal areas.

XS2: Permanently submerged - For reinforced surfaces completely submerged and remaining saturated.

XS3: Tidal, splash and spray zones - For reinforced surfaces in the upper tidal zones and the splash and spray zones.

XF: Freeze-thaw attack - For surfaces exposed to significant attack from freeze-thaw cycles whilst wet.

XF1: Moderate water saturation without de-icing agent - For vertical concrete surfaces such as façades and columns exposed to rain and freezing

XF2: Moderate water saturation with de-icing agent - For concrete surfaces such as parts of bridges, which would otherwise be classified as XF1, but which are exposed to de-icing salts either directly or as spray or run-off.

XF3: High water saturation without de-icing agent - For horizontal concrete surfaces, such as parts of buildings, where water accumulates, and which are exposed to freezing and concrete surfaces subjected to frequent splashing with water and exposed to freezing.

XF4: High water saturation with de-icing agent or sea water - Horizontal concrete surfaces, such as roads and pavements, exposed to freezing and to de-icing salts either directly or as spray or run-off and concrete surfaces subjected to frequent splashing with water containing de-icing agents and exposed to freezing.

XA: Chemical attack::

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