Exposure Conditions Affecting the Durability of Concrete

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Different exposure conditions are established based on the degree of severity of the environmental conditions of the area in which the structure is located.

Various codes such as Indian Standard, American Concrete Institute, and European Standard have set similar exposure categories for reinforced concrete structures.

The exposure conditions govern durability requirements, materials selection and proportion, design, and construction of the structure.

Therefore, the exposure category, if selected appropriately, would ensure high durability of concrete structures, and hence, decrease the maintenance work and cost.

Exposure Conditions Affecting the Durability of Concrete

1. Exposure Conditions Based on IS 456

There are five levels of environmental exposure of concrete upon which the durability requirements of concrete are based. Mild, moderate, severe, very severe, and extreme exposure are different exposure categories established by the Indian standards based on the environment. Table-1 explains each exposure condition along with its description of the environmental conditions.

Table-1: Environment and Exposure Conditions Based on Indian Standard

Environment	Exposure Conditions
Mild	Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal areas.
Moderate	Concrete surfaces sheltered from severe rain or freezing while wet, concrete exposed to condensation and rain, concrete in contact, or buried under non-aggressive soil/groundwater.
Severe	Concrete surfaces exposed to severe rain, alternate wetting and drying, or occasional freezing while wet or severe condensation.
Very severe	Concrete completely immersed in seawater, concrete exposed to the coastal environment, concrete surfaces exposed to seawater, corrosive fumes, or severe freeing conditions while wet.
Extreme	Concrete in contact with or buried under aggressive sub-soil/groundwater, the surface of members in the tidal zone, members in direct contact with liquid/solid aggressive chemicals.

2. Exposure Categories and Classes Based on ACI 318-19

The ACI 318-19 presents four different exposure categories based on the environmental condition, and each exposure category divided into different classes based on the severity of the environmental situation.

When a structure falls into two or more classes, the most undesired exposure class is considered. The exposure classes are crucial since they are the base for the selection of concrete cover, materials used, mix design, and many other design and construction measures.

Table-2: Exposure Category and their Classes Based on ACI 318-19

Exposure Category	Category classes and their condition
Freezing and thawing (F)	Class F0: Concrete not subjected to freezing-and-thawing cycles. Class F1: Concrete experience freezing-and-thawing cycles with limited exposure to water. Class F2: Concrete exposed to freezing-and-thawing cycles with frequent exposure to water. Class F3: Concrete exposed to freezing-and-thawing cycles with continual exposure to water and exposure to deicing chemicals
Sulfate (S)	Class S0: Water-soluble sulfate (SO4^2–) in soil, percent by mass, is smaller than 0.10. Class S1: Water-soluble sulfate (SO4^2–) in soil, percent by mass, is equal or greater than 0.10 but lower than 0.2. Class S2: Water-soluble sulfate (SO4^2–) in soil, percent by mass, is equal or greater than 0.2 but equal or smaller than 2. Class S3: Water-soluble sulfate (SO4^2–) in soil, percent by mass, is greater than 2.
In contact with water (W)	Class W0: Concrete dry in service. Class W1: Concrete contacted with water where low permeability is not needed. Class W2: Concrete in touch with water where low permeability is required.
Corrosion protection of reinforcement (C)	Class C0: Concrete protected from moisture or dry in service. Class C1: Concrete in contact with moisture, but the external source of chloride does not reach it. Class C2: Concrete subjected to moisture and an external source of chlorides such as deicing chemicals, salt, brackish water, seawater, or spray from these sources.

3. Exposure Categories Based on EN 206.1-2000

The exposure classes, which are used for the durability design of concrete structures, are composed of six classes based on EN 206.1-2000. Table-3 presents exposure classes, a description of conditions, and informative examples where the exposure class may occur.

Table-3: Exposure Classes Based on EN 206.1-2000

Class/designation	Description of environment	Informative example where exposure classes may occur
1. No risk of corrosion or attack
X0	For concrete without reinforcement or embedded metal: all exposures except where there is freeze/thaw, abrasion or chemical attack	-
X0	For concrete with reinforcement or embedded metal: very dry	Concrete inside buildings with very low air humidity
2. Corrosion induced by carbonation (Where concrete containing reinforcement or other embedded metal  exposed to air and moisture)
XC1	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
3. Corrosion induced by chlorides other than from seawater (Where concrete containing reinforcement or other embedded metal is subject to contact with water containing chlorides, including deicing salts from sources other than seawater)
XD1	Moderate humidity	Concrete surfaces exposed to airborne chlorides
XD2	Wet, rarely dry	Swimming pools. Concrete exposed to industrial waters containing chlorides
XD3	Cyclic wet and dry	Parts of bridges exposed to spray containing chlorides. Pavements. Car park slabs
4. Corrosion induced by chlorides from seawater (Where concrete containing reinforcement or other embedded metal is subject to contact with chlorides from seawater or air carrying salt originating from seawater)
XS1	Exposed to airborne salt but not in direct contact with seawater	Structures near to on the coast
XS2	Permanently submerged	Parts of marine structures
XS3	Tidal, splash and spray zones	Parts of marine structures
5. Freeze/thaw attack with or without deicing salts (Where concrete is exposed to significant attack from freeze-thaw cycles whilst wet)
XF1	Moderate water saturation, without deicing agents	Vertical concrete surfaces exposed to rain and freezing
XF2	Moderate water saturation, with deicing agents	Vertical concrete surfaces of road structures exposed to freezing and airborne deicing agents
XF3	High water saturation, without deicing agents	Horizontal concrete surfaces exposed to rain and freezing
XF4	High water saturation, with a deicing agent or seawater	Road and bridge decks exposed to deicing agents. Concrete surfaces exposed to direct spray containing deicing agents and freezing. Splash zones of marine structures exposed to freezing
6. Chemical attack
XA1	Slightly aggressive chemical environment
XA2	Moderately aggressive chemical environment
XA3	Highly aggressive environment

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