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The ACI 318-19 set durability demand for concrete based on the category exposure and class exposure of the structure, dependent on the ground and weather situation of the area. Each exposure category contains different class exposure.

The class exposure describes the degree of severity of the condition of the environment. If a structural member falls into more than one exposure class, then the most severe exposure class is used for the member.

The exposure class influences proportion of mixture, type of cement and cementitious materials, and percentage of chemical admixtures like air-entrainment admixture.

Types of Exposure Categories

The ACI 318-19 presents four different exposure categories to make sure that a structure possesses sufficient durability and resists aggressive elements that may come up during the life span of the structure. These exposure categories embrace different exposure classes to address the degree of severity of environmental circumstances.

1. Exposure Category F

This category of exposure applies to concrete exposed to moisture and series freezing and thawing with or without deicing chemicals. There are four classes of category exposure (F). Table 1 presents various exposure class and their exposure situations.

Table 1 Exposure Category F of Concrete and its Exposure Class

Exposure ClassesConditions of exposure
F0Concrete not subjected to freezing-and-thawing cycles
F1Concrete experiences freezing-and-thawing cycles with limited exposure to water
F2Concrete exposed to freezing-and-thawing cycles with frequent exposure to water
F3Concrete exposed to freezing-and-thawing cycles with continual exposure to water and exposure to deicing chemicals

2. Exposure Category S

It uses for concrete in contact with water or soil that contains sulfate ions that are able to solute in water. The degree of severity of the exposure of concrete to sulfate make four classes that are presented in Table 2.

Table 2 Exposure Category F of Concrete and its Exposure Class

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Exposure classesConditions of exposureConditions of exposure
Water-soluble sulfate (SO4^2–) in soil, percent by massDissolved sulfate (SO4^2–) in water, ppm
S0Smaller than 0.10Smaller than 150
S1Equal or greater than 0.10 but smaller than 0.2Equal or greater than 150 but lower than 1500
S2Equal or greater than 0.2 but equal or smaller than 2Equal or greater than 1500 but equal or lower than 10000
S3 Greater than 2Greater than 10000

3. Exposure Class W

This exposure category applies to concrete in contact with water. It is divided into three classes, as presented in Table 3.

Table 3 Exposure Category W of Concrete and its Exposure Class

Exposure classesConditions of exposure
W0Concrete dry in service
W1Concrete contacted with water where low permeability is not needed. A structural member that does not require low permeability and subject to constant or occasional sources of water or can absorb water from soils around fall into this exposure class.
W2Concrete in touch with water where low permeability is required. If the penetration of water into concrete declines serviceability or durability, then the structural element falls into this class W2.

4. Exposure Category C

This category applies to non-prestressed and prestressed concrete subjected to situations that need extra protection to avoid reinforcement corrosion. The degree of condition severity creates three classes within the exposure category C (protection against corrosion reinforcement), as demonstrated in Table 4.

Table 4 Exposure Category C of Concrete and its Exposure Class

Exposure classesConditions of exposure
C0concrete protected from moisture or dry in service.
C1Concrete in contact with moisture, but the external source of chloride does not reach it.
C2Concrete subjected to moisture and an external source of chlorides such as deicing chemicals, salt, brackish water, seawater, or spray from these sources.
Durability Exposure Classes for Concrete Elements
Fig. 1: Durability Exposure Classes for Concrete Elements

Durability Requirement Based on Exposure Class

A specific exposure class of concrete structure may govern several aspects of concrete, for instance, mix proportion, strength, addition of mineral and chemical admixture.

The durability requirement of concrete necessitates low water to cement ratio and addition of cementitious materials, for instance, silica fume, fly ash, and ground granulated blast furnace slag to decrease water penetration into the concrete. Table 5 presents different cementitious materials that can withstand sulfate attacks.

Sometimes, exposure class of concrete dictates its strength to make sure the structural elements have adequate strength and resistance against aggressive elements, as presented in Table 6.

Additionally, durability requirements may enforce the use of admixtures such as air-entrainment admixture to improve resistance against freezing and thawing, Table 7.

Table 5 Selection of Cementitious Materials Based on the Exposure Requirement of Concrete Structure

Exposure ClassCement Types Defined in ASTM C 150Cement Types Defined in ASTM C 595Cement Types Defined in ASTM C 1157
S1IITypes with (MS) DesignationMS
S2VTypes with (HS) DesignationHS
S3, option 1V plus pozzolan or slag cementTypes with (HS) Designation plus, pozzolan or slag cementHS plus pozzolan or slag cement  
S3, option 4VTypes with (HS) DesignationHS

Note:

  1. ASTM C150 provides the composition of type II and V cement. They are suitable for moderate sulfate resistance and high sulfate resistance, respectively.
  2. Different types of cement defined in ASTM C 595 and used for sulfate resistance purposes are Portland blast-furnace slag cement (IS), Portland-pozzolan cement (IP), Portland-limestone cement (IL), and ternary blended cement (IT).
  3. Different types of cement defined in ASTM C 1157 and employed for sulfate resistance reasons are medium sulfate resistance (MH) and high sulfate resistance (HS)

Table 6 Water to Cementitious Material Ratio and Minimum Concrete Strength of Concrete Based on Exposure Class

Exposure classesWater to cementitious materials ratio, %Minimum strength of concrete, MPa
F017
F10.5524
F20.4531
F30.4035
S017
S10.5028
S20.4531
S3, option 10.4531
S3, option 20.4035
W017
W117
W20.5028
C017
C117
C20.4035

 Table 7 Total Air Content for Concrete Exposed to Cycles of Freezing and Thawing

Nominal maximum aggregate size, mmTarget air content, %Target air content, %
F1F2 and F3
9.56.07.5
12.55.57.0
195.06.0
254.56.0
37.54.55.5
504.05.0
753.54.5

Read more:

15 Factors Affecting Durability of Concrete

Durability of Reinforced Concrete in Different Environmental Conditions

Madeh Izat Hamakareem

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