🕑 Reading time: 1 minute
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 Classes||Conditions of exposure|
|F0||Concrete not subjected to freezing-and-thawing cycles|
|F1||Concrete experiences freezing-and-thawing cycles with limited exposure to water|
|F2||Concrete exposed to freezing-and-thawing cycles with frequent exposure to water|
|F3||Concrete 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
|Exposure classes||Conditions of exposure||Conditions of exposure|
|-||Water-soluble sulfate (SO4^2–) in soil, percent by mass||Dissolved sulfate (SO4^2–) in water, ppm|
|S0||Smaller than 0.10||Smaller than 150|
|S1||Equal or greater than 0.10 but smaller than 0.2||Equal or greater than 150 but lower than 1500|
|S2||Equal or greater than 0.2 but equal or smaller than 2||Equal or greater than 1500 but equal or lower than 10000|
|S3||Greater than 2||Greater 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 classes||Conditions of exposure|
|W0||Concrete dry in service|
|W1||Concrete 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.|
|W2||Concrete 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 classes||Conditions of exposure|
|C0||concrete protected from moisture or dry in service.|
|C1||Concrete in contact with moisture, but the external source of chloride does not reach it.|
|C2||Concrete subjected to moisture and an external source of chlorides such as deicing chemicals, salt, brackish water, seawater, or spray from these sources.|
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 Class||Cement Types Defined in ASTM C 150||Cement Types Defined in ASTM C 595||Cement Types Defined in ASTM C 1157|
|S1||II||Types with (MS) Designation||MS|
|S2||V||Types with (HS) Designation||HS|
|S3, option 1||V plus pozzolan or slag cement||Types with (HS) Designation plus, pozzolan or slag cement||HS plus pozzolan or slag cement|
|S3, option 4||V||Types with (HS) Designation||HS|
- ASTM C150 provides the composition of type II and V cement. They are suitable for moderate sulfate resistance and high sulfate resistance, respectively.
- 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).
- 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 classes||Water to cementitious materials ratio, %||Minimum strength of concrete, MPa|
|S3, option 1||0.45||31|
|S3, option 2||0.40||35|
Table 7 Total Air Content for Concrete Exposed to Cycles of Freezing and Thawing
|Nominal maximum aggregate size, mm||Target air content, %||Target air content, %|
|-||F1||F2 and F3|