Concrete cover is assumed to be the first measure considered to prevent influences of aggressive elements on the reinforcement steel bars. That is why utmost cautions are needed during concrete cover thickness design and its construction.
Commonly, the thickness of concrete cover is specified based on the type of the reinforced concrete member and the environment in which the structure is constructed.
In this article, concrete cover specifications based on various most commonly used codes will be discussed.
The specification of different codes regarding concrete cover is commonly dependent on nature of the structure, method of construction, environmental factors around the structure, and quality of concrete utilized in the construction.
Determination of Concrete cover thickness according to the specifications of the following codes will be explored:
- ACI code
- European Code
- British standard
- Special specifications for structures subjected to considerably sever conditions
Concrete Cover Specifications as per ACI code
The American code institute (ACI 318-11) provides minimum concrete cover thickness for different elements of reinforced concrete structure. These minimum concrete covers can be found in the Table 1.
The ACI code specifies certain condition in which this minimum cover thickness would not be adequate and hence it needs to be increased. These conditions include corrosive environment, fire protection, and future extension.
For example, for reinforced concrete member subjected to sea water, the ACI code specifies that thickness of concrete cover should be at least 50mm.
It is desired to set minimum thickness of concrete cover as 65mm to account for possible construction errors and hence make sure that 50mm cover thickness is achieved.
The code allows the use of cover thickness smaller than those provided in Table-1 for precast concrete members.
Table-1: Minimum Cover Thickness for Cast-in-Place Concrete
|Type of structure||Concrete over, mm|
|Concrete cast against and permanently in contact with ground||75|
|Concrete in contact with ground or water|
|No. 19 through No. 57 bars||50|
|No. 16 bar and smaller||40|
|Concrete not exposed to weather or in contact with ground|
|Slabs, walls, joists|
|No. 43 and No. 57 bars||40|
|No. 36 bar and smaller||20|
|Primary reinforcement, ties, stirrups, spirals||40|
|Shells, folded plate members|
|No. 19 bar and larger||20|
|No. 16 bar and smaller||13|
Concrete Cover Specifications as per European Code
The euro code provides exact recommendation and details regarding the thickness of concrete cover. The code specified minimum concrete cover along with water to cement ratio, concrete strength, and minimum cement content based on the type of weather condition to which the structure is exposed.
This can be clearly observed in Table-2 that provided minimum concrete cover thickness along with other parameters based on the weather condition.
Table-2: Minimum concrete cover according to the European code and British specifications
|Exposure condition||Maximum W/C ratio||Minimum cement content, Kg/m3||Minimum concrete cover, mm||Concrete grade|
It is proven by number of researchers like Browne et. el. and Van Daver that concrete cover in majority of structures are less than that specified in design.
For example, Browne et. el. reported that average cover thickness of observed structures was 13.9mm instead of 25mm that specified in the design.
That is why European code states that minimum cover thickness should be increased by permitted standard deviation which ranges from 5-10mm for cast in situ concrete and 0-5mm for precast concrete.
Concrete Cover Specifications as per British standard
According to the British standard thickness of concrete cover is based on weather condition in which the structure is constructed. In addition to the concrete strength, and quality of concrete that is dependent on cement content and water to cement ratio.
Table-3 provides minimum concrete cover thickness determined according to the British standard.
Table-3: Thickness and properties of concrete cover according to British standard
|Environmental conditions||Concrete grade, MPa|
|Water/ cement ratio|
|Minimum cement content, Kg/m3|
|Concrete cover thickness|
1 concrete surface protected from external weather or hard condition
2 concrete surfaces protected from rain or freezing and concrete under water or concrete adjacent to no affected soil
3concrete surface exposed to rain and wetting and drying
4concrete exposed to seawater spray or melting ice by salt or freezing
5concrete surface exposed to abrasion such as seawater containing solid particles, or moving water at pH 4.5 or machines or cars
Special specifications for structures subjected to considerably sever conditions
Commonly, offshore structures are directly subjected to sea water and specific protection measure should be considered to prevent corrosion of embedded steel bars.
There are three major regions of severe weather condition to which reinforced concrete structure may be exposed.
These severe weather conditions include air, splash zone, and immersed in water. The most dangerous condition is the splash zone since the structure subjected to seawater and air combination.
ACI 357 provides recommendation regarding cover thickness in severe weather conditions based on the concrete strength, location of structure, and maximum water to cement ratio, see Table 4.
Table-4: ACI 357 recommendations for cover thickness in offshore structure
|Location||Maximum w/c ratio||Minimum concrete strength at 28 days||Cover thickness|
|Immersed in water||0.45||35||50||75|
Apart from ACI 357, British standard provides recommendation about the selection of concrete cover thickness for private structures.
The determination of concrete cover thickness takes several parameters into consideration for example degree of mixing for each concrete quality, life expectancy of the structure, and chloride diffusion factor in concrete.
Read More: Minimum Concrete Cover for Reinforcement