Reinforced Cement Concrete uses reinforcement in the form of bars. These bars needs to be covered sufficiently under the concrete so as to protect it from atomospheric agencies which make it to get corrode. This minimum depth or thickness of concrete is known as cover. There are different types of cover
Nominal cover is the design depth of concrete cover to all steel reinforcements, including stirrups. It shall be not less than the diameter of the bar.
It is the distance between the exposed concrete surface without finishes to the surface of reinforcing bar. e.g. distance between the exposed beam surface to the stirrups in beam. In any case it shall not be less than diameter of the bar.
Nominal Cover as Per IS- 456
- For column, nominal cover for longitudnial reinforcing bar shall not be less than 40 mm or less than the diameter of the bar.
- In case of columns having minimum dimenstion of 200 mm or less, having reinforcing bars of diameter not greater than 12 mm, nominal cover of 25 mm may be provided.
- For footings minimum cover required is 50 mm.
Nominal cover provided to the concrete depends on the exposure condition. As per Indian Standard 456 Exposure conditions are of following types:
|Sr. No.||Environment||Exposure Conditions|
|i)||Mild||Concrete surfaces protected against weather or aggressive conditions, except those situated in coastal area.|
|ii)||Moderate||Concrete surfaces sheltered from severe rain or freezing whilst wet.|
Concrete exposed to condensation and rain.
Concrete continuously underwater .
Concrete in contact or buried under non aggressive soil/ground water.
Concrete surfaces sheltered from saturated salt air in coastal area.
|iii)||Severe||Concrete surfaces exposed to severe rain, alternate wetting and drying or occasional freezing whilst wet or severe condensation. Concrete completely immersed in seawater.|
Concrete exposed to coastal environment.
|iv)||Very severe||Concrete surfaces exposed to sea water spray, corrosive fumes or severe freezing conditions whilst wet.|
Concrete in contact with or buried under agressive sub-soil/ground water.
|v)||Extreme||Surface of members in tidal zone.|
Members in direct contact with liquld/solid aggressive chemicals.
Nominal Cover as per IS- 456 for Durability requirement
|Exposure condition||Minimum Nominal Cover|
1 For mild exposure condition having main reinforcement up to 12 mm diameter, the nominal cover can be reduced by 5 mm.
2 Actual concrete cover at site should not deviate from the specified nominal cover by + I0 mm
3 For concrete grade M35 and above, for Severe and Very severe exposure conditions reduction upto 5 mm in cover is permissible.
Nominal Cover as per IS- 456 to Meet Specified Period of Fire Resistance
Minimum nominal cover to meet specified period of fire resistance as per IS-456 is as below: