What is Concrete Cover for Reinforcement?

The concrete cover can be defined as the distance between the reinforcement or rebar embedded in the concrete to the outer surface of the concrete surface as per the code ACI 130. The cover in reinforced concrete structures serves the major role to enhance the durability property of the structure. The figure-1 below shows a beam cross-section four main reinforcement as shown.
Concrete Cover for Reinforcement
 Concrete Cover for Reinforcement

The reinforcement bar that is closer to the outer surface of the concrete structure is taken into consideration

What is the Requirements of Concrete Cover?


The main requirements of Concrete cover are:
  1. The cover thickness act as a barrier from the attack of chlorides, sulfates, moisture and other gaseous and liquid pollutants.
  2. The cover avoids the penetration of materials that can make the steel to corrode when coming to contact.
  3. The cover helps the reinforcement bars to be properly embedded to prevent splicing of bars.
For more aggressive environment, a larger cover depth is required. But one factor to be considered is that large cover depth upon flexural loading results in larger cracks width that promotes the ingress of moisture and then comes in contact with steel causing corrosion. This means, cover depth must be selected by maintaining a balance between the crack width and cover depth.

What is Nominal Cover, Clear Cover and Effective Cover of Reinforcement?


Nominal Cover and Clear Cover of Reinforcement

The nominal cover and clear cover are same parameters given in different names. It is the clear distance between the outer surface of the concrete to the nearest reinforcement bar to the surface. This reinforcement bar can be either main reinforcement, stirrups or links.

Effective Cover of Reinforcement

Effective cover is the distance between the outermost surface of the concrete structure or outermost side of the compression face of structure to the center of the nearby main reinforcement embedded.

Nominal and Effective Cover
Nominal and Effective Cover


Nominal Cover for Different Concrete Structures

Most of the codes specifies nominal cover value for construction. In no way the value of nominal cover provided vary less from the standard code value by 0 to +10mm unless specified by the design requirements.
Nominal cover is primarily provided to meet the durability requirements of the concrete structures. Durability of the structures is designed based on the exposure conditions it is subjected to. Based on the conditions of exposure, the minimum nominal cover required are: 

As per IS:456-2000 , Table-16


 Exposure Condition Mild Moderate  Severe Very SevereExtreme 
 Minimum Nominal Cover in mm  20      30    45    50      75

Notes: 
  1. When Main Reinforcement Bar diameter is >= 12mm, reduce the nominal cover by 5mm for mild exposure.
  2. For severe and very severe exposure conditions and for grades greater than M35, a reduction of 5mm can be given from the values of the table above.


How to Provide Cover to Reinforcement bars in Concrete Structures?


All the steel reinforcement must be provided with a nominal cover as stated by the standard codes of the country area. The steel reinforcement including the links must take this cover value.
The cover to reinforcement is provided either by placing spacers, chair or any other form of supports. The spacers or cover blocks are placed at a spacing not greater than 1m . The minimum spacing of cover blocks vary based on the type of reinforcement congestion. 
The spacers or supports used to maintain the concrete cover must have the strength similar to the concrete to be placed.

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