What is Concrete Cover? – Clear Cover, Nominal Cover, Effective Cover

What is Concrete Cover?

While studying RCC detailed drawings or preparing bar bending schedules, you encounter the following terms: Concrete Cover, nominal cover, Clear cover and effective cover. Today in this post, we are going to discuss these terms.

Let us consider the below column Cross-section for a better understanding.

Concrete Cover

Concrete Cover 

In RCC structures, the reinforcement is enclosed by the adequate concrete cover to protect the bars from atmospheric conditions, making reinforcement corrode.

Different ways are available to measure concrete cover, and other terminologies like Clear cover, Nominal Cover, and the effective cover is used to define this.

What is Concrete Clear cover?

It is the distance measured between the exposed concrete face and the nearest steel reinforcement surface. In the following image, Clear cover is explained clearly.

concrete cover

What is Nominal cover?

As per IS 456:2000 (Clause 26.4.1), the term clear cover is followed by Nominal cover.

The nominal cover is the distance measured between the exposed concrete face and the nearest steel reinforcement, even links ( Column ties or shear stirrups). It should not be less than the bar diameter in any condition.

Nominal Cover As Per IS 456:2000 To Meet Durability Requirements

ExposureNominal Concrete Cover Not Less Than (mm)
Mild20
Moderate30
Severe45
Very Severe50
Extreme75

Nominal Cover As Per IS 456:2000 To Meet Specified Period of Fire Resistance

What is Concrete Cover? - Clear Cover, Nominal Cover, Effective Cover Civil Lead

What is Effective Cover ?

The effective cover is mainly employed in design calculations. It is the distance measured between the exposed concrete face and the centroid of the main steel reinforcement. (or)

It is the distance measured between the outermost face of RCC member and the centroid of the area of main tension reinforcement. In the following image, the effective cover is described for the beam cross-section.

concrete cover

In the above image, φis the stirrup diameter and φ is the main bar diameter. We all know the centroid of the main reinforcement that is the bar centre φ/2.

therefore 

Effective cover = Clear cover + φ’ + φ/2

What is the purpose of Concrete Cover?

Covering is provided to restrict the attack of corrosion to reinforcement. Reinforcements are prone to corrosion and fire due to atmospheric conditions.

If we do not offer proper concrete cover, then there is a high possibility of rusting and hardened RCC cracks. The cover also provides protection against fire.

The cover is also essential for developing adequate bond strength along the steel bar surface area.

The concrete cover is essential for every building element (Slabs, Beams, footings), where the reinforcements are present. We use concrete covers to maintain the proper arrangement of steel bars and provide a cover for bars.

Different Types of Concrete Cover Blocks

Concrete Cover

Based on material, various types of cover blocks are available, which are listed below.

  1. Cement Mortar Cover Block
  2. PVC Cover Block
  3. Aluminum Cover Block
  4. Steel Cover Block
  5. Stone Cover Block

Concrete Cover for Steel Reinforcement In Different Structural Member

Concrete cover for steel reinforcement is necessary to fight against corrosion and to give protection against fire. Cover thickness varies according to the structural member and depending on the environmental condition.

The minimum required thickness of cover for steel reinforcement is adopted from the drawing or should be taken from the relevant code of practice.

Cover for Beam and Column

In a beam for a longitudinal reinforcing bar, the cover should be less than 30 mm or not less than the bar diameter.

In a column for longitudinal steel, reinforcement cover should not be less than 40 mm or not less than such bar diameter.

In the situation when the column dimension is 200 mm or under, which steel reinforcements don’t exceed 12 mm, 25 mm cover for reinforcement should be provided.

Cover For Slab

In a slab for compressive shear, tensile or other reinforcement, the cover should not be less than 15 mm or not less than such bar diameter.

concrete cover

For any other steel, reinforcement cover should not be less than 15 mm or not less than such bar diameter.

Cover For Footing

For the structural member where the placing of concrete is directly against the ground, cover for bottom reinforcement should be 75 mm. In the situation where concrete is poured in a layer of lean concrete cover, 50 mm may be adopted.

concrete cover

The concrete surface that is exposed to the weather like footing top and side, grade beam and retaining wall which comes in direct contact with the ground after removal of formwork, cover shall not be less than 50 mm.

Minimum Concrete Cover For Steel Reinforcement

The minimum thickness of the cover at each end face of steel reinforcement should not be less than 25 mm or double the diameter of the bar should be implemented.

The concrete surface that is exposed to the weather like footing top and side, grade beam and retaining wall which comes in direct contact with the ground after removal of formwork, cover shall not be less than 50 mm.

The concrete surface that is exposed to the act of harmful chemicals (Like saline water, acid, alkali, smoke and sulphur) in such locations increased cover thickness should be implemented as given on the drawing.

For the structure that is used for liquid storage purpose cover, 40 mm or main bar diameter, whichever is greater should be provided for all reinforcing bars.

In the area where the water of corrosive nature, oil and seawater are present, the cover should be increased by 10 mm.

The concrete surface exposed to the harmful surrounding, in such case protection to reinforcement may also be provided by using a dense impermeable concrete with an approved protective coating. In such conditions, extra cover, as mentioned above(2,9), may be reduced.

The required cover should be applied by using cover blocks ( cement mortar cube or other approved means). Precast concrete blocks approved by EIC should support steel reinforcement of grade beam, footing and slab on a subgrade. 

Many times unskilled labours use pebbles or stones instead of a concrete cover block that should not be allowed.

The required minimum cover for steel reinforcement should follow the code IS 456:2000 or as indicated in the drawing.

Also, Read

What is Development Length?- Complete Guide

What Is Lap Length? How to Calculate it?

Difference Between Lap Length and Development Length

Difference Between One way and Two way Slab

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