In construction plans and when preparing for the placement of reinforcement bars, you’ll encounter terms like Concrete Cover, nominal cover, Clear cover, and effective cover. In this discussion, we’ll break down these terms and their significance.
To grasp the concept of Concrete Cover, let’s examine the cross-section of a column as an example.
In RCC structures, the reinforcement is surrounded by a sufficient layer of concrete cover. This acts as a protective barrier against environmental conditions that could cause corrosion of the reinforcement bars.
Various terms are used to define concrete cover, including Clear cover, Nominal Cover, and Effective Cover.
Clear cover refers to the space between the exposed concrete surface and the nearest steel reinforcement surface. The image below provides a clear illustration of this concept.
According to IS 456:2000 (Clause 26.4.1), nominal cover is a crucial consideration following clear cover. It represents the distance between the exposed concrete surface and the nearest steel reinforcement, including links such as column ties or shear stirrups. This distance should never be less than the diameter of the reinforcement bar under any circumstance.
Now, let’s delve into the specifics of Nominal Cover as per IS 456:2000.
| Durability Requirement | Minimum Nominal Cover (mm) |
|---|---|
| Mild Exposure | 20 |
| Moderate Exposure | 30 |
| Severe Exposure | 45 |
| Fire Resistance Period (in minutes) | Minimum Nominal Cover (mm) |
|---|---|
| 60 | 40 |
| 90 | 45 |
| 120 | 50 |
Effective cover plays a crucial role in design calculations. It represents the distance between the exposed concrete surface and the centroid of the main steel reinforcement. Another way to visualize it is as the distance between the outermost face of the RCC member and the centroid of the area of the main tension reinforcement.
In conclusion, understanding and implementing concrete cover standards is vital for the durability, safety, and longevity of RCC structures. Adherence to these specifications is a cornerstone of responsible construction practices.
| Location/Exposure Condition | Minimum Cover (mm) |
|---|---|
| Top and side of footings, grade beams, and retaining walls in contact with ground after formwork removal | 50 |
| End face of steel reinforcement | 25 or double the bar diameter (whichever is greater) |
| Exposed to harmful chemicals, saline water, acid, alkali, smoke, or sulphur | As indicated on the drawing |
| For liquid storage structures | 40 or main bar diameter (whichever is greater) |
| In areas with corrosive agents like oil, seawater, or water of corrosive nature | Increase cover by 10 mm |
| In areas with harmful surroundings | Extra cover may be reduced if using dense impermeable concrete with approved protective coating |
| Bottom reinforcement where concrete is directly against the ground | 75 |
| Bottom reinforcement where concrete is poured into a layer of lean concrete cover | 50 |
| Any other steel reinforcement | 15 or not less than bar diameter |
Remember, using approved cover blocks is essential to ensure the required minimum cover for steel reinforcement as per IS 456:2000. Avoid using pebbles or stones as substitutes for concrete cover blocks, as they may not provide adequate protection. Always refer to the relevant code of practice and drawing specifications for precise cover requirements.
By adhering to these cover standards, we contribute to the structural integrity and safety of the built environment.