When a building structure ages or sustains damage for any reason, you may notice sections of the slab, column, or beam deteriorating, with the reinforced steel becoming exposed and corroded. There are many reasons for this, but the primary cause is often the failure to provide proper concrete cover blocks for each structural component. Ensuring adequate column cover, slab cover, and beam cover is essential to protect the reinforcement from fire and corrosion.
What Are Cover Blocks? : Different Sizes Of Cover Blocks.
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What Are Cover Blocks?
Cover blocks are the spacers provided at a design depth of concrete to all the reinforcement including stirrups or links depending on the condition of exposure.
In construction, cover blocks are used to ensure that reinforcing bars, or rebars, are embedded in concrete at the correct depth. Maintaining the proper depth is crucial to protect the rebar from environmental factors such as weathering, erosion, and corrosion. If the rebar is not adequately covered, the structure can become weaker and more susceptible to failure.
Purpose of Cover Blocks in Construction :
Keeping A Gap Between Reinforcement Bars :
- It can maintain a particular gap between reinforcement bars. It helps maintain the correct gap between the reinforcement and the concrete. Structural collapse can be avoided by maintaining the proper gap using a cover block.
Increasing Structural Strength :
- The correct concrete cover play a crucial role in protecting the structure from environmental factors such as humidity, chemicals, and extreme temperatures, thereby preventing the early degradation of reinforced steel.
- Ensuring proper reinforcement cover help the reinforcement bars provide optimal support, strengthening the overall framework of the structure and enhancing its durability and lifespan.
Protection of Reinforcement from Corrosion :
- They create a barrier between the reinforcement bars and the external environment, especially moisture. This barrier helps prevent the steel from coming into contact with water, reducing the risk of corrosion, which can compromise the strength of the structure over time.
Fire Damage Prevention :
- Cover blocks are essential for fire damage prevention which acts as a protective layer against high temperatures during a fire. This concrete barrier significantly delays the exposure of the steel reinforcement to heat, helping the structure retain its integrity for a longer period during a fire.
- By preventing direct contact with flames and reducing the rate of heat transfer to the steel, cover blocks help maintain the strength of the structure, thus preventing rapid collapse. This additional fire resistance is critical in providing extra time for evacuation and minimizing damage to the building.
Common Sizes Used in Various Types of Construction :
As per IS code 456 : 2000, cover blocks are available in various sizes depending on the type of construction and the structural element they are used for. The sizes vary depending on the type of structure (slabs, beams, columns, or footings) and the required concrete cover as per construction standards. Below are common sizes of cover blocks and their recommendations based on different types of construction in India :
| Sr. NO | Structural Elements | Cover Size (mm) |
|---|---|---|
| 1 | Footing | 50-75 |
| 2 | Column | 40 |
| 3 | Beam | 25 |
| 4 | Slab | 20 |
| 5 | Retaining Wall | 25 |
| 6 | Raft foundation | 80 |
Note :- In case of columns of minimum dimensions of 200 mm or under, whose reinforcement bars do not exceed 12 mm, a cover block of 25 mm may be used.
For footings minimum cover shall be 50 mm.
For different climatic conditions, there’s a specific cover size should be used given below as per the code 456 : 2000.
| Sr. No | Exposure | Concrete Cover (mm) |
|---|---|---|
| 1 | Mild | 20 |
| 2 | Moderate | 30 |
| 3 | Severe | 45 |
| 4 | Very Severe | 50 |
| 5 | Extreme | 75 |
How To Choose the Right Cover Blocks?
When choosing cover blocks, it’s important to not only focus on protecting reinforcement bars from corrosion and external damage but also to ensure the concrete cover is adequate. Here are the key factors to keep in mind when selecting the right cover blocks :
1. Cost :
- When choosing the right cover block for a construction project, the cost is a key factor. Prices can vary based on the materials, size, and shape of the blocks.
- Plastic cover blocks tend to be the most affordable, while concrete blocks, though pricier, offer superior strength.
2. Material :
- The material of the cover block needs to withstand harsh weather and support the heavy load of the concrete.
- Plastic is lightweight and economical, while concrete offers unmatched durability.
3. Shape and Size :
- The shape of the cover block should complement the reinforcement bars it supports, ensuring a snug fit. The size, meanwhile, should align with the required concrete thickness as dictated by design specifications.
4. Durability :
- Durability is essential in cover blocks, as they need to endure harsh conditions, including moisture, chemicals, and varying temperatures. Concrete cover blocks are often the most durable and resilient, while plastic or rubber blocks may degrade faster over time. Choosing the right material ensures long-term performance.
5. Load Capacity :
- Cover blocks must be able to handle the combined weight of both the concrete and the reinforcement bars without cracking or weakening. It’s important to check the block’s load capacity according to the project’s design specifications.
At the end, choosing an appropriate and adequate size for the specified condition and structural elements is important to prevent disaster. It helps to increase durability, stability and longevity of the building.
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