Cover block in construction is a small but extremely important component used in reinforced concrete structures to maintain the required clear cover between reinforcement bars and the concrete surface. It ensures durability, strength, and corrosion resistance of RCC members such as slabs, beams, columns, footings, and walls. Proper use of cover blocks is mandatory in quality construction work and is strictly guided by IS codes.
What is a Cover Block in Construction?
A cover block is a spacer placed between reinforcement steel and formwork to maintain the specified concrete cover during concreting. The concrete cover protects steel bars from corrosion, fire, moisture, and environmental effects while ensuring proper bond between steel and concrete. Without cover blocks, reinforcement may shift during concreting, leading to insufficient cover and reduced structural life.
Purpose of Cover Block
The main purpose of a cover block in construction is to keep reinforcement bars at the correct position and distance from the concrete surface. It ensures protection of steel against corrosion, improves fire resistance, maintains structural strength, and helps achieve durability as per design and IS code requirements. Cover blocks also ensure uniform concrete cover throughout the structure.
Importance of Cover Block in RCC Structures
Cover blocks play a vital role in maintaining the durability and safety of RCC structures. Insufficient cover can cause early corrosion of steel, leading to cracks, spalling, and structural failure. Excessive cover, on the other hand, may reduce effective depth and structural strength. Hence, accurate placement of cover blocks is essential for long-term performance of concrete structures.
Types of Cover Blocks in Construction
Concrete Cover Blocks
Concrete cover blocks are the most commonly used type. They are made from cement, sand, and aggregates with strength equal to or higher than the parent concrete. These blocks provide excellent bond, durability, and compatibility with RCC structures.
PVC Cover Blocks
PVC cover blocks are lightweight, corrosion-resistant, and easy to install. They are widely used in slabs and columns for faster construction. However, they should be used only where permitted, as they do not contribute to structural strength.
Stone Cover Blocks
Stone cover blocks are made from hard stones or precast pieces and are sometimes used in footing or foundation works. Their use is limited due to bonding issues with concrete.
Fiber or Plastic Cover Blocks
These are specially manufactured spacers used in modern construction for uniform cover. They are resistant to moisture and chemicals and are mainly used in slabs and precast elements.
Cover Block Sizes as per IS Code
The size of a cover block depends on the type of RCC member and exposure condition. As per IS 456:2000, recommended nominal covers are:
| RCC Member | Nominal Cover |
|---|---|
| Slab | 15–20 mm |
| Beam | 25–30 mm |
| Column | 40 mm |
| Footing | 50 mm |
| These values may increase depending on environmental exposure and fire resistance requirements. |
Cover Block for Different RCC Members
Cover Block in Slab
In slabs, cover blocks are placed below bottom reinforcement to maintain a cover of 15–20 mm. Proper spacing ensures uniform thickness and crack control.
Cover Block in Beam
In beams, cover blocks maintain side and bottom cover, usually 25–30 mm. They help in proper anchorage of reinforcement and improve durability.
Cover Block in Column
Columns require higher cover, typically 40 mm, due to exposure and fire safety considerations. Cover blocks are tied securely to reinforcement to prevent movement during concreting.
Cover Block in Footing
Footings require maximum cover, generally 50 mm or more, as they are in direct contact with soil and moisture. Concrete cover blocks with higher strength are preferred.
IS Code Provisions for Cover Block
As per IS 456:2000, Clause 26.4, the nominal cover should be provided to all reinforcement to ensure durability and fire resistance. The code specifies minimum cover based on exposure conditions such as mild, moderate, severe, and extreme environments. Proper cover block placement ensures compliance with these provisions.
Advantages of Using Cover Block
Using cover blocks ensures correct reinforcement placement, improves durability, prevents corrosion of steel, enhances fire resistance, and increases the overall service life of the structure. They also help maintain design dimensions and improve construction quality.
Problems Caused by Improper Cover Blocks
Improper or missing cover blocks can lead to exposed reinforcement, corrosion, cracking, spalling of concrete, reduced bond strength, and failure to meet IS code requirements. This results in costly repairs and reduced structural life.
Best Practices for Cover Block Placement
Cover blocks should be made of the same or higher grade concrete than the parent structure. They must be placed at proper intervals and securely tied to reinforcement. Broken bricks or stones should never be used as cover blocks. Adequate inspection before concreting ensures correct cover maintenance.
Cover Block vs Spacer
| Aspect | Cover Block | Spacer |
|---|---|---|
| Material | Concrete/PVC | Plastic/Metal |
| Purpose | Maintain concrete cover | Maintain bar spacing |
| Structural Role | High | Limited |
| Usage | RCC works | Precast and slab works |
Conclusion
Cover block in construction is a small but essential element that directly affects the strength, durability, and safety of RCC structures. Proper selection, sizing, and placement of cover blocks as per IS code ensure protection of reinforcement, compliance with standards, and long-term structural performance. Ignoring cover blocks or using improper substitutes can severely compromise construction quality, making them a non-negotiable part of good construction practice.
