What is a Clear Cover?
Clear cover refers to the thickness of concrete that surrounds and protects the steel reinforcement in a concrete structure. It is a critical parameter in the design and construction of concrete structures, as it affects the durability, strength, and overall performance of the structure. It is also known as a concrete cover in a concrete structure.
Importance of Clear Cover in Concrete:
Clear cover plays a significant role in the durability of concrete structures. It provides protection to the reinforcement against corrosion, fire, and other environmental factors. Insufficient cover can result in corrosion of the reinforcement, which can weaken the structure and cause cracking and spalling of the concrete.
The cover also affects the strength of the concrete structure. If the cover is inadequate, it can lead to reduced bond strength between the reinforcement and the concrete, which can lead to premature failure of the structure.
Different Types of Cover:
There are different types of cover that are specified depending on the intended use and exposure of the concrete structure.
- Nominal cover: This is the minimum thickness of concrete cover specified by building codes or standards. It is intended to provide basic protection to the reinforcement and ensure adequate bond strength between the steel and concrete. However, the nominal cover is not always sufficient for structures exposed to aggressive environments or those requiring high durability.
- Effective cover: This refers to the actual distance between the reinforcement and the nearest exposed surface of the concrete. Effective cover accounts for variations in the placement of the reinforcement and the thickness of the concrete cover.
- Minimum cover: This is the smallest thickness of cover that can be used without compromising the durability and strength of the structure. The minimum cover is calculated based on the size of the reinforcement, the type of exposure, and the required design life of the structure.
- Maximum cover: This is the largest thickness of cover that can be used without causing excessive weight and reduced bond strength between the concrete and the reinforcement. Maximum cover is typically specified for structures with lightweight concrete, where excessive cover can result in a decrease in structural performance.
- Special cover: This refers to the thickness of the cover specified for special applications, such as nuclear power plants or marine structures. The special cover takes into consideration the specific exposure conditions and the required durability and strength of the structure.
Nominal Cover
From the consideration of durability, the minimum value of the nominal cover to all the normal weight aggregates is as shown in the table below as per IS 456: 2000 of Table 16.
| Exposure Condition | Minimum Cover (mm) |
|---|---|
| Mild | 20 |
| Moderate | 30 |
| Sever | 45 |
| Very Sever | 50 |
| Extreme | 75 |
Nominal cover to meet the specified period of fire resistance as per table 16.A of IS 456:2000 is a follow
| Fire Resistance | Beam | Slab | Ribs | Column | |||
|---|---|---|---|---|---|---|---|
| Simply Supported Beam | Continuous Beam | Simply Supported Slab | Continuous Slab | Simply Supported Ribs | Continuous Slab | ||
| h | mm | mm | mm | mm | mm | mm | mm |
| 0.5 | 20 | 20 | 20 | 20 | 20 | 20 | 40 |
| 1 | 20 | 20 | 20 | 20 | 20 | 20 | 40 |
| 1.5 | 20 | 20 | 25 | 20 | 35 | 20 | 40 |
| 2 | 40 | 30 | 35 | 25 | 45 | 35 | 40 |
| 3 | 60 | 40 | 45 | 35 | 55 | 45 | 40 |
| 4 | 70 | 50 | 55 | 45 | 65 | 55 | 40 |
Also, read: How to Prepare Structural Drawings?
Clear Cover for Different Structure Elements:
- Cover for Slabs: The cover for beams and slabs is generally determined based on the exposure conditions of the structure. For instance, for a concrete slab in a dry environment, a minimum clear cover of 20 mm is recommended. However, for slabs in coastal areas, the minimum cover should be at least 50 mm.
- Cover for Beams: In addition, cover requirements for beams and slabs may also be influenced by the diameter and spacing of the reinforcement bars used.
- Cover for Columns: The required clear cover for columns is determined based on the exposure conditions, the type of reinforcement used, and the structural requirements. Typically, the cover for columns should be higher than that of beams and slabs. A minimum clear cover of 40 mm is recommended for columns in normal environments, and at least 50 mm for coastal areas.
- Cover for Footings: The cover (clear) requirements for footings are determined based on the type of foundation used, the soil type, and the loads imposed on the structure. A minimum clear cover of 50 mm is recommended for footings in normal environments.
- Cover for Walls: The required clear cover for walls is influenced by the exposure conditions and the thickness of the wall. For instance, for a 100mm thick wall in a normal environment, a minimum clear cover of 20 mm is recommended. However, for a 200mm thick wall in a coastal environment, the minimum cover should be at least 40 mm.
Also, read: Structure Drawing Detailing for Reinforced Concrete Structures
Clear Cover for Different Structure Elements As per IS456: 2000.
| Structure Element | Cover (mm) |
|---|---|
| Foundation | 50-75 |
| Column | 40 |
| Beam | 20-25 |
| Slab | 15-20 |
| Wall | 15-20 |
Conclusion
The clear cover is an essential parameter in the design and construction of concrete structures. It plays a critical role in ensuring the durability, strength, and overall performance of the structure. The required clear cover is influenced by various design considerations, such as the exposure conditions, the type of reinforcement, and the structural requirements. An adequate clear cover should be provided to ensure the long-term durability and strength of the concrete structure.
Also, read: Reinforcement Detailing Requirements for Concrete Structure
