Short Answer:

Cover in reinforcement means the distance between the outer surface of concrete and the nearest steel bar inside. It protects the reinforcement bars from weather, moisture, fire, and corrosion. This layer of concrete is very important for the safety and durability of the structure.

Cover also helps in proper bonding between steel and concrete. It ensures the bars do not get exposed during construction or over time. If the cover is too little, steel can rust; if it’s too much, structural strength may reduce. Therefore, providing the right cover is necessary in all concrete works.

Detailed Explanation:

Cover in Reinforcement and Its Importance

In reinforced concrete construction, steel bars are placed inside concrete to resist tension forces, while concrete resists compression. To protect these steel bars from environmental effects and ensure proper performance, they must be embedded at a certain distance from the concrete’s outer surface. This protective distance is called cover or concrete cover.

There are two main types of cover:

• Clear Cover: The distance between the exposed surface of concrete and the nearest steel bar.
• Nominal Cover: The cover provided according to design codes, considering durability and fire resistance.

Importance of Providing Cover

1. Protection from Corrosion:
   Steel can rust when exposed to air or moisture. Rust causes expansion, which leads to cracks in concrete and weakens the structure. Proper cover keeps moisture away from the bars.
2. Fire Resistance:
   In case of a fire, the concrete cover delays heat reaching the steel bars. A thicker cover provides more time before the bars heat up, maintaining structural stability during emergencies.
3. Durability:
   Cover increases the life of concrete structures by shielding the reinforcement from harsh weather, chemicals, or salty water in coastal areas.
4. Bond Strength:
   Adequate cover ensures strong bonding between concrete and steel, which is needed for the structure to carry loads as designed.
5. Preventing Exposure:
   During concrete placing and vibration, if cover is insufficient, bars may become visible on the surface. This leads to early corrosion and poor appearance.

Cover Requirements

Standard codes like IS 456:2000 provide minimum cover values for different structural elements. Some typical values include:

• 20 mm for slabs
• 25 mm for beams
• 40 mm for columns
• 50 mm or more in footings or structures exposed to severe weather

These values can change based on exposure conditions (mild, moderate, severe), concrete type, bar size, and construction type.

How Cover Is Maintained

To ensure proper cover during construction:

• Cover blocks made of cement mortar or plastic are used under and around bars.
• These blocks hold the reinforcement in place during shuttering and concreting.
• Supervision is essential to check that bars are not displaced while pouring concrete.

Consequences of Inadequate Cover

• Too little cover: Causes corrosion, weakens the structure, reduces fire resistance, and leads to cracks.
• Too much cover: Reduces the effective depth of the section, affects structural capacity, and causes problems in bonding.

Conclusion:

Cover in reinforcement is a vital part of concrete construction. It protects steel bars from corrosion, fire, and environmental damage while ensuring bond strength and durability. Providing the right amount of cover, as per design codes, is necessary for a safe and long-lasting structure. It is a small but crucial factor in maintaining the integrity and performance of reinforced concrete.