17/06/2026
A cover block is a small precast concrete or durable non-corrosive spacer placed between the reinforcement and the formwork to maintain the specified clear cover during concreting. It ensures that the reinforcement remains at the correct position and is fully embedded within the concrete.
The cover block should have strength equal to or greater than the surrounding concrete and should not adversely affect the durability of the structure.
Functions of Cover Blocks
1. Protection Against Corrosion
The primary function of a cover block is to provide sufficient concrete cover around reinforcing steel, preventing moisture, oxygen, and aggressive chemicals from reaching the reinforcement and causing corrosion.
2. Fire Resistance
Concrete cover acts as thermal insulation, protecting steel reinforcement from high temperatures during fire and allowing the structure to maintain its load-carrying capacity for a longer duration.
3. Durability
Adequate cover improves the service life of RCC structures by protecting reinforcement from carbonation, chloride attack, sulphate attack, and other environmental effects.
4. Proper Bond Development
Maintaining the specified cover ensures effective bond between concrete and steel, enabling proper transfer of stresses.
5. Correct Reinforcement Position
Cover blocks prevent displacement of reinforcement during concreting and vibration, ensuring the reinforcement remains at the designed location.
Recommended Clear Cover (Typical Values)
According to Indian Standards, the nominal cover depends on the exposure condition, structural member, and fire resistance requirements. Typical values are:
Slabs: 20–25 mm
Beams: 25–40 mm
Columns: 40 mm (minimum)
Footings: 50–75 mm, depending on whether cast against soil or on PCC
Relevant Indian Codes
IS 456:2000 – Plain and Reinforced Concrete – Code of Practice.
*IS 13920:2016 – Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces
SP 34:1987 – Handbook on Concrete Reinforcement and Detailing
