Cover blocks, also known as spacers or chairs, are small structural elements placed between reinforcement bars and formwork to maintain the specified concrete cover. They ensure that reinforcement is properly positioned at the designed distance from the concrete surface, protecting steel from environmental exposure and ensuring adequate fire resistance and structural integrity.

Proper concrete cover is essential for protecting reinforcement from corrosion, providing fire resistance, and ensuring proper bond between steel and concrete. Cover blocks help achieve consistent cover throughout the structure, preventing issues like spalling, rust staining, and reduced structural capacity. They are critical quality control elements in reinforced concrete construction.

Cover blocks should be placed at regular intervals along reinforcement bars, typically at spacings of 500-1000mm (20-40 inches) depending on the structural element and bar size. For slabs, blocks are placed in a grid pattern to support the reinforcement mesh. In beams and columns, they should be positioned to maintain cover on all exposed faces.

The spacing between cover blocks should be close enough to prevent reinforcement from sagging or moving during concrete placement. For horizontal slabs, closer spacing may be needed if workers will walk on the reinforcement. Ensure blocks are securely tied or attached to the reinforcement to prevent displacement during concreting. Always verify that the block thickness matches or slightly exceeds the specified cover requirement.

Use cover blocks of the same concrete grade as the structural element or higher to prevent weak points in the structure. Plastic cover blocks offer advantages in corrosive environments as they don't rust or cause staining. Ensure blocks are clean and properly positioned before concrete placement - they should not be moved or adjusted once concreting begins.

For critical structures or harsh exposure conditions, increase the number of cover blocks beyond minimum requirements to ensure reinforcement stays in position. Document the type, size, and spacing of cover blocks used for quality assurance records. In areas with multiple layers of reinforcement, use different sized blocks for each layer to maintain proper spacing between steel layers as well as from formwork surfaces.