What is a Cover Block?
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A cover block is a small precast concrete or plastic spacer tied to the reinforcement to hold the steel a fixed distance off the formwork, so the concrete forms the specified clear cover around every bar. Without cover blocks the steel drifts against the shutter during casting, ends up at the surface, and corrodes — so this tiny, cheap piece protects the whole member's durability.
Why the cover matters
Clear cover is the layer of concrete between the outermost steel and the surface. It is the reinforcement's protection against carbonation, chloride and fire. Because carbonation advances roughly with the square root of time, cover is the single biggest lever on how long the steel stays un-corroded — losing 10 mm of it silently shortens the structure's life. Cover blocks are what guarantee it is actually there.
Cover block sizes
The block thickness equals the required clear cover for that member (see reinforcement details):
| Member | Clear cover / block size |
|---|---|
| Slab | 20 mm |
| Beam | 25 mm |
| Column | 40 mm |
| Footing | 50 mm |
These rise with exposure severity per IS 456. Blocks are tied to the steel on all faces at about 1 m spacing so the cage cannot sag or drift to the form during the pour.
Cover block vs chair
They are both spacers, but for different directions:
- Cover block: holds steel off the side/bottom form to keep the horizontal/bottom cover.
- Chair (bar spacer): holds the top mat up from the bottom mat at the correct depth.
Skip either and the steel ends up where it does no good — top steel that sags to mid-depth is the classic reason a slab cracks over its supports.
Frequently asked questions
What is a cover block in RCC? A small precast concrete or plastic spacer tied to the reinforcement to keep it off the formwork and maintain the specified clear cover.
Why are cover blocks used? To guarantee the clear cover of concrete over the steel, which protects it from corrosion and fire — without them the steel drifts to the surface during casting.
What size cover block should I use? Match the block thickness to the required clear cover — 20 mm for slabs, 25 mm for beams, 40 mm for columns, 50 mm for footings, more in severe exposure.
What is the difference between a cover block and a chair? A cover block keeps steel off the side or bottom form; a chair holds the top mat up from the bottom mat at the correct depth.
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CivilSite Editorial Team✓ Engineer reviewed
Written and reviewed by practising civil engineers with 10+ years of Indian residential construction experience.