Kumar IronUpdated
In this guide
Why cover matters
Concrete protects steel from air and moisture. If cover is too thin, water and carbon dioxide reach the bar, it rusts, and rust swells to several times the bar's volume. That cracks and spalls the concrete, and you see brown stains and flaking on the slab underside. Too much cover is also a problem, because it reduces the effective depth and strength of the member.
Cover thickness by member
| Member | Common cover |
|---|---|
| Slab | 20 mm |
| Beam | 25 mm |
| Column | 40 mm |
| Footing (against earth) | 50 mm |
IS 456 sets nominal cover by exposure condition and also by fire rating. The clause for columns gives 40 mm as the general minimum and footings in contact with the ground need at least 50 mm. For moderate exposure (for example, exposed to rain or coastal air in some places), the code table asks for more than the mild-exposure figures, often 30 mm for slabs and beams. Bathrooms, balconies and parapets also see more moisture. If your drawing shows a different number, follow the drawing. If you are unsure, ask your engineer rather than guess.
Types of cover blocks
- Cement mortar blocks: small cubes cast from cement and fine sand with a tie wire loop. The most common and the best match for concrete.
- Plastic cover blocks and wheel spacers: quick to fix, consistent size, good for slab bottoms and walls. Check that they are rated for the cover you need.
- Slab casting blocks and chairs: bar chairs and rods that hold the top steel at the right height in slabs and rafts.
- Concrete blocks of matching strength: for footings and heavier members.
Spacing and placement
As a practical guide, place cover blocks about 600 to 1000 mm apart in both directions under a slab, and closer on thin slabs or heavy steel. Put them under the main bars, tie them in, and walk on planks while concreting so they do not get kicked out. For top steel in cantilevers and chajjas, use chairs so the bars stay at the top and do not get pushed down while casting.
Check cover with a gauge or a block of the right thickness before the pour. Pushing steel up from below after concreting starts does not work.
Why broken bricks and stones are a bad idea
- Bricks and stones soak water from the concrete, creating a wet path straight to the bar.
- They are uneven, so the cover varies and bars sag between supports.
- They create weak spots and can crack the slab underside.
- They do not bond with the concrete like a mortar block does.
A bag of proper cover blocks costs very little compared with repairing a rusting slab. Make a habit to check them before the shuttering is closed.
Pre-pour checklist
- Bars tied firmly with binding wire at crossings.
- Cover blocks fixed at the right spacing and thickness.
- Chairs in place for top steel.
- No loose wire ends or sticks pointing into the cover.
- Cure the slab properly afterwards, see the curing guide.
Frequently asked questions
- What cover should a roof slab have?
- Commonly 20 mm, but check the drawing. Slabs exposed to weather or moisture may need more.
- How far apart should cover blocks be?
- As a practical guide about 600 to 1000 mm in both directions, closer for thin slabs or heavy steel.
- Can I use broken bricks as cover?
- No. They absorb water, give uneven cover and invite rust.
- Are plastic cover blocks acceptable?
- Yes, if sized for the required cover and stable under foot traffic. Mortar blocks are the traditional choice.