How we supply it: Centering and shuttering are arranged through a partner centering contractor who quotes, brings the material and labour, and bills the work, and pays us a referral commission. We time the 20 mm jalli and M-Sand, or the RMC booking, to the day the centering is passed.
In one line: Centering is the temporary platform of sheets, joists and props that holds up a slab or beam until the concrete can carry itself; shuttering is the formwork that shapes the sides of columns, beams and walls.
Stripping times from IS 456
IS 456 notes these periods suit OPC at normal temperatures. With PPC or PSC, or in cool, wet weather, your engineer will extend them.
| Formwork | Minimum period |
|---|---|
| Vertical forms: columns, walls, beam sides | 16–24 hours |
| Slab soffit sheets (props re-fixed at once) | 3 days |
| Beam soffit forms (props re-fixed at once) | 7 days |
| Props to slabs spanning up to 4.5 m | 7 days |
| Props to slabs spanning over 4.5 m | 14 days |
| Props to beams spanning up to 6 m | 14 days |
| Props to beams spanning over 6 m | 21 days |
Centering vs shuttering
Centering is the temporary platform of sheets, joists and props that holds up a slab or beam until the concrete can carry itself; shuttering is the formwork that shapes the sides of columns, beams and walls.
A wet slab weighs about 2.4 tonnes per cubic metre. A 125 mm slab puts about 300 kg on every square metre of centering before anyone walks on it.
Formwork materials
| Material | Typical size | Use |
|---|---|---|
| Steel centering sheets | 3 × 2 ft (about 900 × 600 mm) | Slab soffits |
| Adjustable steel props | Telescopic, pinned | Supporting slab joists and beams |
| Shuttering plywood (IS 4990) | 8 × 4 ft, 12 or 18 mm film-faced | Beam sides, smooth soffits |
| Column boxes | Steel or ply, by column size | Columns |
| Timber or casuarina poles | Varies | Traditional props, now less common |
Hire through a contractor or buy?
Centering contractors usually supply sheets, props and labour together and quote per sq ft of slab. Buying your own only pays back with repeated slabs.
Sheets and props for 1,000 sq ft
Typical estimate: ; the contractor's layout and the slab's beams change the count.
- 3 × 2 ft sheets (6 sq ft each): about 167 sheets for 1,000 sq ft, plus edge pieces
- Props at roughly 0.9–1.2 m centres both ways: about 65–115 props
- Concrete for the same slab at 125 mm: about 11.6 m³
Checks before the pour
- Props stand plumb on base plates or planks, never on loose fill; on upper floors they sit directly above the props below.
- Sheets are clean, coated with release agent and tightly joined so slurry cannot leak.
- Levels are checked against the slab thickness at several points.
- Edge shuttering, sunken portions, ducts and openings are fixed.
- Cover blocks and chairs are in place, and walkway planks keep the crew off the mesh.
Handling formwork between slabs
- Clean and oil sheets after each use and stack them flat.
- Keep props and clamps sorted; rusted threads jam.
- Never stack materials on a freshly stripped slab that is still on re-props.
Formwork mistakes
- Pulling props early to reuse them on the next slab.
- Props standing on loose fill that sinks when wet.
- Bricks or sand packed under props instead of proper base plates.
- Ignoring the slower strength gain of PPC and PSC in cool weather.
- Not re-propping after the soffit sheets come off.