Formwork is expensive to hire and slow to cycle, so there is always pressure to strip it early and move it to the next pour. The time limits exist because fresh concrete looks hard long before it can carry load. Strip too soon and a slab can sag permanently or crack across its underside.
IS 456 Stripping Times (OPC, Normal Weather)
| Formwork | Minimum time |
|---|---|
| Vertical faces of columns, walls, beam sides | 16 – 24 hours |
| Slab soffit (props left under) | 3 days |
| Beam soffit (props left under) | 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 and arches spanning up to 6 m | 14 days |
| Props to beams and arches spanning over 6 m | 21 days |
These assume Ordinary Portland Cement and normal temperatures (around 15°C and above). The props are what carry the load; the soffit sheets can come out earlier only if the props stay in place.
Why Do Props Stay Longer Than Sheets?
The plywood or steel sheets under a slab only shape it. The props under them hold its weight. Once the concrete has set, the sheets can be released and reused while the props, or re-props, keep supporting the slab until it is strong enough to span on its own. Removing the props is the moment the slab starts carrying itself — and anything stacked on it.
How Do PPC and Weather Change the Times?
| Situation | Effect |
|---|---|
| PPC or slag cement | Slower early strength; extend stripping times |
| Cold weather (below about 15°C) | Slower strength gain; extend |
| Hot weather | Faster early strength, but curing must continue |
| High early strength cement or accelerator | May shorten, on the engineer's instruction |
Hyderabad's climate is warm for most of the year, so the OPC figures generally apply; PPC — the most common cement for houses here — warrants extra days, particularly for the props.
What Else Affects When You Can Strip?
- Loads on the slab. Stacking bricks, blocks or cement on a newly de-propped slab can exceed what it can carry at that age. Keep heavy stacks off young slabs.
- The next floor. When casting the floor above, the slab below carries the fresh concrete and formwork. Keep props or re-props in place under it as the engineer directs.
- Cantilevers. Sunshades and balconies should keep their props longer; they have no support at the free end.
- Cube results. Where available, 7-day cube strengths are a better guide than the calendar.
How Should Formwork Be Removed?
- Remove vertical forms first, carefully, without levering against the concrete.
- Release slab and beam soffit sheets while leaving props in place.
- Remove props gradually and evenly, starting from mid-span, not all at once from one end.
- Start curing exposed faces immediately after each removal.
The Short Version
Sides come off in a day; soffits in days; props in one to three weeks depending on span and cement. The props are what matter. With PPC or in cold weather, wait longer, keep heavy material off young slabs, and let the engineer decide anything faster.
