Curing and formwork stripping are where schedules quietly steal strength from a building. Both are cheap to do right and very expensive to get wrong.
Minimum Curing (IS 456, Clause 13.5.1)
| Condition | Keep exposed surfaces continuously moist for at least |
|---|---|
| Ordinary Portland Cement (OPC) | 7 days |
| Mineral admixtures or blended cement (e.g. PPC) | 10 days (extending to 14 days is recommended) |
| Concrete exposed to dry and hot weather | 10 days |
Most residential work in Kolkata uses PPC, so 10 days is the practical minimum on most of our sites, not the 7 days many crews assume.
Formwork Removal (IS 456, Clause 11.3.1)
Minimum times for OPC concrete at normal temperatures (above about 15 °C), where props are left in place as stated:
| Member | Minimum period before striking |
|---|---|
| Vertical formwork to columns, walls, beams | 16 – 24 hours |
| Soffit formwork to slabs (props left under) | 3 days |
| Soffit formwork to beams (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 periods can change for other cement types, admixtures or cold weather. The engineer’s instruction on site takes priority, and cube results can justify early striking.
What We Check on Site
- Curing starts as soon as the surface can take it, not the next morning
- Ponding or wet hessian on slabs, and wet hessian or curing compound on columns
- A curing log per pour with the date started and the date completed
- Striking dates recorded on the pour card and approved before props come out
- No stacking of bricks or materials on young slabs
Reference
- IS 456:2000, clauses 11.3 (Stripping time) and 13.5 (Curing)
Summary for site use. Always work from the current edition of the standard and your project specification. Where they differ, the structural engineer’s specification governs.