What is the minimum period before striking the formwork to RCC slabs (with props to be refixed immediately after removal of formwork)?
Correct Answer: A. 3 days
📚 Detailed Explanation: Soffit Formwork to Slabs — Minimum 3 Days
Why A (3 days) is correct: IS 456:2000 specifies that soffit formwork (bottom shuttering) to slabs can be removed after just 3 days, provided the props are refixed immediately. Slabs are thinner and more uniformly loaded than beams, and by 3 days OPC concrete has sufficient early strength to span between the re-erected props without cracking.
IS 456:2000 Formwork Stripping Times
| S.No. | Type of Formwork | OPC (Minimum Period) | RHC (Minimum Period) |
|---|---|---|---|
| 1 | Vertical formwork to columns, walls, beams | 16–24 hours | 12 hours |
| 2 | Soffit formwork to slabs (props refixed immediately) | 3 days | 2 days |
| 3 | Soffit formwork to beams (props refixed immediately) | 7 days | 5 days |
| 4a | Props to slabs: spanning up to 4.5 m | 7 days | 5 days |
| 4b | Props to slabs: spanning over 4.5 m | 14 days | 10 days |
| 5a | Props to beams and arches: spanning up to 6 m | 14 days | 10 days |
| 5b | Props to beams and arches: spanning over 6 m | 21 days | 14 days |
Key Distinction: Slab vs. Beam Soffit Formwork
| Member | Min. Stripping Time (OPC) | Reason |
|---|---|---|
| Slab soffit | 3 days | Thinner section, lower self-weight, shorter span between props |
| Beam soffit | 7 days | Heavier section, greater span, needs more strength development |
- Condition: props must be refixed immediately after removing formwork. If props are not refixed, the slab must carry its own weight unsupported — a different (longer) duration applies.
- 3 days applies to OPC; Rapid Hardening Cement allows removal after just 2 days.