The minimum stripping time of soffit formwork to beams (props to be refixed immediately after removal of formwork) is:
Correct Answer: B. 7 days
📚 Detailed Explanation: Soffit Formwork to Beams — Minimum Stripping Time
Why B (7 days) is correct: IS 456:2000 Table 1 specifies minimum periods for striking formwork. Beams carry significantly greater loads than slabs, so their soffit formwork (bottom shuttering) must stay in place longer to allow the concrete to gain sufficient strength before the weight of the beam transfers to it. With ordinary Portland cement, the minimum is 7 days.
IS 456:2000 Formwork Stripping Times (Full Table)
| 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 |
Why 7 Days for Beam Soffits?
By 7 days, OPC concrete reaches approximately 65–70% of its 28-day characteristic strength — enough to carry its own weight and the newly refixed props without excessive deflection or cracking. Removing it earlier risks:
- Premature loading before concrete gains adequate strength.
- Deflection and sagging of the beam soffit.
- Cracking at the tension zone of the beam.
- Slab soffit (lighter load) = 3 days; Beam soffit (heavier load) = 7 days.
- Props must be refixed immediately after striking — concrete is not fully cured yet.