Clause 11 specifies requirements for formwork design, construction, and removal. Table 11 gives the minimum period before striking formwork, which depends on the type of member, span, and ambient temperature. Formwork removal must not compromise strength or serviceability.
Key Requirements
•Formwork must be designed to carry all loads during construction including dead weight of forms, live load of workers, and impact
•Stripping shall not commence until concrete has reached sufficient strength — minimum periods per Table 11
•For members less than 10°C ambient, stripping times are doubled
•Props must not be removed until the member can bear its self-weight and any construction loads
•When using rapid-hardening cement, stripping times may be reduced — subject to engineer's approval and cube tests
Reference Tables
Table 11 — Minimum Period Before Striking Formwork (Clause 11.3)
Type of Formwork
Minimum Period
Vertical formwork to columns, walls, beams (sides)
16–24 hours
Soffit formwork to slabs (props left under)
3 days
Soffit formwork to beams (props left under)
7 days
Props to slabs — span up to 4.5 m
7 days
Props to slabs — span over 4.5 m
14 days
Props to beams and arches — span up to 6 m
14 days
Props to beams and arches — span over 6 m
21 days
For ambient temperature below 10°C, these periods should be doubled. Where OPC is used, periods are for OPC 43/53 grade. With PPC or PSC, extend by 50% unless cube tests confirm adequate strength.
Practical Notes
✓In hot Indian summers (35°C+), concrete gains strength faster — but IS 456 Table 11 periods still apply as minimums. Do not reduce without cube test evidence.
✓Re-propping is a practical alternative: strip soffit forms at 3 days (slabs) or 7 days (beams) for reuse, but leave props at prescribed intervals. This is standard practice on multi-storey sites.
✓Side forms to columns and walls can be removed at 16 hours in normal conditions — this allows early finishing and repair of honeycombing.
Common Mistakes
⚠Removing slab props at 7 days when span exceeds 4.5 m — the code requires 14 days for longer spans.
⚠Not extending stripping times in cold weather (below 10°C) — periods should be doubled.
⚠Using PPC or PSC (blended cements) without extending the stripping time — these gain early strength slower than OPC.