Formwork Removal Time as per IS 456 for Slab, Beam and Column

As per IS 456:2000, with ordinary Portland cement (OPC) and normal weather, formwork can be removed after: 16–24 hours for vertical faces of columns, walls and beams; 3 days for slab soffits and 7 days for beam soffits (with props refixed immediately); 7 days for props under slabs spanning up to 4.5 m and 14 days above 4.5 m; and 14 days for props under beams spanning up to 6 m and 21 days above 6 m. These periods are for OPC with adequate curing in normal (not cold) weather. Cold weather, PPC/PSC cement and heavy construction loads all call for longer periods.

The basic rule behind the table

IS 456 states the principle first. Forms should not be released until the concrete has reached a strength of at least twice the stress to which it will be subjected at the time of removal. The table of days is only a guide for normal conditions. The real test is strength, which is why good sites cast extra cubes, cure them next to the structure, and test them before de-propping critical members.

IS 456 stripping times (OPC, normal temperature)

IS 456 adds that for other cements and lower temperatures these times may be suitably modified. It also asks that the props left in place be enough to carry the full dead load of the member plus any live load expected during curing or further construction.

Why vertical forms come off early

Column and beam side forms carry almost no load once the concrete has set. Removing them after a day lets curing start on those faces and frees the shuttering for reuse. IS 456 also advises removing formwork with re-entrant corners as soon as possible after setting, so that the restraint does not cause shrinkage cracks. Strip carefully: green concrete edges chip easily.

Effect of temperature

Cement hydration slows down as it gets colder. A common way to estimate the effect is the maturity (Nurse–Saul) method, which assumes strength depends on time × (temperature + 10 °C). Taking 20 °C as the reference, the approximate multiplier on stripping time is:

These multipliers are estimates, not code values, and real behaviour depends on cement, mix and curing. Use them to plan, then confirm with cube tests. In hot weather do not strip earlier than the IS 456 periods. Early heat helps strength, but the concrete also dries out faster, so curing becomes more important, not less.

Effect of cement type

The IS 456 periods assume OPC. Fly-ash based PPC and slag-based PSC gain strength more slowly in the first week. The minimum mortar-cube strengths in the cement standards show this clearly:

At 7 days, PPC's minimum is 22/33 = 67% of OPC 43's. Most site engineers therefore extend slab and beam prop times when using PPC or PSC, often by a few days. There is no single code figure for this, so follow the designer or base the decision on cube results. Read more in OPC vs PPC vs PSC cement.

Worked example: de-shuttering schedule

Job: first-floor roof slab of a house. Slab panels span 4.2 m and the main beam spans 5.4 m. OPC 43, M20. The slab and beams are cast on Monday 2 November, with average temperature around 25 °C.

1. Beam sides and column forms: after 16–24 hours, so Tuesday 3 November.
2. Slab soffit sheets: 3 days, so Thursday 5 November, but only if props are refixed at once. With ordinary plywood and props, wait for step 4.
3. Beam soffit: 7 days, so Monday 9 November, with props refixed.
4. Slab props: span 4.2 m is not more than 4.5 m, so 7 days, giving Monday 9 November.
5. Beam props: span 5.4 m is not more than 6 m, so 14 days, giving Monday 16 November.

Same job in winter (north India), cast on Monday 4 January with an average temperature of about 10 °C. Using the 1.5 multiplier:

1. Vertical forms: 24 h × 1.5 = 36 h, so take them off on Wednesday 6 January (2 days).
2. Slab soffit with re-propping: 3 × 1.5 = 4.5, so 5 days, giving Saturday 9 January.
3. Slab props: 7 × 1.5 = 10.5, so 11 days, giving Friday 15 January.
4. Beam props: 14 × 1.5 = 21 days, giving Monday 25 January.

If the slab spanned 5 m instead (more than 4.5 m), the props would stay 14 days in normal weather and about 21 days in this winter case.

Cantilevers and multi-storey work

• Cantilevers (chajjas, balconies, canopies) are not listed separately in the table. They have no back-up path if they crack, so most engineers keep their props much longer, often until the concrete has reached most of its design strength. Follow the designer's instruction.
• Floors above: wet concrete and workers on the next slab load the slab below before it is mature. Do not remove all props from the lower floor while the upper floor is being cast. Keep it back-propped as directed by the engineer.
• Heavy loads: do not stack bricks, steel or cement bags on a young slab. Its props are sized for self-weight and light construction load only.

Safety tips for stripping

• Release props gradually by loosening the screw jacks, not by knocking them out.
• For slabs and beams, start de-propping at mid-span and work towards the supports. For cantilevers, start at the free end and work back to the support. This lets the member pick up its load gradually, the way it was designed to.
• Do not lever the formwork off the concrete with crowbars. Use release agent before casting so the forms come away cleanly.
• Watch for sagging or new cracks while removing props. If you see either, stop and re-prop.
• Continue curing immediately on newly exposed faces. Removing forms early only helps if curing starts at once.
• Keep people out of the area below during stripping, and stack removed shuttering safely.

Common mistakes

• Using the 3-day slab soffit figure as a "props out in 3 days" rule.
• Counting the 4.5 m and 6 m spans loosely. A 4.8 m slab needs 14 days, not 7.
• Applying OPC times to PPC concrete in December.
• Stripping on schedule without checking cubes on large-span or heavily loaded members.

Plan shuttering area, concrete and steel for the whole building with our house estimator.