A slab is struck twice, not once, and that is where sites go wrong. Under IS 456:2000 the shuttering removal time for slab soffit plates is three days, with the props refixed immediately as the plates come out. The props themselves stay far longer: seven days for a slab spanning up to 4.5 m and 14 days for a slab spanning over 4.5 m. The familiar “14 days for a slab” is the prop figure, not the plate figure, and treating the whole assembly as one item is what puts a young slab on the floor.

The IS 456 shuttering removal time for slab and beam work

The clause is 11.3.1 of IS 456:2000, the Bureau of Indian Standards code of practice for plain and reinforced concrete. It lists the periods below as deemed to satisfy the general rule in 11.3.

What is being struck Minimum period
Vertical formwork to columns, walls and beams 16 to 24 hours
Soffit formwork to slabs (props refixed immediately) 3 days
Soffit formwork to beams (props refixed immediately) 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

Two rows do most of the work on an ordinary building. The slab soffit at three days, and the prop under it at 7 or 14 days depending on whether the span crosses 4.5 m. Measure the span before you argue about the date. If you hire props out rather than own them for your own jobs, these two rows also decide how long your stock is tied up on each slab, which is the arithmetic behind a shuttering business.

The full text, including clause 11.3 and the curing clauses beside it, is in the copy of IS 456:2000 published under the Right to Information.

Soffit and props: two items with two clocks

The soffit is the formwork that forms the underside of a slab or beam: the plates, plywood sheets or steel panels the concrete is poured onto. The props are the vertical supports, adjustable steel props or timber ballies, that hold the soffit and the slab up. Clause 11.3.1 times them separately because they do different jobs. The soffit gives the concrete its shape and is finished once the surface has set hard enough to stand. The props carry the weight, and the slab cannot carry itself until it has gained real strength.

For a slab and beam poured on 1 November, with ordinary Portland cement, nights above 15 degrees Celsius and proper curing, the table above gives these earliest striking dates. Each one is counted in calendar days from the pour.

What comes out Minimum period Earliest date
Side forms to columns, walls and beams 16 to 24 hours 2 November
Slab soffit plates, props refixed at once 3 days 4 November
Beam soffit, props refixed at once 7 days 8 November
Slab props, span up to 4.5 m 7 days 8 November
Slab props, span over 4.5 m 14 days 15 November
Beam props, span up to 6 m 14 days 15 November
Beam props, span over 6 m 21 days 22 November

On PPC, in a cold month or with patchy curing, these are the earliest dates and not the planned ones. They are also the hire period: a prop under a 5 m slab is tied up for 14 days whatever happens to the plates above it, which is why prop hire is priced separately from sheeting in the shuttering rate per square metre.

Striking the soffit is also the first look at the underside of the slab. Voids and exposed stone that show when the plates come off are honeycombing, and the causes, grades and repair are covered in honeycombing in concrete.

Refixed immediately means immediately

The phrase in brackets against the slab and beam rows is not a footnote. The code says props are to be refixed immediately after removal of the formwork, which describes a specific operation: the plates and sheets come out from between the props, panel by panel, and every prop goes straight back under the slab it was holding.

What it does not describe is the practice most sites actually follow, which is to drop the whole table of props and plates together on day three, clear the floor, and re-prop later if someone asks. At three days a slab has nothing like the strength it needs to span unaided. It is being asked to carry its own weight, plus whatever is stacked on it, on concrete that is roughly a third of the way to its design strength.

Clause 11.3.2 sets the standard for what stays. It gives no count and no spacing, deliberately. It says the number of props left under, their sizes and their disposition shall be such as to be able to safely carry the full dead load of the slab, beam or arch, together with any live load likely to occur during curing or further construction. That last phrase is the one to read twice on a site where the next floor’s steel and shuttering are stacked on the slab you just struck.

The rule under the table is a strength rule

Clause 11.3 itself sets no days at all. It says forms shall not be released until the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at the time of removal of formwork. The days in 11.3.1 are a convenience, a deemed-to-satisfy default for normal circumstances.

The code is also specific about which strength counts. It says the strength referred to shall be that of concrete using the same cement, aggregates and admixture, in the same proportions, and cured under conditions of temperature and moisture similar to those existing on the work. A cube that has sat in a curing tank at controlled temperature does not qualify. If your case for striking early rests on cube results, the cubes have to have lived the same week your slab did.

This matters commercially, because formwork is usually the constraint on cycle time. Shuttering sets are expensive, and every extra day they stay under one floor is a day they are not earning under the next. That pressure is real and it is exactly why the code put a strength rule above the calendar rather than below it. If you are pricing the item, the striking cycle drives the number of sets you need to buy or hire, and the rate analysis for a non-scheduled item should carry that hire period honestly.

Where the table stops applying

Clause 11.3.1 opens with three conditions, and if your site fails any of them the numbers in the table are not yours to use. Ambient temperature must not fall below 15 degrees Celsius. The cement must be ordinary Portland cement. Adequate curing must be being done.

The last sentence of the clause says that for other cements and lower temperature, the stripping time recommended above may be suitably modified. Read in context that means extended, and it covers a large share of Indian work. A north Indian winter pour in December or January falls below 15 degrees on most nights. More to the point, most bags arriving on site today are PPC or slag cement rather than OPC, and those gain strength more slowly at early ages. On a PPC slab, the three-day and 14-day figures are floors under conditions you are not actually meeting.

The code does point to a method for the cold-weather case. Clause 14.1 says that during hot or cold weather the concreting should be done as per the procedure set out in IS 7861, Part 1 for hot weather and Part 2 for cold, and those are the documents to read before you fix a winter striking date.

Adequate curing is the third condition and it is not a formality either. The striking table assumes the slab has been kept wet, because concrete that dried out is concrete that stopped gaining strength. The periods and methods the code requires are set out in full for the curing period for a slab, and the two clauses run at the same time rather than one after the other: the curing clock starts at placing, so by the time the plates come off at three days, three of your seven or 10 curing days are already spent.

One shape gets the opposite treatment. Clause 11.3.3 says that where the element has re-entrant angles, the formwork shall be removed as soon as possible after the concrete has set, to avoid shrinkage cracking caused by the restraint the form itself imposes. That is a case for striking early, not late.

Before you strike, tell someone

There is a procedural clause that rarely gets quoted and is worth knowing if you work for a government department or a large contractor. Clause 13.6.1 says that before any important operation, such as concreting or stripping of the formwork, adequate notice shall be given to the construction supervisor.

On a departmental job that notice is what puts the striking date into the record. If a slab is struck without it and something goes wrong afterwards, the absence of notice becomes part of the finding. It costs nothing to write the date and the span into the site order book before the props come out, and it is the cheapest protection available on a clause that is otherwise argued from memory.

For pavement and road slabs the governing specification is the road contract rather than IS 456 alone, and the sequence differs; the case is covered in CC road construction.

When can you strike plates and props on a slab?

Strike a slab twice. Plates at three days with every prop going straight back under; props at seven days up to a 4.5 m span and 14 days beyond it. Check the three conditions before you use those numbers at all, and if you are on PPC, in a cold month, or curing badly, extend them. If you want to go earlier than the table, the route is site-cured cube results and the engineer-in-charge’s agreement, not a busy programme.

Plan the pour that comes before the strike. Compare concrete equipment models and prices or a concrete pump and connect with a dealer for the cycle your programme actually needs.

Code references are to IS 456:2000 as published; standards, specifications and site conditions change, and your own contract specification may be stricter than the code minimum. Confirm current requirements with the engineer-in-charge, the department or the official source before deciding. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.