Kumar IronUpdated
In this guide
What IS 456 says in simple words
IS 456:2000 clause 11.3 deals with stripping time. The main rule is that formwork must not be removed until the concrete has enough strength to carry its own weight and the load on it safely. In the code, concrete should have reached at least twice the stress it will carry at the time of removal.
As a guide, the code gives minimum times for ordinary Portland cement at normal temperature, about 15 degrees C and above. These are minimum stripping times, not targets. Strike later if in doubt.
| Member | Time |
|---|---|
| Vertical formwork: columns, walls, beam sides | 16 to 24 hours |
| Slab soffit formwork (props left under or refixed straight after) | 3 days |
| Props under slabs, span up to 4.5 m | 7 days |
| Props under slabs, span over 4.5 m | 14 days |
| Beam soffit formwork | 7 days |
| Props under beams, span up to 6 m | 14 days |
| Props under beams, span over 6 m | 21 days |
Cantilevers and long span beams need extra caution. Ask your engineer, and keep props under a cantilever longer than the table above.
How it works on a normal house slab
For a typical room slab up to about 4.5 m span, the flat sheet or plank shuttering can be taken off around day 3, but the props are put back or kept under it. The props carry the slab until around day 7, as per the table. Because the load is shared, many masons keep the props on a typical 4 inch or 5 inch slab for longer, about 10 to 14 days, as a safe margin.
Beam sides come off in about a day. Beam bottoms and their props stay much longer, because the beam carries the slab load.
What changes the timing
- Cement type: the table is for OPC. PPC and other blended cements gain strength more slowly at early age. IS 456 says the time should be suitably modified for other cements. Many practice guides add around 50 percent to the times, but your engineer or a cube test should decide.
- Cold weather: below about 15 degrees C hardening slows down. In west UP winter, from December to January, wait longer. Some guides double the times below 15 degrees C.
- Hot dry weather: concrete may harden fast but loses water. Curing matters more than the stripping date. See our concrete curing guide.
- Load on the slab: do not stack bricks, sand or cement bags on a slab just after removing shuttering. Construction loads can be higher than the slab design load.
- Rain or poor curing: weaker concrete means stripping later.
Why early removal cracks slabs
Young concrete has not reached its design strength, and its stiffness is low. When props are removed, the slab bends under its own weight. Hairline cracks appear in the bottom of the slab at mid span, near beams, and over the supports at the top.
A slab that sags in the first month often stays sagged. Deflection from early loading does not fully come back. It also puts extra stress on the walls built above, and wall cracks follow.
A safe routine on site
- Note the casting date on the wall in chalk, and the cement brand used.
- Cure by ponding from the day after casting. Curing is not stripping, it carries on after the shuttering is off.
- Remove side forms first, then slab soffit sheets, leaving props at the span centre and near supports. Do not remove all props at once.
- Take the props out from the middle outward, slowly, and watch for cracking or sag.
- Do not remove props if the weather has been cold or the mix was lean. Wait a few more days.
- For beams and long spans, ask for a cube test report if you need to strip early.
Frequently asked questions
- How many days before I remove the slab shuttering?
- As per IS 456, slab soffit formwork can go in about 3 days if props are left under the slab. The props themselves stay until around 7 days for spans up to 4.5 m, and 14 days for longer ones.
- Does PPC cement need more days?
- Usually yes. PPC gains early strength more slowly than OPC, so add margin. The code says to modify the times for other cements, and many practice guides add about half again. Check with your engineer.
- Can I remove props earlier if the slab looks hard?
- No. Surface hardness does not tell you the strength inside. Use the minimum times, or a cube test if you need to strip early.
- How long should I keep curing after shuttering is off?
- Keep the slab wet through the curing period. See our guide on concrete curing days for the details.