It is one of the most argued questions on site: "Can we take this formwork off tomorrow?" The answer is not in the calendar, it is in the concrete. Formwork is struck when the concrete has reached the strength to carry itself and whatever will be placed on it; the number of days that have passed is only a rough guide.
What drives the decision?
The same concrete reaches the same strength in different times on two different sites. The difference comes from:
- Concrete class and cement type: the rate of strength gain changes.
- Air temperature: setting and strength gain slow down in the cold; the same number of days is not enough in winter.
- The member and its span: the side form of a column and the soffit of a wide-span slab are not the same thing.
- The load that follows: if material will be stacked on that floor right after striking, the calculation changes.
For this reason striking time is not judged by eye on site; it follows the structural design and the approval of the site engineer. TS EN 13670, which governs the execution of concrete structures, likewise ties striking to strength achieved, not to elapsed time.
How is the strength known?
In practice there are a few routes:
- Test cube results from samples taken at the pour.
- Site-cured samples, kept in the same conditions as the member rather than in the laboratory; this gives a result closer to the real situation.
- The maturity method, estimating strength from a temperature record.
Without one of these the decision rests on assumption, and that is usually where early striking comes from.
Side forms and soffits are not the same
This is the point most often confused on site. The side forms of columns and walls can come off relatively early, because the concrete only has to hold itself. But the shoring under a slab (the shoring towers, table formwork or telescopic props) must stay until the concrete can carry that span on its own.
This is why on many sites the panels are struck while the props are left in place, or are re-propped straight afterwards, to keep supporting the young slab for a while longer. Which props stay is also a decision for the design, not for the eye.
We compared the options under a slab in shoring towers or table formwork.
What early striking looks like on site
- Deflection in the slab: visible sag in the ceiling once the formwork is away.
- Spalled edges and corners: a piece of concrete coming away with the panel.
- Cracking, particularly on wide spans and cantilevers.
None of it can be undone; the day gained turns into weeks of strengthening work.
In short
- Strength decides, not time; the day count only gives an idea.
- Cold weather extends it.
- Side forms come off early, the shoring stays.
- The decision belongs to the structural design and the site engineer.
If you need help choosing or taking off the shoring system, send us the project and we will work out what suits the spans together.