When can I take the formwork off?

Edge forms to a ground-bearing slab come off once the concrete can hold its own shape and arris. Anything that supports load is a different question with a specified answer.

Difficulty: intermediate
Time: 15 min
Read: 2 min
Safety: warning

Short answer

For simple edge shuttering to a slab or path on the ground, the forms can usually come away once the concrete is hard enough that the edge does not slump or crumble when they do — often the following day, and later in cold weather. Curing continues afterwards, so protect the concrete once the forms are off.

The important distinction is between formwork that shapes concrete and formwork that holds concrete up. Edge boards around a path are the first kind: taking them off early spoils an edge. Props under a beam are the second kind, and taking those off early can drop the beam. Only the first kind is a judgement call.

Safety

Striking formwork early on anything that supports a load — a lintel, a beam, a suspended slab — can cause it to fail. Those are engineered elements and their striking times are specified by the designer, not estimated from a guide. This guide covers unsupported edge formwork to ground-bearing concrete only.

Step by step

Establish which kind of formwork you have.Does anything rest on it? Edge boards to a ground-bearing slab, a path or a base support nothing but the wet concrete's own pressure. Props, soffit forms and anything under a span are structural and outside this guide.
Understand that set and cured are different.Concrete stiffens within hours and continues gaining strength for weeks. Striking edge forms depends on the first; the strength for use depends on the second. GuideHQ covers when concrete can be walked or driven on separately.
Test the edge before removing anything.Ease one board away at a corner and look. If the arris holds its shape cleanly, carry on. If it slumps, crumbles or is still soft, put it back and wait.
Allow longer in cold weather.Strength gain slows substantially as temperature falls and nearly stops near freezing. Concrete poured in cold conditions may need considerably longer before the edge will hold.
Strike gently and work along the form.Release the fixings, then ease the board away parallel to the face rather than levering it off. Green concrete edges chip very easily.
Keep curing after the forms come off.Removing the forms exposes new faces to drying. Cover with plastic or a curing membrane, and keep it damp in warm or windy weather — that is when surface problems like dusting begin.
Leave anything structural to the specification.Striking times for beams, lintels and suspended slabs are set by the designer against strength tests, not by feel. Do not guess at those.

Tips

Leaving edge forms on longer than necessary does little harm to the concrete but can make them harder to release. Clean them the same day they come off.
If you need a crisp arris, chamfer strips in the form corners protect the edge far better than a square corner does.

Common mistakes

Treating a published time as a rule regardless of conditions. — Temperature changes strength gain dramatically. The same mix in the same forms behaves very differently in July and January.
Stopping curing once the forms are off. — The newly exposed faces are the ones most at risk of drying too fast, which causes surface dusting and crazing.

Questions people ask

Can I walk on it once the forms are off?

Not necessarily — they are separate questions. GuideHQ has a guide on how long before new concrete can be walked or driven on.

What if the edge crumbles when I strike it?

It was too early, or the mix was too wet or poorly compacted at the edge. Repair it with a suitable mortar once cured, and leave the next one longer.