How do I stop concrete sticking to the formwork?

Concrete sticks because the form face is absorbent or rough. A release agent and a smooth, sound face are what let the formwork come off cleanly.

Difficulty: intermediate
Time: 30 min
Read: 3 min
Safety: caution

Short answer

Apply a release agent to every face the concrete touches, in a thin even film. Use smooth, sound boards or faced ply rather than rough sawn timber, and take the forms off at the right time — too early damages green concrete, and much too late makes them harder to remove.

Formwork that will not come off usually damages the very edge it was there to form, which is the visible part. The two things that decide it are the face of the form and what is on it, and both are settled before any concrete is poured. GuideHQ covers building formwork separately; this is about getting it off again.

Safety

Wet concrete is strongly alkaline and burns skin on prolonged contact. Wear gloves, boots and eye protection, keep it off bare skin, and wash any splashes off promptly. Release agents are chemicals — use them with ventilation and keep them off the reinforcement and away from where you will later be bonding anything.

Step by step

Choose a form face that is smooth and non-absorbent.Faced or film-coated plywood releases far better than rough sawn timber. Bare absorbent timber draws water out of the concrete at the face and grips it.
Apply a proper release agent, not used engine oil.Purpose-made release agents are designed to break the bond without staining the concrete or contaminating it. Waste oil stains, is an environmental problem and can affect any later coating.
Apply it thinly and evenly.A thin film everywhere is what you want. Puddles and runs cause surface blemishes and can affect the concrete's surface hardness where they pool.
Keep release agent off the reinforcement.Anything that stops concrete bonding to the form also stops it bonding to steel. Mask or shield reinforcement while applying it.
Seal the joints between boards.Grout escaping through gaps forms ridges that lock the form in place and leave honeycombed edges. Tape or seal the joints before pouring.
Do not over-fix the formwork.Use enough fixings to hold the shape against the pressure and no more, and put them where you can reach them. Formwork you cannot dismantle is as much a problem as formwork that sticks.
Ease the forms off rather than levering against the concrete.Loosen fixings and work the form away steadily. Levering a bar against a young concrete edge chips exactly the arris you were forming.
Clean and re-treat the forms before reusing them.Hardened residue on a form face is what makes the next pour stick. Scrape them clean while the residue is still fresh.

Tips

Plastic sheet stapled to rough boards is a cheap way to get a smooth non-absorbent face for a one-off pour, though it can leave a slightly wrinkled finish.
Chamfer strips in the internal corners of the form give a bevelled arris that is far less likely to chip than a sharp square edge.

Common mistakes

Using old engine oil as a release agent. — It stains the concrete, is bad practice environmentally, and leaves a contaminated surface that later coatings and screeds will not bond to.
Applying release agent so heavily it puddles. — Pooled release agent can soften the surface of the concrete against it and leaves visible marks on the finished face.

Questions people ask

Do I need a release agent on plastic or steel forms?

Usually still yes, though less is needed. Even smooth non-absorbent faces bond to some degree, and a light film makes striking much easier.

Will release agent affect what I put on the concrete later?

It can. If the surface is to be tiled, screeded or coated, check what the product says about subsequent bonding, and be prepared to clean or mechanically prepare the face.