What is formwork, and why does concrete need it?
Formwork is the temporary mould that holds concrete in shape until it can hold itself. Why the pressure of wet concrete is the whole design problem, and what makes a home-made form fail.
- Difficulty
- intermediate
- Time
- 8 min
- Read
- 4 min
- Safety
- warning
Short answer
Concrete arrives as a heavy liquid and has to be held in the shape you want until it has gained enough strength to stand up. Formwork is that mould, plus everything bracing it against the outward pressure. The pressure is the difficult part: it rises with the depth of concrete and with how fast it is poured, and a form that looks strong enough usually is not.
Formwork is invisible in the finished job, which is why people underestimate it. On a small slab it is four boards and some pegs. On anything taller than it is wide it becomes the most likely thing to go wrong, because a cubic metre of wet concrete weighs well over two tonnes and it behaves like a liquid until it does not.
Safety
Step by step
- Understand what the form is resisting.Wet concrete exerts pressure sideways like any liquid, and that pressure increases with depth. A shallow slab pushes very little; a metre-deep column form pushes a great deal at the bottom, which is where forms burst.
- Note that speed of pouring matters.If concrete is placed slowly enough that the lower part is already stiffening, it stops behaving as a liquid and the pressure at the base is lower. Pouring a deep form quickly is what produces the highest pressures.
- Brace outwards, not just upwards.Most failed home-made forms did not break; they spread. Stakes driven outside the boards, struts across the top, and ties through the form are what hold the width. A board on edge with nothing behind it will bow.
- Fix the form to hold its line and its level.The top edge of the form is usually the level you will screed to, so it is set to the finished level before the concrete arrives, not adjusted afterwards.
- Decide whether it comes out.Striking formwork means removing it once the concrete has enough strength. Some formwork is permanent — insulated formwork and trench-lining systems stay in the ground. Anything meant to be struck needs a release agent or a lining so the concrete does not bond to it.
- Leave it long enough.Concrete gains strength over days, not hours. Striking a form early on anything that has to carry itself is how edges break away. The time depends on the mix, the temperature and what the element has to do, and on real work it is specified rather than guessed.
- Know where it stops being DIY.Ground slabs, paths, small bases and garden steps are ordinary work. Anything vertical and tall, anything that supports formwork above people, and anything structural is designed formwork with a designed sequence.
Tips
- Plywood is the usual face material because it gives a smooth finish and can be reused. The cost of a sheet is trivial next to the cost of a bowed foundation edge.
- A form filled with water for ten minutes before pouring stops dry timber sucking water out of the mix at the face, which is what causes a dusty, weak surface against the boards.
- Mark the finished concrete level inside the form with a chalk line or a nail before the pour. Trying to judge it in a moving grey soup is much harder.
Common mistakes
- Pegging only on the inside of the boards — The pressure pushes outwards, so the restraint has to be outside. Inside pegs get buried in the concrete and hold nothing.
- Pouring a deep form in one go — It maximises the pressure at the base at exactly the moment the concrete is most fluid. Filling in lifts, allowing each to stiffen, is how deep forms are done.
- Striking the form to admire it the next morning — Green concrete has very little tensile strength, and arrises break off cleanly and permanently. There is no repair that looks like the original.
If it doesn't work
The form has bowed and the edge of the slab is fat
Cause: Insufficient outside bracing for the pressure — Fix: There is no fixing it wet without making it worse. Once cured, the excess is cut back with a diamond blade and the arris made good — which is why bracing costs less than not bracing.
Concrete has leaked out under the boards
Cause: The form was not bedded tight to the ground — Fix: Fill the gap from outside quickly with soil or sand to stem it. Before the next pour, bed the form on a sand fillet or seal the base.
The face against the form is dusty and weak
Cause: Dry timber drew water out of the surface, or the mix was too wet and fines washed to the face — Fix: Wet timber forms or use a release agent before the pour, and control the water in the mix. A dusty surface can be sealed but not restored.
Questions people ask
Do I need formwork for a concrete path?
Yes, unless the excavation itself holds the shape. The boards give you the line and the level as well as containing the concrete, and getting a straight edge without them is very difficult.
What is permanent formwork?
Formwork that is left in place. Insulated concrete formwork blocks that stay as the insulation, and trench sheets or lining systems that remain in the ground, are the common examples.
Can I use scaffold boards?
For a slab edge, yes, and they are stiff which helps. They are rough, so the concrete face will be rough, and they will need cleaning immediately after the pour if you want them back.