Formwork and shuttering

Frequently asked questions

Why is formwork measured in area when concrete is measured in volume?

Because the cost follows the FACE, not the fill. What a formwork item buys is a surface: a sheet of material held rigidly in position, sealed at its edges, treated with a release agent, and taken off again afterwards. Doubling the depth of a slab doubles the concrete and barely changes the formwork, while splitting the same volume of concrete into more and thinner elements multiplies the formwork without changing the concrete at all. That is why a quantity of cubic metres tells you very little about what a concrete element will cost, and why two designs with identical concrete volumes can differ sharply in price. The ratio of formed area to volume is the number worth looking at, and it is worst on thin walls, small isolated bases and anything with a lot of edges.

What actually decides when formwork can be struck?

The strength the concrete has reached, which is not the same as the time that has passed — and the difference is temperature. Concrete gains strength through a chemical reaction that runs faster when warm and very slowly when cold, so a striking time that is safe in summer can be badly unsafe in winter on the same mix. The governing question is also different for different parts of the pour: a vertical face carries no load and only needs the concrete stiff enough not to slump or be damaged, so it can come off within a day, whereas the SOFFIT of a slab or a beam is holding the element's own weight and everything on it, and cannot come off until the concrete can carry that itself. This is why striking times are specified against strength, why maturity is monitored in cold weather, and why back-propping keeps load on a young slab after the forms below it are gone.

What pressure does wet concrete put on a wall form?

Far more than people expect, and it depends on how fast the pour goes. Wet concrete behaves like a dense liquid, so the pressure at the bottom of a form rises with the height of fluid concrete above it — and concrete is roughly two and a half times as dense as water, so a form a few metres tall sees pressures that will burst an underbuilt panel. What limits it is that concrete stiffens as it sets: once the lower part is no longer fluid it stops transmitting pressure from above, so a SLOW pour in shallow lifts develops much less pressure than a fast one, and cold weather raises the pressure by delaying that stiffening. Vibration temporarily re-liquefies concrete and pushes the pressure back up, which is why over-vibrating a tall form is a common cause of a blow-out. The pour rate is therefore part of the formwork design, not a site decision.