Column formwork is the temporary mould in which a concrete column is cast: four vertical faces that set the shape, size and surface of the column and resist the pressure of the wet concrete until it has hardened enough to stand alone.
Types
- System formwork — steel or aluminium panels on a fixed module, quick to clamp together and reusable many times. Standard on repetitive work.
- Traditional formwork — made up on site from film-faced plywood on timber bearers, held by steel column clamps. Flexible for non-standard sizes.
- Cardboard tube formwork — disposable formwork for circular columns; the tube is stripped away once the concrete has set.
- Permanent formwork — the mould stays in place and becomes part of the finished element.
Concrete pressure: the critical issue
Fresh concrete exerts a pressure much like a liquid. It increases with height and, crucially, with the rate of pour: the faster a column is filled, the higher the pressure on the formwork. Low temperature (slower setting) and heavy poker vibration also increase it.
Formwork suppliers therefore always state a permissible concrete pressure and a matching maximum rate of rise. Those figures are a limit, not advice: formwork failing during a pour is a well-known cause of serious accidents.
Practical points
- Set out from grid lines — always set out columns from the main grid, never from a previously cast column, or dimensional errors accumulate.
- Plumb and brace — check with a long spirit level or laser and fit at least two props on adjacent faces.
- Seal the kicker — the joint at the slab must be tight, otherwise cement paste leaks out and you get honeycombing at the base.
- Release agent — apply thinly and evenly. Too much causes staining and a dusty surface; too little causes the concrete to stick and damages fair-faced work.
- Striking — columns carry their own weight fairly quickly, so side formwork can usually come off after one to a few days, provided the concrete has gained enough strength. In frost or with a low cement content it takes longer. The engineer or specification sets the striking times.
- Curing — protect against drying out immediately after striking, or you get shrinkage cracks and a weak surface layer.
Related terms
- Formwork
- Film-faced plywood
- Formwork release oil
- Column
- Adjustable prop
- Fair-faced concrete
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