Formwork

Published on 11 March 2008
Formwork platform for a high-rise in Toronto...

The principle of formwork is simple: a mould is made of the construction to be built from "form panels" (panels of the mould), then the concrete is poured. It is only then necessary to let the concrete dry and subsequently un-mould the element required.

Formwork enables the construction of many elements : partitions, walls, columns, bridge piers, stairs, cornices (by the side of motorways, for example). The current trend on sites is to make bigger and bigger formwork so as to build the abutments (concrete walls done with formwork) and high-level columns, in a single cast.

The preparation of the formwork is a critical phase; the workers assemble the form panels which are modular elements 2 to 3 metres high and which are fixed together with elements called strut connectors. Once these form panels are installed, they must be supported so that they will resist the pressure of the concrete being poured into them, especially the lower section where this is most important. It is equally important to ensure that the internal part of the formwork is clean as any dirt present may leave marks.

 
... and bridge formwork (D.R. PERI)

Casting is carried out in accordance with two methods; pouring from the top of the formwork and injection at the foot of the form panel. In both cases a vibrator is used to distribute the concrete evenly in the formwork, whilst at the same time eliminating any air bubbles so as to guarantee the best quality mixture possible. Once the concrete has "cured" then the removal of formwork phase is reached, for which un-moulding agents are sometimes used (with a mineral or vegetable oil base), preventing the concrete from sticking.

In fact, the surface finish, the requirements for which are stricter and stricter, is closely linked to the quality of concrete and formwork removal techniques. With new technology it has thus become possible to give concrete the look of other materials ... such as wooden planks for example !