Formwork is used to contain the freshly placed concrete in the exact shape and place desired. Manufactured forms can be rented or purchased and they account for most of the residential formwork used today. The many proprietary systems available fall into five types: plywood on metal frame; all aluminum; unframed plywood with attached steel hardware and/or braces; unframed plywood with loose hardware; and all steel.
DESIGNING AND ERECTING FORMWORK
The formwork must be made to accommodate various materials that will be embedded or inserted in the concrete. It may also have to accommodate a number of openings for pipes, ducts and cables. Planning formwork involves selecting their sizes, lengths and spacing as well as making decisions about connection details. If the site is complex, formwork drawings and specifications may be needed.
During construction the formwork must support vertical and horizontal loads and also the lateral pressures exerted by fresh concrete. Vertical loads include both dead load and live load. The weight of formwork plus the weight of freshly placed concrete is dead load. Live load includes the weight of workmen, equipment, stored material and runways as well as impact load. Braces and shores should be designed to resist foreseeable horizontal loads. These include loads from wind, dumping of concrete, and workers climbing on the forms.
Form ties resist the lateral pressure exerted by freshly placed concrete. The number of form ties and their spacing may vary with the size and type of form used.
Before using forms it is necessary to apply a form release agent to minimize adhesion by the concrete and to facilitate stripping.