The formation of openings in concrete, either to reduce the weight of the structure or to provide access for services, calls for particular attention in the selection of the former material, positioning and subsequent withdrawal if required. The voids may or may not be sufficiently large for access and fixing of conventional formwork. With the smaller voids the choice of forming method is confined to a permanent form or one which can be removed from one or both ends of the void. Where the void is totally surrounded by concrete the use of a permanent form is unavoidable.
Materials available include:
- cardboard (known as fibreboard)
- thin sheet steel formers of various shapes
- thin sheet steel in helically welded tubes
- inflatable tubes
- foamed flexible and rigid plastic
- reusable plastic castings with mechanical or magnetic fixings.
Cavities formed in the concrete should not be totally enclosed, and some provision should be made to drain the moisture which may collect within the void, leak through the concrete and cause staining. Foamed plastics have an advantage in this respect as they may be regarded as ‘solid holes’ which do not require draining. Normal requirements regarding cover must be observed where steel formers which can rust are used.