How can one be sure that void formers keep their proper and rigid location until the concrete has been placed and hardened?
Void formers should be supported at the right height and then tied down to prevent floating to the surface. Manufacturers often make a special chair for their product. Removable void formers can be located by means of small precast spacing blocks wired to the main reinforcement with loops cast into the back of each block. Another way is to tie down the void former with Number 12 gage wire running through holes drilled in the formwork and if necessary over temporary bulkheads laid over the top surface. The ends of the wire must be bent away from contact with the rubber of the void former to prevent damaging it. After the concrete hardens, any protruding wire should be chiseled back about one-half inch and the hole grouted flush to prevent rusting.