Once upon a time, the only way to form cores and ducts through concrete was to use steel or fabric forms and leave them in position. This was an expensive procedure and a complicated one when the runs were anything but straight. Today, light, reusable, highly flexible forms are available- made of rubber. Natural rubber can withstand repeated contact with green concrete; it needs no form oil, and a light brushing will remove any cement dust that might adhere to it. Some producers of rubber forms guarantee them for 100 uses; many contractors have gotten over a thousand uses for one length. Normally the pneumatic duct tubes are inserted in the forms deflated. After the ends are secured, the tubes are inflated. Before the concrete is poured, it is usually necessary to hold the tubes in position with temporary locating devices. Otherwise they may be displaced as the concrete is poured. When the concrete has set, the tubes are deflated and withdrawn. They are then ready for immediate reuse.