There are a number of ways to form service voids in concrete. Steel pipe is sometimes used; so are pater tubes and wooden plugs. Lately plastics- in the form of polyethylene sleeving- have entered the field. The majority of plastic sleeves on the market are made from linear polyethylene and are as rigid as sheet metal. Plastic will not rust nor discolor concrete, and is an ideal insulator for copper pipe and electrical conduit. Plastic also makes a non-abrasive insulator for high silicone iron pipe and for glass pipe which is now being used for corrosive waste. In many cases sleeving plays a dual role. Its prime function is to allow access to the floor above and, secondly, it acts as a moisture barrier. This second function is important in service areas where there is a danger of water spillage, or in locations that require floor mopping, such as kitchens and laundry room.