Voids are usually formed in concrete either to provide a service duct or to obtain a major structural benefit. Frequently, a void can fulfill both functions. Today, the two most common methods used by contractors to produce a void in concrete consist of using either a multi-use removable former or a single-use expendable former. For site concreting, the removable types are usually made of rubber and the expendable types of fiberboard. Both methods are considerably cheaper than the old time technique of forming a void with timber, steel plates, or of casting steel or asbestos cement pipe permanently in place. When used as a service duct, a cast in void has the advantage of convenience. The electrical trades, for example, do not need additional conduit for wiring and conduit installation is easier and more flexible . Voided construction also offers greater insulating value. This applies to both the thermal insulating properties and to reduced sound transmission. A typical 6 inch voided slab will give a coefficient of thermal conductivity of around 1.25, which is substantially better than the value of 7 usually accepted for solid concrete. On most jobs the advantage of placing services within a void can readily be combined with the structural benefits of voided construction. These are by far the greater, inasmuch as they offer real and immediate savings. The main structural advantage of voided construction lies primarily in the immense weight savings possible by the inclusion of voids. A 20 foot by 20 foot voided flatplate floor slab will have, for example, an average dead weight of 80 pounds per square foot, whereas for the same load conditions as conventional solid slab will exceed 100 pounds per square foot.