Basically, there are two types of void formers- the removable type and the expendable formers. The removable void formers can obviously be reused time and again. The expendable types are designed to be left in place after the concrete has been cast. For site casting, two main types of removable formers are available: those made of rubber, pneumatically inflated; and those which are designed to collapse after use by some mechanical arrangement. One major advantage that the inflatable formers have over other types is that they are easily curved and compressed; this means not only that they can be made to pass under or through reinforcement, existing services, or other obstructions in the form, but also that the duct can be made to follow any desired path. When forming a curved duct, it can generally be reckoned that the inside radius of the curve should not be less than 4 times the tube diameter. This does, however, vary widely for different proprietary products, and thus one manufacturer offers a large size former that can be bent through a radius of roughly 20 times the diameter. A removable void former implies that there is sufficient space for the withdrawal to take place- the inflatable types generally requiring less space than the rigid ones. Thus, there are formers suitable only for jobs where a continuous open ended void or duct is needed; or alternatively, placing must take place in stages. The expendable type avoids these problems since they are cast permanently in place. The commonest type of expendable former is the fiber tube. This is usually fabricated from 5 or 6 inch wide helically-wound strips of strong kraft paper between .037 and .015 inch thick. Special weather resistant types are available for use in exposed locations; normally this weather resistance is achieved by including a moisture-proof ply impregnated with a medium based on a micro-crystalline wax.