and 36 these tension members can be formed of any suitable material such as steel as shown in fig3 a the tension members can be simply located in place using standard reinforcing bar supports or chairs placed across the surface 14 the chairs are used to locate the reinforcing some distance inside the outer surface of the cast so that the reinforcing material is protected from heat during fire the dimension of this fire cover is set by building codes alternatively the tension members can be pre tensioned pre attaching a fixed length of tension reinforcing material to either end of the mold and formed into the curvature required by locating members pressing down on the tensioning cables through the area within which the struts 18 are to be formed thus at each post a pressure member not shown is applied which presses down on the reinforcing rods or cables to pre tension them and hold them in place above the surface 14 during the pour with the cables in place as shown in fig3 a the second side of the mold is added as shown in fig3 so as to press the surfaces 22 together at the area 23 where the concrete is to be excluded turning now to fig4 a an alternative arrangement for the exclusion zone indicated at 23 a is provided in which there is provided a bead or gasket 23 b located at the edge 31 of the raised portion 21 on top of the fabric the bead thus pinches the fabric 25 at the edge 31 and more.