tapered cylindrical shape formed by the two identical semicircular mold halves 116 are two identical semicircular sleeve inserts 122 these semicircular sleeve inserts 122 are substantially shorter than the two semicircular mold halves 116 in this preferred embodiment the sleeve inserts 122 are approximately 4 feet long and the sleeve inserts 122 are also of slightly smaller diameter than the mold halves 116 and have the same taper such that when the sleeve inserts 122 are bolted into the mold halves 116 they fit snugly inside the mold halves 116 the sleeve inserts 122 are mounted at the top end 120 of the respective mold halves 116 such that the upper end of the sleeve inserts 122 is exactly even with the upper end 120 of the cylinder formed by the assembled mold halves 116 the sleeve inserts 122 have an outer surface 124 and an inner surface 126 in this preferred embodiment the inner surface 126 defines a multisided polygonal shape which in this case is a dodecagon or twelve sided figure end plates 128 130 lie adjacent to the respective ends 118 120 of the mold as shown in fig5 a the reinforcing strands 139 extend through holes 140 in the end plate 130 the same is true of the other end which is not shown in this view the reinforcing strands 139 that are tensioned are held in position by chucks 142 and this tension holds the end plates 128 130 on the ends of the mold as will be noted in fig5 a an additional ring.