pair of opposing recesses or apertures 19 for cooperatively engaging the open ends of the bracket 152 to lock a transverse beam 21 or 22 with respect to the corresponding channel 18 the open ends of the bracket 152 are engaged with the apertures 19 in the channel 18 the adjusting bolt is threaded until it contacts the outer end of the transverse beam thereby precluding motion of the movable wall away from the corresponding fixed wall as shown in fig1 and 5 vibrators 158 are attached to the movable walls 120 160 along upper and lower rows the vibrators are external impact vibrators such as ar 06 460 vibrators manufactured by the wacker corporation of menomnee fall wis the vibrators 158 consolidate and compact the concrete in the mold to reduce the number and size of the trapped air pockets at the interface of the mold and the concrete in addition the vibrators 158 enhance the flow of concrete within the mold during the casting process as shown in fig4 and 5 each of the fixed and movable walls include a plurality of push off valves 180 referring to fig6 and 7 the push off valves 180 include air actuated poppets 182 in the default position the poppets 182 are coplanar with the surrounding portion of the skin plate or mold surface such that the local area of the poppet and the skin plate define a planar surface actuation of the push off valve 180 disposes the poppet 182 intermediate of the movable wall and fixed wall.