the c shaped beam has a relatively thick rib 8 which includes two slots 9 10 an approximately square cross section is suitable for the rib 8 the lower slot 9 engages the upper edge of a flexible sealing strip 11 which is conveniently an elastomer extrusion and which extends across the width of the forming section it is also desirable that the sealing strip is one piece if more than one piece is used the ends should be closely butted together and preferably cemented to provide a water tight seal the lower edge of the sealing strip 11 is engaged into a suitable slot 12 in the base beam 1 the upper slot 10 carries the fabric engaging blade 13 a pressurized loading tube 14 is located between the base beam 1 and the rib portion 8 of the c shaped beam 3 the tube is conveniently retained in place by a suitable recess 15 in the base beam 1 the loading tube also conveniently includes a pressure rib 16 on its top surface referring now to fig2 and 3 as the loading tube 14 is pressurized for example with air or with a liquid under pressure it expands and engages the underside of the rib portion 8 of the second edge of the c shaped beam 3 as the pressure is increased the tube 14 expands.