An extruder for the fabrication of concrete slabs, especially hollow-cored slabs, movable on a casting bed and incorporating a feed hopper or similar, at least one feeder, especially an auger, for generating internal pressure in the cast concrete, and at least one coring member or similar, for generating the desired slab cross-section.The extruder in accordance with the invention comprises an assembly of at least two wedged surfaces, of which at least one is fitted in conjunction with the coring member, and the space between the surfaces is continuously changing due to a cyclic longitudinal movement, generating inside the space a compacting shear action in the concrete mix.