An apparatus designed especially for mechanically breaking up reinforced concrete and comprising a cylinder containing a hydraulically-operated piston connected to a thrust wedge and acting upon two insertion pieces to be inserted in a borehole in the reinforced concrete or in stone. In order to avoid excessive wear and tear on the components by reducing the stresses to which they are subjected, the apparatus comprises a bracing part rigidly joined to the cylinder by tie plates, and the two end faces of the insertion pieces remote from the cylinder are braced against this bracing part, through which the thrust wedge passes when the apparatus is in operation. Thus the end faces of the insertion pieces can slide on the surface of the bracing part when they are driven apart by the advancing wedge.