Screw fixing device for a concrete construction element

A screw fixing device for a concrete construction element, comprising a screw having a cylindrical core and a sheath of synthetic material intended to be fixed into the concrete element, this sheath possessing internally a thread corresponding to the thread of the screw and, externally, peripheral anchoring ribs of triangular cross section and two longitudinal ribs, the sheath of synthetic material further being surrounded by a metal sheath comprising two shells which are mutually assembled by crimping along the longitudinal ribs of the sheath of the synthetic material, wherein the upper part of the sheath possesses no peripheral ribs, wherein the inclination of the flanks of the peripheral ribs facing the entry to the sheath is approximately 30.degree., wherein the metal sheath bears of these flanks but has a play (Jd) relative to the other flanks of the peripheral ribs, and wherein the threads of the screw and of the sheath are trapexoidal in cross section, the section of the rib constituting the inlet of the thread of the sheath decreasing progressively, the dimensions of the said section decreasing according to a similarity transformation towards the sheath inlet.