Abstract: A composite casing voussoir of the type constituted by a mass of concrete or like material and a metal reinforcement. The latter comprises a web having the general shape of a portion of a cylinder and a peripheral radially-extending fixing frame which has a curvilinear rectangular shape. The mass of concrete occupies a cavity of the reinforcement which cavity is open at least partly in a radial direction. According to the invention, the reinforcement has means for hooking the mass of concrete constituted by a projecting flange located over said cavity.