Abstract: In a method of using an elongate steel core for making passages in a body of castable material, preferably concrete which is cast in an outer mould, the core is prestressed prior to casting, and to disengage the core from the finished body, the prestress is relieved against the body or the outer mould when the material has set sufficiently. The core is provided at its ends with anchoring means for one or more tensioning members extending therebetween for prestressing the core prior to casting, the prestress for disengaging the core from the finished body being relievable against said body or said outer mould by means of an abutment engageable therewith.