Method of and parts used in the construction of a prestressed concrete structure

Method of constructing a longitudinally extending structure of prestressed concrete comprising building formwork for at least a longitudinally extending portion of the structure, placing at least one thin-walled sheathing tube into the formwork extending in the longitudinal direction for forming a duct extending therethrough, pouring concrete into the formwork and incorporating a sheathing tube therein, inserting a tendon into the duct formed by the sheathing tube after the concrete has set for a desired period and tensioning the tendon, wherein the improvement comprises stiffening the sheathing tube before pouring the concrete by inserting smooth-walled tubes of plastic or the like through the sheathing tube for the length thereof to be incorporated into the concrete and fitting the smooth-walled tubes within the sheathing tube with little play therebetween, retaining the smooth-walled tubes within the sheathing tube during the placing and setting of the concrete, and axially displacing the smooth-walled tubes out of the sheathing tube so that the tendon can be inserted therein and subsequently tensioned.