Hollow formwork elements for producing concrete structures

A re-usable hollow elongated casing formwork element of elastic material, adapted for placement into a concrete pipe line and adapted to be rendered pressure resistant by filling it with a gaseous or liquid fluid medium, comprising an elongated, elastic outer casing and at least two elongated elastic hollow cylinders positioned side-by-side within and extending over the entire length of said outer casing whereby at least two elongated communicating cavities are formed intermediate said outer casing and said cylinders and are adapted to be pressurized by said fluid medium, at least two of said hollow cylinders being interconnected by openings therebetween whereby said fluid medium can communicate between said two interconnected hollow cylinders, a first valve for introducing said fluid medium into one of said two hollow cylinders for pressurizing both said cylinders and a second valve for introducing said fluid medium into one of said hollow cavities for pressurizing both said cavities.