Reinforced concrete cracks under tension, and pipes of reinforced concrete therefore find their main use in nonpressure sewers and drains – though they have also been used for low-pressure duties of up to about 75 psi (5 kg/cm2). They are made by placing a steel reinforcing cage inside a steel cylinder, and then introducing high-quality concrete into the cylinder while it is spun in a nearly horizontal position.
When the concrete has set, the pipe is withdrawn from the cylinder and is then either steam- or water-cured. Prestressed concrete pipes are designed to resistmoderate internal pressures of up to about 220 psi (15 kg/cm2), and are used primarily for large diameter water transmission. Diameters range from 24 in. (600 mm) to more than 160 in. (4000 mm). There are two types of prestressed concrete pipe: cylinder and noncylinder.
Cylinder pipes have a thin-wall steel cylinder core which is manufactured in the same way as steel pipes. A thick lining of concrete is then spun centrifugally into the steel cylinder, and allowed to cure. High-tensile steel wire is wound helically around the full length of the cylinder at a predetermined pitch and tension. Dense cement mortar is finally sprayed into the revolving pipe (guniting) to provide a protective coating at least 1 in. (25 mm) thick. For higher pressures a second layer of prestressing wire and mortar coating is applied.
Noncylinder pipes, as the name implies, have no steel core. Longitudinal high-tensile steel wires are placed inside a cylindrical steel mold and tensioned. The mold is transferred to a spinning machine, and concrete is fed into it as the mold rotates. After a curing period, the mold is removed and high-tensile steel wire is wound around the pipe helically in the same way as for cylinder pipes. Finally, a dense protective cement mortar coating is applied.
Asbestos cement pipes, like prestressed concrete, are made to resist moderate pressures of up to about 220 psi (15 kg/cm2). Pressure pipes do not usually exceed 40 in. (1000 mm) in diameter, but nonpressure pipes are often made in much larger sizes. Pipes are manufactured from ordinary cement reinforced with about 15 per cent by volume of asbestos fibers.
A slurry of cement, asbestos fibers and water is fed into a tank from which a revolving cylindrical sieve picks up a thin film of material. The film is next transferred onto a polished steel mandrel by a felt conveyer belt, and the pipe is gradually built up to the required thickness. Rollers apply pressure over the full length of the mandrel to insure cohesion between successive layers, and to produce a dense pipe. The pipe is removed from the mandrel, and a split wooden former is introduced to prevent deformation before initial hardening has occurred. The former is removed after about 24 hours, and the pipes are then cured either by immersion in water for 7 to 14 days, or by high-pressure steam curing.
Glass-reinforced concrete is a new pipe material which has so far only been developed for nonpressure purposes. These pipes are centrifugally spun, and are reinforced on both the inner and outer faces with alkali-resistant glass fiber.