#015 Prestressed concrete pipes.

Prestressed concrete pipes are used for purposes like liquid retaining structures, such as circular pipes, tanks and pressure vessels etc. which are suited for circular prestressing. The circumferential hoop compression produced in concrete by prestressing counterbalances the hoop tension developed due to the internal fluid pressure. A reinforced concrete pressure pipe requires a large amount of reinforcement to ensure low-tensile stresses resulting in a crack-free member. Circular prestressing produces the required condition of a crack-free member and the material is used more efficiently. Shrinkage cracks also are eliminated in such a situation.

In circular prestressing, tendon wires are wrapped under tension over the concrete pipes which are pre-cast. The tension in the tendon wires is produced by pulling it through a die. Prestressed concrete pipes are ideally suited for a pressure range of 0.5 to 2 N/mm2. For this pressure range, cast iron pipes and steel pipes are not economical. Reinforced concrete pipes also are not practicable for use as these have limited tensile strength. The technique of prestressing of pipes was first introduced in 1930 and numerous pipelines have been installed since then According to the Indian standard code IS: 784, the design of prestressed concrete pipes should cover the following five stages:

(a) Circumferential prestressing, winding with or without longitudinal prestressing,

(b) Handling stresses with or without longitudinal prestressing,

(c) Condition in which a pipe is supported by saddles at extreme points with full water load but zero hydrostatic pressure,

(d) Full working load conforming to the limit state of serviceability, and

(e) The first crack stage corresponding to the limit state of local damage.