6.9 PRESTRESSED CONCRETE PIPES

6.9.1 General

While RCC pipes can cater to the needs where pressures are upto 3.0Kg/cm2 and C.I and steel pipes cater to the needs of higher pressures around 24 Kg/cm2, the P.S.C pipes cater to intermediate pressure range for which the metallic pipes are expensive while RCC pipes would not be suitable.

The strength of a P.S.C pipe is achieved by helically binding high tensile steel wire under tension around a concrete core there by putting the core into compression. When the pipe is pressurised the stresses induced relieve the compressive stress but they arae not sufficent to subject the core to tenslie stresses. The prestressing wire is protected against corrosion by a surround of cementatious cover coat giving at least 25mm of cover.

P.S.C competes economically with steel for pipe diameters of 600 mm and above. It is a unique combination of durability of concrete and high strength of steel combined with economy in raw materials. The P.S.C pipes are ideally suited for water supply mains whre pressures in the range of 6 Kg/cm2 to 20 Kg/cm2 are encountered.

Two types of P.S.C pipes are in use today.

i) Cylinder type : Consists of a concrete lined steel cylinder with steel joint rings welded to its ends warpped with a helix of highly stressed wire and coated with dense cement mortar concrete.

ii) Non cylinder type : Consists of a concrete core which is pre-compress both in longitudinal and circumferential directions by a highly stressed with the wire wrapping is protected by coat of cement mortar or concrete.

Physical behaviour of P.S.C pipes under internal and external load is superior to R.C.C pipes. The P.S.C pipe wall is always in a strength of compresion which is the most favourable factor for impermeability. These pipes can resist high external loads. The protective cover cement sand mortar which covers the tensions wire wrapping by its ability to create and maintain alkaline environment around the steel inhibits corrosion. P.S.C pipes are jointed with flexible rubber rings.

The deflection possible during laying main is relatively small and the pipes cannot cut to size to close gaps in the pipeline. Special closure units (consisting of a short double space piece and a plain ended concrete lined steel tube with a follower-ring assembled at each end)are manufactured for this prupose. the closure unit(minimum length 1.27m) must be ordered specially to the exact length.

Speicals such as bends,bevel pipes, flanged tees tapers and adapters to flange the couplings are generally fabricated as mild steel fittings lined and coated with concrete.

It is worth while when designing the pipe line to make provision for as many branches as likely to be required in the future and then to install sluice valves or blank flanges on these branches. It is possible to make connections to the installed pipe line by empting, breaking out and using a special closure unit but this is a costly item.