Design of pipes
Pre-stressed concrete pipes are suitable when the internal pressure is within 0.5 to 2.0
Mpa. There are two types of pre-stressed concrete pipes:
1. Cylinder type which has a steel cylinder core, over which the concrete is cast and pre-stressed.
2. Non-cylinder type which is made of pre-stressed concrete only.
IS:784-2001 Pre-stressed Concrete Pipes (Including Specials), provides guidelines for the design of pre- stressed concrete pipes with the internal diameter ranging from 200 mm to 2500 mm. The pipes are designed to withstand the combined effect of internal pressure and external loads. The minimum grade of concrete in the core should be M40 for non-cylinder type pipes.
The pipes are manufactured either by,
1. Centrifugal method: In the centrifugal method the mould is subjected to spinning till the concrete is compacted to a uniform thickness throughout the length of the pipe.
2. Vertical casting method: In the vertical casting method, concrete is poured in layers up to a specified height.
After adequate curing of concrete, first the longitudinal wires are pre-stressed. Subsequently, the circumferential pre-stressing is done by the wire wound around the core in a helical form. The wire is wound using a counter weight or a die. Finally a coat of concrete or rich cement mortar is applied over the wire to prevent from corrosion. For cylinder type pipes, first the steel cylinder is fabricated and tested. Then the concrete is cast around it.
Example 1 - non-cylinder
Design a non-cylinder pre-stressed pipe for the following specifications: R = 300 mm, p = 1.05 MPa, fi = 1000 MPa, fe = 800 MPa, f ct = -14 MPa, f cw =-0.7 MPa, Es = 2.1x105 MPa, Ec = 0.35 x 105 MPa and 2.5 mm wires are used. And what would be the internal pressure 'p' required to balance the pre-stress at transfer before losses to maintain a stress of -0.7 in concrete?
Example 2 - non-cylinder
Design a non-cylinder pre-stressed pipe for the following specifications: R = 800 mm, p = 1 MPa, fi = 1000 MPa, fe = 800 MPa, fct = -12 MPa, fcw = 0, Es = 2.1x105 MPa, Ec = 0.35 x 10 5 MPa and 5 mm wires are used. If cracking stress is +2 MPa, what is the F.S against cracking?
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