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Prestressed concrete pipes and tanksCircular prestressing

When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing in pipes, tanks, silos, containment structures and similar structures is a type of circular prestressing. In these structures, there can be prestressing in the longitudinal direction (parallel to axis) as well. Circular prestressing is also applied in domes and shells.

The circumferential prestressing resists the hoop tension generated due to the internal pressure. The prestressing is done by wires or tendons placed spirally, or over sectors of the circumference of the member. The wires or tendons lay outside the concrete core. Hence, the centre of the prestressing steel (CGS) is outside the core concrete section. The hoop compression generated is considered to be uniform across the thickness of a thin shell. Hence, the pressure line (or C-line) lies at the centre of the core concrete section (CGC).

The following sketch shows the internal forces under service conditions. The analysis is done for a slice of unit length along the longitudinal direction (parallel to axis).To reduce the loss of prestress due to friction, the prestressing can be done over sectors of the circumference. Buttresses are used for the anchorage of the tendons. The following sketch shows the buttresses along the circumferencePrestressed Concrete Cylinder Pipe (PCCP) consists of a concrete core, a thin steel cylinder, high tensile prestressing wires and a mortar coating. The concrete core is the main structural load-bearing component with the steel cylinder acting as a water barrier between concrete layers, the prestressing wires produce a uniform compressive pressure in the core that offset tensile stresses in the pipe, and the mortar coating protects the prestressing wires from physical damage and external corrosion.Prestressed Concrete Cylinder Pipe (PCCP) was first manufactured in 1942 as lined cylinder pipe. The prestressing wire in lined cylinder pipe is wrapped directly around the steel cylinder. A second type of PCCP was developed in 1952 that has concrete encasement of the steel cylinder on both sides.

Known as embedded cylinder pipe, it differs from lined cylinder pipe by the encapsulation of its steel cylinder in a concrete core. Therefore, the prestressing wire is wrapped around the concrete core rather than the steel cylinder as in lined cylinder pipe. The typical diameter ranges for lined and embedded cylinder pipe are between 16 to 60-inches and 30 to 256-inches, respectively.

Internal forces under service conditionsUse of buttress in circumferential prestressingAdvantagesDisadvantagesResistant to physical damageRapidly and economically installedGood corrosion resistanceCan resist high internal pressure and external loadingWide range of pipe diameters available (up to 144-inches)Typically more economical than properly lined and coated ferrous pipesRequires careful installation to avoid crackingHeavySusceptible to attack by Hydrogen Sulfide (H2S) and acids when pipes are not coatedMain Forms Failure in PCCP

Broken prestressing wires due to corrosion or poor material qualityJoint leaksPoor beddingExcessive external loadingHydrogen sulfide (H2S, wastewater applications)Poor quality mortar coatingCorrosive environment (corrosive/ aggressive soil)Construction damage (coating damaged and not repaired)Design of non-cylinder pipesThe Basic Principle is to adopt wall thickness circumferential and longitudinal Prestress to ensure freedom from tensile stresses in the core pipe under the most severe combination of sustained internal pressure and external loads likely to occur in service.When transient loads such as surge or traffic are added to sustained loads, tensile stresses up to 30 kg/cm2 maximum may be permitted in concrete.It is usual to divide pipelines in steps of Internal Pressure and design pipe for each section taking advantage of economy to be gained by tailoring design to suit actual needs without reducing safety. Where local high pressures or heavy external loads occur, pipes in these places can be strengthened without affecting cost of the remainder of the pipeline.

The following load components are taken into account.Mass of PipeBedding and BackfillMass of water in pipeConstruction traffic wheel loadsSustained Internal PressureHighway traffic wheel loadsSurge

Types of prestressed concrete pipesMonolyte construction :It is based on the principal that a mix of fresh concrete subjected to triaxial pressure behaves like a solid body.If steel is embedded in such a mass of concrete ,which is deformed while the pressure is maintained , the steel also experiences the deformation of the surrounding concrete. The manufacturing process consists of pouring concrete under high frequency vibration in a vertically-placed steel mould consisting of an inner and outer shell.The outer shell, consisting of longitudinal sections held together by spring assembles ,permits the mould to expand while the inner steel mould is covered with an expansible rubber membrane.The main advantage of this method is that stressing and production are achieved in a single cycle.

2. two-stage construction:This is the method of manufacturing of a non-cylinder pipe in the first stage, the concrete is cast over a tensioned longitudinal reinforcement. In the second stage,the concrete pipes after curing are circumferentially stressed by means of a spiral wire wound under tension and protected by a coat of mortar.The main function of the longitudinal prestress is to prevent cracking in concrete during circumferential winding and cracking due to bending stresses developed during the handling and installation of pipes.The longitudinal section of a non-cylinder pipe with socket and spigot joints is shown in fig below.The technique of double winding and double coating is employed wherever high pressures are involved with larger diameter of pipes.

Typical longitudinal section of non cylinder prestressed concrete pipe

Typical longitudinal section of prestressed concrete embedded cylinder pipeDesign of prestressed concrete pipes Criteria of design :The design of prestressed concrete pipes should cover the following five stages:Circumferential prestressing, winding with or without longitudinal prestressing.Handling stresses with or without longitudinal prestressing.Condition in which a pipe is supported by saddles at extreme points with full water load but zero hydrostatic pressure.Full working pressure conforming to the limit state of serviceability 5. The first crack stage corresponding to the limit state of local damage6. To examine the stage of bursting or failure of pipes corresponding to the limit state of collapse, mainely to ensure a desirable load factor against collapse.

General features of prestressed concrete tanksApplications:Prestressed concrete tanks have been widely used for the storage of fluids, such as water,oil,gas,sewage,granular materials like cement,process liquids and chemicals, slurries and cryogens.Water storage tanks of large capacity are invariably made of prestressed concrete.Recent applications include special forms of prestressed concrete tanks, which are triaxially prestressed and serve as containment vessels and biological shields for nuclear reactors.Prestressed concrete tanks are generally cylindrical and capacities of about 50 million litres. In sanitary structures like sludge digestion tanks , spherical shapes are preferred and the tank is made up of a top and bottom conical shell connected by a circular cylindrical intermediate portion.Prestressed concrete , although water tight , is not gas tight where vapours under pressure are to be stored.

Shapes of prestressed concrete tanksCylindrical tanks are most commonly used types from structural and constructional considerations.Cylindrical shape is well suited for circumferential wire wrapping .Square or rectangular tanks , spanning either vertically or horizontally , are required for industrial use.Square tanks are advantageous for storage in congested urban and industrial where land space is a major constraint.Multi celled tanks have been constructed using interlocking polygons and circular shapes , especially for the storage of cement in silo construction.The hexagonal units are prestressed together to achieve monolithic action by transverse and vertical tendons.The advantage of hyperboloidal shape is the reduction in the thickness of concrete shell and the use of the same set of straight wires to produce circumferential and vertical prestress.Doubly curved shells have also been used to take advantage of the efficiency of the shell action of the concrete.Some typical shapes of prestressed concrete tanks are shown below:

Tank floorsThe base slab forming the floor or the tank is generally made of reinforced concrete constructed on a flat bituminuous surfacing or on a thin concrete binding with the interposition of a sliding layer such as oil paperso that the slab can move over the compacted soil bed.The slab should be sufficiently flexible so that it can adapt itself to the local deformations of the pre-compacted sub-soil.The reinforcements in the slab should be well distributed to control the cracking of the slab due to shrinkage and temperature changes.

Typical construction joints in tank floor slabsJunctions of tank wall and base slabThe joint between the walls of the tank and floor slab may be any one of the following three types:Fixed baseHinged baseSliding base

The ring tension and baending moments developed in the walls of the tank are mainely influenced by the type of connection between the walls and the base slab.The junction between the tank wall and footing is the most vulnerable location as far as leakage is concerned and hence in the case of tanks storing penetrating liquids, it is necessary to form the wall and footing in monolithic construction.

Tank wall with fixed baseHinged baseThe hinged base is not generally adopted for prestressed concrete. In this type, the tank wall is supported over an annular bearing resting on the footing from which the base slab is isolated by a joint containing a compressible filling.This arrangement facilitates the junction between the wall and base slab to rotate about the annular bearing.The hinged joint can also be formed by circumferential wire wrapping to the bottom portion of the wall and then packing the groove with cement mortars.

Tank wall with hinged baseSliding baseIn case of large tanks and especially for those which have to store hot liquids, a movable or sliding joint is the ideal solution to minimise or completely eliminate the moments at the base of the wall.A sliding joint is made by interposing rubber or neoprene pads at the junctions of the wall and the base.The main function of these pads is to allow for free horizontal movement of the wall relative to the base by shear deformation of the rubber joint, which does not exceed a critical value of 30 degrees.The horizontal shear force developed for producing this deformation is influenced by the thickness of the pad and the shore hardness of the material.

Tank wall with sliding baseCircumferetial wire winding methodsThe most common of wire wrapping for circular tanks consists of a traction machine.The machine is suspended from a trolley which runs along the top of the tank walls.The high-tensile wire is drawn through a die while it is wound on the tank to achieve the designed tension in the wire.The B.B.R.V tank winding machine is a lighter and simpler version with winding speed of 1m/s in which the prestressing wire is unwound from a pulley whose circumference is smaller than that of the drive wheel by an amount corresponding to the extension of the wire required to obtain the desired prestress.

Wire winding machine for circular cylindrical tanksCircumferential prestressing is also possible by an ingenious method, which involves the barrel hoop principle. In this method, the successive turns of wire wound round the tank walls having an inward batter are knocked downwards to achieve the desired extension and force in the wire.Circumferential prestressing is also possible by the use of embedded tendons enclosed in sheaths which are prestressed and anchored against plaster vertical ribs on the outside face of tank walls.It is preferable to limit the curved lengths of the cable by providing jacking points spaced at everyone-third of the circumference of the tank wall, mainly to overcome the considerable friction losses.In case of rectangular tanks, either cast in situ or precast and assembled, it is more advantageous to use tendons embedded in cables sheaths.

Analysis of prestressed concrete tanksThe bending moments and ring tension, developed in circular water tanks due to the hydrostatic pressure ,depend upon factors, such as the type of fixity between the tank wall and the base slab, the diameter of the tank, the thickness of the wall and the elastic constants of the material forming the walls.The vertical bending moment Mw and the ring tension Nd developed at a distance x from the base of the tank are expressed as:

Analysis of circular cylindrical tankM0 and N0 are the moment and shear acting at the base of the tank, with their values depending upon the pressure distribution and the conditions of fixity at the base.A diagrammatic representation of the variation of bending moments and ring tension in the walls of tanks for different types of bases shown in fig.Maximum bending moment develop in the case of tanks with a fixed base while the ring tension is maximum for the free-base condition.In the case of tanks with walls resting on rubber or neoprene pads, a comparatively smaller magnitude of bending moments is generated due to the radial frictional force developed at the base junction.

Ring tension and bending moments in cylindrical tank walls