Prestressed concrete poles ,piles, sleepers, pressure vessels and pavements

Prestressed concrete poles ,piles, sleepers, pressure vessels and pavementsPrestressed concrete polesThe main advantages of prestressed concrete poles are:Resistance to corrosion in humid and temperate climates and to erosion in desert areas.Freeze-thaw resistance in cold regions.Easy handling due to less weight than other polesFire resistant , particularly to grass and bush fires near the ground line.Easily installed in drilled holes in ground with or without concrete fill.Lighter because of reduced cross-section when compared with reinforced concrete poles.Clean and neat in appearance and requiring negligible maintenance for a number of years, thus ideally suited for urban installations.

Have increased crack resistance , rigidity and can resist dynamic loads better than reinforced concrete poles.Shapes of prestressed concrete polesThe maximum moment of resistance in a pole is generally required at the base and consequently , the maximum cross sectional area is required at the base section.Poles are generally tapered with a hollow core to reduce the weight , which also helps in providing a race way for electric wires.Typical cross sections of transmission line poles widely employed in different countries as shown in fig. 1.For small lengths of up to 12 m, the square or rectangular sections are preferable as they are easily manufactured and occupy less space in transportation.

Fig.1 cross sections of prestressed concrete polesDesign considerationsPrestressed concrete poles for power transmission lines are generally designed as members with uniform prestress since they are subjected to bending moments of equal magnitude in opposite directions.The poles are generally for the following critical load conditions:Bending due to wind load on the cable and on the exposed face.Combined bending and torsion due to eccentric snapping of wiresMaximum torsion due to skew snapping of wiresBending due to failures of all the wires on one side of the pole Handling and erection stresses. The indian standard IS:1678-1960 provides for a load factor of 2.5 for transverse bending strength.The flexural strength of the pole in the direction of the cable line should be not less than one quarter of the strength in the transverse direction.Smaller load factors ranging from 1.1 to 1.5 are prescribed for failure due to combined bending and torsion as a result of snapping of wires. Under over load conditions, progressive failure of the pole is ensured by designing the critical section as under reinforced , which gives ample warning before failure.The use of mild or high-strength deformed bars, in addition to the high tensile wires , would impart sufficient ductility to the member.In the case of tapered poles with a reduced cross-section towards the top, the effective prestressing force should reduced in proportion to cross section by the techniques of debonding or by dead ending or looping some of the tendons at mid height.Prestressed concrete piles Advantages of prestressed concrete piles:High load and moment carrying capacityStandardization in design for mass productionExcellent durability under adverse environmental conditionsCrack-free characteristics under handling and driving resistance to tensile loads due to upliftCombined load moment capacityGood resistance to hand driving loads and penetration into hard strataPiles can be lengthened by splicingEase of handling , transporting and drivingOverall economy in production and installation

Overall economy in production and installationAdaptability to both developed and developing countries in tropical, sub-arctic and desert regionsUse of solid and hollow cross-sectional configurations to suit design requirementsEase of connections with pile caps to form pier ,trestle and jetty bents to support bridge or wharf decksEffective use of fender piling to resist the kinetic energy of ship impactParticularly advantageous for deep foundations to carry heavy loads in weak soils

Types of prestressed concrete pilesBearing pilesSheet pilesCombined hearing and sheet pilesPier trestle and jelly bent piles (pile bents)High tower and stack piles (subject to uplift)Caisson pilesAnchor pilesFender piles

cross sectional shapes of prestressed concrete pilesNo.Cross sectional shape of pile

Meritsdemerits1.Triangular High ratio of skin friction perimeter to cross sectional area

Low manufacturing cost but low bending resistance

2Square

Good ratio of skin friction perimeter to cross sectional area

Low manufacturing cost ,good bending resistance on major axes3Pentagon or octagon

Approximately equal bending strength on all axes , Good penitrating ability ,Good columnn stability

Prone to surface defects during casting due to large no faces and edges

No.Cross sectional shape of pile

Merits

Demerits4.Cirular Equal bending strength on all axes with absense of corners , good aesthetics and high durability , min. wave and current loads, good column stability manufacturing costs generally higher, surface defects are unavoidable.

5.Rectangular with or without semi circular endsGreater bending strength about the shorter axis, min. surface to wave and current forcesdifficulty of orientation

6.I and starHigh bending resistance high manufacturing costs, difficulty of orientationPile shoes Pile shoes are required for driving through extremely sof materials like buried timbers and rocky strata.However , for driving prestressed concrete piles into sands, silts, clays and soft shales, pile shoes are unnecessary.Pile shoes are formed by thick steel plates or stubs welded to the reinforcing bars anchors and firmly embedded into the pile tip as shown in fig 1.The shape of tip may be varied to suit driving conditions.Field experience indicates that a square tip with chamfered corners is preferable to a pointed or wedge shaped tip since the latter causes the pile to deflect and develops high bending stresses which result in the failure of the shoe and pile.

Types of pile shoesTypes of prestressed sleepersTwo block sleepers connected by a pipe filled with concrete and containing high-tensile bars compressing the concrete in the blocks as shown in fig 2(a).Longitudinal sleepers located continuously under the rails and connected by flexible tie bars for gauge retention are shown in fig 2(b).Beam-type single piece prestressed concrete sleepers ,which are quite similar to the conventional wooden type sleepers in shape, length and supporting area as shown in fig 2 (c). These are flexurally stiff over their entire length and providing greater measure of rigidity to the track if the rails are tightly fastented to the sleepers, preventing rotation at the seatings and buckling of the rails.

Fig 2 (a)Two block R.C.C with prestressed tie

Fig.2 (b) longrine laval longitudinal blocks

Fig. 2 (c) beam type sleepersPrestressed concrete pressure vesselsAdvantages:Reduced possibility of sudden bursting failures triggered by local cracking , due to the high redundancy of the tendon concrete system.High degree of strength reliability , since at the time of intial tensioning , each tendon is proof tested in tension and concrete in compression.The unbonded tendon system provides for periodic surveillance so that retensioning or replacement of damaged tendons is possible during the service life of the reactor.The concrete in the vessel walls being under triaxial compression , higher load factors are available against cracking and failure of the material.

Ability to adequately anchor high pressure piping directly to the concrete through penetration sleeves.Experimental investigations indicate that stress concentrations in concrete due to circular holes do not influence the static and fatigue strength of concrete.Excellent radiation absorption characteristics of high-strength and high-density concrete.Durability and resistance of concrete to moderate temperatures.Feasibilty of construction bigger vessels to withstand higher pressures in the larger plants of the future.

Prestressed concrete pavementGeneral features:The provision of joints at close intervals in a concrete pavements to permit expansion or contraction is detrimental from the considerations of strength and riding quality.

The stresses in a slab resting on an elastic medium under the action of a concentrated load increases as the load approaches the free edge and is a maximum when the load is at the corner zones. Hence smaller slab sizes in a long roadway result in a large number of high stress zones, such as corners and edges.

Due to the differential vertical movement between adjacent slabs, the riding quality of the road deteriorates as the number of joints increases within a given stretch of the road.

Advantages of Prestressed concrete pavement To prevent the formation of shrinkage and temperature cracks during the early life of the concrete.Longitudinal prestressing can effectively eliminate the formation of cracks in slabs.Expansion joints and weak edge zones are entirely eliminated by the introduction of moderate pre-compression in the concrete slabs.

Methods of prestressing pavementsLongitudinal prestressing of the slabs is achieved either by external prestressing against rigid abutments or by internal prestressing by means of tensioned bars or cables.The method of external prestressing by using flat jacks against fixed abutments at the ends of the slab has the following disadvantages:Difficulty of providing unyielding abutments:yielding of abutments reduces the prestress in the slab.The compressive stress in the slab gradually decreases with time due to the shrinkage and creep of concrete.The system of external prestressing is applicable only to straight cables lying wholly in one plane. If applied to curved slabs, instability of the systems develops due to the buckling effect.

The method of internal prestressing by different arrangement of cables and anchorages lying within the slabs is ideally suited for long runways. Typical lay-outs of cables and anchorages used in the internal prestressing system is shown in fig 3(a) , (b) , (c). The transverse prestress eliminates the need for using dowel bars across the longitudinal joint.

Fig 3. internal prestressing methods for pavements