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7/31/2019 Msp Prestress
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Asish Kumar PaniAsst.Professor, Civil Engg Deptt.
KIIT University
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Prestressed Concrete member is a member of concrete in whichinternal stresses are introduced in a planned manner, so that stressesresulting from the superimposed loads are counteracted to a desireddegree.
Two examples of prestressed concrete before the development ofprestressed concrete are provided
Force-fitting of metal bands on wooden barrels
The metal bands induce a state of initial hoop compression, tocounteract the hoop tension caused by filling of liquid in the barrels.
Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is applied to such anextent that there will always be a residual tension in the spoke.
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For concrete, internal stresses are induced (usually, by means of tensionedsteel) for the following reasons.
The tensile strength of concrete is only about 8% to 14% of its compressivestrength.
Cracks tend to develop at early stages of loading in flexural members such asbeams and slabs.
To prevent such cracks, compressive force can be suitably applied in theperpendicular direction.
Prestressing enhances the bending, shear and torsional capacities of theflexural members.
In pipes and liquid storage tanks, the hoop tensile stresses can be effectivelycounteracted by circular prestressing.
In order to get the maximum advantage of PSC member it is necessary touse not only high strength concrete but also high strength steel wires.
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Reasons for HSC
Since large prestressing forces are applied to the member by the tendons,
high bearing stresses are developed at the ends by anchoring devices. The
anchorages are generally designed to be meant for use only for HSC work
Bursting stresses liable to at the ends of the beam cannot be satisfactorily
resisted by low strength concrete.
It offers high bond stress which will be useful for the transferring of stress tothe concrete
Shrinkage cracks will be less in this case.
Due to the high modulus of elasticity of HSC ,the elastic & creep strains are
very small resulting in smaller loss of prestress in steel reinforcement.
By using HSC the c/s areas required for members will be reduced resulting
in considerable reduction of dead load moments particularly in long span
beams
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In an RCC beam concrete in compression side is only effective where as in
PSC the entire section is effective.
RCC beams are generally heavy, but the PSC beams are lighter.
HSC is not required in RCC ,but it is required in PSC.
RCC does not require many auxiliary units but PSC requires auxiliary units
like prestressing equipments,anchoring devices etc.
In an RCC beam the lever arm is constant ,the tension in steel or thecompression in concrete depends on the external loading. But in PSC the
tension in high tensile wires is practically constant & also the compressive
force on the concrete,while the lever arm changes with the external
loading.
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WiresPrestressing wire is a single unit made of steel.
Strands
Two, three or seven wires are wound to form a prestressing strand.
Tendon
A group of strands or wires are wound to form a prestressing tendon.
Cable
A group of tendons form a prestressing cable.
Bars
A tendon can be made up of a single steel bar. The diameter of a bar ismuch larger than that of a wire.
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Section remains uncracked under service loads
Reduction of steel corrosion
Increase in durability.
Full section is utilized
Higher moment of inertia (higher stiffness)
Less deformations (improved serviceability).
Increase in shear capacity.
Suitable for use in pressure vessels, liquid retaining structures. Improved performance (resilience) under dynamic and fatigue loading.
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High span-to-depth ratios
Larger spans possible with prestressing (bridges, buildings with large
column-free spaces)
Typical values of span-to-depth ratios in slabs are given below.
Non-prestressed slab 28:1 Prestressed slab 45:1
For the same span, less depth compared to RC member.
Reduction in self weight
More aesthetic appeal due to slender sections
More economical sections.
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Suitable for precast construction The advantages of precast
construction are as follows.
Rapid construction
Better quality control
Reduced maintenance
Suitable for repetitive construction
Multiple use of formwork
Reduction of formwork
Availability of standard shapes.
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Prestressing needs skilled technology. Hence, it is not as common as
reinforced concrete.
The use of high strength materials is costly.
There is additional cost in auxiliary equipments.
There is need for quality control and inspection.
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The various stages of the pre-tensioning operation are summarised as
follows.
1) Anchoring of tendons against the end abutments
2) Placing of jacks
3) Applying tension to the tendons
4) Casting of concrete
5) Cutting of the tendons.
During the cutting of the tendons, the prestress is transferred to the
concrete with elastic shortening and camber of the member.
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The relative advantages of pre-tensioning as compared to post-tensioning are as follows.
Pre-tensioning is suitable for precast members produced in bulk.
In pre-tensioning large anchorage device is not present .
The relative disadvantages are as follows.
A prestressing bed is required for the pre-tensioning operation.
There is a waiting period in the prestressing bed, before the concrete attains sufficientstrength.
There should be good bond between concrete and steel over the transmission length.
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The various stages of the post-tensioning operation are summarized asfollows.
Casting of concrete.
Placement of the tendons.
Placement of the anchorage block and jack.
Applying tension to the tendons.
Seating of the wedges.
Cutting of the tendons.
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The relative advantages of post-tensioning as compared to pre-tensioning
are as follows.
Post-tensioning is suitable for heavy cast-in-place members.
The waiting period in the casting bed is less.
The transfer of prestress is independent of transmission length.
The relative disadvantage of post-tensioning as compared to pre-tensioning
is the requirement of anchorage device and grouting equipment.