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7/24/2019 Pree Stress
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Introductionto
Prestressed
CONCRETE
Presented by,Sasikumar.S
07da004
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Outline
Introduction .
Definition.
Reinforced concrete vs. prestressed concrete.
dvanta!es.
"imitations.
Prestressin! met#ods.
$ypes of $endons. %ire resistance of prestressed concrete.
pplications of Prestressin!.
&onclusions.
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Introduction
The prestressing and precasting of concrete are inter-related features of the modern building industries.
Prestressing of concrete is the application of a compressiveforce to concrete members and may be achieved by either
pretensioning high tensile steel strands before the concretehas set, or by post-tensioning the strands after the concretehas set.
Although these techniques are commonplace,misunderstanding of the principles, and the way they areapplied, still exists. This paper is aimed at providing aclear outline of the basic factors differentiating eachtechnique and has been prepared to encourageunderstanding amongst those seeing to broaden theirnowledge of structural systems.
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!efinition
Prestressing of concrete is defined as the
application of compressive stresses to
concrete members. Those "ones of the
member ultimately required to carry tensile
stresses under woring load conditions are
given an initial compressive stress before the
application of woring loads so that thetensile stresses developed by these woring
loads are balanced by induced compressive
strength.
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Reinforcedvs.Prestressed
&oncrete
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Advantages
The use of prestressed concrete offers distinct advantages
over ordinary reinforced concrete. These advantages can be
briefly listed as follows#
Prestressing minimi"es the effect of cracs in concreteelements by holding the concrete in compression.
Prestressing allows reduced beam depths to be achieved for
equivalent design strengths.
Prestressed concrete is resilient and will recover from theeffects of a greater degree of overload than any other
structural material.
If the member is sub$ect to overload, cracs, which may
develop, will close up on removal of the overload.
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%ontd&
Prestressing enables both entire structural elements and
structures to be formed from a number of precast units, e.g.
'egmented and (odular %onstruction.
)ighter elements permit the use of longer spanning
members with a high strength to weight characteristic.
The ability to control deflections in prestressed beams and
slabs permits longer spans to be achieved.
Prestressing permits a more efficient usage of steel and
enables the economic use of high tensile steels and high
strength concrete.
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)imitations
The limitations of prestressed concrete are few and
really depend only upon the imagination of the
designer and the terms of his brief. The only real
limitation where prestressing is a possible solution
may be the cost of providing moulds for runs of
limited quantity of small numbers of non-standard
units.
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*
(ethods of Prestressing
Pre'tensionin!is used to describe a method ofprestressing in which the tendons are tensioned beforethe concrete is placed, and the prestress is transferred to
the concrete when a suitable cube strength is reached.
Post'tensionin!is a method of prestressing in whichthe tendon is tensioned after the concrete has reached asuitable strength. The tendons are anchored against thehardened concrete immediately after prestressing.
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Pre-tensioning (ethod
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Post-tensioning (ethod
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(aterials of prestressed concrete
+igh strength concrete.
+igh tensile steel.
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'trength equirements
The minimum -day cube compressive
strength prescribed in I'#/010-/*2 is
1234mm for pre-tensioned members and0234mm for post-tensioned members.
To safeguard against excessive shrinage,
the code prescribes that the cement contentin the mix should preferably not exceed
502g4m0.
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'hrinage of concrete
It is due to gradual loss of moisture which
results in changes in volume.
The phenomenon of shrinage of being timedependent ,only the total residual shrinage
strain is considered in the computation of
losses of the prestress to be used in design.
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%ontd..
The values of total residual shrinage strain
recommended in the I' code for the purpose
of design are 0.26/2-1 for pre-tensionedmembers and 7.26/2-184log7t98 for post
tensioned members where t is the age in
days of the concrete at transfer.
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+igh Tensile steel
The higher tensile strength is generally
achieved by marginally increasing the
carbon content in steel in comparison withmild steel.
+igh tensile steel usually contains2.: to
2.5 percent carbon,2.; to / percentmaganese,2.25 percent of sulphur and
phosphorous with traces of silicon.
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Types of Tendons
$#ere are t#ree types of tendons
(ars
)ires
Strands
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=ire
)ire, mainly used in post-tensioning
systems for prestressing concrete, is cold
drawn and stress relieved with a yield stressof about /022 (Pa. =ire diameters most
commonly used are 5mm, ;mm, and mm.
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'trand
Strand,which is used in both pre and post-tensioning ismade by winding seven cold drawn wires together on a
stranding machine.
'ix wires are wound in a helix around a centre wire which
remains straight. 'trands of /* or 0; wires are formed by
adding subsequent layers of wire.
%ables are formed by arranging wires or strands in bundles
with the wires or strands parallel to each other.
In use the cable is placed in a preformed duct in the
concrete member to be stressed and tensioned by a suitable
posttensioning method.
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%ontd&
Tendons whether bars, wires, strands, or made up cables
may be used either straight or curved.
'traight steel tendons are still by far the most commonly
used tendons in pre-tensioned concrete units. %ontinuously curved tendons are used primarily in post-
tensioning applications. %ast-in ducts are positioned in the
concrete unit to a continuous curve chosen to suit the
varying bending moment distribution along the members
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%ire resistance of prestressed concrete
All concrete is incombustible.
In a fire, failure of concrete members usually occurs due
to the progressive loss of strength of the reinforcing steel
or tendons at high temperatures. Also the physical
properties of some aggregates used in concrete can change
when heated to high temperatures.
fire resistance than structural steel or timber
Prestressed concrete has been shown to have at least the
same fire resistance as ordinary reinforced concrete.
>reater cover to the prestressing tendons is necessary
however, as the reduction in strength of high tensile steel
at high temperatures is greater than that of ordinary mild
steel.
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Applications
The construction possibilities of prestressed concrete are as vast as
those of ordinary reinforced concrete. Typical applications of
prestressing in building and construction are#
'tructural components for integration with ordinary reinforced
concrete construction, e.g. floor slabs, columns, beams.
'tructural components for bridges.
=ater tans and reservoirs where water tightness 7i.e. the absence of
cracs8 is of paramount importance.
%onstruction components e.g. piles, wall panels, frames, windowmullions, power poles, fence posts, etc.
The construction of relatively slender structural frames.
(a$or bridges and other structures.
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conclusion
Prestressed concrete design and construction is precise. The high
stresses imposed by prestressing really do occur. The following points
should be carefully considered#
To adequately protect against losses of prestress and to use the
materials economically requires that the initial stresses at prestressingbe at the allowable upper limits of the material. This imposes high
stresses, which the member is unliely to experience again during its
woring life.
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eferences
Prestressed concrete by 3.?rishnara$u, Tata
(c>raw-+ill publishing %ompany 1th@d.
undamentals of prestressed concrete by3.%.'inha B '.?.oyC '.%hand B %o, 3ew
!elhi.
www.vsl.com
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