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

    http://www.vsl.com/http://www.vsl.com/
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