Prestress Concrete Ankita

8/13/2019 Prestress Concrete Ankita

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SUBMITTEDBY:-ANKITASHARMAMAHESHWARIGUPTASHOEBAHMAD


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INTRODUCTION

A pre-stressed concrete member is

a member of concrete in which internal stressesare introduced in a planned manner, so that

stresses resulting from the super imposed loads

counteracted to a desired degree. Concrete has a

high compressive strength compared to its very

low tensile strength.Prestressed decreases diagonal tensile stresses. This has led to adopt

modified and T sections in which there is a sustainable deduction in web area.

To get the maximum advantage of prestressed concrete members , it is necessary

to use high strength concrete and high tensile steel wires.

TendonA high strength steel

strand or bar for pre-

stressing concrete

AbutmentA structure for anchoring

the reinforcing tendons in

the pre- tensioning of a

concrete member Jacking force

The tensile force

exerted temporarily

by a jacking the

pre-stressing of aconcrete member

Anchor

A mechanical device for

locking of a stressed

tendon in position


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NEED OF PRE-STRESSING

To offset the deficiency of tensile strength in concrete, steel reinforcement is

provided near the bottom of simple beams to carry the tensile stresses.

ADVANTAGES OF PRESTRESSED CONCRETE

1. Durability

As this technique eliminates weakness of concrete in tension, such members

remain free from cracks; hence can resist the effects of impact, shock, and

reversal of stresses more efficiently than R.C.C. structure.

They provide reliable long-term performance in extremely harsh conditions

that could destroy lesser materials.

They are resistant to deterioration from weather extremes, chemical attack,

fire, accidental damage and the determined efforts of vandals.

Winter construction can proceed with few weather delays as pre-cast

components are Prefabricated in heated plants.


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2. ADAPTABILITY

Pre-cast pre-stressed concrete products can be designed and manufactured for

any application, ranging in size from short span bridges to some of the largest

projects in the world.

Permits pre-cast manufacturers to vastly expand the design variety possible

using pre-cast components.

the inherent plasticity of concrete permits to create pre-cast components in

shapes and sizes, which would be prohibitively expensive using other materials

3. FIRE RESISTANT

Pre-stressed concrete bridges are not easily damaged by fire. Have excellent

fire resistance, low maintenance costs, elegance, high corrosion resistance, etc.

4. FAST AND EASY CONSTRUCTION

Pre-cast concrete components lend themselves to fast construction schedules.

Pre-cast manufacturing can proceed while site preparation is underway.

Pre-cast units can be delivered to the jobsite and installed the moment they

are needed in any weather.

Fast construction means earlier completion and the resulting cost savings.

Saves the cost of shuttering and centring for large structures.


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5. AESTHETICS

Pre-cast components can be delivered with a wide range of shapes and finishes

ranging from smooth dense structural units to any number of architectural

treatments.Strikingly rich and varied surface textures and treatments can be achieved by

exposing colure sands, aggregates, cements and colourings agents using

sandblasting and chemical retarders.

custom form liners can be used to introduce reveals, patterns and other

architectural effects.

Stone, tile brick and other materials can be cast into pre-cast panels at the

factory,enabling designers to achieve the expensive look of masonry.


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DISADVANTAGES OF PRESTRESSED CONCRETE

Although pre-stressing has many advantages, there are still some drawbacks

of this process.

The unit cost of high strength materials being used is higher as mostly high

tensile steel is used.

extra initial cost is incurred due to use of pre-stressing equipment and its

installation.

extra labour and transportation cost for pre-stressing is also there.pre-stressing is uneconomical for short spans and light loads.


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Principles of pre-stressing : -

Large pre-stressing force are applied to the member by the

tendons, high bearing stresses are developed at the ends by the

anchoring devices. The anchorages are generally designed to be

meant for use only for high strength concrete work.

Busting stresses liable to at the ends of the beam cannot be

satisfactorily resisted by low strength concrete work.

When stress transfer to concrete has to take place by bond

action, the concrete should have a high strength concrete.

Shrinkage cracks will be very little when high strength concrete is

used.

Due to the high modules of elasticity of high strength concrete,the elastic and creep strain are very small resulting in smaller loss

of pre-stress in all steel reinforcement.


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There are many ways of classifying prestressed concrete members based on

the method of design, construction and application of pre-stress. These are

explained below :

A. EXTERNALLY AND INTERNALLY PRESTRESSED MEMBERS :A member can be prestressed either by external reaction offered by rigid

abutments or by tensioned tendon. The former is called external prestressing

and the latter is called internal prestressing.

In the external prestressing instead of providing a tendon from which the

prestress can be applied , the necessary prestressing force can be applied by

compressing the members by jacking against abutments. This method is difficult

to be adopted , though this principle is adopted in the case of arches. It is also

found that shrinkage and creep of concrete are likely to effect the initially

applied prestress.

B. LINEAR OR CIRCULAR PRESTRESSING:The term circular prestressing is applied to prestressing circular structure like

cylindrical tanks and pipes in this case the tenders are provided in the form of

Rings

Linear prestressing is a term applied to prestressing straight members like beams

and slabs.


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Pre-tensioned members-In these, the tendons are tensioned even before casting the concrete.

One end of the reinforcement (i.e. tendon) is secured to an abutment while theother end of the reinforcement is pulled by using a jack and this end is then fixedto another abutment. The concrete is now poured. After the concrete has cured and hardened, theends of the reinforcement are released from the abutments. The reinforcement which tends to resume its original length will compress theconcrete surrounding it by bond action. The prestress is thus transmitted to

Concrete entirely by the action of bond between the reinforcement and thesurrounding concrete.

C. PRE-TENSIONING AND POST TENSIONING


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POST TENSION MEMBERIt is one in which the reinforcement is tensioned after the

concrete has fully hardened. The beam is first cast leaving ducts for placing the tendons. The ducts are made in a number of ways - by leaving corrugated steel tube

in the concrete, by providing steel spirals, sheet metal tubing, rubber have or anyother duct forming unit in the form work. When the concrete has hardened and developed its strength, the tendon is passed

through the duct. One end is provided with an anchor and is fixed to one end of the member.

Now, the other end of the tendon is pulled by a jack which is butting against the endof the member.

The jack simultaneously pulls the tendon and compresses the concrete. After the tendon is subjected to the desired stress, the end of the tendon is also

properly anchored to the concrete. To avoid crushing of concrete due to excessive bearing stress, a distribution plate is

provided at each end.


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SYSTEMS OF POST- TENSIONING

1. THE FREYSSlNET SYSTEM

High tension steel wires 5 mm to 8 mm diameter about 12

in number are arranged to form a group into a cable with aspiral spring inside.

The spiral spring providesproper clearance between the wiresand thus provides a channel which canbe cement grouted.

It further assists to transfer thereaction to concrete.

The anchorage consists of a good quality concrete cylinder and is provided with

corrugations on the outside. It has a central conical hole and is provided with heavy hoopreinforcement.The conical plugs are pushed into the conical holes after cables are tightened. The centralhole passing axially through the plug permits cement grout to be injected through it.In this way the space between the wires will be filled with the grout. This providesadditional restraint against the slipping of the tendons.

The whole thing is enclosed in thin metalsteel.


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ADVANTAGES OF THE SYSTEM

(i) Securing the wires is not expensive.

(ii) The desired stretching force is obtained quickly.

(iii) The plugs may be left in the concrete and they do not project beyond the ends ofthe member.

DISADVANTAGES OF THE SYSTEM

(i) All the wires of a cable are stretched together. Hence the stresses in the wires maynot be exactly the same.

(ii) The greatest stretching force applied to a cable is from 250 KN to 500 kN. This maynot be sufficient.

(iii) The jacks used are heavy and expensive.


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3. GIFFORD UDALL SYSTEM

This method offers the following three methodsof prestressing:

First method

This is earliest of the three methods of this system.In this method the wires are stressed and anchored one by one in a separate cylinder usingsmall wedging grips called udall grips.

Each grip consists of two-half cones.The bearing plate bears against a thrust ring which is cast into the concrete.The duct end is encircled by a helix.Anchorages are supplied to suit cables of 2, 4, 6 and 12 wires.

Second method

In this method, the wires are anchored by wedges which fitdirectly into tapered recesses made in the bearing plate.

The bearing plate bears against a tube unit containing the tube unit and the helix.This tube unit is cast into the concrete.Anchorages are supplied for cables of 8 to 12 wires.This arrangement is compact and minimizes the congestion of the steel wires in anchor

block.


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4. P.S.C. MONOWIRE SYSTEM

MONOWIRE SYSTEM

In this system also the wires are tensionedindividually. The anchorage consists of a single piece collet

sleeve wedging in a conical hole. A steel truncated guide leads each wire fromthe cable to the anchorage point along a gentlecurvature.In addition to the guide a central block is alsoprovided to anchor the central wires.

5. ELECTRICAL PRESTRESSING

This is a method of post tensioning without the use of jacks introduced by Bittner andCarlson,Steel bars are provided with a coating of sulphur, before they are embedded in

concrete.After the hardening of concrete electric current of low voltage and high amperage isused to heat the bars to a temperature of 1700 C.As the bars expand longitudinally, the nuts on the projecting ends are tightenedagainst heavy washers.As the temperature falls, the prestress is developed in the bars and the bond is againrestored by the resolidification of the sulphur coating.


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LOSS OF PRE-STRESS

A reduction in initial pre-stress resulting from the combined effect of creep,shrinkage or elastic shortening of the concrete, relaxation of the reinforcing steel,frictional losses resulting from the curvature of the draped tendons and slippage at theanchorage.

The steel wires of a pre-stressed concrete member do not retain all the preliminarypre-stress .

The initial pre-stress in concrete undergoes a gradual reduction with time from thestage of transfer due to various causes.

A loss of pre-stress will affect the stress distribution on the section of the member.


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The loss of pre-stressed takes place due to many causes. In general these can be classifiedas:

Loss of pre-stress during the tensioning process

Loss of pre-stress at the anchoring stage.

Losses occurring subsequently

PRE-TENSIONING POST-TENSIONING

Elastic deformation of concrete

Relaxation of stress in steel

Shrinkage of concreteCreep of concrete

No loss due to elastic deformation if allthe wires are simultaneously tensioned. Ifthe wires are successively tensioned there

will be loss of pre-stress due to elasticdeformation of concrete

1.Relaxation of stress in steel2.Shrinkage of concrete3.Creep of concrete4.Friction5.Anchorage slip


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In addition there may be losses of pre-stress due to sudden changes in temperature,especially in steam curing of pre- tensioned units.

The rise in temperature causes a partial transfer of pre-stress (due to elongation of thetendons b/w adjacent units in the long line process) which may cause a large amount of

creep if the concrete is not properly cured.

LOSS OF PRE-STRESS DURING THE TENSIONING PROCESS DUE TO FRICTION

Friction in the jacking and anchoring system and on the walls of the duct where thewires fan out at the anchorage with the result, the actual stress in the tendons is less than

what is indicated by the pressure gauge.

The losses due to friction in the jack and at the anchorage are different for differentsystem of pre-stressing.

This loss due to friction may be classified into:

Loss Due To Length Effect

The extent of friction met with in a straight tendon due to slight imperfection of the duct(the straight tendon).

Hence the cable will touch the duct or concrete, wobbing effect, or wave effect


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Loss due to curvature effect

In the case of curved ducts, the loss of pre-stress depends upon the radius ofcurvature of the duct and the coefficient of friction between the duct surface and thetendons.

LOSS OF PRESTRESS AT THE ANCHORING STAGE This loss is due to the fact that the anchorage fixtures themselves are subjected to astretch.

It is also possible that the friction wedges holding the wires the wires may slip a little

The necessary additional elongation may be provided for at the time of tensioning to

compensate for this loss.LOSS OF PRESTRESS OCCURIING SUBSEQUENTLY

The loss which occur subsequently to pre-stress are:

Loss Of Stress Due To Shrinkage Of Concrete:

Contraction of concrete due to chemical changes and drying. This depends only on the

interval of time and the moisture conditions, but is independent of the stresses in themembers due to loads


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Uses

These tanks are used in portable water treatment and distribution system,

wastewater collection and treatment system and storm water management.

They are also used in a variety of commercial applications including thermalenergy storage, LNG containments, large industrial process tanks and large

bulk storage tanks.

Water

Pre-stressed concrete is the most efficient material for water tanks and

coupled with the circular shape, eliminates all stress conditions.

By placing the steel of the pre-stressed strands in tension and the concrete in

compression, both materials are in an ideal states and the loads are uniformly

distributed around the tank circumference.

PROPERTIES

Low maintenance can be enjoyed

throughout the life as these are built with

concrete, durable material that never

corrodes and does not require coatings

when in contact with water or the

environment.


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Pre-stressing counteracts the differential temperature and

dryness loads that a tank core wall experience. The tank walls are

wet on the inside and dry on outside and the temperature varies

between the two sides. If not properly accounted for, thesemoisture and temperature differential will cause a tank wall to bend

and crack. Counteract these force in both the vertical and

horizontal direction and diminish subsequently the cracking and

leaking

Tanks are very ductile, enabling to withstand seismic forces and

varying water backfill.

Tanks utilize material efficiently steel in tension, concrete in

compression

Pre-cast tanks can store or treat anything from potable water to

hazardous waste to solid storage bins.

Storage capacities can range from 0.4 to 120 mega liters

Diameters of the tank can vary upto 90 m

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Prestress Concrete Ankita