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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