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					                       PRE-STRESSING
INTRODUCTION

The process of induction of compressive
stresses in the structure before it is put to its
actual use is known as Pre-stressing.
Pre-stressed Concrete member is a
member of concrete in which internal
stresses are introduced in a planned manner,
so that the stresses resulting from the
superimposed loads are counteracted to a
desired degree
•Pre-stressing is the intentional creation of permanent stress in a structure or
assembly, for improving its behaviour and strength under various service conditions.
•In ordinary reinforced concrete, consisting of concrete and mild steel as basic
components, the compressive stresses are born by concrete while tensile stresses are
born entirely by steel. The concrete only acts as a binding material; it does not take
part in resisting the external forces.
•In pre-stressed concrete, compression is induced prior to loading in the zones
where external loads would normally cause tensile stresses.
•In the case of long beams, where large shear forces exist, the beam sizes have to be
large to limit the diagonal tensile stresses under certain limits. Pre-stress decrease
diagonal tensile stresses. This has led to adopt modified I-section and T-section in
which there is substantial reduction in web area.
•In order to get the maximum advantage of a pre-stressed concrete member, it is
necessary to use not only high strength concrete but also high tensile steel wires.
•Concrete used for pre-stressed work should have cube strength of 35N/m m2 for
post-tensioned system and 45N/m m2 for pre-tensioned system.
•In the design of a pre-stressed concrete member, the estimated loss of pre-stress
due to shrinkage of concrete and creep of concrete and steel is at the order of nearly
200N/m m2
                                                               Abutment
                           Tendon                              A structure for anchoring
                           A high strength steel               the reinforcing tendons in
                           strand or bar for pre-              the pre- tensioning of a
                           stressing concrete                  concrete member




             Anchor
             A mechanical device for locking of a stressed     Jacking force
             tendon in position and delivering the pre-        The tensile force exerted
             stressing force to the concrete either            temporarily by a jacking
             permanently in a post tensioned member or         the pre-stressing of a
             temporarily during hardening of a pre-
             tensioned concrete member
                                                               concrete member
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.
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 resistance
• Pre-stressed concrete bridges are not easily damaged by fire. Have excellent fire
resistance, low maintenance costs, elegance, high corrosion resistance, etc.
4. impacts local economy directly
•Pre-stressed concrete is produced by local small business - employing local labour.
•Most of its raw materials are also locally purchased and the health of the local pre-
stressed concrete industry directly impacts further on the local economy.
•Due to smaller loads, due to smaller dimensions being used, there is a considerable
saving in cost of supporting members and foundations.
•standard structural shapes such as hollow core, double tees, beams, columns and
panels can be mass-produced at low cost.
5. 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.
6. 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.
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.
     COMPARISON OF PRESTRESSED CONCRETE BEAMS WITH RCC
                            BEAMS
1.    In RCC beams, the concrete in the compression side of the neutral axis alone is
      effective and the concrete in the tension side is ineffective. But, in the pre-
      stressed beams, the entire section is effective.
2.    Reinforced concrete beams are generally heavy. Pre-stressed concrete beams
      are lighter.
3.    RCC beams being heavy and massive are more suitable in situations where the
      weight is more desired than strength. Pre stressed beams are very suitable for
      heavy loads and long spans.
4.    In RCC beams, there is no way of testing the steel and the concrete. In pre-
      stressed concrete beams, testing can be done while pre stressing.
5.    RCC construction does not involve many auxiliary units. But pre-stressed
      beams require many auxiliary units.
    Assumptions in design of pre-stressed concrete members
    Pre-stressed concrete members are analysed and designed on the basis of the
       following assumptions given below:
•      A transverse plane section of the member will remain a plane after bending
       also.
•      Within the limits of the deformation taking, Hook’s law is applicable to
       concrete and steel components.
•      The stress in the reinforcement does not change along the length of the
       reinforcement. Stress changes take place for the concrete component only.
       Variation of stress in the reinforcement due to changes in the external loading is
       ignorable.
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.
        CLASSIFICATION OF PRE-STRESS CONCRETE

There are many ways of classifying pre-stress concrete members based of the
   member of design, construction and application of pre-stress, some of them are
   as follows:-


1. External or internal pre-stressing

2. Linear or circular pre-stressing

3. Pre-tensioning and post tensioning
     Pre-tensioning system
       (fully bonded constructions)
     Post –tensioning system
       (end anchored constructions)

4. Full pre-stressing or partial pre-stressing
A. EXTERNAL AND INTERNLLY PRESTRESSED MEMBERS

•     In External Prestressing, the member
     is prestressed by external reaction offered
     by rigid abutments.
•     In this, the necessary prestressing force can be
     applied by compressing the member by jacking against abutments.
•     A sliding surface may be provided underneath the beam.
•     After the prestressing is over, the space between the end of the beam and the
      abutment may be packed with concrete and the jack recovered.

     Disadvantages
     •Prestress transferred to the member is likely to be lost due to any possible
     outward displacements of the abutments.
     •Shrinkage and creep of concrete are
      likely to affect the initially applied prestress.
     •Even slight vertical deformations of the supports
     will disturb the stresses seriously.

    In Internal Prestressing tendon is provided from which the prestress
    can be applied
B. LINEAR OR CIRCULAR PRESTRESSING.

Linear prestressing is a term applied to prestressing straight members like
beams and slabs.




The term circular prestressing is applied to prestressing circular structures
like cylindrical tanks, silos and pipes. In this case, the tendons are provided in the
form of rings
  C.   PRE-TENSIONING AND POST TENSIONING


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 the other end of
   the reinforcement is pulled by using a jack and
   this end is then fixed to another abutment.

  The concrete is now poured. After the concrete has cured and hardened, the
 ends of the reinforcement are released from the abutments.


                    The reinforcement which tends to resume its
                    original length will compress the concrete
                   surrounding it by bond action. The prestress
                    is thus transmitted to concrete entirely by the
                   action of bond between the reinforcement and
                               the surrounding concrete.
     Post-tensioned member - It 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
      any other 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 end of 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.
   SYSTEMS OF PRESTRESSING
      A system of prestressing means the actual process adopted in making a
      prestressed beam. A system of prestressing involves :
                • process of tensioning the tendons
                • securing them firmly to the concrete

   A. Pre-tensioning system
   HOYER SYSTEM
   The Hoyer system is usually adopted for the production of pre-tensioned members
   on a large scale like precast beams.
  • Wires are stretched between two bulkheads at large distance apart.
  • The concrete is now poured so that a number of beams can be produced in one
  line, by providing suitable shuttering between them.
                      • After the concrete has hardened, the wires are released from
                      the bulkheads and between the different units in one line of
                      beams.
                      • The prestress is transferred to the concrete through bond
                      between the tendons and the concrete.

Pre-tensioning system is found uneconomical for large spans.
In order to grip the pre-tensioned wires properly to the bulkhead the devices are
shown
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 a spiral spring inside.
                                           •The spiral spring provides
                                                proper clearance between the wires
                                                and thus provides a channel which
                                                can be cement grouted.
                                                 It further assists to transfer the
                                                reaction to concrete.
                                           The whole thing is enclosed in thin metal
                                           steel.
•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
hoop reinforcement.
•The conical plugs are pushed into the conical holes after cables are tightened. The
central hole 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 provides
additional restraint against the slipping of the tendons.
Advantage 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
      of the member.


Disadvantage of the system

(i) All the wires of a cable are stretched together. Hence the stresses in the wires
      may not be exactly the same.
(ii) The greatest stretching force applied to a cable is from 250 KN to 500 kN. This
      may not be sufficient.
(iii) The jacks used are heavy and expensive.
2. THE MAGNEL BLATON SYSTEM

• In this system a cable of rectangular
section is provided, which contains layers of
wires 5 mm to 8mm diameter.
• The wires are arranged with four wires
per layer. The wires in the same layer and the
wires in adjacent layers are separated with a
clearance of 4mm.
•   The geometric pattern of the wires is maintained throughout the length of the
    cable by providing grills or spacers at regular intervals. The grills do not offer
    any appreciable frictional resistance to the wires which can be moved relative to
    each other during the tensioning process.
•   The wires are anchored by wedging, two at a time into sandwich plates (25
    mm thick and are provided with two wedge-shaped grooves on its two faces).
•   The wires are taken two in each groove and tightened by jacking two wires at a
    time.
•   Then a steel wedge is driven between the tightened wires to anchor them
    against the plate.
•   Each plate can anchor eight wires. The various sandwich plates forming a unit
    are arranged one above the other against a distribution plate.
3. GIFFORD UDALL SYSTEM

This method offers the following three methods
of 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 using small 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
                             fit directly 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.
4. P.S.C. MONOWIRE SYSTEM

                                      In this system also the wires are tensioned
                                      individually.
                                      • The anchorage consists of a single piece
                                      collet sleeve wedging in a concial hole.
                                      • A steel truncated guide leads each wire
                                      from the cable to the anchorage point along a
                                      gentle curvature.
               MONOWIRE SYSTEM        •In addition to the guide a central block is also
                                      provided to anchor the central wires.

5. ELECTRICAL PRESTRESSING

This is a method of post tensioning without the use of jacks introduced by
Bittner and Carlson,
•Steel bars are provided with a coating of su]phur, before they are embedded in
concrete.
•After the hardening of concrete electric current of low voltage and high
amperage is used to heat the bars to a temperature of 1700 C.
•As the bars expand longitudinally, the nuts on the projecting ends are tightened
against heavy washers.
•As the temperature falls, the prestress is developed in the bars and the bond is
again restored by the resolidification of the sulphur coating.
Post-tensioning system
In order to ensure the correct prestress in the reinforcement, it is preferable to
measure the load applied by the jack as well as the consequent extension of the
reinforcement.
     Extension measurements give an idea as to how much of the steel is being
     properly stretched.
     For instance the sides of the ducts may obstruct the stretching of the
     reinforcement particularly at the end remote from the jack and this part of the
     reinforcement may not receive the full tension. This defect is liable to occur
     when curved cables are provided.
 On the other hand; it may be possible that the extension might have taken place
due to a certain part of the tendon being overstressed. For this reason, it is
necessary to measure the prestressing force applied.
Excessive bearing stresses will be produced at the ends of the members due to
the anchoring devices which bear against the concrete. In this part of the member it
will be necessary to provide additional reinforcement to prevent local splitting and
failure by shear.
SUITABILITY OF THE SYSTEM
Pre-tensioning and post tensioning are the only practically adopted
methods of making prestressed concrete members.

In pre-tensioning, the compressive force in concrete is due to the bond between
concrete and the steel wires, which are kept stretched between buttresses.
     This method is economically adopted in mass production in factories which
     make concrete products of limited size.
     This is so because, handling as well a transporting large products are highly
     expensive and may be practically impossible, if thee members are too large.
     The length of a prestressed concrete product that can be economically made by
     this method may not exceed 20 m. Even this length involves considerable
     difficulty in transport.00

 In post tensioning, the prestressing wires are stretched after the hardening of
 concrete. This can be done by many ways like the following:
   These methods can be adopted in sites or on the ground, and later hoisted into
   position. For instance, if timber is costly or scarce it may be economical to make
   the member on the ground in steel moulds.
MATERIALS FOR PRE-
STRESSED CONCRETE:
1- High-Strength Concrete:
High strength concrete mix:
Pre-stressed concrete requires
concrete which has a high
compressive strength, with
comparatively higher tensile strength.
Low shrinkage, minimum creep
characteristics and a high value of
Young’s modulus are generally
deemed necessary for concrete used
for concrete used for pre-stressed
members.
A minimum cement content of 300 to 360 kg/m3 is prescribed
mainly to cater to the durability requirements. In high-strength
concrete mixes, the water content should be as low as possible
with due regard to adequate workability.
To safeguard against excessive shrinkage, the code prescribes
that the cement content in the mix should preferably not
exceed 530kg/m3.
Aggregate of rock types having high moduli of elasticity and low
values of differed strain are more effective in restraining the
contraction of the cement paste and their use reduces the
shrinkage of concrete. The commonly used aggregates, in
increasing order of effectiveness in restraining shrinkage, are
sand-stone, basalt, granite, quartz and limestone.

The values of total residual shrinkage strain recommended in the
I.S. code for the purpose of design are 3.0X10-4 for pre-tensioned
members and(2.0X10-4)/log(t+2) for post-tensioned members,
where t is the age in days of the concrete at transfer.

Light-weight Aggregate Pre-stressed Concrete:
The main advantage of light weight concrete is that it reduces the
self –weight of the structure, thus minimizing the amount of concrete
and steel required for carrying the load.

The light weight criterion becomes important especially in long span
structures where dead load forms the major portion of the total design
load on the structure, or when the self-weight of the member is a
factor to be considered in the transportation and erection, as in pre-
cast concrete construction.
The light-weight aggregates, generally used for pre-
stressed concrete are foamed slag, lytag and aglite.

The modulus of elasticity of light-weight concrete is
about 50 to 55 percent of the modulus of elasticity of
normal- weight concrete and hence the loss of pre-
stress due to elastic deformation is higher and
deflections of flexural members are comparatively
higher due to the lower values of modulus of elasticity.

The unit weight of light-weight concrete varies
considerably between 1450 and 1750 kg/m3.

The shrinkage and creep of light-weight concrete is
comparable, with marginal variations, to that of sand
and gravel concrete.
  2- High-Tensile Steel:
For pre-stressed concrete members, the
high tensile steel used generally consists of
wires, bars, or strands.
The higher tensile strength is generally
achieved by marginally increasing the
carbon content in steel in comparison with
mild steel.
High tensile steel usually contains 0.60 to
0.85 percent carbons, 0.70 to 1.00 percent
manganese, and 0.05 percent of sulphur and
phosphorus with traces of silicon.
The high-carbon steel ingots are hot-rolled into rods and cold-
drawn through a series of dies to reduce the diameter and increase
the tensile strength.
The process of cold-drawing through dies decreases the durability
of the wires. The cold-drawn wires are subsequently tempered to
improve their properties. Tempering or ageing or stress relieving
by heat treatment of the wires at 150-420oC enhances the tensile
strength. The cold drawn stress relieved wires are generally
available in nominal sizes of 2.5,3,4,5,7 and 8mm diameter and
they should confirm to the Indian standard code IS: 1785-1983.
The hard drawn steel wires which are
indented or crimped are preferred for
pre-tensioned elements because of their
superior bond characteristics. The small
diameter wires of 2 to 5 mm are mostly
used in the form of strands comprising
two, three or seven wires.

The high-tensile steel bars commonly
employed      in     pre-stressing are
manufactured in nominal sizes of
10,12,16,20,22,25,28 and 32mm diameter
and are covered in IS:2090-1983.

The ultimate tensile strength of a plain-
drawn steel wire varies with its diameter.
The tensile strength decreases with
increases in the diameter of the wires.
And referred in the relevant Indian
standard codes.
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 the anchorage.
The steel wires of a pre-stressed concrete member do not retain all
the preliminary pre-stress .




 The initial pre-stress in concrete undergoes a gradual reduction with
 time from the stage of transfer due to various causes.
 A loss of pre-stress will affect the stress distribution on the section of
 the member.
The loss of pre-stressed takes place due to many causes. In general these can be
classified as:
•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              1.No loss due to elastic deformation if all
                                              the wires are simultaneously tensioned. If
                                              the wires are successively tensioned there
                                              will be loss of pre-stress due to elastic
                                              deformation of concrete
 •Relaxation of stress in steel               2.Relaxation of stress in steel
 •Shrinkage of concrete                       3.Shrinkage of concrete
 •Creep of concrete                           4.Creep of concrete
                                              5.Friction
                                              6.Anchorage slip
 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 the
tendons 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 the
wires 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 different
system 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
 Loss due to curvature effect
            In the case of curved ducts, the loss of pre-stress depends upon the radius
of curvature of the duct and the coefficient of friction between the duct surface and the
tendons.
LOSS OF PRESTRESS AT THE ANCHORING STAGE
 This loss is due to the fact that the anchorage fixtures themselves are subjected to a
stretch.
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 the
members due to loads
By minimizing the water cement ration and proportion of cement, the shrinkage can
be reduced.
•Loss Of Stress Due To Creep Of Concrete
Creep of concrete means the deformation of concrete, which depends upon the
interval of time to which the member is loaded
This additional deformation of the stressed member is remaining in a stressed state is
called CREEP.
•Loss Of Stress Due To Elastic Shortening Of Concrete
(a) Pre-tensioned member
Due to the pre-stress transfer to the concrete, the concrete will shorten. This results
in a corresponding shortening of steel


(b) Post tensioned member
Suppose only a single tendon has been provided in a member, the concrete gets
shortened as the tendon is jacked against it.
Hence, after tightening, no more shortening of concrete can take place
     Loss Of Stress Due To Creep Of Steel(Stress Relaxation)
Total loss of pre-stress depends on many factors such as properties of concrete and
steel, moisture and curing condition, the magnitude and system of pre-stress
 The first three types of losses take place due to reduction in the length of
concrete resulting in reduction in reduction of the initial extension of the steel.
 The loss of stress due to elastic shortening of concrete is maximum in pre-
tensioned members.
In the case of post –tensioned members those losses occur only when a number
of cables are progressively stressed one after another.
APPLICATIONS OF THE PRE-STRESSED CONCRETE:
                                     Concrete is an all-round construction
                                     material. Almost every building contains
                                     some concrete, but its questionable
                                     application in certain buildings-for example
                                     in its use in the style of brutalism - has
                                     brought it into discredit. Its dull grey colour
                                     has contributed to the fact that the word
                                     concrete has become a synonym for ugly. In
                                     the field of bridges, concrete deserves a more
                                     favourable judgement.
MEGA FLOOR,the Prestressed                Not all concrete bridges have turned out
slab                                    to be beauties, but pleasing bridges can be
Slab:Hollow core slab, Preslab          built with concrete if one knows the art.
or predalle , prestressed ribs and      Concrete is poured into forms as a stiff but
blocks , lintels.                       workable mix, and it can be given any
Beam: Prestressed rectangular           shape; this is an advantage and a danger.
beam and I-beam for bridges             The construction of good durable concrete
Other prestressed components:           requires special know-how - which the
Lintels , Wineyard stud.                bridge engineer is assumed to have.
  Pre-stressed concrete - if correctly designed - also has high fatigue strength under
the heaviest traffic loads.
  Pre-stressed concrete bridges soon became much cheaper than steel bridges, and
they need almost no maintenance - again assuming that they are well designed and
constructed and not exposed to de-icing salt.
 In bridge building, concrete beams and arches predominate. The
shaping of concrete is usually governed by the wish to use formwork,
which is simple to make. Plain surfaces, parallel edges and constant
thickness are preferred. This gives a stiff appearance to concrete
bridges, and avoiding this is one task of good aesthetic design
 All types of structures can be built with reinforced and pre-stressed
concrete: columns, piers, walls, slabs, beams, arches, frames, even
suspended structures and of course shells and folded plates.
 Tanks
 Foundation panels
 Poles
 Modular block retaining wall system
•Wall panels
•Concrete units
•Slabs
• Roofing and flooring
• Lintel and sunshade
• Beams
• Columns girders
TANKS
  STRAND Wrapped circular pre-stressed concrete
tanks are long life liquid storage structure with virtually
no maintenance.
  Concrete construction makes for a substantial, sturdy
tank structure that easily contain the internal liquid
pressure while comfortably resisting external forces
such as earthquake, wind.
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 thermal energy
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.
 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,
these moisture 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
METHOD
Pre-cast concrete wall elements are usually pre tensioned vertically in the
plant and post tensioned horizontally through ducts cast in the panels.
Vertical pre stressing diminishes vertical bending in the wall and subsequent
leakage
Circumferential pre-stressing counteracts bursting loads from interior liquid.
Joints closures are usually poured concrete on site. This method of sealing
the joints allow the tank to [perform (after post – tensioning) as a monolithic
structure to resist hydraulic, temperature and seismic forces.
SHAPE OF THE TANKS
 FOUNDATION PANELS
 PRECAST CONCRETE foundation panels consist of
steel reinforced concrete studs, reinforced tops and
bottom beams and concrete facing.
Insulation can be placed between the studs.
A typical panellised foundation can be erected in four to
five hours, according to a manufacturer, with no on site
concrete work (the panels sit on gravel bed in lieu of
footings.)




                                  DETAILS
WALL PANELS
                                        Ideally suited for many Building
 Fast Track Easy Installation
                                        Application such as Industrial, Light
 Low Cost Walling                      Industrial & Commercial, Heavy Duty
 Durable Low Maintenance Walling       Warehousing, Bulk Storage and Waste
                                        Transfer Stations
 All Weather Construction
                                        Pre-stressed Concrete Panel
 No Trades – Foundation Free Walling   Division are manufacturers and
 High Impact Resistance – Cladding     suppliers of pre-stressed concrete
Protection                              panels for use in both agricultural and
                                        construction industries.
 Added Security and Fire Stop
Properties                                    These panels are used in a
•Steel Frame Compatible Walling         variety of applications including fast
System                                  track wall construction, retaining
                                        walls, bulk storage bunkers, grain
•Fast Simple and Robust Angle Bracket   stores and silage clamps.
Connection
                                        Manufactured in lengths of up to
•Tongue and Groove Joints
                                        7000mm, the pre-stressed panels are
•Variable Height Walling                offered in a selection of thickness to
•Simple Top Lifter Installation         suit various applications and a range
                                        of widths. Their simple clamp bracket
•Any Length                             connection and tongue/grooved profile
                                        make for a fast and efficient
                                        installation system.
POLES
Low maintenance, competitive price and aesthetic appearance of the pre-cast concrete
poles make them superior to steel or wood for use in utility, communication area lighting
application
The use of concrete poles preserve our forest requires no chemical treatment and
utilizes environmentally safe materials in he production and placement.
 Some other benefits are corrosion resistance, long service life
Because of mineral vibration and deflection, pre-cast concrete poles offer greater
service life to ballast for light and this in turn means less down time and less costly
equipment repair.
Pre-cast concrete poles can save erection time and money by eliminating the need for
anchor based structure, which may take days or weeks to install.

  PRECAST STAIRS AND LANDINGS




Straight Flights       Integral Landings           Separate Landings
                       (quarter/half landings)
MODULAR BLOCK RETAINING WALL SYSTEM
Modular block or segmental, retaining walls employ
interlocking concrete units that tie back into the earth to
efficiently resist loads.
These pre- engineered modular systems are an
attractive, economical and durable an alternatives to
atone or poured concrete retaining walls.
The inherent design flexibility can accommodate a
wide variety of site constraint, project sizes and
aesthetic preferences.

BENEFITS
Controlled manufacturing conditions ensures a durable, damage resistant product.
These systems allow for some flexibility, such as curved walls .
Construction is generally faster than poured in place concrete or stonewalls
Site conditions have a major impact on cost
APPLICATIONS (some examples):
The following fig. Shows the typical pre-stressed concrete flat slab floor construction
using the lift slab technique


                          13-storey apartment building with pre-cast
                          Post tensioned lift slab construction, 203mm
                          Thick lightweight concrete slab of 8.6m spans,
                          SAN FRANSICO


                                          Pre-stressed concrete twin box
                                          girder bridge construction using
                                          the segmentally cast cantilever
                                          method



                           Chaco-corrientes Bridge,
                           Argentina, The longest pre-cast
                           pre-stressed concrete
                           Cable stayed box Girder Bridge
                           In South America


                                                                             Pre-stressed concrete
                                                                             folded plate roof
                                                                             structure

				
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