Prestressing Method

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PRESTRESSING METHOD IN MULTI-STORIED BUILDING FRAME

History of Pre-stressing

The art of pre-stressing concrete evolved over many decades and from manysources, but we can point to a few select instances in history that brought about this

technology.

In the United States, engineer John Roebling established a factory in 1841 for making

rope out of iron wire, which he initially sold to replace the hempen rope used for hoisting cars

over the portage railway in central Pennsylvania. Later, Roebling used wire ropes as

suspension cables for bridges, and he developed the technique for spinning the cables in

place.

During the 19th century, low-cost production of iron and steel, when added to the

invention of portland cement in 1824, led to the development of reinforced concrete. In

1867, Joseph Monier, a French gardener, patented a method of strengthening thin concrete

flowerpots by embedding iron wire mesh into the concrete. Monier later applied his ideas to

patents for buildings and bridges.

Swiss engineer Robert Maillartsuse of reinforced concrete, beginning in 1901,

effected a revolution in structural art. Maillart, all of whose main bridges are located in

Switzerland , was the first designer to break completely with the masonry tradition by puttingconcrete into forms technically appropriate to its properties yet visually surprising. His

radical use of reinforced concrete revolutionized masonry arch bridge design.

The idea of pre-stressing concrete was first applied by Eugene Freyssinet, a French

structural and civil engineer, in 1928 as a method for overcoming concretes natural

weakness in tension. Pre-stressed concrete can now be used to produce beams, floors or

bridges with a longer span than is practical with ordinary reinforced concrete.

PRE-STRESSED CONCRETE

Pre stressed concrete, like reinforced concrete, is a composite material which uses to

advantage the compressive strength of concrete, whilst circumventing its weakness in

tension. Pre stressed concrete is made from structural concrete, usually of high strength, and

high strength steel tendons which may or may not be grouped together. Prior to external

loading the tendons are tensioned in one of two ways. With pretensioning the tendon are

tensioned prior to the casting of the concrete and using post tensioning techniques

the tendons are tensioned after the concrete has hardened. Some ordinary reinforcing

steel is also often included both as subsidiary longitudinal reinforcement and

as transverse stirrups to resist shear.

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Pre-stressed concrete is a method for overcomingconcrete's natural weakness

intension. It can be used to producebeams,floors orbridges with a longerspan than is

practical with ordinary reinforced concrete. Pre-stressing tendons (generally of

hightensilesteelcable or rods) are used to provide a clamping load which produces

acompressive stress that offsets thetensile stress that the concretecompression

member would otherwise experience due to a bending load. Traditionalreinforced concrete is

based on the use ofsteel reinforcement bars, inside poured concrete. The basic purpose of

pre-stressing is to improve the performance of concrete members and this is achieved by

inducing in the beam initial deformation and stresses which tend to counteract those

produced by the service loads.

Since concrete is weak in tension in normal reinforced concrete construction cracks

develop in the tension zone at working loads and therefore all concrete in tension is ignored

in design.

Pre-stressing involves inducing compressive stresses in the zone, which will tend to

become tensile under external loads. This compressive stress neutralizes the tensile

stress so that no resultant tension exists, (or only very small values, within the tensile

strength of the concrete). Cracking is therefore eliminated under working load and all of the

concrete may be assumed effective in carrying load. Therefore lighter sections may be used

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to carry a given bending moment, and pre-stressed concrete may be used for longer span

than reinforced concrete.

The pre-stressing force also reduces the magnitude of the principal tensile stress in

the web so that thin-webbed I - sections may be used without the risk of diagonal tensionfailures and with further savings in self-weight.

The pre-stressing force has to be produced by a high tensile steel, and it is necessary

to use high quality concrete to resist the higher compressive stresses that are developed. As

the name itself suggests pre-stressing is the technique of stressing a structural member prior

to loading to resist excessive tensile stresses.

THE ADVANTAGES OF PRE-STRESSED CONCRETE AS

A CONSTRUCTION MATERIAL IN MULTI STORIED FRAME CAN BE LISTEDAS FOLLOWS:

Maximum utilization of provided section of the member.

Provision of slender member for long span beams as compared to RCC.

Use of high strength materials contribute to the durability of the structure.

Pre-stresses concrete has considerable resilience and impact resistance.

Proves to be economical only in long span beam-column frames compared to other

materials.

The intermediate distance between the columns can be in increased by using pre-

stressed concrete as compared to reinforced cement concrete.

Architectural design provisions and specifications can be achieved using pre-stressed

concrete.

Dead weight of concrete is reduced to a higher rate using pre-stressed concrete.

PRINCIPLE OF PRESTRESSING

The function of pre-stressing is to place the concrete structure under compression in

those regions where load causes tensile stress. Tension caused by the load will first have to

cancel the compression induced by the pre-stressing before it can crack the concrete. Figure

(a) shows a plainly reinforced concrete simple-span beam and fixed cantilever beam cracked

under applied load. Figure (b) shows the same unloaded beams with pre-stressing forces

applied by stressing high strength tendons. By placing the pre-stressing low in the simple-

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span beam and high in the cantilever beam, compression is induced in the tension zones;

creating upward camber.

Figure (c) shows the two pre-stressed beams after loads have been applied. The

loads cause both the simple-span beam and cantilever beam to deflect down, creating tensilestresses in the bottom of the simple-span beam and top of the cantilever beam. The

structural Designer balances the effects of load and pre-stressing in such a way that tension

from the loading is compensated by compression induced by the pre-stressing. Tension is

eliminated under the combination of the two and tension cracks are prevented.

Also, construction materials(concrete and steel) are used more efficiently; optimizing

materials, construction effort and cost.

Fig 1. - Comparison of Reinforced and Prestressed Concrete Beams

Pre-stressing can be applied to concrete members in two ways, by pre-tensioning or

post-tensioning. In pre-tensioned members the pre-stressing strands are tensioned against

restraining bulkheads before the concrete is cast. After the concrete has been placed,

allowed to harden and attain sufficient strength, the strands are released and their force is

transferred to the concrete member. Pre-stressing by post-tensioning involves installing and

stressing pre-stressing strand or bar tendons only after the concrete has been placed,

hardened and attained a minimum compressive strength for that transfer.

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METHODS AND SYSTEM OF PRE-STRESSING

There are two methods of pre-stressing concrete: -

1) Pre-cast Pre-tensioned

2) Pre-cast Post-tensioned

Both methods involve tensioning cables inside a concrete beam and then anchoring the

stressed cables to the concrete.

Pre-cast Pre-tensioned: -Pre-tensioning is a method of pre-stressing in which the steel tendons are tensioned

before the casting of the member. In this method the tendons are tensioned using hydraulic

jacks, which bear on strong abutments between which the moulds are placed. After the

concrete attains full strength the tendons are released and the stress is transferred to the

concrete by bond action.

Procedure of precast pre-tensioned concretingStage 1

Tendons and reinforcement are positioned in the beam mould.

Stage 2Tendons are stressed to about 70% of their ultimate strength.

Stage 3Concrete is cast into the beam mould and allowed to cure to the required initial strength.

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Stage 4When the concrete has cured the stressing force is released and the tendons anchorthemselves in the concrete.

Pre-cast Post-tensioned: -

Post-tensioning is a method of pre-stressing in which the steel tendons are tensioned after the

casting of the member. In this method ducts or sheaths are placed in the required profile in the mould

and the tendons are passed through the ducts. After the concrete had attained sufficient strength the

tendons are tensioned using hydraulic jacks which bear on the member itself. The stress is transferred

to the concrete by bearing action of tendons which are anchored using suitable anchorages. Finally the

ducts are grouted and the anchor plates concealed by cement mortar.

Procedure of precast post-tensioned concreting

Stage 1

Cable ducts and reinforcement are positioned in the beam mould. The ducts are usually

raised towards the neutral axis at the ends to reduce the eccentricity of the stressing force.

Stage 2

Concrete is cast into the beam mould and allowed to cure to the required initial strength.

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Stage 3

Tendons are threaded through the cable ducts and tensioned to about 70% of their ultimate

strength.

Stage 4

Wedges are inserted into the end anchorages and the tensioning force on the tendons is

released. Grout is then pumped into the ducts to protect the tendons.

TYPES OF POST TENSIONED CONCRETING METHODS

1. BONDED POST-TENSIONED CONCRETE METHOD

2. UNBONDED POST-TENSIONED CONCRETE METHOD

BONDED POST-TENSIONED CONCRETE

Bonded post-tensioned concrete is the descriptive term for a method of

applyingcompression after pouring concrete and the curing process (in situ). The concrete is

cast around a plastic,steel oraluminium curved duct, to follow the area where otherwise

tension would occur in the concrete element. A set of tendons are fished through the duct

and the concrete is poured. Once the concrete has hardened, the tendons are tensioned

byhydraulicjacks that react against the concrete member itself. When the tendons have

stretched sufficiently, according to the design specifications, they arewedged in position and

maintain tension after the jacks are removed, transferring pressure to the concrete. The duct

is thengrouted to protect the tendons fromcorrosion. This method is commonly used to

create monolithic slabs for house construction in locations where expansive soils (such

asadobeclay) create problems for the typical perimeter foundation. All stresses from

seasonal expansion and contraction of the underlying soil are taken into the entire tensioned

slab, which supports the building without significant flexure. Post-stressing is also used in the

construction of various bridges, both after concrete is cured after support byfalsework and bythe assembly of prefabricated sections, as in the bridge. The advantages of this system over

unbonded post-tensioning are.

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1. Large reduction in traditional reinforcement requirements as tendons cannot destress in

accidents.

2. Tendons can be easily 'weaved' allowing a more efficient design approach.

3. Higher ultimate strength due to bond generated between the strand and concrete.

4. No long term issues with maintaining the integrity of the anchor/dead end.

UNBONDED POST-TENSIONED CONCRETE

Unbonded post-tensioned concrete differs from bonded post-tensioning by providing

each individual cable permanent freedom of movement relative to the concrete. To achieve

this, each individual tendon is coated with a grease (generallylithium based) and covered by

a plastic sheathing formed in anextrusion process. The transfer of tension to the concrete is

achieved by the steel cable acting against steel anchors embedded in the perimeter of the

slab. The main disadvantage over bonded post-tensioning is the fact that a cable can

destress itself and burst out of the slab if damaged (such as duringrepair on the slab). The

advantages of this system over bonded post-tensioning are:

1. The ability to individually adjust cables based on poor field conditions (For example: shifting

a group of 4 cables around an opening by placing 2 to either side).

2. The procedure of post-stress grouting is eliminated.

3. The ability to de-stress the tendons before attempting repair work.

Picture number one shows rolls of post-tensioning (PT) cables with the holding end

anchors displayed. The holding end anchors are fastened to rebar placed above and below

the cable and buried in the concrete locking that end. Pictures numbered two, three and four

shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is

placed above and below the cable both in front and behind the face of the pulling end anchor.

The above and below placement of the rebar can be seen in picture number three and the

placement of the rebar in front and behind can be seen in picture number four. The blue

cable seen in picture number four is electrical conduit. Picture number five shows the plastic

sheathing stripped from the ends of the post-tensioning cables before placement through the

pulling end anchors. Picture number six shows the post-tensioning cables in place for

concrete pouring. The plastic sheathing has been removed from the end of the cable and the

cable has been pushed through the black pulling end anchor attached to the inside of the

concrete floor side form. The greased cable can be seen protruding from the concrete floor

side form. Pictures seven and eight show the post-tensioning cables protruding from thepoured concrete floor. After the concrete floor has been poured and has set for about a week,

the cable ends will be pulled with a hydraulic jack, shown in picture number nine, until it is

stretched to achieve the specified tension.

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1. Rolls of post-tensioning cables

2. Pulling anchors for post-tensioning cables

3. Pulling anchors for post-tensioning cables

4. Pulling anchors for post-tensioning cables

5. Post-tensioning cables stripped for placement in pulling anchors

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6. Positioned post-tensioning cables

7. Post-tensioning cable ends extending from freshly poured concrete

8. Post-tensioning cable ends extending from concrete slab

9. Hydraulic jack for tensioning cables

10. Cable conduits in formwork

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THE ADVANTAGES OF POST-TENSIONING COMPARED TO PRE-TENSIONING IN THE

MULTI STORIED FRAME CONSTRUCTION CAN BE LISTED AS FOLLOWS

a) Tendons can be provided in any desired profile.

b)Stage pre-stressing can be adopted conveniently.

c) Costly factory equipments are not required.

d)Cast-in-situ construction procedure can be conveniently adopted.

e) It is possible to fabricate a beam with pre-cast and cast-in-situ elements, which are

post-tensioned together to form a single structural unit.

f) Number of systems is available in this method.

Systems of pre-stressing are as given below

Hoyer systemusually adopted for pre-tensioned members.

o The system listed below are adopted for post-tensioning

Freyssinet system

Magnel Balton system

Gifford Udall system

PSC monowire system

CLL standard system

Lee -Macall system

ADVANTAGES OF PRECAST CONCRETE ELEMENTS IN BUILDING CONSTRUCTION

Lower construction cost

Thinner slabs, which are especially important in high-rise buildings where floor

thickness savings can translate into additional floors for the same or lower cost

Fewer joints since the distance that can be spanned by post-tensioned slabs exceeds

that of reinforced construction with the same thickness

Longer span lengths increase the usable unencumbered floorspace in buildings and

parking structures

Fewer joints lead to lower maintenance costs over the design life of the structure,

since joints are the major locus of weakness in concrete buildings.

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One-stop shopping sources much of a building's shell in one efficient, precast contract.

Fabrication of precast elements during permitting and/or site preparation saves time

resulting in fast efficient construction regardless of weather conditions.

Designing precast systems is easier.

Precast components can be erected in winter conditions, maintaining tight schedules.

With total precast systems, speedy erection allows the contractor to enclose the

building quickly, giving interior trades faster access.

Precast components are naturally fire protected, because they will not burn. Precast's

inherent fire resistance eliminates the messy and time-consuming fireproofing required

for a steel structure and subsequent repairs caused by other trades.

LOSS OF PRESTRESS

When the tensioning force is released and the tendons are anchored to the concrete

a series of effects result in a loss of stress in the tendons. The effects are :

Relaxation of the steel tendons

Elastic deformation of the concrete

Shrinkage and creep of the concrete

Slip or movement of the tendons at the anchorages during

anchoring

Other causes in special circumstances, such as when steam curing

is used with pre-tensioning.

Total losses in pre-stress can amount to about 30% of the initial tensioning stress.

Freyssinet system is the most widely adopted system in the construction of pre-

stressed concrete structures. Pre-stressed Concrete is an architectural and structural

material possessing great strength. The unique characteristics of pre-stressed concrete allow

predetermined, engineering stresses to be placed in members to counteract stresses that

occur when the unit is subjected to service loads. This is accomplished by combining the best

properties of two quality materials: high strength concrete for compression and high tensile

strength steel strands for tension.

REASONS FOR USING PRESTRESSED CONCRETE

Column-Free Long Spans

With fewer columns and more usable floor space, precast, prestressed concrete

provides greater freedom for space utilization.

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Conserves Energy

Pre-stressed concrete components can improve the thermal storage potential of a

building. It effectively conserves energy required for heating and cooling.

Maintenance Free

Precast concrete does not require painting and is free from corrosion. Its durability

extends building life.

Resists Fire

Durability and fire resistance mean low insurance premiums and greater personnel

safety. Those who investigate life cycle costing will appreciate the precast concrete's

excellent fire resistance characteristics.

Rapid Construction

Precast concrete construction gets the job done sooner. The manufacturing of

prestressed members and site preparation can proceed simultaneously. Early occupancy

provides obvious benefits to the client.

Versatility of Design

Precast concrete buildings are not only functional but beautiful as well. Numerous

panel configuration design possibilities are available.

Sustainability

As with all concrete wall systems, precast offers high durability and strength as well

as thermal mass, which contributes to increased energy efficiency. Precast systems use

locally derived materials, and can incorporate recycled supplementary cementitious materials

like fly ash and slag cement, one of the key reasons why they are often used in sustainable

or green buildings.

Variety, Flexibility, Utility

One of the biggest benefits of precast systems is in their design: tight controls mean

more efficient mix designs, resulting in smaller structural members and longer spans.

Construction waste is reduced because the exact amount of necessary components is

delivered to the site; any spare components can be recycled, and their materials used again

in another structure. Precast systems can adopt almost any aesthetic, incorporating a variety

of colours and textures, or emulating natural stone. By crafting systems that not only look

great, but also act as structural walls and support floor loads, designers can reduce material

redundancyand project costs.

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Quality in Manufacturing

Because components are precast at manufacturing facilities, quality control measures

ensure that every piece is made to specifications. The components can be cast and

transported to the job site while designs are still being finalized, helping to speed constructionschedules. Evolutions in self-consolidating concrete also promise to offer new options and

challenges for designers using precast.

APPLICATIONS

Prestressed concrete is the predominating material for floors in high-rise

buildings and concrete chambers innuclear reactors,as well as in columns and shear walls

in thebuildings intended for a high degree ofearthquake andblast protection.

Unbonded post-tensioning tendons are commonly used inparking garages as barrier

cable. Also, due to its ability to be stressed and then de-stressed, it can be used to

temporarily repair a damaged building by holding up a damaged wall or floor until permanent

repairs can be made.

GENERAL PRECAUTIONS IN PRESTRESSED CONCRETING

Working platforms

To provide a safe working environment, working platforms need adequate working

space, appropriate edge protection, and safe access and egress.

They must also be designed and constructed to safely support all expectedloads, including impact loads. Factors that will determine the selection of an appropriate

working platform include:

The type and number of items of stressing equipment that may be in use,

The number of people required, or likely to be, on the work platform at any

one time, and the likely material storage on the platform.

The platform should be large enough to enable the operators to remain

clear of anchorages during stressing, and it should be at a height which eliminates or

minimises risk of injury from over-reaching or awkward postures.

Formwork

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In the case of cast in-situ concrete constructions, working platforms may be

built integrally with the formwork.

Formwork and propping systems should be designed by a structural

engineer experienced in formwork design.

Assembled formwork and propping systems should be checked by a

competent person for compliance with the formwork design drawings and documented

proof of such on-site checks should be readily available.

Provisions for stressing

Anchorages for stressing should be set out and tendon spacing marked on the ends.

Since the wedging forces at anchorages are high, anti-burst provisions, such as special

reinforcement, need to be installed and secured into position.

Pre-stressing ducts should be laid in accordance with the specified profile and

adequately secured. Inadequate cover to duct tubes can result in concrete blow-outs during

grouting operations.

Pre-stressing safety considerations

Stressing operations and associated preparations for stressing involve a variety of

tasks which if, appropriate precautions are taken, could endanger the health and safety of

workers carrying them out and/or those in the vicinity.

The area where preparations and stressing are intended to take place must

be fully barricaded with solid panels and signage prominently posted to keep

unauthorised personnel clear of this area.

All personnel involved in the tasks should wear the personal protective

equipment. This will generally include safety goggles, gloves, sturdy protective footwear

and safety helmets.

All personnel involved in the tasks should have adequate training in

identifying the hazards of stressing and their associated risks.

Uncoiling, Cutting and Placing Strands

Coils are very heavy, typically weighing 3 to 4 tonnes, and therefore the structural

adequacy of the area where coils are to be placed must be verified. Manual handling

issues associated with handling the coils should be controlled.

Ensure that the coils are restrained with uncut restraining straps when placing

them onto the strand frame. Uncontrolled release of the coil can result in whip-back with

sufficiently high force to cause serious injury.Strands should not be cut by heat-type cutting equipment such as oxy-acetylene

or LP gas torches, as this may compromise their load-bearing capacity under tension.

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When assembling tendons, thoroughly inspect each individual wire or strand for

obvious flaws.

Pushing Strands into Ducts

All strands for each of the tendons should be pushed into place in

accordance with the drawings, making sure all personnel are kept clear of the direct line

of ducting to prevent injury from strands exiting from the other end of the duct.

Once the specified number of strands is in place, ensure that a "dead end"

is created for each strand by securing them at the end of the element.

In strand set-ups where ends will protrude above the face of the concrete

element and may create a hazard, they should be boxed or barricaded to prevent injury.

Concrete PourConcreting may be placed with either pumps or kibbles.

When concreting is being pumped, "chairs" or other means to support

concrete pump lines above the reinforcement and tendons should be in place and well

secured.

Where kibbles are used to place concrete, avoid dropping concrete in one

place as tendons could be displaced. Concrete should always be allowed to flow in a

controlled manner.

During concrete pouring:

Ensure that the ducts and strands are not damaged during the pour. All

damage should be promptly notified to the contractor's supervisor for repair. However,

concrete around the anchorages needs adequate vibration to ensure a safe and sound

seating for the anchorage.

Concrete test cylinders should be taken at agreed intervals for storing and

curing on site under conditions similar to those applying to the element being poured.

Stressing operationsBefore stressing operations.

Prior to commencing stressing operations, the post-tensioning supervisor should verify that:

Concrete around the anchorages has been examined. The principal

contractor should be notified if the concrete is of poor quality.

All concrete test cylinders have achieved the specified strength.

The grips in the jacks on the stressing equipment are clean and free fromdirt or grit and in good condition.

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The stressing equipment, i.e. jacks and their gauges, has appropriate

service records and up-to-date calibration certificates. All jacks should have a durable

tag securely attached which clearly shows the following information:

Final stressing pressure Diameter and grade of the strand for which the jack is being

used

Jack number

Corresponding gauge number

Date calibration expires

The operator of the stressing equipment has documented evidence of appropriate training.

A "NO GO" area of at least 2 metres radius is in place around the anchorages at the deadand live ends, with barricades behind the line of jacks and "Stressing in Progress. Keep

Clear" signage prominently displayed at all appropriate locations

During stressing operations

Tendons should be stressed in order from the furthest to the closest reachable to ensure that

no person is standing in direct line of the jack or previously stressed strands.

Ensure stress is applied gradually and evenly to tendons.

Ensure that the specified initial and final stressing levels are not exceeded.

After stressing operations

Gain the design engineer's approval prior to cutting off excess tendons.

Seal anchorage recesses following approval and prior to grouting the ducts.

Do not perform tasks requiring impact, such as hammering, drilling or coring in the vicinity

until the grouting of the ducts has been completed.

Grouting

Build-up of excessive pressure during grouting can result in "blow-outs" of the concrete,

which could injure personnel in the vicinity.

To prevent blowouts

Ducts should be blown through to ensure there are no blockages.

Avoid non-continuous grouting to ensure no blockages or voids are in the tubes.

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Monitor the gauge of the equipment throughout grouting to ensure that excessive pressure

does not develop.

Retain barricades used during stressing operations and also barricade at a lower level if

formwork has already been removed.