PRESTRESSED PRESTRESSED CONCRETECONCRETE

STRUCTURESSTRUCTURES

BY,BY,MANJUNATH.SMANJUNATH.S

IST SEM MTECH(STRUCTURES)IST SEM MTECH(STRUCTURES)BANGALORE INSTITUTE OF TECHNOLOGY.BANGALORE INSTITUTE OF TECHNOLOGY.

BANGALORE.BANGALORE.

Under The Guidance of:Under The Guidance of:Prof. K.JAYARAMProf. K.JAYARAM

Prof. & HeadProf. & HeadDepartment of Civil EngineeringDepartment of Civil Engineering

CONTENTSCONTENTS

IntroductionIntroduction

How to make prestressed concreteHow to make prestressed concrete

AdvantagesAdvantages

Difference between RCC & PSCDifference between RCC & PSC

PSC in Construction IndustryPSC in Construction Industry

INTRODUCTION- INTRODUCTION- A Brief HistoryA Brief History

Eugene Freyssinet (1879-1962) devoted much of his Eugene Freyssinet (1879-1962) devoted much of his life to the research development and construction of life to the research development and construction of prestressed concrete structures and is rightly named prestressed concrete structures and is rightly named “the father of prestressed concrete”. It is through his “the father of prestressed concrete”. It is through his dedicated research that many of the techniques and dedicated research that many of the techniques and practices used today to build spectacular and practices used today to build spectacular and aesthetic structures are possible.aesthetic structures are possible.

Freyssinet achieved the first success with prestressed Freyssinet achieved the first success with prestressed concrete in marine construction, having secured the concrete in marine construction, having secured the tender to correct the threatened foundations of the tender to correct the threatened foundations of the LeHarve marine terminal using prestress techniques.LeHarve marine terminal using prestress techniques.

The The beginningbeginning

Eugene freyssinet(1879-1962)Eugene freyssinet(1879-1962)

Precast Precast pretensioned pretensioned hollow formed hollow formed

slabsslabs Prestressed concrete is used today for Prestressed concrete is used today for large building works in many civil large building works in many civil engineering fields such asengineering fields such as .. ..

RoadsRoads BuildingsBuildings BridgesBridges WaterWater EnergyEnergy Public facilities etc..Public facilities etc..

TodayToday

La grande arch in La grande arch in Paris was Paris was constructed using constructed using massive 70m span massive 70m span post tensioned I post tensioned I beams to carry the beams to carry the upper decks of three upper decks of three floors.floors.

INTRODUCTIONINTRODUCTION contd…contd…

A prestressed concrete member is one in which there have A prestressed concrete member is one in which there have been introduced internal stresses of such a magnitude, and also been introduced internal stresses of such a magnitude, and also distribution, that the stresses resulting from external loading are distribution, that the stresses resulting from external loading are counteracted to a desired degree. counteracted to a desired degree.

WHY PRESTRESS ?WHY PRESTRESS ? Prestressing applies an initial compressive axial force to the Prestressing applies an initial compressive axial force to the

concrete which greatly reduces or eliminates the internal concrete which greatly reduces or eliminates the internal stresses.stresses.

It does this applying a tensile stress to a steel cable running It does this applying a tensile stress to a steel cable running through the concrete. The steel cable is then anchored and the through the concrete. The steel cable is then anchored and the compressive force is then transferred to the concrete by the compressive force is then transferred to the concrete by the bond forces…bond forces…

The benefits of prestressing are :-The benefits of prestressing are :-1.1. Cracking is greatly reduced or eliminated, andCracking is greatly reduced or eliminated, and2.2. Applying the prestressing force below the Applying the prestressing force below the

neutral axis induces the moments which oppose neutral axis induces the moments which oppose those caused by the externally applied loads, those caused by the externally applied loads, thus significantly reducing deflection.thus significantly reducing deflection.

3.3. Member sections are smaller than reinforced Member sections are smaller than reinforced concrete sections for the same imposed loads..concrete sections for the same imposed loads..

Based on the method of applying Based on the method of applying prestress to the concrete member, the prestress to the concrete member, the Prestressing can be classified as:-Prestressing can be classified as:-

PRE-TENSIONING METHOD .PRE-TENSIONING METHOD .

POST-TENSIONING METHOD.POST-TENSIONING METHOD.

PRE-TENSIONINGPRE-TENSIONING Pre tensioning is one method of applying prestress,thus –Pre tensioning is one method of applying prestress,thus –

Tendons either pass through a single mould or a line of moulds Tendons either pass through a single mould or a line of moulds for multiple members arranged end to end and can be attached for multiple members arranged end to end and can be attached at one end to fixed anchorage. at one end to fixed anchorage.

The tendons are then tensioned from the fixed anchorage The tendons are then tensioned from the fixed anchorage between an external independent anchorage to give the between an external independent anchorage to give the required tensile force in the tendon.required tensile force in the tendon.

The tendons are then held in place while the concrete is The tendons are then held in place while the concrete is poured.poured.

When the concrete has hardened sufficiently the ends of the When the concrete has hardened sufficiently the ends of the tendons are slowly released from the anchorages.The tendons tendons are slowly released from the anchorages.The tendons are restrained from gaining their original length by the are restrained from gaining their original length by the development of bond stresses between the concrete and the development of bond stresses between the concrete and the tendon, and it is these bond stresses that transfer the tendon, and it is these bond stresses that transfer the compressive stress to the concrete.compressive stress to the concrete.

The tendons are then finally trimmed off as The tendons are then finally trimmed off as shown belowshown below..

POST-TENSIONINGPOST-TENSIONING Post-Tensioning is another method of applying prestress to a concrete Post-Tensioning is another method of applying prestress to a concrete

member, thus-member, thus-

The tendon is placed in the correct position in the formwork The tendon is placed in the correct position in the formwork with the dead-end anchorage and live end anchorage, through with the dead-end anchorage and live end anchorage, through which the tendon passes.which the tendon passes.

The concrete is then poured and left to harden.The concrete is then poured and left to harden. When the concrete has gained sufficient strength a jack is When the concrete has gained sufficient strength a jack is

attached to the live end anchorage and the tendon stressed to attached to the live end anchorage and the tendon stressed to the required force. The operation is to be carefully done as any the required force. The operation is to be carefully done as any error could impair the structural integrity of the member.error could impair the structural integrity of the member.

The tension force in the tendon is transferred to the concrete as The tension force in the tendon is transferred to the concrete as a compressive force by the reactions at the anchorages.The jack a compressive force by the reactions at the anchorages.The jack is then removed.is then removed.

Dead-end Dead-end anchorageanchorage

To prevent the tendon To prevent the tendon from slipping back it is from slipping back it is locked into the locked into the anchorage by means of anchorage by means of a split wedge located in a split wedge located in the barrel of the the barrel of the recessed anchorage.recessed anchorage.

Tendons used for prestressing generally Tendons used for prestressing generally consists of several high strength steel wires of consists of several high strength steel wires of small diameter wrapped around a similar wire.small diameter wrapped around a similar wire.

This is called a This is called a prestressing prestressing strand / strand / Tendons. Made Tendons. Made of high strength of high strength steel, it will steel, it will soon be soon be embedded in embedded in concrete.concrete.

Based on how the tendons are used ,they can be Based on how the tendons are used ,they can be classified as classified as

1.1. Bonded tendonsBonded tendons2.2. Unbonded tendons.Unbonded tendons.

Bonded tendons can also be used. The process is Bonded tendons can also be used. The process is essentially the same except that the prestressing essentially the same except that the prestressing force is transferred to the concrete both by reactions force is transferred to the concrete both by reactions at the anchorages and by the bond stresses between at the anchorages and by the bond stresses between the concrete and the bonded tendon.the concrete and the bonded tendon.

Unbonded tendons are normally assembled under Unbonded tendons are normally assembled under

factory conditions. They consists of strands or wires factory conditions. They consists of strands or wires that are wrapped or encased in plastic having been that are wrapped or encased in plastic having been first coated with grease or a bituminous material.first coated with grease or a bituminous material.

In this floor In this floor construction construction bonded tendons bonded tendons are provided in the are provided in the beams and the beams and the unbonded tendons unbonded tendons in the ribs of the in the ribs of the trough slab.trough slab.

BONDEDBONDED

UNBONDEDUNBONDED

The photograph The photograph

shows the shows the tendon layout tendon layout for a post-for a post-tensioned flat tensioned flat slab.slab.

The tendons are placed in position before the concrete is The tendons are placed in position before the concrete is poured.When the concrete has gained sufficient strength, poured.When the concrete has gained sufficient strength, the tendons are jacked to a predetermined prestress force.the tendons are jacked to a predetermined prestress force.

When concrete has gained sufficient strength the tendon is When concrete has gained sufficient strength the tendon is stressed and then the sleeve is filled with grout under pressure stressed and then the sleeve is filled with grout under pressure through tubes placed at intervals along the sleeve.through tubes placed at intervals along the sleeve.

The grout bonds the tendons to the sleeve thus enabling the The grout bonds the tendons to the sleeve thus enabling the transfer of stress from the tendons to the concrete.transfer of stress from the tendons to the concrete.

How Prestressed concrete is madeHow Prestressed concrete is made

It all begins at the prestressed concrete plant.

The prestressing The prestressing strand is stretched strand is stretched across the casting across the casting bed. bed.

Anchorages embedded in the concrete member are generally Anchorages embedded in the concrete member are generally required only for post tensioning. Pre tensioned members are required only for post tensioning. Pre tensioned members are manufactured and are anchored by bond stresses rather than manufactured and are anchored by bond stresses rather than

anchorages.anchorages.

One anchorage simply holds the end of the tendon in position , One anchorage simply holds the end of the tendon in position , while the other is used with the jack to stress the tendon. Each while the other is used with the jack to stress the tendon. Each

anchorage is attached to the end of the tendon before the anchorage is attached to the end of the tendon before the concrete is poured .The type of anchorage used depends on the concrete is poured .The type of anchorage used depends on the

number of tendons and whether they are strands or wires. number of tendons and whether they are strands or wires.

A prestressing jack is A prestressing jack is attached to the outside attached to the outside plate of the anchorage plate of the anchorage through which the ends through which the ends of the tendons project. of the tendons project. Once the jack has been Once the jack has been connected to the connected to the tendons, stressing can tendons, stressing can begin. The begin. The predetermined stressing predetermined stressing force can be applied to force can be applied to each tendon individually. each tendon individually. The force is applied The force is applied according to the design according to the design engineer’s engineer’s specifications.specifications.

Special trucks bring Special trucks bring the concrete to the the concrete to the casting bed where casting bed where the pouring begins. the pouring begins.

Once the Once the pouring is pouring is complete, a complete, a tarp is placed tarp is placed over the form over the form and applied to and applied to cure the cure the cement cement

The prestressing strands The prestressing strands are cut and the concrete are cut and the concrete form is removed from the form is removed from the casting bed.casting bed.

The ends are The ends are cleaned and the cleaned and the prestressing prestressing strands are sealed strands are sealed with a protective with a protective coating coating

The end-product The end-product is shipped to a is shipped to a building site. building site.

Bond StressesBond Stresses The prestress force in a bonded tendon is The prestress force in a bonded tendon is

transferred to the concrete by bond stresses.transferred to the concrete by bond stresses. There are two stages at which bond stresses are There are two stages at which bond stresses are

considered in the design of prestressed concrete considered in the design of prestressed concrete elements.elements.

At Transfer, At Transfer, the tendons of a predetermined the tendons of a predetermined element are released, and the prestress force is element are released, and the prestress force is transferred to the concrete by bond stresses.transferred to the concrete by bond stresses.

Flexural bond stressesFlexural bond stresses develop when members are develop when members are subjected to external loading. However ,such subjected to external loading. However ,such stresses are likely to be negligible until the stresses are likely to be negligible until the member has developed flexural cracks.member has developed flexural cracks.

ADVANTAGESADVANTAGES

Improved resistance to shearing forcesImproved resistance to shearing forces

Reduce the shear forces developed at the support Reduce the shear forces developed at the support sectionssections

StifferStiffer

Indirect long term savings is more.Indirect long term savings is more.

Economy for long span structuresEconomy for long span structures

COMPARISON BETWEEN RCC & PSCCOMPARISON BETWEEN RCC & PSC

Tensile StrengthTensile Strength StiffnessStiffness ShearShear Cracking & deflectionCracking & deflection ServiceabilityServiceability EconomyEconomy SafetySafety

Prestressed concrete in construction Prestressed concrete in construction industryindustry

It is well established fact that the basic economy of It is well established fact that the basic economy of

prestressed concrete lies in its high strength to weight prestressed concrete lies in its high strength to weight and strength to cost ratios , its resistance to fire & and strength to cost ratios , its resistance to fire & corrosion, & its versatility and adaptability.corrosion, & its versatility and adaptability.

It is ideally suited forIt is ideally suited for

Long span BridgesLong span Bridges

Multistory buildings-Flat slabsMultistory buildings-Flat slabs

Highway 50 Bridge over Osage River in Missouri Land Crossing - West Span and Abutment

The Flat Slab flooring system incorporates prestressed concrete units designed for easy and fast placement while requiring the minimum on site work to obtain a finished floor and ceiling.

The large component units are craned into position, providing an immediate working platform which, with the addition of an in situ concrete topping, gives a suspended floor with a flat, steel formed soffit. Temporary propping is required for longer spans.

SummarySummary How to deal with the low tensile strength of concrete by How to deal with the low tensile strength of concrete by

prestressing the member.prestressing the member. The mechanics of the prestressing system using a row of The mechanics of the prestressing system using a row of

books as example.books as example. How the prestressing force is applied to a member at site.How the prestressing force is applied to a member at site. A brief history of the development of prestressing from its A brief history of the development of prestressing from its

inventor Eugene Freyssinet up to the modern day.inventor Eugene Freyssinet up to the modern day. The methods of prestressing-The methods of prestressing- Pre tensioned prestressing,Pre tensioned prestressing, Post tensioned Prestressing,Post tensioned Prestressing, Bonded and unbonded tendons,Bonded and unbonded tendons, Anchorages,Anchorages, Bond stresses.Bond stresses.

THANK YOUTHANK YOU