Post-Tensioning Systems

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1.4 1.4 Post-tensioning

Post-tensioning Systems and Devices

Systems and Devices

This section covers the following topics

•

• IntroductionIntroduction •

• Stages of Post-tensioningStages of Post-tensioning •

• Advantages of Post-tensioningAdvantages of Post-tensioning •

• Disadvantages of Post-tensioningDisadvantages of Post-tensioning •

• DevicesDevices •

• Manufacturing of a Post-tensioned Bridge GirderManufacturing of a Post-tensioned Bridge Girder

1.4.1 Introduction

Prestressing systems have developed over the years and various companies have Prestressing systems have developed over the years and various companies have patented their products. Detailed information of the systems is given in the product patented their products. Detailed information of the systems is given in the product catalogues and

catalogues and brochures published brochures published by companies. by companies. There are There are general guidelines ofgeneral guidelines of prestressing in

prestressing in Section 12Section 12 ofof IS 1343: 1980IS 1343: 1980. . The information The information given in given in this section this section isis introductory in nature, with emphasis on the basic concepts of the systems.

introductory in nature, with emphasis on the basic concepts of the systems.

The prestressing systems and devices are described for the two types of prestressing, The prestressing systems and devices are described for the two types of prestressing, pre-tensioning and

pre-tensioning and post-tensioning, separately. post-tensioning, separately. This This section section covers covers post-tensioning.post-tensioning. Section 1.3,

Section 1.3, “Pre-tensioning Systems “Pre-tensioning Systems and and Devices”, covers Devices”, covers pre-tensioning. pre-tensioning. In In post- post-tensioning, the tension is applied to the tendons after hardening of the concrete. The tensioning, the tension is applied to the tendons after hardening of the concrete. The stages of post-tensioning are described next.

stages of post-tensioning are described next.

1.4.2 Stages of Post-tensioning

In post-tensioning systems, the ducts for the tendons (or strands) are placed along with In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the reinforcement before the

the reinforcement before the casting of concrete. casting of concrete. The tendons The tendons are placed in are placed in the ductsthe ducts after the casting

after the casting of concrete. of concrete. The duct The duct prevents contact between prevents contact between concrete and theconcrete and the tendons during the tensioning operation.

tendons during the tensioning operation.

Unlike pre-tensioning, the tendons are pulled with the reaction acting against the Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened concrete.

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If the ducts are filled with

If the ducts are filled with groutgrout, , then then it is it is known known as as bonded bonded post-tensioning. post-tensioning. The The groutgrout is a

is a neat cement neat cement paste or paste or a sand-cement a sand-cement mortar containing suitable admixture. mortar containing suitable admixture. TheThe grouting operation

grouting operation is discussed is discussed later in later in the section. the section. The The properties of properties of grout aregrout are discussed in Section 1.6, “Concrete (Part-II)”.

discussed in Section 1.6, “Concrete (Part-II)”.

In unbonded post-tensioning, as the name suggests, the ducts are never grouted and In unbonded post-tensioning, as the name suggests, the ducts are never grouted and the tendon is

the tendon is held in tension solely by held in tension solely by the end anchorages. the end anchorages. The following sketch showsThe following sketch shows a schematic representation of

a schematic representation of a grouted post-tensioned member. a grouted post-tensioned member. The profile of The profile of the ductthe duct depends on

depends on the support conditions. the support conditions. For a For a simply supported member, the simply supported member, the duct has duct has aa sagging profile between the ends.

sagging profile between the ends. For a continuous member, the For a continuous member, the duct sags in duct sags in the spanthe span and hogs over the support.

and hogs over the support.

Figure 1-4.1

Figure 1-4.1 Post-tensioning (Reference: VSL International Ltd.)Post-tensioning (Reference: VSL International Ltd.)

Among the following figures, the first photograph shows the placement of ducts in a box Among the following figures, the first photograph shows the placement of ducts in a box girder of a simply supported bridge.

girder of a simply supported bridge. The second photograph shows the The second photograph shows the end of the end of the boxbox girder after the post-tensioning of some tendons.

girder after the post-tensioning of some tendons.

Figure 1-4.2

Figure 1-4.2 Post-tensioning ducts in a box girderPost-tensioning ducts in a box girder

(Courtesy: Cochin Port Trust, Kerala)

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Figure 1-4.3

Figure 1-4.3 Post-tensioning of a box girderPost-tensioning of a box girder

(Courtesy: Cochin Port Trust, Kerala)

The various stages of the post-tensioning operation are summarised as follows. The various stages of the post-tensioning operation are summarised as follows.

1)

1) Casting Casting of of concrete.concrete. 2)

2) Placement of Placement of the the tendons.tendons. 3)

3) Placement of Placement of the anchorage the anchorage block and block and jack.jack. 4)

4) Applying tension Applying tension to to the the tendons.tendons. 5)

5) Seating Seating of of the the wedges.wedges. 6)

6) Cutting Cutting of of the the tendons.tendons.

The stages

The stages are shown are shown schematically in the schematically in the following figures. following figures. After anchoring a After anchoring a tendontendon at one end, the tension

at one end, the tension is applied at the other end by is applied at the other end by a jack. a jack. The tensioning of tendonsThe tensioning of tendons and

and pre-compression of pre-compression of concrete occur concrete occur simultaneouslysimultaneously. . A A system of system of self-equilibratiself-equilibratingng forces develops after the stretching of the tendons.

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Casting bed

Duct

Side view

Casting bed

Duct

Side view

(a) Casting of concrete

Jack

(b) Tensioning of tendons (b) Tensioning of tendons

Anchor

(c)

(c) Anchoring the tendon at the stretching endAnchoring the tendon at the stretching end

Figure 1-4.4

Figure 1-4.4 Stages of post-tensioning (shown in elevation)Stages of post-tensioning (shown in elevation)

1.4.3 Advantages of Post-tensioning

The relative advantages of post-tensioning as compared to pre-tensioning are as The relative advantages of post-tensioning as compared to pre-tensioning are as follows.

follows.

•

• Post-tensioning is suitable for heavy cast-in-place members.Post-tensioning is suitable for heavy cast-in-place members. •

• The waiting period in the casting bed is less.The waiting period in the casting bed is less. •

• The transfer of prestress is independent of transmission length.The transfer of prestress is independent of transmission length.

1.4.4 Disadvantage of Post-tensioning

The relative disadvantage of post-tensioning as compared to pre-tensioning is the The relative disadvantage of post-tensioning as compared to pre-tensioning is the requirement of anchorage device and grouting equipment.

requirement of anchorage device and grouting equipment.

1.4.5 Devices

The essential devices for post-tensioning are as follows. The essential devices for post-tensioning are as follows.

1)

1) Casting Casting bedbed 2) Mould/Shuttering 2) Mould/Shuttering 3) Ducts

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4)

4) Anchoring Anchoring devicesdevices 5) Jacks

5) Jacks 6)

6) Couplers Couplers (optional)(optional) 7)

7) Grouting Grouting equipment equipment (optional).(optional).

Casting Bed, Mould and Ducts Casting Bed, Mould and Ducts

The following figure shows the devices. The following figure shows the devices.

Casting bed

Mould

Duct

Casting bed

Mould

Duct

Figure 1-4.5

Figure 1-4.5 Casting bed, mould and ductCasting bed, mould and duct

Anchoring Devices Anchoring Devices

In post-tensioned members the anchoring devices transfer the prestress to the concrete. In post-tensioned members the anchoring devices transfer the prestress to the concrete. The devices are based on the following principles of anchoring the tendons.

The devices are based on the following principles of anchoring the tendons. 1)

1) Wedge Wedge actionaction 2)

2) Direct Direct bearingbearing 3)

3) Looping Looping the the wireswires

Wedge action

The anchoring device based on wedge action consists of an anchorage block and The anchoring device based on wedge action consists of an anchorage block and wedges.

wedges. The strands are held The strands are held by frictional grip of the wby frictional grip of the wedges in the anchorage edges in the anchorage block.block. Some examples of systems based on the wedge-action are Freyssinet, Gifford-Udall, Some examples of systems based on the wedge-action are Freyssinet, Gifford-Udall, Anderson and

Anderson and Magnel-Blaton anchorages. Magnel-Blaton anchorages. The following The following figures show figures show some patentedsome patented anchoring devices.

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Figure 1-4.6

Figure 1-4.6 Freyssinet “T” system anchorage conesFreyssinet “T” system anchorage cones

(Reference: Lin, T. Y. and Burns, N. H.,

(Reference: Lin, T. Y. and Burns, N. H., Design of Prestressed Concrete Structures Design of Prestressed Concrete Structures ))

Figure 1-4.7

Figure 1-4.7 Anchoring devicesAnchoring devices

(Reference: Collins, M. P. and Mitchell, D.,

(Reference: Collins, M. P. and Mitchell, D., Prestressed Concrete Structures Prestressed Concrete Structures ))

Figure 1-4.8

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Direct bearing

The rivet or bolt heads or button heads formed at the end of the wires directly bear The rivet or bolt heads or button heads formed at the end of the wires directly bear against a

against a block. block. The The B.B.R.V post-tensioning system B.B.R.V post-tensioning system and the and the Prescon system Prescon system areare based on this

based on this principle. principle. The following figure shows The following figure shows the anchoring by the anchoring by direct bearing.direct bearing.

Figure 1-4.9

Figure 1-4.9 Anchoring with button headsAnchoring with button heads

Looping the wires

The Baur-Leonhardt system, Leoba system and also the Dwidag single-bar anchorage The Baur-Leonhardt system, Leoba system and also the Dwidag single-bar anchorage system, work

system, work on on this principle this principle where where the the wires are wires are looped looped around the around the concrete. concrete. TheThe wires are looped

wires are looped to make a to make a bulb. bulb. The following photo shows The following photo shows the anchorage by the anchorage by loopinglooping of the wires in a post-tensioned slab.

of the wires in a post-tensioned slab.

Figure 1-4.10

Figure 1-4.10 Anchorage by looping the wires in a slabAnchorage by looping the wires in a slab

(Courtesy : VSL India Pvt. Ltd.)

The anchoring devices are tested to calculate their strength. The following photo shows The anchoring devices are tested to calculate their strength. The following photo shows the testing of an anchorage block.

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Figure 1-4.11

Figure 1-4.11 Testing of an anchorage deviceTesting of an anchorage device

Sequence of Anchoring

The following figures show

The following figures show the sequence of the sequence of stressing and anchoring the stressing and anchoring the strands. strands. TheThe photo of an anchoring device is also provided.

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Figure 1-4.12

Figure 1-4.12 Sequence of anchoringSequence of anchoring

(Reference: VSL International Ltd.)

Figure 1-4.13

Figure 1-4.13 Final form of an anchoring deviceFinal form of an anchoring device

(Reference: VSL International Ltd)

Jacks Jacks

The working of a jack and measuring the load were discussed in Section 1.3, The working of a jack and measuring the load were discussed in Section 1.3, “Pre-tensioning Systems and Devices”.

tensioning Systems and Devices”. The following figure shows an The following figure shows an extruded sketch of theextruded sketch of the anchoring devices.

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Figure 1-4.14

Figure 1-4.14 Jacking and anchoring with wedgesJacking and anchoring with wedges

Couplers Couplers

The couplers are used to connect strands or bars. They are located at the junction of The couplers are used to connect strands or bars. They are located at the junction of the members, for example at or near columns in tensioned slabs, on piers in the members, for example at or near columns in tensioned slabs, on piers in post-tensioned bridge decks.

tensioned bridge decks.

The couplers are tested to transmit the full capacity of the strands or bars. A few types The couplers are tested to transmit the full capacity of the strands or bars. A few types of couplers are shown.

of couplers are shown.

Figure 1-4.15

Figure 1-4.15 Coupler for strandsCoupler for strands

(Reference: VSL International Ltd)

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Figure 1-4.16

Figure 1-4.16 Couplers for strandsCouplers for strands

(Reference: Dywidag – Systems International)

Figure 1-4.17

Figure 1-4.17 Couplers for strandsCouplers for strands

(Reference: Dywidag – Systems International)

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Grouting Grouting

Grouting can be defined as the filling of duct, with a material that provides an Grouting can be defined as the filling of duct, with a material that provides an anti-corrosive alkaline environment to the prestressing steel and also a strong bond between corrosive alkaline environment to the prestressing steel and also a strong bond between the tendon and the surrounding grout.

the tendon and the surrounding grout.

The major part of grout comprises of water and cement, with a water-to-cement ratio of The major part of grout comprises of water and cement, with a water-to-cement ratio of about 0.5, together with some water-reducing admixtures, expansion agent and about 0.5, together with some water-reducing admixtures, expansion agent and pozzolans.

pozzolans. The properties The properties of grout of grout are discussed are discussed in Section in Section 1.6, “Concrete 1.6, “Concrete (Part-II)”.(Part-II)”. The following figure shows a grouting equipment, where the ingredients are mixed and The following figure shows a grouting equipment, where the ingredients are mixed and the grout is pumped.

the grout is pumped.

Figure 1-4.18

Figure 1-4.18 Grouting equipmentGrouting equipment

(Reference: Williams Form Engineering Corp.)

1.4.6 Manufacturing of Post-tensioned Bridge Girders

The following photographs show some steps in the manufacturing of a post-tensioned The following photographs show some steps in the manufacturing of a post-tensioned I-girder for a

girder for a bridge (Courtesy: Larsen bridge (Courtesy: Larsen & Toubro). & Toubro). The first photo The first photo shows the shows the fabricatedfabricated steel reinforcement with the

steel reinforcement with the ducts for ducts for the tendons the tendons placed inside. placed inside. Note the Note the parabolicparabolic profiles of the duct for

profiles of the duct for the simply supported girder. the simply supported girder. After the concrete is cast After the concrete is cast and curedand cured to gain sufficient strength, the tendons are passed through the ducts, as shown in the to gain sufficient strength, the tendons are passed through the ducts, as shown in the second photo.

second photo. The tendons are anchored The tendons are anchored at one end at one end and stretched at the and stretched at the other end byother end by a hydraulic jack.

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(a) Fabrication of reinforcement

(b) Placement of tendons

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(c) Stretching and anchoring of tendons

Figure 1-4.19

Figure 1-4.19 Manufacturing of a post-tensioned bridge I-girderManufacturing of a post-tensioned bridge I-girder

The following photos show the construction of post-tensioned box girders for a bridge The following photos show the construction of post-tensioned box girders for a bridge (Courtesy: Cochin Port

(Courtesy: Cochin Port Trust). Trust). The first The first photo shows photo shows the fabricated steel the fabricated steel reinforcementreinforcement with the ducts

with the ducts for the tendons placed for the tendons placed inside. inside. The top flange The top flange will be constructed later.will be constructed later. The second photo

The second photo shows the formwork shows the formwork in the pre-casting yard. in the pre-casting yard. The formwork for The formwork for thethe inner sides of the webs and

inner sides of the webs and the flanges is yet to be the flanges is yet to be placed. placed. In the third photo a In the third photo a girder isgirder is being post-tensioned after adequate curing.

being post-tensioned after adequate curing. The next photo The next photo shows a crane shows a crane on a bargeon a barge that transports a girder to

that transports a girder to the bridge site. the bridge site. The completed bridge can The completed bridge can be seen in be seen in the lastthe last photo.

photo.

(a) Reinforcement cage for box girder

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(b) Formwork for box girder

(c) Post-tensioning of box girder

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(d) Transporting of box girder

(e) Completed bridge

Figure 1-4.20