1.2 advantages tadvantages_types of prestressing.docypes of prestressing

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tressed Concrete Structures

1.2 Advantages and Types of Prestressing

This section covers the following topics.

Definitions

Advantages of Prestressing

• Limitations of Prestressing

Types of Prestressing

1.2.1 Definitions

The terms commonly used in prestressed concrete are eplained. The terms are placed

in groups as per usage.

Forms of Prestressing Steel

!ires

Prestressing wire is a single unit made of

steel. Strands

Two" three or seven wires are wound to form a prestressing

strand. Tendon

A group of strands or wires are wound to form a prestressing tendon.

Ca#le

A group of tendons form a prestressing ca#le.

$ars

A tendon can #e made up of a single steel #ar. The diameter of a #ar is much

larger than that of a wire.

The different types of prestressing steel are further eplained in Section 1.%"

Prestressing Steel.

Nature of Concrete-Steel Interface

$onded tendon

!hen there is ade&uate #ond #etween the prestressing tendon and concrete" it is called

a #onded tendon. Pre'tensioned and grouted post'tensioned tendons are #onded

tendons.

n (nstitute of Technology )adras

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http://var/www/apps/conversion/tmp/scratch_5/advantages%20of%20Prestressing

http://var/www/apps/conversion/tmp/scratch_5/types%20of%20Prestressing

http://var/www/apps/conversion/tmp/scratch_5/definitions



tressed Concrete Structures Dr. Amlan * Sengupta and Prof. Devdas )

+n#onded tendon

!hen there is no #ond #etween the prestressing tendon and concrete" it is called

un#onded tendon. !hen grout is not applied after post'tensioning" the tendon is an

un#onded tendon.

Stages of Loading

The analysis of prestressed mem#ers can #e different for the different stages of loading.

The stages of loading are as follows.

1, (nitial - (t can #e su#divided into two stages.

a, During tensioning of steel

#, At transfer of prestress to concrete.

2, (ntermediate - This includes the loads during transportation of the

prestressed mem#ers.

., /inal - (t can #e su#divided into two stages.

a, At service" during operation.

#, At ultimate" during etreme events.

1.2.2 Advantages of Prestressing

The prestressing of concrete has several advantages as compared to traditional

reinforced concrete 0C, without prestressing. A fully prestressed concrete mem#er is

usually su#ected to compression during service life. This rectifies several deficiencies

of concrete.

The following tet #roadly mentions the advantages of a prestressed concrete mem#er

with an e&uivalent C mem#er. /or each effect" the #enefits are listed.

1) Section remains uncraced under service loads

¾ eduction of steel corrosion

• (ncrease in dura#ility.

¾ /ull section is utilised

• 3igher moment of inertia 0higher stiffness,

• Less deformations 0improved servicea#ility,.





¾ (ncrease in shear capacity.

¾ Suita#le for use in pressure vessels" li&uid retaining structures.

¾ (mproved performance 0resilience, under dynamic and fatigue loading.

2) !ig" s#an-to-de#t" ratios

Larger spans possi#le with prestressing 0#ridges" #uildings with large column'free

spaces,Typical values of span'to'depth ratios in sla#s are given #elow.

4on'prestressed sla# 25-1

Prestressed sla# 67-1

/or the same span" less depth compared to C mem#er.

• eduction in self weight

• )ore aesthetic appeal due to slender sections

• )ore economical sections.

$) Suita%le for #recast construction

The advantages of precast construction are as follows.

• apid construction

• $etter &uality control

• educed maintenance

• Suita#le for repetitive construction

• )ultiple use of formwor8

⇒ eduction of formwor8

• Availa#ility of standard shapes.

The following figure shows the common types of precast sections.




T-section

Double T-

section

Hollow core Piles

L-section Inverted T-section I-girders

Figure 1-2.1 Typical precast mem#ers

1.2.$ Limitations of Prestressing

Although prestressing has advantages" some aspects need to #e carefully addressed.

• Prestressing needs s8illed technology. 3ence" it is not as common as reinforced

concrete.

• The use of high strength materials is costly.

• There is additional cost in auiliary e&uipments.

• There is need for &uality control and inspection.

1.2.& '(#es of Prestressing

Prestressing of concrete can #e classified in several ways. The following classifications

are discussed.

Source of #restressing force

This classification is #ased on the method #y which the prestressing force is generated.

There are four sources of prestressing force- )echanical" hydraulic" electrical and

chemical.




*ternal or internal #restressing

This classification is #ased on the location of the prestressing tendon with respect to the

concrete section.

Pre-tensioning or #ost-tensioning

This is the most important classification and is #ased on the se&uence of casting the

concrete and applying tension to the tendons.

Linear or circular #restressing

This classification is #ased on the shape of the mem#er prestressed.

Full+ limited or #artial #restressing

$ased on the amount of prestressing force" three types of prestressing are defined.

,nia*ial+ %ia*ial or multi-a*ial #restressing

As the names suggest" the classification is #ased on the directions of prestressing a

mem#er.

The individual types of prestressing are eplained net.

Source of Prestressing Force3ydraulic Prestressing

This is the simplest type of prestressing" producing large prestressing forces. The

hydraulic ac8 used for the tensioning of tendons" comprises of cali#rated pressure

gauges which directly indicate the magnitude of force developed during the tensioning.

)echanical Prestressing

(n this type of prestressing" the devices includes weights with or without lever

transmission" geared transmission in conunction with pulley #loc8s" screw ac8s with or

without gear drives and wire'winding machines. This type of prestressing is adopted for

mass scale production.





9lectrical Prestressing

(n this type of prestressing" the steel wires are electrically heated and anchored #efore

placing concrete in the moulds. This type of prestressing is also 8nown as thermo'

electric prestressing.

*ternal or Internal Prestressing

9ternal Prestressing

!hen the prestressing is achieved #y elements located outside the concrete" it is called

eternal prestressing. The tendons can lie outside the mem#er 0for eample in ('girders

or walls, or inside the hollow space of a #o girder. This techni&ue is adopted in #ridges

and strengthening of #uildings. (n the following figure" the #o girder of a #ridge is

prestressed with tendons that lie outside the concrete.

Figure 1-2.2 9ternal prestressing of a #o girder

0eference- :SL (nternational Ltd.,

(nternal Prestressing

!hen the prestressing is achieved #y elements located inside the concrete mem#er

0commonly" #y em#edded tendons," it is called internal prestressing. )ost of the

applications of prestressing are internal prestressing. (n the following figure" concrete

will #e cast around the ducts for placing the tendons.





Figure 1-2.$ (nternal prestressing of a #o girder

0Courtesy- Cochin Port Trust" *erala,

Pre-tensioning or Post-tensioning

Pre'tensioning

The tension is applied to the tendons #efore casting of the concrete. The pre'

compression is transmitted from steel to concrete through #ond over the transmission

length near the ends. The following figure shows manufactured pre'tensioned electric

poles.

Figure 1-2.& Pre'tensioned electric poles

0Courtesy- The Concrete Products and Construction Company" C;PC;" Chennai,





Post'tensioning

The tension is applied to the tendons 0located in a duct, after hardening of the concrete.

The pre'compression is transmitted from steel to concrete #y the anchorage device 0at

the end #loc8s,. The following figure shows a post'tensioned #o girder of a #ridge.

Figure 1-2. Post'tensioning of a #o girder

0Courtesy- Cochin Port Trust" *erala,

The details of pre'tensioning and post'tensioning are covered under Section 1." <Pre'

tensioning Systems and Devices<" and Section 1.6" <Post'tensioning Systems and

Devices=" respectively.

Linear or Circular Prestressing

Linear Prestressing

!hen the prestressed mem#ers are straight or flat" in the direction of prestressing" the

prestressing is called linear prestressing. /or eample" prestressing of #eams" piles"

poles and sla#s. The profile of the prestressing tendon may #e curved. The following

figure shows linearly prestressed railway sleepers.





Figure 1-2. Linearly prestressed railway sleepers

0Courtesy- The Concrete Products and Construction Company" C;PC;" Chennai,

Circular Prestressing

!hen the prestressed mem#ers are curved" in the direction of prestressing" the

prestressing is called circular prestressing. /or eample" circumferential prestressing of

tan8s" silos" pipes and similar structures. The following figure shows the containment

structure for a nuclear reactor which is circularly prestressed.

Figure 1-2./ Circularly prestressed containment structure" *aiga Atomic Power Station" *arnata8a

0eference- Larsen > Tou#ro Ltd" 9CC Division" 60 Landmark Years,





Full+ Limited or Partial Prestressing

/ull Prestressing

!hen the level of prestressing is such that no tensile stress is allowed in concrete under

service loads" it is called /ull Prestressing 0Type 1" as per IS01$&$ - 13,.

Limited Prestressing

!hen the level of prestressing is such that the tensile stress under service loads is

within the crac8ing stress of concrete" it is called Limited Prestressing 0Type 2,.

Partial Prestressing

!hen the level of prestressing is such that under tensile stresses due to service loads"

the crac8 width is within the allowa#le limit" it is called Partial Prestressing 0Type ,.

,nia*ial+ 4ia*ial or 5ultia*ial Prestressing

+niaial Prestressing

!hen the prestressing tendons are parallel to one ais" it is called +niaial Prestressing.

/or eample" longitudinal prestressing of #eams.

$iaial Prestressing

!hen there are prestressing tendons parallel to two aes" it is called $iaial

Prestressing. The following figure shows the #iaial prestressing of sla#s.

Non-prestressed reinforcement

Duct for

prestressi

ng tendon

Figure 1-2. $iaial prestressing of a sla#

0Courtesy- :SL (ndia Pvt. Ltd." Chennai,





)ultiaial Prestressing

!hen the prestressing tendons are parallel to more than two aes" it is called )ultiaial

Prestressing. /or eample" prestressing of domes.

1.2 advantages tadvantages_types of prestressing.docypes of prestressing

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