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8/13/2019 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.
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+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,.
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tressed Concrete Structures Dr. Amlan * Sengupta and Prof. Devdas )
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
• $etter &uality control
• educed maintenance
• Suita#le for repetitive construction
• )ultiple use of formwor8
⇒ eduction of formwor8
• Availa#ility of standard shapes.
The following figure shows the common types of precast sections.
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T-section
Double T-
section
Hollow core Piles
L-section Inverted T-section I-girders
Figure 1-2.1 Typical precast mem#ers
1.2.$ 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.
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*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.
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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.
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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,
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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.
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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,
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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,
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)ultiaial Prestressing
!hen the prestressing tendons are parallel to more than two aes" it is called )ultiaial
Prestressing. /or eample" prestressing of domes.