Pre stressed concrete

Introduction
 Concrete is strong in compression and weak in
tension.
 In this reason, reinforcement is provided in concrete
members.
 When ever service load is placed on RC structures, it
will undergo deformation.

 This leads to causing Tensile cracks in the RC
members
 Generally, steel bars are provided to limits the crack
width and resist the tensile forces.

What is Prestressing?
 Prestressing is the application of an initial load on a
structure, to enable it to counteract the stresses arising
from subsequent loads during its service period

Terms
Wires: 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 prestresssing tendon.
Cable : A group of tendons form a prestressing cable

End Block: An end section of a prestressed member that houses one or more
anchorage assemblies.

Advantages Of Prestressed Concrete
 Section remains uncracked under service loads
 Reduction of steel corrosion
 Increase in durability
 Full section is utilised
 Higher moment of inertia(Higher Stiffness)
 Less deformation(Improved serviceability)
 Increase in shear capacity
 It suitable for use in pressure vessels, liquid retaining structures
 Improved performance under dynamic and fatigue loading.

 High span to depth ratios
 Large spans possible with prestressing(45:1)
 Reduction in self weight
 More economical section
 Suitable for precast construction
Rapid construction
Better quality

Limitation of prestressing
 Skilled technology
 Use of high strength materials is costly
 There is additional cost in auxiliary equipments
 Need for quality control and inspection

Types of prestressing
Prestressing of concrete can be classified in several ways.
1) Source of prestressing force
This classification is based on the method by which the
prestressing force is generated. There are four sources of
prestressing forces.
1. Mechanical
2. Hydraulic
3. Electrical
4. Chemical

2) Pre-tensioning or Post-tensioning
This is based on the sequence of casting the concrete
and applying tension to the tendons.
Pre-tensioning (M-40):
The tension is applied to the tendons before casting
of the concrete. The pre-compression is transmitted from
steel to concrete through bond.
Post-tensioning(M-30):
The tensionn is applied to the tendons after hardening
of the concrete. The pre-compression is transmitted from
steel to concrete by the anchorage device

3)Linear or circular prestressing
The classification is based on the shape of the member
prestressed.
4)Full, Limited or partial prestressing
based on the amount of prestressing force.

Necessity of High grade of concrete and steel
 Higher the grade of concrete higher the bond strength
which is very essential in pretensioned concrete.
 Higher bearing strength which is vital in
post-tensioned concrete.
 Further creep and shrinkage losses are minimum with
high-grade concrete.
 Generally M30 grade concrete is used for post-tension
And M40 grade concrete is used for pretensioning.

 The losses in prestress members due to various reasons
are generally in the range of 250 N/mm2 to 400 N/mm2
 If mild steel or deformed steel is used the residual stress
after losses is either zero or negligible.
 Hence high tensile steel wires are used which varies
from 1600 to 2000 N/mm2
 Steel wire of dia 2.5, 3, 4, 5, 7, and 8mm are available.

Pre-
tensioning
System
Hoyer’s Long
Line method
Shorer
System

Hoyer’s Long Line method
 Hoyer’s long line method is often adopted in pre-
tensioning.
 Two bulk heads or abutments independently anchored
on the ground several meters apart, says 100m and
wire stretched between the bulkheads.
 Moulds are placed enclosing the wires. Concrete is
placed surrounding the wires.
 With this system, several members can be produced
can be produced along one line. And it is economical

 For tensioning, a hydraulic jack is used.
 Wires are gripped at the bulkheads, using split-cone
wedges.
 These wedges are made from tapered conical pins.
 Flat surface of the pin carries serrations to grip the
wire

 The advantage in pre-tensioning system is that there is
no expenditure on end anchorages
 Disadvantages in this system are that the end
abutments should be very strong and are provided
only in precast factories.
 This naturally limits the size of the member as large
sizes are difficult to transport from factory to the site
of construction.
 Loss is more in pre-tensioned members.

Post-Tensioning System
 A metal tube or a flexible hose following intended profile is placed inside
the mould and concrete is laid.
 Flexible hose is then removed leaving a duct inside the member. Steel cable
is inserted in the duct.
 The cable is anchored at one end of the member and stretched using a
hydraulic jack at the other end. After stretching the cable is anchored at the
other end also.
 Therefore post tensioning system consists of end anchorages and jacks.

The popular post-tensioning systems are the following:
 Freyssinet system
 Magnel Blaton system
 Gifford-Udall system
 Lee-McCall system

Post-
Tensioning
System
Freyssinet
System
Lee-McCall
System
Gifford-Udall
System
Magnel Blaton
System

Freyssinet System
 Freyssinet system was introduced by the French
Engineer Freyssinet and it was the first method to be
introduced.
 High strength steel wires of 5mm or 7mm diameter,
numbering 8 or 12 or 16 or 24 are grouped into a cable
with a helical spring inside.
 Spring keeps proper spacing for the wire. Cable is
inserted in the duct.

 Anchorage device consists of a concrete cylinder with a
concentric conical hole and corrugations on its
surface, and a conical plug carrying grooves on its
surface .
 Steel wires are carried along these grooves at the ends.
Concrete cylinder is heavily reinforced.

 Wires are pulled by Freyssinet double acting jacks
which can pull through suitable grooves all the wires
in the cable at a time.
 One end of the wires is anchored and the other end is
pulled till the wires are stretched to the required
length.
 An inner piston in the jack then pushes the plug into
the cylinder to grip the wires.

Magnel Blaton system
 In Freyssinet system several wires are stretched at a time. In
Magnel Blaton system, two wires are stretched at a time.
 This method was introduced by a famous engineer, Prof.
Magnel of Belgium.
 In this system, the anchorage device consists of sandwich
plate having grooves to hold the wires and wedges which
are also grooved. Each plate carries eight wires.

 Between the two ends the spacing of the wires is
maintained by spacers. Wires of 5mm or 7mm are adopted.
Cables consists of wires in multiples of 8 wires.
 Cables with as much as 64 wires are also used under special
conditions.
 A specially devised jack pulls two wires at a time and
anchors them.
 The wires with the sandwich plate using tapered wedge

Gifford Udall System
 This system originated in Great Britain, is widely used
in India.
 This is a single wire system. Each wire is stressed
independently using a double acting jack.
 Any number of wires can be grouped together to form
a cable in this system.
There are two types of anchorage device in this system.
 a) Tube anchorages
b) Plate anchorages

 Tube anchorage consists of a bearing plate, anchor wedges
and anchor grips.
 Anchor plate may be square or circular and have 8 or 12
tapered holes to accommodate the individual prestressing
wires.
 These wires are locked into the tapered holes by means of
anchor wedges.

 In addition, grout entry hole is also provided in the
bearing plate for grouting.
 Anchor wedges are split cone wedges carrying
serrations on its flat surface.
 There is a tube unit which is a fabricated steel
component incorporating a thrust plate, a steel tube
with a surrounding helix.
 This unit is attached to the end shutters and form an
efficient cast-in component of the anchorage

Lee McCall System
•This method is used to prestress steel bars.
•The diameter of the bar is between 12 and 28mm. bars provided with
threads at the ends are inserted in the performed ducts.
•After stretching the bars to the required length, they are tightened
using nuts against bearing plates provided at the end sections of the
member

 In pre-stressed concrete applications, most important
variable is the pre-stressing force. In the earlier days, it
was observed that the pre-stressing force does not stay
constant.
 • Even during pre-stressing of the tendons and the
transfer of pre-stress to the concrete members, there
is a drop of the pre-stressing force from the recorded
value in the jack gauge.

 The various reductions of the pre-stressing force are
termed as the losses in pre-stress.
 Early attempts to produce prestressed concrete
was not successful due to loss of prestress
transferred to concrete after few years.

 Prestress loss is nothing but the reduction of initial applied
prestress to an effective value.
 In other words, loss in prestress is the difference between initial
prestress and the effective prestress that remains in a member.
 Loss of prestress is a great concern since it affects the strength
of member and also significantly affects the member’s
serviceability including Stresses in Concrete, Cracking,
Camber and Deflection.

Elastic Shortening
1. Pre-tensioned Members: When the tendons are cut and the
prestressing force is transferred to the member, concrete
undergoes immediate shortening due to prestress.
2. Tendon also shortens by same amount, which leads to the
loss of prestress.

Elastic Shortening
1. Post-tensioned Members: If there is only one tendon,
there is no loss because the applied prestress is recorded
after the elastic shortening of the member.
2. For more than one tendon, if the tendons are stretched
sequentially, there is loss in a tendon during subsequent
stretching of the other tendons.

Fp= Change in Prestress/Losses in prestress
m= modular ratio Es/Ec
fc= Prestress in concrete

Frictional Loss
 In Post-tensioned members, tendons are housed in ducts or
sheaths.
 If the profile of cable is linear, the loss will be due to
straightening or stretching of the cables called Wobble
Effect.
 If the profile is curved, there will be loss in stress due to
friction between tendon and the duct or between the
tendons themselves.

Shrinkage
 The shrinkage of concrete is defined as the contraction
due to loss of moisture.
 Due to the shrinkage of concrete, the prestress in the
tendon is reduced with time.

Creep of Concrete
 Time-dependent, increase of deformation under
sustained load.
 Due to creep, the prestress in tendons decreases with
time.

 The ratio of the ultimate creep strain to the elastic strain is
defined as the ultimate creep coefficient or simply creep
coefficient, θ.
εcr,ult = θεel

Relaxation
Relaxation is the reduction in stress with time at
constant strain.
– decrease in the stress is due to some of the initial
elastic strain is transformed in to inelastic strain under
constant strain.
– stress decreases according to the remaining elastic
strain.

 This losses is generally 2% to 8% of the initial stress.
Initial Stress Relaxation stress
0.5fy 0
0.6fy 35
0.7fy 70
0.8fy 90

Type of Losses
Percentage loss of stress
Pretensioning Post Tensioning
Elastic Shorting of
concrete
3 1
Creep of concrete 6 5
Shrinkage of concrete 7 6
Creep in steel 2 3

Pre stressed concrete

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