UNIT-1 PSC.pptx

PRESTRESSED CONCRETE
(Professional Elective – II)
B.Tech. III Year II Sem
(R-18)
Mrs.B.Lavanya
M.Tech-SE
Assistant Professor
Dept of Civil Engineering
AY:2020-2021

SYLLABUS
CE611PE - PRESTRESSED CONCRETE
Pre-Requisites: Reinforced Concrete Design
Course Objectives: The objectives of the course are to
 Understand the principles & necessity of
prestressed concrete structures.
 Know different techniques of prestressing.
 Get the knowledge on various losses of
prestress.
 Understand Analysis and design of
prestressed concrete members.

Course Outcomes: After the completion of
the course student should be able to
oAcquire the knowledge of evolution of
process of prestressing.
oAcquire the knowledge of various
prestressing techniques.
oDevelop skills in analysis design of
prestressed structural elements as per the
IS codal Provisions

UNIT I:
Introduction: Historic development- General principles
of prestressing pretensioning and post tensioning-
Advantages and limitations of Prestressed concrete-
General principles of PSC Classification and types of
prestressing- Materials- high strength concrete and high
tensile steel their characteristics.
UNIT II:
Methods and Systems of prestressing: Pretensioning
and Posttensioning methods and systems of prestressing
like Hoyer system, Magnel Blaton system, Freyssinet
system and Gifford- Udall System- Lee McCall system.

Losses of Prestress: Loss of prestress in pretensioned
and posttensioned
members due to various causes like elastic shortage of
concrete, shrinkage of concrete, creep of concrete,
relaxation of stress in steel, slip in anchorage, frictional
losses.
UNIT III:
Flexure: Analysis of sections for flexure- beams
prestressed with straight, concentric, eccentric, bent and
parabolic tendons- stress diagrams- Elastic design of PSC
slabs and beams of rectangular and I sections- Kern line –
Cable profile and cable layout.

Shear: General Considerations- Principal tension and
compression- Improving shear resistance of concrete
by horizontal and vertical prestressing and by using
inclined or parabolic cables- Analysis of rectangular
and I beams for shear – Design of shear
reinforcements- IS Code provisions.
UNIT IV:
Transfer of Prestress in Pretensioned Members:
Transmission of prestressing force by bond –
Transmission length – Flexural bond stresses – IS
code provisions – Anchorage zone stresses in post
tensioned members – stress distribution in End block
– Analysis by Guyon, Magnel, Zienlinski and Rowe’s
methods – Anchorage zone reinforcement- IS
Provisions

UNIT V:
Composite Beams: Different Types- Propped and
Unpropped- stress distribution- Differential
shrinkage- Analysis of composite beams- General
design considerations.
Deflections: Importance of control of deflections-
Factors influencing deflections – Short term
deflections of uncracked beams- prediction of long-
time deflections- IS code requirements.
Text Books & References:
1. Prestressed concrete by Krishna Raju****, Tata Mc Graw
Hill Book – Co. New Delhi.
2. Design of prestress concrete structures by T.Y. Lin and
Burn, John Wiley, New York.
3. Prestressed concrete by S. Ramamrutham Dhanpat Rai &
Sons, Delhi.
4. Prestressed Concrete by N. Rajagopalan***** Narosa
Publishing House

****Code of Practice for PSC
IS 1343:1980 Bureau of Indian Standards
Allied codes
IRC 18:2000 Design Criteria for PS road
Bridges(Post tensioned concrete) The Indian
Road Congress.
IRS-Concrete Bridge Code:1997, Indian
Railway Standard Code of Practice for Plain,
RCC & PSC for General Bridge Construction,
Ministry of Railways.

Introduction:
 Historic development
 Advantages and limitations of
Prestressed concrete-
 General principles of PSC
Classification and types of
prestressing-
 Materials- high strength concrete
and high tensile steel their
characteristics.
1. Introduction
2. What is
Prestressing?
3. Difference between
PSC &RCC
4. Pre Tensioning &
Post tensioning
5. Examples
6. Pretensioning
Devices
7. Advantages &
Disadvantages
UNIT-1
PRESTRESSED CONCRETE
(Professional Elective – II)

What is Prestressed
Concrete ???
 Concrete in which
reinforcing steel bars
are stretched and
anchored to compress
it and thus increase its
resistance to stress.
UNIT-1
1. Introduction
2. What is
Prestressing?
3. Difference between
PSC &RCC
4. Pre Tensioning &
Post tensioning
5. Examples
6. Pretensioning
Devices
7. Advantages &
Disadvantages
“The process of prestressing consists in applying
forces to the concrete structure by stressing tendons
relative to the concrete member”.
or

Main difference Between
REINFORCED CONCRETE
&
PRE-STRESSED CONCRETE
• Both utilizes the structural capabilities of
steel and concrete.

Ordinary Reinforced Concrete
• Beam supports a load
by developing
compressive stresses
at the top, but since
the concrete cannot
resist the tension at
the bottom, it cracks
there.
• Reinforcing steel bars
are placed within this
tension zone to resist
the tension and
control the cracking.

Pre-stressed Concrete
• It involves the application of forces tending
to bend and compress a concrete element
in order to counteract bending which
results from loading.
• The forced applied is the tensioning or
stretching of the steel component which
usually in the form of high tensile strands,
wires or bars.

Reinforced
Concrete
Prestressed
Concrete
Before Loading
Prestressed Concrete After
Loading

History Of PSC
Development Of PSC

Before the development of prestressed concrete, two
significant developments of reinforced concrete are the
invention of Portland cement and introduction of steel in
concrete.
1824 Aspdin, J., (England)
Obtained a patent for the manufacture of Portland cement.
1857 Monier, J., (France)
Introduced steel wires in concrete to make flower pots, pipes,
arches and slabs.

The following events were significant in the
development of prestressed concrete.
1886 Jackson, P. H., (USA)
Introduced the concept of tightening steel tie
rods in artificial stone and concrete arches.
Figure 1-1.6 Steel tie rods in arches
History…

 1888 Dohring, C. E. W., (Germany)
Manufactured concrete slabs and small beams with
embedded tensioned steel.
 1908 Stainer, C. R., (USA)
Recognized losses due to shrinkage and creep, and
suggested retightening the rods to recover lost prestress.
 1923 Emperger, F., (Austria)
Developed a method of winding and
pretensioning high tensile steel wires around concrete
pipes.
History…

 1924 Hewett, W. H., (USA)
Introduced hoop-stressed horizontal reinforcement
around walls of concrete tanks through the use of
turnbuckles.
 Thousands of liquid storage tanks and concrete pipes
were built in the two decades to follow.
 1925 Dill, R. H., (USA)
Used high strength unbonded steel rods.
The rods were tensioned and anchored after hardening of
the concrete.
History…

• Used high tensile steel
wires, with ultimate
strength as high as 1725
MPa and yield stress over
1240 MPa.
• In 1939, he developed
conical wedges for end
anchorages for post-
tensioning and developed
double-acting jacks.
Father of Prestressed Concrete
1926 Eugene Freyssinet (France)
History…

Developed ‘long line’ pre-tensioning method.
1938 Hoyer, E., (Germany)
Developed an anchoring system for post- tensioning,
using flat wedges.
1940 Magnel, G., (Belgium)
In India, the applications of prestressed concrete
diversified over the years.
• First prestressed concrete bridge -1948
under the Assam Rail Link Project
History…
Bogibeel road and rail bridge over the
Brahmaputra river in Assam

⚫Among bridges, the Pamban Road Bridge at
Rameshwaram, Tamilnadu, remains a classic
example of the use of prestressed concrete
girders.
Pamban Road Bridge at
Rameshwaram, Tamilnadu
applications of prestressed concrete …

Need for High Strength Steel & Concrete
• As normal loss of stress in steel is 100 to
240N/mm2, the stress in steel in initial
stages must be very high about 1200 to
2000N/mm2
• high resistance in Tension, shear, bond &
bearing.
• Use of HSC results in reduction of Cross
sectional dimensions of PSC elements.

1.Tendon
2.Anchorage
3.Pretensioning
4.Post-tensioning
5.Bonded Prestressed concrete
6.Non-Bonded Prestressed concrete
Basic Terminology

7.Partial Prestressing
8.Moderate Prestressing
9. Axial Prestressing
10. Eccentric Prestressing
11.Uniaxial,Biaxial and Triaxial Prestressing
12.Circular Prestressing
Basic Terminology……..

13.Debonding
14.Proof stress=residual strain of
0.2 % of original gauge length
15.Creep coefficient
=Total creep strain/ elastic strain
Basic Terminology……..

⚫APPLICATIONS:
Prestressed concrete was started to be used in
 building frames
 parking structures
 stadiums, railway sleepers
 transmission line poles
 and other types of structures and elements.

Advantages of pre-stressed concrete.
o Factory products are possible.
o Long span structure are possible
o Members are tested before use.
o Dead load are get counter balanced by eccentric pre- stressin
o It has high fatigue resistance.
o High ability to resist the impact.
o It has high live load carrying capacity.
o It free from cracks from service loads and enable entire
section to take part in resisting moments.
o Member are free from the tensile stresses

o Lower construction cost.
o Thinner slabs, which are especially important in high-rise
buildings where floor thickness savings can translate into
additional floors for the same or lower cost
o Longer span lengths increase the usable unencumbered
floorspace in buildings and parking structures
o Fewer joints lead to lower maintenance costs over the
design life of the structure, since joints are the major
locus of weakness in concrete buildings.

Disadvantages of pre-stressed concrete.
⚫ Required skilled builders & experienced
engineers.
⚫ Availability of experienced engineers is less.
⚫ Required complicated formwork.
⚫ It requires high strength concrete & steel.
⚫ Pre-stressed concrete is less fiber resistant.

 Major disadvantage construction machineries like
jacks anchorage etc.
 Advanced technical knowledge and strict supervision
is very important.
 For concrete prestressing, high tensile reinforcement
bars are needed which costs greater than generally
used mild steel reinforcement bars.
 Highly skilled labor is needed for prestressed
concrete constructions.

Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is
applied to such an extent that there will always be a
residual tension in the spoke.
Spokes in a bicycle wheel

Types of Pre-stressing
1.External or internal pre-stressing.
It is based on the location of the pre-stressing tendons
with respect to concrete section.
2.*****Pre-tensioning or post-tensioning.
It based on the sequence of casting the concrete and
applying tension to the tendons.
3.Linear or circular pre-stressing.
It based on the shape of the member pre-stressed.
4.Full, limited or partial pre-stressing.
It based on the pre-stressing force.
5.Uniaxial, biaxial or multi-axial pre stressing.
It based on the direction of the pre-stressing member.

External Prestressing
• When the prestressing is achieved by elements
located outside the concrete, it is called external
prestressing.
• The tendons can lie outside the member (for
example in I-girders or walls) or inside the
hollow space of a box girder.
• This Technique is adopted in bridges and
strengthening of buildings. In the following
figure, the box girder of a bridge is prestressed
with tendons that lie outside the concrete.

External prestressing of a box girder

Post-tensioning of a box girder

Internal prestressing of a box girder
Internal Prestressing
When the prestressing is
achieved by elements
located inside the
concrete member
(commonly by embedded
tendons it is called as
internal prestressing.
• Most of the prestressing are internal prestressing.
• In the following figure, concrete will be cast around
the ducts for placing the tendons.

Linearly prestressed railway sleepers
Linear Prestressing
When the prestressed
members are straight
or Flat in the direction of
prestressing is called
linear prestressing.
For example,
prestressing of beams,
piles, poles and slabs.
The profile of the
prestressing tendon may
be curved.

Circularly prestressed containment structure
• When the prestressed members are curved, in the
direction of prestressing, the prestressing is
called circular prestressing.
• For example, circumferential prestressing of
tanks, silos, pipes and similar structures.

Pre-tensioning:
 A method of prestessing concrete in which
tendons are tensioned before the concrete is
placed.
 Usually done in industry
Post-tensioning:
• A method of prestessing concrete in which
tendons are tensioned after the concrete has
place.
• Done in Site only

1)Anchoring the tendons against the end abutments.
2)Placing of jacks.
3)Applying tension to the tendons.
4)Casting of concrete.
5)Cutting of the tendons.
Method of pre-tensioning:

Travelling pre-tensioning
stress bench
Anchoring of strands

Pouring of concrete
Steam curing chamber

1)Casting of concrete.
2)Placement of tendons.
3)Placement of the anchorage block and jack.
4)Applying tension to the tendons.
5)Seating of the wedges.
6)Cutting the tendons.
Method of post-tensioning:

Systems of prestressing
Many systems are in practice.
i. Freysinetsystem
ii. MagnelBlatonsystem
iii.GifordUdalsystem
It is the process of tensioning of tendons.
Secures firmly to concrete till the lift of
member.

Popular post-tensioning system:
1) Freyssinet system 2) Magnel blaton
system

i.TheFreyssinetsystem:
a. High tensile wires12No.
b. Arranged to form a group into cables with a
Spiral spring inside to give clearance between the wires
c. They will be inserted in a metal sheeting cables
d. The cable will be free to move initially and after
prestressing it will be grouted with cement mortar
e. The anchorages consists of a cylinder with
centralconicalhole

Freyssinet System
Fluted male
cone
Duct
former
Female anchorage
with steel spirals

Sandwich
Plate
Steel
wedge
Distribution
Plate
Magnel Blaton System
Magnel Blaton System–where 8 wires
can be prestressed individually

3) Gifford-udall
system
4) Lee-mccall system

Section for Pre-tensioning
Section for Post-tensioning

Process Post-tensioning
1.Rolls of post tensioning
cables

2. Pulling anchors
for post- tensioning cables
3.Positioned
post- tensioning cables

4. Post-tensioning cable ends
extending from freshly
poured concrete
5. Hydraulic Jack are used
to pull the Cables

Tensioning Devices
1.Mechanical devices: The mechanical devices
generally used include weights with or without lever
transmission, geared transmission in conjunction
with pulley blocks, screw jacks with or without gear
devices and wire-winding machines. These devices
are employed mainly for prestressing structural
concrete components produced on a mass scale in
factory.
2.Hydraulic devices: These are simplest means
for producing large prestressing force, extensively
used as tensioning devices.

3.Electrical devices: The wires are
electrically heated and anchored before
placing concrete in the mould. This
method is often referred to as thermo-
prestressing and used for tensioning of
steel wires and deformed bars.
4.Chemical devices: Expanding cements
are used and the degree of expansion is
controlled by varying the curing
condition. Since the expansive action of
cement

EQUIPMENTS :-
T6Z-08 Air Powered Grout Pump
Pumps cement grout only, no sand.
32 Gallon Mixing Tank. Mixes up to
2 sacks of material at once and
allows for grout to be pumped
during mixing or mixed without
pumping.

T7Z Hydraulic Jacks
• Used for testing and pre-stressing anchor bolts.
• Available with up to 5-1/8" center hole.
• Unit comes with ram, pump, gauge, hoses, jack stand,
high strength coupling, high strength test rod, plate,
hex nut and knocker wrench.
• Note: Jack pull rods should have a higher capacity than
the anchor rod.

T80 Post-Tensioning Jacks
• With the T80 series the enclosed bearing housing
contains a geared socket drive to tighten the bolt hex
nut during tensioning.
• Test jack housing will accommodate up to a 9” deep
pocket.
T80 Post-Tensioning Jacks

• PSC -
Materials,
• High strength
concrete
• High tensile
steel
&
• Their
Characteristics
UNIT-1
Materials of PSC
o Cement
o Steel
o Concrete
 Cement
 Ordinary Portland
cement
 High Strength Ordinary
Portland cement
 Rapid hardening
Portland cement

o Concrete
o M35 grade with low shrinkage
o Steel
o 5mm-7mm high tensile steel with proof
stress of 1000-2000 N/mm2

The desirable properties of grout are as follows.
1)Fluidity
2)Minimum bleeding and segregation
3)Low shrinkage
4)Adequate strength after hardening
5)No detrimental compounds
6)Durable.

Prestressing Steel
o Steels with yield strength levels in excess of 550 MPa –AHSS
(“ultrahigh-strength steels”) for tensile strengths exceeding
strengths exceeding 780 MPa.
o AHSS -tensile strength of at least 1000
o It is an alloy of iron, carbon, manganese and
optional materials.
o conventional non-prestressed reinforcement
is used for flexural capacity (optional), shear
capacity, temperature and shrinkage
requirements.

Wires
o A prestressing wire is
a single unit made of
steel.
o nominal diameters of
the wires are 2.5, 3.0,
4.0, 5.0, 7.0 and 8.0
mm.
1)Plain wire: No
indentations on the
surface.
2)Indented wire: There
are circular or elliptical
indentations on the
surface.

Strands
A few wires are spun together in a helical form to
form a prestressing strand.
1)Two-wire strand: Two wires are spun together to
form the strand.
2)Three-wire strand: Three wires are spun together
to form the strand.
3)Seven-wire
strand: In this type
of strand, six
wires are spun
around a central
wire. The central
wire is larger than
the other wires.

Tendons
• A group of strands or wires are placed together to form
a prestressing tendon.
• The tendons are used in post-tensioned members.
• Cables
• A group of tendons form a prestressing cable.
• The cables are used in bridges

• High-tensile steel usually contains 0.6 to 0.85% carbon,
0.7 to 1 % manganese, 0.05% of Sulphur and
phosphorus.
• The durability of wires gets improved due to the cold-
drawing operation.
• Cold-drawn wires are then tempered to improve their
properties.
High-tensile Strength
steel

• Cold-drawn wires -nominal sizes of 2.5, 3, 4, 5, 7 and
8 mm diameter.
• Tempering or ageing or stress relieving by heat
treatment of wires at 150-420°C improves the tensile
strength.
• High-tensile steel bars - nominal sizes of 10, 12,
16, 20, 22, 25, 28 and 32 mm diameter.
• minimum characteristic tensile strength of high-
tensile strength bars as per code is 980 N/mm2

• proof stress should not be less than 80% of
the minimum specified tensile strength
• Elongation at rupture should be 10% for the
specified gauge length.

PSC Materials
High-Strength Concrete Mixes
1.Erntroy & Shacklock
2.American Concrete Institute mix design for no
slump concrete
3. British D.O.E method
4. Indian Standard code method.

The prestressing steel, as per the code,
should be any one of the following types:
•Plain hard-drawn steel wire conforming to
IS: 785(Part 1)-1966 and IS: 1785(Part 2)-
1967,
•Cold-drawn indented wire,
•High tensile steel bar conforming to IS:
2090- 1962, and
•Uncoated stress relieved strand conforming
to IS: 6006-1970.

******Characteristics of Materials
o Indian Standard Code-IS:1343
o IS Code:456-2000 & IS: 1343
1.fck-5% Test results are expected to fall
• 2.Strength Requirements:
• Minimum compressive strength for post-tensioned
-30 N/mm2
• Minimum compressive strength for pre-tensioned
members- 40 N/mm2
• Ratio of cylinder to cube strength- 0.8
• Min cement contet-300-360kg/m3

• 3.Permissible strength in concrete
• Max permissible compressive stress in flexure -
0.41 for M30- 0.35 for M60
At transfer Compressive stress 0.54-0.37 fci for post tensioned
work & 0.51-0.44 fci for
pretensioned work
Tensile stress ………………….
At service
Load
Compressive stress Varies linearly 0.41-0.35 fck
1 2 3 IS:1343
Tensile stress Type-1-None
Type-2-Not exceeds 3N/mm2
Type-3 M30-3.2N/mm2 -7.3
N/mm2
M50- depends on limiting
crack-width

• The permissible compressive & tensile stress in concrete
at transfer & service loads are defined as corresponding
compressive strength fci of concrete at each stage.
• 4.Shrinkage of concrete
• Due to gradual loss of moisture results in changes in
volume
• Drying shrinkage depends on
• Type of aggregate
• Quantity
• Relative humidity
• w/c ratio
• Mix
• Time of exposure
• Phenomenon of Shrinkage –Time dependent

• Total residual shrinkage strain –IS Code
• -3.0 x 10-4 for pretensioned member &
• (2.0 x 10-4 ) / log(t+2) for post tensioned member
• t- age of concrete
• An Estimate of shrinkage of symmetrical RCC section
–(εsh/1+Kρ)
• εsh Shrinkage of plain concrete
• ρ-Area of steel relative to concrete
• t- age of concrete
• K- Coefficient : 25 –internal exposure & 15- external
exposure

• 5.Creep of concrete
• 55% - 20year creep
• 76%- 1 year
• After 1 year- Unity
• After 10 year-Avg-1.26
• After 30 year-Avg-1.36
• BS- 8110-1985
• εcc=(Stress/Et) x Φ
Et= Modulus of elasticity at the
age of loading t
Φ =Creep coefficient

• 6.Deformation of concrete
• IS- 1343 : Ec= 5000√fck N/mm2
• ACI- 318M-2011 : Ec= 5050√fck N/mm2
• FIP: recommends value of secant modulus
for short term loading refer(fig-2.3)
• Compressive strength of concrete -20-80
N/mm2

• Durability, Fire Resistance & Cover
Requirements for PSC Members
• IS- 1343 : Min cover-20mm protected
pretensioned members
• 30mm or size of cable- whichever is greater
• if exposed to aggressive environment-
10mm

Fire Resistance
BS-476-part-8) 1972 & ASTME 119-1979
Fire resistance of structural elements
influenced by
o Size & shape of element
o Detailing, Type & quality of reinforcement of
prestressing tendons
o The level of load supported & pattern of
loading
o Type of concrete & aggregate
o Conditions at end bearing
o Protective cover to reinforcement

Protection of prestressing steel, sheathing
& Anchorages
Non reactive materials- epoxy or zinc or
zinc aluminium
o Non corroding sheathing material- HDPE
o Sheathing & Duct-
o Corrosion inhibition materials- Grease wax
or petroleum jelly
o Anchorages- Casing
o Film Guards

• Water Cement Ratio and Cement Content
As per IS : 456-2000

UNIT-1 PSC.pptx

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