Introduction to prestressed concrete structures

INTRODUCTION TO
PRESTRESSED CONCRETE
STRUCTURES

02/06/18 SPK-PSG College of Technology 2

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

• Reinforced concrete
– Combines concrete and steel bars by simply putting them
together and letting them act together as they may wish.
• Prestressed concrete
– Combines high -strength concrete with high-strength steel in
an “active manner”.
– This is achieved by tensioning the steel and holding it
against the concrete, thus putting the concrete into
compression . This active combination results in a much
better behavior of the two materials.
– Steel is ductile and now is made to act in high tension by
prestressing.
– Concrete is a brittle material with its tensile capacity now
improved by being compressed , while its compressive
capacity is not really harmed.
– PC is ideal combination of two modern high strength
materials

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. He is often referred to as the
Father of Prestressed concrete.
1938 Hoyer, E., (Germany)
Developed ‘long line’ pre-tensioning
method.
1940 Magnel, G., (Belgium)
Developed an anchoring system for post-
tensioning, using flat wedges.
Eugene Freyssinet
(France)

• In India, the applications of prestressed concrete diversified over
the years. The first prestressed concrete bridge was built in 1948
under the Assam Rail Link Project. 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

General principle of prestressing-
barrel construction
 When ropes or metal bands were
wound around wooden staves to form
barrels.
 When the bands were tightened,
they were under tensile prestress
which in turn created compressive
prestress between staves and thus
enabled them to resist hoop tension
produced by internal liquid pressure.
 In other words, the bands and the
staves were both prestressed before
they were subjected to any service
loads.

Terminology
• Tendon: A stretched element used in a concrete member of
structure to impart prestress to the concrete.
• Anchorage: A device generally used to enable the tendon to impart
and maintain prestress in concrete.
• Pretensioning: A method of prestressing concrete in which the
tendons are tensioned before the concrete is placed. In this
method, the concrete is introduced by bond between steel &
concrete.
• Post-tensioning: A method of prestressing concrete by tensioning
the tendons against hardened concrete. In this method, the
prestress is imparted to concrete by bearing

Examples….

Definition-Prestressed concrete-
ACI Committee
• Concrete in which there have been introduced internal stresses
of such magnitude and distribution that the stresses resulting
from given external loadings are counteracted to a desired
degree. In reinforced concrete members the prestress is
commonly introduced by tensioning the steel reinforcement.

Fundamentals of Prestressing

Principles of prestressing..
• Pre-stressing is a method in which compression force is applied
to the reinforced concrete section.
• The effect of pre stressing is to reduce the tensile stress in the
section to the point till the tensile stress is below the cracking
stress. Thus the concrete does not crack.
• It is then possible to treat concrete as a elastic material.
• The concrete can be visualized to have two compressive force
i . Internal pre-stressing force.
ii . External forces (d.l , l.l etc )
• These two forces must counteract each other.

Stress in concrete when pre stressing is applied at the
c.g of the section

Stress in concrete when pre stressing is applied
eccentrically with respect to the c.g of the section .

Basic Concepts
• First Concept
– Prestressing to transform concrete into a elastic material
• Second Concept
– Prestressing for combination of high strength steel with
concrete
• Third Concept
– Prestressing to achieve load balancing

First Concept-
Prestressing to transform concrete into a elastic
material
• Concrete which is transformed from a brittle material into an
elastic one by the precompression given to it.
• Concrete –weak in tension- strong in compression.
• Two systems of forces: Internal prestress and external load,
with the tensile stresses due to the external load counteracted
by the compressive stresses due to the prestress.
• Cracking in concrete is prevented or delayed by precompression
produced by the tendons.

First Concept-
Prestressing to transform concrete into a elastic
material

Second Concept-
Prestressing for combination of high strength steel with
concrete
Case-1-Reinforced concrete & prestressed concrete
• Steel-tensile force
• Concrete- compressive force
• Two forces forming a couple with a lever arm between them.

Second Concept-
Prestressing for combination of high strength steel with
concrete
Case-2-High Tensile steel
• High tensile steel- elongated a great deal before its strength is fully utilized.
• Conventionally High tensile steel is embedded in concrete, this tends to form
cracks in surrounded concrete before the full strength is developed by steel.
• Hence it is necessary to prestretch the steel wrt concrete.
• By prestrecthing and anchoring the steel against the concrete, we produce
desirable stresses and strains in both materials: compressive stresses and
strains in concrete and tensile stresses and strains in steel.
• This permits safe and economical utilization of two materials which cannot be
done in RC.

Third Concept-
Prestressing to achieve load balancing
• Overall design of a prestressed concrete structure, the effect of
prestressing is viewed as the balancing of gravity loads so that members
under bending such as slabs, beams, and girders will not be subjected to
flexural stresses under a given loading condition.
• This enables the transformation of a flexural member into a member
under direct stress and thus greatly simplifies both the design and analysis
of complicated structures.

Forms of Prestressing Steel
Wires
•A prestressing wire is a single unit made of steel. The
nominal diameters of the wires are 2.5, 3.0, 4.0, 5.0,
7.0 and 8.0 mm. The different types of wires are as
follows.
Plain wire: No indentations on the surface.
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. The different types of strands are
as follows.
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. The following figure shows the
cross section of a typical tendon. The strands are
placed in a duct which may be filled with grout after
the post-tensioning operation is completed

Cables
•A group of tendons form a prestressing cable. The
cables are used in bridges.
Bars
•A tendon can be made up of a single steel bar. The
diameter of a bar is much larger
•than that of a wire. Bars are available in the following
sizes: 10, 12, 16, 20, 22, 25, 28 and 32 mm.

Nature of Concrete-Steel Interface
Bonded tendon
•When there is adequate bond between the prestressing
tendon and concrete, it is called a bonded tendon. Pre-
tensioned and grouted post-tensioned tendons are
bonded tendons.
Unbonded tendon
•When there is no bond between the prestressing
tendon and concrete, it is called unbonded tendon.
When grout is not applied after post-tensioning, the
tendon is an unbonded tendon.

Tensioning Devices
• The various types devices used for tensioning steel are grouped under four
principal categories, viz.
• 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 while setting is restrained, it induces tensile
forces in tendons and compressive stresses in concrete

Stages of loading…

Types of prestressing….
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 of a box girder

Internal prestressing of a box girder

Pre-tensioned electric poles

Methods of pre-tensioning:
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.
Methods of post-tensioning:
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.

In pre-tensioning, the tendons are tensioned even
before casting the concrete
One end of tendon is secured to abutment. The
other end is pulled with jacks

In post tensioning, the beam is cast first leaving
ducts for placing the tendons
Depending upon forces, there may be number of
ducts

In post tensioning, not a solid beam but a series of
blocks
Cables are inserted and will be prestressed
02/06/18 SPK-PSG College of Technology 38Post Tensioning in Blocks

End Block
Whatever may be the shape of beam, the end block
is a rectangular section. The entire prestressing will
be transferred by the end block

Systems of prestressing
It is the process of tensioning of tendons. Secures
firmly to concrete till the lift of member. Many
systems are in practice.
i. Freyssinet system
ii. Magnel Blaton system
iii.Gifford Udall system

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

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

• Gifford Udall System

Post-tensioning of a box girder

Linearly prestressed railway sleepers

Circularly prestressed containment structure

Advantages…..
 Factory products are possible.
 Long span structure are possible so that saving of wt is significant & thus it
become economical.
 Pre-stressed member are tested before use.
 Dead load are get counter balanced by eccentric pre-stressing
 It has high ability to resist the impact.
 It has high fatigue resistance.
 It has high live load carrying capacity.
 It free from cracks from service loads and enable entire section to take
part in resisting moments.
 Member are free from the tensile stresses.

Limitations…..
 Required skilled builders & experienced engineers.
 Initial equipment cost is very high.
 Availability of experienced engineers is less.
 Required complicated formwork.
 It requires high strength concrete & steel.
 Pre-stressed concrete is less fiber resistant.

CODAL PROVISIONS-
IS 1343-2012

References
• Prestressed concrete-K.U.Muthu, Azmi Ibrahim,
Maganti Janardhana and M.Vijayanad (Based on IS
1343-2012)
• Design of prestressed concrete structures- T.Y.Lin
and NED.H.Burns.
• Fundamentals of Prestressed Concrete –N.C.Sinha and
S.K.Roy
• Prestressed concrete –N.Rajagopalan
• Prestressed Concrete- N.Krishna Raju
• Reinforced concrete –Limit State Design-Ashok K Jain
• IS 1343-2012-Prestressed Concrete Code of Practice

Thanks for listening-
All the best

Introduction to prestressed concrete structures

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