Prestressed concrete ,post tensioning ,pre tensioning, where normal concrete can not be used and need of more strength is required this type of concrete are used. Metal bars are replaced by the tendoms which are generally used to create tension in concrete. So because of that beam bends in upward direction and when load is applied it come in normal conditon.

1. Prestressed Concrete
Prepared by :
Dhrumil Pandya
181160720011
M.E. (Structural Engineering)
Guided By:
Prof. Kunal Shukla
Assistant Professor
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2. Prestress Concrete :
• Prestressed concrete is generally used when there is a need of
overcoming concrete’s natural weakness in tension.
• In 1904, Freyssinet attempted to introduce permanent acting forces
in concrete to resist elastic forces under loads and was named “Pre
Stressing”.
• It can be used to produce beams, floors or bridges with a longer span
when there is difficult to construct it with ordinary reinforced
concrete.
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3. Prestress Concrete :
• The application of permanent compressive stress to a material like
concrete , which is strong in compression but weak in tension ,
increases the apparent tensile strength of that material , because the
subsequent application of tensile stress must first nullify the
compressive stress
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4. Difference between ordinary and Prestressed concrete
• A prestressed concrete structure is different from a conventional
reinforced concrete structure due to the application of an initial load
on the structure prior to its use.
• The initial load or ‘prestress’ is applied to enable the structure to
counteract the stresses arising during its service period.
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5. Introduction
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Image source : https://civilengineersforum.com/
Fig 1

6. Terminology
Tendon: A stretched element used in a concrete member of structure to
impart prestress to the concrete.
• Generally high tensile steel wires , bars cables or strands are used as
tendons.
Anchorage: A device generally used to enable the tenfdon to impart and
maintain prestress in the concrete
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7. Terminology
Axial prestressing: Members in which the entire cross section of
concrete has a uniform compressive prestress.
• In this type of prestressing , the centroid of the tendons coincides with
that that of the concrete section
Eccentric prestressing: A section at which the tendons are eccentric to
the centroid resulting in a triangular or trapezoidal compressive stress
distribution.
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8. Terminology
• Relaxation in steel : Decrease of stress in steel at constant strain.
• Degre of Prestressing: A measure of the magnitude of the prestressing
force related to the resultant stress occurring in the structural member
at the working load.
• Shrinkage of concrete: Contraction of concrete on drying
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9. Uniaxial, biaxial and triaxial prestressing :
The term refer to the cases where concrete is prestressed (i) in only one
direction (ii) in two mutually perpendicular directions and (iii) in three
mutually perpendicular direction.
Cracking load:
The load on the structural element corresponding to the first visible crack.
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Terminology

10. Types of Prestressing Used in Concrete
Externally or internally prestressing :
• Externally prestressing is a term applied to prestressed structures
where stressing is applied externally. Similarly, internally prestressing
is used to describe the method of prestressing internally.
• Most of the prestressed concrete structures are internally prestressed.
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11. Types of Prestressing Used in Concrete
Circular and linear prestressing :
• Circular prestressing refers to prestressing in circular or round
structures like tanks, pipes etc. Where the prestressing reinforcement
wound around in circle.
• Prestressing all other members like beams slaps, etc. termed as liner
prestressing.
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12. Types of Prestressing Used in Concrete
Pre tensioning and Post tensioning:
• The term pre tensioning is used to describe any method of prestressing
in which the reinforcement are stretched or tensioned before the
concrete is placed. The steel rods are temporarily anchored against
some posts when tensioned and the prestress transferred to the concrete
after it has set.
• In contrast to pre tensioning, post tensioning is a method of
prestressing in which the reinforcement is tensioned after the concrete
has hardened.
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13. • Partial or fully Prestressing
• when a member is designed so that under the working load there is on
tensile stress in it then the concrete is said to be fully prestressed.
• If some tensile stresses will be produced in the member under working
load then it is termed partially prestressed.
Types of Prestressing Used in Concrete

14. Pre-Tensioning in Prestressed Concrete
• Pre-tensioning is accomplished by stressing wires or strands, called
tendons, to predetermined amount by stretching them between two
anchorages prior to placing concrete as shown in fig.1. The concrete is
then placed and tendons become bonded to concrete throughout their
length.
• After concrete has hardened, the tendons are released by cutting them
at the anchorages. The tendons tend to regain their original length by
shortening and in this process transfer through bond a compressive
stress to the concrete.
• The tendons are usually stressed by the use of hydraulic jacks
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15. 15MEFGI,RAJKOT
Pre-Tensioning in Prestressed Concrete
Image source : https://theconstructor.org
Fig 2
Fig 3

16. • The alternative to pre-tensioning is post-tensioning.
• In a post-tensioned beam, the tendons are stressed and each end is
anchored to the concrete section after the concrete has been cast and
has attained sufficient strength to safely withstand the prestressing
force.
• In post-tensioning it is necessary to use some types of device to attach
or anchor the ends of the tendons to the concrete section. These
devices are usually referred to as end anchorages.
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Post-Tensioning in Prestressed Concrete

17. • In post-tensioning method, tendons are coated with grease or a
bituminous material to prevent them from becoming bonded to
concrete
• After the tendon has been stressed, the void between the tendon and
the sheath is filled with grout. Thus the tendons become bonded to
concrete and corrosion of steel is prevented.
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Post-Tensioning in Prestressed Concrete

18. 18MEFGI,RAJKOT
Post-Tensioning in Prestressed Concrete
Image source : https://www.concretenetwork.com/post-tension/
Fig 4

19. • Post-tension prestressing can be done at site.
• This procedure may become necessary or desirable in certain cases.
For heavy loads and large spans in buildings or bridges, it may be very
difficult to transport a member from precasting plant to a job site.
• On the other hand, pre-tensioning can be used in precast as well as in
cast-in-place construction.
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Post-Tensioning in Prestressed Concrete

20. Losses in prestressed concrete
• The initial prestress in concrete undergoes a gradual reduction with time
from the stage of transfer due to various causes.
• The different types of losses encountered in the pre-tensioning and post-
tensioning system are complied in table.
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21. 21MEFGI,RAJKOT
Pre-Tensioning Post-Tensioning
Elastic deformation of concrete No loss due to elastic deformation of
concrete
Relaxation of stress in steel Relaxation of stress in steel
Shrinkage of concrete Shrinkage of concrete
Creep of concrete Creep of concrete
Friction
Anchorage slip
Losses in prestressed concrete

22. Because concrete shortens when the prestressing force is applied to it,
the tendon attached to it also shortens, causing stress loss.
Loss of stress in steel = αe fc
αe = Es / Ec = Modular Ratio
Es = Modulus of elasticity of steel
Ec = Modulus of elasticity of concrete
fc = prestress in concrete at the level of steel
Strain in concrete at the level of steel = fc / Ec
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Losses due to Elastic deformation of concrete

23. Shrinkage : concrete shrinks over time due to loss of water, leading to stress loss on
attached tendons.
Loss of stress = εcs Es
εcs = 300 x 10-6 for pre-tensioning
εcs = [ 200 x 10-6 / log10(t + 2) ] for post-tensioning
εcs =Total residual shrinkage strain in concrete
Es = Modulus of elasticity of steel
t = Age of concrete at transfer in days
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Losses due to Shrinkage of Concrete

24. • Creep of Concrete : It is defined as deformation of structure under
sustained load. Basically as long term pressure or stress
on concrete can make it change shape.
Ultimate creep strain method
Loss of stress = εcc fc Es
εcc =Ultimate creep strain for a sustained unit stress
Es = Modulus of elasticity of steel
fc = prestress in concrete at the level of steel
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Losses due to creep of Concrete

25. Creep coefficient method
Loss of stress = εc Es = φ ( fc / Ec) Es = φ fc αe
εc =Creep strain
Φ = creep coefficient
Es = Modulus of elasticity of steel
fc = prestress in concrete at the level of steel
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Losses due to creep of Concrete
Age at Loading Creep Coefficient
7 days 2.2
28 days 1.6
1 year 1.1

26. Application of prestressed concrete
• Bridges
• Slabs in buildings
• Water Tank
• Concrete Pile
• Thin Shell Structures
• Offshore Platform
• Nuclear Power Plant
• Repair and Rehabilitations 26MEFGI,RAJKOT

27. Advantages of prestressed concrete
• Take full advantages of high strength concrete and
• High strength steel
• Need less materials
• Smaller and lighter structure
• No cracks
• Use the entire section to resist the load
• Better corrosion resistance
• Very effective for deflection control
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28. Disadvantages of prestressed concrete
• The major problem with prestressed concrete is that it needs
specialized 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.
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29. Conclusion
• Thus, pre-stressed concrete increases the quality, strength, span of the
structure.
• Since the cost of it is near about to regular concrete which is used
widely on recent days.
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30. References
• N Krishna Raju- “Prestressed concrete”
• Edward G Nawy- “Prestressed concrete”
• https://www.scribd.com/document/318933089/Terms-in-Pre-Stressed-Concrete
• https://civilengineersforum.com/what-is-prestressed-concrete
• https://www.designingbuildings.co.uk/wiki/Prestressed_concrete
• https://www.concretenetwork.com/post-tension/
• https://theconstructor.org/concrete/prestressed-concrete-pre-and-post-
tensioning
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31. THANK YOU
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32. Introduction
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