The document provides information about prestressed concrete design. It discusses various topics related to prestress loss including immediate losses like elastic shortening, anchorage slip, and friction; and time-dependent losses like creep, shrinkage, and relaxation of steel. It describes the different types of prestressing systems and losses associated with pre-tensioning and post-tensioning. Methods to estimate total prestress losses including lump sum approximations and refined estimations are also presented.

1. AHSANULLAH UNIVERSITY OF
SCIENCE & TECHNOLOGY
Course No : CE-416.
Course Title : Prestress Concrete Design
Sessional.
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2. 2

3. Welcome To Our Presentation
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4. Group Members
10.01.03.062
10.01.03.063
10.01.03.065
10.01.03.066
10.01.03.069
10.01.03.072
10.01.03.075
10.01.03.076
- Shahriar Mujthahid Hossain.
- H.M.Suruzzaman.
- Sabbir Bin Delwar.
- Md. Imran Islam.
- Tasfia Ahmed.
- Mohammad Shakib Rahman.
- Munshi Md. Rasel.
- Md.Moinul Islam.
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5. What is Pre-stressed Concrete?
–
–
Internal stresses are
induced to counteract
external stresses.
In 1904, Freyssinet
attempted to introduce
permanent acting force in
conc. to resist elastic
forces under loads and
was named
“Pre stressing”.
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6. Introduction
• In prestressed concrete applications, most important
variable is the prestress.
• Prestress does not remain constant with time.
• Even during prestressing of tendons, and transfer of
prestress, there is a drop of prestress from the
initially applied stress.
• Reduction of prestress is nothing but the loss in
prestress.
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7. Prestress Loss
• loss in prestress is the difference between initial prestress and
the effective prestress.
• Loss of prestress affects
– the strength of member and
– member’s serviceability [ Stresses in Concrete, Cracking, Camber and Deflection ]
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8. Types
Loss of prestress is classified into two types:
1. Immediate Losses
 immediate
losses
occur
during
prestressing
of
tendons, and transfer of prestress to concrete member.
2. Time Dependent Losses
 Time dependent losses occur during service life of
structure.
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9. Types According To Time
Prestress Losses
Time
Dependent
Immediate
Elastic
Shortening
Friction
Anchorage
Slip
Creep
Shrinkage
Relaxation
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10. Types According To Material
Prestress Losses
Concrete
Elastic
Shortening
Creep
Steel
Shrinkage
Friction
Anchorage
Slip
Relaxation
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11. Types of Prestressing Systems
I . Pre-tensioning:
 In Pre-tension, the tendons are tensioned before the
concrete is place. After the concrete hardened, the
tension force is released.
II . Post tensioning:
 In Post tension, the tendons are tensioned after the
concrete has hardened.
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12. Example of Pre-tensioning
Fig : Pre-tensioned electric pole
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13. Example of Post-tensioning
Fig : Post-tensioning of a box girder
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14. Losses in Various Prestressing Systems
Type of Loss
Pre-tensioning
Post-tensioning
i.
No, if all the cables are
simultaneously tensioned.
If the wires are tensioned
in stages loss will exist.
1. Elastic Shortening
Yes
2. Anchorage Slip
3. Friction Loss
No
No
Yes
Yes
4. Creep and Shrinkage
of Concrete
Yes
Yes
5. Relaxation of Steel
Yes
Yes
ii.
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15. Immediate Loss
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16. Elastic Shortening
• It is the shorten of concrete member, when the prestress is transferred
to concrete, the member shortens and the prestressing steel also
shortens in it. Hence there is a loss of prestress.
Original length of member at transfer of prestress
Pi
Length after elastic shortening
P0
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17. Elastic Shortening at Pre-tensioned Members
 When the tendons are cut and the prestressing
force is transferred to the member, concrete
undergoes immediate shortening due to prestress.
 Tendon also shortens by same amount, which leads
to the loss of prestress.
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18. Elastic Shortening at Pre-tensioned Members
Prestressing bed
Pre-tensioning of a member
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19. Elastic Shortening at 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.
 For more than one tendon, if the tendons are
stretched sequentially, there is loss in a tendon
during subsequent stretching of the other tendons.
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20. Elastic Shortening at Post-tensioned Members
Duct
Anchorage
jack
Casting bed
Post-tensioning of a member
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21. Anchorage Slip
• In most Post-tensioning systems when the prestress force is
transferred from the jack to the anchoring ends, the wedges
slip over a small distance.
• Loss of prestress is due to the consequent reduction in the
length of the tendon.
• Amount of slip depends on type of anchorage system.
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22. Force variation diagrams for various
stages
a) The initial tension at the right
end is high to compensate for the
anchorage slip. It corresponds to
about initial prestress. The force
variation diagram (FVD) is linear.
b) After the anchorage slip, the
FVD drops near the right end till
the length lset.
Note : Effect of anchorage slip is present
up to a certain length, called the setting
length lset.
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23. Force variation diagrams for various
stages
c) The initial tension at the left end
also corresponds to about initial
prestress. The FVD is linear up to
the centre line of the beam.
d) After the anchorage slip, the
FVD drops near the left end till the
length lset. It is observed that after
two stages, the variation of the
prestressing force over the length
of the beam is less than after the
first stage.
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24. Typical values of anchorage slip
Anchorage System
Freyssinet system
12 - 5mm Φ strands
12 - 8mm Φ strands
Magnel system
Dywidag system
Anchorage Slip (Δs)
4 mm
6 mm
8 mm
1 mm
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25. Frictional Loss
•
The friction generated at the interface of
concrete and steel during the stretching
of a curved tendon in a post-tensioned
member.
• The friction in the jacking anchoring system is
generally small.
• More serious frictional loss occurs between the
tendon and its surrounding material.
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26. Frictional loss occurs only in Posttensioned Members
• The loss due to friction does not occur in pretensioned members because there is no concrete
during the stretching of the tendons.
•
Friction is generated due to curvature of
tendon, and vertical component of the prestressing
force.
A typical continuous post-tensioned member
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27. Frictional Loss
Frictional Loss is the summation of
– Friction Loss Due to length Effect.
– Friction Loss Due to Curvature Effect.
• Length Effect: If the profile of cable is linear, the loss
will be due to straightening or stretching of the
cables.
• Curvature 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.
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28. Length & Curvature Effects
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29. Methods available to “Reduce” the
frictional losses
1. Cables should pass through metal tubes.
2. The bends should be through as small an
angle as possible.
3. Radius of curvature for bends should be
large.
4. Prestressing the wire from both ends.
5. Over-tensioning the wires.
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30. Time Dependent Losses
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31. Creep of Concrete
• The Continuous deformation of concrete with time under
sustained load.
 Factors affecting creep of concrete
• Age
• Applied Stress level
• Density of concrete
• Cement Content in concrete
• Water-Cement Ratio
• Relative Humidity and
• Temperature
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32. Condition for calculating the loss of
prestress due to creep.
• Creep is due to sustained (permanent) loads only. Temporary
loads are not considered in calculation of creep.
• Since the prestress may vary along the length of the
member, an average value of the prestress is considered.
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33. Shrinkage of Concrete
• Shrinkage of
defined
as
concrete
the
is
contraction due to loss of
moisture.
• Due to the shrinkage of
concrete, the prestress in the
tendon is reduced with time.
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34. Shrinkage of Concrete
• For pre-tensioned members, transfer
commonly takes place after 24 hours after
casting and nearly all shrinkage takes place
after that.
• For post-tensioned members, stressing may
takes place after one day or much later, thus a
large percentage of shrinkage may already
taken place by them.
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35. Relaxation
 Relaxation is the reduction in stress with time at
constant strain.
– decrease in the stress is due to the fact that
some of the initial elastic strain is transformed in
to inelastic strain under constant strain.
– Percentage of relaxation varies from 1 to 5%.
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36. Factors effecting Relaxation :
• Time
• Initial stress
• Temperature and
• Type of steel.
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37. Method Available to Reduce The Loss
due to Relaxation
• Choice of proper steel helps to reduce this
loss.
• Prestressed wires have lesser creep.
• Galvanised wires also have no creep.
• overstressing steel about 10% above its initial
stress and then releasing it to the initial stress
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38. Calculation of Total Amount of Loss
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39. Loss of Prestress=Initial StressEffective Stress
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40. Initial Prestress
• Deducting the loss due to anchorage take-up and
friction, initial prestress is obtained.
• If prestress is measured at the time of pulling the
wire, the stress is termed as the jacking stress.
• if jacketing stress is treated as the initial
stress, effective stress is jacketing stress minus all
losses.
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41. Effective Prestress
• Initial Prestress in steel minus the losses is
known as the effective or design prestress.
Effective prestress=Initial prestress-Losses
Note: For Pre-Tension system , Pretension Losses are used
instead of losses.
For Post-Tension system , Post-Tension Losses are used
instead of losses.
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42. Total Amount Of Losses According To
Tensioning System
• Total pretension losses=Loss due to creep +
Elastic shortening + Shrinkage +
Steel Relaxation.
• Total post-Tension Loss=Loss due to creep +
Elastic shortening + Shrinkage +
Steel Relaxation +Anchorage slip +
Friction.
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43. Total Losses
• It is difficult to generalize the amount of loss
of prestress, because it is dependent on so
many Factors :
The properties of concrete & steel.
Curing & moisture condition.
Magnitude & time of application of prestress.
Process of prestress.
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44. Method of Loss Estimation
• There are two methods that can be used to
estimate losses in prestressed concrete:
(a)lump sum approximations;
(b)refined estimations.
One should keep in mind that all estimates
for prestress loss are just that – ESTIMATIONS .
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45. Lump Sum Estimation For Prestress
Loss
• First introduce by the ACI-ASCE Committee 423 in
1958.
Table : AASHTO Lump Sum Losses.
Total Loss
Types of
prestressing steel
Pretensioning
strand
Posttensioning wire
or strand
Bars
f’c=28 MPa
f’c=35 MPa
310 MPa
220 MPa
230 Mpa
150 MPa
160 Mpa
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46. AASHTO-LRFD Specifications
For Lump-Sum estimates following conditions should met :
1. Members that are post-tensioned must be non-segmental
members with spans less than 160 feet and concrete
stressed an age of 10-30 days.
2. Members that are pretensioned must be stressed at an age
where the concrete strength is not less than 3,500-psi.
3. Members must be made from normal weight concrete.
4. Members cannot be steam-cured, nor moist-cured.
5. The prestressing steel must be normal or low-relaxation.
6. There must be Average exposure conditions at the site.
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47. Thumb rule of Losses
• For average steel and concrete properties ,the tabulated
percentages may be taken as representative of the average
losses.
Pretensioning ,%
Posttensioning. %
Elastic shortening &
bending of concrete
4
1
Creep of concrete
6
5
Shrinkage of concrete
7
6
Steel relaxation
8
8
25
20
Total Loss
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48. Comparison Between RCC & Prestress
Concrete
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49. Thank You
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