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Prestressed concrete!
Prestressed
concrete is a
concrete members
with associated
cables to encounter
the bending
moment ...
3
LOSSESS IN PRESTRESS
The prestressing force must be
available all the time if the steel is to be
prevented from contract...
4
LOSSESS IN PRESTRESS
In a prestressed concrete beam, the loss is due to
the following:
Elastic shortening
Frictional l...
5
LOSSESS IN PRESTRESS
Types of Losses of Prestress
S.No. TYPES OF LOSSES PRE-TENSIONING POST-TENSIONING
1
Elastic deforma...
6
LOSSESS IN PRESTRESS
7
8
Elastic Shortening
Concrete is subjected to a compressive
force, which results in an instantaneous
shortening of the mem...
9
Elastic Shortening
Pre-tensioned Members
10
Elastic Shortening
Post-tensioned Members
Jacking of PT Strands
11
Many things occur simultaneously
Jacking > Friction > Elastic Shortening
The jack bears against t...
Friction Losses
12
Overcoming friction:
 Over‐tensioning (limited)
 Stressing from both ends
Friction Losses
13
Friction Losses are Function of:
Curvature friction coefficient
Angular change over length of strand
...
Anchorage Slip
14
In a post-tensioned member, when the
prestress is transferred to the concrete
• Wedges slip through a li...
Anchorage Slip
15
ANCHORAG
E DEVICES
Anchorage Slip
16
ANCHORAG
E DEVICES
Concrete Shrinkage
17
Shrinkage of concrete is defined as
the contraction due to loss of
moisture.
Due to the shrinkage of...
Concrete Shrinkage
18
Moisture
L
L
’
L
LL
L
L
sh
'




Concrete Creep
19
Creep of concrete is defined as the
increase in deformation with time
under constant load.
Due to the cr...
Concrete Creep
20
Applicable considerations calculating the loss of
prestress due to creep:
• Creep is due to the sustaine...
Concrete Creep
21
L
L1’
Shrinkage Specimen
L
Creep Specimen
L2’
P
ε1
ε2
Concrete Creep
22
Specimen 2
Specimen 1
Cast
Concrete
EndCuring
(StartDrying)
Apply
Load
Strain
Time
Shrinkage
Elastic
Cre...
Concrete Creep
23
Time, t
ConcreteStrain
(SumofElasticandCreepResponse)
Instantaneous
application of stress, fc
Elastic St...
Relaxation of Steel
24
 Steel Relaxation is defined as the
decrease in stress with time under
constant strain.
 Prestres...
Total Time-dependent Loss
25
Losses of prestress due to creep and
shrinkage of concrete and the
relaxation of the steel ar...
Total Time-dependent Loss
26
To consider the inter-relationship of
the cause and effect, the calculation
can be done for d...
Computation of Losses
27
Step Beginning End
1
Pre-tension: Anchorage of
steel
Post-tension: End of curing
Age of prestress...
Computation of Losses
28
Pre-tensioned Members
Computation of Losses
29
Post-tensioned Members
Computation of Losses
30
Post-tensioned Members
Computation of Losses
31
Elastic Shortening – Pre-tensioned Member
Computation of Losses
32
Elastic Shortening – Pre-tensioned Member
Computation of Losses
33
Elastic Shortening – Post-tensioned Member
Computation of Losses
34
Steel Stress Relaxation R
Computation of Losses
35
Steel Stress Relaxation R
Computation of Losses
36
Steel Stress Relaxation R
Computation of Losses
37
Steel Stress Relaxation R
Computation of Losses
38
Creep Losses
Computation
of Losses
39
Creep Losses
Computation of Losses
40
Creep Losses
Computation of Losses
41
Creep Losses
Computation of Losses
42
Creep Losses
Computation of Losses
43
Shrinkage Losses
Computation of Losses
44
Shrinkage Losses
Computation of Losses
45
Shrinkage Losses
Computation of Losses
46
Friction Losses F
A) Curvature Effect
Computation of Losses
47
Friction Losses F
A) Curvature Effect
Computation of Losses
48
Friction Losses F
B) Wobble Effect
Computation of Losses
49
Friction Losses F
B) Wobble Effect
• Jacking Force Pi applied to
tendon is measured by the
pressure gauge
mounted on the hydraulic jack
and this force cannot...
• “Loss of Prestress, Emphasis on
Items Specific to Post‐Tensioned
Systems” Developed by the PTI
EDC-130 Education Committ...
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Loss of pre stress in pre-stressed concrete

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Loss of pre stress in pre-stressed concrete

  1. 1. 1
  2. 2. 2 Prestressed concrete! Prestressed concrete is a concrete members with associated cables to encounter the bending moment on the beam span. Prestressed concrete has many applications in
  3. 3. 3 LOSSESS IN PRESTRESS The prestressing force must be available all the time if the steel is to be prevented from contracting. This reduction in the prestressing force is called loss in prestress Contraction of steel wire occurs due to several causes
  4. 4. 4 LOSSESS IN PRESTRESS In a prestressed concrete beam, the loss is due to the following: Elastic shortening Frictional loss Anchorage Slip Shrinkage of concrete Creep of concrete Relaxation of steel
  5. 5. 5 LOSSESS IN PRESTRESS Types of Losses of Prestress S.No. TYPES OF LOSSES PRE-TENSIONING POST-TENSIONING 1 Elastic deformation of concrete Yes No loss due to elastic deformation if all the wires are simultaneously tensioned. If the wires are successively tensioned, there will be loss of prestress due to elastic deformation of concrete. 2 Friction No Yes 3 Anchorage Slip No Yes 4 Concrete Shrinkage Yes Yes 5 Concrete Creep Yes Yes 6 Relaxation of stress in steel Yes Yes
  6. 6. 6 LOSSESS IN PRESTRESS
  7. 7. 7
  8. 8. 8 Elastic Shortening Concrete is subjected to a compressive force, which results in an instantaneous shortening of the member Since the tendons are bonded to the concrete, they will lose an equal amount of deformation, meaning a reduction of induced stress
  9. 9. 9 Elastic Shortening Pre-tensioned Members
  10. 10. 10 Elastic Shortening Post-tensioned Members
  11. 11. Jacking of PT Strands 11 Many things occur simultaneously Jacking > Friction > Elastic Shortening The jack bears against the concrete Concrete is compressed gradually as the strand is tensioned
  12. 12. Friction Losses 12 Overcoming friction:  Over‐tensioning (limited)  Stressing from both ends
  13. 13. Friction Losses 13 Friction Losses are Function of: Curvature friction coefficient Angular change over length of strand Wobble friction coefficient Length from jack to point of interest
  14. 14. Anchorage Slip 14 In a post-tensioned member, when the prestress is transferred to the concrete • Wedges slip through a little distance before they get properly seated in the conical space • Also the anchorage block moves before it settles on the concrete
  15. 15. Anchorage Slip 15 ANCHORAG E DEVICES
  16. 16. Anchorage Slip 16 ANCHORAG E DEVICES
  17. 17. Concrete Shrinkage 17 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.
  18. 18. Concrete Shrinkage 18 Moisture L L ’ L LL L L sh '    
  19. 19. Concrete Creep 19 Creep of concrete is defined as the increase in deformation with time under constant load. Due to the creep of concrete, the prestress in the tendon is reduced with time.
  20. 20. Concrete Creep 20 Applicable considerations calculating the loss of prestress due to creep: • Creep is due to the sustained loads only Temporary loads are not considered in the calculation of creep. • Average Value of the prestress can be considered, since the prestress may vary along the length of the member. • The prestress changes due to creep is related to the instantaneous prestress, to consider this interaction, the calculation of creep can be iterated
  21. 21. Concrete Creep 21 L L1’ Shrinkage Specimen L Creep Specimen L2’ P ε1 ε2
  22. 22. Concrete Creep 22 Specimen 2 Specimen 1 Cast Concrete EndCuring (StartDrying) Apply Load Strain Time Shrinkage Elastic Creep
  23. 23. Concrete Creep 23 Time, t ConcreteStrain (SumofElasticandCreepResponse) Instantaneous application of stress, fc Elastic Strain c c el E f  Creep Strain   c c icr E f tt,  ti Linetype Key: Model Baseline Effect of Decreasing f’c Effect of Decreasing H Effect of Decreasing V/S Effect of the same applied stress, fc, at a later time Total Strain   i c c total tt E f ,1   Creep strain is calculated by a creep coefficient, 𝜓(𝑡, 𝑡𝑖), that expresses creep strain as a function of elastic
  24. 24. Relaxation of Steel 24  Steel Relaxation is defined as the decrease in stress with time under constant strain.  Prestress in the tendon is reduced with time due to the relaxation of steel  Relaxation depends on: • Type of steel
  25. 25. Total Time-dependent Loss 25 Losses of prestress due to creep and shrinkage of concrete and the relaxation of the steel are all time- dependent and interrelated to each other. If the losses are calculated separately and added, the calculated total time- dependent loss is over-estimated.
  26. 26. Total Time-dependent Loss 26 To consider the inter-relationship of the cause and effect, the calculation can be done for discrete time steps The results at the end of each time step are used for >> next time step
  27. 27. Computation of Losses 27 Step Beginning End 1 Pre-tension: Anchorage of steel Post-tension: End of curing Age of prestressing 2 End of Step 1 30 days after prestressing or when subjected to superimposed load 3 End of Step 2 1 year of service 4 End of Step 3 End of service life PCI step-by-step procedure, a minimum of four time steps are considered in the service life of a prestressed member
  28. 28. Computation of Losses 28 Pre-tensioned Members
  29. 29. Computation of Losses 29 Post-tensioned Members
  30. 30. Computation of Losses 30 Post-tensioned Members
  31. 31. Computation of Losses 31 Elastic Shortening – Pre-tensioned Member
  32. 32. Computation of Losses 32 Elastic Shortening – Pre-tensioned Member
  33. 33. Computation of Losses 33 Elastic Shortening – Post-tensioned Member
  34. 34. Computation of Losses 34 Steel Stress Relaxation R
  35. 35. Computation of Losses 35 Steel Stress Relaxation R
  36. 36. Computation of Losses 36 Steel Stress Relaxation R
  37. 37. Computation of Losses 37 Steel Stress Relaxation R
  38. 38. Computation of Losses 38 Creep Losses
  39. 39. Computation of Losses 39 Creep Losses
  40. 40. Computation of Losses 40 Creep Losses
  41. 41. Computation of Losses 41 Creep Losses
  42. 42. Computation of Losses 42 Creep Losses
  43. 43. Computation of Losses 43 Shrinkage Losses
  44. 44. Computation of Losses 44 Shrinkage Losses
  45. 45. Computation of Losses 45 Shrinkage Losses
  46. 46. Computation of Losses 46 Friction Losses F A) Curvature Effect
  47. 47. Computation of Losses 47 Friction Losses F A) Curvature Effect
  48. 48. Computation of Losses 48 Friction Losses F B) Wobble Effect
  49. 49. Computation of Losses 49 Friction Losses F B) Wobble Effect
  50. 50. • Jacking Force Pi applied to tendon is measured by the pressure gauge mounted on the hydraulic jack and this force cannot entirely be transmitted to the concrete because some losses of prestress occur during the process of stretching and anchoring the tendons. • These losses are estimated by calculations to be taken into conclusion
  51. 51. • “Loss of Prestress, Emphasis on Items Specific to Post‐Tensioned Systems” Developed by the PTI EDC-130 Education Committee Lead Author: Brian Swartz. • “Prestressed Concrete Structures” by Dr. Amlan K Sengupta and Prof. Devdas Menon • “Prestressed Concrete: A Fundamental Approach” by Edward G Nawy, 4th edition references
  52. 52. THANK YOU
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Loss of pre stress in pre-stressed concrete

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