Loss of pre stress in pre-stressed concrete

1. 1

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
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
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
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
LOSSESS IN PRESTRESS

7. 7

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
Elastic Shortening
Pre-tensioned Members

10. 10
Elastic Shortening
Post-tensioned Members

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. Friction Losses
12
Overcoming friction:
 Over‐tensioning (limited)
 Stressing from both ends

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. 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. Anchorage Slip
15
ANCHORAG
E DEVICES

16. Anchorage Slip
16
ANCHORAG
E DEVICES

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. Concrete Shrinkage
18
Moisture
L
L
’
L
LL
L
L
sh
'





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. 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. Concrete Creep
21
L
L1’
Shrinkage Specimen
L
Creep Specimen
L2’
P
ε1
ε2

22. Concrete Creep
22
Specimen 2
Specimen 1
Cast
Concrete
EndCuring
(StartDrying)
Apply
Load
Strain
Time
Shrinkage
Elastic
Creep

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. 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. 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. 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. 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. Computation of Losses
28
Pre-tensioned Members

29. Computation of Losses
29
Post-tensioned Members

30. Computation of Losses
30
Post-tensioned Members

31. Computation of Losses
31
Elastic Shortening – Pre-tensioned Member

32. Computation of Losses
32
Elastic Shortening – Pre-tensioned Member

33. Computation of Losses
33
Elastic Shortening – Post-tensioned Member

34. Computation of Losses
34
Steel Stress Relaxation R

35. Computation of Losses
35
Steel Stress Relaxation R

36. Computation of Losses
36
Steel Stress Relaxation R

37. Computation of Losses
37
Steel Stress Relaxation R

38. Computation of Losses
38
Creep Losses

39. Computation
of Losses
39
Creep Losses

40. Computation of Losses
40
Creep Losses

41. Computation of Losses
41
Creep Losses

42. Computation of Losses
42
Creep Losses

43. Computation of Losses
43
Shrinkage Losses

44. Computation of Losses
44
Shrinkage Losses

45. Computation of Losses
45
Shrinkage Losses

46. Computation of Losses
46
Friction Losses F
A) Curvature Effect

47. Computation of Losses
47
Friction Losses F
A) Curvature Effect

48. Computation of Losses
48
Friction Losses F
B) Wobble Effect

49. Computation of Losses
49
Friction Losses F
B) Wobble Effect

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. • “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. THANK YOU