PSC Design and construction.ppt

PSC Design and construction

1. PSC BASICS, DESIGN ASPECT, CONCRETE
BRIDGE CODE, CONSTRUCTION ASPECT
AvinashKumar
Prof Track-II

2. CONTENTS
I.BasicConceptofprestressing
II.BasicDesignconcept
III.LossesinPrestressedstructure
IV.ProvisionsofIRS:CBCandIS1343-2012
V.ConstructionAspect

3. What is Pre Stressing ?
It is application of a predetermined force to introduce stresses of suitable
magnitude and distribution so that the stresses resulting from External
Loads can be counteracted to a desired degree.
PP

4. Force-fitting of metal bands on wooden barrels.
The metal bands induce a state of initial hoop compression, to counteract
the hoop tension caused by filling of liquid in the barrels.

5. Pre-tensioning the spokes in a bicycle wheel
The pre-tension of a spoke in a bicycle wheel is applied to such an extent
that there will always be a residual tension in the spoke.

6. •Concretehasestablisheditselfauniversalbuilding
materialbecauseofitshighcompressivestrengthand
capabilitytotakeanyform&shape,
But
•Ithaslowtensilestrength,whichisgenerally
compensatedbyreinforcementandtheresultant
compositemassisknownasRCC–Reinforcedcement
concrete.

7. .

8. RCC What’s bad about it
•Cracks are Inevitable
[Moment resisted by concrete in Compression zone above
Neutral axis and by reinforcement in tension (Cracked) zone]
•Crack widths proportional to strain of steel and therefore the
steel stresses to be limited to low value in service.
•Loss of stiffness due to cracking [Net effective concrete section]
•Large deflections [Reduced Stiffness]
•Unsuitable for large spans as Dead Load becomes very high

9. INITIAL EFFORTS OF ACHIEVING PSC FAILED
Mainly because of Shrinkage & Creep losses of concrete were not known
Case-IIf Mild Steel bar is Used,
Stress = 124 Mpa,
Elongation = Stress/Modulus of Elasticity
= 140/200000 = 0.0007
Loss due to Creep and Shrinkage = 0.0005
Stress after loss = Negligible
Case-II If HSS is used, Stress = 1200 Mpa
Elongation = 1200/200000 = 0.006
Loss due to Creep and Shrinkage = 0.0005
Stress after loss = More than 90%
Hence HSS became necessary for prestressed concrete.

10. Why High Strength Concrete Needed in PSC structures ?
I.In anchorage area, stresses are very high, so high grade
concrete is a necessity
II.Less shrinkage and Creep in Concrete
III.More Modulus of Elasticity, Less Elastic Shortening

11. Advantages of Prestressing
I.Improved durability –No corrosion as no cracks in concrete
II.Full section utilized hence effective saving in material
(RCC –only part of section uncracked carries compression).
III.Less Deflection (More stiffness as entire section uncracked and
effective)
IV.Better shear resistance (more shear for uncracked section and due to
vertical component of curved tendon)
V.Lighter & slender members due to high strength concrete and
steel.

12. Advantages Contd…..
VI.Economicalforhighspans
VII.Betterfatiguestrengthfordynamicallyloadedstructures(as
sectionremainsun-cracked)
VIII.Lowermaintenancecomparedwithsteelstructures
IX.Suitableforpre-castconstruction,hencefasterexecutionis
possible
X.Steelandconcretearetestedinservice(astheyaresubjectedto
considerableloadingconditionsbeforeapplicationofexternal
loads)

13. Disadvantages of Prestressed Concrete
I.The unit cost of high strength materials being used is higher.
II.Extra initial cost is incurred due to use of prestressing
equipment and its installation.
III.Extra labourcost for prestressing is also there.
IV.Prestressing is uneconomical for short spans and light loads.

14. Case-I[No Prestress]

15. Case-II[Concentric prestress, e=0]
Stress Distribution

16. Case-III[Eccentric Prestress]
When e = d/6
Stress Distribution

17. Type of Prestressing
Source
Based
Location
Based
Stressing
Sequence
Based
Direction
Based
Shape
Based
Mechanical Internal Pre Tensioned Uni-axial Linear
Hydraulic External Post Tensioned Biaxial Circular
Electrical Multi-axial
Chemical

18. Definitions: Stages of Loading
The analysis of prestressed members can be different for the
different stages of loadings.
1)Initial : It can be subdivided into two stages.
a) During tensioning of steel
b) At transfer of prestress to concrete.
2) Intermediate: This includes the loads during transportation and
erection of the prestressed members.
3) Final : It can be subdivided into two stages.
a) At service, during operation.
b) At ultimate Load

19. Definitions: 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.

20. Pre-tensioning or Post-tensioning
•Pre-tensioning:Thetensionisappliedtothetendons
beforecastingoftheconcrete.Thepre-compressionis
transmittedfromsteeltoconcretethroughbondoverthe
transmissionlengthneartheends.
•Post-tensioning:Thetensionisappliedtothetendons
(locatedinaduct)afterhardeningoftheconcrete.The
pre-compressionistransmittedfromsteeltoconcretebythe
anchoragedevice(attheendblocks).

21. Pre -Tensioning
•Theabutmentsarefixedattheendsofaprestressingbed.
•Thehigh-strengthsteeltendonsarepulledbetweentwoend
abutmentspriortothecastingofconcrete.Afterthatconcreteis
cast.
•Oncetheconcreteattainsthedesiredstrengthforprestressing,the
tendonsarecutloosefromtheabutments.
•Theprestressistransferredtotheconcretefromthetendons,due
tothebondbetweenthem.
•Duringthetransferofprestress,thememberundergoeselastic
shortening.
•Ifthetendonsarelocatedeccentrically,thememberislikelyto
bendanddeflect.

22. Stages of pre-tensioning

23. Various stages of the post-tensioning operation
Placement of reinforcement cage and sheathing duct
Casting of concrete.
Placement of the tendons.
Placement of the anchorage block and jack.
Applying tension to the tendons.
Seating of the wedges.
Cutting of the tendons and grouting.

24. Stages of post-tensioning

29. Internal Prestressing

30. External Prestressing
Cable outside
concrete

31. Circular Prestressing

32. Biaxial Prestressing

33. Loss of Prestressing
Immediate
Elastic Shortening
Anchorage slip
N
o
Creep
Y
e
s
Ye
s
Friction
Shrinkage
Relaxation
N
o
o
Yes
Time dependent

34. Loss due to Elastic Shortening
Pre-tensionedMembers
WhentendonsarecutandPSforceistransferredtothemember,theconcrete
undergoesimmediateshorteningduetothepre-stress.Thetendonalsoshortens
bythesameamount,whichleadstothelossofpre-stressduetoelastic
shortening.
Post-tensioned Members
If there is only One tendon, No Loss because applied pre-stress is recorded after
elastic shortening of member.
For more than one tendon, if tendons are stretched sequentially, there is loss
recorded in a tendon during the subsequent stretching of other tendons

35. Loss Due to Friction
Pre-tensionedMembers
NoLossbecausethereisnoconcretepresentduringstretchingofthetendons
Post-tensioned Members
•FrictiongeneratedatinterfaceofConcreteandsteelduringstretchingofacurvedtendon
leadstoadropinPrestressingforcealongthememberfromstretchingend.
•Thefrictionisgeneratedduetothecurvatureofthetendonandtheverticalcomponent
ofthePSforce.

36. Loss Due to Friction
•In addition to friction, stretching has to overcome wobble effect of tendon
which refers to change in position of the tendon along the duct.
•The losses due to friction and wobble are grouped together under friction

37. Loss Due to Friction
Pre-stressing force variation diagram after stretching
Approximately
;
Pressure at a distance x from stretching end;
LossinPSforceduetofrictionatadistancexfromstretchingend;ΔP=P
o(μα+kx)
x

39. Anchorage Slip
InmostPost-tensioningsystemswhenthetendonforceis
transferredfromthejacktotheanchoringends,thefriction
wedgesslipoverasmalldistance.
Anchorageblockalsomovesbeforeitsettlesonconcrete.
Lossofprestressisduetotheconsequentreductioninthe
lengthofthetendon.
Certainquantityofprestressisreleasedduetothisslipofwire
throughtheanchorages.–Amountofslipdependsontypeof
wedgeandstressinthewire.

40. Loss due to Creep
Time-dependent increase of deformation under sustained load.
Due to creep, the prestress in tendons decreases with time.
Factors affecting creep and shrinkage of concrete
•Constituent of Concrete
•Size of the member
•Environmental condition
•Total amount of water in concrete
•Cement Content in concrete
•Stress in Concrete
•Age of Loading
•Duration of Loading
For Creep only

41. Loss due to Creep

42. Shrinkage loss
Time-dependent strain measured in an unloaded and unrestrained specimen
at constant temperature.
Loss of prestress (Δfp ) due to shrinkage is as follows
Δfp = Ep x ε,sh
Ep is the modulus of prestressing steel.
Thefactorsresponsibleforcreepofconcretewillhaveinfluenceon
shrinkageofconcretealsoexcepttheloadingconditions
Totalshrinkagestraininplainconcrete,reinforcedconcreteandpre-
tensionedprestressedconcrete:0.0003.

43. Shrinkage Loss

44. Relaxation in steel
Change in stress with time in steel under a constant strain (elongation) or
a plastic flow
•Relaxation do not occur below 0.5fy
•Relaxation at 1000 hrat 30
0
C

45. Relaxation in steel ..contd
•1000 hr value obtained from manufacturer
•It is 4% for normal steel & 2.5 % for low relaxation steel
•This is used to obtain value at about 0.5 x 10
6
hr (~57 yrs)
•It is = 2.5 times the 1000 hr value for normal steel & 3.0 times for
low relaxation steel
•Theabovevalueisatinitialstresslevelof70%ofthe
characteristicstrength
•Itreducesto0at50%.Inbetweenvaluesareobtainedfrom
interpolation.

46. Average losses
Type of loss Pre-
tensioning
(%)
Post-
tensioning
(%)
Elastic Shortening of Concrete 4 1
Creep of concrete 6 5
Shrinkageof Concrete 7 6
Steel Relaxation 8 8
Total Loss 25 20
Loss due Friction and Anchorages have been considered to be
overcome by over tensioning

47. Some Relevant Clauses of CBC

48. Some Relevant Clauses of CBC

49. Some Relevant Clauses of CBC

50. Some Relevant Clauses of CBC

51. Some Relevant Clauses of CBC

52. Some Relevant Clauses of CBC

53. Some Relevant Clauses of CBC

55. Prestressing Steel

56. •SEVEN WIRE STRAND
•Outer wires enclose inner wire in a helix with a uniform pitch
of 12 to 16 times nominal diameter
DIA. OF CENTRAL WIRE IS 1.5%
MORE THAN THE SURRONDING
WIRE

57. IS 14268-2017 provisions
( Uncoated Stress Relieved Low Relaxation Seven Wire(Ply) Strand for
Prestressed Concrete)

58. Cont..

59. 7.2.6.5.6 Measurements of Prestressing Force
The force induced in the prestressing tendon shall be determined by means of gauges
attached to the tensioning apparatus as well as by measuring the extension of the steel and
relating it to its stress-strain curve.
The variation between the two measurements should be within +-5%. It is essential that
both methods are used jointly so that the inaccuracies to which each is singly susceptible
are minimized
If the variation of two measurements exceeds 5% then
I.the cause shall be ascertained.
II.the cable should be released and re-stressed.
III.even then, if the variation does not come within 5% then the cable is to be rejected.

60. Difference between pressure and elongation
The difference
between the
elongation and
the pressure
should not be
more than 5%

61. Measurement of Prestressing Force
( Clause 13.2.1.3 of IS 1343-2012)
•In practice, the force and elongation of tendon may not exactly match with
the expected values given in stressing schedule.
•In such cases either the force (or the elongation) will be achieved first and
the other value lag behind. In such cases the force (or elongation) shall be
further increased, but not exceeding 5 percent of the design value till the
elongation (or force), whichhas lagged behind reaches the design value.
•If, even after reaching 5 percent extra value of the force (or elongation),
the other lagged quantity does not reach the design value, reference
should be made to the designer for review and corrective action.

63. 0
16
30
46
60
76
78
15.33
31.33
45.33
61.33
75.33
91.33
93.33

64. Correction as per IS 1343-2012 and CBC
0
30
16
60
46
76
15.33
78
93.33

65. Post-tensioning Systems and Devices

66. Wedge action: Anchoring Devices

67. Sequence of Anchoring

68. Sequence of Anchoring

70. CASTING OF PSC GIRDER

71. CASTING OF PSC GIRDER
VIDE
O

72. CURING OF PSC GIRDER

73. CASTING YARD OF PSC GIRDER

74. Post Tensioning Work

75. PSC Slab

76. HDPE Sheathing for Duct

77. Measurement of Elongation of Cables

78. EMERGENCY CABLES
•Should be symmetrically placed
•Should be capable of generating additional pre-stressing force
of about 4% of design value
•Stressed only those required to make up deficiency
•Remaining pulled out & hole grouted

79. FUTURE CABLES
•For easy installation at later date
•Made in girders to cater for increased pre-stress force required
in future due to revision of loading standard, strengthening etc.
•Provision of 15% (minimum) of design pre-stressing force.

80. Cable Layout
•Cable layout means
–Deciding about the location of cable at various section
•Vertical profile
•Horizontal profile
–The locations between which the cable will be in straight and on curve
•Working out the ordinates at every meter and at every change
of curvature from curved to straight and vice versa in vertical as
well horizontal plane

82. How Proper Positioning of Cable is ensured
•Cable tends to sag due to its self weight if not supported properly on
reinforcement chairs and supports
•Cable tends to float and move upwards due to buoyancy effect when
concrete is poured (and is in liquid form), if not tied down properly
•So cable has to be secured against downward as well as upward
movement.

83. Why Reinforcement is required in PSC?
In the end block
End Block is a highly stressed zone Extending from point of application of
Prestress to the section where linear distribution of stress is assumed to
occur.
Reinforcement is provided to
To take the local transverse tension around the tendon behind the
anchorage
To cater for the tension developed between two or more anchorages,
which tends to split the member

84. PSC Slab

85. Cracks in End Zone

87. END BLOCK DEATAILS

89. Shear Reinforcement in PSC Girders

90. Load Test of Structures or Parts of Structures [CBC-18.2]
The tests described in this clause are intended as a check on structures where
there is doubt regarding serviceability or strength.
The test should be carried out as soon as possible after the expiry of 28 days
from the time of placing the concrete.
The test load should be equal to the sum of the characteristic dead load plus
1.25 times the characteristic imposed load and should be maintained for a
period of 24 h.
Measurements of deflection and crack width should be taken immediately after
the application of load, at the end of 24 hours loaded period, just after
removal of load and after the 24 hours recovery period.

91. Acceptance Criteria
Forprestressedconcretestructures,novisiblecracksshouldoccurunderthetest
loadforlocaldamage
Formembersspanningbetweentwosupports,thedeflectionimmediatelyafter
applicationofthetestloadshouldbenotmorethan1/500oftheeffectivespan.
Ifthemaximumdeflectionshownduringthe24hunderloadislessthan
40(L²/h)itisnotnecessaryfortherecoverytobemeasured.(Linmandhin
mm)
If,within24hoftheremovalofthetestloadtheconcretestructuresdoesnot
havearecoveryofatleast85%ofthemaximumdeflectionshownduringthe24
hunderload,Theloadingshouldberepeated.
Thestructureshouldbeconsideredtohavefailedtopassthetestiftherecovery
afterthesecondloadingisnotatleast85%ofthemaximumdeflectionshown
duringthesecondloading.