It is the presentation based on pre- stressed concrete construction which includes each and every point and scope which may be useful to civil engineering students

1. Pre-stressed concrete construction

2. What is Pre-stressedConcrete?
2
– Internal stressesare
induced to counteract
external stresses.
– In 1904, Freyssinet
attempted to introduce
permanent acting forcein
conc. to resist elastic
forces under loads and
was named
“Pre stressing”.

3. Introduction
3
• 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.

4. Reinforced concrete:
• Concrete is strong in compression weak intension.
• Steel in strong intension
• Reinforced concrete usesconcrete to resist compression
and to hold bars in position and uses steel to resist tension.
• Tensile strength of concrete is neglected (i.e. zero)
• R.Cbeams allows crack under serviceload.

5. ComparisonBetweenRCC&Prestress Concrete
5

7. Concept of pre-stressing:
i . Theconcept of pre stressing was invented
invented years ago when metal
wound
around wooden pieces to formbarrels.

brands were


 ii . Themetal brands
were tighten under tensile stresswhich
creates compression between the
staves allowing them to resist internal
liquid pressure.

8. Principle of pre-stressing:
• Pre-stressing is amethod in which compression forceis
applied to the reinforcedconcrete section.
• The effect of pre stressing is to reduce the tensile stress in the
section to the point till the tensile stress is below the cracking
stress. Thusthe concrete does not crack.
• It is then possible to treatconcrete asaelastic material.
• Theconcrete can be visualized to have two compressiveforce
i .Internal pre-stressing force.
ii . External forces (d.l , l.l etc )
• Thesetwo forces must counteract eachother.

9. Principle ofPre-stressing:
• Stress in concrete when pre stressing is applied at thec.g of
the section

10. Principle of Pre-stressing:
• Stress in concrete when pre stressing is appliedeccentrically
with respect to the c.g of the section.

11. Types of Prestressing Systems
11
I..Pre-tensioning:
 In Pre-tension, the tendons are tensioned before the
concrete is place.After the concrete hardened, the
tension force isreleased.
II..Post tensioning:
 In Posttension, the tendons are tensioned afterthe
concrete hashardened.

12. Types of pre-stressing:
I .Pre-tensioning
• InPre-tension, the tendonsaretensioned againstsome abutments before the
concrete isplace.After the concrete hardened, the tension force isreleased.
Thetendon tries to shrink backto the initial length but the concrete resistsit
through thebond between them, thus,compressionforce is induced in
concrete. Pretension isusuallydonewithprecast
members

14. • Pre tensioned Concrete
In which the tendons are tensioned
before the concrete is placed, tendons
are temporarily anchored and
tensioned and the prestress is
transferred to the concrete after it is
hardened.

15. Pre-tensioning Method
Stage 1 Stage 2 Stage 3 Stage 4
Tendons and
reinforcement
are positioned
in the beam
mould.
Tendons are
stressed to about
70% of their
ultimate strength.
Concrete iscast
into the beam
mould and allowed
to cure to the
required initial
strength.
When the
concrete has
cured the
stressing force is
released and the
tendons anchor
themselves in the
concrete.

16. Exampleof Pre-tensioning
16
Fig: Pre-tensioned electricpole

17. II .Posttensioning
• In Post tension, the tendons are tensioned after the
concrete has hardened. Commonly, metal or plastic
ducts are placed inside the concrete before casting.
After the concrete hardened and had enough strength,
the tendon was placed inside the duct, stressed, and
anchored against concrete. Grout may be injected into
the duct later. Thiscan be done either asprecast or
cast-in-place.

18. Post tensioning

20. POST –TENSIONING METHOD

21. Post-tensioning Method
Stage 1 Stage 2 Stage 3 Stage 4
Cable ducts and Concrete is cast Tendons are Wedges are inserted
reinforcement are into the beam threaded through the into the end
positioned in the beam mould and allowed cable ducts and anchorages and the
mould. The ducts are to cure to the tensioned to about tensioning force on
usually raised towards required initial 70% oftheir the tendons is
the neutral axis at the strength. ultimate strength. released. Grout is
ends to reduce the then pumped into
eccentricity ofthe the ducts to protect
stressing force. the tendons.

22.  Forms
• Wires
• Strands
• Tendons
• Cables
• Bars
 Source of Force
• Mechanical
• Hydraulic
• Electrical
Prestressing Steel
(High Strength steel)
tendons
Mechanicaljacks
tendons
wires

24. Exampleof Post-tensioning
24
Fig: Post-tensioning of aboxgirder

25. Advantages:
•Takefull advantages of high strengthconcrete
and high strength steel
•Need less materials
•Smaller and lighter structure
•No cracks
•Usethe entire section toresist the load
•Better corrosion resistance
•Good for water tanks and nuclearplant
•Very effective for deflectioncontrol
•Better shearresistance

26. Disadvantagesof
Pre-stressed
concrete
• Needhigherqualitymaterials
• More complextechnically
• Moreexpensive
• Hardertore-cycle

27. Application:
• Bridges
• Slabsin buildings
• WaterTank
• ConcretePile
• ThinShellStructures
• OffshorePlatform
• NuclearPowerPlant
• RepairandRehabilitations

28. Losses in Various Prestressing Systems
28
Type of Loss Pre-tensioning Post-tensioning
1. Elastic Shortening Yes
i. No, if all the cables are
simultaneously tensioned.
ii. If the wires are
tensioned in stages loss
will exist.
2. AnchorageSlip No Yes
3. Friction Loss No Yes
4. Creep and Shrinkage
of Concrete
Yes Yes
5. Relaxation of Steel Yes Yes

29. ElasticShortening
29
Length after elasticshortening
• 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 ofprestress
Pi
P0

30. Elastic Shortening at Pre-tensionedMembers
30
 When the tendons are cut and the prestressing
force is transferred to the member, concrete
undergoes immediate shortening due toprestress.
 Tendon also shortens by same amount, which leads
to the loss ofprestress.

31. Prestressing bed
Pre-tensioning of amember
Elastic Shortening at Pre-tensionedMembers
31

32. Elastic Shortening at Post-tensionedMembers
 If there is only one tendon, there is no loss because the
applied prestressis recordedaftertheelastic shortening of
themember.
 For more than one tendon, if the tendons are stretched
sequentially, there is loss in a tendon during subsequent
stretchingoftheothertendons. 32

33. Duct
jackAnchorage
Casting bed
Post-tensioning of amember
Elastic Shortening at Post-tensionedMembers
33

34. TERMINOLOGY
1.Tendon: A stretched element used in aconcrete
member of structure to impart prestress to the
concrete.
2.2.Anchorage: A device generally used to enable the
tendon to
• impart and maintain prestress in concrete.

35. Anchorage Slip
35
• In most Post-tensioning systems when the prestress force is
transferred from the jack to the anchoring ends, the wedges
slip over a smalldistance.
• Loss of prestress is due to the consequent reduction in the
length of thetendon.
• Amount of slip depends on type of anchorage system.

36. Forcevariation diagramsforvarious stages
22
length lset.
a)Theinitial tension at the right
end is high to compensate forthe
anchorage slip. It corresponds to
about initial prestress. The force
variation diagram (FVD)is linear.
b)After the anchorage slip, the
FVDdrops near the right endtill
the lengthlset.
Note : Effect of anchorage slip is present
up to a certain length, called the setting

37. Forcevariation diagramsforvarious stages
37
c)Theinitial tension at the leftend
also corresponds to about initial
prestress. The FVD is linear up to
the centre line of thebeam.
d)After the anchorage slip, the
FVDdrops 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.

38. Typical values of anchorage slip
38
Anchorage System Anchorage Slip (Δs)
Freyssinet system
12 - 5mm Φ strands
12 - 8mm Φ strands
4 mm
6 mm
Magnel system 8 mm
Dywidag system 1 mm

39. Frictional Loss
39
• Thefriction generated at the interfaceof
concrete and steel during the stretching
of acurved tendon in apost-tensioned
member.
• Thefriction in the jacking anchoring systemis
generally small.
• More serious frictional loss occurs betweenthe
tendon and its surroundingmaterial.

40. Frictional loss occurs only inPost-
tensioned Members
40
• Theloss due to friction does not occur in pre-
tensioned members becausethere is noconcrete
during the stretching of thetendons.
• Friction is generated due to curvature of tendon,
and vertical component of the prestressing force.
A typical continuous post-tensioned member

41. FrictionalLoss
•Frictional Lossis the summation of
– Friction LossDueto lengthEffect.
– Friction LossDueto CurvatureEffect.
• LengthEffect:If theprofileofcableislinear,the loss
41
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 tendonsthemselves.

42. Length& CurvatureEffects
42

43. Methods availableto “Reduce”the frictional
losses
43
1. Cablesshould passthrough metal tubes.
2. Thebends should be through assmall an
angle aspossible.
3. Radiusof curvature for bends shouldbe
large.
4. Prestressing the wire from bothends.
5. Over-tensioning the wires.

44. Creep of Concrete
• TheContinuousdeformationofconcretewithtimeunder sustainedload.
 Factorsaffectingcreepofconcrete
• Age
• Applied Stresslevel
• Density ofconcrete
• CementContent inconcrete
• Water-CementRatio
• Relative Humidityand
• Temperature
44

45. Condition for calculating the loss of
prestress due to creep.
45
• Creep is due to sustained (permanent) loads only.Temporary
loads are not considered in calculationof creep.
• Since the prestress may vary along the length of the
member, an average value of the prestress isconsidered.

46. Shrinkage of Concrete
46
• Shrinkage
defined
of concrete is
as the
contraction due to loss of
moisture.
• Due to the shrinkage of
concrete,the prestress in the
tendon is reduced withtime.

47. Shrinkageof Concrete
47
• For pre-tensioned members, transfer
commonly takes place after 24 hours after
casting and nearly all shrinkage takesplace
after that.
• For post-tensioned members, stressing may
takes place after one day or much later, thusa
large percentage of shrinkage may already
taken place by them.

48. Relaxation
48
 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 constantstrain.
– Percentage of relaxation varies from 1 to5%.

49. Factors effecting Relaxation :
49
• Time
• Initial stress
• Temperatureand
• Typeof steel.

50. Method Availableto ReduceTheLossdueto
Relaxation
50
• Choiceof proper steel helps to reducethis
loss.
• Prestressed wires havelessercreep.
• Galvanised wires also have no creep.
• overstressing steel about 10%above its initial
stressand then releasing it to the initial stress

51. Initial Prestress
51
• Deducting the loss due toanchorage take-up and
friction, initial prestress isobtained.
• If prestress is measured at the time of pulling the
wire, the stress is termed asthe jackingstress.
• if jacketing stress is treated astheinitial
stress, effective stress is jacketing stress minusall
losses.

52. EffectivePrestress
52
• Initial Prestressin steel minus the lossesis
known asthe effective or designprestress.
Effective prestress=Initial prestress-Losses
Note: For Pre-Tension system , Pretension Lossesare used
instead of losses.
For Post-Tension system , Post-Tension Lossesare used
instead of losses.

53. TotalAmount OfLossesAccordingToTensioningSystem
53
• Total pretension losses=Lossdue to creep+
Elastic shortening +Shrinkage+ Steel
Relaxation.
• Total post-Tension Loss=Lossdue to creep+
Elastic shortening +Shrinkage+ Steel
Relaxation +Anchorageslip +
Friction.

54. Prestress Loss
54
• loss in prestress is the difference between initial prestress and
the effectiveprestress.
• Loss of prestressaffects
– the strength of memberand
– member’sserviceability [Stresses in Concrete, Cracking,Camber and Deflection]

55. Types
55
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.

56. Prestress Losses
Immediate
Time
Dependent
Relaxation
Anchorage
Slip
Elastic
Shortening
Friction
ShrinkageCreep
TypesAccordingToTime
56

57. Prestress Losses
Concrete Steel
Relaxation
Shrinkage
Elastic
Shortening
Creep
Anchorage
Slip
Friction
TypesAccordingToMaterial
57

58. TotalLosses
58
• It is difficult to generalize the amount of loss
of prestress, becauseit is dependent on so
many Factors :
Theproperties of concrete & steel.
Curing & moisture condition.
Magnitude & time of application of prestress.
Processof prestress.

59. Method of LossEstimation
59
• There are two methods that canbe usedto
estimate lossesin prestressedconcrete:
(a)lump sum approximations;
(b)refined estimations.
Oneshould keep in mind that allestimates
for prestress loss are just that – ESTIMATIONS.

60. LumpSumEstimationForPrestress Loss
60
• First introduce by the ACI-ASCECommittee 423in
1958.
Table:AASHTO Lump SumLosses.
TotalLoss
Typesof
prestressingsteel
f’c=28 MPa f’c=35 MPa
Pretensioning
strand
310MPa
Posttensioningwire
or strand
220MPa 230Mpa
Bars 150MPa 160Mpa

61. AASHTO-LRFDSpecifications
61
For Lump-Sum estimates following conditions should met:
1. Members that are post-tensioned must benon-segmental
members with spansless than 160 feet and concrete
stressed an ageof 10-30days.
2. Members that are pretensioned must be stressed at an age
where the concrete strength is notless than 3,500-psi.
3. Members must be made from normalweight concrete.
4. Members cannot be steam-cured, normoist-cured.
5. Theprestressing steel must be normal orlow-relaxation.
6. There must beAverage exposure conditions at thesite.

62. Thumbrule ofLosses
62
• For average steel and concrete properties ,the tabulated
percentages may be taken asrepresentative of theaverage
losses.
Pretensioning,% Posttensioning.%
Elasticshortening &
bendingof concrete
4 1
Creepof concrete 6 5
Shrinkageof concrete 7 6
Steel relaxation 8 8
TotalLoss 25 20

63. Methods of Corrosion Protection

Epoxy Coating
Fusion bonded epoxy coating of steel bars to help prevent corrosion has
been successfully employed in many applications because of the
chemical stability of epoxy resins. Epoxy coated bars and fasteners
should be done in accordance with ASTM A- 775 or ASTM 934. Coating
thickness is generally specified between 7 to 12 mils. Epoxy coated bars
and components are subject to damage if dragged on the ground or
mishandled.
Heavy plates and nuts are often galvanized even though the bar may be
epoxy coated since they are difficult to protect against abrasion in the
field. Epoxy coating patch kits are often used in
the field for repairing nicked or scratched epoxy surfaces.
Cement Grout filled corrugated polyethylene tubing is often used to provide an
additional barrier against corrosion attack in highly aggressive soils. These
anchors are often referred to as MCP or Multiple Corrosion Protection
anchors. The steel bars are wrapped with an internal centralizer then placed
inside of the polyethylene tube where they are then factory pre-grouted. When
specifying couplings with MCP ground anchors, verify coupling locations with
a Williams representative.
Pre-Grouted Bars

64. 

Williams strand tendons contain an extruded high density polyethylene
sheathing around each individual strand in the free-stressing portion of
the anchorage. The sheathing is minimum 60 mils thick and applied once
the 7-wire strand has been coated with a corrosion inhibiting compound.
Extruded polyethylene sheathing provides a moisture tight barrier for
corrosion protection and allows the strand to
elongate freely throughout the free-stressing length during the
prestressing operation
Hot Dip Galvanizing
Zinc serves as a sacrificial metal corroding preferentially to the
steel. Galvanized bars have excellent bond characteristics to
grout or concrete and do not require as much care in handling
as epoxy coated bars. However, galvanization of anchor rods is
more expensive than epoxy coating and often has greater lead
time. Hot dip galvanizing bars and fasteners should be done in
accordance with ASTM A-153. Typical galvanized coating
thickness for steel bars and components is between 3 and 4
mils. 150 KSI high strength steel bars should always be
mechanically cleaned (never acid washed) to avoid
problems associated with hydrogen embrittlement.
Extruded Polyethylene

65. Corrosion Inhibiting Wax or Grease with
Sheath
Williams corrosion inhibiting compounds can be placed in the
free stressing sleeves, in the end caps, or in the trumpet
areas. Often bars are greased/waxed and PVC is slipped
over the greased/waxed bar prior to shipping. Each are
of an organic compound with either a grease or wax
base. They provide the appropriate polar moisture
displacement and have corrosion inhibiting additives with
self-healing properties. They can be pumped or applied
manually. Corrosion inhibiting compounds stay
permanently viscous, chemically stable and non-reactive
with the prestressing steel, duct materials or grout. Both
compounds meet PTI standards for Corrosion Inhibiting
Coating.
Coal Tar Epoxy
Coal tar epoxy has shown to be abrasion resistant,
economical and durable. This product when specified
should meet or exceed the requirements of (a) Corp of
Engineers C-200, C200a and (b) AWWA C-210-92 for
exterior. Typically the thickness is between 8 and 24
mils. Make sure that the surfaces of the bar are clean
and dry before coating.

66. 

Heat Shrink Tubing
Heat Shrink Tubing provides a corrosion
protected seal when connecting smooth or
corrugated segments.
Epoxy Coating PatchKits
EpoxyCoatingPatchKitsare availableupon request.
Anchor Head Protection
The most important section of a ground anchor that needs adequate
corrosion protection is the portion of the anchor exposed to air/oxygen.
This is typically defined as the "anchor head", which generally consists of a
steel bearing plate, a hex nut and washer for a bar system, or a wedge
plate and wedges for a strand system. For permanent ground anchors it is
best to galvanize the hex nut and plates even if the bar is epoxy coated.
Galvanized components, if scratched during shipping, are less likely to
provide corrosion protection at otherwise exposed anchor ends.
FiberReinforced
cause corrosion concerns than scratched epoxy coated components. TheNylon Cap
end of the steel bar protruding out from the hex nut is often protected by
the use of a plastic or steel end cap packed with grease or cement grout.
Williams offers several different types of PVC and metal end caps to
Strand
EndCap
Screw-On
PVCCap
Steel Tube Welded on Flangewith
Threaded Screw Connections

67. 

Field Splice for Bars
Continuous corrosion protection can even be
accomplished for the MCP Pregrouted anchors
manufactured from Williams Form Engineering. To
achieve the equivalent levels of corrosion protection
the coupled sections of bar anchors can be wrapped
in a grease impregnated tape that is further protected
with heat shrink sleeving. This scheme is acceptable
by most governing agencies and is specified in the
PTI Recommendations for Prestresed Rock and Soil
Anchors.

68. 1) Jenil
2) Nalin
3) Sid
4) Riyan
5) Panjala