I. Basic Concept of prestressing
II. Basic Design concept
III.Losses in Prestressed structure
IV.Provisions of IRS:CBC and IS 1343-2012
V. Construction Aspect
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.
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. • Concrete has established itself a universal building
material because of its high compressive strength and
capability to take any form & shape,
• It has low tensile strength, which is generally
compensated by reinforcement and the resultant
composite mass is known as RCC – Reinforced cement
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-I If 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
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
12. Advantages Contd…..
VI. Economical for high spans
VII.Better fatigue strength for dynamically loaded structures (as
section remains un-cracked )
VIII.Lower maintenance compared with steel structures
IX. Suitable for pre-cast construction, hence faster execution is
X. Steel and concrete are tested in service (as they are subjected to
considerable loading conditions before application of external
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 labour cost for prestressing is also there.
IV. Prestressing is uneconomical for short spans and light loads.
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
20. Pre-tensioning or Post-tensioning
• Pre-tensioning: The tension is applied to the tendons
before casting of the concrete. The pre-compression is
transmitted from steel to concrete through bond over the
transmission length near the ends.
• Post-tensioning: The tension is applied to the tendons
(located in a duct) after hardening of the concrete. The
pre-compression is transmitted from steel to concrete by the
anchorage device (at the end blocks).
21. Pre - Tensioning
• The abutments are fixed at the ends of a prestressing bed.
• The high-strength steel tendons are pulled between two end
abutments prior to the casting of concrete. After that concrete is
• Once the concrete attains the desired strength for prestressing, the
tendons are cut loose from the abutments.
• The prestress is transferred to the concrete from the tendons, due
to the bond between them.
• During the transfer of prestress, the member undergoes elastic
• If the tendons are located eccentrically, the member is likely to
bend and deflect.
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.
34. Loss due to Elastic Shortening
When tendons are cut and PS force is transferred to the member, the concrete
undergoes immediate shortening due to the pre-stress. The tendon also shortens
by the same amount, which leads to the loss of pre-stress due to elastic
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
No Loss because there is no concrete present during stretching of the tendons
• Friction generated at interface of Concrete and steel during stretching of a curved tendon
leads to a drop in Prestressing force along the member from stretching end.
• The friction is generated due to the curvature of the tendon and the vertical component
of the PS force.
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
Pressure at a distance x from stretching end;
Loss in PS force due to friction at a distance x from stretching end; ΔP = Po(μα + kx)
39. Anchorage Slip
In most Post-tensioning systems when the tendon force is
transferred from the jack to the anchoring ends, the friction
wedges slip over a small distance.
Anchorage block also moves before it settles on concrete.
Loss of prestress is due to the consequent reduction in the
length of the tendon.
Certain quantity of prestress is released due to this slip of wire
through the anchorages. – Amount of slip depends on type of
wedge and stress in the wire.
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
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.
The factors responsible for creep of concrete will have influence on
shrinkage of concrete also except the loading conditions
Total shrinkage strain in plain concrete, reinforced concrete and pre-
tensioned prestressed concrete: 0.0003.
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 hr at 300C
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 106 hr (~ 57 yrs)
• It is = 2.5 times the 1000 hr value for normal steel & 3.0 times for
low relaxation steel
• The above value is at initial stress level of 70% of the
• It reduces to 0 at 50%. In between values are obtained from
46. Average losses
Type of loss Pre-
Elastic Shortening of Concrete 4 1
Creep of concrete 6 5
Shrinkage of 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
59. 18.104.22.168.6 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
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.
61. Measurement of Prestressing Force
( Clause 22.214.171.124 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), which has 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.
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
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
Reinforcement is provided to
To take the local transverse tension around the tendon behind the
To cater for the tension developed between two or more anchorages,
which tends to split the member
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
For prestressed concrete structures, no visible cracks should occur under the test
load for local damage
For members spanning between two supports, the deflection immediately after
application of the test load should be not more than 1/500 of the effective span.
If the maximum deflection shown during the 24 h under load is less than
40(L²/h) it is not necessary for the recovery to be measured. (L in m and h in
If ,within 24 h of the removal of the test load the concrete structures does not
have a recovery of at least 85% of the maximum deflection shown during the 24
h under load, The loading should be repeated.
The structure should be considered to have failed to pass the test if the recovery
after the second loading is not at least 85% of the maximum deflection shown
during the second loading.