This document provides an introduction to prestressed concrete, including: 1) A prestressed concrete structure differs from reinforced concrete in applying an initial load to counteract stresses from use. 2) Prestressing concepts include transforming concrete to an elastic material, combining high strength steel with concrete, and load balancing. 3) Methods include pre-tensioning and post-tensioning, and materials include high strength steel tendons or bars, concrete, and grout.

1. INTRODUC
TION TO
PRESTRESS
ED
CONCRETE
UNIT 5

2. Principle of Prestressing –
Methods of Prestressing,
advantages and
disadvantages.

3. Prestressin
g
Concrete is weak in tension and strong in
compression.
Prestressed concrete is a method for
overcoming concrete's natural weakness
in tension.

4. A prestressed concrete structure is different
from a conventional reinforced concrete
structure due to the application of an initial load
on the structure prior to its use.
The initial load or ‘prestress’ is applied to
enable the structure to counteract the stresses
arising during its service period.

5. Prestressed concrete is a modified form
of reinforced concrete. it not only
eliminates the weakness of concrete,
but also makes it possible to take full
advantage of compressive strength of
concrete.

6. The term “Prestressed Concrete” is
applied to those reinforced concrete
members in which concrete is
subjected to compressive stresses,
before the external load are applied, by
inducing tensile stresses in the
reinforcement to counteract tensile
stresses in the concrete caused by
external loads.

10. 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.
Force-fitting of metal bands on wooden barrels

11. 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.
Spokes in a bicycle wheel

12. BASIC CONCEPTS OF
PRESTRESSING
There are three basic concepts of prestressing:
First Concept: Prestressing to transform
concrete into an Elastic Material.
Second Concept: Prestressing for Combination
of High Strength Steel with Concrete.
Third Concept: Prestressing to Achieve load
Balancing.

14. Principle of
Prestressin
g
Pre-stressing tendons (generally of high tensile
steel cable or rods) are used to provide a
clamping load which produces a compressive
stress that balances the tensile stress that the
concrete compression member would otherwise
experience due to a bending load.

18. A prestressed concrete structure is different from a
conventional reinforced concrete structure due to the
application of an initial load on the structure prior to its
use. The initial load or ‘prestress’ is applied to enable
the structure to counteract the stresses arising during its
service period.
The prestressing of a structure is not the only instance
of prestressing. The concept of prestressing existed
before the applications in concrete.

19. 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.
Force-fitting of metal bands on wooden barrels

20. 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.
Spokes in a bicycle wheel

21. Basic Concepts of Prestressing
There are three basic concepts of prestressing:
First Concept: Prestressing to transform
concrete into an Elastic Material.
Second Concept: Prestressing for Combination
of High Strength Steel with Concrete.
Third Concept: Prestressing to Achieve load
Balancing.

22. First Concept
Prestressing to transform
concrete into an Elastic
Material.

23. Second Concept:
Prestressing for Combination of
High Strength Steel with
Concrete.

24. Third Concept:
Prestressing toAchieve
load Balancing.

25. Advantages of
Prestressing
.

26. 1) Section remains uncracked under service
loads
 Reduction of steel corrosion
 Full section is utilised
Higher moment of inertia (higher stiffness)
Less deformations (improved serviceability).
 Increase in shear capacity.
 Suitable for use in pressure vessels, liquid retaining
structures.
 Improved performance (resilience) under dynamic and
fatigue loading.

27. 2) High span-to-depth ratios
(bridges,
Larger spans possible with prestressing
buildings with large column-free spaces)
3) Suitable for precast construction
 Rapid construction
 Better quality control
 Reduced maintenance
 Suitable for repetitive construction
 Availability of standard shapes.

28. Limitations of Prestressing
Prestressing needs skilled technology. Hence,
it is not as common as reinforced concrete.
The use of high strength materials is costly.
There is additional cost in auxiliary
equipment.
There is need for quality control and
inspection.

29. Types of Prestressing
Source of PrestressingForce
1)Hydraulic Prestressing
This is the simplest type of prestressing,
large prestressing
jack used
forces.
Th
e tensioning
of
producing
hydraulic
tendons, comprises
for the
of calibrated pressure
gaugeswhich directly indicate the magnitude of
force developed during the tensioning.

30. 2)Mechanical Prestressing
In this type of prestressing, the devices includes
weights with or without lever transmission, geared
transmission in conjunction with pulley blocks,
screw jackswith or without gear drives and wire-
winding machines. Thistype of prestressing is
adopted for massscaleproduction.
3)Electrical Prestressing
In this type of prestressing, the steel wires are
electrically heated
and anchored before placing concrete in themolds.
Thistype of prestressing is also known asthermo-
electric prestressing.

31. External or Internal Prestressing
External Prestressing
When the prestressing is achieved by elements
located outside the concrete, it is called external
prestressing. The tendons can lie outside the
member (for example in I-girders or walls) or
technique is adopted in bridges
inside the hollow space of a box girder. This
and
strengthening of buildings. In the following
figure, the box girder of a bridge is prestressed
with tendons that lie outside the concrete.

32. External prestressing of a box girder

33. Internal Prestressing
When the prestressing is achieved by elements
located inside the concrete member (commonly,
by embedded tendons), internal
prestressing. Most of
it is called
the applications of
prestressing are internal prestressing. In the
following figure, concrete will be cast around the
ducts for placing the tendons.

34. Internal prestressing of abox girder

35. 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. The
following figure shows manufactured pre-
tensioned electric poles.

36. Pre-tensioned electric poles

37. 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). The following figure shows a post-
tensioned box girder of a bridge.

38. Post-tensioning of a box girder

39. Linear or Circular Prestressing
Linear Prestressing
When the prestressed members are straight or
prestressing is called linear prestressing.
flat, in the direction of prestressing, the
For
example, prestressing of beams, piles, poles and
slabs. The profile of the prestressing tendon may
be curved. The following figure shows linearly
prestressed railway sleepers.

40. Linearly prestressed railway sleepers

41. Circular Prestressing
When the prestressed members are curved, in the
direction of prestressing, the prestressing is
called circular prestressing. For example,
circumferential prestressing of tanks, silos, pipes
and similar structures.

42. Circularly prestressed containment structure

43. Full, Limited or Partial Prestressing
Full Prestressing
When the level of prestressing is such that no tensile
stress is allowed in concrete under service loads, it is
called Full Prestressing.
Limited Prestressing
When the level of prestressing is such that the tensile
stress under service loads is within the cracking stress of
concrete, it is called Limited Prestressing.
Partial Prestressing
When the level of prestressing is such that under tensile
stresses due to service loads, the crack width is within the
allowable limit, it is called Partial Prestressing.

44. Uniaxial, Biaxial or Multiaxial
Prestressing
Uniaxial Prestressing
When the prestressing tendons are parallel to one axis, it is
For example, longitudinal
called Uniaxial Prestressing.
prestressing of beams.
Biaxial Prestressing
When there are prestressing tendons parallel to two axes, it
is called Biaxial Prestressing. The following figure shows
the biaxial prestressing of slabs.
Multiaxial Prestressing
When the prestressing tendons are parallel to more than two
axes, it is called Multiaxial Prestressing. For example,
prestressing of domes.

45. Pre-tensioning Systems and Devices
In pretensioning, the tension is applied to
the tendons before casting of the concrete.

46. Stages of Pre-tensioning
The various stages of the pre-tensioning operation
are summarized as follows.
1) Anchoring of tendons against the end abutments
2) Placing of jacks
3) Applying tension to the tendons
4) Casting of concrete
5) Cutting of the tendons.

47. Stages of pre-tensioning

48. Advantages
The relative advantages of pre-tensioning as
compared to post-tensioning are as follows.
Pre-tensioning is suitable for precast members
produced in bulk.
In pre-tensioning large anchorage device is not
present.

49. Disadvantages of Pre-tensioning
The relative disadvantages are as follows.
A prestressing bed is required for the pre-
tensioning operation.
There is a waiting period in the prestressing bed,
before the concrete attains sufficient strength.
There should be good bond between concrete and
steel over the transmission length.

50. Travelling pre-tensioning stress bench Anchoring of strands

51. Stretching of strands

52. Pouring of concrete Steam curing chamber

53. Demoulding of sleeper Storage

54. Post-tensioning Systems and Devices
In posttensioning, the tension is applied to the
tendons after hardening of the concrete.

55. Stages of Post-tensioning
The various stages of the post-tensioning operation
are summarized as follows.
1) Casting of concrete.
2) Placement of the tendons.
3) Placement of the anchorage block and jack.
4) Applying tension to the tendons.
5) Seating of the wedges.
6) Cutting of the tendons.

58. Advantages of Post-tensioning
The relative advantages of post-tensioning as
compared to pre-tensioning are as follows:
1)Post-tensioning is suitable for heavy cast-in-
place members.
2)The waiting period in the casting bed is less.
3)The transfer of prestress is independent of
transmission length.

59. Devices
The essential devices for post-tensioning are as
follows.
1) Casting bed
2) Mould/Shuttering
3) Ducts
4) Anchoring devices
5) Jacks
6) Couplers (optional)
7) Grouting equipment (optional).

62. Materials

63. Concrete
Concrete is a composite material composed of
gravels or crushed stones (coarse aggregate), sand
(fine aggregate) and hydrated cement (binder). It
is expected that the student of this course is
familiar with the basics of concrete technology.

64. Concrete

65. Aggregate
The coarse aggregate are granular materials
obtained from rocks and crushed stones. They
may be also obtained from synthetic material
like slag, shale, fly ash and clay for use in light-
weight concrete.
The sand obtained from river beds or quarries is
used as fine aggregate. The fine aggregate
along with the hydrated cement paste fill the
space between the coarse aggregate.

66. The nominal maximum coarse aggregate size is
limited by the lowest of the following
quantities.
1)1/4 times the minimum thickness of the
member
2)Spacing between the tendons/strands minus 5
mm
3) 40 mm.

67. Cement
In present day concrete, cement is a mixture of
lime stone and clay heated in a kiln to 1400 –
1600 ºC.

68. Water
Water used for mixing and curing shall be clean
and free from injurious amounts of oils, acids,
alkalis, salts, sugar, organic materials or other
substances that may be deleterious to concrete and
steel.

69. Admixtures
The admixtures can be broadly divided into two
types: chemical admixtures and mineral admixtures.
The common chemical admixtures are as follows.
1) Air-entraining admixtures
2) Water reducing admixtures
3) Set retarding admixtures
4) Set accelerating admixtures
5) Water reducing and set retarding admixtures
6) Water reducing and set accelerating admixtures.

70. The common mineral admixtures are as
follows.
1) Fly ash
2) Ground granulated blast-furnace slag
3) Silica fumes
4) Rice husk ash
5) Metakoline

71. Properties of Hardened Concrete
1) High strength
2) Durability
3) Stiffness
4) Minimum shrinkage and creep

72. High strength
The maximum grade of concrete is 60 MPa.
The minimum grades of concrete for
prestressed applications are as follows.
1)30 MPa for post-tensioned members
2)40 MPa for pre-tensioned members.

73. Stiffness of Concrete
The stiffness of concrete is required to estimate
the deflection of members. The stiffness is
given by the modulus of elasticity.

74. Durability of Concrete
The durability of concrete is of vital importance
regarding the life cycle cost of a structure. The
life cycle cost includes not only the initial cost
of the materials and labour, but also the cost of
maintenance and repair.

75. Creep of Concrete
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. Hence, the study of creep is
important in prestressed concrete to calculate the
loss in prestress.
The creep occurs due to two causes.
paste
1.Rearrangement of hydrated cement
(especially the layered products)
2. Expulsion of water from voids under load

76. Shrinkage of Concrete
Shrinkage of concrete is defined as the contraction
due to loss of moisture. The study of shrinkage is
also important in prestressed concrete to calculate
the loss in prestress.
The shrinkage occurs due to two causes.
1. Loss of water from voids
2. Reduction of volume during carbonation

77. Grout
Grout is a mixture of water, cement and optional
materials like sand, water-reducing admixtures,
expansion agent and pozzolans. The water-to-
cement ratio is around 0.5. Fine sand is used to
avoid segregation.

79. The desirable properties of grout are as
follows.
1) Fluidity
2) Minimum bleeding and segregation
3) Low shrinkage
4) Adequate strength after hardening
5) No detrimental compounds
6) Durable.

80. Prestressing Steel
The development of prestressed concrete was
influenced by the invention of high strength
steel. It is an alloy of iron, carbon, manganese
and optional materials. In addition to
prestressing steel, conventional non-prestressed
reinforcement is used for flexural capacity
(optional), shear capacity, temperature and
shrinkage requirements.

81. Wires
A prestressing wire is a single unit made of
steel. The nominal diameters of the wires are
2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm. The different
types of wires are as follows.
1)Plain wire: No indentations on the surface.
2)Indented wire: There are circular or elliptical
indentations on the surface.

82. Strands
A few wires are spun together in a helical form to
form a prestressing strand. The different types of
strands are as follows.
1)Two-wire strand: Two wires are spun together to
form the strand.
2)Three-wire strand: Three wires are spun
together to form the strand.
3)Seven-wire strand: In this type of strand, six
wires are spun around a central wire. The central
wire is larger than the other wires.

83. Tendons
A group of strands or wires are placed together
to form a prestressing tendon. The tendons are
used in post-tensioned members.
Cables
A group of tendons form a prestressing cable.
The cables are used in bridges

84. Bars
A tendon can be made up of a single steel bar.
The diameter of a bar is much larger than that of
a wire. Bars are available in the following sizes:
10, 12, 16, 20, 22, 25, 28 and 32 mm.