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Prestressing Concept, Materilas and Prestressing System
Prestressing Concept, Materials
and Prestressing System
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
The prestressing of a structure is not the only instance
of prestressing. The concept of prestressing existed
before the applications in concrete.
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
Force-fitting of metal bands on wooden barrels
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
Spokes in a bicycle wheel
1) Section remains uncracked under service
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
Improved performance (resilience) under dynamic and
2) High span-to-depth ratios
Larger spans possible with prestressing (bridges,
buildings with large column-free spaces)
3) Suitable for precast construction
Better quality control
Suitable for repetitive construction
Availability of standard shapes.
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
There is need for quality control and
Types of Prestressing
Source of Prestressing Force
This is the simplest type of prestressing,
producing large prestressing forces. The
hydraulic jack used for the tensioning of
tendons, comprises of calibrated pressure
gauges which directly indicate the magnitude of
force developed during the tensioning.
In this type of prestressing, the devices includes
weights with or without lever transmission, geared
transmission in conjunction with pulley blocks,
screw jacks with or without gear drives and wire-
winding machines. This type of prestressing is
adopted for mass scale production.
In this type of prestressing, the steel wires are
and anchored before placing concrete in the molds.
This type of prestressing is also known as thermo-
Internal or External Prestressing
When the prestressing is achieved by elements
located inside the concrete member (commonly,
by embedded tendons), it is called internal
prestressing. Most of the applications of
prestressing are internal prestressing. In the
following figure, concrete will be cast around the
ducts for placing the tendons
Pre-tensioning or Post-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 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
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
inside the hollow space of a box girder. This
technique is adopted in bridges and
strengthening of buildings. In the following
figure, the box girder of a bridge is prestressed
with tendons that lie outside the concrete.
Linear or Circular Prestressing
When the prestressed members are straight or
flat, in the direction of prestressing, the
prestressing is called linear prestressing. For
example, prestressing of beams, piles, poles and
slabs. The profile of the prestressing tendon may
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.
Full, Limited or Partial Prestressing
When the level of prestressing is such that no tensile
stress is allowed in concrete under service loads, it is
called Full 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.
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.
Uniaxial, Biaxial or Multiaxial
When the prestressing tendons are parallel to one axis, it is
called Uniaxial Prestressing. For example, longitudinal
prestressing of beams.
When there are prestressing tendons parallel to two axes, it
is called Biaxial Prestressing. The following figure shows
the biaxial prestressing of slabs.
When the prestressing tendons are parallel to more than two
axes, it is called Multiaxial Prestressing. For example,
prestressing of domes.
Pre-tensioning Systems and Devices
In pretensioning, the tension is applied to
the tendons before casting of the concrete.
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.
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
Disadvantages of Pre-tensioning
The relative disadvantages are as follows.
A prestressing bed is required for the pre-
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.
Travelling pre-tensioning stress bench Anchoring of strands
Post-tensioning Systems and Devices
In posttensioning, the tension is applied to the
tendons after hardening of the concrete.
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.
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-
2)The waiting period in the casting bed is less.
3)The transfer of prestress is independent of
The essential devices for post-tensioning are as
1) Casting bed
4) Anchoring devices
6) Couplers (optional)
7) Grouting equipment (optional).
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.
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-
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.
The nominal maximum coarse aggregate size is
limited by the lowest of the following
1) 1/4 times the minimum thickness of the
2) Spacing between the tendons/strands minus 5
3) 40 mm.
In present day concrete, cement is a mixture of
lime stone and clay heated in a kiln to 1400 –
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
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.
The common mineral admixtures are as
1) Fly ash
2) Ground granulated blast-furnace slag
3) Silica fumes
4) Rice husk ash
Properties of Hardened Concrete
1) High strength
4) Minimum shrinkage and creep
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.
Stiffness of Concrete
The stiffness of concrete is required to estimate
the deflection of members. The stiffness is
given by the modulus of elasticity.
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.
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.
1. Rearrangement of hydrated cement paste
(especially the layered products)
2. Expulsion of water from voids under load
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
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
The desirable properties of grout are as
2) Minimum bleeding and segregation
3) Low shrinkage
4) Adequate strength after hardening
5) No detrimental compounds
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
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.
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.
A group of strands or wires are placed together
to form a prestressing tendon. The tendons are
used in post-tensioned members.
A group of tendons form a prestressing cable.
The cables are used in bridges
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.