This paper deals with different methods of prestressing in concrete and the advantages and disadvantages of external prestressing.

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External Prestressing of Concrete
Members
Praveen.S.K Vishwanath.H Vinuth.M
Abstract
The normal interaction between steel and concrete is lost because the tendons
are unbonded with the concrete cross section. External pre-stressing has been
proven cost effective and technically attractive worldwide, as it leads to substan-
tial cost savings and a large decrease in construction time. If the purpose of the
project is to improve the load carrying capacity of existing bridges, then tendons
are usually, placed outside the bridge girders, tensioned and anchored at their
ends. The external tendons are usually made of steel, but also fibre reinforced
polymeric (FRP) materials can be used to replace steel which would prove more
economical.
I. INTRODUCTION
P
re-stressed concrete is reinforced
concrete in which the steel re-
inforcement has been tensioned
against the concrete, which counteracts
the downward deflection caused due
to its dead load and the imposed live
load.
II. SYSTEM OF PRESTRESSING
A prestressed system induces a known
amount of stresses in a concrete mem-
ber before the live load is applied.
These stresses are induced by high
strength steel strands by mechanical
means. These cables are generally
placed inside a cylindrical duct made
out of either metallic or HDPE mate-
rial. The Anchorages, one of the main
components of the prestressing activ-
ity, are used to anchor the cable af-
ter inducing the load. The whole as-
sembly of the Anchorage and the H.T.
Cable is named as "TENDON". The
Prestress system counterbalances the
stresses, produced by additional load-
ing on the members like live/imposed
load, thereby improving the range of
stresses to which a structural member
can safely be subjected.
III. METHODS OF PRESTRESSING
IN CONCRETE
Prestressing System can be classified
by two basic methods as:
I. 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 or
inside the hollow space of a box girder.
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This technique is adopted in bridges
and strengthening of buildings.
II. Internal Prestressing
When the prestressing is achieved by
elements located inside the concrete
member (commonly, by embedded ten-
dons), it is called internal prestressing.
Most of the applications of prestressing
are internal prestressing.
IV. EXTERNAL PRESTRESSING
External prestressing refers to a post-
tensioning method in which tendons
are placed on the outside of a struc-
tural member.The first use with exter-
nal steel tendons was in the 1950s, but
after that it lay dormant for some time.
Now external prestressing techniques
with steel tendons have been widely
used with success to improve existing
structures in the United States, Japan
and Switzerland. However there can be
a problem with corrosion in the steel
that forces the use of steel protection
on the external tendons, for example
by plastic sheeting. This problem can
be resolved by the use of FRP materi-
als. Therefore research in the area has
been conducted since the early 1970s.
In the beginning, glass FRP was used
but at the moment aramid and carbon
are mainly used due to higher modu-
lus of elasticity. FRP tendons lack the
ductility under extreme loading exhib-
ited by steel, which means that a FRP
prestressed beam may simultaneously
provide greater ultimate load capacity
and lower energy absorption than a
similar steel prestressed beam.
V. NEED FOR EXTERNAL
PRESTRESSING
• Because of the absence of bond be-
tween the tendon and structure,
external prestressing allows the re-
moval and replacement of one or
two tendon at a time so that the
bridge could be retrofitted in the
event of deterioration and their ca-
pacity could be increased easily.
This is essential for bridges in ur-
ban areas where traffic disruption
is undesirable.
• It usually allows easy access to an-
chorages and provides the ease of
inspection.
• It allows the adjustment and con-
trol of tendon forces easily.
• It permits the designer more free-
dom in selecting the shape of cross
section of bridges.
• Members could be made thinner
so that there is a reduction of dead
load.
• Concreting of new structures is im-
proved as there are no or few ten-
dons and bars in the section
• Friction losses are significantly re-
duced because external tendons
are linked to the structure only
at the deviation and anchorage
zones.
VI. DISADVANTAGES OF
EXTERNAL PRESTRESSING
• External tendons are more easily
accessible than internal ones, be-
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cause of that they are more vulner-
able to corrosion.
• Anchorage zones are vital addi-
tions to the cross section. These
elements must be designed to sup-
port large longitudinal and trans-
verse forces.
• In the case of internal tendons
with grouting, the long-term fail-
ure of anchor heads has limited
consequences because prestress-
ing may be transferred to the struc-
ture by bond. In the case of exter-
nal tendons, the behaviour of an-
chor heads is much more critical.
• At ultimate limit states, failure
with little warning due to insuf-
ficient ductility is a major concern
for externally prestressed struc-
tures.
VII. FUTURE DEVELOPMENT OF
EXTERNAL PRESTRESSING
The use of Fibre Reinforced Polymers
(FRP) as prestressing material has a lot
of advantages over conventional pre-
stressing materials such as the corro-
sion of prestressing steel is one of the
major reason for adopting FRP as pre-
stressing material. However the use
of FRP materials can be limited due
to some drawbacks such as relaxation
losses. These need to be further inves-
tigated.
VIII. CONCLUSION
External Prestressing has been widely used in construction of new bridges as
well as strengthening of existing bridges. Use external prestressing has resulted
in construction of several innovative bridges with large eccentricities and light
weight concrete.
While this technology is widely used in construction of bridges, there is need
for further research especially in the use of alternate prestressing material.
References
1) Prof. Yogesh Ravindra Suryawanshi, Ms. Manisha Bhise "Study of
Parameters Which Affect the Strength of Concrete by External Prestressing"
2) HÃˇekan Nordin "Strengthening structures with externally prestressed
tendons"
3) Thiru Aravinthan "Overveiw of External Post-Tensioning in Bridges"
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