Losses in Prestress

1. Sharad Institute of Technology
College of Engineering ,Yadrav
Losses in Prestress
Presented By
Mr Y S Patil
Department of Civil Engineering
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2. The losses are majorly classified into two groups,
immediate (Short term) and time-dependent (Long
term). The immediate losses occur when the tendons
are pre stressed and pre stress is transferred to the
concrete part, and time-dependent occurs when the pre
stress members are used or in its service life. The
various types of losses are listed below.
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3. Immediate (Short term)
Losses due to friction.
Losses due to elastic shortening of concrete.
Losses due to anchorage slip.
Time-Dependent Losses (Long term)
Losses due to creep of concrete.
Losses due to shrinkage of concrete.
Losses due to creep or relaxation of steel.
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4.  Pre-tensioning and post-tensioning are the two
procedures for imparting prestressing to a concrete
member. Pre-tensioning occurs before the concrete is
cast, while post-tensioning occurs after the concrete has
hardened. Various forms of losses can occur depending
on the pre stressing technique.
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5. Losses in Pre-tensioning
1. Losses due to elastic shortening of concrete.
2. Losses due to creep or relaxation of steel.
3. Losses due to shrinkage of concrete.
4. Losses due to creep of concrete.
Losses in Post-tensioning
1. Losses due to friction.
2. Losses due to creep or relaxation of steel.
3. Losses due to shrinkage of concrete.
4. Losses due to creep of concrete.
5. Losses due to anchorage slip.
When all the bars are tensioned simultaneously, there are no losses owing to elastic
shortening in the post-tensioning. However, if the bars are successfully
tightened, prestress will be lost owing to elastic shortening.
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6. Loss in Prestress Due to the Friction
During stretching a curved tendon in a post-tensioned part, friction
is formed at the interface of concrete and steel. There is a reduction
in prestress along with the member from the stretched end. The
stretching must also overcome the tendon’s wobbling in addition to
friction. The change in location of the tendon along the duct is
referred to as the wobble. Friction and wobbling losses are grouped
together under friction. Frictional losses do not arise in pre
tensioned members since the tendon is not concretely tensioned in
pre-tensioned members.
Loss of prestress due to the friction (σf)= (P0/A)(Kw X+αμ)
where,
Kw= wobbles friction loss
X = L: When jacking from one end
X = L/2: when jacking from both the ends
α=Angle of a tendon (Radian)
μ=Coefficient of friction
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7. Losses in Prestress Due to Anchored Slip
Anchorage is a component that is used to attach the tendons
to the concrete while terminating them. When the stressing
process is over, the major role of anchorage is to transfer
the stressing force to the concrete. Tendons are provided
inside the ducts of a precast concrete member in the case of
a post-tensioning system. To transfer the stressing force to
the concrete, anchoring is supplied at both ends of the
tendon. If the anchorage moves from its original position,
the tendons loosen, resulting in prestress loss. The loss of
stress due to anchorage slip does not occur in the pre-
tensioned concrete. The tendons are monolithically
implanted in the concrete when pre-tensioning.
The loss due to anchorage slip (σa)= (Δl/l)Es
where,
Δl = anchorage slip in mm
l = Length of cable in mm
Es= Young’s modulus of steel (N/mm2)
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8. Losses in Prestress Due to Elastic Shortening of Concrete
The loss is only encountered in the Pre-tension member;
when the tendons are cut, and the prestressing force is
transferred to the member, the concrete undergoes
immediate shortening due to the prestress. In addition, the
tendon shortens by the same amount. This results in a loss
of prestress. Whereas if there is only one tendon, there is no
loss because the applied prestress is recorded in the Post-
tensioned member after the elastic shortening of the
member. When more than one tendon is stretched
sequentially, a tendon is lost during the successive
stretching of the other tendons.
Loss of prestress due to elastic shortening of concrete
(σe)= mσc
where:
m = Modular ratio
σc= Stress in the concrete at the level of a steel tendon
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9. Creep is the deformation that occurs over time due to a
constant force. Prestress is a permanent force in prestressed
concrete that causes compressive stress at the steel level. As
a result, the member is under stress. Loss of prestress due to
concrete creep occurs in both the pretension and post-
tension member.
Loss of prestress due to creep of concrete
(σcr)= θmσc
where,
m = Modular ratio
σc= stress in the concrete at the
level of a steel tendon
θ = Creep coefficient
(depends on the age of loading)
Age of loading Creep
Coefficient
7 Days 2.2
28 Days 1.6
1 Year 1.1
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10. Steel relaxation is described as a decrease in stress over
time when under constant tension; due to the relaxation
of steel, the pressure in the tendon is reduced with time.
The stress loss due to relaxation depends on the types
of steel, initial pressure, and temperature. Based on the
observation, about 2 per cent loss occurs in pre-
tensioned members, whereas about 3 per cent loss of
stress occurs in post-tension members.
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11. Losses in Prestress Due to Shrinkage of Concrete
The stress loss is aided by the shortening of tensioned
wires caused by concrete shrinkage in prestressed
members. In the case of pre-tensioned members,
moist curing is usually used to keep them from
shrinking until they are transferred. As a result, the
total residual shrinkage strain in pre-tensioned
members after prestress transfer will be greater than
in post-tensioned members, where a portion of
shrinkage will have already occurred at the time of
stress transfer.
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12. Sharad Institute of Technology College of Engineering ,Yadrav
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