losses in prestressed concrete. All losses explained.

1. Losses in Prestressed
concrete
Shubham Singh
16CM22F
National institute of technology Karnataka, surathkal

2. Introduction
 Prestressed
concrete is
a concrete
construction
material which is
placed
under compression
prior to it
supporting any
applied loads.

3. Types of Prestressed concrete
Pre-tension Post-tension
Pre-tensioned concrete is a
variant of prestressed
concrete where the tendons
are tensioned prior to the
concrete being cast.
Minimum grade of concrete
used is M40.
Concrete element is
manufactured remotely from
the final structure location
and transported to site once
cured.
Post-tensioned concrete is a
variant of prestressed
concrete where the tendons
are tensioned after the
surrounding concrete
structure has been cast.
Minimum grade of concrete
used is M30.
Concrete element can be
manufactured at the site
hence it can reduce
transportation cost.

4. Losses in Prestressed concrete
Pre-tension Post-tension
Short term
 Elastic shortening loss
 No frictional loss
 No anchorage slip
Long term
 Creep loss
 Shrinkage loss
 Relaxation loss
Short term
 No elastic shortening if all
bars are tensioned at same
time
 Frictional loss
 Anchorage slip
Long term
 Creep loss
 Shrinkage loss
 Relaxation loss

5.  High strength tendon bars have to use. As initial
prestress is around 1500-2000 N/mm2.
 Total number of loss is more in Post-tensioned concrete
compare to Pre-tensioned concrete.
 But total loss of prestress is more in Pre-tensioned
concrete compare to Post-tensioned concrete.
 Total loss of prestress is around 15-20%.
 Transportation of prestressed concrete is also a big
challenge.
 Heavy equipment and precise design.

6. Elastic Shortening loss
 In pre-tensioned concrete, when the prestress is transferred
to concrete, the member shortens and the prestressing steel
also shortens in it. Hence there is a loss of prestress.
 In case of post-tensioning, if all the cables are tensioned
simultaneously there is no loss since the applied stress is
recorded after the elastic shortening has completely
occurred.
 If the cables are tensioned sequentially, there is loss in a
tendon during subsequent stretching of other tendons.
 Loss due to elastic shortening is quantified by drop in
prestress (Δfp) in a tendon due to change in strain in tendon
(Δεp).
 The change in strain in tendon is equal to the strain in
concrete (εc) at the level of tendon due to prestressing
force.

7. Strain compatibility
 Loss due to elastic shortening is quantified by the drop in
prestress (∆fp) in a tendon due to change in strain in tendon
(∆εp).
 Change in strain in tendon is equal to strain in concrete (εc) at
the level of tendon due to prestressing force, which is called
strain compatibility between concrete and steel.
 Strain in concrete at the level of tendon is calculated from the
stress in concrete (fc) at the same level due to the prestressing
force.
 A linear elastic relationship is used to calculate the strain from
the stress.
Δfp=EpΔεp
=Epεc
=Ep(fc/Ec)
Δfp= mfc

8. Anchorage slip loss
 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.
 Percentage loss is higher for shorter members.
 Due to setting of anchorage block, as the
tendon shortens, there develops a reverse friction.
s
, = Slip of anchorage
L= Length of cable
A= Cross-sectional area of the cable
E = Modulus of Elasticity of steel
P = Prestressing Force in the cab
sP E
A L
where
 
 
 

le.

9. Frictional loss
Post-tensioned Members
 Friction is generated due to curvature of tendon, and
vertical component of the prestressing force.
 The magnitude of prestressing
force, Px at any distance, x from
the tensioning end follows an
exponential function of the
type.
A typical continuous post-tensioned member
 
o, P = Prestressing force at the jacking end
= Coeficient of friction between cable and the duct
umulative angle in radian throug
kx
x oP P e
where
C



 

 h which
the tangent to the cable profile has turned
between any two points under consideration
k = Friction coefficient

10. Relaxation loss
 Relaxation is the reduction in stress with time at constant
strain.
decrease in the stress is due to the fact that some of the initial
elastic strain is transformed in to inelastic strain under
constant strain.
stress decreases according to the remaining elastic strain.
 Factors effecting Relaxation :
Time
Initial stress
Temperature and
Type of steel.
 Relaxation loss can be calculated according to the IS 1343-1980
code.

11. Creep and Shrinkage loss
 Time-dependent increase of deformation under sustained load.
 Due to creep, the prestress in tendons decreases with time.
 For stress in concrete less than one-third of the characteristic
strength, the ultimate creep strain (εcr,ult) is found to be
proportional to the elastic strain (εel).
 The ratio of the ultimate creep strain to the elastic strain is
defined as the ultimate creep coefficient or simply creep
coefficient, θ.
εcr,ult = θεel
 IS: 1343 considers only the age of loading of the prestressed
concrete structure in calculating the ultimate creep strain.
 Creep is due to sustained (permanent) loads only. Temporary loads
are not considered in calculation of creep.

12.  Since the prestress may vary along the length of the member,
an average value of the prestress is considered.
 Prestress changes due to creep, which is related to the
instantaneous prestress.
 To consider this interaction, the calculation of creep can be
iterated over small time steps.
 The approximate value of shrinkage strain for design shall be
assumed as follows (IS 1383):
 For pre-tensioning = 0.0003
 For post-tensioning =
Where t = age of concrete at transfer in days.
10
0.002
( 2)Log t 