prestressing

1. Introduction to Prestressing
CE 572
Purdue University
School of Civil Engineering
Julio Ramirez

2. CE572/Spring 04/JAR 2
Definition of Prestressing
It consists of preloading the structure
before application of design loads in such
a way so as to improve its general
performance.

3. CE572/Spring 04/JAR 3
Objectives of Prestressing
Control or eliminate tensile stresses in the
concrete (cracking) at least up to service
load levels.
Control or eliminate deflection at some
specific load level.
Allow the use of high strength steel and
concrete.

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Net Result of Prestressing
Improved performance of concrete in
“ordinary” design situations (compared to
R/C).
Extended range of application of structural
concrete (longer spans).
Innovative forms of structures.

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Extended Range of Application

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Examples of Structures

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Partial Prestressing
Intermediate state, in which tension and
usually flexure cracking is allowed at full
service load level. Some advantages are:
- Prevents excessive camber at more
typical loads less than full service
loads.
- Reduced prestressing force (less $$$)
compared to full prestressing (zero
tensile stresses at full service load).

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Applications of Prestressed
Concrete in the US and Canada –
Figure 1
About 200,000 tons of prestressing steel used in
US/Canada each year (~ ¼ of the total world
consumption)

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Prestressing Methods
Pretensioning: suited for mass production.
*Video (PCI)

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Prestressing Methods
Post-Tensioning: jacking against the member
and the eccentricity is easily varied along the
length of the member.

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Post-Tensioning Sequence
Place reinforcing cage and post-tensioning ducts
inside the formwork.
After casting and curing of the concrete, the
tendons are tensioned and anchored with
special jacks that react against the member.
Unless unbonded tendons are being used, the
duct is then grouted to complete the post-
tensioning operation.
Slides (PTI)-------

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Profiles of Post-Tensioned Tendons

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Geometry of Parabolic Profiles

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Location and Eccentricity of
Tendon in Duct After Stressing

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Materials: Types of Prestressing
Steel

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Production of Seven Wire Strand
Low relaxation versus stress-relieved

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Concrete

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Hognestad Parabola
fc ε( ) fcmax 2
ε
εo
⋅






ε
εo






2
−






⋅






:=
Calculate εo (strain corresponding to peak stress, fcmax)
using the secant Modulus of Elasticity at 0.5fcmax. Thus, fit
The Hognestad expression through a point corresponding to
0.5fcmax and 0.5fcmax/Ec.
Ec 57000
2
fcmax( )
1000
⋅:=