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Prestressed Concrete
Preliminaries
Fawad MuzaffarM.Sc. Structures (Stanford University)
Ph.D. Structures (Stanford University)
Civil Engineering
Department
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Why Do We Need to Prestress?
Concrete is strong in compression and weak in tension
Tensile Strength of Concrete is 8 to 14 % of Compressive Strength
Flexural Cracks Develop Early
Can be counteracted by application of compressive stresses
Prestressing allows optimal utilization of compressive stresses inconcrete
Some Examples
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Materials Concrete
Qualities of InterestStrength , Endurance
Categories of Concrete Properties
i. Short Term PropertiesCompressive, Tensile and Shear
Strength, Modulus of Elasticity
ii. Long Term Properties Creep and Shrinkage
Compressive Strength,
Depends onMix Design, Aggregate Properties, Time and Quality of
Curing.
Varies from 4,000 to 12,000 psi (can be as high as 20,000 psi)
Determined using 6 in 12 in Cylindrical Specimens
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Materials Tensile Strength,
Difficult to measure because of gripping problems
Brazilian or Splitting Cylinder Test
For flexural members, Modulus of Rupture is used instead of
is obtained by lateral loading of 6 in square beams of 18 in span
at their third points
0.10 0.20
= 7.5 (Normal Weight); = 0.75 7.5
(Light Weight)
= 0.85 7.5 (Sand-Light Weight)
Shear Strength:
Difficult to measure, because it is difficult to isolate shear effects from
other stresses
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Materials From literature, Shear Strength varies from 20 to 85 % of
depending on loading conditions
Shear Strength:
Difficult to Measure, because it is Difficult to Isolate Shear Effects from
other Stresses Shear Strength Rarely
Controls Design
Typical StressStrain Curves of
Concrete
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Materials
Modulus of Elasticity, Tangent Modulus:
Initial Tangent Slope
Secant Modulus:
Slope of Line Connecting aCertain Point (Usually 0.4
)to
origin.
For Light Weight Concrete, ACI
Building Code gives
For Normal Weight Concrete
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Materials Creep:
Definition: Increase in compressive strain with sustained compressive
Load
An example of the relative numerical values
Stress Strain relationship for short
Term loading lose significance with time. Creep Recovery
Note:
Concrete resistant to shrinkage is
also presents a low creep tendency
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Materials Shrinkage:
Two types of shrinkage effects exist
i. Plastic Shrinkage
Occurs during the first few hours of placing the concrete in
forms
Associated with
ii. Drying Shrinkage
Occurs after the concrete has already attained its final set.
Represents Moving of Water In and Out of Concrete
Swelling Increase in volume of concrete due to hydration of concrete
Loss of volume due to shrinkage is not completely reversible
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Materials Non-Prestressing Reinforcement
Parameters of Interest
Typical Reinforcement Used are Grade 40, 60 and 75 steels.
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Materials Prestressing Reinforcement
Three types of Prestressing Reinforcement Exists.
Stress-Relieved and Low Relaxation Wires and Strands
7-wire strands are made by twisting 6 cold-drawn wires around a
single slightly larger control wire.
The assembly of wires can be drawn through a die to compact the
strand to maximize steel area.
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Materials Typical Stress-Strain Plots
Steel Relaxation
Decrease in tensile steel stress due to constant tensile steel strain
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Materials
Low Relaxation Steel: Strands that are Stabilized by stretching to upto
70% of their ultimate strength at 20 to 1000
C has a relaxation loss that is25% of the normal stress-relieved strands. These strands are known as
Low Relaxation Strands.
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Prestressing Systems and Anchorages
Pretensioning:
What is Pretensioning?
Hold Down Devices For Harped Profile
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Prestressing Systems and Anchorages
Posttensioning:
What is Post Tensioning?
Jacking Systems:
Typical Capacity: 10 to 500 Tons
Stroke Length: 6 to 48 in
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Prestressing Systems and Anchorages
A 500 Ton Jack:
Anchoring Systems:
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Prestressing Systems and Anchorages
Ducts:
Placed in concrete to form cavities in post-tensioned members.
Duct Size: For Tendons, the duct area should be at least twice the net area
of prestressing steel.
Grout Openings or Vents: Vents should exist at both ends.
For draped profiles, all high points should have a
grout vent.
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Prestressing Systems and Anchorages
Grout vents or drain holes should be
provided at low points if post tensioning is
to be done in freezing climate.
Placement of Ducts:
Ducts should be securely fastened at close intervals to avoid
displacement during concreting.
Holes in ducts must be repaired prior to placement of concrete.
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The Ten Principles
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The Ten Principles
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