Prestressed Concrete Preliminaries

8/9/2019 Prestressed Concrete Preliminaries

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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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Posttensioning:

What is Post Tensioning?

Jacking Systems:

Typical Capacity: 10 to 500 Tons

Stroke Length: 6 to 48 in

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A 500 Ton Jack:

Anchoring Systems:

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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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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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Prestressed Concrete Preliminaries

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