Introduction to Prestressed Concrete

Introduction to PrestressingCE 572

Purdue UniversitySchool of Civil Engineering

Julio Ramirez

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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.

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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 PrestressingIntermediate 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 (zerotensile 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 MethodsPretensioning: suited for mass production.

*Video (PCI)

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

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Post-Tensioning SequencePlace 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 StrandLow relaxation versus stress-relieved

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Concrete

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Hognestad Parabola

fc ε( ) fcmax 2εεo⋅

⎛⎜⎝

⎞⎟⎠

εεo

⎛⎜⎝

⎞⎟⎠

2−

⎡⎢⎣

⎤⎥⎦

⋅⎡⎢⎣

⎤⎥⎦

:=

Calculate εo (strain corresponding to peak stress, fcmax) using the secant Modulus of Elasticity at 0.5fcmax. Thus, fitThe Hognestad expression through a point corresponding to 0.5fcmax and 0.5fcmax/Ec.

Ec 570002 fcmax( )

1000⋅:=

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Prestress Force Levels

Po = Jacking Force

Anchor Slip Friction Elastic Shortening

Pi = Initial (Transfer) Force

Creep Shrinkage Relaxation

Pe = Effective Force

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