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Introduction to PrestressingCE 572

Purdue University

School of Civil Engineering

Julio Ramirez

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Definition of Prestressing

It consists of preloading the structurebefore application of design loads in sucha way so as to improve its general

performance.

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Objectives of Prestressing

Control or eliminate tensile stresses in theconcrete (cracking) at least up to serviceload levels.

Control or eliminate deflection at somespecific load level.

Allow the use of high strength steel andconcrete.

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Net Result of Prestressing

Improved performance of concrete inordinary design situations (compared to

R/C).

Extended range of application of structuralconcrete (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 andusually flexure cracking is allowed at fullservice load level. Some advantages are:

- Prevents excessive camber at moretypical loads less than full serviceloads.

- 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 1About 200,000 tons of prestressing steel used in

US/Canada each year (~ 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 memberand the eccentricity is easily varied along thelength of the member.

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Post-Tensioning Sequence

Place reinforcing cage and post-tensioning ductsinside the formwork.

After casting and curing of the concrete, the

tendons are tensioned and anchored withspecial jacks that react against the member.

Unless unbonded tendons are being used, theduct 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( ) fcmax 2

o

o

2

Calculate o strain corresponding to peak stress, fcmax)using the secant Modulus of Elasticity at 0.5fcmax. Thus, fit

The Hognestad expression through a point corresponding to0.5fcmax and 0.5fcmax/Ec.

Ec 57000

2fcmax( )

1000