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

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CE572/Spring 04/JAR2 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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CE572/Spring 04/JAR3 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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CE572/Spring 04/JAR4 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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CE572/Spring 04/JAR5 Extended Range of Application

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CE572/Spring 04/JAR6 Examples of Structures

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CE572/Spring 04/JAR7 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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CE572/Spring 04/JAR8 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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CE572/Spring 04/JAR9 Prestressing Methods Pretensioning: suited for mass production. *Video (PCI)

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CE572/Spring 04/JAR10 Prestressing Methods Post-Tensioning: jacking against the member and the eccentricity is easily varied along the length of the member.

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CE572/Spring 04/JAR11 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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CE572/Spring 04/JAR12 Profiles of Post-Tensioned Tendons

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CE572/Spring 04/JAR13 Geometry of Parabolic Profiles

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CE572/Spring 04/JAR14 Location and Eccentricity of Tendon in Duct After Stressing

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CE572/Spring 04/JAR15 Materials: Types of Prestressing Steel

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CE572/Spring 04/JAR16 Production of Seven Wire Strand Low relaxation versus stress-relieved

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CE572/Spring 04/JAR19 Concrete

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CE572/Spring 04/JAR20 Hognestad Parabola Calculate ε 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 to 0.5fcmax and 0.5fcmax/Ec.

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Prestress Force Levels Po = Jacking Force Anchor SlipFriction Pi = Initial (Transfer) Force CreepShrinkage Pe = Effective Force Relaxation Elastic Shortening

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