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Factors Affecting Bond and Friction Losses in Multi-Strand Post-Tensioning Tendons Including the Effect of Emulsifiable Oils

Luthi, Tanya

2005

Emulsifiable oils are often used in post-tensioned construction to reduce friction losses and to provide temporary corrosion protection for tendons during the period of time between stressing and grouting. In the past, oils were flushed from the ducts with water prior to grouting. This practice often led to voids in the grout and created environmental problems related to the disposal of the flushing water.

This thesis addresses the effect of emulsifiable oils on corrosion, bond, and friction losses and is a summary of work done by multiple researchers at Pennsylvania State University and The University of Texas at Austin. Based on preliminary corrosion and pullout tests at Pennsylvania State University, two oils were chosen for large-scale bond and friction tests at The University of Texas at Austin. Large-scale tests investigated the effects of duct type and oil on bond and friction losses.

Overall bond test results indicate that corrugated galvanized ducts provide better development than corrugated HDPE ducts. Rigid steel pipes performed poorly due to failure at the duct-concrete interface, indicating the need for shear studs or connectors to provide better anchorage for smooth steel deviator pipes. Even though such studs anchor the pipe effectively, the plane of failure changed to the inside of the pipe-grout interface, and bond results were substantially below those for corrugated ducts.

Bond test results also indicate that the strength of post-tensioned specimens with oiled tendons is similar to or better than the strength of specimens with unoiled tendons. Specimens with oiled tendons did experience large amounts of slip in comparison to specimens with unoiled tendons. However, because service load level cracking often will not occur in precast segmental structures and can easily be controlled with additional mild steel in cast-in-place posttensioned applications, slip behavior is less important than strength.

Overall friction test results indicate that current design values for the coefficient of friction for steel pipes and galvanized ducts are accurate. However, the coefficient of friction for HDPE ducts measured from this test program was significantly less than the value recommended by AASTHO.

Friction tests also indicate that lubrication reduces the friction coefficient on the order of 15% if the tendon is stressed when the oil is fresh. Friction loss reductions were significant in rigid steel pipes and HDPE ducts but were relatively insignificant in galvanized steel ducts.

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