Post-Fire Deterioration and Prestress Loss in Steel Tendons used in Post-Tensioned Slabs

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Title: Post-Fire Deterioration and Prestress Loss in Steel Tendons used in Post-Tensioned Slabs

Author(s): K.J.N. MacLean, L.A. Bisby, and C.C. MacDougall

Publication: Special Publication

Volume: 255

Issue:

Appears on pages(s): 147-174

Keywords: assessment; fire; high-temperature creep; post-tensioned slabs; prestressing steel; relaxation; repair

Date: 10/1/2008

Abstract:
Unbonded post-tensioned (PT) concrete slabs have been widely used in Canada and the United States since the 1960s, as they allow increased span-to-depth ratios and excellent control of deflections compared to non-prestressed reinforced concrete flexural members. The satisfactory fire performance of unbonded, PT concrete slabs in North America was established by a series of standard fire tests performed in the United States during the 1960s. However, there is a paucity of data on the effect of elevated temperatures on cold-drawn prestressing steel, both in terms of post-fire residual mechanical properties and high-temperature stress relaxation, which can lead to significant prestress loss both during and after a fire. To aid in the post-fire evaluation of PT concrete floors, a series of high-temperature residual tension tests on prestressing steel is presented, along with a second series of tests that illustrate the irrecoverable and significant loss of prestress force that may result from steel relaxation (creep) during a fire. A preliminary model is presented that can be used to predict the change in prestress force and allow for the computation of flexural capacity of a PT slab after a fire.