A Comparison of Methods for Experimentally Determining Prestress Losses in Pretensioned Prestressed Concrete Girders

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Title: A Comparison of Methods for Experimentally Determining Prestress Losses in Pretensioned Prestressed Concrete Girders

Author(s): E. Baran, C.K. Shield, and C.E. French

Publication: Special Publication

Volume: 231

Issue:

Appears on pages(s): 161-180

Keywords: flexural cracking; prestress losses; prestressed concretegirders; testing

Date: 10/1/2005

Abstract:
This paper presents a description and comparison of several experimentaltechniques used to determine the effective prestressing force in pretensionedprestressed concrete girders. The effective prestressing force was determined by threemethods: (1) using vibrating wire strain gages that were embedded in the girders duringfabrication; (2) load testing the girders to determine flexural cracking and crack re-opening loads and then back calculating the losses; and (3) exposing a length ofstrand, attaching resistance strain gauges on the strands, and flame-cutting theinstrumented strands.Several instruments were used to determine the flexural crack initiation and crack re-opening loads. These included crack detection gages, concrete surface strain gauges,and LVDTs, as well as visual observations. Use of data from the strain gauges placed atthe bottom surface of the girders was determined to be the most effective way ofdetecting flexural crack initiation and re-opening. Cracking loads determined fromvisual observation were significantly larger than those determined from the straingauge data.The back-calculated prestress losses determined from the measured flexural crackinitiation and re-opening loads of the girders were significantly larger than thosedetermined from other experimental methods and those predicted by the PCICommittee and AASHTO LRFD methods. Losses determined by the other experimentalmethods and the predictions correlated more closely with those back-calculated usingvisually-observed cracking loads.These results indicate that prediction of losses based on the measured flexuralcracking and crack re-opening loads using the basic theory of mechanics results in anoverestimation of prestress losses. Consequently, girders may undergo flexuralcracking and crack re-opening at lower loads than predicted using the basic theory ofmechanics.