, Professor, Civil, Environmental and Geo-Engineering
During fabrication of prestressed concrete bridge girders, the strands are pretensioned in the precasting bed prior to casting the concrete. Because the beds are of fixed length, changes in temperature affect the mechanical stress in the strands. Currently, the strand stress is adjusted during tensioning to account for the difference in temperature between strand pull and the concrete at placement. Since it is believed that temperatures may increase to the hydration temperature (e.g., 160 degrees Fahrenheit) prior to bonding, the implication is that the strand may undergo larger losses in prestress prior to bonding that are currently unaccounted for in design.
The Minnesota Department of Transportation has reported erection cambers of many prestressed concrete bridge girders that were much lower than anticipated. A previous University of Minnesota study attributed the discrepancies to inaccurate estimates of the concrete strength and stiffness at release and strand force loss due to temperature during fabrication. The objective of this study was to further investigate the effects of temperature on strand force and camber during precast, prestressed girder fabrication and to make recommendations for the design and fabrication processes to reduce the potential loss of prestress due to temperature effects during fabrication and to improve the release camber estimation. A thermal effects analysis was developed based on four key steps in the girder fabrication process: tensioning, concrete-steel bond, release, and normalization. The study included fabricating six short, prestressed concrete segments released at early ages to determine the time/temperature associated with bonding the prestressing strand to the concrete. To investigate the non-recoverable prestress losses during girder fabrication, four sets of girders (MN54 and 82MW) were instrumented with thermocouples, strain gages, and in some cases load cells, which were monitored during the fabrication process to separate the thermal and mechanical strain components. Effects investigated included casting during a cold season, casting during a warm season, casting with the free length of strand covered, and casting with different bed occupancy during any season. A recommended procedure for adjusting strand force during tensioning was proposed to account for non-recoverable strand force changes due to temperature changes between tensioning and bond.