Research Reports

Effects of Pre-Release Cracks in High-Strength Prestressed Girders

Principal Investigator:

Tina Wyffels, Catherine E French, Carol K Shield

January 2000

Report no. MnDOT 2000-25

Projects: Effect of Pre-release Cracks on Prestressed Bridge Girder Performance including Remaining Fatigue Life

Topics: Bridge design and sensing

Pre-release cracks have been observed during the fabrication process of some prestressed concrete girders. The pre-release cracks were observed to begin at the top flange and extend into the depth of the section, sometimes penetrating through the entire depth. The cracks close due to the effects of prestressing and girder self-weight when the prestressing strands are released. The objective of this report was to determine the effects these pre-release cracks have on girder camber, flexural cracking capacity, and steel stress ranges. The research included a parametric study investigating stress ranges in the prestressing strands in uncracked, cracked, and partially cracked girder sections to determine if steel fatigue was a concern. An analytical study was also performed which modeled several pre-release cracks, including models of two experimental girders that developed pre-release cracks, to determine the effect of various cracks on girder stress and camber. It was found that steel fatigue in the prestressing strand is a concern in girders that become cracked in service. Fatigue of the steel strands has typically not been a concern in prestressed girders because the girders are designed so the section remains uncracked under service load. However, a loss of compressive stress is believed to occur in the bottom fiber of the girder due to pre-release cracks, which may result in the section cracking at a lower applied load. The loss of compressive stress in the bottom fiber of girders with pre-release cracks was determined using finite element modeling. Additional results of the analytical models were that pre-release cracks result in a loss of girder camber, the effects of the pre-release cracks remained local to the crack location, non-linear stress distributions occurred during the process of crack closure, and the magnitude of the pre-release crack effects was dependent on the number of cracks, the crack width, and the crack depth. Keywords: pre-release cracks, prestressed concrete girders, flexural cracking, fatigue

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