Experimental Investigation of the Effect of Vertical Pre-Release Cracks in Prestressed Bridge Girders

Project summary:

The proposed project is an experimental study to investigate the effect of vertical pre-release cracks on girder strains, flexural cracking loads, and steel strand fatigue. Vertical pre-release cracks have been found to develop during construction of prestressed bridge girders. They develop at the top flange and penetrate vertically downward toward the bottom flange. The cracks are believed to generate as a result of shrinkage and cooling of the girders while they are still restrained in the bed (prior to release). Because the cracks close upon the release of the strands leaving no visible trace, they may often go undetected, and the effect of the cracks has been unclear. In a previous study regarding high-strength prestressed bridge girders, the researchers suspected that the observed pre-release cracks were the cause for the girder to undergo flexural cracking at a lower load than predicted. To further investigate the effect of the pre-release cracks, an analytical/experimental study was proposed. MnDOT funded the initial analytical phase of the research as s pilot study before funding the experimental portion (focus of current proposal). The finite element analytical investigation of the effect of the vertical pre-release cracks in prestressed bridge girder construction confirmed that local stress concentrations develop in the bottom flange of girders in the vicinity or pre-release cracks. The result is that flexural cracks will develop in the bottom flange of the girder earlier than predicted. The reduction in flexural cracking load was found to be dependent on the size and depth of the flexural cracks. The occurrence of pre-release cracks may have detrimental effects on long-term durability and fatigue life of bridge girders. A reduction in the flexural cracking capacity may cause the girders to crack under service loads or more likely under an overload. Once the girder is cracked, the concrete no longer has any tension capacity, and lower loads ma