Effect of Vertical Pre-Release Cracks on Prestressed Bridge Girders
Eray Baran, Catherine E French, Carol K Shield
Report no. MnDOT 2003-33
Topics: Bridge design and sensing
Vertical cracks near the midspan of large-sized prestressed concrete bridge girders may develop during the curing process and can extend through the depth of the girder. The cracking is attributed to restrained shrinkage and thermal effects prior to release of the prestressing strands. Eighteen full-scale MnDOT Type 28M prestressed concrete beams were tested to investigate the effects of the cracks on the performance of the beams. Thirteen beams tested in this study incorporated manmade pre-release cracks. All of the beams were tested under static loading to investigate the effects of pre-release cracks on concrete strains, flexural crack initiation and re-opening loads, overall beam stiffness, and ultimate flexural capacity. Three of the beams were subjected to cyclic testing to evaluate possible effects of the pre-release cracks on the strand stress ranges and fatigue life of the beams. Unlike the field observations, the pre-release cracks in the test beams did not close completely under the beam's weight and pre-stressing force. The pre-release cracks were found to cause changes in beam strains around the crack locations. The overall stiffnesses of the beams were also affected by the reduction in the moment of inertia of the pre-release crack section. Following pre-release crack closure, the beams recover the stiffness comparable to that of the uncracked beams. No significant effect of pre-release cracks was observed on the behavior of the beams near the ultimate capacity. Results from the cyclic testing of three beams indicated that a beam that develops pre-release cracks is more likely to experience fatigue problems and tend to cause a reduction in the beam's fatigue life. Guidelines are proposed for the assessment of girders that develop pre-release cracks during production.