Analysis of the effects of vertical pre-release cracks on prestressed concrete bridge girders

Vertical cracks, termed "pre-release cracks," beginning at the top flange and propagating downward into the section depth have been observed to develop during the production of long-span prestressed concrete bridge girders. The cracking, which is attributed to the restrained shrinkage of the concrete and thermal effects during the curing period prior to release of the prestressing strands, tends to be more critical for long-span girders with deep sections and large amounts of prestressing strands. Studies were conducted at the University of Minnesota to investigate the effects of pre-release cracks on the behavior of bridge girders. The effects of the size of pre-release cracks on beam stresses and crack-closure behavior were studied with finite element models. Full-scale Mn/DOT Type-28M prestressed concrete bridge girders incorporating purposely-made pre-release cracks were tested under four-point bending to verify the findings. Closing of the pre-release cracks and the beam strains and deflections were monitored continuously during the load tests. The pre-release cracks were found to cause local changes in the beam strains near the crack locations. Beams with pre-release cracks were observed to undergo flexural crack initiation and crack re-opening at lower loads than similar beams without pre-release cracks.