Causes of excessive detensioning stresses in northeast extreme tee (NEXT) beams

During prestress release, northeast extreme tee (NEXT) bridge beams crack in their flanges and webs at beam ends in recurrent patterns. These cracks may lead to corrosion in pretensioned and mild reinforcement. Causes of flange cracks in NEXT beams were investigated in this paper using nonlinear finite element analysis that captures stress redistribution in concrete upon cracking and using field measurements of reinforcing bar strains. A torsion-based analytical method was also used to explore the impact of differential camber between stems of skewed beams. The individual contributions of prestress, uncut strand restraint, skew, thermal strains, formwork restraint, and torsion due to differential camber to detensioning cracks were studied. Torsion with formwork restraint and high skew angles were the largest contributors to end cracking. Flange cutouts, flexible supports, or removable formwork are recommended for crack control. The torsion-based method may also be suitable for predicting flange stresses and detailing reinforcement.