Effects of production practices on camber of prestressed concrete bridge girders

Abstract

Although predicting camber can be difficult because it depends on so many random variables, it is necessary to be accurate since too much discrepancy between the predicted and actual camber can result in deck construction problems. In this paper, the factors related to prestressed concrete girder production that could affect the camber are investigated. Camber prediction methods also are discussed. The actual concrete compressive strength was found to be an average of 25% higher than the specified transfer strength at prestress transfer, thus affecting the camber predictions. At 28 days, the actual compressive strength was an average of 45% higher than the specified 28-day strength. The study also showed that camber behavior was varied depending on different girder types and curing methods. Camber was affected significantly by the deformation of internal void forms in box beams and cored-slab girders due to the hydrostatic pressure of the fresh concrete during casting. Strand debonding and transfer length also were important in camber prediction for some girders. The authors propose a refined camber prediction method that uses creep coefficients and prestress losses based on the 2010 AASHTO LRFD Bridge Design Specifications. An approximate method based on PCI Design Handbook: Precast and Prestressed Concrete camber multipliers was also developed. To evaluate these methods, the measured cambers of 382 prestressed concrete bridge girders were compared with predicted values. Both methods were found to provide reasonable estimates of camber. However, for most girder types and curing methods, the refined method was more accurate.