Post-tensioned box girder bridges are one of our most economical highway design solutions. In 2000, Caltrans began construction on 35 structures of this type, totaling over $88M in construction costs. In the new AASHTO LRFD Specification, the lump sum approximate method for evaluation of losses only applies to spans less than 160 ft (50m), and the refined method for spans less than 240 ft (75m). Research is needed to either confirm the application of these methods to boxgirders and segmental bridges with longer span lengths and higher concrete strengths, or suggest an alternative. If losses in any bridge are over-estimated, excessive camber and improper drainage could result. If losses in the same structures are under-estimated, stresses will be underestimated, and cracking could be excessive - even to the point of failure. If designers must individually investigate this topic for each bridge designed, production will decrease.

The overall reearch objective is to asess the time-dependent lump sum and refined prestress loss estimates based ont he current AASHTO LRFD Bridge Design Specifications [3]. Previous or more recent research that shows promise will be reviewed. Changes to the current lump sum and refined prestress loss models will be suggested, if necessary, for application to cast-in-place post-tensioned bridges. The prestress loss models should be assessed by means of long-term field measurements and computer time-step analyses of cast-in-place post-tensioned box girder bridges.

1) Review previous work, and NCHRP 18-07 "Prestress Losses in Pretensioned High-Strength Concrete Bridge Girders". This effort would in essence repeat 18-07, but for post-tensioned rather than prestressed superstructures. 2) Summarize and comment on current methods for estimating losses in post-tensioned bridges, as they apply to boxgirders, segmental structures, post-tensioned structures that utilize high-strength concrete, and span lengths in excess of present limitations in the AASHTO LRFD Specification. 3) Field-measurements: The study of cast-in-place post-tensioned concrete must be executed in partnership with construction contractors. Measurement devices would be embedded prior to concrete placement. Data would be collected after stressing, dropping of falsework, placement of barriers and wearing surface, and periodically once in service, for two-to-three years. Field measurements will be compared to both approximate lump-sum estimates and those using refined methods per the AASHTO LRFD Specifications, plus any alternative proposed method, for at least three (3) structures of various span lengths. 4) Make design recommendations in the form of proposed changes to the AASHTO LRFD Specifications.