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For this research, prestress losses were monitored in six HPC bridge girders. These measured losses were compared to predicted losses according to four sources. Prestress loss predictive methods considered for this research were: 1- AASHTO LRFD 2004, 2- AASHTO LRFD 2004 Refined, 3- AASHTO LRFD 2007, and 4- AASHTO LRFD Lump Sum method. On the other hand, the camber prediction methods used in the present research were: 1- Time dependent method described in NCHRP Report 496, 2- PCI multiplier method, and 3- Improved PCI Multiplier method. For the purpose of this research, long-term prestress losses were monitored in select girders from Bridge 669 located near Farmington, Utah. Bridge 669 is a three-span prestress concrete girder bridge. The three spans have lengths of 132.2, 108.5, and 82.2 feet long, respectively. Eleven AASHTO Type VI precast prestressed girders were used to support the deck in each span. The deflection of several girders from a three-span, prestressed, precast concrete girder bridge was monitored for 3 years. Fifteen bridge girders were fabricated for the three span-bridge. Ten girders from the exterior spans had span length of 80 feet, and five girders from the middle span had span length of 137 feet. From the results of this research, in both the 82- and 132-foot-long, the AASHTO LRFD 2004 Refined Method does a better job predicting the prestress loss and it can be concluded that all the prediction methods do a better job predicting the loss for the larger girders. The Lump Sum method predicted very accurately the long term prestress loss for the 132-foot-long girders.
Pretensioned concrete girders are currently fabricated using 0.5- or 0.6-in. diameter prestressing strands. In recent years, however, it has become of interest to employ larger-diameter 0.7-in. diameter strands to reduce the number of strands and improve the efficiency of pretensioned concrete members. Such a transition requires a considerable initial investment that needs to be justified based on the benefits obtained. Furthermore, the use of 0.7-in. strands would increase the stresses within the end-region of pretensioned elements, which could lead to undesirable cracking and impact the serviceability of the girders. The work presented in this thesis consists of 1) a comprehensive parametric investigation to evaluate the benefits and limitations of using 0.7-in. strands in pretensioned bridge girders, and 2) a full-scale experimental study to investigate the behavior of pretensioned concrete girders with 0.7-in. strands at the time of prestress transfer. The parametric investigation was accomplished by designing thousands of bridge girders with different span lengths, concrete release strengths, and transverse spacings. The results showed that the most noticeable benefit of 0.7-in. strands over 0.6-in. strands was a reduction of up to 35 percent in the number of strands. However, the difference in the total weight of prestressing steel was insignificant. Increasing the release strength of concrete, at least to 7.5 ksi, was found essential to observe benefits in design aspects other than the number of strands. The experimental investigation involved the fabrication of two Tx46 and two Tx70 specimens at the Ferguson Structural Engineering Laboratory. All specimens employed 0.7-in. strands on a 2- by 2-in. grid and the standard detailing currently used for girders with smaller-diameter strands. The observed crack widths in the specimens upon prestress transfer did not exceed those typically observed in Tx-girders with smaller-diameter strands. Therefore, the use of 0.7-in. strands does not seem to trigger a need to modify the end-region detailing in Tx-girders. However, noticeably greater bursting and spalling forces were observed in the end regions of the specimens compared to the demands predicted by AASHTO LRFD provisions. The measured 24-hour transfer length from the specimens also exceeded estimates by AASHTO LRFD and ACI 318-14 provisions.
Bridges B-0071, and B-0171 in Hamilton County, Ohio have been in service for about fifty years. They are short span bridges with prestressed concrete girders. Until late 2001, they had conventional reinforced concrete decks. On November third of that year the ribbon was cut to reopen the bridges, now with Fiber Reinforced Polymer Decks. One of the bridges also had the girders replaced. These are the only bridges in existence that have FRP decking on concrete girders. The Hamilton County Engineers Office contracted with the Civil Engineering Department at the University of Cincinnati to perform research on these bridges. Information gained from this research will seek to confirm the safety of the new technology, approve construction and design techniques with reference to the FRP deck, and determine overall performance of the bridge to provided understanding of the system. The 50-year-old prestressed concrete girders were subjected to destructive load testing. The girders showed little loss of strength or stiffness from aging. The information on the performance of the girders was used in the analysis of the bridge system. Two of the bridges were subjected to nondestructive load testing. A three-dimensional finite element model was then created to replicate the performance of the bridges. Data from the bridge tests provided enough information to create an accurate model of the bridge girders, but not the deck. Using the finite element model, a Load Rating was performed. The bridges were found to be sufficiently strong to resist the loads that may be applied to them. The deck showed no signs of separation from the concrete girders as was previously suspected. The bridge system acted as a fixed end beam because of the semi-integral end abutments for the range of loads tested. The deck was not adding any strength tot the girders through composite action. The load transfer from one girder to another was not provided by the deck as was assumed in the design process, but by the concrete diaphragms used for lateral stability. Further testing will be needed to understand the deck performance better so that a full bridge analysis may be performed.