The prestress loss estimation provision in the AASHTO LRFD Bridge Design Specifications was recalibrated in 2005 to be more accurate for "high-strength [conventional] concrete." Greater accuracy may imply less conservatism, the result of which may be flexural cracking of beams under service loads. Concern with a potential lack of conservatism and the degree of complexity of these recalibrated prestress loss estimation provisions prompted the investigation to be discussed in this dissertation. The primary objectives of this investigation were: (1) to assess the conservatism and accuracy of the current prestress loss provisions, (2) to identify the benefits and weaknesses of using the AASHTO LRFD 2004 and 2005 prestress loss provisions, and (3) to make recommendations to simplify the current provisions. These objectives were accomplished through (1) the fabrication, conditioning, and testing of 30 field-representative girders, (2) the assembly and analysis of a prestress loss database unmatched in size and diversity when compared with previously assembled databases, and (3) a parametric study investigating the design implications and sensitivity of the current loss provisions. Based on the database evaluation coupled with the experimental results, it was revealed that the use of the AASHTO LRFD 2005 prestress loss provisions resulted in underestimation of the prestress loss in nearly half of all cases. A loss estimation procedure was developed based on the AASHTO LRFD 2005 provisions to greatly simplify the procedure and provide a reasonable level of conservatism. / text

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.

The Varina-Enon Bridge is a cable-stayed, post-tensioned segmental box girder bridge in Richmond, Virginia. A large flexural crack was noted by inspectors in July 2012 which prompted a number of investigations into the current condition of the bridge. Particular focus has been put on prestress losses which have a significant impact on the strength and serviceability of the bridge. Previous work has been conducted to monitor the behavior of the bridge and to back-calculate effective prestress. This was done using field data from a long-term data collection system in the bridge as well as a finite element model which includes a staged-construction analysis. Creep and shrinkage are accounted for using the CEB-FIP '90 model code. Effective prestress in the Varina-Enon Bridge is back-calculated using live load strain data from the long-term data collection system. Previous work has overestimated live load moment since the influence of the crack opening has not been accounted for. This research refines the methods used to determine live load moment from live load strain. Two new methods are developed based on influence lines matching crack gauge data during a live load event. The new methods are compared to the method used in previous studies. Results using two elastic moduli for concrete are compared for each method of live load moment calculation. Finally, back-calculated effective prestress values are compared against effective prestress from the finite element model. Depending on the method used for live load moment calculation, back-calculated effective prestress ranged from 167.4 ksi to 170.8 ksi. Both new methods for live load moment calculation yielded slightly smaller values for effective prestress compared to the method used previously. Increasing the elastic modulus from 6000 ksi to 6200 ksi increased back-calculated effective prestress values from an average of 168.3 ksi to 168.6 ksi. For elastic moduli of 6000 ksi and 6200 ksi, the finite element model returned an effective prestress of 170.3 ksi and 170.8 ksi, respectively. / Master of Science / Prestressing in concrete uses steel tendons to apply a compressive force to a structure. This technique allows for stiffer and lighter structures with longer span lengths to be built. The force in the steel tendons decreases over time, and this is called prestress loss. Prestress losses can have a significant impact on the strength and service life of a structure, so estimating the magnitude of prestress loss is of great importance in prestressed concrete structures. The Varina-Enon Bridge is a cable-stayed, prestressed concrete box-girder bridge in Richmond, Virginia. In July 2012, cracking was observed in the bridge, and this prompted several investigations into its performance. This research calculates effective prestress (prestress force leftover after prestress loss) in several ways. A long-term data collection system collects sensor data which is used to calculate effective prestress experimentally, and a computer model is used to determine effective prestress computationally. Effective prestress results from sensor data are slightly smaller than results from the computer model. However, the differences in results are fairly small, and all values are within expectations, so it is concluded that the Varina-Enon Bridge has not experienced more than expected prestress losses.

As the aging bridges and infrastructure within the US continue to deteriorate, traffic delays due to construction will become more and more common. One method that can reduce delays due to bridge construction is to use precast deck panels. Precast deck panels can significantly reduce the overall length of the construction project. The panels can be manufactured ahead of time, and with higher quality control than is possible in the field. One of the reasons precast deck panels are not widely accepted is because of a lack of research concerning the required post-tensioning force, shear stud pocket placement, and proper joint design. In a recent dissertation (Swenty 2009) numerous recommendations were made for joint design, shear stud pocket design, and post-tensioning force for full-depth precast deck panel bridges. Design drawings were included for the replacement of a bridge located in Scott County, Virginia. The research in this report focuses on the short-term and long-term testing of this bridge. The short-term testing involved performing a live load test in which two trucks of known weight and dimensions were positioned on the bridge in order to maximize the negative moment at the joints over the piers and document strains and deflections at a number of other critical locations. The long-term testing involved monitoring the strains within the deck and on one of the six girders for a number of months in order to document the changes in strain due to creep and shrinkage. The results of these tests were compared to 2D beam-line models and to the parametric study results of Bowers' research on prestress loss within full-depth precast deck panel bridges. It was determined that the bridge was acting compositely and that the post-tensioning force was sufficient in keeping the joints in compression during testing. / Master of Science

Post-tensioned concrete is a building technology which provides a compressive force to concrete via steel tendons. This combination of steel and concrete allows for the construction of lighter and stiffer structures. Post-tensioned concrete is widely utilized throughout the United States highway system and bridge construction. Over time, the force in the prestressing strands is reduced by delayed strains in the concrete. The accurate estimation of this prestress loss is vital for making good decisions about the remaining capacity of a structure and the infrastructure system at large. The Varina-Enon Bridge is a post-tensioned concrete box-girder bridge in Richmond Virginia. Cracks in the bridge prompted an investigation into the magnitude of prestress loss experienced by the structure. To estimate prestress loss, a computer model of the structure was created. In addition, data from sensors previously installed on the bridge were used to back calculate prestress loss. It was found that the estimation of losses from the field closely matched those estimated at the construction of the bridge. Additionally, more updated loss models estimated similar, or slightly smaller values for prestress loss. / Master of Science / Post-tensioned concrete is a building technology which provides a compressive force to concrete via steel tendons. This combination of steel and concrete allows for the construction of lighter and stiffer structures. Post-tensioned concrete is widely utilized throughout the United States highway system and bridge construction. Over time, the force in the prestressing strands is reduced by delayed strains in the concrete. The accurate estimation of this prestress loss is vital for making good decisions about the remaining capacity of a structure and the infrastructure system at large. The Varina-Enon Bridge is a post-tensioned concrete box-girder bridge in Richmond Virginia. Cracks in the bridge prompted an investigation into the magnitude of prestress loss experienced by the structure. To estimate prestress loss, a computer model of the structure was created. In addition, data from sensors previously installed on the bridge were used to back calculate prestress loss. It was found that the estimation of losses from the field closely matched those estimated at the construction of the bridge. Additionally, more updated loss models estimated similar, or slightly smaller values for prestress loss.

The major cause of cracking in bridge decks, concrete pavements, as well as slabs on grade, is restrained shrinkage of the concrete. The resulting steel corrosion problem causes tremendous increase of maintenance and replacement cost. Shrinkage-compensating concrete (SHCC) and fiber-reinforced polymer (FRP) are explored to develop a hybrid slab system as one possible method of delaying the cracking and eliminating corrosion. To achieve the objective, a hybrid FRP reinforced SHCC structural system was developmed, and short-term and long-term behavior of this hybrid FRP-SHCC beams were investigated in this dissertation. In the first-stage development, a series of “coffee can” tests were carried out to measure and compare the expansion of SHCC from two candidate materials which were ettringite-system SHCC and lime-system SHCC. The selected SHCC candidate mix was then optimized to get the maximum expansion as well as a reasonable concrete strength. The optimized SHCC mix was used to make FRP-SHCC beams. The expansion of the concrete was measured through strain gauges on the FRP composite sheets during curing. Both glass FRP (GFRP) composite sheets and carbon FRP (CFRP) composite sheets were used for comparison. A series of third-point loading experiments were conducted to study the behavior of the proposed hybrid FRP-SHCC beams. In the second-stage development, long term prestress loss and static structural test of the proposed beams are investigated. Test results were evaluated based on maximum expansion strain, cracking load, crack width, load-deflection and ultimate load.The results indicate that the proposed system is promising in terms of its ability to develop a residual pre-stressing effect. Tests also show that the pre-stressing effect from the expansion of SHCC increases as the axial stiffness of the FRP reinforcement increases. A lime-system SHCC structural system shows higher prestress strain and less prestress loss than an ettringite-system SHCC system over the long term.

The Varina-Enon Bridge is a cable-stayed, precast, segmental, post-tensioned box girder bridge located in Richmond, Virginia. Inspectors noticed flexural cracking in July of 2012 that prompted concerns regarding long-term prestress losses in the structure. Prestress losses could impact the future performance, serviceability, and flexural strength of the bridge. Accurately quantifying prestress losses is critical for understanding and maintaining the structure during its remaining service life. Long-term prestress losses are estimated in the Varina-Enon Bridge using two methods. The first utilizes a time-dependent staged-construction analysis in a finite element model of the full structure to obtain predicted prestress losses using the CEB-FIP '90 code expressions for creep and shrinkage. The second method involves collecting data from instrumentation installed in the bridge that is used to back-calculate the effective prestress force. The prestress losses predicted by the finite element model were 44.9 ksi in Span 5, 47.8 ksi in Span 6, and 45.3 ksi in Span 9. The prestress losses estimated from field data were 50.0 ksi in Span 5, 48.0 ksi in Span 6, and 46.7 ksi in Span 9. The field data estimates were consistently greater than the finite element model predictions, but the discrepancies are relatively small. Therefore, the methods used to estimate the effective prestress from field data are validated. In addition, long-term prestress losses in the Varina-Enon Bridge are not significantly greater than expected. / Master of Science / Post-tensioned concrete uses stressed steel strands to apply a precompression force to concrete structures. This popular building technology can be used to create lighter, stiffer structures. Over time, the steel strands experience a reduction in force known as prestress losses. Accurately quantifying prestress losses is critical for understanding and maintaining a structure during its remaining service life. The Varina-Enon Bridge is a cable-stayed, prestressed box girder bridge located in Richmond, Virginia. Inspectors noticed cracking in July of 2012 that prompted concerns regarding long-term prestress losses in the structure. Prestress losses were estimated using two methods. The first method utilized a computer model of the full bridge. The second method used data from sensors installed on the bridge to back calculate prestress losses. It was found that the prestress losses estimated from field data were slightly greater than, but closely aligned with, the computer model results. Therefore, it was concluded that the Varina-Enon Bridge has not experienced significantly more prestress losses than expected.

The use of high compressive strengths in prestressed bridge girders can lower costs by allowing for longer spans, increased girder spacing, and smaller cross-sections. If high strength lightweight concrete (HSLWC) is used, these advantages are further enhanced due to the corresponding reduction in self-weight. Additional benefits can then be realized in the form of more traffic lanes, increased load capacity, smaller substructures, reduced crane capacity requirements, and lower shipping costs. Despite the possible economic savings, HSLWC has been used infrequently in prestressed bridge girder applications across the nation. While recent research has been performed to extend the applicability of current bridge design specifications to normal weight concretes with strengths as high as 18 ksi, little has been done by comparison with regards to HSLWC. The purpose of the research in this report was to assess whether current bridge design specifications for transfer length, development length, prestress loss, camber, and flexural capacity are satisfactory for use with fully-bonded, pretensioned flexural members consisting of HSLWC and to make recommendations for improvements where necessary. Twelve high strength pretensioned beams of variable unit weight (eight lightweight beams and four normal weight beams) and strand size (eight beams with 0.5-in. strand and four beams with 0.6-in. strand) were cast at the Thomas M. Murray Structural Engineering Laboratory at Virginia Tech. These beams were allowed to sit for a period of several months after fabrication while measurements were taken regarding transfer length, prestress loss, and camber. After this period, the beams were load tested to collect development length data, flexural data, and further data related to prestress loss. In addition to the laboratory cast beams, prestress loss and camber data from six full-size bridge beams (five lightweight beams and one normal weight beam) cast as part of a separate project at Virginia Tech was examined. Analysis of the results for all beams shows that with a few caveats, the current AASHTO LRFD Specifications and other design methods examined regarding the topics under consideration are satisfactory for use in the design of HSLWC pretensioned bridge girders with properties similar to those of the beams studied. / Ph. D.

To make sound decisions about the remaining life of a structure, the precise calculation of the prestress losses is very important. In post-tensioned structures, the prestress losses due to creep and shrinkage can cause serviceability issues and can reduce flexural capacity. The Varina-Enon Bridge is a cable-stayed, precast, segmental, post-tensioned box girder bridge located in Richmond, Virginia. Observation of flexural cracks in the bridge by inspectors promoted a study regarding long-term prestress losses in the structure. For understanding and sustaining the structure throughout its remaining service life, accurately quantifying prestress losses is important. Two approaches are used to predict long-term prestress losses on the Varina-Enon Bridge. The first approach involves a finite element computer model of the bridge which run a timedependent staged-construction analysis to obtain predicted prestress losses using the CEB-FIP '90 code expressions for creep and shrinkage. The second approach involves the compilation of data from instrumentation mounted in the bridge to back calculate the effective prestress force. The analysis using the computer model predicted the prestress losses as 44.6 ksi in Span 5, 47.9 ksi in Span 6, 45.3 ksi in Span 9, and 45.9 ksi in Span 11. The prestress losses estimated from field data were 50.0 ksi in Span 5, 48.0 ksi in Span 6, 46.7 ksi in Span 9, and 49.1 ksi in Span 11. It can be seen that relative to the results of field data estimations, the finite element analyses underestimated prestress loss, but given the degree of uncertainty in each form of estimation, the results are considered to fit well. / Master of Science / In order to apply a precompression force to concrete structures, post-tensioned concrete employs stressed steel strands. To construct lighter, stiffer structures, this popular building technology can be used. The steel strands undergo a reduction in force known as prestress losses over time. To make good decisions about the remaining life of a structure, the precise calculation of the prestress losses is very important. The Varina-Enon Bridge is a post-tensioned concrete box-girder bridge in Richmond Virginia. In July of 2012, observation of flexural cracks in the bridge by the inspectors promoted a study regarding long-term prestress losses in the structure. Two techniques are used to predict long-term prestress losses for this bridge. A computer model of the bridge is used in the first method to calculate losses using the design code. In order to measure prestress losses, the second technique used data from sensors mounted on the bridge. It was found that the estimation of losses closely matched those predicted at the time of the bridge construction and the computer model results. Based on this the final conclusion is made that the prestress loss in the Varina-Enon Bridge is not significantly more than expected.

Precast, prestressed concrete panels (PCPs) have been widely used in Texas as stay-in-place formwork in bridge deck construction. Although PCPs are widely popular and extensively used, Texas is experiencing problems with collinear cracks (cracks along the strands) in panels. One reason for the formation of collinear cracks is thought to be the required level of initial prestress. Currently, PCPs are designed assuming a 45-ksi, lump-sum prestress loss. If the prestress losses are demonstrated to be lower than this value, this could justify the use of a lower initial prestress, probably resulting in fewer collinear cracks. For this purpose, 20 precast, prestressed panels were cast at two different plants. Half of those 20 panels were fabricated with the current TxDOT-required prestress of 16.1 kips per strand, and the other half were fabricated with a lower prestress of 14.4 kips per strand based on initially observed prestress losses of 25 ksi or less. Thirteen of those panels were instrumented with strain gages and monitored over their life time. Observed losses stabilized after five months, and are found to be about 24.4 ksi. Even with the reduced initial prestress, the remaining prestress in all panels exceeds the value now assumed by TxDOT for design. / text