Full-depth precast concrete panels offer an efficient alternative to traditional cast-in-place concrete for replacement or new construction of bridge decks. Research has shown that longitudinal post-tensioning helps keep the precast bridge deck in compression and avoid problems such as leaking, cracking, spalling, and subsequent rusting on the beams at the transverse panel joints. Current design recommendations suggest levels of initial compression for precast concrete decks in a very limited number of bridge configurations. The time-dependent effects of creep and shrinkage in concrete and relaxation of prestressing steel complicate bridge behavior, making the existing recommendations for post-tensioning in precast deck panels invalid for all bridges with differing girder types, sizes, spacings, and span lengths. Therefore, the development of guidelines for levels of post-tensioning applicable to a variety of bridge types is necessary so designers may easily implement precast concrete panels in bridge deck construction or rehabilitation. To fulfill the needs described, the primary objective of this research was to determine the initial level of post-tensioning required in various precast concrete bridge deck panel systems in order to maintain compression in the transverse panel joints until the end of each bridge's service life. These recommendations were determined by the results of parametric studies which investigated the behavior of bridges with precast concrete decks supported by both steel and prestressed concrete girders in single spans as well as two and three continuous spans. The three primary variables in each parametric study included girder type, girder spacing, and span length. The age-adjusted effective modulus method was used to account for the ongoing effects of creep and shrinkage in concrete. Results from the Mathcad models used in the parametric studies were confirmed through comparison with results obtained from finite element models generated in DIANA. Initial levels of post-tensioning for various bridge systems are proposed based on the trends observed in the parametric studies. The precast decks of the simple span bridges with steel girders and the one, two, and three span bridges with prestressed concrete girders needed only 200 psi of initial post-tensioning to remain in compression under permanent and time-dependent loads throughout each bridge's service life. The precast decks of the two and three span continuous bridges with steel girders, however, needed a significantly higher level of initial compression due to the negative moments created by live loads. / Master of Science

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.

Effective determination of long-term prestress losses is important in the design of prestressed concrete bridges. Over-predicting prestress losses results in an overly conservative design for service load stresses, and under-predicting prestress losses, can result in cracking at service loads. Creep and shrinkage produce the most significant time-dependent effect on prestress losses, and research has shown that high performance and high strength concretes (HPC and HSC) exhibit less creep and shrinkage than conventional concrete. For this reason, the majority of traditional creep and shrinkage models and methods for estimating prestress losses, over-predict the prestress losses of HPC and HSC girders. Nine HPC girders, with design compressive strengths ranging from 8,000 psi to 10,000 psi, and three 8,000 psi lightweight HPC (HPLWC) girders were instrumented to determine the changes in strain and prestress losses. Several creep and shrinkage models were used to model the instrumented girders. For the HPLWC, each model over-predicted the long-term strains, and the Shams and Kahn model was the best predictor of the measured strains. For the normal weight HPC, the models under-estimated the measured strains at early ages and over-estimated the measured strains at later ages, and the B3 model was the best-predictor of the measured strains. The PCI-BDM model was the most consistent model across all of the instrumented girders. Several methods for estimating prestress losses were also investigated. The methods correlated to high strength concrete, the PCI-BDM and NCHRP 496 methods, predicted the total losses more accurately than the methods provided in the AASHTO Specifications. The newer methods over-predicted the total losses of the HPLWC girders by no more than 8 ksi, and although they under-predicted the total losses of the normal weight HPC girders, they did so by less than 5 ksi. / Ph. D.

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.

In post-tensioned (PT) bridges, prestressing steel tendons are the major load carrying components. These tendons consist of strands, ducts, and cementitious grout that fill the interstitial space between the strands and ducts. However, inspections on PT bridges have reported the presence of voids, moisture, and chlorides inside grouted ducts as the major cause of accelerated corrosion of strands. Corrosion of the strands has resulted in PT bridge failures in Europe and tendon failures in the United States. As most of the PT bridges have high importance measures and the consequences of failure are significant, it is important to maintain high levels of safety and serviceability for these bridges. To meet this goal, bridge management authorities are in dire need of tools to quantify the long-term performance of these bridges. Time-variant structural reliability models can be useful tools to quantify the long-term performance of PT bridges. This doctoral dissertation presents the following results obtained from a comprehensive experimental and analytical program on the performance of PT bridges. 1) Electrochemical characteristics of PT systems 2) Probabilistic models for tension capacity of PT strands and wires exposed to various void and environmental conditions 3) Time-variant structural reliability models (based on bending moment and stress limit states) for PT bridges 4) Time-variant strength and service reliabilities of a typical PT bridge experiencing HS20 and HL93 loading conditions and different exposure conditions for a period of 75 years The experimental program included exposure of strand specimens to wet-dry and continuous-atmospheric conditions. These strand specimens were fabricated to mimic void and/or grout-air-strand (GAS) conditions inside the tendons. It was found that the GAS interface plays a major role in strand corrosion. The GAS interfaces that are typically located in the anchorage zones of harped PT girders or vertical PT columns can cause aggressive strand corrosion. At these locations, if voids are present and the environment is relatively dry, then limited corrosion of the strands occurs. However, if the presence of high relative humidity or uncontaminated and chloride-contaminated water exists at these interfaces, then corrosion activity can be high. The strands were exposed for a period of 12, 16, and 21 months, after which the remaining tension capacity was determined. The analytical program included the development of probabilistic strand capacity models (based on the experimental data) and the structural reliability models. The timevariant tension capacity predicted using the developed probabilistic models were reasonably consistent with the tendon failures observed in PT bridges in Florida and Virginia. The strength reliability model was developed based on the moment capacity and demand at midspan. Service reliability model was developed based on the allowable and applied stresses at midspan. Using these models, the time-variant strength and service reliabilities of a typical PT bridge were determined based on a set of pre-defined constant and random parameters representing void, material, exposure, prestress, structural loading, and other conditions. The strength and service reliabilities of PT bridges exposed to aggressive environmental conditions can drop below the recommended values at relatively young ages. In addition, under similar conditions the service reliability drops at a faster rate than the strength reliability.

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

Fibre-reinforced polymers (FRPs) are becoming increasingly accepted in structural engineering applications. In particular, Carbon-FRP (CFRP) tendons are proving to be promising as prestressing reinforcement for concrete structures. While several studies have been conducted on CFRP-prestressed concrete beams, very little attention has been given to their long-term behaviour at low temperatures. This thesis investigates the behaviour of CFRP prestressed concrete beams in two studies: (a) under sustained loading at low temperature, and (b) under high-cycle fatigue at low temperature. Seven 13 year old, 4.4 m long precast concrete T beams were tested, of which five were prestressed to various levels with CFRP tendons and two with conventional steel strands. In the first study, three beams were exposed to −27 °C while being subjected to a sustained load of 25% of their flexural capacity for 163 days. The sustained load produced cracking in two beams with lower prestress levels. Results were compared to those obtained from three similar beams subjected to the same sustained load at room temperature. Deflection increase under sustained load at low temperature was generally small and similar to that at room temperature. Prestressing strain had a direct relationship with temperature in the CFRP prestressed beams. After being subjected to sustained loading, all seven beams were tested in the second study. Only three of the five CFRP prestressed beams were subjected to cyclic loading, one at −28 °C and two at room temperature, while only one of the two steel prestressed beams was subjected to cyclic loading, at −28 °C. Cyclic loading consisted of 3 million cycles at a frequency of 0.85 Hz. The load range represented 21 to 42% of the flexural capacity of the CFRP prestressed beams and 30 to 60% of that of the steel-prestressed beam. Monotonic tests were run every 1 million cycles. Finally, all seven beams were monotonically loaded to failure. All CFRP prestressed beams survived the 3 million cycles but the steel prestressed beam failed after 185,000 cycles. However, the CFRP concrete bond was weakened by high prestress levels, cyclic loading, and low temperature during sustained loading and loading to failure. This resulted in bond failure at loads ranging from 69 to 91% of the full flexural capacity. Stiffness and camber gradually decreased during cyclic loading. / Thesis (Master, Civil Engineering) -- Queen's University, 2009-12-21 15:16:33.381

Estruturas Mistas de Aço-Concreto têm sido usadas extensivamente na construção de pontes e viadutos urbanos, especialmente a partir da segunda metade do século XX. A popularidade desse tipo de solução, com seções caixão, cresceu devido a sua alta capacidade à flexão, rigidez à torção e uma seção transversal fechada que reduz a superfície exposta a corrosão. Este trabalho discorre sobre o comportamento estrutural, procedimentos de análise e verificação em Estado Limite Último (ELU) de pontes mistas de seção caixão com aplicação de protensão externa. Em vista da escassez de literatura sobre o assunto e inexistência de norma brasileira, o trabalho objetiva produzir um roteiro de análise para a determinação da capacidade à flexão em ELU de estruturas mistas de seção caixão protendidas. Embasado na norma americana AASHTO-LRFD:2012 e na revisão bibliográfica, propôs-se um estudo de caso para verificação/dimensionamento analíticos da estrutura, tratando de Momentos Fletores Resistentes (positivos e negativos), Esforço Cortante Resistente e conectores de cisalhamento. Após esta etapa inicial, aplicou-se protensão à estrutura e, por meio de métodos analíticos, e auxílio do método dos trabalhos virtuais, obtiveram-se as perdas de protensão e a relação entre a deformação adicional do cabo de protensão em função do momento externo aplicado à estrutura. Deste modo pôde-se fazer o equilíbrio de forças horizontais, através do método da Bissecção, e obter-se o valor de incremento de Momentos Fletores Positivos e Negativos Resistentes da estrutura. Observou-se, com a protensão, um aumento de resistência importante na região de Momentos Fletores Negativos em ELU (~40%); para a região de flexão positiva esse incremento foi pouco superior a 7%, em relação à estrutura nãoprotendida. Por fim, modelou-se a estrutura em elementos finitos de casca com o software SAP2000, a fim de confrontar a análise inicial, feita em modelo de barras de pórtico espacial, preconizada pela norma AASHTO-LRFD:2012. Os resultados mostram que o modelo em barras de pórtico espacial, em termos de deslocamentos e tensões, é adequado à análise deste tipo de estrutura. / Steel-Concrete Composite Strutures have been used extensively in the construction of bridges and urban viaducts, especially from the second half of the twentieth century. The popularity of this type of solution, with box sections, has increased due to its high flexural capacity and torsion stiffness combined with a closed cross section that reduces the exposed surface to corrosion. This paper discusses the structural behavior, analysis and verification procedures in the Ultimate Limite State (ULS) of Composite Box Girder Bridges with application of external prestressing. In view of the paucity of literature on the subject and the absence of Brazilian standard, this work aims to produce a analysis script to determine the flexural capacity of prestressed composite box girder structures in ULS. Grounded in the American Standard AASHTO-LRFD:2012 and the literature review, we propose a case study for analytical verification/dimensioning of the structure, concerning positive and negative bending moments, shear and shear connectors. After this initial stage, prestressing was applied to the structure, and with de aid of analytical methods, and the virtual work method, the prestress losses and the relation between the additional strain of the tendons and the external applied moment were obtained. Thus, it was possible to make the horizontal forces balance through the Bisection Method and obtain the increment of positive and negative flexion strength. It was observed, with prestressing, an important increase of capacity in the negative bending region for ULS (~40%); for the positive bending region, the increase was somewhat higher than 7%, compared with the non-prestressed structure. Finally, a finite element model with shell elements was held with aid of the software SAP2000 to confront the initial analysis, made in space frame bars model, recommended by AASHTO-LRFD:2012 standard. The results show that the space frame bars model, in terms of displacements and stresses, is appropriate to analyze this type of structure.

Estruturas Mistas de Aço-Concreto têm sido usadas extensivamente na construção de pontes e viadutos urbanos, especialmente a partir da segunda metade do século XX. A popularidade desse tipo de solução, com seções caixão, cresceu devido a sua alta capacidade à flexão, rigidez à torção e uma seção transversal fechada que reduz a superfície exposta a corrosão. Este trabalho discorre sobre o comportamento estrutural, procedimentos de análise e verificação em Estado Limite Último (ELU) de pontes mistas de seção caixão com aplicação de protensão externa. Em vista da escassez de literatura sobre o assunto e inexistência de norma brasileira, o trabalho objetiva produzir um roteiro de análise para a determinação da capacidade à flexão em ELU de estruturas mistas de seção caixão protendidas. Embasado na norma americana AASHTO-LRFD:2012 e na revisão bibliográfica, propôs-se um estudo de caso para verificação/dimensionamento analíticos da estrutura, tratando de Momentos Fletores Resistentes (positivos e negativos), Esforço Cortante Resistente e conectores de cisalhamento. Após esta etapa inicial, aplicou-se protensão à estrutura e, por meio de métodos analíticos, e auxílio do método dos trabalhos virtuais, obtiveram-se as perdas de protensão e a relação entre a deformação adicional do cabo de protensão em função do momento externo aplicado à estrutura. Deste modo pôde-se fazer o equilíbrio de forças horizontais, através do método da Bissecção, e obter-se o valor de incremento de Momentos Fletores Positivos e Negativos Resistentes da estrutura. Observou-se, com a protensão, um aumento de resistência importante na região de Momentos Fletores Negativos em ELU (~40%); para a região de flexão positiva esse incremento foi pouco superior a 7%, em relação à estrutura nãoprotendida. Por fim, modelou-se a estrutura em elementos finitos de casca com o software SAP2000, a fim de confrontar a análise inicial, feita em modelo de barras de pórtico espacial, preconizada pela norma AASHTO-LRFD:2012. Os resultados mostram que o modelo em barras de pórtico espacial, em termos de deslocamentos e tensões, é adequado à análise deste tipo de estrutura. / Steel-Concrete Composite Strutures have been used extensively in the construction of bridges and urban viaducts, especially from the second half of the twentieth century. The popularity of this type of solution, with box sections, has increased due to its high flexural capacity and torsion stiffness combined with a closed cross section that reduces the exposed surface to corrosion. This paper discusses the structural behavior, analysis and verification procedures in the Ultimate Limite State (ULS) of Composite Box Girder Bridges with application of external prestressing. In view of the paucity of literature on the subject and the absence of Brazilian standard, this work aims to produce a analysis script to determine the flexural capacity of prestressed composite box girder structures in ULS. Grounded in the American Standard AASHTO-LRFD:2012 and the literature review, we propose a case study for analytical verification/dimensioning of the structure, concerning positive and negative bending moments, shear and shear connectors. After this initial stage, prestressing was applied to the structure, and with de aid of analytical methods, and the virtual work method, the prestress losses and the relation between the additional strain of the tendons and the external applied moment were obtained. Thus, it was possible to make the horizontal forces balance through the Bisection Method and obtain the increment of positive and negative flexion strength. It was observed, with prestressing, an important increase of capacity in the negative bending region for ULS (~40%); for the positive bending region, the increase was somewhat higher than 7%, compared with the non-prestressed structure. Finally, a finite element model with shell elements was held with aid of the software SAP2000 to confront the initial analysis, made in space frame bars model, recommended by AASHTO-LRFD:2012 standard. The results show that the space frame bars model, in terms of displacements and stresses, is appropriate to analyze this type of structure.

Estruturas Mistas de Aço-Concreto têm sido usadas extensivamente na construção de pontes e viadutos urbanos, especialmente a partir da segunda metade do século XX. A popularidade desse tipo de solução, com seções caixão, cresceu devido a sua alta capacidade à flexão, rigidez à torção e uma seção transversal fechada que reduz a superfície exposta a corrosão. Este trabalho discorre sobre o comportamento estrutural, procedimentos de análise e verificação em Estado Limite Último (ELU) de pontes mistas de seção caixão com aplicação de protensão externa. Em vista da escassez de literatura sobre o assunto e inexistência de norma brasileira, o trabalho objetiva produzir um roteiro de análise para a determinação da capacidade à flexão em ELU de estruturas mistas de seção caixão protendidas. Embasado na norma americana AASHTO-LRFD:2012 e na revisão bibliográfica, propôs-se um estudo de caso para verificação/dimensionamento analíticos da estrutura, tratando de Momentos Fletores Resistentes (positivos e negativos), Esforço Cortante Resistente e conectores de cisalhamento. Após esta etapa inicial, aplicou-se protensão à estrutura e, por meio de métodos analíticos, e auxílio do método dos trabalhos virtuais, obtiveram-se as perdas de protensão e a relação entre a deformação adicional do cabo de protensão em função do momento externo aplicado à estrutura. Deste modo pôde-se fazer o equilíbrio de forças horizontais, através do método da Bissecção, e obter-se o valor de incremento de Momentos Fletores Positivos e Negativos Resistentes da estrutura. Observou-se, com a protensão, um aumento de resistência importante na região de Momentos Fletores Negativos em ELU (~40%); para a região de flexão positiva esse incremento foi pouco superior a 7%, em relação à estrutura nãoprotendida. Por fim, modelou-se a estrutura em elementos finitos de casca com o software SAP2000, a fim de confrontar a análise inicial, feita em modelo de barras de pórtico espacial, preconizada pela norma AASHTO-LRFD:2012. Os resultados mostram que o modelo em barras de pórtico espacial, em termos de deslocamentos e tensões, é adequado à análise deste tipo de estrutura. / Steel-Concrete Composite Strutures have been used extensively in the construction of bridges and urban viaducts, especially from the second half of the twentieth century. The popularity of this type of solution, with box sections, has increased due to its high flexural capacity and torsion stiffness combined with a closed cross section that reduces the exposed surface to corrosion. This paper discusses the structural behavior, analysis and verification procedures in the Ultimate Limite State (ULS) of Composite Box Girder Bridges with application of external prestressing. In view of the paucity of literature on the subject and the absence of Brazilian standard, this work aims to produce a analysis script to determine the flexural capacity of prestressed composite box girder structures in ULS. Grounded in the American Standard AASHTO-LRFD:2012 and the literature review, we propose a case study for analytical verification/dimensioning of the structure, concerning positive and negative bending moments, shear and shear connectors. After this initial stage, prestressing was applied to the structure, and with de aid of analytical methods, and the virtual work method, the prestress losses and the relation between the additional strain of the tendons and the external applied moment were obtained. Thus, it was possible to make the horizontal forces balance through the Bisection Method and obtain the increment of positive and negative flexion strength. It was observed, with prestressing, an important increase of capacity in the negative bending region for ULS (~40%); for the positive bending region, the increase was somewhat higher than 7%, compared with the non-prestressed structure. Finally, a finite element model with shell elements was held with aid of the software SAP2000 to confront the initial analysis, made in space frame bars model, recommended by AASHTO-LRFD:2012 standard. The results show that the space frame bars model, in terms of displacements and stresses, is appropriate to analyze this type of structure.