Precast, prestressed concrete panels (PCPs) are used in 85% of bridges in Texas. The goal of this thesis is to reduce collinear cracking (cracks propagating parallel to strands) in PCPs. One way to reduce collinear cracking would be to reduce the initial prestress force. In design, TxDOT conservatively assumes total prestress losses of 45 ksi. Based on eight panel specimens, instrumented and fabricated at two different precast plants in Texas, actual prestress losses were measured as at most 25 ksi. This difference (about 20 ksi) is consistent with a reduction in initial prestress force from 16.1 kips per strand to 14.4 kips per strand. Another way to reduce collinear cracking would be to provide additional transverse reinforcement in the end regions of the panels. By comparing crack spacings and crack widths in current and modified panel specimens, it was found that additional reinforcement consisting of one or two #3 bars placed transverse to strands at panel ends would effectively control collinear cracking in PCPs. / text

Prestressed concrete construction relies on the application of compressive stresses to concrete elements. The prestressing force is typically applied through the tensioning of strands that react against the concrete and induce compression in the concrete. Loss of prestress is the decrease of this pre-applied stress. The conservative estimation of the prestress losses is imperative to prevent undesired cracking of the prestressed element under service loads. A large fraction of the prestress losses is a consequence of concrete deformations. This fraction of the losses can be identified as strain-related losses, and these occur due to instantaneous elastic shortening, and time-dependent creep and shrinkage. Creep and shrinkage of concrete depend on many factors that are extremely variable within concrete structures. The time-dependent behavior of concrete is not well-understood, but recent findings in the topics of concrete creep and shrinkage provide a better understanding of the underlying mechanisms affecting the nature of these two phenomena. However, current design practices and prestress loss estimation methods do not reflect the state-of-the-art knowledge regarding creep and shrinkage. The main objective of this dissertation was the study and estimation of strain-related prestress losses in simply supported pretensioned bridge girders. Simply supported pretensioned girders are widely designed, produced and frequently used in bridge construction. Due to this common use, pretensioned concrete bridge girders has become fairly standardized elements, which results in a reduced variability in the behavior of pretensioned bridge girders, as compare to that of less standardized concrete structures. Hence, a simplified method was calibrated to estimate prestress losses within pretensioned girders to an adequate level of accuracy. To achieve an acceptable accuracy experimental data from the monitoring of pretensioned simply supported girders was used for the calibration of the method. The accuracy of this simplified method is comparable to that achievable using more elaborate methods developed for generic concrete structures. / text

The first objective of this thesis was to assess the effect of casting orientation on bond strength in pretensioned prestressed concrete members. The "top strand effect" was evaluated through transfer and development length tests of prestressed concrete beams. Eight beams were cast with normal orientation, while four beams were cast with inverted orientation so that a significant depth of fresh concrete was placed below prestressing strands. Discrete transfer lengths were determined at the ends of each beam by measuring concrete surface strains. Inverted casting orientation caused an average 70 percent increase in transfer length. Some transfer lengths in beams with inverted casting orientation exceed current ACI and AASHTO code provisions. All measured transfer lengths were less than 90 strand diameters (45 in. for 0.5 in. diameter strands). Ranges of development length were determined through iterative load testing. The top strand effect on development length was more qualitative than quantitative. Ranges of development length in normal beams were conservatively less than code provisions. Ranges of development length in beams with inverted casting orientation were much closer to and sometimes exceeded code provisions. It is recommended that ACI and AASHTO code provisions for the development length of prestressing strand be modified to include the same magnification factors that are specified for the development length of deformed bars with twelve or more inches of fresh concrete placed below. The second objective of this thesis was to compare experimentally measured prestress losses to theoretical calculations. Theoretical prestress losses were calculated according to PCI and AASHTO Refined methods. These methods produced similar results. Prestress losses were experimentally measured by vibrating wire gages and flexural load testing. Vibrating wire gages were used to monitor internal concrete strains. Two methods were used to reduce vibrating wire gage data: an upper/lower bound method and a basic method. The upper/lower bound method produced distorted data that was unreasonable in some cases. The basic method was more reasonable, but resulted in some prestress loss measurements that were greater than theoretical predictions. Flexural load testing was used to back calculate prestress losses from crack initiation and crack reopening loads. Prestress losses measured by crack initiation loads were generally greater than theoretical values. Losses measured by crack reopening loads were distorted. The distortion was attributed to difficulty in isolation of the correct crack reopening load. Large measurements of prestress losses by the basic vibrating wire gage and crack initiation methods suggested that losses occurred between the time when concrete was poured and prestress transfer occurred. Such losses are not accounted for in current code provisions. More research is recommended to determine the magnitude of these additional losses and their effect on design. / Master of Science

The research conducted was part of a project sponsored by the Virginia Department of Transportation and the Virginia Transportation Research Council. One PCBT-53 girder was fabricated with lightweight, self-consolidating concrete. An additional composite cast-in-place lightweight concrete deck was added at the Virginia Tech Structures and Material Laboratory. The project had two specific goals. The first was to experimentally determine the shear strength of the bridge girder. The initial tests focused on the web-shear strength of the girder, and the second tests focused on the flexure-shear strength. The theoretical predictions for the web shear strength were all conservative when compared to the experimentally measured failure strength. The theoretical predictions of the flexure-shear strength were typically unconservative because during the flexure-shear test the girder reached the nominal flexural strength, and a failure occurred in the previously damaged region of the beam. Shear strength was also predicted using the design material properties. Results from these calculations suggested that the equation for the steel contribution to shear strength proposed in the NCHRP Simplified Method were unconservative. Further investigation into the results from the web-shear test showed that the maximum nominal shear strength calculated using the AASHTO LRFD Specifications was typically unconservative. Test results from this project suggested that the constant multiplier of 0.25 used in the LRFD equation for Vnmax may be too high. Further research may be needed to accurately quantify an upper limit on the shear strength. Additionally, predictions of the initial web-shear cracking load were conservative when using the AASHTO Standard Specifications and the NCHRP Simplified Method. The initial web-shear crack angle was under-predicted using the AASHTO LRFD Specifications. The second goal was to monitor the change in prestress over time (and hence the prestress loss) occurring in the PCBT-53 girder. Prestress losses were experimentally measured by vibrating wire gages (measured changes in concrete strain) and flexural load testing. Measured prestress losses were compared to a theoretical prediction calculated using the AASHTO Refined Method. The amount of prestress recorded at any given time using vibrating wire gages was greater than predictions from the AASHTO Refined method. The effective prestress measured just prior to deck placement was higher than the theoretical prediction, and the measured effective prestress at the time of testing was also higher than the theoretical effective prestressing force. The effective prestress value calculated using the flexural crack initiation method was significantly lower than the effective prestress values predicted by both the code provisions and the vibrating wire gages; however, the effective prestress value calculated using the flexural crack re-opening method corresponded very well with the effective prestress values predicted by the code provisions and measured by the vibrating wire gages. The discrepancy in the crack initiation effective prestress values may be due to prestress losses occurring between placement of the concrete and transfer of the prestress force. These losses are not taken into account when using current code provisions to estimate prestress losses. Additional research is recommended to determine if these losses occur in bulb-tee girders, and if so, to quantify them. Finally, from test results within the scope of this research project, design of prestressed bulb-tee girders with lightweight, self-consolidating concrete is practical. The current AASHTO LRFD Specifications provided conservative results when predicting the shear strength of the PCBT-53. Additionally, prestress losses in PCBT girders fabricated with lightweight, self-consolidating concrete were less than those predicted using the AASHTO Refined method. / Master of Science

The first objective of this thesis was to determine the effect of using self-consolidating concrete versus normal concrete on transfer and development lengths, and flexural strengths of prestressed members. Three small rectangular members were made, two cast with SCC mixes and one cast with a conventional mix, to determine the transfer length of each mix. Transfer lengths of both ends of each member were determined by measuring the concrete surface strains. The change in the transfer length was monitored by determining the transfer length of each member at prestress release, 7 days after release, and 28 days after release. All concrete mixes had lower than code determined transfer lengths at prestress release. Each concrete mix showed between a 12 to 56 percent increase in transfer length after 28 days. One SCC mix exceeded the ACI code stipulated 50 strand diameters 7 days after prestress transfer. The other SCC mix was consistently below the transfer length of the conventional concrete. Separate development length members were cast in a stay-in-place steel form used for creating structural double tees. Each development length member was a stub tee. Iterative load testing was performed to determine the development length of each SCC and conventional mix. Development lengths for both SCC mixes were approximately 20 percent shorter than ACI and AASHTO code predictions. A development length for the conventional concrete was not determined due to non-repeating test data. The flexural strength of each member was determined during load testing. All concrete mixes achieved higher than the ACI predicted strengths. The second objective of this thesis was to experimentally measure prestress losses and compare these experimental values to theoretical models. Crack initiation and crack reopening tests were performed to experimentally determine the prestress losses in each member. Three theoretical models were evaluated, the sixth edition PCI Design Handbook suggested model, a 1975 PCI Committee on Prestress Losses model, and the AASHTO LRFD prestress loss model. The crack initiation experimental values tended to be between 10 and 15 percent lower than theoretical models. In general, the crack reopening prediction of the effective prestress had a good correlation with theoretical models. This suggests crack reopening tests can be used as predictors of effective prestress, and as such, predictors of prestress losses in future experimental research. Additionally, the concrete type was shown to affect the prestress losses determined in the development length members. The SCC members tended to have higher effective prestress forces than the conventional concrete members, and thus had less prestress losses due to creep and shrinkage than the conventional concrete members. / Master of Science

Accurate camber prediction in prestressed concrete bridge beams is important to all parties involved in bridge design and construction. Many current prestress loss prediction methods, necessary for proper camber calculation, were developed many years ago and are predicated on assumptions that may no longer be valid as higher strength concrete, wider beam spacing, and longer span lengths become more commonplace. This throws into question which models are appropriate for use in camber calculation by the bridge engineers and contractors of today. Twenty-seven high-strength concrete modified 79 in. Bulb Tee beams with a design compressive strength of 9,000 psi were periodically measured to determine camber growth. Most available models for concrete creep and shrinkage were used to calculate creep and shrinkage strain. The modulus of elasticity equation of each model was used to predict modulus of elasticity of the studied mix. The Shams and Kahn compressive strength and modulus of elasticity equations were modified in order to approximate measured modulus of elasticity. The creep, shrinkage, and modulus of elasticity equations were used as inputs to an incremental time step method. The time-dependent change in beam curvature calculated by the time step method was used to calculate theoretical camber using the Moment-Area method. Predicted camber, using inputs from each considered model, was then compared with measured camber to determine the most accurate camber prediction models. Season of casting was also examined to determine what, if any, affect ambient temperature has on camber growth. For the studied beams, the Shams and Kahn Model for creep, shrinkage, and modulus of elasticity, used as inputs for an incremental time step analysis, were found to most accurately predict camber values. Lower concrete compressive strength was observed for test cylinders from beams cast in summer versus beams cast in winter. Differences in beam deflection based on season of casting showed mixed results. / Master of Science

This multi-scale investigation aimed to provide new knowledge and understanding of creep and shrinkage of high performance lightweight concrete (HPLC) by assessing prestress losses in HPLC prestressed members in a large-scale study; by quantifying the effect of the constituent materials and external conditions on creep and shrinkage in a medium-scale study; and by improving the fundamental understanding of creep and shrinkage in a small-scale study. Creep plus shrinkage prestress losses were between two and eight times lower than those estimated for the design standards and approximately 50% of those measured in similar strength normal weight high performance concrete girders. The lower creep and shrinkage exhibited by HPLC was found to be caused by a synergy between the pre-soaked lightweight aggregate and the low water-to-cementitious material ratio matrix. That is, the water contained in the lightweight aggregate contributes to enhance hydration by providing an internal moist curing. The water in the aggregate also contributes to maintain a high internal relative humidity which reduces or eliminates autogenous shrinkage. This higher internal relative humidity also reduces creep by preventing load-induced water migration. Finally, lightweight aggregate exhibits a better elastic compatibility with the paste than normal weight aggregate. This improved elastic matching and the enhanced hydration are believed to reduce peak deformations at the ITZ which further decreases creep and shrinkage.

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.

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

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.