The ultimate shear capacity of prestressed concrete beams is difficult to predict accurately, especially after being in service for an extended period of time. The Utah Department of Transportation asked researchers at Utah State University to experimentally determine the existing shear capacity of 41-year-old prestressed, decommissioned concrete bridge girders and then provide recommendations on how to increase that ultimate shear capacity. This thesis presents the research findings that relate to the existing shear capacity of the prestressed concrete girders. Eight AASHTO Type II bridge girders were tested up to failure by applying external loads near the supports to determine their ultimate shear capacities. The measured results were then compared to calculated values obtained using the AASHTO LRFD bridge design code, and the ACI 318-08 design code. Prestress losses were also measured by means of a cracking test and then compared to values calculated according to the AASHTO prestress loss equations. Both the ultimate shear capacities and the residual prestress forces were used to evaluate the girders after being in service for more than 40 years.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21355号 / 工博第4514号 / 新制||工||1703(附属図書館) / 京都大学大学院工学研究科社会基盤工学専攻 / (主査)教授 KIM Chul-Woo, 教授 杉浦 邦征, 講師 張 凱淳 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Optical fibers are getting significant considerations in the field of the sensors and sensing beyond its applications in optical communications. Because of several advantages, e.g., low profile of the sensors, immunity to electromagnetic noises, the ability of multiplexing, etc., the use of the fiber optic sensor is increasing in the field of physical, chemical, and biomedical sensing. In this study, we have developed two new fiber optic sensors based on fiber loop ringdown technique (FLRD) and have demonstrated their applications in the field of sensing. In the first part of this study, we report on the development of a high-sensitivity FLRD strain sensor. For the design of the strain sensor, the fiber loop was cut at the middle, and then the two fiber ends from broken fiber loop were cleaved and aligned carefully to couple the light from one end to another end. Any strain during the measurement changes the alignment of the fiber ends, consequently, the ringdown time changes. With this scheme, the FLRD strain sensor has shown the strain detection limit of 65 nanostrain, which is five times better than any FLRD strain sensors reported in the literature. Furthermore, The FLRD strain sensors were successfully embedded into prestressed concrete-beams.The FLRD strain sensor was able to monitor stress on a post-tensioned rod, as well as the load applied on the concrete-beam during the three-point loading test, thus exhibiting immense potential in structural health monitoring. For the chemical sensor, a new scheme of interrogation for a fiber optic surface plasmon sensor was developed with the use of the FLRD technique. A gold nanolayer was deposited on an uncladded fiber section, and the fiber section was integrated into the FLRD system as a sensor head. The gold layer facilitates for increased interaction of sample of interest, with the light pulse confined in the fiber waveguide. Moreover, with the affinity of the gold with specific biomolecules, the sensor has the potential for applications in biochemical sensing. In the experiment, the SP-FLRD sensor was used for refractive index sensing, and index detection limit of 4.6×10-5 RIU was achieved.

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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 purpose of this study was to investigate the influence an increase in strand strength and the effect the as-cast vertical location had on transfer length, development length, and flexural strength and to resolve the discrepancies regarding the definition of the top-bar/strand effect. Two types of test specimens were fabricated and tested investigating each respective item. The increase in strand strength was found to influence transfer length, development length, and flexural strength, while the as-cast vertical location was only found to influence transfer length, and in turn development length. Contrary to the historical definition, the top-bar/strand effect was found to be more dependent on the amount of concrete cast above the strand than the amount below it, with transfer lengths showing a steady increase with a decrease in the amount of concrete cast above the strand. As a result of the findings of this study, a new transfer length equation was proposed and a previously proposed flexural bond length equation was recommended for use in lieu of the current code provisions. The current equations for flexural strength were found to give adequate estimates for flexural strength, although a decrease in ductility was noted. / Ph. D.