O presente estudo, in vitro, teve como objetivo avaliar a influência da variação da densidade de potência (300, 600, 1000 e 1550 mW/cm2) na contração de polimerização e na pigmentação de duas resinas compostas, Filtek Z250 e Filtek Z350XT (3M/ESPE). Para avaliação das forças geradas durante a contração de polimerização foram utilizadas duas bases de aço, em forma retangular (6 X 2 mm), posicionadas paralelamente na máquina de ensaios (Emic - DL 500), uma conectada à célula de carga de 100 Kg e, a outra, ao braço fixo. As resinas compostas foram introduzidas entre as bases metálicas, com altura de 1mm, volume de 12mm3 e fator C de 1,5. Para fotoativação foi utilizado o aparelho LED Blue Star 3 (Microdont Micro Usinagem de Precisão LTDA), determinando-se o tempo de 20 s para as diferentes densidades de potência. As bases ficaram fixas durante o ensaio (120 s) e as forças geradas durante a polimerização foram registradas numericamente e por meio de gráficos [força (N) x tempo (s)], num total de dez ensaios para cada grupo. Para os testes de pigmentação foram confeccionados, em uma matriz pré-fabricada, cinco espécimes circulares (6 mm X 2 mm) para cada grupo. As avaliações das alterações de cor (_E) foram realizadas com o Espectrofotômetro Easyshade com o sistema CIE-Lab, em diferentes períodos (inicial, após 6 e 12hs, 1, 2, 3, 4 e 5 dias de imersão em café). Os resultados obtidos foram submetidos à análise estatística ANOVA a dois critérios e teste Tukey para comparações individuais (p<0,05). Os resultados evidenciaram que a variação da densidade de potência influenciou diretamente na indução da contração de polimerização, com geração de forças significantemente maiores à medida que se aumentou os valores de irradiância, independente da resina composta utilizada. De forma semelhante, as alterações de cor apresentaram uma relação com a variação da densidade de potência, porém de forma indireta, evidenciando-se que o aumento progressivo da irradiância inferiu em menores valores de _E, assim como uma pigmentação mais acentuada para a resina composta nanoparticulada em relação à resina micro-híbrida. Considerando a observação de efeitos distintos com emprego de diferentes densidades de potência, dentro de um mesmo tempo de fotoativação das resinas compostas analisadas, tanto em relação à indução da contração de polimerização quanto de alteração de cor, deve ser concebido que o fator densidade de potência não pode ser estabelecido de forma independente aos demais parâmetros técnicos relacionados à dose de irradiância, à sequência de energia necessária e ao espectro de luz requerido para efetivação da polimerização das diferentes formulações de compósitos disponíveis. / The aim of this in vitro study was to evaluate the influence of the power density (300, 600, 1000 and 1550 mW/cm2) on polymerization shrinkage and pigmentation of two composite resins, Filtek Z250 and Filtek Z350XT (3M/ESPE). The experimental groups were set based on different combinations of the influencing factors. For the polymerization shrinkage test two steel bases with a parallel disposition were used, the movable one was connected to a 100 kg load cell and the other one was fixed using a universal testing machine (EMIC - DL 500). The resin composite was inserted between the bases and light-cured for 20 s with a LED unit (Blue Star 3, Microdont) The bases stayed fixed during the test (120 s) and the polymerization stresses generated were recorded in graphs [curves force (N) x time (s)]. A total of ten tests for each group were made. For the pigmentation tests five circular specimens (6 mm X 2 mm) for each group were made using a prefabricated mold. Using the color-measuring device Easyshade with the CIE-Lab system, evaluations were made at different times (baseline, after 6 hours, 12 hours, 1, 2, 3, 4 and 5 days of coffee immersion). Statistical analysis was performed with two-way ANOVA and Tukey test for individual comparisons (p <0.05). The results showed that the power density variation directly influenced the induction of polymerization shrinkage, with significantly greater generation force as it increased the values of irradiance, independent of the resin used. Similarly, color changes showed a relation with the power density variation, however demonstrating indirectly that the gradual increase in irradiance inferred at lower values of _E, as pigmentation was more pronounced for the composite with nanoparticles compared to a micro-hybrid resin. Considering the observation of various effects when using different power densities within the same light-curing time of the composite resins analyzed, in relation to the induction of polymerization shrinkage and color change, should be understood that the power density factor can not be established independently of other technical parameters related to the irradiation dosage, the sequence of necessary energy and light spectrum required to effecting the polymerization of composites with different available formulations.

Obtaining high spatial or spatiotemporal resolution along with good slice coverage is challenging in dynamic magnetic resonance imaging, MRI, due to the slow nature of the acquisition process. In recent years, there has been a rapid growth of MRI techniques that allow faster scan speed by exploiting spatial or spatiotemporal redundancy of the images. These techniques can improve the performance of imaging significantly across multiple clinical applications, including cardiac functional examinations, perfusion imaging, blood flow assessment, contrast-enhanced angiography, functional MRI, and interventional imaging, among others. The ultimate goal of this thesis is to develop novel algorithms to reconstruct heavily undersampled sparse imaging. The designed schemes aim to achieve a shorter scan duration, higher spatial resolution, increased temporal resolution, signal-to-noise ratio and coverage in multidimensional multichannel MRI. In addition to improving patients comfort and compliance while imaging under the MRI device, the newly developed schemes will allow patients with arrhythmia problems, pediatric and obese subjects to breath freely without the need for any breath-hold scans. Shortening examination periods also reduces patient's stress, lowers the entire visit to the clinic and finally decreases the associated economic costs. Rapid imaging acquisitions will also allow for efficient extraction of quantitative information needed for the patients' diagnosis eg. tumor characterization and veins blockages through myocardial perfusion MRI. Current applications of interests include real-time CINE MRI and contrast changing perfusion MRI.

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Multi-beam bridges made from precast concrete box girders are one of the most common bridge types used in the United States. One problem that affects these bridges is the development of longitudinal or reflective cracks on the road surface because of failure of the shear keys. Some states have attempted to correct this problem by redesigning the shear key or adding post-tensioning, but the problem persists in many new bridges. The purpose of this study is to investigate why these shear key failures are occurring. This project studies two types of box girder designs, the common Precast/Prestressed Concrete Institute (PCI) box girder bridges and the Texas Department of Tranportation (TxDOT) box girder bridge. In the past, reflective cracking has occurred in bridges of both types. The analysis procedure involves finite element analyses of bridge models with realistic support and loading conditions, and comparing the PCI and TxDOT bridges. The results indicate that both PCI and TxDOT box girder have sufficient strength to resist cracking from vehicular loads, but uneven temperature changes and shrinkage strains cause high tensile stresses in the shear key regions and lead to reflective cracking. The analyses showed the highest stresses were often times near the supports, rather than at midspan. Past studies have proposed using larger composite deck slabs, transverse posttensioning, or full-depth shear keys to prevent shear key failure. Composite slabs were the most effective way to reduce high stresses in shear keys, and were effective for all loading cases considered. Post-tensioning and full-depth keys also showed a reduction in shear key stresses, but were less effective.

This research was undertaken to study the effects of high performance concrete (HPC) mix design modifications on the propensity of early-age cracking. Seven mixtures were tested: one 35 MPa conventional concrete (CC) mixture made with ordinary Portland cement with blended slag; one typical 50 MPa HPC mixture containing slag and silica fume; and five modified HPC mixtures using extra set-retarder, increased slag replacement, shrinkage-reducing admixture (SRA), pre-saturated lightweight aggregate (LWA), and decreased cement paste content to improve thermal and/or shrinkage properties. The mixtures were tested for durability, mechanical, thermal, and shrinkage properties. All modified HPC mixtures showed reduced shrinkage relative to the HPC control mixture, and the most shrinkage mitigation was observed in the mixture containing LWA. While SRA reduced restrained shrinkage in HPC to the magnitude of CC, it provided very low rapid chloride penetrability, and using LWA in HPC resulted in significant restrained shrinkage reduction compared to CC.

This research was undertaken to study the effects of high performance concrete (HPC) mix design modifications on the propensity of early-age cracking. Seven mixtures were tested: one 35 MPa conventional concrete (CC) mixture made with ordinary Portland cement with blended slag; one typical 50 MPa HPC mixture containing slag and silica fume; and five modified HPC mixtures using extra set-retarder, increased slag replacement, shrinkage-reducing admixture (SRA), pre-saturated lightweight aggregate (LWA), and decreased cement paste content to improve thermal and/or shrinkage properties. The mixtures were tested for durability, mechanical, thermal, and shrinkage properties. All modified HPC mixtures showed reduced shrinkage relative to the HPC control mixture, and the most shrinkage mitigation was observed in the mixture containing LWA. While SRA reduced restrained shrinkage in HPC to the magnitude of CC, it provided very low rapid chloride penetrability, and using LWA in HPC resulted in significant restrained shrinkage reduction compared to CC.

When portfolio selection is implemented by using the past sample values, parameter uncertainty may lead to suboptimal portfolios. Previous studies of portfolio selection demonstrate that classical approach based on the simple mean estimator is less reliable cause of inherent estimation error. In this paper, we investigate a shrinkage estimator based on Stein’s idea in measuring the expected returns. We apply the research of Jorion (1985) to Taiwan Stock market, present the effects of estimation error on the portfolio selection and demonstrate that the shrinkage estimator is robust and dominates the classical estimator on the MSE criterion. In addition, we also examine the effect of different shrinkage target on the performance of the Bayes-Stein estimator and find that this estimator still has lower risk than the classical sample mean.

Geopolymer cements offer an alternative to, and potential replacement for, ordinary Portland cement (OPC). Geopolymer technology also has the potential to reduce global greenhouse emissions caused by OPC production. There is already a considerable amount of work and research conducted on geopolymers in the past decades, and it is now possible to implement this technology commercially. However, to ensure that geopolymer becomes commercially available and able to be used in the world, further understanding of its ability to provide durable and long lasting materials is required. One main property which is still relatively unexplored compared to other properties is its shrinkage properties. The objective of this thesis is therefore to examine the shrinkage of geopolymers and factors which might influence it. / The factors which influence geopolymer strength were investigated as being the factors which may influence shrinkage. The selection of the activating solution is an important factor in forming the final product of a geopolymer. Activating solution SiO2/Na2O ratio is determined to be an important influence on the shrinkage of geopolymer. SEM images of the samples enable observation of the sample topology and microstructure. An important observation was the existence of a ‘knee point’ which also occurs in OPC shrinkage. The ‘knee point’ is the point where the shrinkage goes from rapid shrinkage to slow shrinkage. From SEMs it is noted that the samples past the knee point are shown to have a smoother topology which means it is more reacted. / Autogenous shrinkage is an important issue for OPC containing a high amount of silica, and is also a key factor in geopolymer shrinkage. Autogenous shrinkage is tested by keeping samples in a sealed environment where water lost to drying is kept to a minimum. It is noted that sealing and bagging the samples reduces the shrinkage considerably. The water to cement ratio, which is an important factor in OPC shrinkage, is also explored for the case of geopolymers. Water content plays an important role in determining early stage shrinkage, and has little to no effect on the later stage shrinkage. The water loss from the samples during drying on exposure to environment is noted and compared. The addition of more water did not necessary means that more water was lost. / Addition of slag is known to be beneficial to geopolymers by giving early structural strength and faster setting time. Commercial geopolymer concrete will also include the use of slag. However, the addition of slag up to a certain extent gives a deleterious affect on shrinkage. / A different type of Class F fly ash source with different composition data was used to see its effect on shrinkage, with only a slight influence observed between the two ashes tested. Fly ash was also ground for different lengths of time before use in geopolymerization, with grinding for less than 12 hours giving higher shrinkage than an unground sample, but shrinkage the decreasing with grinding for 18 or 24 hours. This initial higher shrinkage has been attributed to the mechanism of grinding which resulted in unevenly shaped fly ash particles taking up a larger initial volume resulting in higher shrinkage. The sample grinded for 24 hours showed higher shrinkage due to the particle size to be so fine that agglomerates may have form during mixing which would result in a lower reaction rate which increases the shrinkage. Elevated curing temperatures also reduce geopolymer shrinkage. / Thus, it is clear that the shrinkage of geopolymers is influenced by a wide range of variables, and more notably by a few important variables: activating solution ratio, addition of water, grinding and bagging. The shrinkage of geopolymers can be correlated to the strength to a certain extent. However, the understanding of the shrinkage of geopolymers is still at a very initial phase, and further research is required.

This work deals with problem of analyzing cooling curves of ductile iron and discovering possibility to predict shrinkage tendency of melt from thermal analysis results. This work was executed in foundry production conditions where obtained data from thermal analysis data linked to experimental castings where amount of shrinkage was estimated. Data were analyzed and looked for correlations between acquired thermal data and shrinkage defects.