Evaluation of the mechanical properties of self compacting concrete using current estimating models. Estimating the modulus of elasticity, tensile strength, and modulus of rupture of self compacting concrete

This study includes an analysis of the applicability of current models used for estimating the mechanical properties of conventional concrete to self-compacting concrete. The mechanical properties evaluated are: modulus of elasticity, tensile strength, and modulus of rupture. An extensive database which included the dosifications and the mechanical properties of 627 mixtures from 138 different references, was used. The models considered are: ACI, EC-2, NZS 3101:2006 (New Zealand code) and the CSA A23.3-04 (Canadian code). The precision in estimating the modulus of elasticity and tensile strength is acceptable for all models; however, all models are less precise in estimating the modulus of rupture.

Values of the deformation modulus of soil at different ways of determining this amount

In accordance with Russian, Kazakh and European building regulations, the deformation modulus of soil should be determined in the laboratory and also apply in situ soil tests on the stages of engineering geology prospecting. In this paper we present data on the determination and adjustment of the deformation characteristics at different stages of construction

Evaluation of the influence of visual parameters on wave transmission velocity in sawn chestnut timber

Non-destructive, visual evaluation and mechanical testing techniques were used to assess the structural properties of 374 samples of chestnut (Castanea sativa). The principal components method was applied to establish and interpret correlations between variables obtained of modulus of elasticity, bending strength and density. The static modulus of elasticity presented higher correlation values than those obtained using non-destructive methods. Bending strength presented low correlations with the non-destructive parameters, but there was some relation to the different knot ratios defined. The relationship was stronger with the most widely used ratio, CKDR. No significant correlations were observed between any of the variables and density.

Polypropylene fiber-reinforced self-compacting concrete for the reconstruction of the cathedral of La Laguna, Canary Island, Spain

High performance materials are needed for the reconstruction of such a singular building as a cathedral, since in addition to special mechanical properties, high self compact ability, high durability and high surface quality, are specified. Because of the project’s specifications, the use of polypropylene fiber-reinforced, self-compacting concrete was selected by the engineering office. The low quality of local materials and the lack of experience in applying macro polypropylene fiber for structural reinforcement with these components materials required the development of a pretesting program. To optimize the mix design, performance was evaluated following technical, economical and constructability criteria. Since the addition of fibers reduces concrete self-compactability, many trials were run to determine the optimal mix proportions. The variables introduced were paste volume; the aggregate skeleton of two or three fractions plus limestone filler; fiber type and dosage. Two mix designs were selected from the preliminary results. The first one was used as reference for self-compactability and mechanical properties. The second one was an optimized mix with a reduction in cement content of 20 kg/m3and fiber dosage of 1 kg/m3. For these mix designs, extended testing was carried out to measure the compression and flexural strength, modulus of elasticity, toughness, and water permeability resistance

Influence of the molecular architecture on the secondary relaxations of Poly(styrene-co-methylmethacrylate) copolymers

The processes of adsorption of grafted copolymers onto negatively charged surfaces were studied using a dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The control parameters in the study of the adsorption are the existence or absence on the molecular architecture of grafted polyethyleneglycol (PEG) chains with different lengths and the chemical nature of the main chain, poly(allylamine) (PAH) or poly(L-lysine) (PLL). It was found out that the adsorption kinetics of the polymers showed a complex behavior. The total adsorbed amount depends on the architecture of the polymer chains (length of the PEG chains), on the polymer concentration and on the chemical nature of the main chain. The comparison of the thicknesses of the adsorbed layers obtained from D-QCM and from ellipsometry allowed calculation of the water content of the layers that is intimately related to the grafting length. The analysis of D-QCM results also provides information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. It is shown that the adsorption of polymers with a charged backbone is not driven exclusively by the electrostatic interactions, but the entropic contributions as a result of the trapping of water in the layer structure are of fundamental importance.

Nanoindentation of Bridgman YBCO samples

In this study, the mechanical properties of YBa2Cu3O7−x, obtained by the Bridgman technique, were examined using a Berkovich tip indenter on the basal plane (0 0 1). Intrinsic hardness was measured by nanoindentation tests and corrected using the Nix and Gao model for this material. Furthermore, Vickers hardness tests were performed, in order to determine the possible size effect on these measurements. The results showed an underestimation of the hardness value when the tests were performed with large loads. Moreover, the elastic modulus of the Bridgman samples was 128 ± 5 GPa. Different residual imprints were visualised by atomic force microscopy and a focused ion beam, in order to observe superficial and internal fracturing. Mechanical properties presented a considerable reduction at the interface. This effect could be attributed to internal stress generated during the texturing process. In order to corroborate this hypothesis, an observation using transmission electron microscopy was performed.

Mechanical properties of self-consolidating concrete using conventional concrete models

The objective of this study is to analyze the applicability of current models used for estimating the mechanical properties of conventional concrete to self-consolidating concrete (SCC). The mechanical properties evaluated are modulus of elasticity, tensile strength,and modulus of rupture. As part of the study, it was necessary to build an extensive database that included the proportions and mechanical properties of 627 mixtures from 138 different references. The same models that are currently used for calculating the mechanical properties of conventional concrete were applied to SCC to evaluate their applicability to this type of concrete. The models considered are the ACI 318, ACI 363R, and EC2. These are the most commonly used models worldwide. In the first part of the study, the overall behavior and adaptability of the different models to SCC is evaluated. The specific characterization parameters for each concrete mixture are used to calculate the various mechanical properties applying the different estimation models. The second part of the analysis consists of comparing the experimental results of all the mixtures included in the database with the estimated results to evaluate the applicability of these models to SCC. Various statistical procedures, such as regression analysis and residual analysis, are used to compare the predicted and measured properties. It terms of general applicability, the evaluated models are suitable for estimating the modulus of elasticity, tensile strength, and modulus of rupture of SCC. These models have a rather low sensitivity, however, and adjust well only to mean values. This is because the models use the compressive strength as the main variable to characterize the concrete and do not consider other variables that affect these properties.

Variation throughout the tree stem in the physical-mechanical properties of the wood of Abies alba Mill. from the Spanish Pyrenees

This study analyses the variation of main physical-mechanical properties of wood along the longitudinal and radial directions of the tree for Abies alba Mill. growing in the Spanish Pyrenees. Small clear specimens were used to study the properties of volumetric shrinkage (VS), density (?), hardness (H), bending strength (MOR), modulus of elasticity (MOE), maximum compressive strength parallel to the grain (MCS) and impact strength (K). Several models of properties variation in the longitudinal and radial directions were analyzed. Main trends of variation of properties throughout the tree stem were identified although none of them could be fitted to predictive statistical models. Along the longitudinal direction, the properties studied followed a downward trend from the base to the crown, which was not significant in all cases, indicating that no differences in quality existed. Throughout the radial direction the trend is upward for the first 40-50 growth rings, after which it slopes downwards, more gently at first until rings 70-75 and then more steeply. This behaviour is related to variation in wood structure from the pith to the bark, depending on whether the wood is juvenile, sapwood or heartwood, and to wood maturity and microfibril angle. Authors encourage carrying further studies on other populations of A. alba in the Spanish Pyrenees to check if the trends found in this study apply to other provenances.

Experimental and numerical study of cracking of concrete due to reinforcement corrosion

La corrosión del acero es una de las patologías más importantes que afectan a las estructuras de hormigón armado que están expuestas a ambientes marinos o al ataque de sales fundentes. Cuando se produce corrosión, se genera una capa de óxido alrededor de la superficie de las armaduras, que ocupa un volumen mayor que el acero inicial; como consecuencia, el óxido ejerce presiones internas en el hormigón circundante, que lleva a la fisuración y, ocasionalmente, al desprendimiento del recubrimiento de hormigón. Durante los últimos años, numerosos estudios han contribuido a ampliar el conocimiento sobre el proceso de fisuración; sin embargo, aún existen muchas incertidumbres respecto al comportamiento mecánico de la capa de óxido, que es fundamental para predecir la fisuración. Por ello, en esta tesis se ha desarrollado y aplicado una metodología, para mejorar el conocimiento respecto al comportamiento del sistema acero-óxido-hormigón, combinando experimentos y simulaciones numéricas. Se han realizado ensayos de corrosión acelerada en condiciones de laboratorio, utilizando la técnica de corriente impresa. Con el objetivo de obtener información cercana a la capa de acero, como muestras se seleccionaron prismas de hormigón con un tubo de acero liso como armadura, que se diseñaron para conseguir la formación de una única fisura principal en el recubrimiento. Durante los ensayos, las muestras se equiparon con instrumentos especialmente diseñados para medir la variación de diámetro y volumen interior de los tubos, y se midió la apertura de la fisura principal utilizando un extensómetro comercial, adaptado a la geometría de las muestras. Las condiciones de contorno se diseñaron cuidadosamente para que los campos de corriente y deformación fuesen planos durante los ensayos, resultando en corrosión uniforme a lo largo del tubo, para poder reproducir los ensayos en simulaciones numéricas. Se ensayaron series con varias densidades de corriente y varias profundidades de corrosión. De manera complementaria, el comportamiento en fractura del hormigón se caracterizó en ensayos independientes, y se midió la pérdida gravimétrica de los tubos siguiendo procedimientos estándar. En todos los ensayos, la fisura principal creció muy despacio durante las primeras micras de profundidad de corrosión, pero después de una cierta profundidad crítica, la fisura se desarrolló completamente, con un aumento rápido de su apertura; la densidad de corriente influye en la profundidad de corrosión crítica. Las variaciones de diámetro interior y de volumen interior de los tubos mostraron tendencias diferentes entre sí, lo que indica que la deformación del tubo no fue uniforme. Después de la corrosión acelerada, las muestras se cortaron en rebanadas, que se utilizaron en ensayos post-corrosión. El patrón de fisuración se estudió a lo largo del tubo, en rebanadas que se impregnaron en vacío con resina y fluoresceína para mejorar la visibilidad de las fisuras bajo luz ultravioleta, y se estudió la presencia de óxido dentro de las grietas. En todas las muestras, se formó una fisura principal en el recubrimiento, infiltrada con óxido, y varias fisuras secundarias finas alrededor del tubo; el número de fisuras varió con la profundidad de corrosión de las muestras. Para muestras con la misma corrosión, el número de fisuras y su posición fue diferente entre muestras y entre secciones de una misma muestra, debido a la heterogeneidad del hormigón. Finalmente, se investigó la adherencia entre el acero y el hormigón, utilizando un dispositivo diseñado para empujar el tubo en el hormigón. Las curvas de tensión frente a desplazamiento del tubo presentaron un pico marcado, seguido de un descenso constante; la profundidad de corrosión y la apertura de fisura de las muestras influyeron notablemente en la tensión residual del ensayo. Para simular la fisuración del hormigón causada por la corrosión de las armaduras, se programó un modelo numérico. Éste combina elementos finitos con fisura embebida adaptable que reproducen la fractura del hormigón conforme al modelo de fisura cohesiva estándar, y elementos de interfaz llamados elementos junta expansiva, que se programaron específicamente para reproducir la expansión volumétrica del óxido y que incorporan su comportamiento mecánico. En el elemento junta expansiva se implementó un fenómeno de despegue, concretamente de deslizamiento y separación, que resultó fundamental para obtener localización de fisuras adecuada, y que se consiguió con una fuerte reducción de la rigidez tangencial y la rigidez en tracción del óxido. Con este modelo, se realizaron simulaciones de los ensayos, utilizando modelos bidimensionales de las muestras con elementos finitos. Como datos para el comportamiento en fractura del hormigón, se utilizaron las propiedades determinadas en experimentos. Para el óxido, inicialmente se supuso un comportamiento fluido, con deslizamiento y separación casi perfectos. Después, se realizó un ajuste de los parámetros del elemento junta expansiva para reproducir los resultados experimentales. Se observó que variaciones en la rigidez normal del óxido apenas afectaban a los resultados, y que los demás parámetros apenas afectaban a la apertura de fisura; sin embargo, la deformación del tubo resultó ser muy sensible a variaciones en los parámetros del óxido, debido a la flexibilidad de la pared de los tubos, lo que resultó fundamental para determinar indirectamente los valores de los parámetros constitutivos del óxido. Finalmente, se realizaron simulaciones definitivas de los ensayos. El modelo reprodujo la profundidad de corrosión crítica y el comportamiento final de las curvas experimentales; se comprobó que la variación de diámetro interior de los tubos está fuertemente influenciada por su posición relativa respecto a la fisura principal, en concordancia con los resultados experimentales. De la comparación de los resultados experimentales y numéricos, se pudo extraer información sobre las propiedades del óxido que de otra manera no habría podido obtenerse. Corrosion of steel is one of the main pathologies affecting reinforced concrete structures exposed to marine environments or to molten salt. When corrosion occurs, an oxide layer develops around the reinforcement surface, which occupies a greater volume than the initial steel; thus, it induces internal pressure on the surrounding concrete that leads to cracking and, eventually, to full-spalling of the concrete cover. During the last years much effort has been devoted to understand the process of cracking; however, there is still a lack of knowledge regarding the mechanical behavior of the oxide layer, which is essential in the prediction of cracking. Thus, a methodology has been developed and applied in this thesis to gain further understanding of the behavior of the steel-oxide-concrete system, combining experiments and numerical simulations. Accelerated corrosion tests were carried out in laboratory conditions, using the impressed current technique. To get experimental information close to the oxide layer, concrete prisms with a smooth steel tube as reinforcement were selected as specimens, which were designed to get a single main crack across the cover. During the tests, the specimens were equipped with instruments that were specially designed to measure the variation of inner diameter and volume of the tubes, and the width of the main crack was recorded using a commercial extensometer that was adapted to the geometry of the specimens. The boundary conditions were carefully designed so that plane current and strain fields were expected during the tests, resulting in nearly uniform corrosion along the length of the tube, so that the tests could be reproduced in numerical simulations. Series of tests were carried out with various current densities and corrosion depths. Complementarily, the fracture behavior of concrete was characterized in independent tests, and the gravimetric loss of the steel tubes was determined by standard means. In all the tests, the main crack grew very slowly during the first microns of corrosion depth, but after a critical corrosion depth it fully developed and opened faster; the current density influenced the critical corrosion depth. The variation of inner diameter and inner volume of the tubes had different trends, which indicates that the deformation of the tube was not uniform. After accelerated corrosion, the specimens were cut into slices, which were used in post-corrosion tests. The pattern of cracking along the reinforcement was investigated in slices that were impregnated under vacuum with resin containing fluorescein to enhance the visibility of cracks under ultraviolet lightening and a study was carried out to assess the presence of oxide into the cracks. In all the specimens, a main crack developed through the concrete cover, which was infiltrated with oxide, and several thin secondary cracks around the reinforcement; the number of cracks diminished with the corrosion depth of the specimen. For specimens with the same corrosion, the number of cracks and their position varied from one specimen to another and between cross-sections of a given specimen, due to the heterogeneity of concrete. Finally, the bond between the steel and the concrete was investigated, using a device designed to push the tubes of steel in the concrete. The curves of stress versus displacement of the tube presented a marked peak, followed by a steady descent, with notably influence of the corrosion depth and the crack width on the residual stress. To simulate cracking of concrete due to corrosion of the reinforcement, a numerical model was implemented. It combines finite elements with an embedded adaptable crack that reproduces cracking of concrete according to the basic cohesive model, and interface elements so-called expansive joint elements, which were specially designed to reproduce the volumetric expansion of oxide and incorporate its mechanical behavior. In the expansive joint element, a debonding effect was implemented consisting of sliding and separation, which was proved to be essential to achieve proper localization of cracks, and was achieved by strongly reducing the shear and the tensile stiffnesses of the oxide. With that model, simulations of the accelerated corrosion tests were carried out on 2- dimensional finite element models of the specimens. For the fracture behavior of concrete, the properties experimentally determined were used as input. For the oxide, initially a fluidlike behavior was assumed with nearly perfect sliding and separation; then the parameters of the expansive joint element were modified to fit the experimental results. Changes in the bulk modulus of the oxide barely affected the results and changes in the remaining parameters had a moderate effect on the predicted crack width; however, the deformation of the tube was very sensitive to variations in the parameters of oxide, due to the flexibility of the tube wall, which was crucial for indirect determination of the constitutive parameters of oxide. Finally, definitive simulations of the tests were carried out. The model reproduced the critical corrosion depth and the final behavior of the experimental curves; it was assessed that the variation of inner diameter of the tubes is highly influenced by its relative position with respect to the main crack, in accordance with the experimental observations. From the comparison of the experimental and numerical results, some properties of the mechanical behavior of the oxide were disclosed that otherwise could not have been measured.

Exact solution for the conjugate fluid-fluid problem in the thermal entrance region of laminar counterflow heat exchangers

A generalized Lévêque solution is presented for the conjugate fluid–fluid problem that arises in the thermal entrance region of laminar counterflow heat exchangers. The analysis, carried out for constant property fluids, assumes that the Prandtl and Peclet numbers are both large compared to unity, and neglects axial conduction both in the fluids and in the plate, assumed to be thermally thin. Under these conditions, the thermal entrance region admits an asymptotic self-similar description where the temperature varies as a power ϳ of the axial distance, with the particularity that the self-similarity exponent must be determined as an eigenvalue by solving a transcendental equation arising from the requirement of continuity of heat fluxes at the heat conducting wall. Specifically, the analysis reveals that j depends only on the lumped parameter ƙ = (A2/A1)1/3 (α1/α2)1/3(k2/k1), defined in terms of the ratios of the wall velocity gradients, A, thermal diffusivities, α i, and thermal conductivities,k i, of the fluids entering, 1, and exiting, 2, the heat exchanger. Moreover, it is shown that for large (small) values of K solution reduces to the classical first (second) Lévêque solution. Closed-form analytical expressions for the asymptotic temperature distributions and local heat-transfer rate in the thermal entrance region are given and compared with numerical results in the counterflow parallel-plate configuration, showing very good agreement in all cases.

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.

Mechanical characterization of GdBCO/Ag and YBCO single grains fabricated by top-seeded melt growth at 77 and 300 K

YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.

Modal analysis of buildings : quantification of uncertainties and model updating

Una estructura vibra con la suma de sus infinitos modos de vibración, definidos por sus parámetros modales (frecuencias naturales, formas modales y coeficientes de amortiguamiento). Estos parámetros se pueden identificar a través del Análisis Modal Operacional (OMA). Así, un equipo de investigación de la Universidad Politécnica de Madrid ha identificado las propiedades modales de un edificio de hormigón armado en Madrid con el método Identificación de los sub-espacios estocásticos (SSI). Para completar el estudio dinámico de este edificio, se ha desarrollado un modelo de elementos finitos (FE) de este edificio de 19 plantas. Este modelo se ha calibrado a partir de su comportamiento dinámico obtenido experimentalmente a través del OMA. Los objetivos de esta tesis son; (i) identificar la estructura con varios métodos de SSI y el uso de diferentes ventanas de tiempo de tal manera que se cuantifican incertidumbres de los parámetros modales debidos al proceso de estimación, (ii) desarrollar FEM de este edificio y calibrar este modelo a partir de su comportamiento dinámico, y (iii) valorar la bondad del modelo. Los parámetros modales utilizados en esta calibración han sido; espesor de las losas, densidades de los materiales, módulos de elasticidad, dimensiones de las columnas y las condiciones de contorno de la cimentación. Se ha visto que el modelo actualizado representa el comportamiento dinámico de la estructura con una buena precisión. Por lo tanto, este modelo puede utilizarse dentro de un sistema de monitorización estructural (SHM) y para la detección de daños. En el futuro, podrá estudiar la influencia de los agentes medioambientales, tales como la temperatura o el viento, en los parámetros modales. A structure vibrates according to the sum of its vibration modes, defined by their modal parameters (natural frequencies, damping ratios and modal shapes). These parameters can be identified through Operational Modal Analysis (OMA). Thus, a research team of the Technical University of Madrid has identified the modal properties of a reinforced-concrete-frame building in Madrid using the Stochastic Subspace Identification (SSI) method and a time domain technique for the OMA. To complete the dynamic study of this building, a finite element model (FE) of this 19-floor building has been developed throughout this thesis. This model has been updated from its dynamic behavior identified by the OMA. The objectives of this thesis are to; (i) identify the structure with several SSI methods and using different time blocks in such a way that uncertainties due to the modal parameter estimation are quantified, (ii) develop a FEM of this building and tune this model from its dynamic behavior, and (iii) Assess the quality of the model, the modal parameters used in this updating process have been; thickness of slabs, material densities, modulus of elasticity, column dimensions and foundation boundary conditions. It has been shown that the final updated model represents the structure with a very good accuracy. Thus, this model might be used within a structural health monitoring framework (SHM). The study of the influence of changing environmental factors (such as temperature or wind) on the model parameters might be considered as a future work.

Application of Bayesian approach in geotechnical engineering

En esta Tesis Doctoral se emplean y desarrollan Métodos Bayesianos para su aplicación en análisis geotécnicos habituales, con un énfasis particular en (i) la valoración y selección de modelos geotécnicos basados en correlaciones empíricas; en (ii) el desarrollo de predicciones acerca de los resultados esperados en modelos geotécnicos complejos. Se llevan a cabo diferentes aplicaciones a problemas geotécnicos, como es el caso de: (1) En el caso de rocas intactas, se presenta un método Bayesiano para la evaluación de modelos que permiten estimar el módulo de Young a partir de la resistencia a compresión simple (UCS). La metodología desarrollada suministra estimaciones de las incertidumbres de los parámetros y predicciones y es capaz de diferenciar entre las diferentes fuentes de error. Se desarrollan modelos "específicos de roca" para los tipos de roca más comunes y se muestra cómo se pueden "actualizar" esos modelos "iniciales" para incorporar, cuando se encuentra disponible, la nueva información específica del proyecto, reduciendo las incertidumbres del modelo y mejorando sus capacidades predictivas. (2) Para macizos rocosos, se presenta una metodología, fundamentada en un criterio de selección de modelos, que permite determinar el modelo más apropiado, entre un conjunto de candidatos, para estimar el módulo de deformación de un macizo rocoso a partir de un conjunto de datos observados. Una vez que se ha seleccionado el modelo más apropiado, se emplea un método Bayesiano para obtener distribuciones predictivas de los módulos de deformación de macizos rocosos y para actualizarlos con la nueva información específica del proyecto. Este método Bayesiano de actualización puede reducir significativamente la incertidumbre asociada a la predicción, y por lo tanto, afectar las estimaciones que se hagan de la probabilidad de fallo, lo cual es de un interés significativo para los diseños de mecánica de rocas basados en fiabilidad. (3) En las primeras etapas de los diseños de mecánica de rocas, la información acerca de los parámetros geomecánicos y geométricos, las tensiones in-situ o los parámetros de sostenimiento, es, a menudo, escasa o incompleta. Esto plantea dificultades para aplicar las correlaciones empíricas tradicionales que no pueden trabajar con información incompleta para realizar predicciones. Por lo tanto, se propone la utilización de una Red Bayesiana para trabajar con información incompleta y, en particular, se desarrolla un clasificador Naïve Bayes para predecir la probabilidad de ocurrencia de grandes deformaciones (squeezing) en un túnel a partir de cinco parámetros de entrada habitualmente disponibles, al menos parcialmente, en la etapa de diseño. This dissertation employs and develops Bayesian methods to be used in typical geotechnical analyses, with a particular emphasis on (i) the assessment and selection of geotechnical models based on empirical correlations; on (ii) the development of probabilistic predictions of outcomes expected for complex geotechnical models. Examples of application to geotechnical problems are developed, as follows: (1) For intact rocks, we present a Bayesian framework for model assessment to estimate the Young’s moduli based on their UCS. Our approach provides uncertainty estimates of parameters and predictions, and can differentiate among the sources of error. We develop ‘rock-specific’ models for common rock types, and illustrate that such ‘initial’ models can be ‘updated’ to incorporate new project-specific information as it becomes available, reducing model uncertainties and improving their predictive capabilities. (2) For rock masses, we present an approach, based on model selection criteria to select the most appropriate model, among a set of candidate models, to estimate the deformation modulus of a rock mass, given a set of observed data. Once the most appropriate model is selected, a Bayesian framework is employed to develop predictive distributions of the deformation moduli of rock masses, and to update them with new project-specific data. Such Bayesian updating approach can significantly reduce the associated predictive uncertainty, and therefore, affect our computed estimates of probability of failure, which is of significant interest to reliability-based rock engineering design. (3) In the preliminary design stage of rock engineering, the information about geomechanical and geometrical parameters, in situ stress or support parameters is often scarce or incomplete. This poses difficulties in applying traditional empirical correlations that cannot deal with incomplete data to make predictions. Therefore, we propose the use of Bayesian Networks to deal with incomplete data and, in particular, a Naïve Bayes classifier is developed to predict the probability of occurrence of tunnel squeezing based on five input parameters that are commonly available, at least partially, at design stages.

Influencia de la adición de nanosílice y microsílice en el comportamiento mecánico, microestructural y durable de un hormigón autocompactante

Desde mediados de la década de los 80 se está investigando sobre el hormigón autocompactante. Cada día, su uso en el mundo de la construcción es más común debido a sus numerosas ventajas como su excelente fluidez ya que puede fluir bajo su propio peso y llenar encofrados con formas complicadas y muy armados sin necesidad de compactaciones internas o externas. Por otra parte, la búsqueda de materiales más resistentes y duraderos, ha dado lugar a la incorporación de adiciones en materiales a base de cemento. En las últimas dos décadas, los ensayos con los nanomateriales, ha experimentado un gran aumento. Los resultados hasta ahora obtenidos pueden asumir no sólo un aumento en la resistencia de estos materiales, pero un cambio es su funcionalidad. Estas nanopartículas, concretamente la nanosílice, no sólo mejoran sus propiedades mecánicas y especialmente sus propiedades durables, sino que pueden implicar un cambio sustancial en las condiciones de uso y en su ciclo de vida. Este trabajo tiene como principal objetivo el estudio de las propiedades mecánicas, características microestructurales y durables de un hormigón autocompactante cuando se le agrega como adición nanosílice, microsílice y mezcla binarias de ambas, como adición al cemento. Para ello se han realizado 10 mezclas de hormigón. Se utilizó como referencia un hormigón autocompactante obtenido con cemento, caliza, árido, aditivo modificador de viscosidad Se han fabricado tres hormigones con la misma dosificación pero con diferentes contenidos de nanosílice. 2,5%, 5% y 7,5% Tres dosificaciones con adición de microsílice 2,5%, 5% y 7,5% y las tres restantes con mezclas binarias de nanosílice y microsílice con respectivamente2,5%-2,5%, 5%-2,5% y 2,5%-5%, sobre el peso del cemento. El contenido de superplastificante se modificó para conseguir las características de autocompactabilidad. Para observar los efectos de las adiciones añadidas al hormigón, se realiza una extensa campaña experimental. En ella se evaluaron en primer lugar, las características de autocompactabilidad del material en estado fresco, mediante los ensayos prescritos en la Instrucción Española del hormigón estructural EHE 08. Las propiedades mecánicas fueron evaluadas con ensayos de resistencia a compresión, resistencia a tracción indirecta y módulo de elasticidad. Las características microestructurales fueron analizadas mediante porosimetría por intrusión de mercurio, el análisis termogravimétrico y la microscopía electrónica de barrido. Para el estudio de la capacidad durable de las mezclas se realizaron ensayos de resistividad eléctrica, migración de cloruros, difusión de cloruros, carbonatación acelerada, absorción capilar y resistencia al hielo-deshielo. Los resultados ponen de manifiesto que la acción de las adiciones genera mejoras en las propiedades resistentes del material. Así, la adición de nanosílice proporciona mayores resistencias a compresión que la microsílice, sin embargo las mezclas binarias con bajas proporciones de adición producen mayores resistencias. Por otra parte, se observó mediante la determinación de las relaciones de gel/portlandita, que las mezclas que contienen nanosílice tienen una mayor actividad puzolánica que las que contienen microsílice. En las mezclas binarias se obtuvo como resultado que mientras mayor es el contenido de nanosílice en la mezcla mayor es la actividad puzolánica. Unido a lo anteriormente expuesto, el estudio de la porosidad da como resultado que la adición de nanosílice genera un refinamiento del tamaño de los poros mientras que la adición de microsílice disminuye la cantidad de los mismos sin variar el tamaño de poro medio. Por su parte, en las micrografías, se visualizó la formación de cristales procedentes de la hidratación del cemento. En ellas, se pudo observar, que al adicionar nanosílice, la velocidad de hidratación aumenta al aumentar la formación de monosulfoaluminatos con escasa presencia de etringita. Mientras que en las mezclas con adición de microsílice se observan mayor cantidad de cristales de etringita, lo que confirma que la velocidad de hidratación en estos últimos fue menor. Mediante el estudio de los resultados de las pruebas de durabilidad, se observó que no hay diferencias significativas entre el coeficiente de migración de cloruros y el coeficiente de difusión de cloruros en hormigones con adición de nano o microsílice. Aunque este coeficiente es ligeramente menor en mezclas con adición de microsílice. Sin embargo, en las mezclas binarias de ambas adiciones se obtuvo valores de los coeficientes de difusión o migración de cloruros inferiores a los obtenidos en mezclas con una única adición. Esto se evidencia en los resultados de las pruebas de resistividad eléctrica, de difusión de cloruros y de migración de cloruros. Esto puede ser debido a la suma de los efectos que producen el nano y micro adiciones en la porosidad. El resultado mostró que nanosílice tiene un papel importante en la reducción de los poros y la microsílice disminuye el volumen total de ellos. Esto permite definir la vida útil de estos hormigones a valores muy superiores a los exigidos por la EHE-08, por lo que es posible reducir, de forma notable, el recubrimiento exigido en ambiente de alta agresividad asegurando un buen comportamiento en servicio. Por otra parte, la pérdida de masa debido a los ciclos de congelación-descongelación es significativamente menor en los hormigones que contienen nanosílice que los que contienen microsílice. Este resultado está de acuerdo con el ensayo de absorción capilar. De manera general, se puede concluir que son las mezclas binarias y más concretamente la mezcla con un 5% de nanosílice y 2,5% de microsílice la que presenta los mejores resultados tanto en su comportamiento resistente con en su comportamiento durable. Esto puede ser debido a que en estas mezclas la nanosílice se comporta como un núcleo de activación de las reacciones puzolánicas rodeado de partículas de mayor tamaño. Además, el extraordinario comportamiento durable puede deberse también a la continuidad en la curva granulométrica por la existencia de la microsílice, el filler calizo, el cemento, la arena y la gravilla con tamaños de partículas que garantice mezclas muy compactas que presentan elevadas prestaciones. Since the middle of the decade of the 80 is being investigated about self-consolidating concrete. Every day, its use in the world of construction is more common due to their numerous advantages as its excellent fluidity such that it can flow under its own weight and fill formworks with complicated shapes and congested reinforcement without need for internal or external compactions. Moreover, the search for more resistant and durable materials, has led to the incorporation of additions to cement-based materials. In the last two decades, trials with nanomaterials, has experienced a large increase. The results so far obtained can assume not only an increase in the resistance of these materials but a change is its functionality. These nano particles, particularly the nano silica, not only improve their mechanical properties and especially its durable properties, but that may imply a substantial change in the conditions of use and in their life cycle. This work has as its main objective the study of the mechanical properties, the microstructural characteristics and durability capacity in one self-compacting concrete, when added as addition to cement: nano silica, micro silica o binary mixtures of both. To this effect, 10 concrete mixes have been made. As reference one with a certain amount of cement, limestone filler, viscosity modifying additive and water/binder relation. Furthermore they were manufactured with the same dosage three mix with addition of 2.5%, 5% and 7.5% of nano silica by weight of cement. Other three with 2.5%, 5% and 7.5% of micro silica and the remaining three with binary mixtures of 2.5%-2.5%, 5%-2.5% and 2.5%-5% of silica nano-micro silica respectively, b weight of cement, varying only the amount of superplasticizer to obtain concrete with characteristics of self-compactability. To observe the effects of the additions added to the concrete, an extensive experimental campaign was performed. It assessed, first, the characteristics of self-compactability of fresh material through the tests prescribed in the Spanish Structural Instruction Concrete EHE 08. The mechanical properties were evaluated by compression strength tests, indirect tensile strength and modulus of elasticity. The microstructural properties were analyzed by mercury intrusion porosimetry, thermogravimetric analysis and scanning electron microscopy. To study the durability, were performed electrical resistivity tests, migration and diffusion of chlorides, accelerated carbonation, capillary suction and resistance to freeze-thaw cycles. The results show that the action of the additions generates improvements in the strength properties of the material. Specifically, the addition of nano silica provides greater resistance to compression that the mix with micro silica, however binary mixtures with low addition rates generate higher strengths. Moreover, it was observed by determining relationships gel/portlandite, that the pozzolanic activity in the mixtures with nano silica was higher than in the mixtures with micro silica. In binary mixtures it was found that the highest content of nano silica in the mix is the one with the highest pozzolanic activity. Together with the foregoing, the study of the porosity results in the mixture with addition of nano silica generates a refinement of pore size while adding micro silica decreases the amount thereof without changing the average pore size. On the other hand, in the micrographs, the formation of crystals of cement hydration was visualized. In them, it was observed that by adding nano silica, the speed of hydration increases with increasing formation monosulfoaluminatos with scarce presence of ettringite. While in mixtures with addition of micro silica, ettringite crystals are observed, confirming that the hydration speed was lower in these mixtures. By studying the results of durability testing, it observed that no significant differences between the coefficient of migration of chlorides and coefficient of diffusion of chlorides in concretes with addition of nano or micro silica. Although this coefficient is slightly lower in mixtures with addition of micro silica. However, in binary mixtures of both additions was obtained values of coefficients of difusion o migration of chlorides lower than those obtained in mixtures with one of the additions. This is evidenced by the results of the tests electrical resistivity, diffusion of chlorides and migration of chlorides. This may be due to the sum of the effects that produced the nano and micro additions in the porosity. The result showed that nano silica has an important role in the pores refining and the micro silica decreases the total volume of them. This allows defining the life of these concretes in values to far exceed those required by the EHE-08, making it possible to reduce, significantly, the coating required in highly aggressive environment and to guarantee good behavior in service. Moreover, the mass loss due to freeze-thaw cycles is significantly lower in concretes containing nano silica than those containing micro silica. This result agrees with the capillary absorption test. In general, one can conclude that the binary mixture and more specifically the mixture with 5% of nano silica and 2.5% silica fume is which presents the best results in its durable behavior. This may be because in these mixtures, the nano silica behaves as cores activation of pozzolanic reactions. In addition, the durable extraordinary behavior may also be due to the continuity of the grading curve due to existence of micro silica, limestone filler, cement, sand and gravel with particle sizes that guarantees very compact mixtures which have high performance.