Analisis seccional de elementos de hormigón armado sometidos a fatiga, incluyendo secciones entre fisuras

El efecto de la fatiga sobre elementos de hormigón armado no es normalmente considerado en la fase de diseño estructural. Incluso en los códigos de diseño más avanzados, el tratamiento está basado en el estado estático de tensiones y en el concepto de las curvas S-N, que proporcionan el número de ciclos de carga resistentes. A pesar de eso, la fatiga del hormigón es un proceso de progresiva microfisuración del material que conlleva, a nivel macroscópico, un cambio continuo de las propiedades mecánicas. Las losas de tableros de puentes en sentido transversal, las losas de transición entre estribo y terraplén en puentes de carretera, estructuras bajo la acción del oleaje o del viento, son elementos de hormigón armado sometidos a alto número de ciclos donde ya se han publicado problemas de fatiga. La evaluación de la capacidad resistente de estructuras antiguas ante incrementos de cargas y el diseño estructural orientado a toda la vida útil requieren también tener en cuenta los mecanismos de degradación como la fatiga. La Tesis aborda el estudio de la influencia de las cargas repetidas en el comportamiento de estructuras de hormigón armado que trabajan fundamentalmente a flexión, tanto en rotura como en servicio. En este tipo de elementos, la fatiga puede tener lugar de tres formas: fatiga del hormigón en compresión, fatiga de la armadura y fatiga de la adherencia hormigón - acero. Una probeta de hormigón sometida a carga repetida experimenta un continuo crecimiento de sus deformaciones máximas y permanentes, a la vez que una reducción de su rigidez. Para evaluar la influencia de la fatiga en elementos armados, se ha elaborado un modelo de material para el hormigón, dependiente del número de ciclos, la calidad del material y los niveles entre los que oscila la tensión. Además, para su aplicación al análisis de elementos estructurales, se ha definido un criterio de acumulación de la degradación que tiene en cuenta la degradación en procesos de límites de la tensión variable. El modelo de material ha sido introducido después en una metodología de análisis seccional que permite reproducir la evolución de la distribución de tensiones y deformaciones durante la vida a fatiga de un elemento, a la vez que calcular el número de ciclos de carga resistentes. El estudio llevado a cabo muestra que dentro de la sección se desarrolla un proceso de redistribución de tensiones a la vez que aumentan las deformaciones. Esta capacidad redistributiva de elementos armados que trabajan a flexión explica que el modo de fallo por fatiga de vigas sea por rotura frágil de la armadura incluso ante cargas elevadas en las que el fallo por fatiga del hormigón podría ser previsible empleando las actuales metodologías basadas en el estado tensional estático. El análisis realizado en la Tesis muestra que el fallo por fatiga del hormigón es posible en elementos sobre-armados ante cargas máximas elevadas, además de elementos comprimidos como soportes. La fragilidad o no del fallo por fatiga es asimismo estudiada en función del tipo de fallo: por el acero o el hormigón. El estudio del comportamiento en servicio de elementos ante cargas repetidas ha requerido analizar la influencia de la colaboración de las secciones entre fisuras, efecto tension - stiffening. Para ello, se ha incluido en la metodología de análisis la progresiva reducción de la adherencia entre acero y hormigón a tracción a través de la evolución en las leyes tensión de adherencia – deslizamiento. El acoplamiento de los fenómenos de degradación de los bloques traccionado y comprimido permiten estudiar la evolución de flechas y fisuras en elementos cargados de forma repetida. El estudio es especialmente importante para calcular los valores residuales de estas variables, que son crecientes con el número de ciclos y definen la respuesta en servicio por razones estéticas y de durabilidad. La Tesis ha contado con dos campañas experimentales, con ensayos de fatiga en tirantes armados a tracción y de vigas de hormigón armado, ambos ante carga repetida. Los ensayos han confirmado los análisis teóricos y han permitido refinar algunas tendencias. Las conclusiones alcanzadas permiten evaluar la sensibilidad a fatiga de elementos de hormigón armado de una forma realista y comprensible. Asimismo, se presentan una serie de diagramas con recomendaciones Abstract The fatigue effect on reinforced concrete elements is not usually considered at the design phase. Even the most advanced codes of practice are based on the static stress state and the well-known concept of S-N curves. These approaches are only focused on the maximum bearable number of load cycles. In spite of this, fatigue of concrete is a process of progressive microcracking which leads to the continuous change of the macroscopic mechanical properties. Bridge deck slabs, approach slabs and offshore structures are reinforced concrete members subjected to highly repeated loading where fatigue problems have been already reported. The evaluation of the carrying capacity of older structures under increasing loads, and a lifetime – oriented structural design, need to account for all the degradation mechanisms like fatigue. The Thesis deals with the response of reinforced concrete elements under bending fatigue, considering both the serviceability and ultimate behavior. The fatigue influence may take place through three mechanisms: fatigue of concrete in compression, brittle fracture of the reinforcement and fatigue of bond between concrete and steel. Under uniaxial compression fatigue, concrete exhibits increasing total and residual strains and decreasing stiffness. To evaluate its influence on structural members, a time – dependent material model for concrete has been proposed. It includes evolution laws for total strains and stiffness as function of the number of cycles, the material properties and the stress levels of the cyclic load. Furthermore, a new accumulation criterion has been introduced in order to account for the degradation under variable stress limits, the so-called equivalent number of cycles. The material model is then introduced in a sectional algorithm, which enables the calculation of the stress and strain evolution over the section depth during the fatigue life. The realistic resistant number of cycles is also investigated based on this evolution. The analysis shows that a process of redistribution of stresses develops within the crosssection while strains increase. Moreover, the sectional capacity for redistribution of stresses is shown to be the governing factor of the mode of failure and explains why most experimental results on beams in the literature failed by brittle fracture of the reinforcement. The study concludes that fatigue failure of concrete in compression may take place in over-reinforced beams under maximum relative load levels, as well as in columns. The brittleness of the fatigue failure is also analyzed with regard to the type of failure: due to either concrete or steel. The study of the serviceability behavior of concrete members under repeated loading has been carried out accounting for the contribution of sections between cracks, the tension-stiffening effect. For this, the progressive reduction of bond between concrete and the reinforcement along the tension chord is considered through the change of the local bond – slip relations. To study the evolution of crack widths and deflections, both degradation mechanisms of the compression and tension chords are coupled. The analysis is particularly interesting for calculating their residual values, since they increase during the fatigue life and define the service state of the structure for aesthetic and durability reasons. Two experimental campaigns have been performed. On the one hand, two reinforced concrete ties have been tested under repeated tension. On the other hand, three reinforced concrete beams have been loaded until fatigue failure. The tests have confirmed the theoretical approaches and refined some trends. The conclusions of the Thesis enable the evaluation of the fatigue sensibility of reinforced concrete elements in a realistic and understandable way. A series of practical proposals and recommendations have been presented in the form of diagrams showing the resistant number of cycles and the mode of fatigue failure

Application of the Mutual Information Minimization to speaker recognition / identification improvement

In this paper we propose the inversion of nonlinear distortions in order to improve the recognition rates of a speaker recognizer system. We study the effect of saturations on the test signals, trying to take into account real situations where the training material has been recorded in a controlled situation but the testing signals present some mismatch with the input signal level (saturations). The experimental results for speaker recognition shows that a combination of several strategies can improve the recognition rates with saturated test sentences from 80% to 89.39%, while the results with clean speech (without saturation) is 87.76% for one microphone, and for speaker identification can reduce the minimum detection cost function with saturated test sentences from 6.42% to 4.15%, while the results with clean speech (without saturation) is 5.74% for one microphone and 7.02% for the other one

Maximum likelihood Linear Programming Data Fusion for Speaker Recognition

Biometric system performance can be improved by means of data fusion. Several kinds of information can be fused in order to obtain a more accurate classification (identification or verification) of an input sample. In this paper we present a method for computing the weights in a weighted sum fusion for score combinations, by means of a likelihood model. The maximum likelihood estimation is set as a linear programming problem. The scores are derived from a GMM classifier working on a different feature extractor. Our experimental results assesed the robustness of the system in front a changes on time (different sessions) and robustness in front a change of microphone. The improvements obtained were significantly better (error bars of two standard deviations) than a uniform weighted sum or a uniform weighted product or the best single classifier. The proposed method scales computationaly with the number of scores to be fussioned as the simplex method for linear programming