A Damage Model for Masonry Infilled Frames

The effect of infill walls on the behaviour of frames is widely recognized, and, for several decades now, has been the subject of numerous experimental investigations. However, the analytical modeling of infilled panels and frames under in-plane loading is difficult and generally unreliable. From the point of view of the simulation technique the models may be divided into micromodels and simplified (or macro-) models. Based on the equivalent strut approach (simplified model), in this paper a damage model is proposed for the characterization of masonry walls submitted to lateral cyclic loads. The model, developed along the lines of the Continuum Damage Mechanics, have the advantages of including explicitly the coupling between damage and mechanical behaviour and so is consistent with the definition of damage as a phenomenon with mechanical consequences.

Fatigue damage modelling of steel structures

In the present work a constitutive model is developed which permits the simulation of the low cycle fatigue behaviour in steel framed structures. In the elaboration of this model, the concepts of the mechanics of continuum medium are applied on lumped dissipative models. In this type of formulation an explicit coupling between the damage and the structural mechanical behaviour is employed, allowing the possibility of considering as a whole different coupled phenomena. A damage index is defined in order to model elastoplasticity coupled with damage and fatigue damage.

Análisis experimental y por el método de los elementos finitos del estado de tensiones en uniones carpinteras de empalme de llave

In this research, the halved and tabled traditional timber scarf joint is analyzed. This joint consists in two end joint pieces usually subjected to tension. Initially, the study is discussed from an experimental point of view. In this way, 3 critical cross-sections are established (section of the notch, section of the horizontal plane and reduced section) and mechanical tests are performed to achieve the failure on each of critical sections by changing the geometry of the joint. The study is completed by developing a finite element model which allows verify experimental results and extend the analysis to other geometries. This model has to simulate the real behavior of the material which is being studied, so mechanical tests are performed to obtain the elastic constants and the coefficients of friction of the material. In the reduced section, an abrupt decrease of the effective cross-section takes place, and this effect is also experimentally analyzed. These tests indicate that a crack is initiated before the bending-tension failure occurs in the reduced section. The test material consists of wood of Pinus sylvestris L. coming from the “Valsaín´s Sawmill” (Segovia) with “premium quality” according to the nonstructural wood visual classification of sawmill. It is observed that initiation of a crack, in the mortise (bottom of reduced section), and shear stress concentration, at the initial part of the heel (beginning of horizontal plane), completely determine the mechanical behaviour of the joint, resulting in 3 failure modes: local compression failure in the section of the notch, shear failure in the horizontal plane, and failure of stresses concentration, mainly perpendicular to the grain tension, at the bottom of reduced section. The geometric optimization is obtained for halved and tabled traditional scarf joint, when the joint has made with similar properties of wood than tested specimens, for any height and width of the cross-section. It is considered the failure due to the initiation of a crack in reduced section, by applying a correction coefficient into the usual equation used to design the members subjected to both tension and bending. Therefore, it is possible to obtain, analytically, the design conditions to be met of the 3 critical cross-sections. According to the theoretical optimization, the tension strength of complete cross-section is reduced until 14%, when using this type of joint. The experimental optimization indicates even a greater reduction, until 6%. En el presente trabajo de investigación se analiza el comportamiento mecánico de las uniones tradicionales de empalme de llave, que consisten en dos piezas unidas por sus testas transmitiéndose entre ellas principalmente un esfuerzo de tracción. Inicialmente, el estudio se aborda desde un punto de vista experimental. De este modo, se establecen las 3 secciones críticas o de estudio (sección del encaje, sección rasante del cogote y sección reducida) y se realizan ensayos mecánicos, variando la geometría de la unión, para alcanzar la rotura en cada una de ellas. Se completa el estudio mediante la elaboración de un modelo por elementos finitos que permite verificar los resultados experimentales y ampliar el análisis a otras geometrías. Este modelo debe simular el comportamiento real del material objeto de estudio, por lo que se realizan ensayos para obtener las constantes elásticas y los coeficientes de rozamiento del mismo. También se analiza, experimentalmente, el efecto entalladura que reduce bruscamente la sección completa del tirante, estableciendo que el fallo por flexotracción en la sección reducida de la pieza, no llega a producirse por el inicio previo de una grieta. El material de ensayo consiste en madera de Pinus sylvestris L. (pino silvestre) procedente del Aserradero de Valsaín (Segovia) y de calidad “Extra” o “Primera” según la clasificación visual no estructural del aserradero. Se observa que el inicio de una grieta en la mortaja del rediente y la concentración de tensiones tangenciales en la parte inicial del cogote, determinan completamente el comportamiento mecánico de la unión, dando lugar a 3 modos distintos de rotura: fallo por compresión en la sección del encaje, fallo por cortante en la sección rasante y fallo por concentración de tensiones, principalmente tracciones perpendiculares, en el rebaje de la sección reducida. Se consigue optimizar geométricamente cualquier empalme de llave confeccionado con madera de características similares a la ensayada, para cualquier valor de la altura y de la anchura de la sección. Se considera el agotamiento en la sección reducida causado por el inicio de grieta, mediante la aplicación de un coeficiente corrector en la expresión habitual de agotamiento por flexotracción, en consecuencia, finalmente es posible obtener, de modo analítico, un valor del índice de agotamiento en cada una de las 3 secciones de estudio. La optimización teórica del empalme de llave indica que la capacidad resistente del tirante bruto se reduce al 14%, cuando se coloca este tipo de unión tradicional. Experimentalmente se obtiene, que, para la sección ensayada, la capacidad resistente del tirante bruto se reduce todavía más, llegando al 6%.

Comportamiento estructural de elementos de fábrica reforzados con materiales compuestos avanzados solicitados a flexión y flexocompresión

El refuerzo de estructuras existentes mediante el encolado exterior de láminas de polímeros reforzados con fibras (FRP) se ha convertido en la aplicación más común de los materiales compuestos avanzados en construcción. Estos materiales presentan muchas ventajas frente a los materiales convencionales (sin corrosión, ligeros, de fácil aplicación, etc.). Pero a pesar de las numerosas investigaciones realizadas, aún persisten ciertas dudas sobre algunos aspectos de su comportamiento y las aplicaciones prácticas se llevan a cabo sólo con la ayuda de guías, sin que haya una normativa oficial. El objetivo de este trabajo es incrementar el conocimiento sobre esta técnica de refuerzo, y más concretamente, sobre el refuerzo a flexión de estructuras de fábrica. Con frecuencia el elemento reforzado es de hormigón armado y las láminas de FRP encoladas al exterior sirven para mejorar su resistencia a flexión, cortante o compresión (encamisados). Sin embargo su empleo en otros materiales como las estructuras de fábrica resulta muy prometedor. Las fábricas se caracterizan por soportar muy bien los esfuerzos de compresión pero bastante mal los de tracción. Adherir láminas de materiales compuestos puede servir para mejorar la capacidad resistente de elementos de fábrica sometidos a esfuerzos de flexión. Pero para ello, debe quedar garantizada una correcta adherencia entre el FRP y la fábrica, especialmente en edificios antiguos cuya superficie puede estar deteriorada por encontrarse a la intemperie o por el propio paso del tiempo. En el capítulo II se describen los objetivos fundamentales del trabajo y el método seguido. En el capítulo III se hace una amplia revisión del estado de conocimiento sobre el tema. En el apartado III.1 se detallan las principales características y propiedades mecánicas de fibras, matrices y materiales compuestos así como sus principales aplicaciones, haciendo especial hincapié en aspectos relativos a su durabilidad. En el apartado III.2 se incluye una revisión histórica de las líneas de investigación, tanto teóricas como empíricas, publicadas sobre estructuras de hormigón reforzadas a flexión encolando materiales compuestos. El apartado III.3 se centra en el aspecto fundamental de la adherencia refuerzo-soporte. Se hace un repaso a distintos modelos propuestos para prevenir el despegue distinguiendo si éste se inicia en la zona de anclaje o si está inducido por fisuras en la zona interior del elemento. Se observa falta de consenso en las propuestas. Además en este punto se relatan las campañas experimentales publicadas acerca de la adherencia entre materiales compuestos y fábricas. En el apartado III.4 se analizan las particularidades de las estructuras de fábrica. Además, se revisan algunas de las investigaciones relativas a la mejora de su comportamiento a flexión mediante láminas de FRP. El comportamiento mecánico de muros reforzados solicitados a flexión pura (sin compresión) ha sido documentado por varios autores, si bien es una situación poco frecuente en fábricas reales. Ni el comportamiento mecánico de muros reforzados solicitados a flexocompresión ni la incidencia que el nivel de compresión soportado por la fábrica tiene sobre la capacidad resistente del elemento reforzado han sido suficientemente tratados. En cuanto a los trabajos teóricos, las diferentes propuestas se basan en los métodos utilizados para hormigón armado y comparten los principios habituales de cálculo. Sin embargo, presentan diferencias relativas, sobre todo, a tres aspectos: 1) la forma de modelar el comportamiento de la fábrica, 2) el valor de deformación de cálculo del refuerzo, y 3) el modo de fallo que se considera recomendable buscar con el diseño. A pesar de ello, el ajuste con la parte experimental de cada trabajo suele ser bueno debido a una enorme disparidad en las variables consideradas. Cada campaña presenta un modo de fallo característico y la formulación que se propone resulta apropiada para él. Parece necesario desarrollar un método de cálculo para fábricas flexocomprimidas reforzadas con FRP que pueda ser utilizado para todos los posibles fallos, tanto atribuibles a la lámina como a la fábrica. En el apartado III.4 se repasan algunas lesiones habituales en fábricas solicitadas a flexión y se recogen ejemplos de refuerzos con FRP para reparar o prevenir estos daños. Para mejorar el conocimiento sobre el tema, se llevan a cabo dos pequeñas campañas experimentales realizadas en el Instituto de Ciencias de la Construcción Eduardo Torroja. La primera acerca de la adherencia de materiales compuestos encolados a fábricas deterioradas (apartado IV.1) y la segunda sobre el comportamiento estructural a flexocompresión de probetas de fábrica reforzadas con estos materiales (apartado IV.2). En el capítulo V se analizan algunos de los modelos de adherencia propuestos para prevenir el despegue del extremo del refuerzo. Se confirma que las predicciones obtenidas con ellos resultan muy dispares. Se recopila una base de datos con los resultados experimentales de campañas sobre adherencia de FRP a fábricas extraídas de la literatura y de los resultados propios de la campaña descrita en el punto IV.1. Esta base de datos permite conocer cual de los métodos analizados resulta más adecuado para dimensionar el anclaje de láminas de FRP adheridas a fábricas. En el capítulo VI se propone un método para la comprobación en agotamiento de secciones de fábrica reforzadas con materiales compuestos sometidas a esfuerzos combinados de flexión y compresión. Está basado en el procedimiento de cálculo de la capacidad resistente de secciones de hormigón armado pero adaptado a las fábricas reforzadas. Para ello, se utiliza un diagrama de cálculo tensión deformación de la fábrica de tipo bilineal (acorde con el CTE DB SE-F) cuya simplicidad facilita el desarrollo de toda la formulación al tiempo que resulta adecuado para predecir la capacidad resistente a flexión tanto para fallos debidos al refuerzo como a la fábrica. Además se limita la deformación de cálculo del refuerzo teniendo en consideración ciertos aspectos que provocan que la lámina adherida no pueda desarrollar toda su resistencia, como el desprendimiento inducido por fisuras en el interior del elemento o el deterioro medioambiental. En concreto, se propone un “coeficiente reductor por adherencia” que se determina a partir de una base de datos con 68 resultados experimentales procedentes de publicaciones de varios autores y de los ensayos propios de la campaña descrita en el punto IV.2. También se revisa la formulación propuesta con ayuda de la base de datos. En el capítulo VII se estudia la incidencia de las principales variables, como el axil, la deformación de cálculo del refuerzo o su rigidez, en la capacidad final del elemento. Las conclusiones del trabajo realizado y las posibles líneas futuras de investigación se exponen en el capítulo VIII. ABSTRACT Strengthening of existing structures with externally bonded fiber reinforced polymers (FRP) has become the most common application of advanced composite materials in construction. These materials exhibit many advantages in comparison with traditional ones (corrosion resistance, light weight, easy to apply, etc.). But despite countless researches have been done, there are still doubts about some aspects of their behaviour and applications are carried out only with the help of guidelines, without official regulations. The aim of this work is to improve the knowledge on this retrofitting technique, particularly in regard to flexural strengthening of masonry structures. Reinforced concrete is often the strengthened material and external glued FRP plates are used to improve its flexural, shear or compressive (by wrapping) capacity. However the use of this technique on other materials like masonry structures looks promising. Unreinforced masonry is characterized for being a good material to support compressive stresses but really bad to withstand tensile ones. Glue composite plates can improve the flexural capacity of masonry elements subject to bending. But a proper bond between FRP sheet and masonry must be ensured to do that, especially in old buildings whose surface can be damaged due to being outside or ageing. The main objectives of the work and the methodology carried out are described In Chapter II. An extensive overview of the state of art is done in Chapter III. In Section III.1 physical and mechanical properties of fibers, matrix and composites and their main applications are related. Durability aspects are especially emphasized. Section III.2 includes an historical overview of theoretical and empirical researches on concrete structures strengthened gluing FRP plates to improve their flexural behaviour. Section III.3 focuses on the critical point of bonding between FRP and substrate. Some theoretical models to prevent debonding of FRP laminate are reviewed, it has made a distinction between models for detachment at the end of the plate or debonding in the intermediate zones due to the effects of cracks. It is observed a lack of agreement in the proposals. Some experimental studies on bonding between masonry and FRP are also related in this chapter. The particular characteristics of masonry structures are analyzed in Section III.4. Besides some empirical and theoretical investigations relative to improve their flexural capacity with FRP sheets are reviewed. The mechanical behaviour of strengthened walls subject to pure bending (without compression) has been established by several authors, but this is an unusual situation for real masonry. Neither mechanical behaviour of walls subject to bending and compression nor influence of axial load in the final capacity of the strengthened element are adequately studied. In regard to theoretical studies, the different proposals are based on reinforced concrete analytical methods and share common design principles. However, they present differences, especially, about three aspects: 1) the constitutive law of masonry, 2) the value of ultimate FRP strain and 3) the desirable failure mode that must be looked for. In spite of them, a good agreement between each experimental program and its theoretical study is often exhibited due to enormous disparity in considered test parameters. Each experimental program usually presents a characteristic failure mode and the proposed formulation results appropriate for this one. It seems necessary to develop a method for FRP strengthened walls subject to bending and compression enable for all failure modes (due to FRP or masonry). Some common damages in masonry subject to bending are explained in Section III.4. Examples of FRP strengthening to repair or prevent these damages are also written. Two small experimental programs are carried out in Eduardo Torroja Institute to improve the knowledge on this topic. The first one is concerned about the bond between FRP plates and damaged masonry (section IV.1) and the second one is related to the mechanical behaviour of the strengthened masonry specimens subject to out of plane bending combined with axial force (section IV.2). In the Chapter V some bond models to prevent the debonding at the FRP plate end are checked. It is confirmed that their predictions are so different. A pure-shear test database is compiled with results from the existing literature and others from the experimental program described in section IV.1. This database lets know which of the considered model is more suitable to design anchorage lengths of glued FRP to masonry. In the Chapter VI a method to check unreinforced masonry sections with external FRP strengthening subject to bending and compression to the ultimate limit state is proposed. This method is based on concrete reinforced one, but it is adapted to strengthened masonry. A bilinear constitutive law is used for masonry (according to CTE DB SE-F). Its simplicity helps to develop the model formulation and it has proven to be suitable to predict bending capacity either for FRP failures or masonry crushing. With regard to FRP, the design strain is limited. It is taken into account different aspects which cause the plate can’t reach its ultimate strength, like intermediate FRP debonding induced by opening cracking or environmental damage. A “bond factor” is proposed. It is obtained by means of an experimental bending test database that includes 68 results from the existing literature and from the experimental program described in section IV.2. The proposed formulation has also been checked with the help of bending database. The effects of the main parameters, like axial load, FRP design effective strain or FRP stiffness, on the bending capacity of the strengthened element are studied in Chapter VII. Finally, the main conclusions from the work carried out are summarized in Chapter VIII. Future lines of research to be explored are suggested as well.

Concrete constitutive model, calibration and applications

A methodology has been developed for characterising the mechanical behaviour of concrete, based on the damaged plasticity model, enriched with a user subroutine (V)USDFLD in order to capture better the ductility of the material under moderate confining pressures. The model has been applied in the context of the international benchmark IRIS_2012, organised by the OECD/NEA/CSNI Nuclear Energy Agency, dealing with impacts of rigid and deformable missiles against reinforced concrete targets. A slightly modified version of the concrete damaged plasticity model was used to represent the concrete. The simulation results matched very well the observations made during the actual tests. Particularly successful predictions involved the energy spent by the rigid missile in perforating the target, the crushed length of the deformable missile, the crushed and cracked areas of the concrete target, and the values of the strains recorded at a number of locations in the concrete slab.

Induction of engineered residual stresses fields and enhancement of fatigue life of high reliability metallic components by laser shock processing

Laser shock processing (LSP) is being increasingly applied as an effective technology for the improvement of metallic materials mechanical and surface properties in different types of components as a means of enhancement of their corrosion and fatigue life behavior. As reported in previous contributions by the authors, a main effect resulting from the application of the LSP technique consists on the generation of relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Additional results accomplished by the authors in the line of practical development of the LSP technique at an experimental level (aiming its integral assessment from an interrelated theoretical and experimental point of view) are presented in this paper. Concretely, follow-on experimental results on the residual stress profiles and associated surface properties modification successfully reached in typical materials (especially Al and Ti alloys characteristic of high reliability components in the aerospace, nuclear and biomedical sectors) under different LSP irradiation conditions are presented along with a practical correlated analysis on the protective character of the residual stress profiles obtained under different irradiation strategies. Additional remarks on the improved character of the LSP technique over the traditional “shot peening” technique in what concerns depth of induced compressive residual stresses fields are also made through the paper

The aggressiveness of pig slurry to cement mortars

The aim was to measure the behaviour of various mortars employed in livestock media in central Spain and to analyse the aggressiveness of pig slurry to cement blended with fly ash mortars. To achieve this, mortar specimens were immersed in ponds storing pig slurry. Mortar specimens, of 40 ? 40 ? 160 mm, were made from four types of cement commonly used and recommended for rural areas. The types were a sulphate-resistant Portland cement and three cements blended in different proportions with fly ash and limestone filler. After 3, 6, 12, 24, 36, 48 and 60 months of exposure, three or four specimens of each cement type were removed from the pond and washed with water. Their compressive strength and microstructure (X-ray diffraction, mercury intrusion pore-symmetry, thermal analysis and scanning electron microscopy) were then measured. Sulphate-resistant Portland cement (SR-PC), found to be more susceptible to degradation due to its greater proportion of macro-pores and increased total porosity, was found not to be suitable for use with livestock. After 60 months of immersion in the pig slurry medium, CEM II-A (40.3%) mortar retained the greatest compressive strength. Mortars with less than 20% replacement of cement by fly ash were found to be the most durable, with the most suitable mechanical behaviour.

Análisis del comportamiento de las tuberías de poliéster reforzado con fibra de vidrio con capa de arena

Aunque las primeras fábricas de tubos de poliéster reforzado con fibra de vidrio en España datan del año 1984, no es sino hasta el año 1996 cuando se comienza su utilización masiva como un sustituto de las tuberías de fribrocemento, que ya habían sido prohibidas por la legislación, debido a los efectos cancerígenos de este material. Desde entonces se ha prodigado la utilización de todas las diferentes tipologías de esta clase de tubería, de conformidad a los procesos de fabricación empleados que se encuentran recopilados en el AWWA Manual M45 (Fiberglass Pipe Design), obteniéndose muy diversos resultados. Durante estos años, ha surgido una creciente preocupación en los usuarios de este tipo de tuberías dadas las continuas y numerosas averías en todo el ámbito geográfico. Esto ha promovido el desarrollo de la presente investigaicón, que se ha dividido en dos partes y que ha concluido con la determinación de un nuevo mecanismo específico de fractura. La primera parte se centró en la obtención y desarrollo del modelo teórico que hemos venido a denominar como "Teoría de la Caja Mecánicamente Contaminada", y que está basado en la contaminación o separación por un impacto de dos de las tres capas que forman la tubería, la capa intermedia de arena y la capa más interna o "inner layer". La consecuencia es la disminución del canto resistente, la rotura del inner layer y la entrada de fluido a la capa de arena. Para la evaluación de la magnitud de esta separación se ha desarrollado un modelo analítico que ha determinado la existencia de una relación cuadrática que la rige, y que ha sido verificado mediante ensayos de impacto sobre probetas de tuberías, alcanzando ajustes de hasta el 92%. Así, se ha determinado que impactos de muy baja intensidad, del entorno de 90 a 160 Julios en tuberías Filament Winding continuo PN 16-20 (de 800 a 1000mm) pueden comprometer seriamente la integridad estructural de la tubería sin dejar, en un principio, muesca o traza alguna que pueda alertar del problema. Los siguientes pasos en el estudio se dirigieron a determinar qué otros mecanismos, aparte del golpe, podrían contaminar la tubería y a estudiar el consiguiente avance de la fractura a las capas externas. Se trataba además de analizar la aparición en el tubo de unas misteriosas manchas en forma de "piel de leopardo" y de otros fenómenos aparecidos en las averías como que algunas de las deformaciones de la rotura por presión interna son hacia el interior del tubo y no al revés, como habría sido de esperar a priori. Se optó entonces por comenzar la que ha constituido la segunda parte de la investigación. Para ello se recurrió a realizar ensayos hidráulicos en banco de pruebas a alta presión, cuyos resultados fueron sorprendentes al descubrir que en el proceso se producía la hidrólisis de la resina de poliéster no catalizada que fluía hacia el exterior del tubo. Como consecuencia se llevaron a cabo nuevos ensayos físicos y químicos para estudiar la migración del material y la hidrólisis producida en el proceso de fractura. En este estudio, resultó muy relevante el hecho de sobrepasar o no la presión que producía el desagarro entre las capas del tubo. En definitiva, en esta investigación, que ha constado de estudios analíticos y estudios experimentales, químicos y numéricos, se ha determinado un nuevo mecanismo de fractura que explica gran parte de los fallos acontecidos en las tuberías de poliéster reforzado con fibra de vidrio. Como aplicación se exponen recomendaciones para mejorar el comportamiento mecánico de esta tipología y evitar así los sobrecostes millonarios producidos por su reposición. Numerous and continuous failures in fiberglass reinforced polyester pipes of different companies and manufacturing processes of the AWWA Manual M45 (Fiberglass Pipe Design), have prompted the development of this research, that has concluded with a specific mechanism describing pipe fractures. This research was carried out via two independent studies. The first one is the development of the hypothesis that turned into the Mechanically Contaminated Layer Theory. This theory describes the fracture mchanism which explains a significant part of massive failures due to the existence of a sand layer placed near the neutral axis in the core making the composite very sensitive to impacts in fibreglass reinforced polyester pipes. These failures create interface delamination and consequently fluid can leak into supporting sand backfill thereby iniating the fracture process. In order to assess the delimination magnitude, an analytic method is developed and a squared root law between delamination and energy applied proposed. Vertical blunt ram testts on samples extracted from complete pipes have been carried out to verify this theory, reaching a goodness of fit up to 92%. It is concluded that low energy impacts, around 90-160J in 800-1000mm diameter PN 16-20 continuous filament winding pipes, can seriously compromise their structural integraty with no external trace. The next step in the study was to determine what other mechanism, apart from the brittle hit, could contaminate the pipe and to analyse the consequente advance of the fracture to the external layers. Another aim was to analyse two phenomena occurred in real pipe failures. The first one is the appearance on the tube of "leopard fur" stains on some of the analysed failures, and the other phenomenon is the "inverse fracture", in which the deformations of the failure due to internal pressure are towards the inside of the tube and not the other way round, as it would be expected. It was then chosen to follow a new branch of the investigation by hydraulic high-pressure bench tests that study seepage and load transmission. The results were very surprising as it was discovered that in the process, hydrolysis of the non-catalysed polyester resin occured, flowing towards the outer of the pipe, which entailed the development of chemical and physical tests of the exuded material to study material migration and hydrolysis of the fracture process. In this particular study it was relevant to exceed or not the pressure that produced the rip between the layers of the tube. In conclusion, a new breakage mechanism in FRP pies with sand-filled layer has been found, which explains a high part of the failure global cases. The whole failure process is justified by the Mechanically Contaminated Layer Theory, which has been corroborated by means of analytical, numerical and experimental studies. Several recommendations are also provided in order to improve the mechanical behaviour of this typology and avoid the millionaire overruns generated by its massive failures.

Asphalt rubber mixtures with warm mix asphalt additives

En los últimos años, debido a la creciente preocupación por el calentamiento global y el cambio climático, uno de los retos más importantes a los que se enfrenta nuestra sociedad es el uso eficiente y económico de energía así como la necesidad correspondiente de reducir los gases de efecto invernadero (GEI). Las tecnologías de mezclas semicalientes se han convertido en un nuevo e importante tema de investigación en el campo de los materiales para pavimentos ya que ofrece una solución potencial para la reducción del consumo energético y las emisiones de GEI durante la producción y puesta en obra de las mezclas bituminosas. Por otro lado, los pavimentos que contienen polvo de caucho procedente de neumático fuera de uso, al hacer uso productos de desecho, ahorran energía y recursos naturales. Estos pavimentos ofrecen una resistencia mejorada a la formación de roderas, a la fatiga y a la fisuración térmica, reducen los costes de mantenimiento y el ruido del tráfico así como prolongan la vida útil del pavimento. Sin embargo, estas mezclas presentan un importante inconveniente: la temperatura de fabricación se debe aumentar en comparación con las mezclas asfálticas convencionales, ya que la incorporación de caucho aumenta la viscosidad del ligante y, por lo tanto, se producen mayores cantidades de emisiones de GEI. En la presente Tesis, la tecnología de mezclas semicalientes con aditivos orgánicos (Sasobit, Asphaltan A, Asphaltan B, Licomont) se incorporó a la de betunes de alta viscosidad modificados con caucho (15% y 20% de caucho) con la finalidad de dar una solución a los inconvenientes de mezclas con caucho gracias a la utilización de aditivos reductores de la viscosidad. Para este fin, se estudió si sería posible obtener una producción más sostenible de mezclas con betunes de alto contenido en caucho sin afectar significativamente su nivel de rendimiento mecánico. La metodología aplicada para evaluar y comparar las características de las mezclas consistió en la realización de una serie de ensayos de laboratorio para betunes y mezclas con caucho y con aditivos de mezclas semicalientes y de un análisis del ciclo de vida híbrido de la producción de mezclas semicalientes teniendo en cuenta la papel del aditivo en la cadena de suministro con el fin de cuantificar con precisión los beneficios de esta tecnología. Los resultados del estudio indicaron que la incorporación de los aditivos permite reducir la viscosidad de los ligantes y, en consecuencia, las temperaturas de producción y de compactación de las mezclas. Por otro lado, aunque la adición de caucho mejoró significativamente el comportamiento mecánico de los ligantes a baja temperatura reduciendo la susceptibilidad al fenómeno de fisuración térmica, la adición de las ceras aumentó ligeramente la rigidez. Los resultados del estudio reológico mostraron que la adición de porcentajes crecientes de caucho mejoraban la resistencia del pavimento con respecto a la resistencia a la deformación permanente a altas temperaturas y a la fisuración térmica a bajas temperaturas. Además, se observó que los aditivos mejoran la resistencia a roderas y la elasticidad del pavimento al aumentar el módulo complejo a altas temperaturas y al disminuir del ángulo de fase. Por otra parte, el estudio reológico confirmó que los aditivos estudiados aumentan ligeramente la rigidez a bajas temperaturas. Los ensayos de fluencia llevados a cabo con el reómetro demostraron una vez más la mejora en la elasticidad y en la resistencia a la deformación permanente dada por la adición de las ceras. El estudio de mezclas con caucho y aditivos de mezclas semicalientes llevado a cabo demostró que las temperaturas de producción/compactación se pueden disminuir, que las mezclas no experimentarían escurrimiento, que los aditivos no cambian significativamente la resistencia conservada y que cumplen la sensibilidad al agua exigida. Además, los aditivos aumentaron el módulo de rigidez en algunos casos y mejoraron significativamente la resistencia a la deformación permanente. Asimismo, a excepción de uno de los aditivos, las mezclas con ceras tenían la misma o mayor resistencia a la fatiga en comparación con la mezcla control. Los resultados del análisis de ciclo de vida híbrido mostraron que la tecnología de mezclas semicalientes es capaz de ahorrar significativamente energía y reducir las emisiones de GEI, hasta un 18% y 20% respectivamente, en comparación con las mezclas de control. Sin embargo, en algunos de los casos estudiados, debido a la presencia de la cera, la temperatura de fabricación debe reducirse en un promedio de 8 ºC antes de que los beneficios de la reducción de emisiones y el consumo de combustible puedan ser obtenidos. Los principales sectores contribuyentes a los impactos ambientales generados en la fabricación de mezclas semicalientes fueron el sector de los combustibles, el de la minería y el de la construcción. Due to growing concerns over global warming and climate change in recent years, one of the most important challenges facing our society is the efficient and economic use of energy, and with it, the corresponding need to reduce greenhouse gas (GHG) emissions. The Warm Mix Asphalt (WMA) technology has become an important new research topic in the field of pavement materials as it offers a potential solution for the reduction of energy consumption and GHG emissions during the production and placement of asphalt mixtures. On the other hand, pavements containing crumb-rubber modified (CRM) binders save energy and natural resources by making use of waste products. These pavements offer an improved resistance to rutting, fatigue and thermal cracking; reduce traffic noise and maintenance costs and prolong pavement life. These mixtures, however, present one major drawback: the manufacturing temperature is higher compared to conventional asphalt mixtures as the rubber lends greater viscosity to the binder and, therefore, larger amounts of GHG emissions are produced. In this dissertation the WMA technology with organic additives (Sasobit, Asphaltan A, Asphaltan B and Licomont) was applied to CRM binders (15% and 20% of rubber) in order to offer a solution to the drawbacks of asphalt rubber (AR) mixtures thanks to the use of fluidifying additives. For this purpose, this study sought to determine if a more sustainable production of AR mixtures could be obtained without significantly affecting their level of mechanical performance. The methodology applied in order to evaluate and compare the performance of the mixtures consisted of carrying out several laboratory tests for the CRM binders and AR mixtures with WMA additives (AR-WMA mixtures) and a hybrid input-output-based life cycle assessment (hLCA) of the production of WMA. The results of the study indicated that the incorporation of the organic additives were able to reduce the viscosity of the binders and, consequently, the production and compaction temperatures. On the other hand, although the addition of rubber significantly improved the mechanical behaviour of the binders at low temperatures reducing the susceptibility to thermal cracking phenomena, the addition of the waxes slightly increased the stiffness. Master curves showed that the addition of increasing percentages of rubber improved the resistance of the pavement regarding both resistance to permanent deformation at high temperatures and thermal cracking at low temperatures. In addition, the waxes improved the rutting resistance and the elasticity as they increased the complex modulus at high temperatures and decreased the phase angle. Moreover, master curves also attest that the WMA additives studied increase the stiffness at low temperatures. The creep tests carried out proved once again the improvement in the elasticity and in the resistance to permanent deformation given by the addition of the waxes. The AR-WMA mixtures studied have shown that the production/compaction temperatures can be decreased, that the mixtures would not experience binder drainage, that the additives did not significantly change the retained resistance and fulfilled the water sensitivity required. Furthermore, the additives increased the stiffness modulus in some cases and significantly improved the permanent deformation resistance. Except for one of the additives, the waxes had the same or higher fatigue resistance compared to the control mixture. The results of the hLCA demonstrated that the WMA technology is able to significantly save energy and reduce GHG emissions, up to 18% and 20%, respectively, compared to the control mixtures. However, in some of the case studies, due to the presence of wax, the manufacturing temperature at the asphalt plant must be reduced by an average of 8ºC before the benefits of reduced emissions and fuel usage can be obtained. The results regarding the overall impacts generated using a detailed production layer decomposition indicated that fuel, mining and construction sectors are the main contributors to the environmental impacts of manufacturing WMA mixtures.

An experimental and numerical study of ductile failure under quasi-static and impact loadings of Inconel 718 nickel-base superalloy

A numerical and experimental study of ballistic impacts at various temperatures on precipitation hardened Inconel 718 nickel-base superalloy plates has been performed. A coupled elastoplastic-damage constitutive model with Lode angle dependent failure criterion has been implemented in LS-DYNA non-linear finite element code to model the mechanical behaviour of such an alloy. The ballistic impact tests have been carried out at three temperatures: room temperature (25 °C), 400 °C and 700 °C. The numerical study showed that the mesh size is crucial to predict correctly the shear bands detected in the tested plates. Moreover, the mesh size convergence has been achieved for element sizes on the same order that the shear bands. The residual velocity as well as the ballistic limit prediction has been considered excellent for high temperature ballistic tests. Nevertheless, the model has been less accurate for the numerical simulations performed at room temperature, being though in reasonable agreement with the experimental data. Additionally, the influence that the Lode angle had on quasi-static failure patterns such as cup-cone and slanted failure has been studied numerically. The study has revealed that the combined action of weakened constitutive equations and Lode angle dependent failure criterion has been necessary to predict the previously-mentioned failure patterns

On the application of meshfree methods to the nonlinear dynamics of multibody systems

Esta tesis propone una completa formulación termo-mecánica para la simulación no-lineal de mecanismos flexibles basada en métodos libres de malla. El enfoque se basa en tres pilares principales: la formulación de Lagrangiano total para medios continuos, la discretización de Bubnov-Galerkin, y las funciones de forma libres de malla. Los métodos sin malla se caracterizan por la definición de un conjunto de funciones de forma en dominios solapados, junto con una malla de integración de las ecuaciones discretas de balance. Dos tipos de funciones de forma se han seleccionado como representación de las familias interpolantes (Funciones de Base Radial) y aproximantes (Mínimos Cuadrados Móviles). Su formulación se ha adaptado haciendo sus parámetros compatibles, y su ausencia de conectividad predefinida se ha aprovechado para interconectar múltiples dominios de manera automática, permitiendo el uso de mallas de fondo no conformes. Se propone una formulación generalizada de restricciones, juntas y contactos, válida para sólidos rígidos y flexibles, siendo estos últimos discretizados mediante elementos finitos (MEF) o libres de malla. La mayor ventaja de este enfoque reside en que independiza completamente el dominio con respecto de las uniones y acciones externas a cada sólido, permitiendo su definición incluso fuera del contorno. Al mismo tiempo, también se minimiza el número de ecuaciones de restricción necesarias para la definición de uniones realistas. Las diversas validaciones, ejemplos y comparaciones detalladas muestran como el enfoque propuesto es genérico y extensible a un gran número de sistemas. En concreto, las comparaciones con el MEF indican una importante reducción del error para igual número de nodos, tanto en simulaciones mecánicas, como térmicas y termo-mecánicas acopladas. A igualdad de error, la eficiencia numérica de los métodos libres de malla es mayor que la del MEF cuanto más grosera es la discretización. Finalmente, la formulación se aplica a un problema de diseño real sobre el mantenimiento de estructuras masivas en el interior de un reactor de fusión, demostrando su viabilidad en análisis de problemas reales, y a su vez mostrando su potencial para su uso en simulación en tiempo real de sistemas no-lineales. A new complete formulation is proposed for the simulation of nonlinear dynamic of multibody systems with thermo-mechanical behaviour. The approach is founded in three main pillars: total Lagrangian formulation, Bubnov-Galerkin discretization, and meshfree shape functions. Meshfree methods are characterized by the definition of a set of shape functions in overlapping domains, and a background grid for integration of the Galerkin discrete equations. Two different types of shape functions have been chosen as representatives of interpolation (Radial Basis Functions), and approximation (Moving Least Squares) families. Their formulation has been adapted to use compatible parameters, and their lack of predefined connectivity is used to interconnect different domains seamlessly, allowing the use of non-conforming meshes. A generalized formulation for constraints, joints, and contacts is proposed, which is valid for rigid and flexible solids, being the later discretized using either finite elements (FEM) or meshfree methods. The greatest advantage of this approach is that makes the domain completely independent of the external links and actions, allowing to even define them outside of the boundary. At the same time, the number of constraint equations needed for defining realistic joints is minimized. Validation, examples, and benchmarks are provided for the proposed formulation, demonstrating that the approach is generic and extensible to further problems. Comparisons with FEM show a much lower error for the same number of nodes, both for mechanical and thermal analyses. The numerical efficiency is also better when coarse discretizations are used. A final demonstration to a real problem for handling massive structures inside of a fusion reactor is presented. It demonstrates that the application of meshfree methods is feasible and can provide an advantage towards the definition of nonlinear real-time simulation models.

New Plaster Composite with Mineral Wool Fibres from CDW Recycling

Over the last decade the intense activity of the building sector has generated large quantities of construction and demolition waste (CDW). In particular, in Europe around 890 million tons of CDW is generated every year; however, only 50% of them are recycled. In Spain, over the last years 40 millions of tons of construction and demolition waste have been generated. On the other hand, since the implementation of the Technical Building Code regulation the use of mineral wools as building insulation materials has become a widespread solution in both rehabilitation and new construction works, and because of that, this kind of insulation waste is increasing. This research analyzes the potential of a new composite (gypsum and fiber waste) including several mineral wools waste into a plaster matrix. For this purpose, an experimental plan, characterizing the physical and mechanical behaviour as well as the Shore C hardness of the new composite, was elaborated fulfilling UNE Standards.

A regularised continuum damage model based on the mesoscopic scale for soft tissue

Material properties of soft fibrous tissues are highly conditioned by the hierarchical structure of this kind of composites. Collagen based tissues present, at decreasing length scales, a complex framework of fibres, fibrils, tropocollagen molecules and amino-acids. Understanding the mechanical behaviour at nano-scale level is critical to accurately incorporate this structural information in phenomenological damage models. In this work we derive a relationship between the mechanical and geometrical properties of the fibril constituents and the soft tissue material parameters at macroscopic scale. A Hodge–Petruska two-dimensional model has been used to describe the fibrils as staggered arrays of tropocollagen molecules. After a mechanical characterisation of each of the fibril components, two fibril failures modes have been defined related with two planes of weakness. A phenomenological continuous damage model with regularised softening was presented along with meso-structurally based definitions for its material parameters. Finally, numerical analysis at fibril, fibre and tissue levels are presented to show the capabilities of the model

Experience and lessons learnt from the SMART 2013 Benchmark

The French CEA, together with EDF and the IAEA, recently organised an international benchmark to evaluate the ability to model the mechanical behaviour of a typical nuclear reinforced concrete structure subjected to seismic demands. The participants were provided with descriptions of the structure and the testing campaign; they had to propose the numerical model and the material laws for the concrete (stage #1). A mesh of beam and shell elements was generated; for modelling the concrete a damaged plasticity model was used, but a smeared crack model was also investigated. Some of the initial experimental results, with the mock-up remaining in the elastic range, were provided to the participants for calibrating their models (stage #2). Predictions had to be produced in terms of eigen-frequencies and motion time histories. The calculated frequencies reproduced reasonably the experimental ones; the time histories, calculated by modal response analysis, also reproduced adequately the observed amplifications. The participants were then expected to predict the structural response under strong ground motions (stage #3), which increased progressively up to a history recorded during the 1994 Northridge earthquake, followed by an aftershock. These results were produced using an explicit solver and a damaged plasticity model for the concrete, although an implicit solver with a smeared crack model was also investigated. The paper presents the conclusions of the pre-test exercise, as well as some observations from additional simulations conducted after the experimental results were made available.

Comportamento mecânico e desempenho estrutural de materiais reciclados e estabilizados com espuma de asfalto.

Em busca de soluções estruturais para restauração de pavimentos rodoviários que que sejam eficientes, mas ao mesmo tempo que sejam econômicas e impactem o mínimo possível na dinâmica de operação da malha rodoviária, tem sido difundida a metodologia de reciclagem a frio de pavimentos com a estabilização com espuma de asfalto. A redução de custos devido a reutilização de material e a menor necessidade de transporte de insumos, além da possibilidade de realização da restauração em um curto espaço de tempo, têm contribuído para a crescente utilização do processo. Este trabalho tem como objetivo avaliar o desempenho e o comportamento mecânico de uma mistura reciclada estabilizada com espuma asfalto, para melhor entender os efeitos do confinamento e do teor de umidade do material, visto que este passa por um processo de cura quando já em serviço. Foi acompanhado um trecho experimental onde o pavimento foi restaurado com a aplicação de uma base reciclada estabilizada com espuma de asfalto. O segmento foi monitorado através do controle tecnológico de execução e de levantamentos deflectométricos com FWD. Verificou-se que as deflexões após quase 24 meses da execução do trecho reduziram consideravelmente. Paralelamente, foram realizados ensaios de resistência à tração por compressão diametral, módulo de resiliência triaxial e de deformação permanente para diferentes procedimentos de cura para verificação do efeito da saída da água nas mudanças de comportamento mecânico do material. Verificou-se ainda o efeito das tensões de confinamento no módulo de resiliência de materiais estabilizados com espuma de asfalto e determinaram-se os parâmetros de cisalhamento do material através de ensaiosTriaxiais Monotônicos. Pode-se concluir que a cura é uma consideração importante tanto com relação a sua duração, quanto com relação ao seu efeito no comportamento do material.