Flexural fatigue of pre-cracked fibre reinforced concrete: experimental study and numerical modelling

Fibre reinforced concrete (FRC) is recognized as suitable material for structural applications. The number of national codes that have approved it is an evidence. Structures where FRC is generally used can be subjected to fatigue loads and are expected to resist millions of cycles during their service life. Cyclic loads affect significantly the characteristics of materials and can cause fatigue failures. The most demanded cross-sections being cracked under tensile stresses due to direct loads or imposed deformations. Commonly, publications report fatigue behaviour of concrete under compression and are valid for uncracked sections. Imprecision in fatigue prescriptions are reflected through formulation of models that contemplate a probabilistic approach, or an introduction of high safety coefficients within construction codes. The aim of the present doctoral thesis is to perform a structural design oriented analysis on the behaviour of pre-cracked FRC subjected to flexural fatigue loads. FRC with steel and polypropylene fibre with different volume content were investigated by means of three-point bending tests, considering an initial crack width accepted in the service limit state. The mechanical behaviour of FRC were analysed in terms of applied load level, crack opening displacement (CMOD) and fatigue life. The residual flexural tensile strength was assessed after these tests to estimate the impact of the cycles in the remaining resistant capacity of the specimens. Results suggest that the mechanism of crack propagation is independent of the fibre type and content and the monotonic load-crack opening displacement curve might be used as deformation failure criterion for FRC under flexural fatigue loading. The conducted probabilistic approach allows predicting the fatigue strength of concrete reinforced with steel fibres. The findings postulate the proposal of a model to predict the evolution of the crack-opening and the remaining resistant capacity. An optimisation procedure is proposed to derive the model parameters using a limited number of initial load cycles. This doctoral thesis provides knowledge and data that may aid further research and contribute to the future development of design recommendations.El hormigón reforzado con fibra (FRC) se reconoce como material adecuado para aplicaciones estructurales. El número de normativas que lo han aprobado es una evidencia. Las estructuras donde generalmente se usa FRC pueden estar sujetas a cargas de fatiga y se espera que resistan millones de ciclos durante su vida útil. Las cargas cíclicas afectan significativamente a las características de los materiales y pueden causar roturas por fatiga. Las secciones transversales más demandadas se fisuran bajo tensión debido a cargas directas o deformaciones impuestas. Comúnmente, las publicaciones informan del comportamiento de fatiga del hormigón bajo compresión y son válidas para secciones no fisuradas. La imprecisión de las recomendaciones se refleja a través de la formulación de modelos que contemplan un enfoque probabilístico o la introducción de altos coeficientes de seguridad dentro de los códigos de construcción. El objetivo de la presente tesis doctoral es realizar un análisis orientado al diseño estructural sobre el comportamiento del FRC pre-fisurado sometido a cargas de fatiga por flexión. Se investigaron FRC con fibras de acero y polipropileno con diferentes contenidos de fibras mediante pruebas de flexotracción a tres puntos, considerando un ancho de fisura inicial aceptado en el estado límite de servicio. El comportamiento mecánico del FRC se analizó en términos de nivel de carga aplicada, desplazamiento de apertura de fisura (CMOD) y vida útil bajo fatiga. La resistencia residual a flexotracción se evaluó después de los ciclos de fatiga para estimar el impacto de los ciclos en la capacidad de resistencia restante de las muestras. Los resultados sugieren que el mecanismo de propagación de fisuras es independiente del tipo y contenido de fibra y la curva monotónica de CMOD podría ser utilizada como criterio de falla de deformación para FRC bajo carga de fatiga por flexotracción. El enfoque probabilístico realizado permite predecir la resistencia a la fatiga del hormigón reforzado con fibras de acero. Los resultados postulan la propuesta de un modelo para predecir la evolución de la apertura de fisura y la capacidad resistente remanente. Se propone un procedimiento de optimización para derivar los parámetros del modelo utilizando un número limitado de ciclos de carga inicial. Esta tesis doctoral proporciona conocimiento y datos que pueden ayudar a futuras investigaciones y contribuir al desarrollo futuro de recomendaciones de diseño.Postprint (published version

Avaliação do desempenho do aditivo superplastificante de base policarboxilato pré-tratado por sonicação no comportamento reológico de pastas de cimento portland

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Civil, Florianópolis, 2015Um dos mais eficientes métodos de dispersão de materiais é o uso de um agente dispersante (em geral, um aditivo superplastificante de base policarboxilato) em conjunto com um equipamento que gera ondas ultrassônicas. Essa dispersão ocorre pela formação de bolhas de cavitação, que colapsam e desaglomeram as partículas. Porém, a literatura sugere que tal método danifica os materiais sonicados e compromete o desempenho de trabalhabilidade e fluidez de pastas cimentícias elaboradas com esse aditivo de base policarboxilato (SP). Neste trabalho, investigaram-se os efeitos das ondas ultrassônicas (sonicação) no aditivo SP utilizado na elaboração de pastas cimentícias. Para isso, primeiramente verificou-se, através da termogravimetria e calorimetria diferencial de varredura, que não há variação de massa nem transição de fase em função da temperatura e fluxo de calor entre 0 e 110º C. Após isso, diversas soluções diluídas e concentradas de aditivo SP foram analisadas por meio de espectrofotometria na região do infravermelho, a fim de definir a sensibilidade da técnica e do equipamento utilizado na detecção de alterações na estrutura polimérica do aditivo sob investigação. Após essa avaliação, o aditivo foi sonicado em diversas amplitudes de sonicação por três tempos de duração distintos, e pastas cimentícias foram elaboradas com cada solução de aditivo sonicado. Através do ensaio de espectroscopia na região do infravermelho, pode-se observar que houve alterações na molécula de policarboxilato, apesar de que seus principais grupos puderam ser distinguidos. Os comportamentos reológicos de pastas no estado fresco foram avaliados por meio de reometria aos 5, 30 e 60 minutos, após o primeiro contato do cimento com a água. Foi possível verificar que certas pastas elaboradas com aditivo pré-tratado por sonicação apresentaram comportamento reológico distinto ao da pasta sem aditivo pré-tratado. Ainda, em certos casos, para uma mesma pasta, o comportamento foi atípico entre seus próprios parâmetros reológicos, dificultando a formulação de hipóteses conclusivas que relacionem a sonicação de aditivo SP e alterações no seu desempenho. Mesmo assim, é possível verificar que a sonicação comprometeu o desempenho reológico das pastas.Abstract : One of the most efficient methods to disperse material is using a dispersing agent (typically a polycarboxylate based superplasticizer) together with an equipment that produces ultrasonic waves. Such dispersion occurs by creating cavitation bubbles, which collapse and deagglomerate the particles. However, the literature suggests that such method damages sonicated materials and compromises the performance of workability and flow of cement pastes prepared with this polycarboxylate base admixture (SP). This study investigated the effects of ultrasound waves (sonication) in SP admixture used to prepare cement pastes. For this, first it was found by thermogravimetry and differential scanning calorimetry, there is no weight change or phase transition as a function of temperature and heat flux between 0 and 110o C. After that, several dilute and concentrated solutions of SP admixture were analyzed by spectrophotometry in the infrared region in order to set the sensitivity of the technique and equipment used in detecting changes in the polymer structure of the admixture under investigation. After this evaluation, the admixture was sonicated in various amplitudes of sonication for three different duration times, and cement pastes were prepared with each sonicated admixture solution. Through the spectroscopy in the infrared region test, it can be seen that are changes in polycarboxylate molecule, even though its major groups could be distinguished. The rheological behavior of fresh pastes were evaluated by rheometry at 5, 30 and 60 minutes after the first contact of the cement with water. It was observed that certain pastes prepared with pre-treated by sonication admixture showed distinct rheological behavior of without pre-treated admixture pastes. Furthermore, in certain cases, for a same cement paste, the behavior was atypical of its own rheologic parameters, making it difficult to formulate conclusive hypotheses relate to sonication SP admixture and changes in its performance. Even then, it s still possible to verify that sonication has compromised the rheological performance of cementitious pastes

Flexural fatigue of pre-cracked fibre reinforced concrete: experimental study and numerical modelling

Fibre reinforced concrete (FRC) is recognized as suitable material for structural applications. The number of national codes that have approved it is an evidence. Structures where FRC is generally used can be subjected to fatigue loads and are expected to resist millions of cycles during their service life. Cyclic loads affect significantly the characteristics of materials and can cause fatigue failures. The most demanded cross-sections being cracked under tensile stresses due to direct loads or imposed deformations. Commonly, publications report fatigue behaviour of concrete under compression and are valid for uncracked sections. Imprecision in fatigue prescriptions are reflected through formulation of models that contemplate a probabilistic approach, or an introduction of high safety coefficients within construction codes. The aim of the present doctoral thesis is to perform a structural design oriented analysis on the behaviour of pre-cracked FRC subjected to flexural fatigue loads. FRC with steel and polypropylene fibre with different volume content were investigated by means of three-point bending tests, considering an initial crack width accepted in the service limit state. The mechanical behaviour of FRC were analysed in terms of applied load level, crack opening displacement (CMOD) and fatigue life. The residual flexural tensile strength was assessed after these tests to estimate the impact of the cycles in the remaining resistant capacity of the specimens. Results suggest that the mechanism of crack propagation is independent of the fibre type and content and the monotonic load-crack opening displacement curve might be used as deformation failure criterion for FRC under flexural fatigue loading. The conducted probabilistic approach allows predicting the fatigue strength of concrete reinforced with steel fibres. The findings postulate the proposal of a model to predict the evolution of the crack-opening and the remaining resistant capacity. An optimisation procedure is proposed to derive the model parameters using a limited number of initial load cycles. This doctoral thesis provides knowledge and data that may aid further research and contribute to the future development of design recommendations.El hormigón reforzado con fibra (FRC) se reconoce como material adecuado para aplicaciones estructurales. El número de normativas que lo han aprobado es una evidencia. Las estructuras donde generalmente se usa FRC pueden estar sujetas a cargas de fatiga y se espera que resistan millones de ciclos durante su vida útil. Las cargas cíclicas afectan significativamente a las características de los materiales y pueden causar roturas por fatiga. Las secciones transversales más demandadas se fisuran bajo tensión debido a cargas directas o deformaciones impuestas. Comúnmente, las publicaciones informan del comportamiento de fatiga del hormigón bajo compresión y son válidas para secciones no fisuradas. La imprecisión de las recomendaciones se refleja a través de la formulación de modelos que contemplan un enfoque probabilístico o la introducción de altos coeficientes de seguridad dentro de los códigos de construcción. El objetivo de la presente tesis doctoral es realizar un análisis orientado al diseño estructural sobre el comportamiento del FRC pre-fisurado sometido a cargas de fatiga por flexión. Se investigaron FRC con fibras de acero y polipropileno con diferentes contenidos de fibras mediante pruebas de flexotracción a tres puntos, considerando un ancho de fisura inicial aceptado en el estado límite de servicio. El comportamiento mecánico del FRC se analizó en términos de nivel de carga aplicada, desplazamiento de apertura de fisura (CMOD) y vida útil bajo fatiga. La resistencia residual a flexotracción se evaluó después de los ciclos de fatiga para estimar el impacto de los ciclos en la capacidad de resistencia restante de las muestras. Los resultados sugieren que el mecanismo de propagación de fisuras es independiente del tipo y contenido de fibra y la curva monotónica de CMOD podría ser utilizada como criterio de falla de deformación para FRC bajo carga de fatiga por flexotracción. El enfoque probabilístico realizado permite predecir la resistencia a la fatiga del hormigón reforzado con fibras de acero. Los resultados postulan la propuesta de un modelo para predecir la evolución de la apertura de fisura y la capacidad resistente remanente. Se propone un procedimiento de optimización para derivar los parámetros del modelo utilizando un número limitado de ciclos de carga inicial. Esta tesis doctoral proporciona conocimiento y datos que pueden ayudar a futuras investigaciones y contribuir al desarrollo futuro de recomendaciones de diseño

Flexural fatigue of pre-cracked fibre reinforced concrete: experimental study and numerical modelling

Fibre reinforced concrete (FRC) is recognized as suitable material for structural applications. The number of national codes that have approved it is an evidence. Structures where FRC is generally used can be subjected to fatigue loads and are expected to resist millions of cycles during their service life. Cyclic loads affect significantly the characteristics of materials and can cause fatigue failures. The most demanded cross-sections being cracked under tensile stresses due to direct loads or imposed deformations. Commonly, publications report fatigue behaviour of concrete under compression and are valid for uncracked sections. Imprecision in fatigue prescriptions are reflected through formulation of models that contemplate a probabilistic approach, or an introduction of high safety coefficients within construction codes. The aim of the present doctoral thesis is to perform a structural design oriented analysis on the behaviour of pre-cracked FRC subjected to flexural fatigue loads. FRC with steel and polypropylene fibre with different volume content were investigated by means of three-point bending tests, considering an initial crack width accepted in the service limit state. The mechanical behaviour of FRC were analysed in terms of applied load level, crack opening displacement (CMOD) and fatigue life. The residual flexural tensile strength was assessed after these tests to estimate the impact of the cycles in the remaining resistant capacity of the specimens. Results suggest that the mechanism of crack propagation is independent of the fibre type and content and the monotonic load-crack opening displacement curve might be used as deformation failure criterion for FRC under flexural fatigue loading. The conducted probabilistic approach allows predicting the fatigue strength of concrete reinforced with steel fibres. The findings postulate the proposal of a model to predict the evolution of the crack-opening and the remaining resistant capacity. An optimisation procedure is proposed to derive the model parameters using a limited number of initial load cycles. This doctoral thesis provides knowledge and data that may aid further research and contribute to the future development of design recommendations.El hormigón reforzado con fibra (FRC) se reconoce como material adecuado para aplicaciones estructurales. El número de normativas que lo han aprobado es una evidencia. Las estructuras donde generalmente se usa FRC pueden estar sujetas a cargas de fatiga y se espera que resistan millones de ciclos durante su vida útil. Las cargas cíclicas afectan significativamente a las características de los materiales y pueden causar roturas por fatiga. Las secciones transversales más demandadas se fisuran bajo tensión debido a cargas directas o deformaciones impuestas. Comúnmente, las publicaciones informan del comportamiento de fatiga del hormigón bajo compresión y son válidas para secciones no fisuradas. La imprecisión de las recomendaciones se refleja a través de la formulación de modelos que contemplan un enfoque probabilístico o la introducción de altos coeficientes de seguridad dentro de los códigos de construcción. El objetivo de la presente tesis doctoral es realizar un análisis orientado al diseño estructural sobre el comportamiento del FRC pre-fisurado sometido a cargas de fatiga por flexión. Se investigaron FRC con fibras de acero y polipropileno con diferentes contenidos de fibras mediante pruebas de flexotracción a tres puntos, considerando un ancho de fisura inicial aceptado en el estado límite de servicio. El comportamiento mecánico del FRC se analizó en términos de nivel de carga aplicada, desplazamiento de apertura de fisura (CMOD) y vida útil bajo fatiga. La resistencia residual a flexotracción se evaluó después de los ciclos de fatiga para estimar el impacto de los ciclos en la capacidad de resistencia restante de las muestras. Los resultados sugieren que el mecanismo de propagación de fisuras es independiente del tipo y contenido de fibra y la curva monotónica de CMOD podría ser utilizada como criterio de falla de deformación para FRC bajo carga de fatiga por flexotracción. El enfoque probabilístico realizado permite predecir la resistencia a la fatiga del hormigón reforzado con fibras de acero. Los resultados postulan la propuesta de un modelo para predecir la evolución de la apertura de fisura y la capacidad resistente remanente. Se propone un procedimiento de optimización para derivar los parámetros del modelo utilizando un número limitado de ciclos de carga inicial. Esta tesis doctoral proporciona conocimiento y datos que pueden ayudar a futuras investigaciones y contribuir al desarrollo futuro de recomendaciones de diseño