Publication: Comportamiento de armaduras de acero inoxidable en morteros con cenizas volantes
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2016-01
Defense date
2016-01-20
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Abstract
La sustitución parcial del cemento por adiciones de cenizas volantes (CV) tiene ventajas de tipo medioambiental (reduce la extracción en canteras, la emisión de CO₂ y reutiliza los residuos), económicas (reduce el gasto en cemento y evita costes de eliminación de residuos) y en algunos casos tecnológicas (dependiendo del tipo de cemento puede mejorar las propiedades mecánicas en hormigones). Sin embargo, el problema potencial de este tipo de morteros es una posible mayor susceptibilidad a la carbonatación (debido a la reacción puzolánica de las CV) que provocaría con facilidad problemas de corrosión en las armaduras de acero al carbono más externas, si se empleara en estructuras de hormigón armado (EHA). En esta Tesis se plantea el estudio de la durabilidad de armaduras de acero inoxidable embebidos en morteros con CV en medios contaminados con cloruros y frente a la carbonatación, que son los ataques de corrosión más frecuentes en EHA. Para ello se han estudiado por separado: las propiedades físicas, químicas y mecánicas de morteros, así como la influencia del conformado en las propiedades mecánicas y de corrosión de los aceros inoxidables. Por un lado, los resultados que se han obtenido para la caracterización de aceros inoxidables tanto conformados en frío como en caliente han demostrado que proceso de conformado de las corrugas modifica la microestructura de los aceros inoxidables y tiene un importante efecto en las características de la capa pasiva, y en el comportamiento a la corrosión. Los resultados de EIS y Ecorr para morteros armados muestran que los aceros inoxidables corrugados de tipo austenítico (304L y 316L) y dúplex (2205) son una interesante opción para asegurar la durabilidad de las EHA en ambientes con cloruros, incluso en armaduras soldadas. Sin embargo, el austenítico 204Cu CW no es recomendable en ambientes de inmersión parcial en presencia de cloruros. Por otro lado, se ha realizado un estudio del comportamiento de los morteros con CV frente a la carbonatación a largo plazo, en un ambiente natural (atmósfera de Madrid durante 4 años). Los morteros se han fabricado con dos cementos (tipos I y IV) y dos tamaños de partículas de CV, en estado de recepción (CV140) y tamizadas (CV20). Este estudio indica que el comportamiento de los morteros fabricados con CEM IV es pobre y no se recomienda el uso de CV con este cemento, porque este ya contiene adiciones en su composición. Los estudios en morteros con cemento tipo I, indican que las CV con menor tamaño de partícula (CV20) en grandes proporciones (50 %) no afectan la densidad del mortero, pero sí son más susceptibles a la carbonatación. Las CV con mayor tamaño de partícula (CV140) son menos susceptibles a la carbonatación que las CV20. Por lo tanto, la densidad y por consiguiente las propiedades mecánicas se ven afectadas con el uso de grandes proporciones (50 %), por lo que es conveniente el uso de estas CV por debajo del 25 %. Además, en estos morteros con diferentes tamaños de CV, también se ha realizado el estudio de su resistencia frente al ataque por sulfatos, tipo de degradación química que sucede en contacto con residuos contaminantes bajo tierra o en suelos o aguas, ricos en sulfatos. Los resultados indican que a medida que el contenido de CV aumenta (sin importar el tamaño de estas), el ataque por sulfatos se ve inhibido debido a la reacción puzolánica de las CV. Además, en morteros carbonatados, el ataque por sulfatos también se inhibe ya que la cantidad de Ca(OH)₂ en los poros es despreciable debido a la carbonatación, independientemente de que haya o no CV. Por último, el estudio electroquímico realizado a los aceros inoxidables embebidos en morteros con CV señala que todas las armaduras (tipo austenítico y dúplex) presentan un comportamiento pasivo después de 2 años aproximadamente de exposición en ambientes agresivos de contaminación de cloruros y carbonatación. Sin embargo, la adherencia armadura/mortero disminuye en los morteros con CV (50 % CV20) en condiciones de alta HR y contaminación con cloruros. Por lo tanto, el interés del empleo de morteros ecológicos con CV (teniendo en cuenta las limitaciones de contenidos así como el tamaño de CV) y haciendo uso de armaduras de acero inoxidable para asegurar la durabilidad en ambientes agresivos, quedan demostrados y han sentado una base para continuar con estos estudios.
The partial substitution of cement with fly ashes (CV) has several environmental (reduction of quarrying and CO₂ emissions and reusing of wastes), economic (reduction of the cement cost and avoiding costs of wastes’ elimination) and, in some cases, technological advantages (depending on the type of cement, CVs can improve the mechanical properties of concrete). However, the potential problem of this type of mortars is a possible higher susceptibility to carbonation (because of the pozzolanic reaction of fly ash), which could easily cause problems of corrosion in the carbon steel reinforcements placed in the more external areas of reinforced concrete structures. In this Thesis, the study of the durability of stainless steel bars in mortars with CV has been proposed for chloride contaminated and carbonated environments. These are the most common corrosion attacks in reinforced concrete structures. Information about the physical and mechanical properties of mortars, as well as the influence of the forming in mechanical and corrosion properties of stainless steel bars was also obtained. On one hand, the obtained results on stainless steel characterization (both cold and hot formed) have proved that the corrugations modify the microstructure, affect the passive layer and the corrosion behavior of these steels. EIS and Ecorr results of reinforced mortars show that corrugated austenitic (304L and 316L) and duplex (2205) stainless steels are an interesting option to ensure the durability of reinforced concrete structures in environments with chlorides, even for welded reinforcements. However, the austenitic CW 204Cu is not recommended for partial immersion environments with the presence of chlorides. On the other hand, it has been made a study of the behavior of fly ash containing mortars under long term carbonation, in a natural environment (Madrid atmosphere for 4 years). The mortars were manufactured with two cements (types I and IV) and two sizes of fly ash particles: reception state (CV140) and sieved (CV20). This study indicates that the behavior of mortars made with CEM IV is poor and the use of fly ash with this cement is not recommended, because it already contains additions in its composition. Studies on mortars with type I cement prove that fly ash with smaller particle size (CV20) and high contents (50 %) does not affect the density of the mortar, but are more sensitive to carbonation. The fly ash with larger particle size (CV140) is less sensitive to carbonation than CV20. Therefore, the density and mechanical properties are affected by the use of larger proportions (50 %), so it is better to use this fly ash in a proportion lower than 25 %. Moreover, in these mortars with different sizes of CV, the resistance to sulphate attack has been studied, a type of chemical degradation that occurs in contact with polluting waste underground or in soils or waters with high content of sulphates. The results indicate that as the content of fly ash increases (regardless of the particle size), the sulphate attack is inhibited by the pozzolanic reaction of fly ash. Furthermore, the attack by sulfates in carbonated mortars is also inhibited because of the amount of Ca(OH)₂ inside of the mortar pores is negligible due to carbonation, regardless of the presence of fly ash. Finally, the electrochemical study of stainless steels embedded in mortars with fly ash states that all reinforcements (austenitic and duplex) have a passive behavior after approximately two years of exposure in aggressive environments (contamination of chlorides and carbonation). However, the adherence between reinforcement and mortar is lower in mortars with fly ash (50 % CV20) under high RH and contaminated with chlorides. Therefore, the importance of using ecological fly ash mortars (taking into account the limitations of content and the fly ash particle size) reinforced with stainless steel to ensure durability in aggressive environments are demonstrated and based on these results it is possible to continue with these studies.
The partial substitution of cement with fly ashes (CV) has several environmental (reduction of quarrying and CO₂ emissions and reusing of wastes), economic (reduction of the cement cost and avoiding costs of wastes’ elimination) and, in some cases, technological advantages (depending on the type of cement, CVs can improve the mechanical properties of concrete). However, the potential problem of this type of mortars is a possible higher susceptibility to carbonation (because of the pozzolanic reaction of fly ash), which could easily cause problems of corrosion in the carbon steel reinforcements placed in the more external areas of reinforced concrete structures. In this Thesis, the study of the durability of stainless steel bars in mortars with CV has been proposed for chloride contaminated and carbonated environments. These are the most common corrosion attacks in reinforced concrete structures. Information about the physical and mechanical properties of mortars, as well as the influence of the forming in mechanical and corrosion properties of stainless steel bars was also obtained. On one hand, the obtained results on stainless steel characterization (both cold and hot formed) have proved that the corrugations modify the microstructure, affect the passive layer and the corrosion behavior of these steels. EIS and Ecorr results of reinforced mortars show that corrugated austenitic (304L and 316L) and duplex (2205) stainless steels are an interesting option to ensure the durability of reinforced concrete structures in environments with chlorides, even for welded reinforcements. However, the austenitic CW 204Cu is not recommended for partial immersion environments with the presence of chlorides. On the other hand, it has been made a study of the behavior of fly ash containing mortars under long term carbonation, in a natural environment (Madrid atmosphere for 4 years). The mortars were manufactured with two cements (types I and IV) and two sizes of fly ash particles: reception state (CV140) and sieved (CV20). This study indicates that the behavior of mortars made with CEM IV is poor and the use of fly ash with this cement is not recommended, because it already contains additions in its composition. Studies on mortars with type I cement prove that fly ash with smaller particle size (CV20) and high contents (50 %) does not affect the density of the mortar, but are more sensitive to carbonation. The fly ash with larger particle size (CV140) is less sensitive to carbonation than CV20. Therefore, the density and mechanical properties are affected by the use of larger proportions (50 %), so it is better to use this fly ash in a proportion lower than 25 %. Moreover, in these mortars with different sizes of CV, the resistance to sulphate attack has been studied, a type of chemical degradation that occurs in contact with polluting waste underground or in soils or waters with high content of sulphates. The results indicate that as the content of fly ash increases (regardless of the particle size), the sulphate attack is inhibited by the pozzolanic reaction of fly ash. Furthermore, the attack by sulfates in carbonated mortars is also inhibited because of the amount of Ca(OH)₂ inside of the mortar pores is negligible due to carbonation, regardless of the presence of fly ash. Finally, the electrochemical study of stainless steels embedded in mortars with fly ash states that all reinforcements (austenitic and duplex) have a passive behavior after approximately two years of exposure in aggressive environments (contamination of chlorides and carbonation). However, the adherence between reinforcement and mortar is lower in mortars with fly ash (50 % CV20) under high RH and contaminated with chlorides. Therefore, the importance of using ecological fly ash mortars (taking into account the limitations of content and the fly ash particle size) reinforced with stainless steel to ensure durability in aggressive environments are demonstrated and based on these results it is possible to continue with these studies.
Description
Mención Internacional en el título de doctor
Esta tesis contiene artículos de investigación en anexo.
Esta tesis contiene artículos de investigación en anexo.
Keywords
Cenizas volantes, Morteros, Armaduras de acero inoxidable, Hormigón armado, Resistencia de materiales, Impacto ambiental