New insights on the fundamentals and modeling of the external sulfate attack in concrete structures

The external sulfate attack (ESA) is a complex degradation process typically compromising the durability of underground foundations, nuclear or industrial waste containments and tunnel linings exposed to sulfate solutions. The structures affected usually remain covered its entire service life, which compromises the detection of this phenomenon before severe material degradation has occurred. Once diagnosed, the large size and criticality of the typical structures affected greatly limit the efficiency of the remedial actions. Consequently, monitoring of the evolution of the structural behavior is often the only applicable measure. This scenario places the development of reliable tools to assist the design of sulfate-resisting concrete structures and assess the risk of ESA in existing properties as key challenges for structural durability. The present thesis aims to advance knowledge in this field by presenting important contributions in three different research lines: numerical modeling of the ESA, role of porosity during the attack and the relevance of reproducing field-like conditions on ESA assessments. Advances on the ESA numerical modelization led to the development of a chemo-transport-mechanical model and a simplified assessment methodology. The former simulates the effects of ionic transport, chemical reactions, degradation mechanisms and the mechanical response of the structure. The validations performed indicate that the model captures the importance of the location of the ettringite formed within the pore network and provides a fair quantification of the overall expansions. The simplified assessment methodology evaluates the risk of failure during the ESA based on the aggressiveness of the media, the reactivity and mechanical properties of the material and the geometric characteristics and service life of the element under attack, without resorting to complex iterative algorithms. Unlike current design guidelines, the application of this simplified procedure allows the definition of flexible and optimized precautionary measures for each application. The second research line involved an extensive experimental program that led to the formulation of a conceptual model to explain the role of porosity during the ESA. The results obtained indicate that high durability against the attack might be achieved by limiting the penetration of sulfates or increasing the capacity of the matrix to accommodate expansive products. Both approaches correspond to opposing pore characteristics of the matrix: the former is usually associated with low porosities while the latter requires matrices with high porosities. These results question the common perception that high porosities are always negative for ESA durability and open up the possibility to design sulfate-resisting materials by increasing the capacity of the matrix to accommodate expansive phases. The third research line evaluates the influence of early sulfate exposure and the effects of confinement on the ESA by two experimental programs. The first study suggests that the delayed exposition to sulfates commonly adopted in accelerated laboratory tests might lead to imprecise damage estimations for structures cast in situ. In these cases, it is recommended to expose the samples to sulfates shortly after casting. The second study suggests that assessing sulfate resistance on specimens in free-expanding conditions might not be representative of the behavior of real structures where the attack is developed in combination with confining conditions. Results indicate that compressive stresses generated by confinement interact with the normal development of the attack by limiting or delaying the appearance of micro-cracks and reducing the amount of ettringite crystals exerting expansive pressures.El ataque sulfático externo (ASE) es un proceso de degradación complejo que afecta principalmente la durabilidad de estructuras de cimentación, contenedores de residuos nucleares o industriales y revestimientos de túneles. Dichas estructuras suelen permanecer enterradas toda su vida útil, lo que compromete la detección del fenómeno antes de que se hayan desarrollado altos niveles de degradación. Una vez detectado, el tamaño e importancia estratégica de las estructuras afectadas limitan las opciones de reparación y su eficacia. Debido a ello, habitualmente la única acción posible consiste en la monitorización de la evolución del comportamiento estructural. Este escenario sitúa el desarrollo de herramientas para el diseño de estructuras resistentes a sulfatos y la evaluación del fenómeno en propiedades existentes como desafíos clave para la durabilidad de estructuras enterradas. Esta tesis doctoral aspira a profundizar el conocimiento en torno a esta temática mediante contribuciones relevantes en tres líneas de investigación: la modelización numérica del ASE, el rol de la porosidad durante el ataque y la relevancia de reproducir condiciones de campo en la evaluación del ASE. Avances en el campo de la modelización numérica han dado lugar a un modelo avanzado y una metodología de evaluación simplificada del ASE. El primero se basa en la simulación de procesos de transporte iónico, reacciones químicas, mecanismos de degradación y respuesta mecánica de la estructura. Las validaciones realizadas indican que el modelo refleja la importancia de la localización de la etringita dentro de la red porosa y proporciona estimaciones ajustadas de las expansiones generadas. La metodología de evaluación simplificada mide el riesgo de fallo estructural basándose en la agresividad del medio, la reactividad y propiedades mecánicas del material y las características geométricas y vida útil del elemento atacado, sin recurrir a algoritmos iterativos. A diferencia de las guías de diseño actuales, su aplicación permite la definición de medidas preventivas ajustadas a cada aplicación. La segunda línea de investigación ha dado lugar a un modelo conceptual que explica el rol de la porosidad durante el ASE. Los resultados obtenidos indican que se pueden alcanzar altas resistencias al ataque mediante la limitación de la penetración de sulfatos en la estructura o incrementando la capacidad de la matriz de acomodar fases expansivas. Los dos enfoques se corresponden a características porosas opuestas de la matriz. La primera se suele asociar a bajas porosidades mientras que la segunda se maximiza en porosidades altas. Los resultados obtenidos cuestionan la idea de que porosidades altas siempre son negativas para la durabilidad ante el ASE y abre la posibilidad de diseñar materiales resistentes a sulfatos incrementando la capacidad de acomodar fases expansivas. La tercera línea de investigación evalúa la influencia de la exposición temprana a sulfatos y los efectos del confinamiento en el ASE. Resultados referentes al primer estudio indican que una exposición tardía a los sulfatos como la empleada en la mayoría de ensayos acelerados puede conllevar a estimaciones incorrectas del daño generado en estructuras fabricadas in situ. El segundo estudio, referente a los efectos del confinamiento, sugiere que el uso de probetas en condiciones de expansión libre puede no ser adecuado para reproducir los efectos del ASE en estructuras donde el ataque se desarrolla en condiciones confinadas. Las tensiones de compresión generadas interactúan con el desarrollo normal del ataque limitando o retardando la aparición de micro-fisuras y reduciendo la cantidad de cristales de etringita creciendo en condiciones confinadas y por ende, ejerciendo presiones expansivas.Postprint (published version

Numerical assessment of external sulfate attack in concrete structures: a review

The slow progress of the external sulfate attack and the large size and criticality of typical structures affected by this phenomenon establish numerical modeling as a key tool to assess the future evolution of concrete structures exposed to sulfate-rich environments. The present review examines a selection of the most relevant numerical models developed during the last two decades to identify and explain the principles and simulation approaches commonly adopted. Assumptions associated with each approach are described to fully disclose the limitations and capabilities of each model. Guidance on model selection is provided based on the outcomes required. Finally, major areas holding significant potential to improve the reliability of the predictions are identified and discussed.Peer ReviewedPostprint (published version

Effects of biaxial confinement in mortars exposed to external sulfate attack

Research on external sulfate attack (ESA) is usually performed on small scale specimens under free expanding conditions. However, most field structures cannot expand freely due to confinement induced by the ground or other elements from the structure. As a result, ESA usually develops in confined conditions. This work aims to assess the interaction of gradual biaxial compression stresses generated by confinement with the ESA. Visual appearance, mass and ultrasonic velocity were monitored to characterize the macro-scale behavior of free and restrained mortar samples. Changes on phase composition and crystal morphology induced by confinement were evaluated by X-Ray diffraction and scanning electron microscopy. The confining stresses generated were estimated during the attack. Results indicate that ESA is not developed equally in free and confined specimens. The confinement limits sulfate availability, reduce the amount of ettringite precipitated and might induce changes on crystal morphology that reduce the degradation caused by the ESA.Peer ReviewedPostprint (author's final draft

The role of porosity in external sulphate attack

Design codes promote a limitation of permeability (indirectly of porosity) to reduce the sulphate ingress and improve the resistance of concrete and mortar to external sulphate attack (ESA). However, porosity could also have a positive effect on durability by generating additional space to accommodate the expansive phases. The aim of this study is to evaluate the role of porosity in ESA. For that, changes at the macro-scale, phase composition and pore network are monitored for mortar compositions with different pore-size distribution. Results indicate the existence of two mechanisms: the capacity to accommodate expansive phases controls the durability during the initial stages of the attack, while at later stages durability is defined by the permeability. Results from specimens with air-entrainer suggest that the intentional increase of porosity towards maximising the capacity to accommodate expansive products might be a valid approach in order to reduce the expansive forces generated during ESA.Peer ReviewedPostprint (author's final draft

Data-driven optimization tool for the functional, economic, and environmental properties of blended cement concrete using supplementary cementitious materials

The need to produce more sustainable concrete is proving imminent given the rising environmental concerns facing the industry. Blended cement concrete, based on any of the prominent supplementary cementitious materials (SCMs) such as fly ash, ground granulated blast-furnace slag, silica fume, calcined clay and limestone powder, have proven to be the best candidates for sustainable concrete mixes. However, a reliable sustainability measure includes not only the environmental impact, but also the economic and functional ones. Within these five SCMs, their environmental, economic and functional properties are found to be conflicting at times, making a clear judgement on what would be the optimum mix not a straightforward path. A recent framework and tool for concrete sustainability assessment ECO2, sets a reliable methodology for including the functional performance of a concrete mix depending on project-based specifications. Therefore, in this study, a recently published regression model, Pre-bcc was used to predict the functional properties of a wide grid search of potentially suitable blended cement concrete mixes. Hence, an open access novel genetic algorithm tool “Opt-bcc” was developed and used to optimize the sustainability score of these mixes based on a set selection of user-defined project-specific functional criteria. The optimized mixes using the Opt-bcc model for each strength class were compared against the mix design proposed by other optimization models from the literature and were found to be at least 70% cheaper and of 30% less environmental impact.Peer ReviewedPostprint (published version

Simplified methodology to evaluate the external sulfate attack in concrete structures

The external sulfate attack is a degradation process that causes expansion and cracking in concrete structures. Due to the absence of simplified methodologies to predict the potential damage, codes specify that sulfate resistant cement should be used whenever the surrounding sulfate concentration surpasses a predefined limit. This may lead to penalizing measures as the size of the element or the mechanical properties of the concrete used are not considered. In the present work, an alternative approach is proposed. A simplified chemo-mechanical methodology is deducted to assess the potential damage in concrete elements exposed to sulfate rich environments. Equations to estimate the penetration of sulfates are derived froma numericalmodel taking into account sulfate consumption, acceleration of the penetration induced by cracking and decrease in diffusivity caused by pore filling. Failure modes associated to this phenomenon are analyzed and a set of equations to assess the risk of failure are deducted. Finally, a parametric studywith different geometries of elements and surrounding sulfate contents is performed. The results show that the criterion included in codesmight be modified depending on the characteristics of the structure.Peer ReviewedPostprint (author's final draft

Modified digital image correlation aided measurement of the transverse to longitudinal deformation ratio for polymeric macro-fibres

Considering the increasing interest of polymeric macro-fibres reinforced concrete, a simple and straightforward method to assess the fibre’s transverse to longitudinal deformation ratio (¿f) becomes a determining aspect in terms of fibre both characterization and fibre–matrix interaction optimization. A noncontact optical measurement procedure to assess this ratio (¿f) - for concrete reinforcement - by a modified 2D Digital Image Correlation method is presented in this paper. This ratio (¿f) has influence on the matrix-fibre interaction and, thus, on the post-cracking tensile response of the concrete composite. Details on the definition, implementation and validation of this measurement procedure are presented. Additionally, an experimental program on 20 selected lab grade and commercial fibres was carried out to analyse the consistency of the measurements and the influence of the fibre’s geometric characteristics (flat, curved, embossed or crimped) on the ¿f. The outcomes of this research and the experimental procedure proposed are meant to assist fibre producers on the design process of optimized fibres to be used in structural concrete.Peer ReviewedPostprint (published version

Synergistic effect of fatigue loading and accelerated corrosion on reinforced concrete beams

Some structures such as bridges and marine structures that are susceptible to corrosion are also subjected to repeated loading. Whenever corrosion damage can occur to the material surface of a dynamically loaded structure, corrosion fatigue can be a serious problem. It should be remembered that both of these phenomena attack the surface of the steel reinforcing bars in the initial stage, corrosion damage involve surface damage reducing the fatigue crack initiation time. Besides, when corrosion is developed along with fatigue loading, there is a synergistic effect between them, resulting in further corrosion and fatigue damage than when considered separately. The present study was designed to investigate the synergistic effect of corrosion and fatigue loading when considered as a coupled phenomena on fatigue strength, deflection evolvement and corrosion onset of reinforced concrete beams. For that, a new corrosion fatigue set up that allows the application of the load and corrosion at the same time is presented and tested. Specimen under 2.5 A, representing the most damaging environment , showed the highest increase in stiffness during the early stages and the highest stiffness degradation during the last 25 % of its life, even though it failed at the lowest corrosion rate. While the specimen under 1.0 A, representing the less damaging environment, showed a more gradual stiffness degradation. Combined with the fatigue strength results obtained, it is believed that the parameter representing corrosion damage should not be the corrosion rate at failure, but a parameter representing the degree of corrosion speed that could be present in a specific environment.Outgoin

Synergistic effect of fatigue loading and accelerated corrosion on reinforced concrete beams

Some structures such as bridges and marine structures that are susceptible to corrosion are also subjected to repeated loading. Whenever corrosion damage can occur to the material surface of a dynamically loaded structure, corrosion fatigue can be a serious problem. It should be remembered that both of these phenomena attack the surface of the steel reinforcing bars in the initial stage, corrosion damage involve surface damage reducing the fatigue crack initiation time. Besides, when corrosion is developed along with fatigue loading, there is a synergistic effect between them, resulting in further corrosion and fatigue damage than when considered separately. The present study was designed to investigate the synergistic effect of corrosion and fatigue loading when considered as a coupled phenomena on fatigue strength, deflection evolvement and corrosion onset of reinforced concrete beams. For that, a new corrosion fatigue set up that allows the application of the load and corrosion at the same time is presented and tested. Specimen under 2.5 A, representing the most damaging environment , showed the highest increase in stiffness during the early stages and the highest stiffness degradation during the last 25 % of its life, even though it failed at the lowest corrosion rate. While the specimen under 1.0 A, representing the less damaging environment, showed a more gradual stiffness degradation. Combined with the fatigue strength results obtained, it is believed that the parameter representing corrosion damage should not be the corrosion rate at failure, but a parameter representing the degree of corrosion speed that could be present in a specific environment.Outgoin

Synergistic effect of fatigue loading and accelerated corrosion on reinforced concrete beams

Some structures such as bridges and marine structures that are susceptible to corrosion are also subjected to repeated loading. Whenever corrosion damage can occur to the material surface of a dynamically loaded structure, corrosion fatigue can be a serious problem. It should be remembered that both of these phenomena attack the surface of the steel reinforcing bars in the initial stage, corrosion damage involve surface damage reducing the fatigue crack initiation time. Besides, when corrosion is developed along with fatigue loading, there is a synergistic effect between them, resulting in further corrosion and fatigue damage than when considered separately. The present study was designed to investigate the synergistic effect of corrosion and fatigue loading when considered as a coupled phenomena on fatigue strength, deflection evolvement and corrosion onset of reinforced concrete beams. For that, a new corrosion fatigue set up that allows the application of the load and corrosion at the same time is presented and tested. Specimen under 2.5 A, representing the most damaging environment , showed the highest increase in stiffness during the early stages and the highest stiffness degradation during the last 25 % of its life, even though it failed at the lowest corrosion rate. While the specimen under 1.0 A, representing the less damaging environment, showed a more gradual stiffness degradation. Combined with the fatigue strength results obtained, it is believed that the parameter representing corrosion damage should not be the corrosion rate at failure, but a parameter representing the degree of corrosion speed that could be present in a specific environment.Outgoin