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

Assessment of mortar evolution in pig slurry by mechanical and ultrasonic measurements

This work presents the results obtained in a long-term experiment focused on the study of the evolution of cementitious materials immersed in pig slurry at real conditions. Cement mortars were made with four different cement types and immersed in pig slurry for 48 months. Furthermore, to separate pure hydration process from pig slurry effect, mortar samples were immersed in water for 12 months at laboratory conditions. Compressive strength, X-ray diffraction and ultrasonic measurements were made in all samples. Ultrasonic measurements were made from ultrasonic images obtained from automatic ultrasonic inspections. Use of ultrasonic images has allowed the extraction of information about the state of the studied materials. An empirical relationship between ultrasonic velocity and compressive strength has been obtained and the long-term effect of pig slurry on cementitious materials has been determined

Caracterización del proceso de descalificación en morteros, mediante ensayos destructivos y no destructivos

Diversas aplicaciones del hormigón requieren una definición de durabilidad y de vida útil prolongada en el tiempo, como es el caso de aquellas estructuras dedicas al confinamiento de residuos de diversos tipos (como aquellos producidos en las plantas de incineración de residuos sólidos urbanos o los derivados de la utilización de los combustibles nucleares), tuberías de canalización, presas y tanques de almacenamiento de agua, entre otras aplicaciones. El principal proceso de degradación que puede afectar a estas estructuras es el ataque producido por aguas naturales, que provoca la descalcificación de la matriz cementicia y puede desembocar en un fallo en servicio del material, por el aumento de porosidad y la pérdida de propiedades mecánicas. En esta tesis doctoral se ha caracterizado el proceso de descalcificación en morteros de cemento mediante técnicas destructivas y no destructivas, con el objetivo de desarrollar un modelo de estimación de la vida útil de los materiales cementicios sometidos a procesos de descalcificación. Para lograr este objetivo se han fabricado varios grupos de probetas, con diferentes tipos de cementos y relaciones agua/cemento, las cuales han sido sometidas a un proceso de degradación acelerada por inmersión en nitrato de amonio. La caracterización microestructural de los materiales degradados y no degradados se ha abordado mediante la combinación de técnicas destructivas de caracterización (entre otras, análisis térmico, difracción de rayos-X, ténicas de medida de la porosidad), con la caracterización no destructiva mediante medidas de velocidad ultrasónica en inmersión. Los modelos desarrollados en esta tesis doctoral toman en cuenta el contenido de CaO del cemento de partida (teniendo en cuenta todos sus componentes) y la porosidad de las muestras no degradadas, para estimar la cinética del proceso de degradación y el grado de degradación de las mismas, estimando el parámetro cinético de avance del espesor degradado y la velocidad ultrasónica en la zona degradada. De esta forma, se ha obtenido un modelo cinético, que permite realizar estimaciones de vida útil de los materiales sometidos a procesos de descalcificación, y un modelo cualitativo, que nos permite estimar el grado de degradación de las muestras y estimar el espesor degradado de las mismas, a partir de medidas no destructivas de velocidad ultrasónica. Several applications of concrete need a deffinition of durability and life-cycle extended on time. Some examples of these applications among others, are those used on waste confinement (as those produced on solid urban residues incineration plants or those derived from nuclear combustible), pipes, water storage tanks and dams. The main degradation process that can affect this structures is caused by natural waters. Water-based degradation processes cause decalcification of the cementmatrix and can lead to a mechanical failure of the structure, caused by the increase on materials porosity and the loss of mechanical properties. Decalcification process in cement mortars has been characterised in this thesis by means of destructive and non-destructive testing. The main aim of this work is to develop a life-cyle estimation model for cement-based materials under decalcification process. To achieve this objective several mortar samples have been made, with different cement types and different water/cement ratios. Samples have been degradated in an accelerated way by inmersion under ammonium nitrate. Microstructural characterization of degraded and non-degraded samples has been tackled by the combination of destructive characterization techniques (thermal analysis, X-ray diffraction, porosity measurements, among others) with non-destructive characterization techniques by means of ultrasonic pulse velocity measurements under water. The models developed in this thesis take in account the CaO content in the cement (accounting for all the components of the cement) and the porosity of non-degraded samples, to estimate the kinetics of the degradation process and the degradation degree of the samples. This model estimates the kinetic parameter from the degraded depth propagation and the ultrasonic velocity in the degradaded zone. Thereby it has been obtained a kinetic model and a cualtitative model for the degradation process. First one allows life-time estimations for materials under decalcification processes. The second one allows to estimate degradation degree and degraded depth from nondestructive ultrasonic measurement

Caracterización microestructural de materiales base cemento mediante ensayos destructivos (Parte I)

El objetivo principal del presente trabajo ha sido la caracterización microestructural de materiales base cemento mediante la aplicación de ensayos destructivos, para obtener una descripción detallada de las características físicas y químicas de dichos materiales que sirva como base para la correlación con otras metodologías de evaluación, principalmente no destructivas. Este trabajo ha sido subdividido en dos partes. En esta primera parte del trabajo se presenta una descripción de las principales características microestructurales de los materiales base cemento que servirá como base teórica para la posterior caracterización de los materiales. Se presentan las principales características de la hidratación de los compuestos del clínker y las principales fases hidratadas presentes. Así mismo se describen las características de la microestructura porosa y los efectos particulares que genera la presencia de los áridos en los procesos de hidratación y de desarrollo de microestructura porosa en la zona interfacial árido-pasta. En una segunda parte se presenta el trabajo experimental desarrollado de caracterización microestructural

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

Development and experimental validation of an overlay mortar with biocide activity

Biodeterioration of concrete by microorganism colonisation may be a problem in several structures, especially in irrigation and hydroelectric canals. The main problem in such structures is the proliferation of algae and cyanobacteria that affect the performance of the structure, increase the maintenance costs and affects its durability. A research was conducted to develop a novel cement-based material with biocide activity that can be used as an overlay mortar in existing structures, such as canals and pipes. With this aim, ten commercial biocides were evaluated in a laboratory campaign to assess the effectiveness of the compounds against the microbial colonisation of concrete. Both mono- and multicomponent formulations were designed from the commercial products, to increase their antimicrobial effect obtaining a set of biocide formulations. The formulations were submitted to a flowchart process to determine their influence on the physical properties of the concrete, evaluate the release of the actives, and their antimicrobial efficiency both before and after accelerated aging processes. During the campaign, some formulations were observed to diminish the strength of the concrete. Such behaviour was normally due to the interaction of the active with the cement hydration process. Other formulations showed a high release of active from the concrete in water, compromising the durability of the treatment. In general, monocomponent formulations did not succeed to fulfill all the requirements, thus multicomponent formulations were analysed. One studied multicomponent formulation presented particularly good results in all properties analysed. This product did not significantly change the properties of concrete and the release of active in water from the concrete was low, while the antimicrobial effects were long lasting.Peer ReviewedPostprint (author's final draft

Adaptive Vectorial Filter for Grid Synchronization of Power Converters Under Unbalanced and/or Distorted Grid Conditions

This paper presents a new synchronization scheme for detecting multiple positive-/negative-sequence frequency harmonics in three-phase systems for grid-connected power converters. The proposed technique is called MAVF-FLL because it is based on the use of multiple adaptive vectorial filters (AVFs) working together inside a harmonic decoupling network, resting on a frequency-locked loop (FLL) which makes the system frequency adaptive. The method uses the vectorial properties of the three-phase input signal in the αβ reference frame in order to obtain the different harmonic components. The MAVF-FLL is fully designed and analyzed, addressing the tuning procedure in order to obtain the desired and predefined performance. The proposed algorithm is evaluated by both simulation and experimental results, demonstrating its ability to perform as required for detecting different harmonic components under a highly unbalanced and distorted input grid voltage

Expansions with different origins in a concrete dam with bridge over spillway

Concrete dams with expansions from different origins are rare and scarcely documented in the literature, therefore their study gains special interest. This study presents the case of a 62-year-old concrete dam with signs of deterioration due to expansive reactions. Several tests performed on the samples extracted from the dam such as X-ray diffraction, scanning electron microscope with energy dispersive spectroscopy mode and petrography suggest that the expansions are due to a delayed ettringite formation and a subsequent alkali-silica reaction. Thermal and mechanical models developed in the study support this diagnosis.Peer ReviewedPostprint (author's final draft

Evaluation of conductive concrete made with steel slag aggregates

The incorporation of black steel slag (waste from steel production) in concrete responds to a greater awareness of sustainability. In addition, it is capable of increasing electrical conductivity. The objective of this work is to achieve multifunctional concretes with higher mechanical performance and which can modify the electrical characteristics. In this work, natural aggregates have been replaced by different aggregates of steel slags from Spanish industries and the mechanical characteristics of compression, modulus of elasticity, indirect traction and electrical characteristics of these concretes have been studied with respect to reference mixtures. Encouraging results have been obtained for concrete with slag and metallic fibers, improving the electrical conductive capacity by almost 70% and improving the mechanical performance by 14%.The authors acknowledge the financial support provided by the project PID2019-106555RB-I00. The first author acknowledges the financial support from Agència de Gestió d'Ajuts Universitaris i de Recerca (AGAUR), with the doctoral fellowship: Personal investigador novell (FI) 2021. We would also like to thank the following companies: Promsa, Sidenor, Calcinor, Hormor and Adec for supplying the slag, without which this study would not be possible.Peer ReviewedPostprint (published version