An RVE-Based Multi-Scale Approach for Concrete Affected by Alkali–Silica Reaction

The alkali-silica reaction (ASR) is a deleterious reaction that occurs in cementitious mixtures like concrete due to the combination of the alkaline solution of the cement paste with the amorphous silica of the aggregates. As a consequence of this reaction a gel is generated that expands through water absorption, leading to pore filling and pore pressure increment. Experimentally, the consequences of ASR are observed in both the micro-cracking path around the aggregate and the stiffness reduction of the overall skeleton or solid phase. To get a proper prediction of the aforementioned effect, it is necessary to consider the kinetics of the chemical reaction and its effect on the mechanical behavior. In this paper, the ASR is modeled introducing a variable that quantifies its progress through a first order kinetic law. This variable affects the volumetric component of the Helmholtz free energy which now shall account for the chemo-mechanical behavior of the material. Thus, an additional term is introduced in the microscopic free energy density related to the chemical reaction process. The proposed free energy equation is implemented in a thermodynamically consistent multi-scale framework accounting for the chemo-mechanical degradation of the micro-structure due to the volumetric expansion of the gel. The cement mortar constitutive relation is reformulated using Biot’s poromechanics theory to include the pore pressure in the mechanical description, and a damage model to reproduce the solid phase degradation. Finally, some numerical examples showing the potential of the presented formulation are discussed.Publicado en: Mecánica Computacional vol. XXXV, no. 23Facultad de Ingenierí

An RVE-Based Multi-Scale Approach for Concrete Affected by Alkali–Silica Reaction

The alkali-silica reaction (ASR) is a deleterious reaction that occurs in cementitious mixtures like concrete due to the combination of the alkaline solution of the cement paste with the amorphous silica of the aggregates. As a consequence of this reaction a gel is generated that expands through water absorption, leading to pore filling and pore pressure increment. Experimentally, the consequences of ASR are observed in both the micro-cracking path around the aggregate and the stiffness reduction of the overall skeleton or solid phase. To get a proper prediction of the aforementioned effect, it is necessary to consider the kinetics of the chemical reaction and its effect on the mechanical behavior. In this paper, the ASR is modeled introducing a variable that quantifies its progress through a first order kinetic law. This variable affects the volumetric component of the Helmholtz free energy which now shall account for the chemo-mechanical behavior of the material. Thus, an additional term is introduced in the microscopic free energy density related to the chemical reaction process. The proposed free energy equation is implemented in a thermodynamically consistent multi-scale framework accounting for the chemo-mechanical degradation of the micro-structure due to the volumetric expansion of the gel. The cement mortar constitutive relation is reformulated using Biot’s poromechanics theory to include the pore pressure in the mechanical description, and a damage model to reproduce the solid phase degradation. Finally, some numerical examples showing the potential of the presented formulation are discussed.Publicado en: Mecánica Computacional vol. XXXV, no. 23Facultad de Ingenierí

An RVE-Based Multi-Scale Approach for Concrete Affected by Alkali–Silica Reaction

The alkali-silica reaction (ASR) is a deleterious reaction that occurs in cementitious mixtures like concrete due to the combination of the alkaline solution of the cement paste with the amorphous silica of the aggregates. As a consequence of this reaction a gel is generated that expands through water absorption, leading to pore filling and pore pressure increment. Experimentally, the consequences of ASR are observed in both the micro-cracking path around the aggregate and the stiffness reduction of the overall skeleton or solid phase. To get a proper prediction of the aforementioned effect, it is necessary to consider the kinetics of the chemical reaction and its effect on the mechanical behavior. In this paper, the ASR is modeled introducing a variable that quantifies its progress through a first order kinetic law. This variable affects the volumetric component of the Helmholtz free energy which now shall account for the chemo-mechanical behavior of the material. Thus, an additional term is introduced in the microscopic free energy density related to the chemical reaction process. The proposed free energy equation is implemented in a thermodynamically consistent multi-scale framework accounting for the chemo-mechanical degradation of the micro-structure due to the volumetric expansion of the gel. The cement mortar constitutive relation is reformulated using Biot’s poromechanics theory to include the pore pressure in the mechanical description, and a damage model to reproduce the solid phase degradation. Finally, some numerical examples showing the potential of the presented formulation are discussed.Publicado en: Mecánica Computacional vol. XXXV, no. 23Facultad de Ingenierí

Modelo termo-químico-mecánico para simular la respuesta de un dique de gravedad sujeto a reacción álcali-sílice

Las reacciones álcali-sílice (RAS) en estructuras de hormigón consisten en reacciones químicas que se producen entre agregados con sílice reactiva y los álcalis que se liberan durante la hidratación del cemento. El producto de estas reacciones es un gel expansivo que conduce a la fisuración del hormigón, comprometiendo su durabilidad. Se trata de un proceso complejo debido a que el acoplamiento entre la difusión del calor, humedad y la cinética del RAS puede ser crítico. La RAS es una reacción química que es activada térmicamente y bajo ciertas condiciones de contenido de agua. La disponibilidad de una herramienta que permita realizar una evaluación cuantitativa, en tiempo y espacio, del impacto de los efectos que dichas expansiones pueden provocar en estructuras de hormigón es de fundamental importancia en la evaluación de la durabilidad de estructuras que manifiestan este fenómeno. Los diques, por ejemplo, son estructuras que potencialmente presentan este tipo de degradación dado que en forma permanente durante al menos por un largo período de su vida en servicio están en contacto con agua o suelos húmedos. En este trabajo se desarrolla un modelo gradiente de daño basado en la mecánica multifásica de medios reactivos porosos, donde las ecuaciones de masa, energía y momento conjuntamente a las relaciones constitutivas y físicas necesarias para modelar la RAS en condiciones ambientales variables se desarrollan de manera acoplada y resuelven mediante el método de los elementos finitos. Simulaciones numéricas de un dique de gravedad sujeto a condiciones variables de temperatura a lo largo de 16 años de vida útil verifican el modelo propuesto.Publicado en: Mecánica Computacional vol. XXXV, no. 25Facultad de Ingenierí

Magnetic exchange interaction in a pair of orbitally degenerate ions: Magnetic anisotropy of [Ti2Cl9]−3

The theory of the kinetic exchange in a pair of orbitally degenerate ions developed by the authors [J. Phys. Chem. A 102, 200 (1998)] is applied to the case of face-shared bioctahedral dimer (overall D3h-symmetry). The effective kinetic exchange Hamiltonian is found for a 2T2–2T2 system taking into account all relevant transfer pathways and charge-transfer crystal field states. The influence of different transfer integrals involved in the kinetic exchange on the energy pattern and magnetic properties of the system is examined. The role of other related interactions (trigonal crystal field, spin–orbit coupling) is also discussed in detail. Using the pseudoangular momentum representation and the technique of the irreducible tensor operators of R3-group we give a general outlook on the nontrivial symmetry properties of the effective Hamiltonian for the D3h-pair, and on the magnetic anisotropy arising from the orbital interactions specific for the case of orbital degeneracy. The magnetic properties of the binuclear unit [Ti2Cl9]−3 in Cs3Ti2Cl9 are discussed with a special emphasis on the magnetic anisotropy experimentally observed in this system. The existing exchange models for [Ti2Cl9]−3 and the concept of the effective Hamiltonian are discussed in the context of the present [email protected] ; [email protected] ; [email protected] ; [email protected]

High‐nuclearity mixed‐valence magnetic clusters : A general solution of the double exchange problem

We report here a general solution of the double‐exchange problem in the high‐nuclearity mixed valence systems containing arbitrary number P of the electrons delocalized over the network of N (P<N) localized spins. The developed approach is based on the successive (chainlike) spin‐coupling scheme and takes full advantage from the quantum angular momentum theory. In the framework of this approach the closed‐form analytical expressions are deduced for the matrix elements of the double exchange interaction, two‐electron transfer, and three‐center interaction that can be referred to as the potential exchange transfer. For the arbitrary nuclearity mixed‐valence systems the matrix elements of all named interactions are expressed in terms of all relevant spin quantum numbers and 6j symbols and do not contain higher order recoupling coefficients. We describe also the combined approach taking into account both angular momentum consideration and advantages of point symmetry adapted basis [email protected] , [email protected] ; [email protected] ; [email protected] ; [email protected]

Chromatin establishes an immature version of neuronal protocadherin selection during the naive-to-primed conversion of pluripotent stem cells.

In the mammalian genome, the clustered protocadherin (cPCDH) locus provides a paradigm for stochastic gene expression with the potential to generate a unique cPCDH combination in every neuron. Here we report a chromatin-based mechanism that emerges during the transition from the naive to the primed states of cell pluripotency and reduces, by orders of magnitude, the combinatorial potential in the human cPCDH locus. This mechanism selectively increases the frequency of stochastic selection of a small subset of cPCDH genes after neuronal differentiation in monolayers, 10-month-old cortical organoids and engrafted cells in the spinal cords of rats. Signs of these frequent selections can be observed in the brain throughout fetal development and disappear after birth, except in conditions of delayed maturation such as Down's syndrome. We therefore propose that a pattern of limited cPCDH-gene expression diversity is maintained while human neurons still retain fetal-like levels of maturation