This study aims at generating numerical 3D samples of concrete so as to study the effects of the granular inclusions shape on the macroscopic kinetics of reactive transport phenomena.Two types of mesostructure configurations are considered: the first one is composed of a matrix of mortar in which are randomly distributed inclusions corresponding to the concrete coarse aggregates, and the second one also includes a steel rebar. The choice of a mesoscopic modeling for the mortar matrix is based on the need to obtain numerical structures of reasonable size. In particular, the Interfacial Transition Zones (ITZ) are ignored, as this hypothesis seems acceptable for coarse aggregates. This study is applied to the case of drying and atmospheric carbonation by using simplified models solved by the finite element code Cast3M. The purpose is to quantify the influence of the aggregate shape on the kinetics of macroscopic transfer and the isovalue lines for some physical variables representative of the reactive transport problems: saturation degree for drying, and porosity, calcite and portlandite concentrations for carbonation. Basic aggregates shapes are studied (spheres, cubes), as well as more complex ones (Voronoi particles) which are supposed to be more representative of real aggregates. The effects of 'non-isotropic' shapes (oblate and prolate ones) are also investigated. It is shown that the influence of the aggregate shapes appears negligibly small on macroscopic indicators, except for oblate shapes with aspect ratios of 3. This latter case also exhibits substantial local delayed effects and a more important variability, which may have some importance for a precise description and estimation of degradation processes related to steel rebar corrosion.

We present molecular dynamics simulations directed at understanding self-limiting oxidation of nanoclusters. Atomic oxygen is inserted in an atom-by-atom way in the silicon bonds to form silicon oxide. First, we focus on planar oxidation to calibrate our model and test its capabilities. Then, we present results on oxidation of 50 Å diam silicon spheres. Kinetic causes of self-limitation are investigated by drawing a map of the local stress in the Si/SiO 2 system. We obtain stresses in contrast to in continuum models. For thin oxides, we find in particular tensile pressure in the silicon core and a pressure gradient in the oxide shell. We investigate the effect of pressure gradient on the O 2 transport within the framework of Nerst-Eintein's transport equation. We find that a pressure gradient compatible with experimental estimates yields self-limitation of the oxidation kinetics.

2005) JERK, an event-based Kinetic Monte Carlo model to predict microstructure evolution of materials under irradiation, Philosophical Magazine, 85:4-7, 549-558To link to this article: http://dx.JERK is a Kinetic Monte Carlo model which aims at describing the evolution of materials under irradiation over large time and length scales. The evolution is calculated by collecting only those events which actually modify the objects making up the microstructure (defect clusters, various complexes, dislocations), by sampling their probability of occurrence, deciding in view of the chosen delays whether the events will take place or not within a given time interval. The details of the atomic transport are ignored and the jumps of mobile species are bunched into trajectories, which comply with continuous diffusion laws. After some tuning, the increase in efficiency over a Kinetic Monte Carlo procedure which processes each jump event after another should be noticeable.

We introduce a class of Monte Carlo algorithms that solve a dynamic problem defined by the transition rates and the initial state of a discrete system. This class contains the method of Bortz, Kalos, and Lebowitz ͑BKL͒ ͓J. Comp. Phys. 17, 10 ͑1975͔͒ as a limit. We show that introducing a constant time step in a Metropolis algorithm leads to an approximation of the solution in which the system relaxation times are underestimated. This can be corrected if the time step is an adequate stochastic variable. Thus, we are able to define kinetic Metropolis algorithms and generalize them in a case of nonconstant numbers of attempt configurations. The algorithm class allows us to introduce a useful method in which the calculation of transition rates are exploited for the next step in an adaptive way. This method corresponds to a kinetic Metropolis algorithm when the rejection probability is reasonable and becomes similar to the BKL method otherwise. We describe and compare four different algorithms applied to a physical example about the diffusion in lattice gases.

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The developed tools are used to create a representative volume element (RVE) of cementitious materials and then assess the diffusive properties. The algorithms developed in Combs target a fast placement of the inclusions and a fast generation of the RVE shape and its mesh. Two application cases are considered: the unaltered material diffusivity and the degraded material diffusivity. The first case of application focuses on the description of the capillary porosity. The second application case focuses on the description of the degradation of cementitious material (mineral and porosity) and the diffusive properties associated. The reliability of the analytical effective medium approximations (MT and SC) is confirmed from 3D finite elements (FE) calculations performed on a matrix-inclusions microstructure obtained by RVE generation with Combs. The results also show the need to take into account the percolation behavior.

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