Prévision et évaluation de la fissuration précoce des ouvrages en béton

L'objectif de ce travail était de concevoir un outil de simulation permettant de choisir les meilleures solutions pour limiter la fissuration au jeune âge des structures en béton.Cet outil a été développé dans le code aux éléments finis CASTEM, il s'organise en deux phases de modélisations successives : un modèle déterminant les champs d'hydratation ainsi que les états hydriques et thermiques de la structure suivi d'un modèle mécanique utilisant ces données pour estimer la fissuration. Le modèle d'hydratation développé est un modèle multiphasique permettant de prévoir les évolutions conjointes de la teneur en eau, de la température et de l'hydratation des différentes phases de liants composés. Il est basé sur la résolution couplée des lois de cinétiques d'hydratation de chaque phase avec les lois de conservation de la masse d'eau et de la chaleur. La loi cinétique utilisée pour modéliser l'hydratation des phases du liant est basée sur une approche phénoménologique des cinétiques de réactions et des interactions entre phases (clinker et composés secondaires). Le modèle a été testé sur une structure massive de 27 m3 coulée in situ. La connaissance des champs hydriques, thermiques et d'hydratation permet ensuite de prévoir les déformations induites par les variations de teneur en eau (retrait de séchage et d'autodessiccation) et de température (déformations thermiques) au sein de la structure. Le risque de fissuration peut alors être évalué avec le modèle mécanique à partir des contraintes induites par les déformations empêchées (gradients thermiques et hydriques ou conditions aux limites mécaniques). Le modèle mécanique proposé est de type viscoélastique non linéaire couplé à un modèle d'endommagement anisotrope. Il permet de traiter de façon globale les phénomènes de retrait et de fluage par l'utilisation d'un modèle rhéologique reproduisant le comportement hydromécanique du béton non saturé.L'objectif de ce travail était de concevoir un outil de simulation permettant de choisir les meilleures solutions pour limiter la fissuration au jeune âge des structures en béton.Cet outil a été développé dans le code aux éléments finis CASTEM, il s'organise en deux phases de modélisations successives : un modèle déterminant les champs d'hydratation ainsi que les états hydriques et thermiques de la structure suivi d'un modèle mécanique utilisant ces données pour estimer la fissuration. Le modèle d'hydratation développé est un modèle multiphasique permettant de prévoir les évolutions conjointes de la teneur en eau, de la température et de l'hydratation des différentes phases de liants composés. Il est basé sur la résolution couplée des lois de cinétiques d'hydratation de chaque phase avec les lois de conservation de la masse d'eau et de la chaleur. La loi cinétique utilisée pour modéliser l'hydratation des phases du liant est basée sur une approche phénoménologique des cinétiques de réactions et des interactions entre phases (clinker et composés secondaires). Le modèle a été testé sur une structure massive de 27 m3 coulée in situ. La connaissance des champs hydriques, thermiques et d'hydratation permet ensuite de prévoir les déformations induites par les variations de teneur en eau (retrait de séchage et d'autodessiccation) et de température (déformations thermiques) au sein de la structure. Le risque de fissuration peut alors être évalué avec le modèle mécanique à partir des contraintes induites par les déformations empêchées (gradients thermiques et hydriques ou conditions aux limites mécaniques). Le modèle mécanique proposé est de type viscoélastique non linéaire couplé à un modèle d'endommagement anisotrope. Il permet de traiter de façon globale les phénomènes de retrait et de fluage par l'utilisation d'un modèle rhéologique reproduisant le comportement hydromécanique du béton non saturé

Microscopic effect of iron dosage on the stability of Fe‐doped ettringite

International audienceAbstract One significant aspect that affects the performance of high‐ferrite cement is the incorporation of iron (Fe) atoms and its stabilization into the cement phases. However, the influence of Fe doping on the stability of key phases, particularly on the ettringite, remains largely unexplored requiring deeper insights into the fundamental mechanisms. The present study explored the stability of Fe‐doped AFt structure based on density functional theory calculations at different Fe dosage. The obtained results showed that the stability of AFt structure decreases by increasing Fe dosage, which is in good agreement with the previous experimental observations. The incorporation of Fe into ettringite induced a series of changes in AFt structural and electronic properties. Bond order analysis underscored stronger covalent Fe–O bonds compared to Al–O, further emphasizing changes in the material's bonding network. Notably, density of states (DOS) analysis highlights the emergence of new occupied states near the Fermi level, primarily contributed by Fe and O atoms. This increased DOS, coupled with higher electron mobility, correlates with a reduction in material stability, as observed through shifts in bonding interactions and alterations in the bonding network. These findings point toward a complex interplay of factors, including altered bonding characteristics, increased electron mobility, and changes in the electronic structure, contributing to the observed instability of Fe‐doped ettringite

Chemo-Mechanical Modeling Requirements for the Assessment of Concrete Structure Service Life

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Material and Geometric Heterogeneity Consideration for Cracking Risk Prediction of Young Age Behavior of Experimental Massive Reinforced Concrete Structure

International audienceThis article presents the application of a thermo-hydro-chemo-mechanical (THCM) model to a real complex structure of reactor confinement (mock-up VERCORS from EDF) by taking into account the specificities of the construction (construction consequences), the distributed reinforcements and the material heterogeneity of massive structure. The experimental campaigns were conducted during and after the construction of VERCORS. The early-age behavior of concrete is first modelled based on a multiphasic hydration model to ensure the thermal evolution. Then a 3D mechanical model is used to predict the consequences of hydration, temperature and water variations on mechanical behavior. An alternative approach to consider the structural effect of distributed reinforcement without explicit meshing of reinforcements is implemented and is able to reproduce the influence of reinforcement on the crack patterns. Moreover, the " Weakest link localization " method is also adapted to deal with a probabilistic scale effect due to the material heterogeneity of massive structure. It permits to assess directly the most likely tensile strength which can treat the first crack in softening part of the loaded volume of structures

Modelling of ageing behaviour of Supplementary Cementitious Materials

International audienceA model of chemo-mechanical evolution of new cementitious materials such as low-heat or low-pH cements is described. The proposed phenomenological model, usable at structure scale, is based at early age on a multiphasic hydration developed for blended cements. At higher ages, the evolution of mechanical properties of such binder with high silica content cannot be explained by pozzolanic reaction (because portlandite is entirely consumed at early ages). At these ages, mineralogy analyses showed that the hydration of remaining anhydrous silica oxide is still developing by consumption of calcium from hydrates with high C/S ratios (for instance C-S-H produced by clinker hydration at early age). These chemical evolutions are modeled taking into account chemical equilibrium of solution and solid phases in terms of calcium concentration. The impact on mechanical properties is then also predicted. Finally the chemo-mechanical model is applied on the prediction of mechanical behavior of nuclear waste storage structures casted with low pH based concrete

Modelling of chemo-mechanical behaviour of low-pH Concretes

International audienceA model of the chemo-mechanical evolution of low-pH cement is clarified in order to be used at a structural scale. The proposed phenomenological model is based on a multiphasic hydration model developed in previous studies to predict the risk of early age cracking of structures cast with blended cements. At later ages, the evolution of mechanical properties cannot be explained only by the pozzolanic reaction usually considered in hydration models (because portlandite is entirely consumed at early ages). At these ages, mineralogical analyses showed that the hydration of remaining anhydrous silicate continued to develop by consumption of calcium from hydrates with high C/S ratios (e.g. C–S–H produced by clinker hydration at early age). A model able to predict these chemical evolutions is thus proposed. It is based on the principle of chemical equilibrium between the solution and the solid phases in terms of calcium concentration. The impact of this chemical evolution on mechanical properties can then be predicted with a better accuracy than with a classical hydration model. Finally the chemo-mechanical model is applied to the prediction of cracking of a large concrete element cast with a low pH based concrete

Multiphasic finite element modeling of concrete hydration

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Model of water transfer in concrete and rock based on a single state variable to consider simultaneous positive pressure and drying boundary conditions

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Nouvelles avancées sur le gel d'aluminosilicate provenant de l'attaque par l'acide acétique du ciment Portland hydraté : Caractérisation expérimentale et thermodynamique

International audienceMany concrete structures in aqueous environments suffer leaching, affecting their microstructure and durability. The resulting chemical and mineralogical degradation are difficult to predict over the long term and for environments of varying chemical composition, especially for severely degraded cement matrices. This is mainly because of the lack of chemical and thermodynamic data on the degraded phases formed during these attacks. In this context, this study aims to evaluate the chemical changes of leached ordinary Portland cement (OPC) paste by combining experimental (batch experiments) and modelling (thermodynamic equilibria calculations) approaches. The ground OPC paste was gradually added to an acetic acid solution. pH and chemical compositions of the solution were monitored throughout the experiment. The solid fraction was characterised over time, with particular attention paid to the phase obtained during the first additions. The latter was found to be an amorphous aluminosilicate gel (Al/Si = 0.3), with major contributions from Si Q 4 and Al IV (obtained by 29 Si and 27 Al NMR analyses respectively)

Interactions between hydrated cement pastes and aggressive ammonium: experimental batches characterization

International audienceAgricultural and food industries concrete facilities face chemically aggressive conditions that can damage their microstructure and reduce their lifespan. They are particularly exposed to ammonium-rich environments from natural microbial activity. The poorly crystalline mineralogy of hydrated cement pastes, the compositional variability of the phases and their reactivity make the geochemical behaviour of such materials difficult to investigate and predict over both large periods of time and wide variety of chemical compositions. This work aims (i) to assess the stability of the cement phases involved in ammonium-rich conditions as well as to identify the alteration products, and (ii) to understand the mechanisms and intensity of alteration. To do this, experiments were carried out both on OPC paste powder and on monolithic OPC pastes, degraded by an ammonium nitrate solution in semi-batch conditions. The powder was gradually added to the aggressive solution while the monoliths were immersed for 16 weeks in regularly renewed solution. The pH and the concentration of the chemical elements in solution were monitored over the experiments. The microstructural, chemical and mineralogical changes of the samples were analysed by scanning electron microscopy, electron probe micro-analysis and X-Ray diffraction and showed phenomena of dissolution, leaching and carbonation