A microstructural insight into the hygro-mechanical behaviour of a stabilised hypercompacted earth

The use of raw earth as construction material can save embodied and operational energy because of low processing costs and passive regulation of indoor ambient conditions. Raw earth must however be mechanically and/or chemically stabilised to enhance stiffness, strength and water durability. In this work, stiffness and strength are enhanced by compacting raw earth to very high pressures up to 100 MPa while water durability is improved by using alkaline solutions and silicon based admixtures. The effect of these stabilisation methods on hygro-mechanical behaviour is explored and interpreted in terms of the microstructural features of the material. Stiffness and strength are defined at different humidity levels by unconfined compression tests while the moisture buffering capacity is measured by humidification/desiccation cycles as prescribed by the norm ISO 24353 (Hygrothermal performance of building materials and products determination of moisture adsorption/desorption properties in response to humidity variation. International Organization for Standardization, Geneva, 2008). As for the microstructural characterisation, different tests (i.e. X-ray diffractometry, Infrared Spectroscopy, Mercury Intrusion Porosimetry, Nitrogen Adsorption) are performed to analyse the effect of stabilisation on material fabric and mineralogy. Results indicate that the use of alkaline activators and silicon based admixtures significantly improves water durability while preserving good mechanical and moisture buffering properties. Similarly, the compaction to very high pressures results in high levels of stiffness and strength, which are comparable to those of standard masonry bricks. This macroscopic behaviour is then linked to the microscopic observations to clarify the mechanisms through which stabilisation affects the properties of raw earth at different scales

AID-Targeting and Hypermutation of Non-Immunoglobulin Genes Does Not Correlate with Proximity to Immunoglobulin Genes in Germinal Center B Cells

Upon activation, B cells divide, form a germinal center, and express the activation induced deaminase (AID), an enzyme that triggers somatic hypermutation of the variable regions of immunoglobulin (Ig) loci. Recent evidence indicates that at least 25% of expressed genes in germinal center B cells are mutated or deaminated by AID. One of the most deaminated genes, c-Myc, frequently appears as a translocation partner with the Ig heavy chain gene (Igh) in mouse plasmacytomas and human Burkitt's lymphomas. This indicates that the two genes or their double-strand break ends come into close proximity at a biologically relevant frequency. However, the proximity of c-Myc and Igh has never been measured in germinal center B cells, where many such translocations are thought to occur. We hypothesized that in germinal center B cells, not only is c-Myc near Igh, but other mutating non-Ig genes are deaminated by AID because they are near Ig genes, the primary targets of AID. We tested this “collateral damage” model using 3D-fluorescence in situ hybridization (3D-FISH) to measure the distance from non-Ig genes to Ig genes in germinal center B cells. We also made mice transgenic for human MYC and measured expression and mutation of the transgenes. We found that there is no correlation between proximity to Ig genes and levels of AID targeting or gene mutation, and that c-Myc was not closer to Igh than were other non-Ig genes. In addition, the human MYC transgenes did not accumulate mutations and were not deaminated by AID. We conclude that proximity to Ig loci is unlikely to be a major determinant of AID targeting or mutation of non-Ig genes, and that the MYC transgenes are either missing important regulatory elements that allow mutation or are unable to mutate because their new nuclear position is not conducive to AID deamination

Influence of cement type on transport properties and chemical degradation: application to nuclear waste storage

International audienceThe geological repository of nuclear waste in concrete containers is a possible storage method explored by ANDRA (Agence Nationale pour la gestion des Déchets RAdioactifs). The concrete must display a high confinement capacity for long periods, characterized by transport properties and by the acido-basic buffer of hydrated cement. During service life, these properties can be endangered by chemical attack of underground water. The cement type has an important influence on the concrete's performances. Then, it is essential to establish appropriate mixtures and select accurate components. In this work an ordinary Portland cement and fly ash and blast furnace slag blended cement are compared. To determine confinement capacities, transfer properties and mortars pore size distributions were investigated. To predict the long term structure behavior, an ammonium nitrate test has been developed to enhance decalcification and to accelerate hydrolysis of cementitious materials. Measurement of degraded depth with time regarding calcium content was carried out. Impact of decalcification on transport properties was evaluated. Fly ash and blast furnace slag provide better properties for native mortars, and more principally diffusion properties, but not as much as necessary to limit leaching in degraded material by chemical attack. Keywords: Fly ash, blast furnace slag, gas permeability, chloride diffusion, mercury intrusion porosimetry, ammonium nitrate degradation. Résumé : Le stockage de déchets radioactifs en formation géologique profonde est une voie d'investigation de l'ANDRA (Agence Nationale pour la gestion des Déchets RAdioactifs). La barrière ouvragée cimentaire doit assurer une capacité de confinement pendant de longues périodes, caractérisée par ses propriétés de transfert et le maintien du tampon acido-basique du ciment hydraté. Pendant la durée de service de l'ouvrage, ces propriétés sont modifiées par l'attaque acide des eaux souterraines. La nature du ciment influençant majoritairement les performances du béton, il apparaît essentiel de sélectionner les composants adéquats pour établir des formulations performantes. Dans cette étude les performances en terme de durabilité d'un ciment Portland ordinaire et un ciment aux cendres volantes et aux laitiers sont comparées. 2 /22 Les propriétés de transfert et la microstructure des mortiers sont mesurées pour en évaluer les capacités de confinement. Afin d'estimer le comportement à long terme de la structure, un essai de dégradation par le nitrate d'ammonium a été développé pour accélérer la décalcification des matériaux cimentaires. L'évolution de l'épaisseur dégradée au cours de l'attaque ramenée à la teneur en calcium est suivie. L'impact de la décalcification sur les propriétés de transfert est évalué. Les cendres volantes et les laitiers semblent améliorer les propriétés des mortiers, notamment en terme de diffusion, mais pas assez pour résister à l'attaque chimique

Influence of cement type on transport properties and chemical degradation: application to nuclear waste storage

International audienceThe geological repository of nuclear waste in concrete containers is a possible storage method explored by ANDRA (Agence Nationale pour la gestion des Déchets RAdioactifs). The concrete must display a high confinement capacity for long periods, characterized by transport properties and by the acido-basic buffer of hydrated cement. During service life, these properties can be endangered by chemical attack of underground water. The cement type has an important influence on the concrete's performances. Then, it is essential to establish appropriate mixtures and select accurate components. In this work an ordinary Portland cement and fly ash and blast furnace slag blended cement are compared. To determine confinement capacities, transfer properties and mortars pore size distributions were investigated. To predict the long term structure behavior, an ammonium nitrate test has been developed to enhance decalcification and to accelerate hydrolysis of cementitious materials. Measurement of degraded depth with time regarding calcium content was carried out. Impact of decalcification on transport properties was evaluated. Fly ash and blast furnace slag provide better properties for native mortars, and more principally diffusion properties, but not as much as necessary to limit leaching in degraded material by chemical attack. Keywords: Fly ash, blast furnace slag, gas permeability, chloride diffusion, mercury intrusion porosimetry, ammonium nitrate degradation. Résumé : Le stockage de déchets radioactifs en formation géologique profonde est une voie d'investigation de l'ANDRA (Agence Nationale pour la gestion des Déchets RAdioactifs). La barrière ouvragée cimentaire doit assurer une capacité de confinement pendant de longues périodes, caractérisée par ses propriétés de transfert et le maintien du tampon acido-basique du ciment hydraté. Pendant la durée de service de l'ouvrage, ces propriétés sont modifiées par l'attaque acide des eaux souterraines. La nature du ciment influençant majoritairement les performances du béton, il apparaît essentiel de sélectionner les composants adéquats pour établir des formulations performantes. Dans cette étude les performances en terme de durabilité d'un ciment Portland ordinaire et un ciment aux cendres volantes et aux laitiers sont comparées. 2 /22 Les propriétés de transfert et la microstructure des mortiers sont mesurées pour en évaluer les capacités de confinement. Afin d'estimer le comportement à long terme de la structure, un essai de dégradation par le nitrate d'ammonium a été développé pour accélérer la décalcification des matériaux cimentaires. L'évolution de l'épaisseur dégradée au cours de l'attaque ramenée à la teneur en calcium est suivie. L'impact de la décalcification sur les propriétés de transfert est évalué. Les cendres volantes et les laitiers semblent améliorer les propriétés des mortiers, notamment en terme de diffusion, mais pas assez pour résister à l'attaque chimique

Approche continue-discrète appliquée au comportement en cisaillement d'un joint rocheux

Le cisaillement de deux surfaces lisses est décrit par un phénomène simple : le glissement. Ce comportement est très bien modélisé par le critère de Coulomb. Cependant, lorsque deux volumes sont cisaillés au niveau de leur surface rugueuse, le glissement se combine à de l'endommagement mécanique. Des méthodes de calcul déterminant la résistance au cisaillement existent comme le critère de Barton où la rugosité et la résistance du matériau sont prises en compte par deux paramètres. Ces méthodes peuvent être utilisées pour modéliser le comportement au cisaillement de joints. Ces modèles peuvent être linéaires ou non, et tenir compte de l'endommagement mécanique des aspérités ou non. Ces critères ou ces modèles considèrent la rugosité comme un paramètre moyenné sur toute la surface et non comme une géométrique variable influençant le comportement du joint localement. Ainsi, ils ne représentent pas le comportement réel du joint en cisaillement. Afin de prendre en compte l'effet local de la rugosité, le modèle développé sous le code de calcul aux éléments finis Cast3M, modélise le joint au travers d'un maillage se basant sur des surfaces réelles numérisées. Ces données proviennent d'une campagne d'essais de cisaillement direct réalisée à l'Université de Sherbrooke, Québec. Les deux surfaces maillées engendrent deux volumes sur lesquels on applique les conditions aux limites adéquates pour réaliser le cisaillement. Un modèle continu d'endommagement unilatéral couplé à la plasticité permet le calcul de l'état de contraintes dans les volumes et la détermination des résultantes sur chaque facette des surfaces de contact. Un modèle de glissement de type Coulomb est simultanément appliqué à ces facettes pour gérer le glissement de manière locale. Ce travail présente les premiers résultats obtenus par ce modèle de cisaillement couplant un endommagement continu à un glissement local, permettant d'observer l'influence des aspérités et plus particulièrement de leur emboîtement

Evaluation of recycling capabilities of bio-stabilised earth building materials

Earth building materials are considered sustainable due to the inherent low embodied and operational energies, the ability to buffer hygrothermal fluctuations and the recycling potential. Despite these advantageous attributes, they are not employed in mainstream construction mainly because of their vulnerability to water. To improve durability in humid environments, earth is often stabilised with chemical binders, such as cement, which results in similar mechanical properties to those of conventional building materials. It has been reported that the addition of about 8-12% of cement to the earth is sufficient to obtain the same mechanical performance of fired bricks1. Cement stabilisation was initially adopted for the manufacture of compressed earth blocks, but it has now become a norm for other forms of earthen construction like adobe. The addition of cement increases durability but also contributes to carbon emissions while increasing embodied energy. Studies have shown that cement-stabilised earthen materials have the same net carbon emissions as lean concrete and, therefore, relatively poor green credentials. Furthermore, cement stabilisation negatively impacts other aspects of material performance by reducing both hygroscopicity and ease of recycling2. Reduced hygroscopicity leads to a poorer ability to buffer hygrothermal fluctuations and, hence, higher levels of operational energy for ensuring adequate levels of indoor comfort. The loss of recycling potential means instead that demolished materials cannot experience a full lifecycle but are either downcycled or dumped in landfills. To reinstate the original sustainability of the material without compromising on durability, it is necessary to look for alternative stabilisation techniques that can ensure engineering performance while retaining hygroscopic and recycling advantages