Failure of rammed earth walls: from observations to quantifications

International audienceNowadays, rammed earth construction is attracting renewed interest throughout the world thanks to its "green" characteristics in the context of sustainable development. Firstly, using a local material (soil on site or near the site), rammed earth constructions have very low embodied energy. Secondly, rammed earth houses have an attractive appearance and present advantageous living comfort due to substantial thermal inertia and the "natural regulator of moisture" of rammed earth walls. This is why several research studies have been carried out recently to study the mechanical and thermal characteristics of rammed earth. However, to our knowledge, there are not yet sufficient studies on the tensile strength and the shear strength of rammed earth. The tensile strength of rammed earth is neglected in general due to its very low value, but in extreme conditions (e.g., seismic conditions), knowing the tensile strength is necessary for structural design. Moreover, the shear strength is required in many cases to check the local failure of rammed earth quickly, which has been observed in old structures (especially those submitted to concentrated loads). This paper presents experimental results on tensile strengths and the Poisson ratio of rammed earth specimens. Local failure tests were also conducted on 1 m × 1 m × 0.3 m wallettes manufactured in the laboratory. The shear strength was then identified using a simple method based on compressive strength, tensile strength and Mohr's circle theory. The approach proposed was validated by tests on the wallettes. Finite Element (FE) modeling was also carried out to confirm the results. Last, the method presented was validated for stabilized rammed earth lintels presented in the literature

Poured Earth as concrete

International audienceIn order to pour an earthen material in a liquid state, as a concrete, technologies used by concrete and ceramic industries can be transferred to the field of earthen construction. Two different methods should be employed simultaneously. The first relates to theories of grain packing that have led to models of Apollonian packing and spaced packing, commonly used for the development of cement concrete (ultra high performance concrete and self-leveling concrete). It concerns the optimization of the granular skeleton of natural materials. The second relates to the dispersion of the colloidal fraction of earthen materials. In natural soils, clays are organized as porous aggregates composed of several tens of particles. These aggregates trap water that is not used to liquefy the mixture. The dispersion of these aggregates, releasing this interstitial water, liquefies the earthen material without adding water. This dispersion is obtained by adding a small proportion (on the order of a few tenths of a percent by mass relative to the dry material) of deflocculating agents such as those commonly used for the development of industrial ceramics. The combined action of these two methods leads to a solid material that does not crack while drying, and can easily be implemented at a viscosity comparable to that of vibrated concrete with the same tools than those employed by the concrete industry (cement mixer, shuttering, vibrating needle). This new technique is particularly suited for the implementation of slabs and other horizontal surfaces, and also for vertical walls

Poured Earth as concrete

International audienceIn order to pour an earthen material in a liquid state, as a concrete, technologies used by concrete and ceramic industries can be transferred to the field of earthen construction. Two different methods should be employed simultaneously. The first relates to theories of grain packing that have led to models of Apollonian packing and spaced packing, commonly used for the development of cement concrete (ultra high performance concrete and self-leveling concrete). It concerns the optimization of the granular skeleton of natural materials. The second relates to the dispersion of the colloidal fraction of earthen materials. In natural soils, clays are organized as porous aggregates composed of several tens of particles. These aggregates trap water that is not used to liquefy the mixture. The dispersion of these aggregates, releasing this interstitial water, liquefies the earthen material without adding water. This dispersion is obtained by adding a small proportion (on the order of a few tenths of a percent by mass relative to the dry material) of deflocculating agents such as those commonly used for the development of industrial ceramics. The combined action of these two methods leads to a solid material that does not crack while drying, and can easily be implemented at a viscosity comparable to that of vibrated concrete with the same tools than those employed by the concrete industry (cement mixer, shuttering, vibrating needle). This new technique is particularly suited for the implementation of slabs and other horizontal surfaces, and also for vertical walls

Dietary transitions among three contemporary hunter-gatherers across the tropics

The diets of contemporary hunter-gatherers are diverse and highly nutritious, but are rapidly changing as these societies integrate into the market economy. Here, we analyse empirical data on the dietary patterns and sources of foods of three contemporary hunter-gatherer societies: the Baka of Cameroon (n=160), the Tsimane' of Bolivia (n=124) and the Punan Tubu of Indonesia (n=109). We focus on differences among villages with different levels of integration into the market economy and explore potential pathways through which two key elements of the food environment (food availability and food accessibility) might alter the diets of contemporary hunter-gatherers. Results suggest that people living in isolated villages have more diverse diets than those living in villages closer to markets. Our results also suggest that availability of nutritionally important foods (i.e., fruits, vegetables and animal foods) decreases with increasing market integration, while availability of fats and sweets increases. The differences found seem to relate to changes in the wider food environment (e.g., village level access to wild and/or market foods and seasonality), rather than to individual-level factors (e.g., time allocation or individual income), probably because food sharing reduces the impact of individual level differences in food consumption. These results highlight the need to better understand the impact of changes in the wider food environment on dietary choice, and the role of the food environment in driving dietary transitions.Peer reviewe

Effect of polysaccharides on the hydration of cement suspension

International audienceThis work compares the effects induced by polysaccharides on the hydration of cement. It also brings new insights into the interaction mechanisms between these two components. Several parameters such as structure, concentration, average molecular weight, and the soluble fraction value of the polysaccharides were examined. The hydration of cement was monitored by conductivity measurement, and ionic chromatography. The influence of polysaccharide structure on the kinetics of cement hydration was revealed. The extent of retardation increases when polysaccharide concentration rises. Dextrins with lower average molecular weights compared with starches favor a higher soluble fraction value and further retard hydration. The growth of hydrates seemed to be more affected by the presence of these admixtures than did the dissolution of anhydrous particles or the nucleation of former hydrate

Peer evaluation can reliably measure local knowledge

We assess the consistency of measures of individual local ecological knowledge obtained through peer evaluation against three standard measures: identification tasks, structured questionnaires, and self-reported skills questionnaires. We collected ethnographic information among the Baka (Congo), the Punan (Borneo), and the Tsimane’ (Amazon) to design site-specific but comparable tasks to measure medicinal plant and hunting knowledge. Scores derived from peer ratings correlate with scores of identification tasks and self-reported skills questionnaires. The higher the number of people rating a subject, the larger the association. Associations were larger for the full sample than for subsamples with high and low rating scores. Peer evaluation can provide a more affordable method in terms of difficulty, time, and budget to study intracultural variation of knowledge, provided that researchers (1) do not aim to describe local knowledge; (2) select culturally recognized domains of knowledge; and (3) use a large and diverse (age, sex, and kinship) group of evaluators.Peer reviewe

Schooling, Local Knowledge and Working Memory : A Study among Three Contemporary Hunter-Gatherer Societies

Researchers have analysed whether school and local knowledge complement or substitute each other, but have paid less attention to whether those two learning models use different cognitive strategies. In this study, we use data collected among three contemporary hunter-gatherer societies with relatively low levels of exposure to schooling yet with high levels of local ecological knowledge to test the association between i) schooling and ii) local ecological knowledge and verbal working memory. Participants include 94 people (24 Baka, 25 Punan, and 45 Tsimane') from whom we collected information on 1) schooling and school related skills (i.e., literacy and numeracy), 2) local knowledge and skills related to hunting and medicinal plants, and 3) working memory. To assess working memory, we applied a multi-trial free recall using words relevant to each cultural setting. People with and without schooling have similar levels of accurate and inaccurate recall, although they differ in their strategies to organize recall: people with schooling have higher results for serial clustering, suggesting better learning with repetition, whereas people without schooling have higher results for semantic clustering, suggesting they organize recall around semantically meaningful categories. Individual levels of local ecological knowledge are not related to accurate recall or organization recall, arguably due to overall high levels of local ecological knowledge. While schooling seems to favour some organization strategies this might come at the expense of some other organization strategies.Peer reviewe

The Adaptive Nature of Culture : A cross-cultural analysis of the returns of Local Environmental Knowledge in three indigenous societies

Researchers have argued that the behavioral adaptations that explain the success of our species are partially cultural, that is, cumulative and socially transmitted. Thus, understanding the adaptive nature of culture is crucial to understand human evolution. We use a cross-cultural framework and empirical data purposely collected to test whether culturally transmitted and individually appropriated knowledge provides individual returns in terms of hunting yields and health and, by extension, nutritional status, a proxy for individual adaptive success. Data were collected in three subsistence-oriented societies: the Tsimane’ (Amazon), the Baka (Congo Basin), and the Punan (Borneo). Results suggest that variations in individual levels of local environmental knowledge relate to individual hunting returns and self-reported health but not to nutritional status. We argue that this paradox can be explained through the prevalence of sharing: individuals achieving higher returns to their knowledge transfer them to the rest of the population, which explains the lack of association between knowledge and nutritional status. The finding is in consonance with previous research highlighting the importance of cultural traits favoring group success but pushes it forward by elucidating the mechanisms through which individual- and group-level adaptive forces interact.Peer reviewe

Relations entre la microstructure des blocs agglomérés et les propriétés électriques des fusibles

Les éléments clés de la protection des circuits électriques contre les surintensités de courant sont constitués d'éléments fusibles (argent ou cuivre) noyés dans une matière de remplissage agglomérée. L'objet de ces travaux est d'étudier l'influence de la microstructure de la matière agglomérée (bloc de sable de silice lié par un silicate de sodium) sur le fonctionnement électrique du fusible au cours de la coupure. Un séquençage de l'arc électrique a été réalisé afin d'en étudier la progression au cours de la coupure, ce qui a permis d'avancer des hypothèses sur les propriétés physiques du bloc aggloméré intervenant au cours de ce phénomène. Ceci a été orienté vers d'une part l'étude de la compacité du bloc (empilement des grains de sable), et d'autre part, vers l'étude du vieillissement (effet de la cristallisation du silicate de sodium). Un fusible modèle a été conçu pour limiter les effets parasites dus à la présence simultanée de plusieurs arcs électriques. Ce fusible est en effet constitué d'uune seule lame d'argent contenant une seule rangée de sections réduites. Plusieurs paramètres ont alors été étudiés : la compacité du bloc de sable : soit en jouant sur la distribution granulométrique du sable, soit en insérant les défauts dans le sable, le vieillissement du liant : en appliquant des traitements thermiques supplémentaires aux fusibles. Concernant la compacité, aucune relation évidente n'a pu être observée entre cette propriété et le comportement électrique des fusibles. Cependant, l'analyse détaillée des oscillogrammes de coupure a permis d'imaginer ce que doit être le comportement idéal en coupure d'un fusible. Ceci a conduit à élaborer des fusibles à deux composants : le premier, dans lequel est noyée la lame, a une porosité plus faible (environ 20%) que celle du bloc aggloméré, ce qui permet de mieux confiner les vapeurs métalliques produites par l'arc électrique, le second étant le sable aggloméré (porosité d'environ 35%), dans lequel est noyé l'ensemble (lame + 1er composant). Ce type de fusibles a donné des performances électriques environ trois fois meilleures que celles obtenues avec les fusibles de référence, ce qui valide la modélisation de l'allumage, de la propagation et de l'extinction de l'arc électrique proposée. La caractérisation de la cristallisation du silicate de sodium a été effectuée par dissolution dans l'acide fluorhydrique. On a montré qu'un traitement thermique accélère la cinétique de cristallisation, mais que l'effet de cette cristallisation sur les performances électriques des fusibles est du second ordre. Ce travail a été réalisé en collaboration avec la Société FERRAZ SHAWMUT avec le soutien de la région Rhône-Alpes, dans le cadre du programme "Poudres et multicomposants".VILLEURBANNE-DOC'INSA LYON (692662301) / SudocST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Influence d'un MHEC sur les propriétés physiques et mécaniques d'un enduit de rénovation de façade (étude de l'adhérence de l'enduit sur une pierre de Tuffeau)

Les enduits de rénovation de façade sont des matériaux complexes. Ils contiennent de nombreuses matières premières : ciment, chaux, charges et adjuvants organiques. Parmi ces derniers, on trouve le rétenteur d'eau, dont le rôle est de retenir l'eau nécessaire à l'hydratation des liants au sein de l'enduit lorsque ce dernier est appliqué sur un support poreux. Le rétenteur d'eau étudié ici est un éther de cellulose (MHEC). Son mode d'action et ses effets sur les propriétés de l'enduit sont mal connus. L'objectif final de cette étude est de mesurer et d'expliquer l'influence de ce MHEC sur l'adhérence de l'enduit sur un support en pierre de tuffeau. Nous avons donc comparé deux enduits, avec et sans MHEC. Cependant, l'ajout de MHEC nécessite d'ajuster la quantité d'eau de gâchage pour obtenir une maniabilité de l'enduit identique. L'étude de l'adhérence des enduits est réalisée à l'aide d'un essai de clivage. Cet essai permet de contrôler la propagation d'une fissure à l'interface enduit/support et donne accès à de nombreuses informations (force, énergie de fissuration, ). La caractérisation des propriétés mécaniques (module) et physiques (variations dimensionnelles, cinétique de prise, ...) des enduits, complétée par l'étude des mouvements d'eau (succion, évaporation) a permis d'aider à la compréhension de leur comportement adhésif. Nous avons aussi pu mettre en évidence les modifications qui sont à attribuer à la présence de MHEC indépendamment de l'augmentation du taux de gâchage. Des observations en microscopie environnementale de l'hydratation des enduits ainsi que des surfaces de rupture enduit/support ont permis de dresser des hypothèses sur le mode d'action du MHECRenovation renders are complex materials. They content many components: cement, lime, aggregates and organic admixtures. One of them, the water retentive, allows to retain the water inside the render when applied on a porous substrate. In this study, the retentive admixture is a cellulose ether (MHEC). Its mode of action and effects on render properties are not well known. The aim of this work is to measure and explain the MHEC influence on the render adhesion when applied on a limestone substrate (tuff). Therefore, we compared Iwo renders, with and without MHEC. However, the adjunction of MHEC demands an addition of water during the render mix to obtain similar render workability. The study of the render adhesion is done thanks to the Wedge Splitting Test. This test allows to control the crack propagation at the render/substrate interface and gives many information (load, stiffness, fracture energy). The characterization of mechanical (elastic modulus) and physical properties (length variations, setting kinetics) of renders completed by the study of water movements (suction, evaporation) helps to understand their adhesive behavior. Also, modifications due to MHEC addition were underlined, separately of the render water content. ESEM observations of the renders hydration and of the renders/substrate fracture surfaces allow to raise assumptions on the MHEC action mode.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF