Thermal conductivity of hemp concretes: Variation with formulation, density and water content

This study investigates the effect of formulation, density and water content on the thermal conductivity of hemp concretes. The investigations are based on experimental measurements and on self-consistent scheme modelling. The thermal conductivity of studied materials ranges from 90 to 160 mW/(m.K) at (23 degrees C; 50%HR). The impact of density on thermal conductivity is much more important than the impact of moisture content. It is shown that the thermal conductivity increases by about 54 % when the density increases by 2/3 while it increases by less than 15 % to 20 % from dry state to 90%RH

LIFE CYCLE ASSESSMENT OF A HEMP CONCRETE WALL: IMPACT OF THICKNESS AND COATING.

International audienceIn a context of sustainable development and energy sparing, a life cycle assessment (LCA) may be useful to make good choices. Thus, this study concerns the LCA of an environmentally friendly material used for building construction, hemp concrete. The functional unit is first defined per square such that the wall may provide the function of bearing wall meter and its thermal performance is described by a thermal resistance of 2.78 m².K/W. The results then showed that the production phase of raw materials is mainly responsible for the environmental impact of the wall, mostly due to the binder production. It was also shown that, compared to traditional construction materials, hemp concrete has a low impact on environment. Moreover, hemp concrete contributes to reduce climate change as photosynthesis-mediated carbon sequestration and carbonation serve to reduce atmospheric carbon dioxide. A sensitivity analysis is performed on three criteria: wall thickness, renewal of coatings and compounds of the indoor coating. Our results show that environmental indicators evolve with wall thickness, except for the climate change indicator. It improves with thickness due to carbon sequestration and carbonation. Moreover the increase in the wall's thermal resistance with wall thickness is not taken into account in such an LCA performed at the material level. The renewal of coating slightly impacts the environmental indicator for small numbers of renewals but it leads to negative effects if they are too numerous. It appears that hemp-lime coating has a greater impact than sand-lime coating as it embeds more binder

Hygrothermal behaviour of a hemp concrete wall : influence of sorption isotherm modelling

International audienceConstructions built with environmentally friendly materials like hemp concrete know currently a real development. The development of numerical models able to evaluate their hygrothermal behaviour turns out to be a precious tool for their study. The model deals with coupled heat, mass and air transfer through multi-layer 1D porous media submitted to climatic variations. The model is used to simulate the behaviour of a hemp concrete wall. Comparison between simulation and experiment is done showing the importance of taking into account hysteresis for sorption isotherm modelling

Hygrothermal transfers through a bio-based multilayered wall: Modeling study of different wall configurations subjected to various climates and indoor cyclic loads

International audienceThe hygrothermal behavior of a bio-based multilayered wall has been studied by numerical simulations. The key point of these research investigations was to properly describe the hygrothermal transfers occurring inside the studied wall solution. In previous work, the case of the wall subjected to a given real climate (Wroughton HIVE demonstrator, UK, Feb 2018) has been investigated. The present work, focused on the moisture regulation capacity of the wall, considers an improved kinetics model of sorption, different layer configurations, one additional climate (Bordeaux, FR, Apr 2008) and the effect of indoor cyclic loads. Compared to the classical approach, the local kinetics approach results in prediction of stronger and steeper hygric dynamics with larger relative humidity variations at small time scales. The study of the different wall configurations allows to determine the best one in terms of moisture damping: the vapor control membrane is advantageously removed provided the OSB3 12 mm layer is replaced by an OSB4 18 mm layer. Moreover, the simulations show that the Moisture Buffer Value characteristic of each material layer is not a sufficient criterion to evaluate hygric performance of the wall; strong hygric interactions occur with the layer's permeability independently of its sorption capacity. Finally, water content hysteresis phenomena are studied and it appears that under usual operating conditions, they can be ignored by adjusting the layers' permeabilities for adequate fits on the Moisture Buffer Value tests

Propriétés physiques de bétons de chanvre

International audienceLe béton de chanvre est un biomatériau composite léger obtenu en mélangeant un liant à base de chaux hydraulique et aérienne, des particules végétales et de l'eau. Ce matériau n'est pas porteur ce qui conduit à l'utiliser principalement en matériau de remplissage associé à une ossature bois, trois procédés de mise en œuvre étant possible. Dans cette étude, différentes formulations utilisées pour le montage de paroi sont étudiées. La mise en œuvre est faite par trois procédés. Une relation de type loi-puissance est mise en évidence entre la résistance à la compression et la densité apparente. Des mesures de la conductivité thermique en fonction de la masse volumique et de la teneur en eau permettent de définir la capacité d'isolation des matériaux. La caractérisation hydrique qui repose sur les capacités de stockage et de transfert d'humidité des matériaux est réalisée par la détermination des isothermes de sorption-désorption et de la perméabilité à la vapeur

Variation de la capacité hydrique tampon de bétons de chanvre en fonction de la formulation

International audienceLe béton de chanvre est un matériau bio-sourcé utilisé en remplissage d'enveloppes de bâtiment. Sa conductivité thermique est de l'ordre de 0,1 W/(m.K) et sa perméabilité à la vapeur est de l'ordre de 2,5.10-11 kg.m-1.s-1.Pa-1 à 0/50 %HR. Ce matériau est composé d'un liant à base de chaux (comme le Tradical 70) et de chènevottes. Sa résistance mécanique se développe essentiellement au cours des premières semaines de maturation et est liée à la teneur en eau initiale du matériau. Afin d'améliorer la résistance mécanique au jeune âge, de nouvelles formulations ont été proposées dans le cadre du projet ANR Betonchanvre. Ces formulations reposent notamment sur la substitution de Microsem 7 au Tradical 70. Il est alors nécessaire de vérifier que les caractéristiques hygrothermiques ne sont pas affectées par la composition du liant. Une étude menée sur la conductivité thermique a montré que celle-ci n'était pas dégradée par ces nouvelles formulations. D'un point de vue hydrique, les perméabilités à la vapeur mesurées sur ces nouvelles formulations sont également comparables à celles mesurées sur des bétons de chanvre de référence. Cette étude quantifie l'impact de la composition du liant sur la capacité tampon hydrique qui caractérise le comportement en régime dynamique. La mesure est réalisée selon le protocole défini dans le NORDTEST project basé sur des cycles d'humidité 75/33% HR pendant 8/16 heures. Pour l'ensemble des formulations étudiées, la stabilité est atteinte à partir du troisième cycle. Les valeurs moyennes des MBV sont comprises entre 1,99 et 2,51 g/(m².%HR) pour les différentes formulations, ce qui classe le matériau comme bon, voire excellent, régulateur hydrique. Les modifications de formulation n'impactent pas notablement la capacité tampon du béton de chanvre qui est de 2,15 g/(m².%HR) pour le matériau de référence

Caractérisation hydrique de béton de chanvre projeté. Possibilité de mesure en régime dynamique ?

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Experimental investigation of moisture buffering capacity of sprayed hemp concrete

International audienceThis study deals with dynamic characterization of hydric behaviour of sprayed hemp concrete. The measurement of moisture buffer value (MBV) is held according to the NORDTEST protocol (Rode, 2005). Specimen are exposed to daily cyclic variation of relative humidity: 8 h at 75%RH and 16 h at 33%RH the test is held under isothermal conditions (23 °C). The experimental set-up is validated by comparison of moisture buffer value measured on cellular concrete with values given in the NORDTEST project for the same kind of material. Results show that sprayed hemp concrete has an excellent moisture buffer capacity (MBV = 2.15 g/(m2 %RH)). So, it is an excellent hydric regulator that improves hygrothermal comfort and reduces energy needs. Moisture effusivity is then calculated from sorption and water vapour permeability value and from dynamic test of MBV. The results (respectively 5.8 * 10^-07 and 4.8 * 10^-07 kg/ (m2 Pa s1/2) show a slight discrepancy between values obtained from steady-state and from dynamic conditions. Finally, the penetration depth for daily cyclic variation is estimated about 5.8 cm for sprayed hemp concrete. Also, the experimental data set can be used for benchmarking buffering models

Experimental Study of Hygrothermal Behavior of a Hemp Concrete Wall (without and with coating)

International audienceThis study deals with hygrothermal behavior of a hemp concrete wall made of precast blocks and of a timber frame. In a first time, measurements are held on uncoated wall, and then the wall is coated. The experimental device is made of two climatic chambers which ambient conditions are selected to induce vapor transfer, heat transfer or simultaneous heat and vapor transfers. The monitoring deals with temperature and relative humidity in thickness of the wall. Kinetics of temperature and of vapor pressure are given. Profiles are drawn at several times of the transient phase. It is shown that i) under isothermal conditions, homogeneous vapor diffusion occurs, ii) under temperature gradient (without vapor pressure gradient between ambient conditions), huge vapor pressure variations appear, probably due to evaporation-condensation phenomena, iii) the frame is not a thermal bridge, iv) the coating reduces and delays vapor diffusion

Effect of coating on moisture buffering of hemp concrete

International audienceThis study deals with dynamic characterization of hydric behaviour of sprayed hemp concrete. In a first time, the study is held on the material itself. Then, the study considers systems made of coated sprayed hemp concrete for two kinds of coating. The measurement of moisture buffer value (MBV) is held according to the NORDTEST protocol (Rode, 2005). The experimental set-up is validated by comparison of moisture buffer value measured on cellular concrete with values given in the NORDTEST project for the same kind of material. Results show that sprayed hemp concrete has an excellent moisture buffer capacity (MBV=2.14 g/(m².%RH). So, it is an excellent hydric regulator that improves hygrothermal comfort and reduces energy needs. Once coated, hemp concrete remains good hygric regulator (for closed coating) and nearly excellent hygric regulator (with open coating). Also, the experimental data set can be used for benchmarking buffering models