Experimental highlight of hygrothermal phenomena in hemp concrete wall

International audienceThis study investigates the hygrothermal behaviour of a timber-framed hemp concrete wall made of precast blocks. Measurements are performed on an uncoated wall as well as on a coated wall. The experimental device consists in two air-conditioned rooms where ambient conditions are selected to induce temperature and/or vapour pressure gradient between the two sides of the wall. The monitoring deals with temperature and relative humidity within the wall. Kinetics of temperature and of vapour pressure are given. Profiles are drawn at several times of the transient phase. In the regular part of the wall, several kinds of hygric behaviours are highlighted such as homogeneous vapour diffusion and huge vapour pressure variations due to evaporation-condensation and/or sorption-desorption phenomena. The results in the line of frame show that the frame doesn't induce disturbances in the hygrothermal behaviour of the wall. It is also shown that the coating reduces and delays vapour diffusion

Effect of Temperature on Moisture Buffering of Hemp Concrete

International audienceHemp concrete is a bio-based building material made of hemp shiv and lime. This material is not used as load-bearing due to low mechanical properties. Hemp concrete is thus mainly used as filling material associated to a wooden structure. The indoor side of the wall can be coated or not. On hygrothermal point of view, hemp concrete shows low thermal conductivity, about 0.1 W/m/K, and can be used without added insulation layer in buildings. The hydric characterization of hemp concrete shows high transfer and storage capacities. Thus, this study deals with hydric characterization of hemp concrete under dynamic conditions. The experimental method is based on the NORDTEST protocol where specimens are exposed to daily cyclic variation of relative humidity. Generally, hygrothermal characterization is held at 23°C. This study deals with the impact of temperature on moisture buffer value of hemp concrete. Several temperatures are investigated from 11°C to 23°C. The results show that moisture buffer value increases with the temperature according an exponential law

Effect of temperature on thermal and hygric properties of hemp-lime plasters

International audienceHemp-lime plasters are bio-based building materials made with a binder and hemp aggregate. Their main qualities are their low environmental impact due to bio-sourced components and their hygrothermal behavior.This study investigates two types of Hemp-lime plasters. The first composition corresponds to a mixture of Tradichanvre lime-based binder with hemp shiv (chanvribat). The second composition corresponds to a mixture of Tradichanvre lime-based binder with fibred hemp shiv (Terrachanvre). This study investigates the effect of temperature on thermal properties (conductivity and thermal diffusivity) and on Moisture Buffer Value (MBV) of hemp-lime plasters. The investigations are based on experimental measurements and are performed at 14°C, 17°C, 20°C and 23°C.The measurements of thermal properties are made at 50% Relative Humidity (RH). To limit the problems of water migration, the measurements are performed with transient state methods. Hot disk is used at all temperatures; it provides both thermal conductivity and diffusivity. Hot wire is used at 23°C, it provides thermal conductivity only.The measurement of moisture buffer value (MBV) is performed following the NORDTEST protocol. Specimens are exposed to daily cyclic variation of relative humidity: 8 hours at 75% RH and 16 hours at 33% RH.Over the considered temperature range (14-23 °C), the impact of temperature on thermal properties is very poorly while it is more sensitive on moisture buffer value. It is shown that the moisture buffer value decreases by about 36% when the temperature decreases from 23°C to 14°C

Performance hydrique de bétons de chanvre : effet de l’enduit sur leur capacité de régulateurs hydriques

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Mixed convection with flow reversal in entrance region of inclined tube

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Hygric and thermal properties of hemp-lime plasters

International audienceHemp-lime mortars are generally used as indoor insulating plasters of building envelopes. They can be applied in thick coat to allow high enough hygrothermal performances to be reached in retrofit case. This study investigates hygric and thermal properties of two hemp-lime plasters. These plasters are made with the same lime-based binder and differ from their kind of hemp shiv. The two kinds of hemp shiv are defibered but one is smaller than the other. This experimental study is based on the measurement of sorption isotherm, water vapour permeability, moisture buffer value and thermal properties (conductivity and diffusivity). The effect of temperature on moisture buffer value and on thermal properties is also investigated. It is shown that hemp-lime plasters are hygroscopic and breathable materials. They are good hygric regulators according to the Nordtest classification and their moisture buffering ability is slightly impacted by temperature in the range from 11° to 23°C. The hemp-lime plaster with the smallest hem

Formulation of binder to improve mechanical properties of hemp concrete at early stage - Effect on thermal and hygric properties in use

International audienceHemp concrete is a bio-based material generally made of lime-based binder and hemp shiv currently used for building envelop. This lightweight material shows a thermal conductivity about 100 mW/(m.K). On hygric point of view, hemp concrete is strongly hygroscopic, with high moisture transfer and storage capacities. These properties allow hemp concrete to moderate ambient relative humidity variations. It is an excellent hygric regulator with moisture buffer values globally higher than 2 g/(m².%RH).On mechanical point of view, hemp concrete is a non-load bearing material. It gains mechanical resistance over the first few weeks after manufacturing. These mechanical performances depend, among other factors, on the properties of components and on the mix proportioning. In order to improve the mechanical resistance of hemp concrete at early stage, this study investigates new formulations starting from reference formulation. The binder composition varies by increasing the substitution rate of lime-based binder for selected calcium sulfate-based binder. This binder leads to interesting hardening at early stage. The effect on mechanical performances at long term appears effective, without large influences on thermal and hygric properties (maximal variation about 20% and 15%). Effective proportioning tool is then available

Comparing hemp composites made with mineral or organic binder on thermal, hygric and mechanical point of view

International audienceThis study investigates the effect of the kind of binder on thermal, hygric and mechanical properties of hemp composites. Three binders are considered: a formulated hydraulic lime based binder, a prompt cement based binder and a peculiar bio-polymer: the Poly-Lactic Acid (PLA). The binder is coupled with hemp shiv to produce biocomposite. For each kind of binder, the specific manufacturing method is presented and discussed.The thermal properties of produced biocomposite are measured with a hot disk method at 23°C, 50%RH. The hygric characterization is based on the measurement of Moisture Buffer Value at 23°C. The mechanical behavior of composite under compression is analyzed.Thermal conductivities range from 0.08 to 0.16 W/(m.K), this allows to use these composites without added insulation layer. Following the classification given in the Nordtest report, the studied materials are very good or excellent hygric regulator. Performances are slightly higher with mineral binders. However, the compressive strength obtained with PLA binder are much higher than the ones with mineral binders

Hysteresis phenomenon in hemp concrete

International audienceHemp concrete participates in a process of sustainable development in the housing field by reducing environmental impact and improving hygrothermal comfort. This kind of hygroscopic bio-material is characterized by a strong hysteretic hydric behavior. At a given relative humidity, hysteresis results in more or less differences between the equilibrium moisture content obtained during adsorption or desorption. This phenomenon affects moisture transfer in hygroscopic building materials. In this paper, experimental and numerical study on hysteretic behavior of the hemp concrete sorption process is presented.Experimental intermediate scanning curves are measured using the glass jar method. This experimental data is used to fit hysteresis models. In this study, one hysteresis model is selected for its suitability to represent the hysteretic hydric behavior of hemp concrete. It is based on the knowledge of the main adsorption and desorption curves respectively built from the dry to the saturated state and from the saturated to the dry state. Nevertheless, as for many porous bio-materials, the saturated state is difficult even impossible to reach. In order to overcome this issue, an original approach is presented leading to the identification of the main desorption curve from a primary desorption curve.The implementation of a suited hysteresis model in a heat and moisture transfer model allow evaluating and discussing the influence of this phenomenon in the transient hygrothermal response of hemp concrete to hydric loadings as on the sample scale as on the wall scale

Influence of hysteresis on the transient hygrothermal response of a hemp concrete wall

International audienceBuilding envelopes with bio-materials like hemp concrete are currently undergoing an increasing development due to their low environmental impact and their interesting hygrothermal properties. This kind of hygroscopic material is characterized by a hysteretic behaviour. In this paper, the influence of such behaviour on the hygrothermal response of a building wall is discussed. An experimental facility is designed to measure temperature and relative humidity within a hemp concrete wall submitted to climatic variations. This facility provides a set of experimental data suited for benchmarking. Numerical simulations are performed with a hysteresis model implemented in a heat and moisture transfer model. Comparisons between numerical and experimental results show that hysteresis modelling is relevant to simulate the hygrothermal response of the wall. The discussion is extended to the influence of the initial moisture content