Effect of water content on the acoustical and thermal properties of hemp concretes

Hemp concrete is a multifunctional ecological material used in buildings. It is obtained by mixing a binder with hemp particles (the non-fibrous fraction of the hemp stem called 'shiv' or 'hurd'). Due to its high porosity (ranging from 60 to 90%), it presents an 'atypical' mechanical behavior and its hygrothermal and acoustical properties are particularly interesting. This paper focuses on the influence of the water content of hemp concretes on their acoustical and thermal properties. Four mixtures of hemp concrete were manufactured using a binder and two shives under two distinct stresses of compaction. It is shown that water content does not affect significantly the acoustical properties of hemp concretes although a swelling effect can be detected by an increase of resistivity and a decrease of porosity. Finally, this study confirms that thermal conductivity rises almost linearly with water content while evolutions of thermal diffusivity and specific heat capacity are different depending on the concrete

Influence of accelerated aging on the properties of hemp concretes

The use of vegetal concretes, such as hemp concrete, is growing for building insulation. They are constituted by vegetal aggregates embedded in a mineral binder. Hemp concretes are multifunctional materials: they have interesting thermal and acoustical performances and their hygrothermal behavior enables a natural moisture regulation. Currently, their use is limited by their lack of guarantees in terms of durability. This paper focuses on the influence of 75 days of wetting and drying cycles on hemp concrete properties. It is shown that this aging induces modifications of the porosity of the materials and of their skeleton density. However, these evolutions of microstructure are not sufficient to lead to modifications of thermal and acoustical performances. This work has also shown that mold growth occurs when the relative humidity is high (98% RH) and when the pH of the binder has decreased due to its carbonation reaction

Cyclic hygrothermal ageing of flax fibers' bundles and unidirectional flax/epoxy composite. Are bio-based reinforced composites so sensitive?

The use of vegetal fibers to reinforce polymeric matrix composites is challenging and goes with questions of durability issues, especially when exposed to damp conditions. The aim of this study is to quantify, up to 1 year, the impact of humidification/drying cycles, i.e. 3.5 days at 90% HR and 3.5 days at 40% HR, both at 55°C, on the longitudinal mechanical properties of a unidirectional flax/epoxy composite. Then, by a multi-scale analysis, the objective is to identify the causes of mechanical properties evolutions. According to the results of this study, a cyclic hygrothermal ageing induces irreversible degradations of fibers (matrix-embedded or not) and interfaces within the composite. Indeed fiber/fiber and fiber/matrix debondings have been observed by morphological analyses and a loss of fibers hydrophilic components has been highlighted by chemical and microstructural characterizations. A plasticization of flax fibers has also been shown during this kind of ageing. All these modifications induce a drop of composite's moduli after the first cycles of exposure and a decrease of ultimate tensile stress and strain after long time exposure. But these mechanical properties evolutions are much lower than the values found in the literature. Decreases of about -10% for the modulus and about -14% for the ultimate tensile stress have been recorded after 1 year of exposure and 52 cycles of ageing. Thus, this study highlighted that a well manufactured flax/epoxy composite is resistant to a cyclic hygrothermal ageing. These results are therefore promising for the development of this type of bio-based materials on a larger scale

Why cyclic hygrothermal ageing modifies the transverse mechanical properties of a unidirectional epoxy-flax fibres composite ?

The development of composites based on plant fibres has considerably grown in last decades; however, their high sensitivity to moisture limits their use in structural applications. The objective of this study is to determine and understand the impact of moisture variations during one-year cyclic hygrothermal ageing i.e. 52 cycles on the transverse properties of a unidirectional epoxy flax composite. A cycle consists of 3.5 days at 90% RH followed by 3.5 days at 40% RH. Compared to the studies usually carried out to determine the impact of ageing on the longitudinal properties of unidirectional composites, the determination of the transverse mechanical properties during this type of ageing allowed to focus on the contribution of the matrix and the interface on the evolution of the composite properties. The multi-scale analyzes provided identification of the phases responsible for the evolution of transverse mechanical properties induced by ageing. The transverse ultimate tensile strength drops by about 20% after the first week of ageing and slightly decreases up to 2 months. This evolution seems to be induced by the creation of flaws, in particular the phenomena of fibre-fibre and matrix-fibre debonding and the creation of cracks in the fibres initiated at the lumen level, revealed by SEM. The transverse modulus also strongly drops after the first week by about 18% to reach about 45 % loss after one year of ageing. These evolutions are mainly linked to the physical plasticization of the epoxy matrix demonstrated by the reversible change of its glass transition temperature, but analyses showed that water does not lead to chemical degradation of the matrix

Assessment of the durability of bio-based insulating materials

First International Conference on Bio-Based Building Materials, Clermont-Ferrand, FRANCE, 22-/06/2015 - 24/06/2015The use of vegetal concretes, such as hemp concrete, is growing for building insulation. They are constituted by vegetal aggregates embedded in a mineral binder. Hemp concretes are multifunctional materials: they have good thermal and acoustical insulating properties and their hygrothermal behavior enables a natural moisture regulation. Unfortunately, the use of hemp concrete is limited by its lack of guarantees in terms of durability. This paper focuses on the study of the evolution of the properties of hemp concretes after accelerated ageing in laboratory. The samples are submitted to wetting/drying cycles and their performances are analyzed periodically. In this article, the properties of the materials after 75 days of ageing are reported. It is shown that this period is too short to observe any significant variation of the properties of the hemp concretes, except for their porosity. The biological ageing of the materials is investigated too. It is shown that the mold growth occurs when the relative humidity is high (98% RH) and when the pH of the binder has decreased due to its carbonation reaction

Effects of cyclic moisture-induced ageing on a bio-based laminated composite

International audienc

UV accelerated aging of unidirectional flax composites: Comparative study between recycled and virgin polypropylene matrix

International audiencePrevious studies on the photooxidation of natural fiber composites have focused on using short fibers and virgin matrices. In this work, unidirectional flax fiber composites with virgin and recycled polypropylene matrix were prepared by thermocompression to compare their accelerated aging. These composites were exposed to two accelerated xenon arc UV exposures where only the irradiance was modified. A multi-scale characterization was carried out to evaluate the consequences of the photooxidation on their mechanical and physicochemical properties with time. Results show that the photooxidation was limited to the surface. In addition, the photo-degradation of the surfaces of both composites was identified by the appearance of cracks, the variation of crystallinity, the decrease in weight average molar mass, and the appearance of new chemical products. These physicochemical variations are more evident with increasing irradiance. Despite these variations, the mechanical tensile properties of recycled matrix composites remain relatively indifferent to the two UV exposures compared to those with virgin polypropylene. It can be concluded that recycled polypropylene, if well selected, can replace virgin polypropylene in natural fiber composites for better mechanical resistance to photooxidation

Evaluation de la durabilité d'isolants contenant des granulats Îgétaux

Conférence Internationale Francophone NoMaD2015 - Nouveaux Matériaux et Durabilté, Douai, FRANCE, 05-/11/2015 - 06/11/2015Les bétons de chanvre sont des matériaux biosourcés écologiques obtenus en mélangeant un liant minéral et des granulats de chanvre (chènevotte). Grâce à leur porosité éleÎe (entre 60 et 90% en volume selon les formulations), leurs propriétés sont multi-fonctionnelles : ce sont de bons isolants thermiques et acoustiques et leur comportement hygrothermique permet de réguler naturellement l'humidité dans un bâtiment. Cependant, leur utilisation est encore limitée par le manque de recul concernant leurs propriétés à long terme. Dans cet article, les premiers résultats d'une étude générale sur la durabilité des bétons Îgétaux utilisés pour l'isolation des bâtiments sont présentés. Des cycles d'humidification/séchage ont été appliqués à ces matériaux pendant 75 jours. Après ce vieillissement accéléré en laboratoire, les performances des matériaux ont été analysées. Parmi les propriétés mesurées, ce vieillissement a un effet significatif sur la porosité des bétons Îgétaux et leur densité de squelette. Aucune variation notable de conductivité thermique ou des propriétés acoustiques n'a pu être mise en évidence. Les essais se poursuivent sur ces matériaux pour des temps de vieillissement plus longs pour observer l'évolution de leurs performances et valider le protocole de vieillissement utilisé en laboratoire. Ce travail permet de montrer que différents paramètres influencent le développement de microorganismes à la surface de ces échantillons : une humidité relative éleÎe, proche de la saturation (98% HR dans cette étude) et une valeur de pH du liant inférieure à 10. La diminution du pH est liée à la carbonatation naturelle du liant. Elle est obserÎe quelques mois après la fabrication des bétons et peut favoriser le développement fongique. Ces résultats seront confirmés et généralisés avec des caractérisations complémentaires pluridisciplinaires pour obtenir une caractérisation multiéchelle du vieillissement des bétons Îgétaux

Development of a thermocompression manufacturing process adapted to flax-Epoxy laminated composites

Second International Conference on Bio-based Building Materials and first conference on Ecological valorisation fo Granular and Fibrous materials (ECOGRAFI), Clermont-Ferrand, France, 21-/06/2017 - 23/06/2017This study focuses on the optimization of a manufacturing process adapted to flax fibers based composites in which the components are controlled. Twelve plies unidirectional composites samples were prepared by thermocompression using unidirectional flax fibers and fluid epoxyamine resin. Temperature cycle was chosen to limit fibers degradation during fabrication. Different parameters have been studied to evaluate their influence on the mechanical properties of composites. 0° tensile specimens were tested at a 1mm/min crosshead speed. Measures were performed with a 50mm gauge length extensometer and a 100kN load cell. Furthermore, DSC analyses were performed to obtain information about cross-linking of the matrix. Properties are in accordance with the literature. Using 3bar pressure, fiber content was found to be around46% (in volume). The density of the composite was measured using Archimedes' principle and was valued to 1.28 when the fibers' one was 1.45