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caractérisation et optimisation d'un composite biosourcé pour l'habitat

Abstract : The building sector (both for the construction and operation) is a major source of CO2 emissions worldwide, due to the energy consumption (heat, air ventilation and cooling) and materials production involved. Furthermore, the depletion of natural resources is becoming a critical issue; the use of renewable and recyclable resources, with low embodied energy is therefore relevant. In that respect, the use of agricultural by-products as lightweight aggregates for thermal insulation purposes is particularly judicious. They have low embodied energy and can be considered as waste with a positive carbon balance as they store CO2. Replacing Portland cement by local crude earth improves the material’s carbon footprint. The work presented here that claims the use of those ecologically friendly materials is carried out in the frame work of BIOCOMP project which aims to develop locally produced biocomposites to be used as houses interior insulating materials. This, to offer refurbishment solutions for vernacular buildings in Touraine, Center of France. In this study, samples have been prepared using 4 different vegetal aggregates (rape straw, sunflower bark, sunflower pith and a mix of sunflower pith and bark) and a characterized crude earth as binder for a same binder/aggregates ratio. The purpose of this study was to compare hygrothermal acoustical and mechanical properties of different materials based on vegetal aggregates and clay in order to characterize the performances of different biocomposites with a view to valorizing agricultural waste. We first analyzed the raw materials in order to get the density, thermal and hydric properties of the vegetal aggregates in one hand and the mineralogy and gravimetric data of the selected crude earth in the other hand. Thermal behavior of earth:vegetal aggregates mixes have been investigated at different humidity rates and different density ranges. Additionally, hydric properties have been measured to produce sorption and desorption curves and moisture buffer values (MBV) in one hand and capillarity of both aggregates and biocomposites on the other hand. Studying those hydric properties is relevant as a suitable insulating material for the refurbishment of the locally porous limestone made houses should offer good water vapor regulation. For the first time, we investigate here the thermal properties of mixes of crude earth with rape-straw and sunflower stem (bark and pith). According to the french NF P 75-101 standard, in order to be considered as building thermal insulator, a material should have a thermal conductivity below 0.065W.m-1.K-1. Lime-vegetal or clay-vegetal plasters can hardly reach those performances, but one of their major advantages is their indoor climate regulation capacity as well as their compatibility with vernacular buildings made with crude earth or limestone. The weaknesses of these materials are their poor mechanical performances avoiding any structural use and also their sensitivity to liquid water. In order to enhance both characteristics, ethyl silicate tends to offer interesting guarantees as an additive that need to be further studied. On the contrary, an addition of a low amount (5%) of lime is counterproductive, decreasing both mechanical and capillarity resistance. Masons use to recover those kind of insulating materials with a finishing plaster, we then found interesting to measure the impact of this coating on the MBV and acoustical (absorption and transmission loss) performances of the biocomposites. It appears that the influence of those coating decrease hydric dynamical regulation and the sound absorption but enhance the transmission loss. Lastly, we investigate the behavior of the thermal flow through a limestone wall covered with a clay:rape-straw plaster in a climatic chamber. We compared the found results with a WUFI 2D software simulation and tried to also simulate the changes in the drying kinetic of a fresh composite covered with different kinds of finishing plasters pointing the important drying delays depending on the coating used. One of the main interests of this study is to confirm that a wide range of locally produced vegetal byproducts could be used as bioaggregates for concretes. Local biomaterials industries could therefore emerge depending on the locally available resources at country scale. Despite its lower mechanical performances and vulnerability to liquid water compared to lime or pozzolanic binders, the use of crude earth as binder for vegetal concretes deserves further studies. Besides its ecological and hygrothermal performances, clay production is practically ubiquitous, enabling the emergence of very local clay-vegetal aggregate concrete industries to develop a green, carbon-light and circular economy.
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Yoann Brouard. caractérisation et optimisation d'un composite biosourcé pour l'habitat. Construction durable. école doctorale université de Tours, 2018. Français. ⟨tel-01835828⟩

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