Sandrine Marceau

This study aims at presenting a new proposal to perform the water capillary absorption test of hemp concretes, and to establish the parameters useful for analyzing the obtained results. Based on the standards of traditional materials, such as concrete and mortar, a protocol was proposed and executed by 8 laboratories through a round robin test. Homogeneous hemp concrete cubic specimens of edge 150 mm were cast, distributed to the laboratories, where they were conditioned before being submitted to the test. By adopting the new test procedure, it was possible to obtain consistent results after analyzing in both square root of time and log-time regimes. For each regime, two parameters couples CA and k (square root of time regime), IRA and K1 (log-time regime) were used to compare the obtained data, and validate successfully the round robin test.

The good insulation properties and the low carbon footprint of vegetal concretes make them promising materials whose use tends to grow continuously. To produce optimized building materials, a better understanding of the interfacial transition zone (ITZ) between vegetal particles and cement paste in terms of the reactions involved and the size of the impacted surface was investigated. This research led to the setting of a reliable visual test to observe ITZ, which enables the monitoring of its appearance and development. Different combinations of vegetal particles and cement pastes were tested to compare the formed ITZ: hemp, rapeseed, and bamboo into Portland and Prompt cement. Finally, a clear link was drawn between the sugar concentration and the size of ITZ. Thanks to image analysis, it was shown that ITZ is due to physico-chemical reactions, with the extraction of free saccharide molecules from the vegetal and water suction followed by their release into the cement paste.

Hemp concretes are the most widely used biobased concretes in France. However, their growth is still limited by the lack of knowledge and high variability of the performances of biobased concretes, especially for their mechanical properties. These results are related to interactions between the mineral binder and plant compounds that modify the hydration of the cement. In this work, the interactions between cement and eight types of hemp shiv, and a flax shiv are studied by isothermal calorimetry. The setting delays observed in the presence of plants are interpreted by analyzing the molecules extracted from these plants in water. A link can be observed between the setting delay and the coloration of the extract solutions or their concentration in reducing sugars and in polyphenols. These results constitute a basis in the objective to define an indicator enabling to predict the compatibility between plants and mineral binders.

Biosourced materials such as vegetal wools offer major thermal insulation advantages in the green buildings field. Experimental characterisations of vegetal wools thermal conductivity as a function of their density show the existence of an optimum conduction-radiation coupled value. This specific point, as well as the properties of vegetal wools are related to the large variability of shapes and sizes of their fibres. In order to take this specificity into account, it seems particularly relevant to use micro-macro modelling methods to predict the thermal conductivities related to both conduction and radiation heat transfer phenomena. In a first time, a self-consistent method based on a cylindrical geometry (SCM_cyl) is used as a modelling approach for conduction transfers. Then, a modelling approach developed by Bankvall and based on an equivalent fibre radius value is used for radiation transfers. So, by coupling these two approaches, it is possible to obtain an equivalent thermal conductivity of fibrous materials as a function of density. Finally, this method is validated by comparison with experimental data.