AN ESTIMATION MODEL FOR THE DRY THERMAL-CONDUCTIVITY OF AUTOCLAVED AERATED CONCRETE

In this article, we present a model of the relationship between the rate of macroporosity and the thermal conductivity of autoclaved aerated concrete (AAC) in a dry state which should provide an efficient frame for improving the thermal performances of this material. Such a model has to be developed because 'classical' models of heat conductivity of heterogeneous materials, although they are very numerous, all predict values, in the case of AAC, which differ much from the experimentally measured data. The main reason why they are not well adapted is due to the particular porous structure of AAC, showing two main types of porosity which are of completely different origins. Therefore, considering the total porosity as the main parameter is not the right approach to understanding the thermal properties of AAC. So, to build our model, we clearly separate micro- and macro-porosity and we consider the internal structure of AAC as a dispersion of air 'bubbles' in a solid skeleton which is assumed to be homogeneous at the scale of these macropores. A 'tortuosity' parameter is introduced to build an 'elementary cell' for which an equivalent heat conductivity can be calculated. To allow an experimental validation of the proposed model, heat conductivity measurements have been carried out on products of various densities (between 0.25 and 0.65) in a standard dry state. For that purpose, we have used a thermal shock probe method in which the heat conductivity is determined in a transient way. Image analysis techniques have been implemented to characterize the rate of macro-porosity of these samples. Non-aerated samples have also been prepared (by the same process as for AAC but with no aluminium powder) to determine the density and heat conductivity of the solid skeleton of AAC. Finally, the proposed model has shown good agreement over the whole range of macroporosity between the values of heat conductivity that can be evaluated by using it and the data actually measured. We therefore consider it is now possible to use this model to improve the thermal conductivity of AAC. Considering the parameters taken into account in this model, three different types of action may be undertaken: increase the rate of macroporosity, reduce the heat conductivity of the solid skeleton, or increase the tortuosity.