Influence de la teneur en eau sur la diffusivité thermique du plâtre

Résumé

La mesure simultanée de la variation des coefficients thermocinétiques (a, λ) du plâtre en fonction de sa teneur en eau pondérale ω est faite par la méthode de la sonde à choc thermique. La diffusivité thermique présente un maximum pour une teneur en eau comprise entre 5 et 10%, la conductivité thermique croît plus rapidement pour les faibles teneurs en eau (ω<10%). L'évolution de la chaleur volumique en fonction de ω vérifie assez bien les lois de la thermodynamique.

Summary

The hygrothermal behaviour of building materials is raising renewed interest at the moment. Indeed, the improivement in thermal insulation of buildings and their protection against deterioratorion are partly based on improved knowledge of the heat and mass transfer phenomena.

Most of the materials used in building are porous and part or possibly all the pores are interconnected. this characteristic makes them susceptible to fixing humidity through the more or less joint action of three fundamental physical mechanisms: adsorption, evaporation-condensation, capillary migration.

For a given material, the thermokinetic coefficients will therefore vary principally with the water content.

To take measurements as fine as possible, the CSTB at Grenoble has used a transient thermal method (thermal shock probe method) with short characteristic time (a few minutes) founded on the study of propagation of thermal flux produced by the Joule effect from a cylindrical heating element. The quasi-punctual measurement of thermokinetic coefficients is simultaneous and only slightly disturbs the thermal and hydric states of the medium.

The first measurements undertaken in the laboratory on a sample of gypsum plaster fully mixed show that the thermal conductivity λ (ω/m. K) increased more rapidly for low ponderal water contents (ω<10%) and that thermal diffusivity a (m ²/s) reached a maximum value for a water content between 5 and 10%.

The quasi-linear progression of heat per unit of volumeC (J/m ³.K) obtained experimentally complies with the approximative theoretical expression C=C_s+θC_e (θ=water content per unit of volume, C_s; C_e; heat per unit of volume respectively of dry gypsum plaster and the water) and justifies the results found for λ and a.

These results confirm those found at INSA in Lyons where research scientists have undertaken overall measurement of the thermal diffusivity of several samples of gypsum plaster in different conditions of water content (humid material), the experimental technique chosen using the classical method of the sinusoidal signal (Angstroëm method) from a Peltier-effect generator on the entry surface of a cylindrical sample. It has been found that a very clear maximum of diffusivity is shown for a water content close oo 5–7% whatever the type of gypsum plaster subjected to tests.

The phenomena observed should not be specific to the case of gypsum plaster and are likely to be found with all humid, porous materials. Their interpretation calls for simultaneous measurement of the thermo-kinetic characteristics (a, λ and C).