Effect of Moisture Movement on Tested Thermal Conductivity of Moist Materials

Thermal conductivity measurements are based on temperature gradients within the tested material. When testing nonsaturated moist porous materials, this gradient causes redistribution of the enclosed moisture and, as a result, the apparent (measured) conductivity is smaller than the actual one. This paper analyzes the dynamic processes and the development of thermal and moisture fields during the test, and shows that changes in the moisture field increase as the ratio of the vapor diffusivity to the hydraulic modulus of the material decreases. It shows, however, that for a wide range of concretes (dry density between 340 and $2\text{,}240\mathrm{kg}/m^3$ ), tested under a commonly used temperature gradient (of about 3–5° C/cm), moisture changes may reach a significant fraction of a percent only when this ratio is smaller than 1.0 mN/gr as may be the case for some highly hydrophobic thermal plasters. This paper suggests a procedure for estimating the dependence of the actual moist thermal conductivity on the moisture content by means of the apparent conductivities and the calculated moisture changes.