Measurement of Thermal Conductivity and Specific Heat of Cement by the Heat Flow Meter Method

摘要

The heat flow meter method, described in the standard test method ASTM C518, is a well-established and reliable method for thermal conductivity measurement of insulating materials such as polymer based foam boards and fiberglass batts. The method is often used for quality control as well as research and development within the field of insulating building materials. The typical range of thermal conductivity covered is approximately 0.005 W/m~*K to 0.5 W/m~*K. For higher thermal conductivity building materials such as cement, concrete and masonry, measurement difficulties are encountered due to much lower sample thermal resistance and rigid and non-smooth surfaces which lead to errors due to interface thermal resistance between the hot and cold plates and sample surfaces. Since these materials can be heterogeneous, the relatively large sample size of the heat flow meter method (e.g. 300 mm by 300 mm) is an advantage over other methods such as flash diffusivity which require small sample size. This work will describe modifications of the method to extend the thermal conductivity range to approximately 2 W/m~*K for samples of sufficient thickness. Along with thermal conductivity, the specific heat or heat capacity of building materials is of interest for modeling temperatures within the building envelope and calculations of heating and cooling loads and efficiency. With proper calibration, the heat flow meter can also be used as a calorimeter by heating the sample step-wise and integrating the recorded heat flux into the sample versus time during the heating transient to determine specific heat and enthalpy for transitions such as melting of phase-change materials. This work will describe modifications of the heat flow meter method to measure specific heat. In order to evaluate these methods, thermal conductivity and specific heat results will be presented for a cement sample and a Pyrex 7740 reference sample with thermal conductivity higher than 1 W/m~*K. Measurement reproducibility and accuracy are assessed.