Thermal properties of concrete from transient conduction of a thin slab

Abstract

The thermal properties of concrete, such as its thermal conductivity, thermal diffusivity, and specific heat, are important in some applications of concrete. The rate at which heat flows into, through, or out of a concrete structure is governed by the thermal conductivity of the concrete. The ease of difficulty with which the concrete undergoes temperature change as a result of heat loss or gain depends also on the thermal diffusivity and heat capacity. Established methods for determining the thermal conductivity of concrete invariably use steady-state heat conduction from which the conductivity under a constant thermal gradient can easily be calculated. Such methods suffer the disadvantage that the time taken to reach thermal equilibrium may be inconveniently long, especially for concrete, which is a poor conductor of heat. The long period to reach steady-state also restricts its use for measurements on damp materials because of redistribution and drying out of moisture within the specimen. Transient heat conduction methods are less well known and seldom used. This paper describes the use of transient heat conduction through a thin slab that is initially at a constant temperature and subsequently heated by free convection in a thermal bath to determine the thermal conductivity (k) and diffusivity (α) of the slab. From the transient heat conduction theory, it is deduced that a thickness-to-length ratio of 0.2 is adequate to achieve one-dimensional heat flow in the middle of a square slab. Using the theory, the nondimensional temperature-time history for the midpoint of the slab is obtained. From these nondimensional plots, a statistical scheme of finding the best fit between experimental data and the theory is described. By obtaining the best fit, the most probable values of the thermal conductivity and diffusivity are obtained directly. Repeated tests on several slabs of different compositions illustrate the usefulness of the method of obtaining k and α of concrete materials. Good comparison with published values of k and α obtained experimentally using similar materials serve to validate the method.