In making calculations of heat transfer in massive concrete why is thermal diffusivity figured instead of thermal conductivity? What is thermal diffusivity?
First of all it should be said that thermal conductivity is the rate at which heat flows through any kind of material, such as concrete. Thermal diffusivity, on the other hand, is the rate at which temperature changes occur in the concrete. In the English system of units thermal conductivity is given in terms of Btu per hour per square foot of surface per inch of thickness per degree difference in Fahrenheit temperature. Thermal conductivity is measured under steady state conditions, using a constant temperature on one side of the concrete and another constant temperature on the other. Where masses of concrete are concerned it is often desirable to calculate the rate at which temperature changes will occur in the concrete. This involves not only the rate at which heat flows, but also how much of the heat is used up in warming or is given off in cooling the concrete. This heat change is the product of the density, d, of the concrete and its specific heat, S. If the thermal conductivity, k, is divided by the product of density and specific heat, the result is the diffusivity, D. Thus,
     D = -----
         S * d