The HeatBox determines the heat of hydration of a concrete mixture based on the principle of semi-adiabatic calorimetry. A specimen of fresh concrete is placed at the center of an insulated container. A portion of the total heat generated by hydration raises the temperature of the concrete specimen and a portion is absorbed by the container and transferred to the surrounding air. The amount of heat absorbed by and transferred through the container can be established by appropriate temperature measurements of the insulation material or it can be determined by numerical simulation provided the thermal conductivity and heat capacity of the insulation are known. The HeatBox system is based on the second approach. The thermal properties of the container are established by a standardization procedure in which the mold is filled with water at an initial temperature of 40 to 45 °C. The water-filled mold is placed in the container and the drop in water temperature is recorded until the water approaches room temperature, which generally takes about 5 days. In the graph to the right, water temperature is shown in red, and ambient air temperature is in green.
The thermal properties of the insulation are determined by an iterative analytical procedure. Values of specific heat and thermal conductivity of the container material are assumed and the temperature drop of the water is calculated using the initial measured water temperature and the measured air temperature as boundary conditions. The calculated temperature-time curve is compared with the measured curve. The thermal properties of the container material are updated and a new temperature-time curve is calculated. The process is repeated automatically by the software until there is an agreement between the measured and calculated temperature history of the water. The final values of the thermal properties of the container material are used in subsequent analyses to determine the heat of hydration.