Quick review of some basics for thermodynamics and heat flow. Heat moves from hot to cold, the difference in temperatures determines the rate of movement in similar substance. Insulation moderates the flow of heat. If your frost penetration is 8 ft. then we can assume that the temperature at eight feet is about freezing point. Move eight feet towards the surface and we have a seventy degree temperature differential in eight feet. Pretty steep slope on the graph. Add to that the fact that sand will provide even more of a heat barrier because of the dead air space between grains. To achieve a seventy degree temperature differential in the Earth use the oil field standard of one degree per one hundred feet and you would have to go about seven hundred feet into solid rock to achieve the same difference. Other variables include the temperature loss rate of the concrete as it cures which is a function of surface area and volume. Rebar in the concrete would accelerate the heat loss from the center towards the cold. Conclusion. No way would a professional engineer risk their licence and approve this without a lot more thought and expense to insulate the concrete from those kinds of surface temperatures. My two cents worth.