The first thing that you should do is lay sheets of rigid foam insulation on the ground. Cover the rigid foam with gravel, and install the pipes in the gravel. Over that, pour the concrete and let it set. This is an excellent way to heat the garage, but there is one hitch: Once you have installed the pipes and covered them with concrete, you are committed to keeping the temperature of those pipes above zero. You must not let them freeze. If they do, the pipes will burst. For safety, you should add bypass, cutoff, and drainage valves.