Radiant Solar Heating
It doesn't require a NASA rocket scientist to figure out how much heat energy is being absorbed on a concrete deck surface. All one has to do is stand, bare-footed, on a hot concrete surface, during springtime.

The heat stored in concrete surfaces can be absorbed through the embedded InDekSolar© tubing and stored in your swimming pool making a longer swim season, all the while lessening your carbon footprint.

Source : Surface and ambient air temperatures associated with different ground material: a case study at the University of California, Berkeley