OK - the traditional method is to use heat tape to keep the pipes warm, particularly if the water is stationary in the pipes for long periods of time.
Another option is adding a loop and a small circulation pump to move the water to where it can get warmed and to keep it in motion to resist freezing.
Adding more insulation underneath if the pipe insulation is not working seems less likely to fix the problem. Heated air rises, so the majority of the heat loss is out the top, though I am unclear how the slab might be currently heated or cooled.
Before looking at insulation as a solution, put together a diagram of the elements of the installation - how deep the section is, the temp of the air above the slab, the temp of the air below the slab, how thick the slab is, and if the current installation is open to the outside air, along with whether this is flowing water and, if it is, if it is exposed to outside air prior to reaching the breezeway. Without that you will have a difficult time knowing the minimum insulation required or if enough insulation could possibly help. Put an insulated container in a freezer and the inside will freeze, if it is there long enough and the air is cold enough.