More thoughts on this.
I have been trying to puzzle out the "why" of the concrete from its first mention. All of the ideas I have come up with result in "It wouldn't have worked".
As we have seen, it didn't protect from discovery from traffic.
It didn't serve to conceal the body, but rather attracted attention, which would be expected.
It wouldn't have protected from animals or insects.
It wouldn't have protected from erosion, or at least not for long. Erosion would have first drawn attention to the spot, and then probably undermined around it.
It certainly wouldn't have accelerated decomposition, nor would it have slowed it down significantly.
There isn't much indication of intelligent planning here.
Even the site itself is remarkably stupid. Far from being "isolated", I can think of few places more likely than a good fishing spot close to town to guarantee visitors at all times of day, night, or year with plenty of time on their hands to ponder their immediate surroundings, and reason to walk around.
When I finally found the location on Google Earth streetview I told (the stunningly beautiful) Mrs. fortytwo that any fisherman would instantly recognize that as a popular place to fish, even if they were moving 60 mph and had never been in the state before.
There were times in my youth when my car's ignition would have cut off on its own just driving past it.