I don’t think concrete will cure all the way to the center if you just dump it into the ground and hope there is enough water to penetrate the whole thing before the shell cures. Just eat your Wheaties, get a friend, and mix up one bag at a time and keep dumping the mix in. Standard concrete takes quite some time to cure, longer than it will take you to mix several bags up.
Of note, you cannot go too think, as the heat of curing can cause the concrete to crack. I don’t know what that limit would be.
I am starting to think that plastic pipe for a drain is problematic from a thermal expansion point of view. If the soils are warn surrounding the pipe in the dam center, but cold water flows through, the pipe may shrink enough to allow water to make its way along the outer edge despite a good installation. Sure the collars should stop it, but only if the soils around the collar are well packed, which seems difficult to do below the pipe. Maybe it would be ideal to box in the collars with concrete when installing? Even if it is just the lower half to seal the collars that are difficult to pack.
Also, concrete may stick to the plastic at first, but a shrinking cold pipe may still pull away.
| 
