I recall that when building my footing and pier, in a manual labour fashion similiar to yours (with lots of rebar and star posts), it dawned on me that nothing short of a nuclear strike on my observatory, or the expanding Sun, is ever likely to remove that concrete. I have a contingency plan, should I or my children ever sell the house, the pier can be converted to a Bird Bath by the addition of a top dish (And I actually already have one from Bunnings sitting at the rear of my block).
Pity the unsuspecting future buyer who might try to remove it.
