Here is what I would do I think. Have a bottom layer of concrete blocks as the foundation with steel rebar or 2x4's running through the holes and tied so they stay flat. Stack and tie the rest of the blocks to the foundation and tied. Then on top tie a truck inter tube to the top of the blocks. When the blocks drop through the ice the tube should keep them upright as they drop to the bottom.
Great idea with the tire on top. Kinda like a parachute.
I am thinking the (unrealistic) fear with excessive amounts of concrete is Calcium Hydroxide (Hydrated Lime) leaching out. Hydrated Lime, on it's own will raise pH very quickly to lethal levels when spread in a pond....I do not see that likely happening, over years, from broken concrete, however.
As soon as water is added to concrete the quick lime changes and will never be quick lime again. The longer is cures or is exposed to air the harder it will get and the less quick lime with be left. The quick lime gos back to calcite again. I would be more worried about the magnesium and potassium that can be released when broke or even the water base sealant used to seal the blocks dissolving again.
Cheers Don
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