In article .com,
wrote:
I'm still calling around for reasonable quotes on a shotcrete cistern.
One question is how to do the top for such a cistern. If I put it
under the patio, I can arrange to have a lightweight wooden deck over
it, which could be supported by steel beams. I was thinking I could
glue the blueboard to the underside of the steel, then glue the EPDM to
the underside of the blueboard, but glue + high temperatures makes me
see delamination in the future. There is also the issue of making sure
that rain and whatnot drains off around the cistern and doesn't pool on
top of it.

You do seem (from here) determined to take the most complicated
approach. To simplify - make your patio from spancrete panels - these
are pre-stressed, pre-cast concrete panels which a crane puts in place.
They are used for ceiling and floor in buildings - getting one to span
your 15 (or less, without rocks) foot hole will be trivial. Or, cast a
top in place, but the spancrete will be easier (no need to form in the
hole). Provide for a slight grade and water will flow off.

Foam will float, so fussing about glue is probably a non-issue. However,
you could glue the foam to the underside of the concrete if you like.
Better yet, use polyurethane spray-on foam, which will glue itself in
place when sprayed. It's also waterproof enough that it's used as
roofing, you you can probably skip the EDPM.

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