It's not the weight of the water at issue, per se. It's how it's going to be supported, and what that support system was designed to endure. Dirt would support a 40000 gal bag of water, but you couldn't replace the bag with plaster, because the dirt could shift and the plaster would crack.
My last post was all conjecture, based on how I've watched plaster pools being built here at TFP. Six to eight inches of solid, one-piece concrete, embedded with a massive amount of rebar. I have assumed the purpose of that construction method was to ensure that it would absolutely not crack (and even those do, sometimes), and because it was all one material, would not expand and contract differently from one area to another. We know that the shell doesn't need to be water tight. The liner/plaster does that. With a liner, which is somewhat flexible, it would remain watertight if the shell was not absolutely rock solid. Like if it flexed, or if two different materials (like slab and block) expanded or contracted in slightly different ways. But those kinds of movements could cause plaster to crack, which would then leak. And we don't know how thick the floor is, or whether it has rebar in it, or how much even if it does, and how the connection was engineered between slab and block, etc... I was just posing the questions: does that matter? Or does it need to be determined ahead of time if plaster can withstand what that shell might subject it to?
What I do know, is that concrete and stone can move. And can move differently than surrounding materials of a different composition. That's why some pools are surrounded by expansion joints. Do pools constructed of slab and block get plastered? Does that work? And work well? That's beyond my knowledge.
Now if that pool had been a plaster pool, one might somewhat safely assume that it could support a fresh coat of plaster. But according to others here, it never was a plaster pool. Is the OP willing to gamble that this non-plaster pool can be successfully converted to a plaster pool?
Has the OP received estimates from any PB willing to use plaster on that shell?
Again, just posin' the questions. I have no idea what the answers are...
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Ha, ya beat me while I was typing! Why did it take me six paragraphs to say that!!