We are building a 10' x 10' slab for our hot tub. It is 4" thick with 2 piles and no rebar. The hot tub weighs around 4200lbs with water. We're wondering if that should be enough to support the weight or should we build the slab thicker?
I don't get the part about the two piles. Is this a typo, or did you drive two piles into the ground to support the slab? As for support, a 4 inch slab should work fine provided it is properly supported by a suitable crushed stone or clean gravel base on adequately compacted and prepared soil. The concrete will be entirely in compression, which is the way concrete is strong, unless you have voids or compressible soil supporting the concrete, in which case you will have serious trouble.
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