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I am planning to build a concrete slab on a soil surface to hold a 400 gallon water tank (Shown Below). I want to make it 7' x 7'. It should hold about 3,500 lbs.

Would I need to put in rebar? I've done much smaller slabs with 80 lbs bags of Quikrete Concrete ready mix. How many bags would I need?

Water Tank

  • You're asking for how many bags to use, but you don't tell us how thick you want the pad. Is the thickness a part of the question? – JPhi1618 Jan 21 '16 at 13:25
  • Would be helpful if you can mention the layers and thickness too. – Rick Jan 21 '16 at 13:26
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My typical answer to these questions is talk to a Licensed Engineer because there are a lot of considerations that are needed. I firmly stand behind that.

On this project make sure you're using the correct information.

Water ~3,300 pounds

Tank ~ 180 pounds,

Concrete (4in thick without reinforcement) ~2,400 pounds

Piping ? other accessories?

Thats only part of the weight that the ground below your tank will support; how sure are you that the ground will hold that? Typically those tanks don't overflow at their rated capacity, they have 5-15% excess capacity, this needs to be designed to passively let the excess overflow or designed for the tank to be filled the overflow level.

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Each bag is .6 cu ft. You have 49 square feet divide by 3 for a 4” slab by 2 for a 6” slab. 49 divided by 3 is 16 cubic feet so it will take just over 27 bags. Expect to use a bit more as you always use more than expected, I would use a wire mesh reinforcement on a hand pour of this size.

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