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I removed a chimney from my house earlier this year. After everything was cleaned up, I was left with this big hole:

The hole where my chimney used to be

I have two questions: (1) How do I know what type of concrete to use? (2) How do I prep the hole before pouring/placing the concrete?

Some info: The hole is in my garage. It's about 2 or 3 feet deep. Surrounded on three sides by chimney brick and one side by the crawlspace foundation of my house. The bottom is dirt. The garage cement is pretty smooth, which you can see from the picture.

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If the bottom is undisturbed earth, you don't need to do anything special, just fill the hole. The concrete mix can just be common 2000psi, again no special requirements. If surface finish is important, only fill to the top of the bricks, come back and place a cap layer much later, as that much concrete will shrink and maybe crack some. –  bcworkz Aug 10 '13 at 19:24
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Actually, it looks as if it could be useful something, especially if it is dry, but nothing comes to mind. Maybe a top-load fireproof safe. I'd probably just frame it with 2x stock for a single or double layer of 3/4" plywood. –  mike Aug 10 '13 at 19:47
    
It would be perfect for a waterproof safe. –  DA01 Aug 11 '13 at 2:50
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1 Answer

up vote 2 down vote accepted

Backfill the hole with dirt/fill to about 10-12" below the surface. You should compact the soil frequently as you go - don't wait until the end. Wetting it will help compact it. After the soil add 6-8" of 1/2" crushed gravel. Compact this. Last, add 4-6" of concrete.

Unless you plan on parking something exceptionally heavy, you can use any concrete really - 2000-5000psi.

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