# What size gravel to use under concrete pad



## woof24 (Jan 4, 2011)

Can I use 3/4" (crushed/angular) gravel under a concrete pad? 

The pad is 5x5ft. I'll pour 6" concrete over 6" of ??. I'm using #3 rebar.

I have a lot of this gravel, and also unsorted sand, so I'm hoping one of those options are ok to use?

If this matters, the ground is very hard (Arizona) - has to be wet overnight to dig it.

Thanks


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## fjacky (Jul 29, 2005)

3/4 is what I used


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## jklingel (Dec 21, 2008)

Woof: 3/4" or less, and compactable, if a house slab. What are you pouring 5' x 5' that requires 6" of concrete? Are you storing Abrams tanks?


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## woof24 (Jan 4, 2011)

*rain tanks*

I am putting in rainwater catchment tanks: 4ft diam culvert x 10 ft tall. The culvert will be stood on end into the wet concrete. 

Thank you both for your help.


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## jklingel (Dec 21, 2008)

Approx numbers: 340 cubic feet. 21,760 lbs. 1813 pounds/sqr ft. I think you'd be safe w/ a steel reinforced 4" slab, and if you go to 6" with only one layer of steel, you may be wasting 2" of concrete. But, I'm not an engineer. Is the #3 bar 12" OC in both directions? Placed below center line of slab (approx 1/3 of the way up)?


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## woof24 (Jan 4, 2011)

Yes to your questions on rebar and placement of it. Since posting I've chatted with an engineer friend who said 4" concrete is plenty. He also suggested 1" sand, then 4" of 3/4" gravel (compacted) then the 4" concrete. Am going to try that! 
Thanks for your help ...


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