What base material under concrete slab?


Old 09-10-13, 07:05 PM
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Join Date: Sep 2013
Location: Canada
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What base material under concrete slab?

Hi All,

Hoping for a bit of advice. I've laid concrete slabs in the past and am about to do one for a 12' x 10' shed that I'm building. The plan is 3 1/2" of concrete over a base material.

In the past I've done this by pouring over several inches (+ or - depending on application) of 3/4" crushed stone that has been well compacted.

Now, I just moved into a new house in a new neighborhood and there's been a lot of road construction and renos going on and recently noticed this:

Name:  aggregate.jpg
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I'm not sure if there's a particular name for it but it appears to be regular 3/4" crushed mixed with sand.

It appears as though this mix (whatever it's called) would pack better but I'm not sure if it's something I should be looking for based on my application.

QUESTION - Should I use good old gravel? This sort of stuff? Or some combination of this stuff packed over top of normal crushed material?

All advice welcome.
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Old 09-10-13, 10:26 PM
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Join Date: Oct 2011
Location: USA
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It's mostly in the semantics--remember S.I.Hayakawa's "Is She Skinny, Thin or Svelte?" But I digress . . . .

What you formerly used and called gravel, was probably a washed rock blend, and had a relatively small percentage of fines (sand). Compared to what you're seeing now in your new neighborhood, which has a higher proportion of fines. The debate regarding which is better for a compacted concrete base could last for years. Coarse aggregate in any gravel is what contributes to most of its bearing strength, while a minimum amount of fines are necessary to enable the rock particles to interlock with each other, without voids between them. Using a uniformly-graded mix from a DOT-approved (or in your case, MOT-approved) pit would be your best course of action, as it will lock up well when compacted while containing a minimum amount of impurities. I personally would stay away from anything having a lot of organic matter in it, as it will settle more and recede under the concrete over time.
Old 09-11-13, 05:53 AM
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That picture looks like what we call crush and run.
Make sure when you get ready to form for that slab that the outside edges are turned down, the outside edges should not be any wider then the buildings going to be, and the slab needs to about 6" above finished grade.
If all of this is not done your going to end up with water coming in under that walls and the sheathing is going to rot out.
turned down slab - Bing Images
Old 09-11-13, 10:23 AM
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Location: Canada
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This is great info. Thanks! I have such a pit in close proximity.
Old 09-11-13, 07:15 PM
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