Tamping crusher dust?

[OptionalStop]

For my new slab I put in 6" of 1" crusher run stone and tamped well in 2" lifts. The stone was hard to get perfectly level after tamping so I brought in some crusher dust to make up about 1" left before I put my foam board down. So I've got roughly 1-2" of crusher dust and I'm not sure if I should bother tamping this or not. I don't want it settling and my slab to crack. Thoughts on tamping this small amount? Should I be OK with the weight of the concrete when it's being poured to compress the dust and set it in?

 

[rburke65]

Why would you rely on the weight of the concrete? I would wet it and tamp it.

 

[rayra]

I think the OP has misunderstood the process and is chasing an unnecessary OCD-level of smoothness. Under his vapor barrier. Under his foam. Under his slab.

 

[OptionalStop]

I think the OP has misunderstood the process and is chasing an unnecessary OCD-level of smoothness. Under his vapor barrier. Under his foam. Under his slab.

No, I just try and do the best job I can. Workmanship...

Trying to avoid my foam sheets lifting and moving up and down as the concrete guys step on them. I'm stapling the pex to them and don't want any staples to pop. Simple really.

 

[like2wheel]

If you're heating the slab, I've read that you need the last layer of stone to be clean (washed) stone to stop the capillary action of moisture away from the vapor barrier under the insulation. No fines.

I just put in a layer of clean #1, & also wondering how perfect I need to get it.
I've read here + or - 1/4", but I'm sure I can't get to that level of accuracy.

 

[mcbane]

If you're heating the slab, I've read that you need the last layer of stone to be clean (washed) stone to stop the capillary action of moisture away from the vapor barrier under the insulation. No fines.

I just put in a layer of clean #1, & also wondering how perfect I need to get it.
I've read here + or - 1/4", but I'm sure I can't get to that level of accuracy.

Not a stone expert but isnt #1 stone ranging in size from 2" to 4"? If you put a vapor barrier directly on top of that it will be punctured in numerous places and not be effective. The recommended practice when using a capillary break is to place a layer of fine grained compactable fill on top of the capillary break. The fine grained material is easier to level and will protect the vapor barrier. See http://www.nrmca.org/aboutconcrete/cips/29p.pdf

 

[rustyjames]

Not a stone expert but isnt #1 stone ranging in size from 2" to 4"? If you put a vapor barrier directly on top of that it will be punctured in numerous places and not be effective. The recommended practice when using a capillary break is to place a layer of fine grained compactable fill on top of the capillary break. The fine grained material is easier to level and will protect the vapor barrier. See http://www.nrmca.org/aboutconcrete/cips/29p.pdf

Exactly, it's often done with sand as well.

 

[like2wheel]

Not a stone expert but isnt #1 stone ranging in size from 2" to 4"? If you put a vapor barrier directly on top of that it will be punctured in numerous places and not be effective. The recommended practice when using a capillary break is to place a layer of fine grained compactable fill on top of the capillary break. The fine grained material is easier to level and will protect the vapor barrier. See http://www.nrmca.org/aboutconcrete/cips/29p.pdf

What I got was clean limestone measuring from 1/4" to 3/4"ish. Just checked the receipt from the quarry, it says #1 stone. Must be a regional thing. My vapor barrier is 15 mil poly so I'm not worried about punctures from that, just not sure how perfect I need to rake it. And how much it'll get stirred up positioning the vb.

Wouldn't putting sand or something fine above the capillary just filter down & ruin the effect of the capillary break anyway? Or possibly create voids if it filters down after the pour?

Here is a couple of the places I saw the order of the layers:

https://www.buildingscience.com/documents/insights/bsi-059-slab-happy

https://www.greenbuildingadvisor.com/article/polyethylene-under-concrete-slabs


I have no experience in this, just trying to learn what's right for my situation



.

 

[mcbane]

"Just checked the receipt from the quarry, it says #1 stone. Must be a regional thing."

Interesting. So ordering numbered stone is like ordering numbered menu items at the Chinese restaurant.

With 1/4 to 3/4 stone you dont need a layer of finer material on top. You should be able to get it flat enough with a large paving rake, the kind used for leveling hot asphalt.