Gravel under concrete?

[noel4747]

Can I use number 28 gravel as a base under concrete? It is half the size of #57 gravel but half the price per ton

 

[GMCGarage]

You can use whatever you want. Results may vary. If compacted correctly, '28' gravel should be acceptable. I have never heard of 28 though, but if similar gradation to 57's should compact nicely.

 

[Firebrick43]

I really don't know what 28's are as they are not a local specification nor can I find any mention of them online.

But you don't want 57's either nor any gravel (other than pea stone and I don't like that either) for the 8-12" directly under a slab.

You want 3/4" minus crushed stone without fines. Known around here as number 8's. The lack of fines allow drainage and the angular faces of the crushed stone allows the stone to interlock and consolidate as its being compacted.

Round natural stone without fines will not compact. Gravel with sand/dirt fines works fine for fill as long as it doesn't have expansive clays as a binder but should not be used directly under a slab but is ok below the above mentioned base.

 

[LX-Markham]

You want 3/4" minus crushed stone without fines. Known around here as number 8's. The lack of fines allow drainage and the angular faces of the crushed stone allows the stone to interlock and consolidate as its being compacted.

Nit picking at the details, but it's actually for water vapor transport, not technically drainage. It acts as a vapor barrier. If you need drainage under your slab, you should have drainage tiles.

We usually specify 3/4" clear stone under the slab. It allows the slab to cure more evenly top/bottom and reduces slab curling.
If compacted granular 'B' is specified, then we use a vapor barrier (poly sheet) under the slab. But it traps moisture under the slab while it's curing. Wet curing the top of the slab helps to even this out.