Hi. I was reading different sites on the internet, and found differing
opinions. One site actually recommends to follow I guess what I will call a
sandwich method. They recommend to place plastic on top of the soil, then
sand, then another layer of plastic, then crushed gravel. Is this a normal
practice or overkill?
If this exterior slab is for a patio or something similar, you should be
able to pour the concrete directly on the ground.
The plastic sheet is a vapor barrier to keep ground moisture from coming up
through the slab. For exterior use, I don't see the need, as it'll be
exposed to rain and other moisture anyway. I also don't see a reason why
you would need two layers of plastic?
When we built our garage, I laid 6 mil plastic sheeting directly on the
ground (overlapping rows by at least 12"). Our soil drains well and didn't
have any rocks or sticks that could puncture the plastic. So, I opted not
to use a sand layer. I then covered that with a layer of gravel, and rented
a plate compactor to get everything nice and level. It worked out great,
and I haven't seen any moisture coming through the slab. (tape down a sheet
of plastic for a couple of days, then see if you have moisture underneath
it when you remove it).
For our sheds and whatnot, I just laid the plastic on the ground, and
poured the concrete directly on top. It's not worth the trouble and expense
of adding a gravel layer. It takes a bit more concrete to fill in the dips
and whatnot, but that's very minor.
For our sidewalks, I just poured the concrete directly on the ground.
as long as your soil has good drainage like the reply poster.
For my driveway and patio, I dug out several inches of impermeable 'clay
like' soil, put down "3/4 minus" road base gravel then used a plate
compactor on that, then formed, added rebar & poured. Came out good.
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