Tolerances for gravel supporting foam below slab?
How critical is the flatness of the gravel bed below 2″ sub-slab foam?
I embedded and leveled two parallel lengths of 3/4″ black pipe in the gravel, then used a 6″ x 10′ aluminum channel to screed the gravel to the pipes. Repeated for the whole slab. After running the plate compactor over everything (It had also been compacted after the rough grading also), there are some humps and valleys. If I walk on the foam I can feel it depress with my weight. For example, across one 4×8 sheet of 25 PSI EPS, I can feel it go down 1/8″ or so in not more than one place. Putting the middle of my 10′ straight-edge on top of a hump, I can see as much as 3/8″ or even 1/2′” on either side when I teeter-totter the straight-edge; since the hump gradually spreads across about 3′ or so, the height of the hump would be about 1/4″ above the level plane of gravel.
When the 4″ slab is poured, will the weight depress the foam enough for acceptably even support? It will have a 1/2″ rebar grid in it by the way.
So how exacting does this have to be? The labor of installation seems to be significant expense. Or am I being too particular?
Thanks very much.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
David, the foam will easily deform under the weight of the concrete. It shouldn't even crack, unless the changes in elevation are sudden. The only thing I would be concerned about is the thickness of the concrete. If your 4" slab ends up at 3 1/2" or less in some areas, it will not be as strong as a full thickness slab. That probably won't be a problem but it depends on site conditions.
If you want to get the gravel flatter, you can just distribute more gravel at the low areas and go over them again with the compacter.
David,
I agree with Michael that you'll probably be OK.
On your next slab, though, you should make the necessary adjustments before the rigid foam is laid in place. If you have a 10-foot straightedge (and a level), you have what you need to deal with the humps and valleys. A rake will easily correct these problems.
Martin, the foam is not in place, and that is why I am asking now (I just put down a few pieces to test the result). I more than used a level and straightedge: I used screeds and got everything dead-nuts level with no small effort. Compacting it a second time made the soft areas that had just been "raked" compress and become depressions. So your solution is to level and rake yet again, taking the top off compacted gravel and filling up slight depressions with non-compacted gravel. If I were to then compact it again, the soft gravel would compress again, in a process that seemingly could go on endlessly.
My question is when is it appropriate to stop? Will the softer, raked gravel provide even support for the foam?
I get the feeling I have already gone for the overkill. I have invested way more than any contractor who is interested in pace (i.e. profit) would have done, although of course efficiency would come with experience.
So please clarify what the tolerances are for the gravel bed below foam, for now and for future reference also.
Thanks very much.
David,
As Michael said, and as I wrote, everything is probably fine.
I'm wondering what you mean by "gravel." (The word has different meaning in different areas.) Where I live, "gravel" is a mixture of sand and small stones.
I don't recommend using gravel under a slab. I prefer to see a 4-inch layer of 3/4-inch crushed stone (with no fines). But maybe that's what you have -- and maybe crushed stone is called "gravel" where you live.
3/4-inch crushed stone does not have to be compacted.