How much gravel under concrete?

[Mosey]

How thick should I put a layer of gravel that I will (someday) pour concrete over? What is the minimum thickness and what is the maximum thickness (if there is one)? It will be at least a year, maybe 2, before the concrete is poured over it.

 

[cowboydoc]

Usually you don't pour concrete over gravel. Is it a building site? The site will probably have to be prepped. We always scrape off whatever is there and then level it. Sometimes we had to use sand to get the sight level.

 

[kevincook]

It depends on what it is for and how much weight it will have on it. In any case you want to use something that will compact like crusher run and not just gravel.

For something like garage 8" of compacted stone and 4" of reinforced concrete would be a minimum and 12" or stone and 6" of concrete would hold just about anything. Somewhere in that range will work for just about anything you want.

 

[gerard]

All residential garages I've ever seen have never been more than 4 inches of concrete. Of course if I was doing my own the extra cost for 6 inches may be worth it. I would also use the fiber mesh concrete, well worth the small added cost. With it you supposedly don't need the mesh or rebar but again, I think overkill is better and I'd use it anyway. As far as the gravel base that's what I've always seen used (not running crush which I think would be more prone to frost heeves in above grade applications. Course if you're in florida or texas that's kind of moot!)and would use at least the same amount of gravel as the concrete. More important is good compaction of the base BEFORE you put the gravel down, then good compaction of the gravel itself. Better to do it in two "lifts", compacting between each one. Iv'e never seen sand used as a base for a concrete pour but maybe that's different for different parts of the country. Check out some of the concrete websites - lot of good info out there.

 

[gerard]

everything you ever wanted to know about concrete and then some!! /w3tcompact/icons/smile.gif

<A target="_blank" HREF=http://www.concrete.com/forum/forum.htm>http://www.concrete.com/forum/forum.htm</A>

 

[alan40]

My 24 X 28 garage in Vt. is almost done. What I did was to peel off the sod and put in about 40 yds of sand to level the pad above ground level for drainage. Of course that properly relates to building on top of ledge,slope etc. The important thing is what ever material you use compact the heck out of it. I rented a plate compacter for a couple of days and also used a sprinkler system to put water on the sand to pack it down.
Good Luck!!!!/w3tcompact/icons/smile.gif

 

[Mosey]

This is for a pole barn/garage site. I'm planning on a 6" thick concrete floor. Nothing heavier than a full size truck or a Suburban will go in it.

There is no sod, I have it built up with fill dirt (clay), which was hauled in 2 years ago. So, it's pretty much settled now. I thought it was pretty level, but checked it with a line level and it's about 10" low on one side. I was going to put just 3" or so of gravel over the whole thing, but now I either need to get some more fill dirt for the low end, or just make up the difference with the gravel. That means I would have 13" of gravel on the low side and just 3" of gravel on the other side. I'll put more fill dirt/top soil around it to contain it, and plant either grass or crown vetch.

I talked to a place that sells gravel and stuff and they recommend a #53, which has lime in it so it will pack down. Is that the same as "crusher run"

 

[Soundguy]

I beleive he meant fill dirt, as opposed to say.. white sand, etc. And for instance, here in florida, he use fill for our slab bases.
I personally wouldn't use gravel in a situation where I wanted compaction. Grab a handfull of small marsh mellows in one hand, and a handfull of roller bearings or bb's in the other hand. Squeeze both well. While the bb's will consolodate, there will be no compaction... the marshmellows compact and fill air voids, etc.

Soundguy

 

[Green_in_MI]

Let me try to clear up a few things here about subbase for a concrete pour. The object is to get a base which is as incompressible as possible yet provides drainage. Gravel best suits this task. Not just ordinary gravel. What is needed is processed gravel of the type that your county will put on its gravel roads. This gravel has a wide gradation of material size including some clay fines. Under proper compaction techniques, ie. plate compactor and correct amount of water, a well compacted gravel base will feel like concrete itself.

Never use the original material excavated from the site as fill to raise the grade. It usually contains soils(organics) which will continue to compress and squeeze out under load, particularly if water is present.

A minimum thickness is 4-6 inches of well compacted gravel should be your target. Any more depth of gravel is ok with the proviso that it be well compacted. That means putting it down in thin layers and compacting the *&$%$# out it each time.

When you are ready to pour your slab after a couple years check the grade and maybe put in a little more gravel if you need it.

Sound guy. Your are quite correct in that you want to eliminate as many air voids as possible. What you need is incompressible material(rock) that fills voids with various sizes of diameter fitting in between the larger ones. And not a material that simply squeezes.

 

[Mosey]

Well, I had 10 yards (15 tons) of #53 limestone delivered yesterday. It has a lot of limestone "powder" and the guy said it will pack real good.

I was going to have him dump it right on the site, but it was too muddy to get all the way to it (he got stuck trying and I had to call my neighbor to come over with his bigger tractor to pull him out after he dumped the load). I guess it's just as well, since now I can shove the fill dirt around until it's level, so I'll have an even 6" layer of the limestone. I'll only have to move the limestone about 20', so that's not too bad.