SUBGRADE Question

[twolabs]

I'm building a 40X60 and have the footing and stem walls done. I'm getting ready to do the slab and I have about 12" I need to fill with subgrade material. The ground under it has poor drainage so I want to fill it with good material for drainage. I have been told a couple different option. One is to use sand and the the other is to use 3/4"-7/8" drain rock. Will either work better than the other?

 

[Richard Cranium]

I would go with the drain rock, that is what we use around here. Central Washington (Yakima Valley)

 

[bcoke]

Rock on!

 

[36truck]

If you have water issues I would go with the rock. That way water can run through the rock and out from under the building. If it's just clay soil that you want to make a better base for sand will work just fine. Compact what ever you use real good. I would use only about 4" of rock close to the slab and sand below.

 

[Toomanytools?]

Might want to use drainage tile around perimeter of the building down by the footings so water can move away from the building. You could use 1 1/2" - 2" rock as a base for 4-6inches then smaller 3/4 -5/8" minus, put down a vapor barrier 6mil or better, 15mil would resist tearing, placed over the 5/8's next to the slab.

 

[ms fowler]

Here is a suggestion---use the rock until about 3"-4" below the bottom of your slab, then a heavy vapor barrier, then a couple of inches of sand. The place your concrete.
Why?you ask--all the extra work.
Well, it may be overkill for a garage, but concrete curls as it curls. Even if you place it perfectly level, the center will drop down, and the edges will curl up, slightly. The reason is that the water in the concrete can only leave by evaporation thru the top side of the slab. If you have a sand ( drainage) layer above your vapor barrier, you allow the water to drain out the bottom as well. This will produce a slab that resists curling.

 

[matt_i]

My thoughts are to use crushed washed limestone, 3/4" nominal. That does a nice job of breaking surface tension with the groundwater. Plate compact it and then lay 10 mil vapor barrier before your reinforcing steel goes in.

 

[Firebrick43]

Here is a suggestion---use the rock until about 3"-4" below the bottom of your slab, then a heavy vapor barrier, then a couple of inches of sand. The place your concrete.
Why?you ask--all the extra work.
Well, it may be overkill for a garage, but concrete curls as it curls. Even if you place it perfectly level, the center will drop down, and the edges will curl up, slightly. The reason is that the water in the concrete can only leave by evaporation thru the top side of the slab. If you have a sand ( drainage) layer above your vapor barrier, you allow the water to drain out the bottom as well. This will produce a slab that resists curling.

While your method works it results in weaker slab. It better to use plastic directly under the slab and put straw or a soaker hose and cover with visqueen on top of the slab as soon as you can walk on it. Water several times a day at least a week

This not only helps keep curing even (therefore less curling) but allows hydration to continue for at least seven days. Concrete allowed to dry out will be up to 50 percent weaker than concrete kept moist for at least 7 days. High strength precast sections many times are cured under water for up to a month!

And yes use crushed stone under the slabs. It's provides the best subgrade. The gold standard is a sub base 3 times as thick as the slab of washed 3/4 minus crushed stone with no fines. This is known as number 8 stone in many local. Vibration Compacted in 4-6" lifts depending on the size of compactor. Most only use double the thickness however.

 

[twolabs]

Thanks for all the info. Any thoughts on using pea gravel?

 

[Firebrick43]

The only reason it's used is it sort of self compacts and is cheap to place. It does settle somewhat if water runs through it leading to voids and doesn't interlock and make the base compacted stone does. Also it leads to uneven slab thickness as it's impossible to keep it flat/smooth while walking on it doing slab prep and concrete placement.

 

[matt_i]

My personal opinion is the angular stones interlock better and tighter than a run of something smooth and rounded almost like ball bearings...

 

[kbs2244]

No sand!
It will wash away.

Pea gravel is what is used around here.
But that is because the local pit sells it cheap.
Crushed rock will work.

As said, you want something that will drain.

If you want to see what the experts do, check out the local RR tracks.
They want those miles of track on a dry bed to prevent frost heaves,
That crushed rock bed they put down drains pretty well.