Concrete foundation advice

dmccarty · Aug 7, 2003

Re: Advice taken, and another Q

Boondox,

My father in law used gravel to fill in under his slab. He had
a depth of at least three feet on one side.

ABC is what the DOT uses for road bed construction and what
I used for my driveway.

I would *** think *** it would be could to use under a slab
but in the back of my mind something is saying that you
should not use ABC in this application. For the life of me I
don't know why it would be a BAD idea. The stuff compacts
real nice and its real hard when compacted. I'll send a note
to my father in law and ask what he used and why....

Later,
Dan

carlinva · Aug 8, 2003

Re: Advice taken, and another Q

If you have a building permit ask the inspector what he wants. I (stupidly!!) backfilled and compacted my slab with gravel only to be told by the building inspector to either dig it out and replace it with sand or get a licensed engineer to approve it. My arms still hurt ten years later from digging it out and my ears still hurt from my wife telling me "I told you not to do that"

The slab is on about three feet of sand and after ten plus years it has never cracked or settled.

alan40 · Aug 11, 2003

Re: Advice taken, and another Q

Hi Pete, up here in Fairfield most everyone reccomended sand so thats the way I went. I also rented a compacter and compacted it for a couple of hours. Seemed to help. The garage floor has not been a problem yet.
If you need a hand let me know my weekends are pretty booked up but for a cup of coffee I could proberly make time. /forums/images/graemlins/grin.gif
Take care.

Boondox · Aug 11, 2003

Re: Advice taken, and another Q

Thanks, Al. /forums/images/graemlins/smile.gif

With all this blasted rain we've been getting I'm wondering if it will ever come together!

Pete

dmccarty · Aug 18, 2003

Re: Advice taken, and another Q

BoonDox,

Did you ever decide on what to put under your slab?

Well, I finally got ahold of my father in law about all of this.

I did remember right to NOT use ABC. The reason is that ABC
needs to be compacted to avoid settling. He speced #57 or
#67 and they put in ABC which settled and he had to redo his
slate floor. ABC can be compacted but it can bust your
foundation walls if not done correctly.

Sand is a bad idea since it can wick to the concrete. Stone
should not do so.

Sorry for the long time to get a response....

Later,
Dan

em14 · Aug 18, 2003

Re: Advice taken, and another Q

I think sand is a very good material to use and it is used quite often. You must compact the sand and use a moisture barrier (plastic sheets IE poly .. vis queen) between the concrete slab and sand. You should also tape the joints to insure a good moisture seal. To use the sand you must trap in which is what your Stem walls would. I just poured 38 yds this way in a shop I'm building. Pasted inspections A-OK.
Leo

em14 · Aug 18, 2003

Re: Advice taken, and another Q

"To use the sand you must trap in which is what your Stem walls would."
Should read:
"To use the sand you must trap it which is what your Stem walls would do."
Sorry about the type-o!
Leo

Soundguy · Aug 19, 2003

Re: Advice taken, and another Q

Keep in mind the properties of the materials you are working with. Sand compacts, to the extent that it fills voids and expels air. Gravel consolidates, doesn't compact.. at all.. and retains lots of voids, clay compacts, but its volums changes with water content, making it plastic.

Some gravel in your base will be ok.. I wouldn't use alot of it though... Good clean fill, in correct compacted lifts, inside a stemwall will be a safe bet.

As for state dot work, comonly, native earth is removed, and or / fill added, then it is mixed with cement powder, some clay, or crushed lime rock to form a stabilized subgrade.
A finish grade of limerock is then used.. then asphalt. Other variations involve the sole use of clay and asphalt, or native soil and cement.. making a soil cement base, with asphalt as a surface course.
A black base road is solely asphalt, in a few very thick lifts... no limerock or clay is used as a sub-base. ( lots of dot work use this in areas where they need nearly -0- downtime, as you can excavate, box, roll, lay asphalt, and roll as you go, pumping out finished turnlanes, etc, as you go.
Occasionally a road surface will be milled then the millings will be reclaimed, or mixed in as stabilizer in the sub-base, and also sometimes the on overlay projects the milled road surface will be coated with a bituminous tack coat, then a very thin layer of gravel.. like 5/7 will be laid.. like 1 diameter thick, to help the new surface adhear, then a few lifts of asphalt, etc.

Personally.. I would avoid an all gravel base with a thin layer of 'icing' fill on top. It will have great drainage, and eventually, all that fill will be filling the voids in the gravel underneath, leaving your slab unsupported in areas. Even with filter fabric to help prevent the soil migration.. it is a gamble at best...

The company I work for has built miles and miles of road, including sections of the interstate system in florida., and countless building pads for both comercial and residential buildings... gravel wasn't used in -any- building pads...

Soundguy

PaulT · Aug 19, 2003

Re: Advice taken, and another Q

Well your experience is yet another example of how different construction techniques can be in different parts of the country, especially between extreme north/south locations. Here in Upstate NY, and probably in Vermont too (where Boondocks is), gravel is what the DOT uses as the road bed. You probably see less use of gravel in Florida than we do, since here in the NE, rocks are more readily available (talk about an understatement /forums/images/graemlins/shocked.gif I think our biggest crop is fieldstone!) It is also widely recommended for a solid bed to pour a slab on, either for a garage, house or barn. Washed or clean gravel will not readily compact, as you say, but gravel with the fines left in will compact quite nicely, especially if it is still damp from the run through the crusher.

For my pole barn, I plan to have a base of what we call "inch and a half minus", which is just everything that comes out of a crusher that is 1.5 inches or less. The stone dust in this eventually sets up very nearly as hard as concrete over time. This will make a good base to pour my slab on.

Of course you want to be sure and let it settle for a few years before going to the expense of putting a slab in (no, not the gravel, the bank account /forums/images/graemlins/blush.gif)

For houses, they try to leave as much of the original soil undisturbed as possible when they excavate for the foundation and footings. In areas where the slab is at grade, they dig down a ways inside the foundation and use clean 1.5 inch gravel (no fines), vapor barrier, rebar, concrete. For a slab that goes over an area that needed excavation and fill, the high-end contractors use clean gravel all of the way up (since it doesn't compact), while some trying to save costs will re-use the excavated earth, but compact it at 4 inch lifts.