I work for an underground utility / sitework contractor and we primarily build residential subdivisions. One of our client’s big callback issues is driveways/sidewalks and patios. I would appreciate if someone could clarify for me what forces affect concrete in the following scenarios:
I am assuming proper compaction of fill, so settlement is not the issue I am trying to think through. We work in Indiana, and the frost line is about 48″. With a standard footer/foundation wall/slab installation, the spread footer is determined on the bearing capacity of the soil and the foundation wall is to transfer the weight of the slab down (true?) What keeps the soil under the slab / inside the foundation wall from freezing and heaving the slab?
I do have an appreciation for how much heat concrete traps in.
So with a slab poured without a foundation, i.e. sidewalks / driveways / patios, what keeps the slab from heaving due to soils expanding when frozen? What keeps the slab from moving away from the house which is a slab on grade?
And what I am really trying to understand is, if you poured a slab with pillars, say the round 18″ x 4′ deep cardboard forms, rebarred them to a slab for a small slab say 12 x 20′, would this keep the slab from moving away from the slab-on-grade? Would frost heaving move it? Do the pillars have any effect at all?
thank you very much. We have a professional engineer and work with civils, and nobody can give me a definitive answer.
remodeler
Replies
Normally would leave this to Gabe or another concrete expert, but stuck in the lounge at Copenhagen airport (2nd day, work 'slowdown by mechanics") and trying to stay just sober enough to get back on the airplane so will kill some time with an answer - there are 4 other engineers on the flight, but they are mostly software 'weenies' and don't know krap about the real world. .
With a standard footer/foundation wall/slab installation, the spread footer is determined on the bearing capacity of the soil and the foundation wall is to transfer the weight of the slab down (true?) Not really it is to ransfer the weight of the walls and second/etc floors to the soil, slab does not really enter into the equation by more than a few percent.
What keeps the soil under the slab / inside the foundation wall from freezing and heaving the slab? In Indiana (and most other places) what happens in real life is that the soil under the slab dries out to a near dry powder condition, so even if the soil under the slab in a garage is at -30C, there is little expansion as there is no moisture to freeze. (note; even in Seattle, with a water table at ground level, the soil within the footing is 'bone dry'). The soil under the footing is wet, so has to be below the freeze line.
So with a slab poured without a foundation, i.e. sidewalks / driveways / patios, what keeps the slab from heaving due to soils expanding when frozen? What keeps the slab from moving away from the house which is a slab on grade? You have stated the crux of the conundrum. The standard practice is for the sub-base to be a pourous material such as crushed rock or coarse sand where the granularity of the material can take the vertical loads but still leave room for any trapped moisture to expand within the trapped air spaces of the gravel/sand without the heaving - selecting fills is a science in itself, there is an Army corps of Eng. document titled "soil engineering" that covers this in detail, avai. from gov printing office.
And what I am really trying to understand is, if you poured a slab with pillars, say the round 18" x 4' deep cardboard forms, rebarred them to a slab for a small slab say 12 x 20', would this keep the slab from moving away from the slab-on-grade?
NO
Would frost heaving move it?
YES
Do the pillars have any effect at all? Only insofar as the thermal conductivity of the concrete heats the upper soil from a heave aspect, from a structural support standpoint the effects are large, but unrelated to frost heaving.
Hope my 'kill the time' comments are of some help, anyway, maybe will generate some other contradictory or other comments.
Junk, when I started reading your post I thought I would have to jump all over you after your disclaimer, instead I may just copy this post to my tips folder.
Maybe it's all those White Russians you had in the lounge. hehehe.
SamT
"Law reflects, but in no sense determines the moral worth of a society.... The better the society, the less law there will be. In Heaven, there will be no law, and the lion will lie down with the lamb.... The worse the society, the more law there will be. In Hell, there will be nothing but law, and due process will be meticulously observed."
Grant Gilmore, The Ages of American Law (New Haven: Yale University Press, 1977), pp. 110-111.From 32866.117
note; even in Seattle, with a water table at ground level, the soil within the footing is 'bone dry')
Thanks for the stab. Your above statement conflicts with something I have observed here - digging down on a foundation, and seeing water pour out of it. Water table 1' below grade or so. I think the water might have been coming from a spring under the slab and in the foundation walls - which are pretty common here too.
Is there a way to pour a slab, say porch etc., adjacent to a slab-on-grade, when one has a goal of not putting a foundation in for the porch slab, and not have them move away from each other or shift in elevation from each other? The pillars don't sound like the answer, more that they would transfer the weight to good bearing soil. Cost is the issue, spread over typically 70 houses in a subdivision section.
thank you again.
remodeler
Remodeler,
Water/frost heaving inside foundation walls: waterproof, insulate, gravel beds and install footing drains to help.
Open slabs: Don't compact the subsoil. excavate all topsoil and down at least 1' below bottom of slab and backfill with engineered soil/ crusher-run gravel. use silt cloth/landscaping cloth.
Is there a way to pour a slab, say porch etc., adjacent to a slab-on-grade, when one has a goal of not putting a foundation in for the porch slab, and not have them move away from each other or shift in elevation from each other?
No.
Plan for expansion joints, insulate under slabs, gravel under slabs, drainage around slabs, slip pins in the slabs at x-joints, will all help.
Frost heaving is an up and down motion for floating slabs. Up only for columns and poorly designed footings.
How good of a job can you afford? more $$$ = more quality.
Or I can let you borrow my magic wand, it seems to have quit working for me. |;>)
SamT
"Law reflects, but in no sense determines the moral worth of a society.... The better the society, the less law there will be. In Heaven, there will be no law, and the lion will lie down with the lamb.... The worse the society, the more law there will be. In Hell, there will be nothing but law, and due process will be meticulously observed."
Grant Gilmore, The Ages of American Law (New Haven: Yale University Press, 1977), pp. 110-111.From 32866.117