slab thickness

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RicodJour wrote:

Absolutely correct, but the problem is that you can't guarantee that something won't move. Thus a little reinforcement is, IMO, cheap insurance.
Matt
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Matt Whiting wrote:

Much like the single guy with no dependents buying life insurance. It's cheap, but is it necessary? Too many people think that it is a panacea for all problems concrete. It's not. Just like painting, slab on grade results are all in the preparation.
R
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RicodJour wrote:

Not a good analogy at all. I've seen many slabs crack and shift up to an inch even with what appeared to be proper preparation. And there is a major road nearby that hs shifted nearly 6' in one set of lanes of a divided highway. I'm sure the construction company thought they had prepared the soil properly also, but my point is that you can't always be sure.
Matt
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scribbled this interesting note:

In our area, with the soil we have, it is impossible to "correctly prepare" what is underneath the slab. The black, clay rich soil we have expands and contracts so much, I remember, as a child, having soccer games cancelled because the cracks in the hard-as-a-rock dirt were so large a kid could have broken an ankle or leg!
In this area, post tensioned slabs always move and crack. Any slab that is built to FHA minimum standards will crack. Those standards must be exceeded by a fair margin to insure the slab stays where you want it, as you want it.
I wish this area had soil that was easier to build on. Doing any kind of digging around here is a major chore.
-- John Willis (Remove the Primes before e-mailing me)
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Bob Morrison wrote:

Yes, unfortunately, a soft spot can go quite deep and be sizeable. It may not even be noticed by most excavators. Unless you dig down 6' and fill with gravel, it is hard to be sure that you won't have problems in the future. This depends greatly, obviously, on location and soil type. Here in PA, it isn't unusual to have challenging soil conditions. I figure a few bucks worth of rebar is cheap insurance. You still want proper preparation, however, I like suspenders with my belt for things that are hard to fix later. And cured concrete that has cracked and heaved an inch isn't cheap to fix.
Matt
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Quoting from my recently purchased "Pocket Ref" book, the recommended thickness for driveways is 6 to 8 inches. Garage floors are 4-5 inches.
Putting in a driveway is no small chore. Or expenditure. I tend to make things stronger and thicker than the minimum. A few hundred bucks spent now could save you some major work and expense later. And, making the forms a couple of inches thicker now is no big deal. Materials can be taken out, or the forms merely made a little higher.
Think long term.
DO IT ONCE. DO IT RIGHT.
Steve
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SteveB wrote:

Steve...what state do you work in?
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Just an observation based on my poured driveway. Had a contractor who normally does commercial work do it, they were slow last winter.
They used 3/8 rebar, tied in 16" squares. The perimeter was 6" thick. Foreman said the thicker perimeter serves two purposes. 1. Like a house concrete perimeter, serves as a beam. 2. Prevents water runoff wash from undermining the slab. The driveway itself is 3" thick. Simlar technique on the apron at the street. No frost line in the area, too warm a climate.
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scribbled this interesting note:

That perimeter bean serves another function as well. A floating slab will do just that...float. That perimeter beam helps keep the monolithic slab exactly where it was placed and it no longer wants to float around on the surface of the ground.
-- John Willis (Remove the Primes before e-mailing me)
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4" is fine if you use 3000lb concrete for slab on grade. 8-10" is for foundation walls.

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