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Old 07-17-2016, 03:33 PM   #21
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Concrete Pad

I went with 6000 psi on the pad with rebar and 4000 psi on the drive. Probably overkill but it didnt cost much more. I also put a drain in the middle and ran extra conduit under for future wiring.
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Old 07-17-2016, 05:41 PM   #22
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I can assure you that there are as many thoughts on this matter as the number of folks that are asked. I can offer a few more from over 40 years as a structural engineer...

It is the nature of concrete to crack, we can only control it. Concrete shrinks as it cures, so that is done either by providing control joints that are usually tooled-in or saw-cut, or restraining the shrinkage with an adequate amount of rebar. The rebar in slabs on grade will not prevent cracks, but keep them tightly closed so as not be a problem. If rebar is used, it should be close to the top with about 1.5" of cover.

I feel the better option is to use control joints to provide a place for the concrete to crack. A rule of thumb spacing is not more than 20x the slab thickness and closer is usually better, otherwise a random crack could form in between. If the joints are saw-cut, be sure this is done as soon as the concrete can stand it, usually the next morning (within 24 hours). Otherwise, the concrete has already determined where it will crack.

A someone mentioned, 4000 psi at 28 days is a good choice. Ask for a light broom finish for skid resistance and apply a membrane forming liquid curing compound as soon as possible after finishing. If this can not be done, it must be kept continuously wet for several days. Proper curing is an absolute must for durability. Since this will be exposed to freeze/thaw cycles, require an air entrainment admixture be added to prevent spalling later on.

As for a plastic sheet (vapor barrier) beneath the slab, that is normally used for interior slabs to retard moisture migration into the floor finishes and should not be used for exterior applications. When the slab has begun curing, excess water has to leave because only a small amount of water is needed for the chemical reaction to take place. Only the least amount of water should be used to make it workable, it should never flow on it's own.

Excess water leaves the slab from the top and bottom, otherwise the slab may curl upwards at the joints as seen in dried clay mud puddles. For this reason, crushed stone is necessary under the slab to provide a place for water to go. If at least 4" of compacted aggregate base course is used, a 6" 4000 psi slab should be fine. Approximately 2/3 of the 28 day strength will be seen in seven days of curing which should be ok for moderate wheel loads, but if it can wait that's even better.

Those are my main points and others have some as well based on their experience, but concrete design and placement is as much an art as a science. At times it seems to have a mind of it's own regardless. Good luck.
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Old 07-17-2016, 05:58 PM   #23
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In Canada, we use 35 MPa concrete for structural applications. That equates to around 5000 psi. I wouldn't use anything less than 4000 psi.

Rebar is used for tension loads in a suspended slab so for a slab on grade I'd say you can do without the rebar and just use the steel mesh, although my FIL used prestressed steel in his garage floor just because he worked for a contractor that does prestressing.

I agree with other posters about drainage and good strength in the granular base materials. Your granulars may be a lot different than ours but I would look for a well-graded crushed material that drains well, not a sandy material. If that's cheaper, use 6" of the crushed material immediately under the concrete slab and use the sandier material below that.

If you live in an urban area, you may be able to get some free advice from the road designers at your town/city hall on what they use for an industrial road. A Class A MH will need a strong base to sit on for extended periods.

Good luck with your project!
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Old 07-17-2016, 06:42 PM   #24
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Ample expansion joints are the only way to ensure it wont crack anywhere else. Fiber and 3500 are good insurance too. The base that the slab rest on is key.
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Old 07-17-2016, 06:44 PM   #25
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WOW someone who knows what he is talking about, REFRESHING!
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Old 07-17-2016, 06:45 PM   #26
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Hexnut!
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Old 07-17-2016, 07:03 PM   #27
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First off the base is important remove all organic material, I like to use 6 inches of compacted rock (2 -3" lifts water and compact) of 411 crushed stone
I would not use rebar but go with the fiber only the rebar is a waste of your money period
Use a 6.5 bag mix or what is called class C
One thing that is that is a for sure concrete WILL crack you cut it in 10 x 10 sections so it cracks where you want it.
U ope this helps
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Old 07-17-2016, 07:05 PM   #28
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Our drives were poured with our heavy equipment company and trucking needs in mind.
We used 6", 3500psi cement with another 6-8" of thickened edge, piers anywhere there was fill and then 1/2" rebar mat that was no more than 18" between bars. The rebars were also drilled and epoxy filled into the existing shop slab. We used a broom finish with minimal saw cuts and picture frame edge. We have sandy soil naturally, but we wet it and compacted it. No slab cracks in the 6 years since it was poured.

(Our shop has 7-8" slab with 11.5" thickened edge, 1/2"rebar and 18"x18" footing. No saw cuts and 22 chain floor pots. I think there are 2 or 3 hairline cracks in it. Pretty good considering we had tons of rain right before that pour.)

(I hate fiber mesh, go crawl under your bus or truck on that and you will too.)
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Old 07-17-2016, 07:32 PM   #29
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What is this fibre mesh everyone is talking about? We use either rebar or steel wire mesh reinforcement. We've experimented with loose fibreglass fibre in concrete and it didn't work very well. Any kind of a synthetic fibre mat would be a geotextile product?

Many posters have mentioned a type of granular material but my experience is that the material has different names depending on where you are. The OP may not be familiar with the names that some folks are using to describe the granular material they're referring to (e.g. "411 crushed", "6-6-6", we call it "Granular A" in Ontario).

I'd go with RubberNail's recommendation on the concrete.
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Old 07-17-2016, 07:38 PM   #30
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You could pay and engineer to write a spec and test the concrete for air, slump, etc. at the site. They can also pour cylinders and do test. That would give you confidence that is was done correctly and have some insurance.
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