Concrete - How thin can I go . . .


I have a garage, 17' X 20' inside area. It is a converted carport . . . There lies the problem . . . The carport was poured with a "hump" in the middle running across the 20'. This was so the rain blowing in from front are rear would drain. Well now I have decided to make this area into my shop, I have a problem. I want it to be level. In the center the thickest I can pour is 1.5". Any thicker and it creates all sorts of problems, like doors not opening. As I go down the slopes it will get to be about 3.5" thick. The existing sloped slab is over 20 years old with no cracks. I have been told that if I pour it this way, it will crack . . . Any suggestions appreciated . . .
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On Mon, 1 Aug 2005 20:31:02 -0500, "Steve DeMars"

http://www.quikrete.com/catalog/ConcreteBondingAdhesive.html
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I have done it down to less than an inch and it works but I have a special procedure to ensure that it is bonded to the old concrete.

I
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RE: Subject
A 4" layer of concrete is considered little more than a dust cover.
If you want to do it to last, think at least 6" with mesh.
Lew
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An update.
I previously wrote:

Don't be surprised if jack hammering out the existing crowned pad and pouring a whole new, mesh reinforced pad doesn't turn out to be the least expense way to do the job.
Lew
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On Tue, 02 Aug 2005 04:26:21 GMT, Lew Hodgett

That was my first thought, but also because once done, the floor can be whatever reasonable thickness is required without the variation otherwise. I didn't suggest it since I thought the OP might object, so offered the other alternative of a bonding material. So, I'd go along with you that this is the best approach in the long run.
So, rent a jack-hammer and go for it. You might have to rent somebody to use the hammer, but again you can do that and then get on with what you really want to do, your woodworking.
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On Tue, 2 Aug 2005 06:44:18 -0600, Guess who wrote

If you pour over the existing slab with the dimensions you stated you'll need to use a latex additive in the concrete along with the bonding agent. Basically you need to mix up a giant batch of tile thin-set.
The only sure way to make it work is with the jack hammer.....
-Bruce
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You need to get the floor perfectly clean to start.
A few key holes to help the new surface lock in may be good.
You want to do it in sections. Leave the middle hump until last (thinnest section)
There are newer patching materials for concrete that can go to 1/2" with no problems but surface preparation is essential.
The cheapest method if you can do the work is to chip out the hump.
Note: If the slab has a foundation at the edges you do NOT want to break up the slab.
Consult a concrete worker in your area,
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On Tue, 02 Aug 2005 08:44:18 -0400, Guess who

Not too hard for a do-it-yourself job, provided you care to take the time. If you get the old slab out, it'd be a whole lot less expensive to have someone come in and pour a new one for you.
Just got done taking out a bunch of concrete that had been installed by a freeway contractor, and that stuff was tough (8" slab with 3/4" thick rebar every 14 inches or so- talk about overkill for a driveway!). Took a day to remove about a garage's worth of slab- the guys I work with said it usually takes an hour or two with a normal driveway, but this one was an irritating exception! Used a Bosch "Brute" hammer, and it worked like a charm- wasn't even sore the next day. Make sure you've got a prybar and some good leather gloves to get the concrete out, though. FWIW, I've never used a jackhammer before Monday (always did it the hard way with a masonry chisel and a sledge in the past), and it's about as simple as it gets.
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Do you realy want a "destroy-any-edge-that-falls-on-it" floor in your workshop? Consider a wooden or OSB floor shimmed level over the concrete.
--
Dr. Juergen Hannappel http://lisa2.physik.uni-bonn.de/~hannappe
mailto: snipped-for-privacy@physik.uni-bonn.de Phone: +49 228 73 2447 FAX ... 7869
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=========To convert a carport into a shop I have to agree.... I would much prefer to be standing on a wood floor all day then a concrete floor...
Can't remember the last time I dropped a chisel however...I just know my legs will feel better at the end of the day...
Bob Griffiths
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I poured a slab in my garage last summer, mine is about 5" thick at a minimum. From the research I did, 1 1/2" is pushing it, and I would be hesitant to go that thin. Can you maintain some of that pitch, and give yourself and extra inch in the high spot? You'd probably never notice such a slight pitch in the floor, it would also give you some drain pitch (might not be necessary) I pitched mine toward the garage doors. Also, I'd suggest you get fiber reinforced concrete.
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If you tear out the old floor, and replace it, 4 inch thick is fine, more than that is over kill. Rather than the mesh, go with #4 (1/2 inch) rebar about 24 to 30 inch on center. Mesh is a royal PITA, and only works if it is suspended in the center of the slab which is almost impossible to do unless you use the heavy sheets of it rather than the rolls. The fibermesh works well in helping to prevent cracks. By varying the thickness of the slab from 1 1/2 inches to 3 1/2 inches you can guarantee cracking. If you want concrete for a floor, replacing would be the best choice. Putting in a wood floor would be a good alternative for your feet and back. You could also shim the machines and benches to keep them level, and work with the uneven floor rather than replace it. robo hippy
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Talk to a concrete guy about levelling the slab. There are compounds which can be poured on top of a concrete slab to level it and can be as thin as a few millimeters. They can just put in a perimeter form and fill it up until the hump is covered. It is more $ than ordinary concrete, but if your existing slab is sound then it would be dumb to remove and replace it.
-j

I
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