Thickness of Concret Slab

Old 10-10-08, 11:43 PM
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Location: Rio Grande Valley, Texas
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Thickness of Concret Slab

I am going to have concret poured on a small lot as a pad for a 38', 27,000 # motorhome and a "Texas Room" in the Texas Rio Grande Valley. Many other similiar sites in the neighborhood have cracks in the concrete. 4" of concrete seems to be the normal debth with wire (no rebar). I have been told that 6 inches of concrete would be better. Others have suggested that fiberglass be imbedded in the concrete (but the developer says fiberglass tends to stick out of the concrete).

Would the added cost of 6" verses 4" of concrete be worth it? And trhe concrete guy tries to avoid fiberglass. Should I insist on it? I would like to avoid ugly cracks.


Steve Turney
Old 10-11-08, 05:20 AM
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Four inches would be sufficient IF:

1) the stone or sand base is properly compacted by the contractor.
2) a good grade of concrete is used. I would suggest at least 4,000 psi. Only coarse fiber would be seen in the finished slab. I use a fine fiber called Nytech which is invisible when finished. BTW, many people mistakenly say fiberglass. It's really nylon or polypropylene fibers that are added to concrete.
3) rebar would be beneficial, but not mandatory. It's not that expensive to put some in there though. You don't necessarily need a 2x2 grid tied or anything, just a few long pieces laid in each direction and supported so it's up in the slab, not laying on the bottom. Make sure there is a piece within about 6 inches of the perimeter of the slab. If wire mesh is used, make sure it's supported as well.
4) crack control joints a minimum of 1 to 1.5 inches deep are put into the slab. This makes the concrete crack in the joint instead of randomly across it.
5) the concrete is allowed to cure sufficiently before driving on it.
If all of these conditions are met, 4 inches should do you. Good luck!

Old 10-11-08, 08:05 AM
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Thickness of Concret Slab

Fibers (glass, poly, etc.) in the concrete are really not reinforcement and are not a substitute for rebar. There are only for micro-cracking. My last personal driveway had both welded wire mesh (6/6 10/10 or similar) and poly fibers. If I had to eliminate one, I would definitely eliminated the fibers and not the wire. - There is nothing wrong with using a few rebars around the perimeter, but try not to run them through where the joint will be sawed.

The sawed control joints (1" to 1.5") are very important, since if there is any cracking, the location will be controlled and occur at the sawed joint where it is not visually objectionable. Saw the joints late the same day as pouring or very early the next day, since it will be much easier. After a few weeks, you can caulk the joints to prevent dirt accumulation, ants and provide a more attractive floor.

Regarding the thickness, going from 4" to 6" is just some extra material. The cost to place it will be a little more, but the labor for finishing ot will be the same.

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