getting ready to cur footings and pour a slab

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Old 06-07-06, 05:39 AM
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getting ready to cur footings and pour a slab

Im in Arkansas. no permits or inspections required. I have brought in 22 dump truck loads of shale to flatten out my land (its on the top of a hill so the edge of my property drops off). The shop will be 30x50. The flat filled area is 6' larger that the building and tapers off at a 35 degree slope or so.

We spread the dirt using a dozer and Ive been compacting it for the past month or so by driving over it with my truck, a backhoe - whatever I'm driving at the time. Had a couple of good rains and it seems pretty compact.

So now I'm ready for footings and the slab. Im going to cut a footing and drop some rebar into it, probably 2 rows around the perimeter.

My concrete guy (a friend too) said that I dont need a rebar grid across the slab and that I just need wire mesh on top of plastic (visqueen? sp?).

Will wire mesh be enough? how much is rebar if I wanted to put that down? If I wanted rebar, what is the grid? 24" or something else?

the slab with will poured at the same time as the footing.
 
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Old 06-07-06, 06:31 AM
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I'm not a concrete finisher or builder but have been around construction most of my life. IMO rebar is better than the wire mesh and not just because it is stiffer. Often when the concrete is poured the workers fail to pull the wire up and if it sets at the bottom [or under] of the slab it isn't very effective.

I also think it is worth spending the extra money for the fiberglass additive. The pros should be along later and I'm sure they will have better more detailed advice for you.
 
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Old 06-07-06, 07:04 AM
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the good thing here is that I will be working along side so I can make sure certain things are done. I heard a good tip for raising the rebar for the footing up is to use concrete bricks standing on their side.

I figured I'd use the same method for the wire mesh (or rebar IF consensus says it is required)
 
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Old 06-07-06, 07:48 AM
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Rebar around the perimeter certainly wouldn't hurt, and a couple of more on the approach apron, along with the wire.
 
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Old 06-07-06, 08:37 AM
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a guy in another forum asked whether I was going to put down compacted stone before the vapor barrier. My concrete guy never mentioned that (nor did anyone else I spoke to out here).

is that something thats required? if so, why?
 
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Old 06-07-06, 10:19 AM
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I am NOT a concrete professional, so please take my comments for whatever you think they are worth. I am replacing the slab and foundations of my detached garage, mostly a DIY job, so I have read a good bit about concrete work.

It sounds like you are planning what is called a "slab on grade" pour. That's how my garage was built, and if it had been done right it would have been fine. However;

-no rebar was used
-slab only 4.5" thick.
-"stemwalls" only about 4" thick, with NO below grade excavation or footer.

The floor has numerous MAJOR cracks and the walls are slumping outward. Got no choice, has to be fixed.

A full-blown professional concrete slab and foundation installation prep would include the following steps:

Excavate the site. [Because you are working on an area of recent fill, check the compaction of the soil at the bottom of the foundation trench carefully. You may have compacted the surface well, but 3 feet down could be a different story. You may need to excavate holes for piers down to undisturbed soil.]

Emplace a layer of compacted crushed rock and/or sand, to provid a level, stable base and drainage for the slab. Various writers recommend different depths, usually 6"-10".

Emplace perimeter forms.

Spread vapor barrier.

Rebar: Mainly used to improve tensile strength of slabs and foundations walls. Concrete has terrific compressive strength, but is relatively quite weak in tensile strength. For garages, usual recommendations is 4" slab with rebar, or 6" without. 18" is widest spacing I can recall seeing recommended.

DON'T recommend using brickbats to supprot rebar. It's cheap and easy, but each one will be a weak point (much thinner slab above it) and a focal point for failure (cracks). They make a special wire cradle for this purpose. I have recently been reading that plastic ties can be used on the rebar instead of wire (which is a pain), and I like the idea.

Wire mesh is used to control/minimize surface cracking. The fiiber-reinforced mix might not be a bad idea instead.

For this big a job, hire an experiencd crew. Unless you have worked with it, you cannot comprehend how heavy wet concrete is, and how much work is involved in finishing a large slab properly before it sets up. Grooves at least every 10' (to control cracks) are recommended.

You may be willing to assume the risk of skimping on any of these steps, but know that doing so involves the risk that the slab may not hold up as it should.
 
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Old 06-07-06, 10:42 AM
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getting ready to cur footings and pour a slab

Concrete brick are adequate to support the rebars (concrete bond to concrete well). The strength loss is negligible for the very few points. The wire chairs have a tendancy to get pushed into the ground if the rebars gets stepped on (and they will), which defeats the purpose. - Its your call-

Saw your control jopints the next day or late the same day if the concrete is cured enough and does not ravel. Make sure the cuts are deep enough to do some good. It is much easier and cheaper to saw if you do not wait too long. After a week or two, depending on curing conditions, caulk the control joints.

Dick
 
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Old 06-07-06, 11:39 AM
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excellent! thanks for the replies. One last question, what size rebar? I hear #4 and 1/2". Same size rebar for footing as well as under slab?

and how far apart do I place the blocks to keep it up (i'm trying to buy materials)
 

Last edited by aarcuda; 06-07-06 at 11:54 AM.
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