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  #16 (permalink)  
Old 08-29-2004, 07:41 AM
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Well, I can only draw from the experience that I had with building my own 30x40 and alot of buildings that I have seen. First thing I am wondering is if this is a pole barn or an I beam barn. With I beam I have never seen any kind of expansion joint at the post. For mine I set 1/2 weld plates with 12 inch j hooks for welding the posts to. I am also kinof wondering about the saw cutting on a small slab like that. I work in commercial buildings and schools so I see all the saw cuts in the large slabs like that. In small shops I have not seen it, and for that matter I have never seen a cut slab that did not have cracks right through the middle of them. Again....not a pro, just some observations.


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Old 08-29-2004, 07:49 AM
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Edge- You're right soft cutting is the best, that's what I almost always do. Sorry, I'm reading my post and it sounds like one should wait 15-24 hours to saw cut. I figure once the slab isn't marking anymore (hard enough not to leave marks with the saw or by walking on it) you can cut it anytime up to 24 hours from the sart of the pour. After 24 hours I figure it's a waste of time, but some guys do it even after that. In some of the residential sites I've been at they never soft cut. It's always the next day or two days later. They're pretty adimant that what they are doing is right, I still disagree.
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Old 08-29-2004, 09:18 AM
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I'd just like to add one thing. You said your base was packed clay. Clay isn't a good substrate becasue it holds moisture, then when it freezes it exapands. I'm from Ontario, up here we put up to a foot of 3/4" clear limestone under our slabs. This allows water to drain and not freeze right under the slab.
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Old 08-30-2004, 02:35 AM
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If you have the room, add a barrier of 1/2" - 1" styrofoam underneath before you pour the slab. This will act as a moisture barrier. And if whoever pours the slab, specify that the corners are higher than the middle. Around here, ob a 24' x 24' garage they will slope the slab 2" from back to opening to allow for any water drainage. The way that they do this is strike a line in the back, drop it down in the front, and pour to the line. It almost always seems that the two corners are lower than the midle, so when ever you wash out the garage the water will pool in those corners. No big problem unless you have a workbench, fridge, or something sitting there.

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Old 08-30-2004, 10:16 AM
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I dunno where u guys come from, but Expansion joint are a MUST......But it depends on it your Building is Metal or Block. If Block then DOWS have to be placed with the proper guage re-bar....We did a garage for this guy whos concrete was pressure rated 4,800 PSI and had dows every 3 foot....and he still had Expansion Joints in his Concrete........Point being not ALL concrete cracks, my grandma has lived in her house for 40 years and the concrete there is just fine, But to avoid a MASSIVE crack throught the whole entire slab, then u will want expansion joints.
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Old 08-31-2004, 06:42 PM
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Finishing that slab

A few more tips from an ACI (American Concrete Institute) Certified Field Tester and construction Quality Assurance manager:

If you use wire, don't lay the wire on the ground, pour over it, then let them pull the wire up into the concrete. Support the wire with plastic "chairs". Same thing if you use rebar.

Use a low slump mix - 6" max - the concrete outfit will know what you're talking about. It's a little harder to work, but will be lots stronger. And, do NOT add much IF ANY water to the mixer truck. You get too high of a water cement ratio along with the higher slump which weakens the concrete.

A common mistake is to get on it too quick with the power trowels. Working it too much brings too much of the paste to the surface which later will crack and spall and flake.

To cure it, immediately cover the whole thing with plastic (make sure it's a little wet underneath or at least moist) as soon as the sawcutting is done, and do that as soon as they can roll the saw on it, and leave it tightly covered for 7 days. If you let the surface dry out, it will shrink crack and the surface will not cure to it's full strength.

When you pour it, do not do it in direct sun and/or windy hot conditions. The surface will dry out too much before you can sawcut and cover it.

Make sure whoever pours the concrete uses vibrators to get all the voids out especially near the forms and in corners, etc. For posts/columns, you can also use round heavy cardboard sleeves they make just for that - that eliminates corners where the cracks are more apt to occur. Also, tie lengths of rebar in an interlocking pattern all the way around your column blockouts.
Also tie a length of rebar into the mat across the points of any inside corners.

Another thing you absolutely want to avoid is a "cold joint" such as you'd have if a truck arrives too late after the previous truck is poured. That's when the concrete starts to set and the next load cannot be vibrated ("consolidated") into it. If a truck is absolutely unavoidable going to be too late, have them build a control joint, and it's best to dowel the two sides of a control joint together if possible.

Depends on where you're at, (especially the southern 1/2 of the country) you might want to lay plastic on the ground which can prevent moisture from coming up thru the floor and making the floor sweat in the spring and fall.

If you want a real smooth finish, have them put discs on the power trowels for a "burnt" finish.

Lastly, after you uncover the slab, treating it with Ashford Formula (that's a brand name) will considerably strengthen the surface and make it much more durable. Any concrete supply place ought to be able to get it. All you have to do is pour a thin layer onto the finished floor, broom it around and let it set awhile. Follow directions.
Makes a chemical reaction with the cement that makes it much harder and durable - this is often used in warehouses with lots of heavy forklift traffic and it wears well for years.

Do it right when you pour it and you will have years of trouble free service. Disregard these "good practices" and you are likely to have problems in short order and be unhappy with it forever.

Good luck!
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