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Tool Talk Discussion Forum

Concrete slab specs

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Bob

11-17-2003 17:45:45




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This is what the contractor is recommending, tell me what you guys think. 40'x100' Metal building already erected, 3/4" crushed rock on grade in place. Want to pour a level 6" slab with fiber. Wirsbo 5/8" radiant tubing going in on 18" centers 250' loops. With 1" insulation under slab (6 mil plastic under insulation). 1/2" rebar grid on 3 foot centers. Going to pour 20 foot by 40 foot sections at a time because of limitations with the building already erected and getting the concrete placed. This means 5 seperate pours. Control joints sawed leaving a 10 foot by 10 foot grid. Does this sound like good recommendations? Especially the rebar on 3 foot grid? This is going to be a farm shop so the weight loads will see occasional loaded grain trucks. Thanks for your insight.

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the tractor vet

11-20-2003 20:24:36




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
It has been years since i poored a slab , but the biggest one was a 40 x72 for a large truck garage for Coal Buckets , we put down a 4 inch base of 57 lime stone and about 15 cases of beer cans, heavy wire mesh and 8 inches of 6 1/2 sack mix and after 4 days we backed in three 6 axel semi dumps with the lightest one weighing around 130000 and to this day sorry to say there is one crack back buy the furnace . If i was to make a pour today for a shop i would not use fiber as ya can never get a smoothe finish and make it had to sweep.

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Ray,IN

11-20-2003 20:10:56




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
I'm negotiating with a builder now for a new shop building, and they say not to put plastic under the concrete. It prevents the concrete from curing properly on the bottom. Thier solution to preventing dampness is top sealer. Supposedly the water settles to the bottom and just sits there. Anyone ever heard this theory?



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Mark

11-20-2003 04:25:19




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
I just did a ton of research on this, since I just poured a 40x64 floor in one shot with mesh, fiber, and pex tubing for a shop. First of all, the pex tubing on 18" centers is really pushing the limit. I would recommend 1/2" on 12" centers at least on the outer perimeter a few feet. 18" centers is likely to have hot and cold spots. Also, you need to have enough tubing in the floor to get enough heat into the room. Your heat from your floor can only be radiated at an amount proportional to the heat of the water going in. To get 10-15 btus per sq foot, ( a typical farm shop value) you need about 90-95 water into the slab. You don't want the slab surface temp to go much over 85F, as it becomes to hot to stand on. If you don't have enough tubing in the slab, you won't be able to get the building to your design temp without cranking the heat in the pex, and you will be uncomfortable. I know the pex is expensive, ( i used 2000 ft on my shop.) but you will be able to heat the shop better. And as far as the floor strength, subsurface prep is very important. keep it compacted and you'll be fine.

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ty

11-19-2003 18:05:33




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
My dad built a shop in 1981. He poured the floor 4" thick with wire mesh. I told him he was crazy but after many years with loaded dumptrucks (74,000#), loaded tractor trailers (80,000#), and a 60,000# forklift (40,000# on one axle) there is not so much as even a crack. I don't think they make concrete like they used to though. It never gets cold enough here to freeze under a slab of any size so yall my have problems further north than we have here (Tennessee).

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VaTom

11-19-2003 19:26:49




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 Re: Re: Concrete slab specs in reply to ty, 11-19-2003 18:05:33  
Naw, he wasn't crazy, just paid attention that the concrete had something firm to sit on. I poured a concrete drive that has had a great many loaded concrete trucks travel over it. It was 4" with no wire mesh. Certainly no rebar. This is outside in Va where we do get some winter and snow every year. If it didn't have something firm to rest on, it wouldn't have lasted. Concrete's very strong in compression, needs help if you put it in tension.

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Gary

11-18-2003 18:52:45




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
Heavy loads such as loaded grain trucks require at least 8" thick, 4000 psi concrete with two mats of #6 bars 12" OC both directions. Don't cheat on the control joints. Forget the fibermesh. Also, when ordering redi-mixed concrete from your supplier request test cylinders with each load. You don't necessarily have to use them but knowing the cylinders are there and the possibility of having them break tested keeps the supplier honest. They cost about a $1 each and make nice containers for fasteners, etc. Use the proper admixtures if your doing cold weather pours and provide proper freeze protection afterwards. Don't place over frozen ground. Stay off concrete with anything heavier than foot traffic at least 28 days.

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Fred P

11-18-2003 09:07:53




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
I put 1/2" rebar on 2' centers and the heat piping also on 2' centers. That way you can tie the pipe to the rebar. Other wise the pipe will want to work up to the top. It helps to fill the pipe with water before you pour the cement.



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jd520

11-18-2003 17:41:27




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 Re: Re: Concrete slab specs in reply to Fred P, 11-18-2003 09:07:53  
they also make a real nice plastic clamp[ it screws into the foam board and clamps around 1/2 '' pex ] red in color but i do not know who make them. they wok very good! you do not have to worry about rebar rubbing a hole in pex or about the tube floating.



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rhudson

11-18-2003 06:13:07




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
don't know anything about fiber. but we spec all industrial floors: 6", 5000 psi compressive strength concrete, single trowel finish. two matts of #3 rebar, 14 inches on center both ways. bottom matt to be two inched off grade, top matt to be two inches below surface.



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Greywolf

11-18-2003 04:42:05




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
I can't comment on the rebar, fiber and such. But I can give personal experience on how a slab holds up.

30 yr old slab, (well actually a complete yard,not a slab). 1000 yd pour was done in 16' wide strips about 200 ft long. 6 sack mix. No rod, fiber, nadda. Was originally poured as cattle feedlot 2 X 4 thick (3.5"). Base under is a sandy clay.

No cattle since 1980, our grain trucks and the last 4 years my semi travels over the pad hard in the fall and winter (some loads will hit 87,000 GVWR 5 axle), but not the summer, then only farm equipment ( no 4WD biggest is Case 2590). There are no expansion joints cut or placed. Each 16' slab is poured next to the existing.

In a few places, there is breakup ( assuming some settling underneath for the cause) but for the most part all is suprisingly intact for the use it gets. This is in southern MN so freeze thaw is present.

I have been told that rebar will eventually deteriate the pour because of rust, not sure if its true or not. But I do know of some using cattle panels for rebar instead, cheaper (I'm told) and with a 2 X 4 grid pattern, better reinforcement and the galvanizing standing up better.

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WD-9

11-17-2003 19:54:38




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
I would recommend leaving out the rebar, the fiber will be sufficient support. Make sure your base (crushed stone) is well compacted with no "spongey" locations.
Sound like a great plan to make a pleasant working environment, good luck.



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Butcher

11-17-2003 18:23:33




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 Re: Concrete slab specs in reply to Bob, 11-17-2003 17:45:45  
Sounds like your contractor is a good one. As an old worn out concrete man, the only thing I would do differently would be to forget the fiber and use 5/8 rebar two foot on center. Either way it sounds good. Make sure they cut the stuff at every post. Concrete cracks anywhere there is a corner. You can not have to many control joints. Also, make sure they connect those 20 by 40 pours with rebar.

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Slowpoke

11-17-2003 22:00:53




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 Re: Re: Concrete slab specs in reply to Butcher, 11-17-2003 18:23:33  
Butcher...Will this work...6 mil vapor barrier under 2"-3" of sand on sandy clay, undisturbed ground, 3" dobies under 1/2" rebar on 16" centers, 6" fiber reinforced concrete, one pour, 42x80'? Slab will be on grade.
I found that much over 16" centers will bend 1/2" rebar when stepped on. Ground gets concrete hard in the summer, and does crack somewhat. When wet, it's easy digging, but cars and trucks don't sink in unless parked for several weeks.
1/2" rebar is $2.85 and up + tax in central Ca. It sure takes a lot to cover the floor area.
I also have some 5/8, 3/4 & 1" rebar which will be used in the center area. I guess those can be 24" oc. BTW, how much rust is acceptable on rebar?
Thanks for your input.

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Butcher

11-18-2003 17:31:47




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 Re: Re: Re: Concrete slab specs in reply to Slowpoke, 11-17-2003 22:00:53  
If you are going to place a vapor barrier under your crete I would put on top of the finish grade. As far as the rebar goes, rust dosnt hurt a thing. The main reason for reinforcement is so that when the concrete cracks at your controll joints (and it will crack esspesially here in north east Iowa) that the sections stay together. There is nothing worse than haveing 1 10x10 section sticking up 1/2 inch past the rest.
As far as bending the rebar when stepped on, You just need to tell your help to stay the H##! off of it.

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