Rebar in concrete slab

coshoo · Mar 6, 2018

Building a shop- 4" slab, will have nothing heavier on it than an automobile (I have a different shop for tractors and heavy stuff). I was thinking 3' x 3' cross hatch of 1/2 inch rebar- what say the experts here assembled?

Oliver 500 Wa · Mar 6, 2018

Not claiming any expertise....

You'll get better results from having the sub-grade properly compacted. 90-95% density. If you did both then you are even better off.

ErnieD · Mar 6, 2018

Any chance of snow/ice melt water with salt? What is sub soil like?
Reinforcing steel will rust from salt and cause slab failure. Use
epoxy coated steel or no steel and thicker or higher psi. Plant had
6" floors on gravel. Wire mesh on bottom (worthless)cast in 14' square
slabs. Held up well to 4T loads on 6T forklift traffic. No idea on psi
rating or additives.

docmirror · Mar 6, 2018

Concrete is a whole system. Not just the rebar. Start with proper compaction of the soil underneath. If you are on decent hardpack then worry about fill. Use the right kind of fill, and compact that. Your rebar spec sounds fine, but the mix of concrete you get will determine how long it lasts. For just car support, 3500PSI pea gravel will do the job. HOWEVER - almost all the time when you order small batch load of concrete from the local vendor, you are not getting actual 3500PSI. You are getting something less. Which means you need to order 4000PSI and tell the vendor you are going to do a slump test on it(google concrete slump test). This will let your concrete vendor know not to cheap out on the cement in the mix.

I'm not a concrete expert, but I've done enough work to know that I've gotten a bad batch a few times, and it really irritates me. My boat ramp was 5000PSI and I had to send one load back. It was far too wet, and way to loose. They don't like that but too bad, I'm not paying for junk that will split after 4 years.

Janicholson · Mar 6, 2018

That (at 4500psi mix, and not wet) would be best with fibermesh in it. Opinions vary, but it sure reduces cracking and edge damage. The rebar should be coated to prevent rust spalling. 12' to 14' squares max. Jim

old · Mar 6, 2018

If it was me I would do a true foundation around the slab and also find the foundation with gravel and use concrete wire in the slab. Foundation down to or below frost level in your area

kyhayman · Mar 6, 2018

We've gone to footers with rebar bent to 90 degrees into slab, below frost line, wire in the slab on saddles, and single fiber mesh for trowel finish and double fiber for broom finish. Control joints on 8 to 10 foot centers, #8 crushed stone under the slab.

Billy NY · Mar 6, 2018

I would second the footing or foundation suggestion, and decrease the spacing on your deformed metal bar reinforcing if can be afforded, 3' x 3' is a bit wide, 1/2" bar is fine. 6" x 6" woven wire mesh would work. When building a slab, consider future or change of use, no harm in overbuilding a slab, pay now or pay a lot more later. Also consider all the mechanical, electrical and plumbing you need and anything of the like for future use if need be.

Traditional Farmer · Mar 6, 2018

Slump test is for testing how much water is in the mix,lower the number equals less water.Slump is how much in inches the concrete goes down after the slump cone is packed and then removed.
If you order 3500 you should get at least 3500 and that is if you keep it around a 4" slump,if you let the finisher put in enough water to raise it to a 6" slump then you won't have
3500 anymore as too much water kills the strength of concrete.The best way to pour concrete is use a Plasticizer you should get the concrete with a 3" slump then put in the
plasticizer and it'll take the mix to about 7" slump for about 30-45 minutes time to get it off the truck and worked down then the concrete starts to go back to a 3" and you'll have a good strong
slab.

flembo · Mar 6, 2018

They did mine a floating slab 4" in the middle and 12" wide x 9" deep around the perimeter. They used wire mesh and fiber reinforced concrete been 12yrs. and no problems I also went with 3 blocks high on the perimeter to raise the building up for a 10' ceiling and 9' over head doors. I don't remember how deep we went with the compacted crushed stone and gravel. The building is 24' x 30' I wish I went bigger I have already added a 20'x 10' shed addition of the back and a few yrs. and 2 more tractors later I added another 20' x 16' lean-to tractor shed on the back of that.

rustyfarmall · Mar 6, 2018

I would go at least 6 inches. Do it right the first time and you'll never need to do it again.

David G · Mar 6, 2018

4" is fine thickness, I would do wire with rebar on the outside.

Michael Soldan · Mar 6, 2018

I did a 24x24 shop, 4" pour on wire mesh. When set I ran saw cuts down the middle each way. The slab cracked along the sawcuts perfectly the first winter, have never had a problem and that was almost 40 years ago

Andy Martin · Mar 6, 2018

(quoted from post at 11:17:59 03/06/18) I would second the footing or foundation suggestion, and decrease the spacing on your deformed metal bar reinforcing if can be afforded, 3' x 3' is a bit wide, 1/2" bar is fine. 6" x 6" woven wire mesh would work. When building a slab, consider future or change of use, no harm in overbuilding a slab, pay now or pay a lot more later. Also consider all the mechanical, electrical and plumbing you need and anything of the like for future use if need be.

A Civil Engineer who I really trust designed a slab for me for a friend's barn, 6" for small equipment (10,000#). He specified #3 on 36" centers. We went ahead and did that and have had no problems. It sure looks skinny but costs a lot less.

Russ from MN · Mar 6, 2018

It depends on the base. 4 inches is fine, but I believe in going much thicker around the outside, 12 inches and a rebar close to the top and bottom.

Ellis Kinney · Mar 6, 2018

6 inches thick and re-bar 12 inches on centre. 4 inches thick and you may as well not bother. At least 4000 mix. Be prepared to saw the next day. Ellis

Andy Martin · Mar 6, 2018

(quoted from post at 14:24:18 03/06/18) 6 inches thick and re-bar 12 inches on centre. 4 inches thick and you may as well not bother. At least 4000 mix. Be prepared to saw the next day. Ellis

For cars? Actually 3-1/2" of 2,300# concrete would do the job fine, but it's harder to finish. When a guy says he wants to use it for automobiles I don't see the value in doubling the cost. Of course, I don't shudder when I see a crack in a floor, either.

Traditional Farmer · Mar 6, 2018

2500 is footer mix,3000 is the minimum that should ever be used for a slab

Phil in Pa · Mar 6, 2018

I do industrial slabs for a living. For car traffic, my suggestion is increase the thickness to 5" and use fiber mesh in the mix and forget the rebar. Sawing control joints asap is critical to avoid cracks.

jm. · Mar 6, 2018

I have several building with 4 inch Placed on a good solid prepared footing and have never had problems even using tractors, ton trucks ect. I think you are over doing a 6 inches. Most folks that use rebar never get it up in the concrete. I had rather have 3,000 lb mix with fiber on a solid prepared surface. Four inches thick and you will not have any problems.

wilson ind · Mar 6, 2018

Agree only fiber, no rebar. 2x6 form boards gives 5 1/2 thickness. Easy to measure. Just water on sand seems to give enough compaction done couple days in advance. Remember adding water to the mix thinking make easier to work will weaken the pour. Nothing wrong with using 2x6 in middle to help level them pulling board makes work much easier than grade stakes, at least for me.

David G · Mar 6, 2018

Tell me about the fiber mesh, what is that?

Hoenes · Mar 6, 2018

There are "standees" you can buy, just for that purpose. Looks like a spider with a saddle.

Hoenes · Mar 6, 2018

I had bad luck with epoxy coated rods in my driveway. They are not made for "slab on grade." They are made for elevated slabs such as bridge decks and post tension parking garages, that flex a lot. Free isn't always better! Also, whatever size rods that are used, you have to have at LEAST that much coverage.

dpendzic · Mar 6, 2018

I agree that the slump test is for water content---you should always make test cylinders to verify the strength of the concrete
use air entrained concrete if subject to freeze-thaw or wet conditions--be sure to properly cure the concrete by moist cure for 7 day and keeping it above 40*F

Stephen Newell · Mar 6, 2018

You need to check with your local building code office. The specifications for a concrete floor can vary area to area even 20 miles apart. You may need to put down gravel or cushion sand first. Where I live you could pour a floor 3" thick with noting more than remesh in it and not have a problem. My soil is sugar sand though. Just 40 miles from me the soil is black gumbo and I saw a garage floor that had a about a about a 5" floor and it's broke in a half dozen sections with wide gaps and one end sinking into the ground.

casenut1 · Mar 6, 2018

Use the fiber mesh in the concrete. Rebar and wire mesh are used to hold the concrete together after it cracks. The mesh actually makes it stronger.

Vern · Mar 6, 2018

I had a garage built with fiber mesh. It cracked badly after about 2 years. 6 mo warranty, wouldn't do anything about it.
Use plenty of re-bar.

Vern

Phil in Pa · Mar 7, 2018

I think they're nylon fibers that are throughout the mix, you get 100% the effect of wire. It's usually cheaper to add than to actually purchase material and install it. It's also difficult to keep wire mesh at the right location in the slab which is middle or lower middle. For a slab that only has vehicle traffic I just don't think rebar is necessary.

docmirror · Mar 7, 2018

I just can't imagine flat-work to be driven on without rebar. Goes against everything I was taught in eng school. I know every nuclear plant ever built with concrete used plenty of rebar. When I see a concrete fwy being built with those giant concrete machines, they are always laying over gobs of rebar. Rebar is cheap, easy to form, and plentiful. Fixing a broke slab is hard, expensive, and messy. Hmmm, choose wisely.

caterpillar guy · Mar 8, 2018

There are only 2 types of concrete cracked and that that is going to crack. Now For cars 4 inches is fine . Since I react to the fiberglass for the fiber they put in I will not use the fiber in concrete. I'm not so sure it is all that it is made out to be any way.
As for cutting for cracking that is all nonsense. The guys that did my approach to the shed cut it and it has cracked right corner ways across it. I would not ever cut a floor I wanted to use a creeper on.

WGM · Mar 8, 2018

There are other types of fiber mesh besides fiberglass.

Rebar in concrete slab

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