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Old 09-30-2010, 12:36 PM   #1
Kemo
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Default Concrete with no rebar?

New guy here with a question about concrete slabs. Ive been surfing around looking at videos on youtube of slabs being poured and noticed that many that are being poured use no rebar or moisture barrier at all. And of course there are plenty of comments stating just how crappy the video was and how its gonna break into a jigsaw puzzle in a week. I am no concrete specialist, but I have been under the impression that rebar is always required. Is there a new (or not so new) method of mixing concrete with certain fibers that eliminate the need for rebar?

Steve
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Old 09-30-2010, 12:40 PM   #2
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Default Re: Concrete with no rebar?

Rebar is not always required. And neither is a vapor barrier. Soil conditions dictate the need for either of these. That said, if I were building I would use #3 rebar on 18 inch centers along with a vapor barrier like Stegeowrap at 15 mil. Assuming money is no object.
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Old 09-30-2010, 12:49 PM   #3
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Default Re: Concrete with no rebar?

Concrete resists compressive forces, but not tension. Fiber reinforced concrete adds some tensile resistance, but it really isn't a replacement for reinforcing.

I see lots of slabs and even some residential footings poured without reinforcing. I also hear a lot of people go on about how there is only two types of concrete - concrete that has cracked and concrete that will crack... Do it properly and you don't have to worry about cracks.

Vapor barrier is the same way. In many soil conditions you won't have too much of a problem, but concrete will wick moisture so the vapor barrier will help in nearly every situation.

In the overall cost of pouring a slab, reinforcing and vapor barrier is a minor percentage. But not putting it in just ends up being more profit for the contractor. They'll cut any corner they can.
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Old 09-30-2010, 03:02 PM   #4
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Default Re: Concrete with no rebar?

Use it.
4 x 4 mesh at the least.
It is false economy to skip it.
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Old 09-30-2010, 05:52 PM   #5
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Default Re: Concrete with no rebar?

It's false economy to do concrete without putting in rebar and mesh. Rebar around the perimeter at the least, and mesh over the rest. Pull it up when pouring so it is not resting on the bottom of the slab. You don't need the fancy rebar supports, a stone or broken brick will work just as well. Rebar is expensive at Home Depot and Lowes, call aroud to steel yards. Most carry rebar and the price difference over retail is significant.
Fun story from San Antonio years ago. A big builder built block after block of houses all the same. They only had one set of mesh and rebar. The inspector would come out and sign off the slab was ready to pour. Once he left, the crew would pick up the mesh and rebar and carry it over to the house slab next door and there it would stay until the foundation passed it's pre-pour inspection. Repeat through the subdivision, I don't know if the last house poured got the steel in the slab or not!
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Old 09-30-2010, 06:28 PM   #6
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Default Re: Concrete with no rebar?

As stated above concrete is great in compression but not in tension (pulling). As a result the placement of the reinforcements (rebar) matters. For example, in a retaining wall where the pressure is coming from one side, the wall wants to bow out. That is tension on the open side and compression on the fill side. Thus the rebar should be off center toward the open side where it can help the concrete resist tensile forces.

I think in a slab the same thinking would put the rebar in the bottom 1/3 of the pour, not vertically centered. But I'm not sure and will defer to anyone here who knows.
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Old 09-30-2010, 06:36 PM   #7
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Default Re: Concrete with no rebar?

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Originally Posted by R-132 Fan View Post
Pull it up when pouring so it is not resting on the bottom of the slab. You don't need the fancy rebar supports, a stone or broken brick will work just as well.
I've seen enough slabwork removed where the mesh was all sitting at the bottom except for "mounds" where it was pulled up into the concrete to advise setting the mesh up on supports of some sort.

Also, get the sheets of mesh. The rolls are too light gage to do much good and even if you ever get them straightened out and support them they also end up at the bottom of the slab.
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Old 09-30-2010, 06:51 PM   #8
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Default Re: Concrete with no rebar?

Almost all slabs on grade/ground (driveways, building floors, highway pavement) are designed ignoring any strength contributed by steel reinforcing. Thus the concrete thickness is determined based on the relatively low tension that concrete can take. But that is also assuming the ground underneath is well prepared (stone, drainage, etc) which of course should always be the goal

Steel reinforcing is added to help control shrinkage cracking during curing and which allows for wider spaced contraction joints (less maintenance for filled joints). Also helps bridge small pockets of soft soil or washed out soil. Code say that for concrete poured on the ground you need 3" from base of concrete to face of any steel reinforcing, thus for 4 or 5" slab the reinforcing isn't deep enough to add a lot of strength.

So if a person feels confident that they have a solid well drained base, doesn't allow water to flow through joints, etc. and tightens up the joints there is no reason why you can't go without reinforcing other than perhaps any local building code requirements. Been done for ages. But need to be more cautious especially about soil types (expansive clays and such).
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Old 09-30-2010, 07:50 PM   #9
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Default Re: Concrete with no rebar?

Only thing I can add to this is that my slab is a full 6" with fiber,no mesh or rebar, and we run those 26,000 lb JD's in and out a lot. It hasnt cracked any place yet.
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Old 09-30-2010, 08:03 PM   #10
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Default Re: Concrete with no rebar?

We had our shop floor put down in 1994, 16 years ago, I did all the forms for a 6 inch slab, I put in 4x4 mash, and 1/2 Re-Bar, and used concret with Fiberglass in it, The guy that did the job was the best we had in town at that time, and he told me that I was doing a Over kill on this, I told him there will be some Big Equipment come in this shop, When I bring in a Bull Doser, or a track hoe, I put down 3/4 plywood so it will not put marks in the floor,
the only Hair line Crack in my shop is under my Desk, Now I'M about 140, and the Wife is about 120, So how did we get the Crack under the DesK ??
Oh the shop is 26 X 50,
Thay say if you use the Fiberglass in the concret you don't need the 4x4 mash or the re-bar. Use it any way, My shop floor cost me $2,000 in 1994
and have had NO Problems, it would now cost about 6 to 8 thousand.
and more if I had to take out the old and put in New now.
DO IT RIGHT THE FIRST TIME, LIKE I DID, IN 16 YEARS YOU WILL BE HAPPY.
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P.S. And use moisture barrier it is a must, I used 3 sheets 6 mill
If you can't pay the price to do it right now, Do it when you can, You will Be A Lot Happer in 16 Years,

Last edited by chief ben; 09-30-2010 at 08:20 PM.
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Old 09-30-2010, 11:41 PM   #11
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Default Re: Concrete with no rebar?

While there is no doubt that reinforcement is good, it should be specified by an engineer unless you just want to throw dollars at a situation without ever knowing if you helped yourself at all. Although you do get bragging rights.... you can tell your buddies about your rebar and welded wire mesh, its a great topic at any neighborhood cook out. If you have a properly compacted, well drained base, the reinforcement doesn't buy you much in a typical residential shop/garage. Conversely, putting all kinds of metal in a slab to make up for poor site preparation doesn't buy you much either.
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Old 10-01-2010, 08:43 AM   #12
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Default Re: Concrete with no rebar?

Quote:
Originally Posted by tcianci View Post
While there is no doubt that reinforcement is good, it should be specified by an engineer unless you just want to throw dollars at a situation without ever knowing if you helped yourself at all. Although you do get bragging rights.... you can tell your buddies about your rebar and welded wire mesh, its a great topic at any neighborhood cook out. If you have a properly compacted, well drained base, the reinforcement doesn't buy you much in a typical residential shop/garage. Conversely, putting all kinds of metal in a slab to make up for poor site preparation doesn't buy you much either.
Are you a Engineer or a concret expert ? Thank You For the Info.
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Old 10-01-2010, 12:41 PM   #13
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Default Re: Concrete with no rebar?

Concrete with no rebar is like a lady with no meat on her bones. She may hold up for awhile, but in the long run she just won't hold up to the poundings...

ps- I have no background in concrete or engineering...
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Old 10-01-2010, 07:40 PM   #14
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Default Re: Concrete with no rebar?

My driveway....no rebar....just mesh...was poured according to the neighbor, sometime in the 60's....that crack is about 1/8".



Some have made comments about rebar rusting....here is one comment I dug up....found several sites that said basically the same thing.

Quote:
Comments shared thus far have merit and can be contributing causes of the rust. Any time rust on re-bar begins to swell and discolor a concrete as well as expand to cause cracks, the REAL answer to what has happened is : The Steel has Lost the Passivating Layer !
Concrete when poured and surrounds the steel has a pH of 13 +or-. This high pH forms a layer on the steel that prevents it from corroding[Passivating Layer] ! With time, the pH begins to fall and this protection is lost.

As stated, salts, atmosphere and carbonation begin to play a role in the degradation of the concrete. The most deletrious affect is moisture. Concrete is a thermodynamically unstable material that absorbs moisture into the concrete matrix. [pour a small amount of water on a surface and observe] As water enters the gel pore capillary system of concrete, it carries contaminates with it, that speed the lowering of a pH significantly. When the pH goes below 11.5, the passivating layer is gone, an electrochemical reaction commences causing rust to form. As the rust progresses, it expands the size of the re-bar exerting pressure on the concrete creating cracks [Spalling]. At this point, additional cracks allow more water/moisture into the matrix and the rust really takes off! At a pH of 13 it takes 8000 ppm of Chlorides to commence corrosion on steel, at a pH of 11.5 that # is 70 ppm !! Drastic Difference

Concrete mix, Depth of cover and good practices all play a roll, however a concrete that absorbs moisture is the major culprit. Use of Epoxy coated re-bar was thought to be the answer! After monumental and catastrophic failures of coated steel was observed, it has been determined that these protective measures do not work. When a coating is chipped or a small nick on a surface is present, the lowered pH then concentrates at that site and instead of even surface rust you wind up with corrosion all the way through the bar and all tension properties are lost, resulting in a structural failure at that point!

Waterproofing of a concrete with StableCrete will prevent moisture intrusion and also prevent internal chemistry from migrating out to a surface reaction with the atmosphere. Freeze thaw does not happen since there is no moisture in the concrete to freeze. Because the concrete is dry, the Ohms resistance of the concrete is maintained at high levels, preventing the flow of electrons required for corrosion cells to form.
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Old 10-01-2010, 08:42 PM   #15
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Default Re: Concrete with no rebar?

Quote:
Originally Posted by Kemo View Post
I am no concrete specialist, but I have been under the impression that rebar is always required. Is there a new (or not so new) method of mixing concrete with certain fibers that eliminate the need for rebar?

Steve
Quote:
Originally Posted by DuluthMN View Post
Concrete with no rebar is like a lady with no meat on her bones. She may hold up for awhile, but in the long run she just won't hold up to the poundings...

ps- I have no background in concrete or engineering...


It seemed to work for the Romans for over 2000 years. But I'm no concrete expert.
http://www.romanconcrete.com/docs/spillway/spillway.htm
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Old 10-02-2010, 06:14 AM   #16
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Default Re: Concrete with no rebar?

We must get at least two of these types of thread a month. In some situations rebar is absolutely required. In others, perfect prep expansion joints and curing will be all that is required.

When in doubt, always err to the side of caution.
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Old 10-02-2010, 07:54 PM   #17
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Default Re: Concrete with no rebar?

My friend has a 30x50 building with an on grade poured floor 4" thick. They used a vapor barier and fiber in the concrete. It has been 16 years and has only 1 hairline crack. I had a 24x32 floor poured using fiber in the concrete 6" at the back 24x8 and 4 " in the rest. I laid down a vapor barier and my concrete guy said it won't do much good because the moisture is mostly condensation from the floor being cold and the inside building being hot during the summer or heated during the winter. I also had expansion joints cut in.
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Old 10-02-2010, 08:33 PM   #18
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Default Re: Concrete with no rebar?

Concrete has exceptional compaction strength, but very little shear strength. This is why it is very easy to break it up with a hammer. When you put rebar, mesh etc. in to it you gain the much needed shear strength. This stops the sections of any given area from moving, causing cracks. When cracks do occur, it minimizes movement.
(definition of shear: to become fractured along a plane as a result of forces acting parallel to the plane)

If you chose not to use rebar, You risk movement, and let's face it Everything moves at some point, be it from frost heave, water migration, static or dynamic hydraulic pressure, weight of heavy objects, machinery, vibrating from them, voids forming underneath due to water, settling, etc. Something's going to give.

You can save the relatively few dollars needed, but sooner or later its going to crack.
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Old 10-03-2010, 06:09 AM   #19
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Default Re: Concrete with no rebar?

Quote:
Originally Posted by tcianci View Post
While there is no doubt that reinforcement is good, it should be specified by an engineer unless you just want to throw dollars at a situation without ever knowing if you helped yourself at all. Although you do get bragging rights.... you can tell your buddies about your rebar and welded wire mesh, its a great topic at any neighborhood cook out. If you have a properly compacted, well drained base, the reinforcement doesn't buy you much in a typical residential shop/garage. Conversely, putting all kinds of metal in a slab to make up for poor site preparation doesn't buy you much either.
If a PE was specing a job for a neighbor, what are the chances he doesn't spec steel in the slab? I'd say none, he's always going to put steel in there whether its mesh or bar, its going in there. He has to design for most every possible circumstance. In Maine an engineer on a local radio home improvement show said you need 7 inches of blue styrofoam under a bare concrete patio to keep it from moving. Didn't care what the soils were under it, he said he had to design for a 100 year event. if I design it I can do what I want but a PE can't.
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Old 10-03-2010, 02:07 PM   #20
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Default Re: Concrete with no rebar?

I didnt get any rebar in my slab either 24x36. They put fiber in it 5000psi. Local code is only 2500psi. I thought you always had to have steel, but Im not a concrete guy and they assured me everything would be good. Very reputable company in the area and the only one I found that guaranteed no cracking (besides the obvious cracking in the expansion joints). They used around 8-10 inches compacted limestone and a vapor barrier. concrete is around 6 inches depending on where you are in the shop. The soil around here is very stable undisturbed clay.

block stem walls btw, and the footing had 2 runs of rebar. I guess we will see in another decade or so. I would always insist in reinforcement in a poured foundation/stemwall just for the peace of mind
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