Concrete Pad - Control/Expansion Joints?

zuiko · Aug 20, 2004

I'm going to be pouring a concrete pad inside a building... Roughly 40x25, 3" thick. Any tips on where and how to put in control joints and if I need any expansion joints? Do I need control joints running in both directions or just one?

Also what size of rebar should I use and how often?

Currmudgeon · Aug 20, 2004

Three inches is about the absolute minimum for a slab, and possibly too thin to include rebar. The rebar does little to improve the strength of a slab, mainly because of the thinness of the slab placing the steel too close to the neutral fibre. (The middle of a beam does no work in bending). Rebar does help control contraction cracking and holds the pieces together when they do crack. Notice I say "when." Concrete will crack. That is a given. That's the purpose of the contraction or control joints. They're based on the principle that if you can't hide it, make it look deliberate.

I'd think about mesh rather than rebar, and place it in the center of the slab. Some people place it on chairs, my preference. Others, with a better eye, hook it up after pouring.

Place the joints at t/24, or two feet per inch of slab thickness, in both directions. Try to make the panels as close to square as works out geometrically.

For a slab that size, expansion joints aren't necessary, unless you have hard points. If you're adjoining an existing slab, or have intrusions like stairs, then I'd put an expansion joint on those lines.

mahlers · Aug 20, 2004

What is the pad going to be used for? Will you be covering it with flooring, or will it be exposed? Will it be load-bearing?

I definitely second the recomendation to use mesh with a pad that thin. You'll save lots of time and money with little loss in structural integrity. If it were me, I would also go with a fiber reinforced concrete for something that thin.

Archdean · Aug 20, 2004

I am pouring a 14'x23'x51/2" this week for my new addition to park my tractor in.

helpful site HERE

em14 · Aug 20, 2004

To anyone,
I was discussing fiber reinforced concrete with a couple concrete finishing companies a while back and they were voicing a problem with the fiber balling up under the finish machine and also leaving a "Fuzz" in and on the concrete. What would cause this?
Leo

Junkman · Aug 20, 2004

( To anyone,
I was discussing fiber reinforced concrete with a couple concrete finishing companies a while back and they were voicing a problem with the fiber balling up under the finish machine and also leaving a "Fuzz" in and on the concrete. What would cause this?
Leo )

There reluctance to new products. I have fiber in my garage floor and there is no balling up of the fibers when the machine was power troweling it, nor are there any appreciable pieces of fiber sticking up from the floor after a month. I have also heard people claim that if you were to lie of the floor, the "fiberglass" would irritate you. Hasn't bothered me or the dog and we both have laid on it for different reasons in the past. If the concrete installer has any abilities to learn, then he shouldn't have any problems. It is like putting plastic under the floor.. they don't like it because it slows the drying process and it takes longer for the concrete to set up. Same with the steel mesh... some don't like it because it takes time to pull it up into the concrete after it is poured. Just like any other industry, there are good and bad working in it... Go with a 3500# mix with fiber and steel mesh in the floor. Power trowel to a smooth finish and then put down the poly sealer the same day. You will never regret a beautiful floor. If the sub base is prepared properly, you will not need the "crack joints"..

neilly2 · Aug 22, 2004

Fibermesh is an alternative to wire mesh -- that's why its called fibermesh. Put one or the other in. Again, fibermesh and WWF(steel mesh) do NOT add structural integrity to a slab. They control shrinkage cracks. That is why you're supposed to pull it up into the center of the slab, where there is no structural benefit.

You will be able to see fibers on the finished concrete, but I've never heard of anyone giving a ****. Its a concrete floor for goodness sake. I managed 6 floors of an office building where we used fibermesh. None of the finishers had problems with troweling. This was about 6 years ago when fiber was the new thing. Don't see much of it anymore.

As for plastic directly under a slab, you are correct. It slows up the curing (by holding in moisture), but only on the bottom of the slab. That is why it technically is an inferior installation technique (though very common). The top of the slab is exposed to the air and will cure more rapidly than the bottom. This differential curing enduces unwanted stress into the slabs and can cause more cracks than normal. In extreme but rare cases the slab will visibly curl. The technically proper installation would be to put 2 inches of clean sand over the plastic than pour concrete directly on the sand. The sand will let the moisture escape the curing concrete.

zuiko · Aug 22, 2004

I think I will bump it up to 5 1/2" thick. I would like to be able to park a tractor or pickup on part of it while I am working on it. The rest will have metalworking tools on it. There will be a couple of pretty light walls (steel stud with panelling probably) and no ceiling on it.

The wire mesh is a good idea, I will use that instead of rebar.

I was planning on using plastic sheeting on the bottom. I have a dirt floor right now that is very dry. I was concerned about it sucking the moisture out of the concrete too fast. Would it be better to put it directly on the floor? Thanks.

neilly2 · Aug 22, 2004

No to concrete directly on the dirt. It's more important to have a vapor barrier than worry about the curing issues.

Just to repeat something-- Re-bar is for re-inforcing a slab to handle heavy loads. Wire mesh/fibermesh does NOT add structural integrity--it only helps with shrinkage cracks.

The statement 'I'll use wire mesh instead of rebar' is not correct. You should have wire mesh in all slabs. You don't need rebar in a garage slab. I think a 4" slab will do you fine as long as the subgrade is well compacted.

Junkman · Aug 22, 2004

( Fibermesh is an alternative to wire mesh -- that's why its called fibermesh. Put one or the other in. .................................)
I put the steel mesh into the concrete even though there was fiber-mesh in the mix because if it cracked, I wanted to know that the two halves wouldn't separate. I don't have much experience with the fiber-mesh other than my own floor. The steel mesh is inexpensive insurance in my opinion. I would rather err on the side of caution.
(
As for plastic directly under a slab, you are correct. It slows up the curing (by holding in moisture), but only on the bottom of the slab. That is why it technically is an inferior installation technique (though very common). The top of the slab is exposed to the air and will cure more rapidly than the bottom. This differential curing enduces unwanted stress into the slabs and can cause more cracks than normal. In extreme but rare cases the slab will visibly curl. The technically proper installation would be to put 2 inches of clean sand over the plastic than pour concrete directly on the sand. The sand will let the moisture escape the curing concrete. )

I put the plastic down and the concrete was poured directly on top of it. I then put a concrete sealer on as soon as the concrete contractor had finished putting the smooth finish on top. This sealed the moisture in from the top and the only place that there could be any moisture leaving was at the sides, the best that I could figure. I even tried spraying the concrete the next day and all the water did was to bead on the surface. I don't understand how this could be an "inferior installation technique".

I have heard your statements before, but the concrete contractor that I used is not known for skimping on materials or labor and prides himself on as perfect a floor that he can do. He agreed with my logic and I am very pleased with my 6" thick smooth as glass floor. /forums/images/graemlins/wink.gif /forums/images/graemlins/wink.gif