Search the forums

Loading

rebar spacing in a slab

Shakesplitter's picture

I was wondering if there were any rules of thumb for the spacing of rebar in an on-ground slab?  The slab in question is 4" thick and is intended for an outside patio.  I will be using 10M (1/2") bar and was going to use a 24" grid.  Is this enough?

(post #100117, reply #1 of 12)

The slab in question is 4" thick and is intended for an outside patio.  I will be using 10M (1/2") bar and was going to use a 24" grid.  Is this enough?


Since this is a patio, you could use 6x6 10/10 wire mesh instead of the rebar. If the slab is poured against an existing foundation wall or another slab, you would use the #4 or #5 rebar to dowel it to the which ever you are butting too. Unless you are going to park a very large truck on the slab the 2'x2' oc #4 rebar is over kill IMHO.


I personally like the ww mesh that is in sheets (panels) vs. the roll type. Lap over each panel to the next by one grid and wire tie them together. set them all on 2" chairs, to keep them in the center of the slab and make your pour.


Keep it simple. you're not building a runway :)


BTW, could you fill out your profile? It will help to know what area of the country you are in, and make make a difference in the answers you get to future questions.


Welcome to Breaktime.


 


Dave

 

(post #100117, reply #3 of 12)

Thanks Dave,


I appreciate your feedback.   Yes I did suspect that it was overkill but I'm o.k. with that.  I suspect that this slab may be enclosed in the future and become a floor that supports walls and structural posts.  Your suggestion to fill out a profile is a good one as I'm up in Canada and the rules/codes and terminology are indeed different.  BTW, what is the American numbering system for rebar?  I noticed that you refered to #4 and #5 rebar.  Could you direct me to a site or information regarding this?


Thanks again,


Shakesplitter

(post #100117, reply #4 of 12)

# 4 is 4/8 aka 1/2 inch . #5 is 5/8, #6  6/8 . None of you mentioned poly fiber . Dont you trust it?

(post #100117, reply #7 of 12)

Good point. As long as the floor isn't going to be an exposed finish, it certainly can't hurt.

(post #100117, reply #8 of 12)

The only note of caution with 6x6 mesh is to make sure that it stays in the middle of the concrete vertically.  The stuff I just demoed last week was basically "mesh on grade" with concrete poured over it.  6x6 makes it pretty much impossible to walk without pushing it to the ground.  Dumping mud on it also tends to push it to the ground. 


Adding structural posts later will definitely require something more than just the slab.  I second the motion to design and provide the perimeter footing now, and save the documentation.  Figure on cutting the slab and pouring the internal stuff when the future building happens.


 


 


-- J.S.


 

 

 

-- J.S.

 

(post #100117, reply #10 of 12)

<<The only note of caution with 6x6 mesh is to make sure that it stays in the middle of the concrete vertically.>>

That's for sure.

I tear up a lot of concrete. The slabs with the mesh in the middle are a lot tougher than the ones with the mesh still on the dirt.

(post #100117, reply #11 of 12)

Walking down mesh has always been a problem. Even good crews that make every effort to hook and hold the mesh up will have high and low spots in the placement of mesh.


I recently started using 5x10 panels and short chairs instead of 5x100' rolls of wire. Placement and tieing is much faster than a couple of men unspooling a roll wire. It is also a lot safer for the wire handlers, with no recoil or cut ends to contend with. I tried spacing it off the bottom with 1 1/2 or 2" concrete bricks, but was no more satisfied with that than I was with the same under roll mesh. I had mil supply company send out 2" rebar chairs  and tied them under the mesh on my last pour. Worked like a charm, but took the crew some getting use too. You have to step a little higher when the mesh is already up, and you still need to lift in few spots where it sags from walking on it. You also have to watch for chairs that get rolled over and kick them back up with the end of the come a long, but over all I satisfied with how well everything worked together.Cost may have been $10 to $20 higher for a eleven yard pour, but worth it IMO.


BTW, I did not make the pour myself. I just prepared everything for it, and subbed out the pour and finishing. I had originally intended to have RFH in this slab but got "sticker shock" when quoted $3600 for a 900 sf slab (turn key for everything). That is why I had prepped everything with panels instead of rolled wire mesh. Two months later got an off the cuff figure of 2K for the same RFH install from the contractor I had originally called, but he failed to return my calls then. Grrrr,  was I pizzed.


Anyway back to the mesh panels. My concrete sub said he has been using them for two years now, and prefers them over rolled wire mesh. So my extra effort was just sop for him.


 


Dave

(post #100117, reply #12 of 12)

That matches my experience, too.

Even the part about the plumbers. <G>

On a job about a year ago we ran into bad soils, a creative architect with great vision and stunningly little technical knowledge, and exposed concrete floor, and a homeowner who would throw a rod if there was more than a couple of very fine hairline cracks.

The archy had spec'd #4 EW @ 16".

And radiant.

I wasn't allowed to talk to the archy but I did talk with the homeowner and explained everything I said in my previous post here, and that there would likely be PEX tubing way too close to the finished floor in places if you tried to tie it on a rebar grid like that.

I asked for a meeting with an engineer so we could change the drawings with or without the archy. The engineer was pretty worried about the soils, and wanted the #4 also until I explained my concerns.

The engineer was also bothered that with radiant and a 4" slab there is no way to get the mesh in the middle of the pour and get a good result.

I asked if we could do an overex and replace to correct the subgrade, lay down 1 layer of remesh panels for the plumber to tie the PEX to, and lay another layer of remesh panels over the PEX so that second layer would work the way it's supposed to.

He thought about it for a while and said "Sure."

A year later the palce looks great.

The overex had to be done anyway. The plumber definitely charged less because he could tie to mesh. And two layers of mesh was still definitely cheaper than a #4 mat.

I'm with you on the panels. If I never again have to watch anyone fight with a roll of remesh that will be fine by me.

(post #100117, reply #2 of 12)

What Dave has said is true, let me add to it a little for explanation.

It's been said here many times, remesh and rebar do not perform the same function in concrete, and more steel is not automatically better.

Concrete has great compressive strength but negligible tensile strength.

Reinforced concrete is not the same material as plain concrete (without steel).

The addition of steel in the right place adds tensile strength.

So if, for example, you are building a footing or a grade beam and the anticipated use may imposed tensile (or in some cases shear) loads, properly sized and placed rebar handles that.

But a 4" slab on a properly prepared subgrade is a different story. Unless you are putting some kind of unusual load on the slab or the subgrade is irreparably bad, you want mesh.

Wire mesh provides some resistance to cracking in properly batched, properly placed, properly cured concrete on a properly prepared subgrade. (My, aren't we proper? <G> But it's all important.)

There is also no doubt that wire mesh provides a certain amount of tensile strength, but not much.

What wire mesh really does is maintain aggregate interlock -- that is, when your concrete does crack, it keeps the crack from seperating more than about half the diameter of your coarse aggregate, which is what prevents vertical displacement. Vertical displacement is the failure that really causes problems in most applications.

Now here is why you do not want rebar.

Besides the fact that it is an expense that doesn't help anything, it can actually cause problems. Here are just a few reasons, if Brownbagg joins this discussion he may provide more.

1) The remesh Dave mentioned has a little square in it every 6 inches. You can buy other patterns, but that one is standard. You get a lot of coverage that way. It actually works better for its intended purpose.

2) For concrete that will be exposed to weather, particularly wet, cold, or otherwise harsh conditions, it's best to have three inches of coverage. You can't get that in a four inch slab.

3) If your rebar is somehow perfectly placed in the center of the slab (unlikely) and your large aggregate is the typical 3/4" or 7/8", then think about the top 1-1/2" of concrete right over the rebar after you finish it. It will be different from the concrete right next to it. If your slab is just a little bit thin, you will most likely get a crack pattern that precisely conforms to your rebar layout.

4) High temperatures or direct sun during the first week will have the same effect -- a crack pattern that matches your rebar.

And your concrete will now actually be weaker than if you used remesh.

If you use remesh, make sure that it is properly placed. It usually ends up laying on the grade under the slab. That is a complete waste.

If you have unusual structural concerns for the slab, that's OK but it's best to address them now in a way that will get you the result you are looking for.

(post #100117, reply #5 of 12)

You mention possibly enclosing this in the future, I would put the footing in now to support the walls and roof in the future and use the rebar in that and the mesh in the floor area. Shoot photos of the footing and rebar for any possible future inspections, who knows?

Never fear the want of business. A man who qualifies himself well for his calling, never fails of employment. Thomas Jefferson 3rd president of US (1743 - 1826)

(post #100117, reply #6 of 12)

Good idea.

(post #100117, reply #9 of 12)

Yes the "footings" are already in.  Two of the outside edges of the slab are going to sit on previously placed concrete retaining walls.  The vertical rebar is coming up from the walls and will tie into the bars in the slab.  The other two edges abutt to the existing foundation wall of the house on one side and a existing patio slab on the other.  I will be tying the rebar into the existing slab by drilling and epoxying the bars into the edge.


Thanks to you all for your ideas and contributions,


Shakesplitter