Rebar vs. Wire Mesh

[medeek]

For residential work (slab-on-grade foundations, garage slabs etc...) I usually call out a 4" or 6" thick slab reinforced with 6x6-W2.9xW2.9 WWM. However, after some recent reading I'm beginning to think that rebar slab reinf. may be better for a number of reasons. One of which is how do you actually get out on a mesh reinf. slab without stepping all over the mesh, bending it and pushing it to the bottom of the slab even it is supported by chairs or dobies?

Perhaps a better option would be #3 bars at 18" o/c or #4 bars at 24" o/c, at least workers can step around the bars.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[cvg]

don't do any residential work, however I never allow mesh except for shotcrete. contractors belly ache, but the results are far superior. light weight mesh at the bottom of the slab is a total waste of time and money. always use #4 minimum. I don't see that you save much by going with #3's

 

[DBuker]

I almost always spec rebar in residential slab on grade applications. #3 @ 12"oc meets the minimum temperature reinforcing requirement of 0.002 * Area for grade 60 bar. I also leave a note for wire mesh specs in my general notes so that when the contractor calls and requests a substitute there's already a basic spec for the product in the plans.

I prefer rebar for all of the reasons that you mentioned. Wire mesh just doesn't produce as nice of a finished product most of the time. But really, there's so much more to a good concrete slab than the reinforcing. W/C ratio, concrete strength, curing conditions, etc, all have a big impact on the final product...

 

[medeek]

There are a number of threads on this same topic but I kind of want to get some current thinking.

Other than material cost and setup labor and time what are the advantage of mesh over the rebar?

Has anyone ever used wire mesh with rebar combined? For example #4 bars @ 48" with WWM to help support the mesh from being trampled?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[RM87]

I would go with #4 @ 16" O.C. each way. Specify GR60 bars, but keep in mind that your contractors may try to cut corners with GR40.

In my experience, if any corrosive elements are exposed to #3 rebar, they won't survive long. Make sure to specify the required base rock, the compaction of the area, and appropriate waterproofing measures, because if the slab starts cracking, things aren't going to be so fun.

 

[manstrom]

If I had to pay for it, 6x6 W1.4 WWM all day. It is pretty standard and produces a reasonable slab. Keep in mind, these things can be poured without any steel at all. The mesh just restrains cracks.

If I was building my dream home (and not paying for it), #3 bars @ 12" o.c. so that I know is will be in the right location and will actually do something. There is a good cost associated with this though.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[medeek]

Wow, everyone is all over the board with the spacing on the rebar. I just got off the phone with a local contractor after picking his brain a bit and he typically will use a #4 bar @ 48" o/c. He says he stays away from the mesh though, too much trouble and he can't get good quality, flat material anymore.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[IRstuff]

I've not seen any construction recently that uses mesh. Our housing development is only about 3 yr old, and everything was rebar'd and post-tensioned.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[Lomarandil]

This is just a gut feel thing, but I'd have trouble convincing myself that anything at 48" would do a great job restraining cracks.

 

[medeek]

The min. reinf. requirements for shrinkage and temp. (ACI318-11, Section 7.12.2.1) specify the following ratios for slabs:

.002 for GR40 or GR50
.0018 for GR60

Also spacing should not be farther apart than five times slab thickness or 18 in. For a 6" slab the 18" requirement controls.

This section deals specifically with structural slabs. Are SOG applicable?

How does the WWM meet this requirement? For residential work it seems like we are constantly breaking all of the rules but no one seems to care.

For a 6" thick slab with GR60 bar per the ACI318 I get the following max. spacings:

#4 bar = 18.17 inches (18" controls)
#3 bar = 10.23 inches

For a 4" thick slab with GR60 bars I get the following max. spacings:

#4 bar = 27.26 inches (18" controls)
#3 bar = 15.33 inches



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

@Lomarandil

I would strongly agree, might as well forego the reinforcement entirely. Funny thing is I just talked with another contractor in the area and his suggestion was to place the bar @ 48" o/c as well. He also avoids the wire mesh entirely and prefers fiber reinf. which he thinks takes the place of the wire. However, once the concrete cracks the fiber is useless and will not hold the crack together so that line of reasoning seems a bit flawed in my opinion.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[DBuker]

I don't think a 4" slab on grade in a residential unit is really considered "structural" and therefore it probably doesn't actually have any reinforcing requirements. Since a slab on grade is continuously supported by soil, the plain concrete chapter in ACI 318 could be referenced but again, if it's not structural then why use the "structural plain concrete" code. A quick check of the IRC didn't turn up any requirements but I rarely look at the IRC, so maybe there are requirements that I'm not aware of.

I know that ACI 302.1 has a lot of information about slab on grade design for various different performance requirements but I don't have a copy of it...

As far as putting a bar at 48" oc, why bother?

To your original question, I think rebar is more likely to provides a better end product than mesh but it's undoubtedly more expensive so one must balance the cost with the desired end product.

 

[KootK]

I've been going pretty liberal for my residential slabs on grade.

Interior = 3.5" with no reinforcing. Really easy to walk through.
Garage = 4" with 10M@1000 (1/2 x 0.2%). That's easy to walk through and provides a bit more durability. Not sure if it's required.

SE's in my market are stodgy and going with an unreinforced slab on grade makes me look like a fool / ballsy maverick. Contractors eat it up. I use the un-conservative tack here in exchange for credibility that I spend elsewhere on issues that are more important to me. I usually have a chat with the client regarding expectations and the fact that they're not buying the main floor slab of the Guggenheim.

I don' believe that either the 0.2% or the 5X apply to slabs on grade. Slabs on grade are not generally reinforced as flexural members and the restraint condition for SOG is much different than it is for suspended slabs (it's worse actually).

Some related ACI info:

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

The consensus seems to be that SOG for residential work is non-structural and no requirement exists for reinforcement.

My only qualm with this line of thought is that when the concrete cracks it will have a tendency to separate and even worse the two edges may settle slightly differently creating an edge that juts up. My own garage has a large crack line running down the center with this exact problem. The mesh or bar would prevent this. Additionally if there are voids beneath the slab the added reinf. should help the concrete bridge these voids. In my opinion a good quality slab should have some reinforcement.

Realistically though I don't think a residential slab needs to comply with the .002 or .0018 ratio, this is probably overkill unless the client has an open check book or has some serious load requirements.

My thinking is mesh is garbage and an outdated method, too hard to place properly and for that reason too easy to mess up. Mesh at the bottom of the slab may be slightly better than no mesh at all but how do you quantify this and why design a defective product from the start.

#3 rebar @ 24" o/c for a 4" residential slab and #4 rebar @ 24" o/c for a 5" or 6" residential slab would probably be my recommendation. Bars need to be placed on chairs or dobie blocks prior to the pour, none of this lifting things up or hooking I've read about. For 4" slab I would place the bar in the center of the slab. For 5" or 6" slab place the bar about 2" from the top. Fiber reinf. conc. with low moisture content, 3000 psi min., use a 6 mil vapor barrier on all interior slabs (living space,garages etc...) In a hot dry environment make sure the concrete stays wet as it cures (ie. burlap).

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

Many times I will specify, "Slab per code. Optionally add #4 @ 24" o.c. each way for greater crack control". Passing the buck to the client :>

 

[JStructsteel]

I have seen alot of 6" slabs installed with no reinforcing at all, with proper saw joints ,cracks are controlled.

If putting in mesh, have it about 2" from the top. Your right, trampled down at the bottom does no good. goto a 12x12 mesh that the workers can step between.

 

[shaneelliss]

The commentary on section 7.12.1 in the ACI specifically states that slabs on grade are not structural slabs and that the provisions for minimum steel don't apply to them. Many homes around here don't use any for slabs on grade. I like to specify #3 @ 18" to provide some crack control.

 

[medeek]

A local contractor says he would like to replace the mesh and/or rebar with fiber reinf. concrete.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[MacGruber22]

medeek - that is an acceptable way to control crack widths from shrinkage and mitigate cracks from temperature changes. Clients of huge warehouses often like to go this route, and it works fine when it is spec'd properly. But, they still need to provide contraction joints.

"It is imperative Cunth doesn't get his hands on those codes."

 

[XR250]

My concrete engineer says you have to spec the fiber at about three times what is normally added for it to do anything. I never count on it.