Rebar vs. Wire Mesh
Rebar vs. Wire Mesh
(OP)
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.
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
www.medeek.com






RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
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...
RE: Rebar vs. Wire Mesh
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
www.medeek.com
RE: Rebar vs. Wire Mesh
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.
RE: Rebar vs. Wire Mesh
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.
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RE: Rebar vs. Wire Mesh
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Rebar vs. Wire Mesh
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RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
.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
www.medeek.com
RE: Rebar vs. Wire Mesh
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
www.medeek.com
RE: Rebar vs. Wire Mesh
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.
RE: Rebar vs. Wire Mesh
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.
RE: Rebar vs. Wire Mesh
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
www.medeek.com
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
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.
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Rebar vs. Wire Mesh
"It is imperative Cunth doesn't get his hands on those codes."
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
I just figured I'd muddy up the water a little more.
RE: Rebar vs. Wire Mesh
It sounds like you are talking about using fibers for primary reinforcement - no one that I am aware of does that.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
My father disliked wwf, at least since about 1951. Installation was always a problem & performance in near vicinity to expansive clays was usually poor.
His 2 comments :
1) Very little benefit if not a very heavy wire & the wwf will double the cost of removal when failure occurs.
2) He NEVER observed a concrete worker with 6 inch feet.
My comments for residential use:
a) I specify either #3 @ 18"c/c or #4 @24"c/c for 4" to 5" slabs. The #3 is preferred as the @4 has been associated with reflective cracking on the slab surface.
b) If curling may be an issue, I keep the rebar at the upper 1/3 zone (hence part of the reflective cracking problem).
c) A light wwf may be used with the rebar matte to support in-floor heat tubing.
RE: Rebar vs. Wire Mesh
Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com
RE: Rebar vs. Wire Mesh
I've been doing a good bit of reading on the subject and a #4 bar in a 4" slab may be a potential problem because of the reflective cracking. A number of other comments by contractors and others lead me to believe that WWM may be the best solution for a typical 4" residential slab because of this particular problem. However, I really would like to get away from calling out the mesh since all of the local contractors seriously dislike the stuff and they probably aren't installing it correctly anyways.
Lately I've had a lot of clients who want large RV garages and many of these have 6" slabs which naturally lend themselves toward rebar reinforcment. For these going to a #4 bar @ 24" o/c is a no brainer.
What is the thinking on fiber (poly) reinforced concrete combined with mesh or bar?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Rebar vs. Wire Mesh
I do have some super small hairline cracks in the basement as they used a pump mix. The first story was done with wheelbarrows and has zero cracks anywhere.
RE: Rebar vs. Wire Mesh
RE: Rebar vs. Wire Mesh
As a whole, most of my work with typical 4" residential slabs has been using the #3 bar matte. A lot of the finishers also use fiber, they seem to think the added expense is offset by some improvement of slab surfaces & a little less raveling at cut & tooled joints.