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

 

[bridgebuster]

NYCDOT standard sidewalk is 4" unreinforced concrete on 6" of gravel, broken stone, or sand. Driveways are 7" unreinforced concrete on 6" base. Bus stops are 12" concrete with WWF4x4-W4-W4 with 4" of cover. Dummy joints are 1/8" wide by 1/2" deep or 3/4" deep if saw cut.

I just figured I'd muddy up the water a little more.

 

[MacGruber22]

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.

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."

 

[BUGGAR]

Yoda speaks: "Proper and thorough curing. That is the Way."

 

[RPMG]

I realize that I'm opening myself up for insult. We use wire mesh as temperature reinforcement, and I have yet to receive any performance problems or contractor complaints. Also, wire mesh is used very commonly with metal decks (elevated floor slabs).

 

[emmgjld]

In my world, Much depends on the subgrade conditions & preparations.
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.

 

[TehMightyEngineer]

XR250, yeah I've done a lot of investigation into research on using macrofibers for strengthening concrete without rebar, it can be done but requires very large dose rates and probably isn't worth it.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[medeek]

@ emmgjld

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

http://www.medeek.com

 

[XR250]

t sounds like you are talking about using fibers for primary reinforcement - no one that I am aware of does that.

I was referring to it for temp. and shrinkage reinforcement. I used #4 @ 24" O.C. in my house in the basement and first floor and have seen no reflective cracking.
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.

 

[TLHS]

You certainly don't need to reinforce a slab on grade if it isn't critical to control cracks and you've followed common practice for crack control joints. For more critical applications, the ACI document on slabs on grade recommends half a percent of reinforcement in each direction at either two inches from the top or two thirds from the bottom (it suggests that both are common).

 

[emmgjld]

@ medeek

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.