reinforcement mesh or rebars? 1

[gmd255]

Hi, when do you guys reinforce slabs with reinforcement mesh and when with rebars only (no mesh)?
I have seen that for large spans (lets say 6 x 6 m) they usually reinforce slabs with rebars, but not meshes. Why is that? Maybe because of large tension forces in rebars which means we need larger lap lenght between meshes?

In my country rebars are up to 6 m long (12 m for special orders).

thank you for help

 

[JedClampett]

You'll hear this in a lot of ways, but most of us think mesh is worthless. It's just too hard to keep it supported. When the laborers pour the concrete, they push it down by walking on it. And that's where it stays. So you have a layer of reinforcing without sufficient cover or at the soil concrete interface. Whenever I've seen mesh reinforced concrete being demolished it's a layer of corroded metal at the very bottom of the concrete.
With reinforcing bars, they're hopefully supported sufficiently that the workers can walk on it without smashing it down.
If you wanted to generalize, I'd say that if the steel is there for strength and not just temperature and shrinkage, it's bars. Mesh is used for slabs on grade. It's a little more economical as it's "pre-tied" and probably lighter than reinforcing bars, but lighter means less steel.

 

[MotorCity]

I never use it in slabs on grade for reasons mentioned above. I don't mind using it for shrinkage steel in elevated slabs.

 

[KootK]

- No technical basis for not using mesh so long as the rules are followed.

- Like the other guys said, there are practical issues associated with using mesh in North American markets.

In the past, when labor here was less expensive, things requiring craftsmanship like this were more palatable. I've seen examples where the mesh was even draped so that it could be both the top and bottom reinforcement. That seems almost unfathomable nowadays.

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.

 

[Seppe]

Agree with KootK.

Save for some moment frame / shear wall restrictions related to high seismicity, usage of "welded deformed wire reinforcement" in ACI 318 is on equal ground to that of rebar. Design equations are the same. ACI does allow you to refine development length and lap splice dimensions by taking advantage of a welded crosswire in the development region, but if you are a traditionalist (and want to avoid the hassle of mats stacking at the lap locations) then you are permitted to simply ignore the welds and calculate these lengths the same as you would for rebar.

Now, as it relates to plain (smooth) wire, especially the commodity stuff that is darned-near unmanageably flexible due to tiny wire diameters, yeah, not a big fan.

I've designed structures using the larger-diameter welded deformed wire mats before. No issues. I guess it just depends on the designer and which end of the product spectrum they're referring to.

 

[dik]

I just don't use WWM... too much of a problem...

Dik

 

[Ingenuity]

Not all WWM/fabric are created equal!

In AU for example, there are many configurations available, most (all?) in flat sheet form, with deformed longitudinal wires up to 12 mm diameter. In the US, the availability of different size mesh configurations is less.

I have designed (and placed) mesh in structural slabs (in AU) - those larger wire diameter sheets can get mighty heavy, and where 4 sheets intersect you have to think about placement order to avoid 'stacking'.

And, check with local supplier/manufacturer on the ductility of the mesh - often it is of lesser (low) ductility and some codes provide limitations.