4" slab with 6 X 6 W 1.4 X W1.4 WWR 2

[davesatt]

What are most people seeing the contractor do when this type floor is specified ?
How close do you need the chairs to hold mesh this light of mesh in place. 12" ? Does anyone allow the contractor to walk in mesh or pull up mesh ? How can the contractor get the chairs in place in an enclosed building (he can't pump from over head or pour from the sides) with a lazer screed and concrete trucks on the grade ?

 

[cvg]

we don't allow walk in or pull up of any reinforcement. In fact, some clients we work for don't allow this type of WWF at all and require bars instead. The only place I use it is for shotcrete slope protection.

 

[DarthSoilsGuy]

I've never seen anyone specify floating it in or pulling it up and noone should. it happens the majority of the time for SOG and people don't fight it to stay away from means/methods fights with the contractor PM.

There's always a way to do it right. The WWF can be set, spaced, and secured piece by piece during the pour or b/n trucks, and it would be the 1st time i've ever heard it happen. shoot, you could even have the spacers pre-tied on sheets of wwf.

 

[concretemasonry]

You you feel you cannot live with pulling it up or using chairs for the approximate height location the option is rebar.

Ir you go with rebar, do not space it at 12" unless you expect the concrete to be placed by ballet dancers. Go to a larger spacing and expect less crack control.

With good control of construction practices and concrete placement, mesh can and does work.

 

[msucog]

if there's wire in the slab, it's more for looks because i'm willing to be it will not be properly placed at the end of the day. the only times i've seen it correct is closely spaced (<4' o.c.) chairs with heavier gauge wwf. if the reinforcement is essential, i suggest specifying rebar if you want it correct. out of all the slabs i've cored, i guess less than 5% of the cores have wwf anywhere near the middle. probably 50% has the wire on the very bottom (<1/4" from bottom). a lot of contractors are using fibermesh in lieu of wwf (i'm not sure how effective this is but the word is that it works at least as good as wwf on the bottom).

 

[sdz]

I believe I have seen chair spacing for mesh recommended at 800mm maximum centers. Light mesh might require closer spacing.

 

[davesatt]

Thanks for the input.
I agree that most mesh end up at the bottom of the slab.
I also think that a 4" floor with mesh is not much better than a 4" plain concrete floor because of this.
I worry about damage to the vapor barrier with wire chairs.
I remember having a guy at the WOC saying how can you stand on wire mesh and lift it 2". I tend to agree with this but I would say it is just as difficult to install chairs while standing on the mesh.
I like the idea of going to larger bars and larger spacing.
Has anyone done this in a 4" slab ?

 

[sdz]

You can chair up mesh; I've done it. Also you can get pans to go under the bar chairs to prevent them punching through the plastic and foundation. Or, better, you can get plastic chairs with a wide base.

http://www.smorgonsteel.com.au/rein....cfm?GroupID=4&ProductLineID=20&ProductID=127

 

[WillisV]

Just use fiber reinforcing and get rid of the mesh.

 

[apsix]

It must be a cultural thing.
In Australia mesh is routinely used and I'm not aware of widespread problems with its positioning.
Typical chair spacing is 1000 x 1000mm, reducing to 800 x 800mm for wire diameter less than 7mm, I believe.

 

[minorchord2000]

I have been a structural engineer for over 30 years and have NEVER EVER seen this WWF or WWR installed properly. It should be BANNED as a reinforcing mechanism for slabs on grade. I will NEVER, EVER specify it for anything other than for 4" concrete slope protection. Engineers here in AZ will not use it. We had a seminar for our local structural engineering association and the speaker was a rep from the welded wire institute. He was roundly and politely booed when he tried to extol its virtue.

If you want to use something to reinforce against shrinkage and plastic cracking, use polypropylene fibers AND a good quality curing compound.

It is a monumental waste of money to put this stuff in slabs. It nevers ends up where the engineer wants it to be. So he wastes time calculating and detailing it, and the contractor wastes time and money putting it in and the owner wastes money thinking he is getting a quality project.

Everybody loses.

I cannot believe that we are still talking about this stuff.

 

[civilperson]

Minorchord has a definite but wrong opinion on WWF. Properly specified and placed, WWF gives value over rebar in cost and strength, (due to ease of placement , lesser laps and higher yield strengths). With continous chairs at small intervals, the location is fixed prior to concrete placement and using proper techniques can eliminate the need to walk on the fabric.

 

[cvg]

I would have to side with minor chord as that is also my view. At least in Arizona, WWF has a bad rap. My clients don't allow it to be used. Note that I come from a heavy civil background, not so much buildings.

Possibly, our local (heavy civil) contractors have never figured out how to do it right.

 

[concretemasonry]

I agree with the ability of certain contractors being able to place/use it correctly.

Commercial contractors are too far away from the placement details and are more concerned with volume production and the big picture. If you do not good control on site, problems with wire are possible, but probably not of the magintude to ever be documented or identified.

Contractors on smaller problems can easily put the wire where it belongs because the concrete it not thrown at them that fast. This is far superior to the vaguaries of fibers and the over mixing/agitation problems that occur. - I can still remeb=mber when the glass fibers disappeared before alkalai resistant glass was used.

I had some work done and all the wire was just below the center of the slab to permit sawing joints and still have enough coverage for the wire. I would never had considered fibers for this application.

Dick

 

[emmgjld]

In my experience, there 2 types of WWF:
- The Flat Sheets, which can be properly positioned (I have never seen a concrete worker who had a small enough foot to fit within the mesh)
- The material which comes in rolls, which I have never seen properly placed, unless the vapor barrier needs reinforcement.

I only specify the Flat Sheets, except for some gunite applications.

 

[davesatt]

The contractor placed this floor they used plastic chairs on 2 to 3 foot centers. They used 2 kinds of plastic chairs the rigid plastic chair which the clips break off when someone steps on and the flexible chairs (they are expensive) which come back up about 80% when stepped on or ran over. With the chairs on 2 foot centers the mesh still deflects almost 2 inches when someone stands on it near mid span. The contractor had 2 labors chairing mesh and one laborer hooking and lifting mesh that had deflected badly.
I would say with a contractor really trying and being watched 50% of the mesh is 2 inches deep 25% is 2 1/2" deep and 25% is 3" deep and I don't think any vapor barrier was ripped.
Can I call that a success ?
Is this what the designer has in mind when he specs this type floor ?

 

[concretemasonry]

You want the mesh at or below the center of the slab.

Assuming a 3 1/2" slab. you got 75% with over 1" cover. You got the remaining 25% with 1" cover. No was laying on the poly. You got 100% uniformity covereage of the total area. I would say that it is reasonable to say it was adequate and the contractor made a good effort to comply. What did the specs say?

The alternate with fibermesh does not have the coverage condition, but you always face the over-mixing problem that is very difficult to determine. Balled fibers can loook a lot like aggregate (my observations). It does not take but balling to lose a substantial amount of the effective fibers.

 

[sdz]

I agree with concretemasonry. The mesh is for crack control so if it is a bit low it will still work. It would be a different matter if you were relying on it for flexural strength.

 

[hokie66]

Slabs on ground crack because of restraint from the subgrade, thickenings, or other restraining features. For subgrade restraint, reinforcement near the bottom has the best chance of controlling cracking, because the tensile force is applied at the bottom surface.

 

[jike]

You are interested in preventing cracks or at least keeping them tight together at the exposed top surface, not at the bottom surface where no one will see it. This is why "Designing Floor Slabs On Grade" by Ringo and Anderson recommend steel at or above mid-depth to control shrinkage cracking.