Rebar or wire mesh 2

[Gillespie]

I have a driveway structure on concrete metal deck and steel beams.
6" thick concrete W3 formlok 18ga. metal deck.
For reinforcing (crack/temperature/shrinkage) what is the better:
Wire Mesh or rebar.

concrete Contractors always seem to complain about the wire mesh.
I've never understood why they can't put the mesh in at the end (before pouring) so it's not all trampled on.

 

[sdz]

Both will work so use the one the contractors prefer; unless you have a technical reason to specify the other.

 

[Gillespie]

I would like to add another question to this thread.

Has anyone ever been sued for doing what the contractor prefers?

 

[TXStructural]

The codes all require that either mesh or bar be supported prior to placement of concrete, such that construction loads will not permanently deform or displace the reinforcement. Using a light mesh requires more support (more closely-spaced chairs) but when supported properly it performs well and provides a reasonably safe walking surface during placement. Mesh comes with wire sized up to about 5/8" diameter (large sheets of which obviously pose a challenge in handling unless a crane is used.)

In certain cases, the tighter spacing of small wires can help reduce crack size, compared to widely-spaced, larger bars. Since you have deck on beams, and most likely have stiffer girders as well, you will need bars extending over the stiffer elements to avoid excessive cracking. Using rebar may be better in your application so you avoid having to place both. Given a particular concrete mix and curing conditions, the sum of widths of all cracks will usually be similar. Closer bar or wire spacing typically results in more, narrow cracks; using widely-spaced bars generally results in fewer, but wider, cracks.

Are you specifying a traffic membrane to keep water intrusion to a minimum? If not, you can expect the deck to begin corroding very soon after the first cracks appear. Without internal reinforcement, moment strength will likely be compromised before the end of a typical design life.

 

[Ron]

Contractors want the fastest, easiest method they can use...they don't care if it works.

WWF is difficult to place such that it works. Contractors like it because it is easy to place, and no one is watching them, so just put it in there...doesn't matter to them whether it is on the bottom or not.

If they use WWF, then make them support it on tied chairs. If they use rebar, make them support it on tied chairs....it all boils down to whether or not you have the gonads to require what is necessary and follow through with it. Be an a$$ if you need to be. There's no harm in forcing the right result.

 

[delagina]

question: which is more expensive? say same area of steel

 

[ergus]

In nearly every building I have seen demolished or altered the mesh is right on the bottom of the slab where it is doing no good, that is why I always insist on using properly chaired rebar instead of mesh. A few years back the Concrete Associations were pushing the idea to cut out putting steel in slabs on grade and add an extra inch to the depth of the concrete instead. That idea was popular for a while but now seems to have faded to a distant memory.

 

[apsix]

If you can insist on properly chaired bars why not also insist on properly chaired mesh?
There is nothing intrinsic to mesh which stops it from being properly chaired.

 

[sdz]

I agree with above comments regarding proper support for both mesh and bars, and both need inspection. Beyond that I would specify what the contractor prefers.

 

[csd72]

Closely spaced smaller bars are better for crack control than the same area of widely spaced larger bars.

Personally I would specify mesh if the sheet is light enough for two men to carry, if not then I would specify bars. The weight is usually the issue that the guys complain about.

As per putting the mesh in last, the problem is that it is the reinforcement that most things like service ducts are tied to.

 

[HSIII]

Everyone that mentioned it is absolutely correct about placing the reinforcing, whether it be rebar or wire mesh, on chairs for adequate support and reinforcing positioning. In my previous years as a concrete field inspector, I actually held up a few slab placements because of the lack of chairs, or supports in some manner. The argument I kept hearing was "well, we'll pick it up as we go along". What contractors don't realize about this, or any other lack of supports, is the one square foot out of 25 that you pick up does absolutely nada, especially when someone else steps right there and, because the concrete is still fresh, pushes the reinforcing right back down to the bottom of the slab.

Bottom line, whichever you choose, insist on supports, and go there after they set the reinforcing to be absolutely sure it's sufficiently on the supports.

 

[bridgebuster]

"Contractors want the fastest, easiest method they can use...they don't care if it works."

At least I'm not the only cynic (or perhaps realist.)

 

[RHTPE]

Everyone that mentioned it is absolutely correct about placing the reinforcing, whether it be rebar or wire mesh, on chairs for adequate support and reinforcing positioning.

If they use WWF, then make them support it on tied chairs. If they use rebar, make them support it on tied chairs.

"Contractors want the fastest, easiest method they can use...they don't care if it works."

I'm not convinced it's a case of contractors not caring as much as it is a case of them not knowing that their methods are ineffective.

The key point is simple - either works, but must be securely tied in the correct position prior to placing concrete. Depending on which reinforcing is used, the support spacing must be suitable for the material AND the foot traffic it has to endure during concrete placement. Light gage mesh doesn't stay in proper position well under the weight applied by a 250 lb worker's foot, even with a reasonable bar support spacing.

WWF in rolls is a PIA - sheets work best and the placer won't have to deal with the mesh trying to spring back to its rolled shape. Bars require a lot of handling and tying of each piece so if you can achieve the steel area with mesh I think you would be better served.

If you want to better understand the final in-place cost, bid it with mesh and require an alternate for bars.

Ralph
Structures Consulting
Northeast USA

 

[jgailla]

Coming from a civil (pavement/slab on grade) perspective, there is no reason to use wire mesh. It is rarely placed correctly. If you need steel, use rebar, which is much more likely to be properly placed than mesh. If you don't need steel, use plastic fiber and properly timed and placed contraction joints. You will generally need contraction joints with wire mesh anyway. I don't specify wire mesh at all. I have observed many slabs being placed and have demoed quite a few slabs as well. The mesh is almost always on the bottom, where it does little good.

 

[kieran1]

The problems i see with mesh in these floor decks is the overlapping of the bars where 4 sheets of mesh meet. You need to use either flying end mesh or provide splice bars for lap. Otherwise you could have problems getting cover for the rebar

Kieran

 

[msquared48]

As for this question, "Has anyone ever been sued for doing what the contractor prefers?", the answer is definitely yes.

Just look at the detail change, for construction convenience, submitted by the contractor, but inadequately reviewed by the engineer, for the Hyatt Regency Walkway collapse back in the 80's. There are many others.

However, I do not think that is an issue here.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

There are times when you just can't seem to win. I inspected a flat plate a few years ago where the bars appeared to be properly chaired everywhere prior to the pour. The concrete crew was a subtrade. On the day of the pour, they thundered around the reinforcement like a heard of elephants. At one point, two guys were vigorously jumping up and down on the rebar where concrete was being pumped into the form. They were trying to make last minute adjustments to the rebar location.

I stopped the pour to make proper adjustments and was subjected to considerable criticism, harassment and abuse by the foreman and some of his crew. There are times when an inspector's lot is not an 'appy one.

When the concreting crew is a subtrade coming in for the day of the pour and leaving immediately afterward, it is sometimes difficult to obtain their full co-operation, especially in the absence of firm direction from the general contractor.

BA

 

[ACtrafficengr]

Are steel fibers an option?

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[csd72]

RHTPE,

Truly, most of the guys on site do not care. As long as they keep their jobs and get home on time they always think of it as someone elses responsibility.

 

[RHTPE]

csd72 I would beg to differ. All of the craftsmen that I know personally do care - especially when they understand the consequences of doing things incorrectly. The key is getting their supervisory personnel to understand this.

ACtrafficengr & jgailla This is clearly and elevated slab, as hinted at by the OP's statement regarding "W3 formlok 18ga. metal deck". Fibers alone would not be a proper option for main reinforcing.

jgailla Since this is an elevated slab, contraction joints would not be used.

kieran1 You are correct that the 4 layers of mesh could be a problem when the sheets are lined up in both directions. This can be minimized to 3 layers simply by shifting the laps in one direction by half a sheet.


Ralph
Structures Consulting
Northeast USA