WWF vs Rebar 4

[STR04]

Can anyone tell me which system you prefer and why? I hear this thing go back and forth about WWF not being installed properly, however it cost much less. I'm working on a design-build project where cost is very important but I'm not sure everyone will buy off with a cheaper solution should quality be effected.

TIA

 

[JAE]

Check out this thread:

thread507-126594

 

[archeng59]

I always specify reinforcing bars instead of WWF. the WWF takes less effort to lay it out, but always ends up in the bottom of the slab where it does absolutely no good. I've tried to support the WWF on blocks, chairs and bolsters but all that accomplishes is creating a trip hazard for the laborers. rebar at 16" o.c. each way allows plastic chairs to be used, everyone can put their feet between the bars to walk around. most contractors I work with like it even tho the bars take longer to place.

 

[ArneG]

WWF is a fine product if installed properly, and it does do something even at the bottom of the slab - unfortunately its not crack control.

Just ask any demo contractor who has demo'd a slab without reinforcing and one with WWF.

Rebar is however hands down better for crack control.

However you need to remember rebar and WWF don't act until the slab cracks. And for some owners, a hairline crack is still a crack.

Also remeber SOG vertical capacity is all subgrade modulus, f'c, and thickness of slab. Its got nothing to do with rebar.

And when in doubt - add (2) control joints.

 

[miecz]

I use rebar, and reduce or eliminate the control joints. When using rebar, does anyone see any point in adding fibers to the mix?

 

[archeng59]

I think adding fibers to the mix is a waste of $$. If the slab is for pedestrian use only, like a normal office, I've used fibermesh but put sawed joints (or a construction joint where it made sense) at 10 ft on center each way. that seemed to help with cracking between the joints. Using fibermesh with sawjoints or construction joints at a greater spacing resulted in some cracking between the joints. Based on conversations with a contractor that I've worked with on several different projects where we used either fibermesh or rebar in a slab-on-grade, he prefers rebar to the extra sawcuts and fibermesh.

 

[archeng59]

in my previous post, I meant using fibers with rebar is a waste of $$.

 

[ArneG]

Fiber and reinforcing do different things.

Fibers are for plastic shrinkage crack control (shrinkage that happens when the concrete initially loses its water) and durability (impact resistance).

Reinforcing is for crack width control.

Its interesting that we structural engineers tend to do one or the other when they arn't for the same thing.

If I'm hard on a project for money, I use unreinforced slabs with a tighter control joint spacing.

I hardly ever use fibers on exposed concrete surfaces, unless they're the kind of fibers designed not to be seen.

 

[SlideRuleEra]

I prefer rebar, but if you need to use WWF for cost control, here are several documents (free. pdf downloads) on how do use it correctly

http://www.wirereinforcementinstitute.org/HTDocs/Pages/pubfullcat.htm

http://www.SlideRuleEra.net

 

[minorchord2000]

My experience with WWF is disastrous. The stuff never ends up where you want it. The contractor had to remove portions of some flat work that had gotten away from him during the finishing process. During the removal of a 50' square portion of concrete, the WWF was at the bottom of the slab not doing a damn thing for which it was intended. I like the fibers for the micro cracks. I always mildly reinforce all slabs on grade even if it is #3 bars. Joint spacing is the most critical element taking special care around re-entrant cornerns. There have been voluminous studies done about the spacing of control joints based on concrete thickness, etc. You can check ACI, and PCA.

 

[ccvasquez46]

What about WWF used in elevated slabs with metal decking? Has everyone had similarly disastrous results?

Most metal floor decking product catalogues seem to give capacities based on wwf reinforcing.

 

[JAE]

In decking for structural slabs, we haven't had near the problems compared with on-grade situations. In fact, most of our non-composite deck designs (using formdeck) utilize either rebar or WWF for the primary reinforcing.

 

[bjb]

In my area, the big box developers are getting away from having any steel in their sogs. It's been my observation that wwf never ends up where it's supposed to be, and that it always ends up at the bottom of the slab. With sog design, I think the most important things are making sure you specify a low shrinkage concrete mix, have a good subbase, and have the proper control joint spacing. WWf or rebar doesn't stop a crack from opening, and it's risky to extend joint spacings because you have some steel in the sog. It is my opinion that wwf is not money well spent.

I generally favor placing joints spaced at 2 to 3 times the thickness of slab (in feet)and using fibers for plastic shrinkage cracking. Euclid makes a macro fiber that is backed up by good test results that show it is effective in reducing plastic shrinkage cracks. We also specify the use of the Soff-Cut saw, and use plate dowels by PNA at our control and construction joints. With a low shrinkage mix, we have had good results using this system and haven't had any serious problems or complaints. Also, when we size the thickness of the slab we use "Designing Floor Slabs on Grade" by Ringo and Anderson.

 

[emmgjld]

I agree with JAE. On-grade applications are typically more troublesome. I tend to require bars for residential/small commercial construction and and allow WWF for projects which will have more field control and inspection. As noted above, I too, have never found a worker whose foot can fit in a 6"x 6" space. The rebar option tends to be less troublesome, even though it is more costly.

 

[VTPE]

A blended fiber mix has really performed well for me. Synthetic Industries makes a blend they call Fibermesh 850 which is a blend of steel and poly fibers. The poly fibers help with plastic shrinkage cracking. The steel fibers help with elastic shrinkage cracking. The beauty of the mix is that it puts the steel everywhere in the slab and is not dependant on a guy standing on top of it trying to pull it into place. There are some tricks with finishing the slab, but the manufacturer will help with those. In addition, joints have to be spaced at a max 3*slab thickness.

 

[STR04]

If most designers are prescribing their control joint spacing according to plain concrete (Table 9.9 from ACI 360 for plain concrete) then why does it matter how the WWF is installed? It would only be a bonus if the contractor installed it properly, right? Why all the complaints about WWF if you really don't need reinforcing at all unless designers are increasing their joint spacing, but how is this justified on paper? Does anyone use Formula 6-3 from ACI 360, As = FLw / (2 x fs) to increase joint spacing?

 

[MikeE55]

I've been using the subgrade drag formula (referenced by STR04) for years with minimal problems. I agree with STR04, if you use cj's at spacings for plain concrete, why put in any reinforcing at all? I don't use plastic fibers for that reason - the manufacturer's recommend you treat the slab as plain concrete. WWF may be successful in a 4" to 6" slab, even if it's on the bottom - but I would limit cj spacing to about 25 feet. For a slab thicker than 6" I would recommend rebar.

 

[bjb]

I never use that formula to increase joint spacing, in fact I can't remember a time where I have tried to increase joint spacing beyond 3 times the slab thickness (in feet, 3x6" slab = 18' joint spacing). This is especially true for something like a sog in a fire station where we really care what it looks like in service. Since I work out my joint spacing based on plain concrete, WWF is typically not needed, so it's not worth the money, so I don't spec it.

If you do want to increase joint spacing, I recommend looking at Concrete Floor Slabs on Grade Subject to Heavy Loads by the Corps. of Engineers (Army TM 5-809-12) that you can download for free from thier website. It has a procedure for using rebar to reduce slab thickness, and if I remember right for increasing joint spacing. I only use reinforcing in a sog if I have a calculated reason for it, like if I am limited to a certain thickness and that thickness of plain concrete isn't strong enough. Then I use rebar according to the above reference. It is much more likely that rebar will end up where you want it.

another good reference for designing sog's is Designing Floor Slabs on Grade by Ringo and Anderson. I strongly recommend it.

Another observation I have is that it seems that no 2 engineers design/detail their sogs the same way, we seem to all have our prefered ways for doing it, and that's fine. My suggestions are based on what I have found to work, but their are other ways to make it work too. I think the most important factor to a successful sog is to see the big picture, and not to focus on one particular thing to the exclusion of others. You need a good low shrinkage mix, proper subgrade and subbase, and good jointing design. Beyond your control, but critically important is a contractor who follows good construction practice. The sog has to be finished properly, cut at the proper time, and properly cured.