A word (or a thousand) about control joints...

Discussion in 'Article Discussion Forum' started by a2mfk74, Aug 28, 2013.

  1. a2mfk74

    a2mfk74 Structural/Forensic Engineer Published

    After reading several posts about this subject, I went through my technical library and did a little refresher reading, and put this article together. I hope you find it useful. This is not about moisture barriers or other subjects on slabs, just about control joints and cracks.

    The Behavior of Concrete Slabs With Control Joints and Cracks

    Please read the article and discuss it below.
  2. Jim McClain

    Jim McClain TFP Owner/Founder Administrator

    Thank you for the time and effort you put in here, a2mfk74. I'm sure the article will be well received.

  3. Good article, like the insight, learned a bit about future behavior of slab, (temp/moisture). Glad your a member!
  4. Barry Carlton

    Barry Carlton I Support TFP Senior Member Published

    Very well written. Only a couple of minor inaccuracies. But the science of concrete is lengthy and constantly evolving to some extent. You packed a lot of info into a concise package. Good job.
  5. Incognito

    Incognito No more Mr. Nice Guy! I Support TFP Senior Member

    Great article. Thanks

    I always thought it was bizarre to sawcut the concrete to allow for cracking/expansion/curling/settling and then filling it with cement based patch. Doesn't make a lick of sense-------without any scientific analysis.

    But that's what they call for and that's sure as shit what we do. Haven't had any "issues" for many years. I'd say roundabout 10-15 years ago we were doing TONS of new retail stores and we did have issues with the patch at the sawcuts, but it never amounted to anything we had to suffer extensive repairs or unsatisfied customers. A few slabs curled, hurled, cracked and shook with earthquakes to boot. But we'd get paid to go back and replace a few cartons of tile here and there on one store out of a dozen or so. Sometimes it would be so minor you'd just fix it when they were paying you to repair the "trade damage" or tiles damaged by the store fixture/merchandise crews. So that all gets rolled in a change order/ EXTRA ticket for repairs.
  6. a2mfk74

    a2mfk74 Structural/Forensic Engineer Published

    Please point out any specific items you find to be inaccurate, or perhaps poorly worded or explained, or tell me if you just flat out disagree with something. I am here to learn too, and I am searching for the truth, not to just be right all the time (though that is fun too!).

    I will research these issues further, and may produce an addendum or erratta if it is warranted. Any further technical citations are also welcomed.

    Thanks to everyone for your compliments, and if you have any other questions that I can incorporate into a revised version I can do that also.
    Last edited: Aug 30, 2013
  7. a2mfk74

    a2mfk74 Structural/Forensic Engineer Published

    Were your callbacks on the control joint patches due to cracks in the patch? As in, the joint opened up slightly and then cracked the compound?

    There is a theory and practice with SOG to just let them crack wherever (no CJs), and then once the cracking is "done", fill with epoxy or other material.
  8. Thing I saw, was chicken wire described as wwf, I think of chicken wire as octagon, now working at lowes years back someone asked me where the hardware cloth was, they quickly said another name I recognized, I said oh, outside garden on the left as you walk out door. So I think it's more like hardware cloth, I'm not a chicken connoisseur but maybe you were talking about chicken flooring, verse chicken walls.
  9. a2mfk74

    a2mfk74 Structural/Forensic Engineer Published


    I am a connoisseur! - fried, grilled, rotisserie, stir fry, you name it, I eat it.

    Perhaps in my attempt to use more common terms non-concrete people may recognize I only caused confusion. And I also probably should have used the newer term- Welded Wire Reinforcement (WWR).

    But if I had said the stuff they put in slabs that looks like "steel field fencing designed to handle the tough task of corralling large animals", I think the overall reaction would be :confused::rolleyes:

    Shop Sierra 47-in x 330-ft Silver Zinc-Coated Galvanized Steel Field Fencing at Lowes.com
  10. Incognito

    Incognito No more Mr. Nice Guy! I Support TFP Senior Member

    Mostly there was just curling at those sawcuts after the fact. Sometimes the slab would expand and squeeze up a little patch, just enough to make a visible lump line from a distance with the high gloss wax/finish. The patch was usually sound underneath when we'd burn out a row to look see, so long as it was decent grade material and mixed correctly. Sometimes guys water down the patch mixture ratio and it's fairly evident when you open it up that it's too SOFT, sinks into the hole, shrinks or just gives from the pallet jack traffic. We had guys who didn't vacuum out the cracks, guys who tried to fill the cracks in one pass, guys who used white patch.........any stupid pet trick you can imagine. We also had quite a few moisture issues that first manifest themselves along the control joints by weakening and degrading the patch. Normally we'd have the waiver signed where the GC opted out of the moisture treatments------or delay. We never fixed ANYTHING substantial without getting paid hourly time and material at union journeyman rates.

    At one time one of the major brands out here was labeled "cementitious" but you could see the way it dried that it had gypsum or some other damn chemical besides Portland cement. That crap turned to mush. It was very nice as far as "workability" though and the stores looked SWEET when they first opened. My "trick" was to find some concrete guys, ceramic or stone work going on and "borrow" a couple or three big healthy scoops of Portland cement. When patching joints you're only able to mix a little at a time. So I'd throw a handful of Portland cement into each batch and that made it "workable" pretty much exactly like Feather Finish. It dried a nice, dark color. It was MUCH harder than the scab brand we were using and it held up better-------few to ZERO call backs.
  11. Barry Carlton

    Barry Carlton I Support TFP Senior Member Published

    Your references to the portland patch and epoxy fillers.

    There are portland patch mixes out there with many times the compressive strength of the average slab and thus on the average control joint will not "pinch" out or crumble if mixed and used properly.

    Epoxies can be used to "weld" the slab as you said, however if the CJ or crack is active at all, generally you will end up with a "sister" crack alongside the original cut or crack. This is a problem with the high compression patches as well. On active joints or cracks, sumptens gotta give.

    But as I said earlier, this is a science that volumes could be and have been written about. You did a great job in making it so concise.
  12. I was told at Mapei class in Deerfield Beach Florida all patches contain Gypsum, maybe different quantities but all do, you may hear a salesman saying no gypsum but maybe not true.
  13. Mr Structural Engineer
    As a token of my appreciation I would like to send you a gift card, any specific one? I think lowes has an endcap full of them, PM me an address to send it to or PayPal or any other method, thanks Mike
    The brew is a given, just as long as your not forensicing my job!

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