Rebar vs. Wire mesh and fiber in concrete

Discussion in 'Heavy Equipment & Pavement' started by concrete man, Feb 18, 2001.

  1. concrete man

    concrete man LawnSite Member
    Posts: 7

    I would like some input on what some of you like to use in your concrete. I use #3 rebar on most all my 4" slab work on 2'to 3'o.c. The inspectors here like to use wire mesh and fiber. In my experience, it is useless. I have tore out numerous slabs containing the mesh and fiber because of settlement and cracking. I know concrete is going to crack, but the rebar I have used has stopped the settltment and stopped the cracks from widening or shifting to where one side is higher than the other.
     
  2. parkwest

    parkwest LawnSite Senior Member
    Posts: 678

    Boy, if wire mesh doesn't work on a slab I think you may have a sub-base and compaction problem if it takes #3 rebar to get the job done.
     
  3. John DiMartino

    John DiMartino LawnSite Silver Member
    Posts: 2,555

    It works for me,if you dont pull it up when you pour the concrete,then it will be t the bottom,but if its in the midle like its supposed to be,it works well.Im sure #3 rebar is ging to help,but I still use wire mesh on every pour i do,which isnt many,I think ive poured about 15 slabs in last 5 yrs-,but all of them are doing fine.Are you using 4000 PSI mix? I find it works much better and is easier to get the disired finish with 4000 mix.
     
  4. WALT

    WALT LawnSite Member
    Posts: 27

    On a 4" slab, I would never use rebar, unless of course there is heavy traffic on it. Then I would think about making then slab thicker to put rebar in. I say that because, and I have no evidence on this but, I think there is not enough crete in a 4" slab when the rebar is encased. To be more clear, 4" thick, rebar about halfway inside, so less than 2" above the rebar, and the possibility of it failing it the thin spots. Thats why if I used rebar, the slab would have to be thicker. Thats just my personal experience with pouring crete, which has been too many to count, in my 9 years. If the slab that is 4" requires re-enforcing, I would only use wire mesh, squares not rolled. Also when using the wire mesh, it was taught to me that it should installed at the top of the bottom third of the crete. I've also used "chairs" to place the wire at that specified height. Altough this is not needed, a rake to pull it up is fine I think. Depends on where and what the slab is used for, I've poured many sidewalks with no re-enfocement at all. I also would like a clairifacation if my theory about the rebar is right. That's just my rule of thumb that I have aquired over the years.
     
  5. John DiMartino

    John DiMartino LawnSite Silver Member
    Posts: 2,555

    Walt,your right,whenever we pour a 4"floor that will carry a wall on the interior,we deepen a channel to 8" about a ft on each side of where the wall will go,then we lay the rebar in the there,that is the only place we use rebar in a 4" floor.
     
  6. WALT

    WALT LawnSite Member
    Posts: 27

    Yeah, Stem Walls, or preparing for a future wall are a excellent situations of making a thickend edge, but keeping most of the main area about 4". I take it you did not mean a concrete wall, so you bend rebar to a "L" shape into the new floor. But also idea when pouring a stem wall and having rebar protruding straight out of the channel for the new wall itself.
    I am looking for some litature about the rebar and reinforcing concrete. It was a long time ago I read this, but I believe it was a Corp of Engineer book. Looking on internet for it too with no luck. :( It's got me curious now, and it doesn't hurt to brush up anyway. Also thinking after I read concreteman's post again, that #3 rebar is rather small, less than 0.4" Dia. Welded wire fabric also comes in large gage steel to if you feel you just had to use something bigger in that pad. Plus saves you time and backaches from tying your rebar grid.

    [Edited by WALT on 02-19-2001 at 02:11 AM]
     
  7. parkwest

    parkwest LawnSite Senior Member
    Posts: 678

  8. concrete man

    concrete man LawnSite Member
    Posts: 7

    Thanks for the responses. In response to keeping the mesh up in the slab, it is easy to pull it up the first time but when you walk back in it to strike it off with a straight edge you walk it back down. The #3 bar is only 3/8" dia. I hate to mess with the aggrevation and danger of rolling out the mesh. Then if you have to wheelbarrow it you run the chance of tripping or hanging the front of the wheelbarrow on it. When I said a 4" slab is approx. it usually runs between 4 & 5". It's so much easier to use the rebar and cheaper. What do you think about the fiber that you can put in the concrete now. I had a salesman tell me that it would replace rebar and mesh. I very rarley use it. The only thing I found it helps is to keep the top from popping off from shirnkage cracks or from where salt eats the top off. If someone could help me find info on the fiber it would be appreciated.
     
  9. WALT

    WALT LawnSite Member
    Posts: 27

    My experiances with fiber crete was on an airfield where strength is a must. The problem with that stuff is the unvoidable contact that happens, and the price is considerbly higher. As you probabley know the fiber in the crete really will irritate the skin, and damage the eyes, more than regular concrete. It's mainly used as a secondary reinforcement, and as use said, reduces cracking, even hairline. Some studies have shown when doing a large pour where joints are needed, that fiber concrete will hold the joint together so tight that allows independent movement of the slabs, (freeze/thaw, or expansion). That would be a disadvantage of the fiber of course.
    I hate using rolled mesh too, thats why I said only use squares and tie these together. Pulling it up can be done, but not advisable of course if walking over it again, even chairs or stands to keep it up. By all meens screed it off from the sides, and/or work the crete from the edges, e.g bull float/skim float, use knee boards, or dangle the luckey finisher choosen from a piece of eqipment (excavator bucket) to get the hard spots HA HA :) I didn't say that did I? :) How do you pull up the rebar grid that you walk on? Or does the chairs able to hold the rebar with weight of a person on them. Also there is less give from rebar if your foot does get tangled in the grid, at least you give chance to save your @ss from tripping and falling.

    I also found this question that was asked on World of Concrete.Com. This kind of supports what I was saying before about cracking above the rebar, Although it is #4 rebar, it's the same idea. I think this will get the point across, and that size rebar you use is cool as long as you have at least 2" above the steel, I wouldn't take the chance.

    We do finish work for a pool contractor who insists on using #4 rebar in all his flatwork. We get hairline cracks above the rebar locations except where concrete is 6 inches thick. We have suggested that he use welded wire mesh but he refuses. What can we do to stop cracks?

    Answer:

    Your suggestion of using wire mesh (which has a smaller diameter than #4 bars)--and your observation that cracking doesn't occur when the steel is deeper in the concrete--both agree with the recommendation of the ACI 302, Section 2.2.2, which appears in Concrete International, June 1980.

    The first paragraph of that section says:

    "Plastic settlement cracking over rebars is caused by inadequate concrete cover over steel, larger rebar size and/or higher slump concrete or a combination of these. To help avoid these problems, concrete cover should be at least 2 inches (50 millimeters) where possible. Other measures to avoid this are smaller bar size, low slump concrete (2 inches or 50 millimeters) and/or revibration of concrete."

    This recommendation is based on data presented by Fadh. H. Dakil, Philip D. Cady and Roger E. Carrier in the article "Cracking of Fresh Concrete as Related to Reinforcement," ACI Journal, August 1975, pages 421-428.

    ACI 302 also says (Section 11.1.3):

    "Some cracking may occur before the concrete has hardened. This may complicate the finishing operations considerably. Some examples are: . . . (2) cracking from settlement of concrete around reinforcing bars or other embedments."

    I hope this helps...


     
  10. DIG'EM

    DIG'EM Guest
    Posts: 0

    Concrete man do you know DIG'EM? How do you like the computer? How do you like lawnsite ?
     

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