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Thread: Concrete question

  1. #1
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    Concrete question

    i have just gotten a mono slab poured on last sat. and have noticed today that i already have 3 cracks and not in the expansion joints like they were supposed to be. they are coming from the very edge where it is over 16inches deep and has 2 rebar in them. if this was to happen after the winter time and spring thaw that would be one thing, but it sure aint winter and not spring time either.. they are ranginging from 3 to 6 ft long already any opinions would be appreciated...to me this is just alittel to soon what would you do????
    If in Doubt, Build it Stout!
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  2. #2
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    These are probably just minor surface shrinkage cracks, Larry. I doubt they go all the way through the slab. It wouldn't surprise me if you get some more, and/or if the ones you have now grow some more. These things often show up in the first few days after a slab is born. Another thing that's possibly contributing to things is the fact that since the edge is thicker, it's expanding and contracting a little differently that the rest of the slab. If possible, keeping the slab moist and cool for a few more days can help reduce the cracking a bit.

    I'm betting these cracks are so thin you can't stick much more than a piece of paper in them. Unless you start seeing the cracks open up wider and wider (how wide would depend on the size and thickness of the slab), I don't think you have anything to worry about. The rebar's gonna keep things from going too far.

    Your slab's just following Benny's Law of Concrete:

    1. It's gray
    2. It gets hard
    3. It cracks

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  3. #3
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    well vaughn they go down the side of the thick part and are thin now but the fact that they are going down the outside portion of the thick part where the bearing walls are to be is what has me wondering i have dealt with concrete before and do know the three laws of concrete,, but havnt had this happen before???? thansk for your input and will see what he guy that poured it has to say?????
    If in Doubt, Build it Stout!
    One hand washes the other!
    Don't put off today till tomorrow!

  4. #4
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    Quote Originally Posted by larry merlau View Post
    well vaughn they go down the side of the thick part and are thin now but the fact that they are going down the outside portion of the thick part where the bearing walls are to be is what has me wondering i have dealt with concrete before and do know the three laws of concrete,, but havnt had this happen before???? thansk for your input and will see what he guy that poured it has to say?????
    Even though they're going down the outside of the slab/foundation, I suspect they're not very deep. Unless you're seeing wide cracks, or if the slab on one side of the crack is higher than the other side of the crack (indicating up and down movement under the slab), I think it'll be fine, even under your bearing walls.

    As Steve asked, where are the joints in the slab? (More specifically, how far apart are they, and how thick is the slab?) Also, for clarity in case others read this in the future, are the joints expansion joints, control joints, or a mix of both? Expansion joints have some type of resilient material between the two pieces of concrete, so the separate pieces can expand without pressing into each other. Control joints are simply tooled or sawn into the surface of the slab, to give the slab a "path of least resistance" for the inevitable cracks.
    When the going gets weird, the weird turn pro. - Hunter S. Thompson
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  5. #5
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    I ALWAYS guarantee my concrete will NOT crack.......until I get out of the driveway.


    Concrete is going to crack, the contol joints are supposed to help but as many pipes as you got sticking out of that crete Larry, don't be surprised that you will see more.

    There is a "formula" so to speak, that for so much area you need a joint at "X" amount of feet. Any time you have a jog in a wall ....there will be a crack. You don't have any jogs, but still a lot of pipe would be a area to me that would be a concern.

    Don't worry...I doubt it is going anywhere. It is just a cosmetic thing you hate to see. I have one crack in my 32 x 48 floor, been there since the first week I poured it and I ain't tripped over it yet no matter how many brewskis I might have swallerd.
    A very wise man once said.......
    "I'll take my chances with Misseurs Smith and Wesson. "

  6. #6
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    Quote Originally Posted by Steve Ash View Post
    I ALWAYS guarantee my concrete will NOT crack.......until I get out of the driveway.
    That's a great line. Spoken with experience, too, I'm sure.

    Quote Originally Posted by Steve Ash View Post
    There is a "formula" so to speak, that for so much area you need a joint at "X" amount of feet. Any time you have a jog in a wall ....there will be a crack. You don't have any jogs, but still a lot of pipe would be a area to me that would be a concern.
    A conservative rule of thumb formula I've used for paving joint layout is to take the thickness of the slab (in inches), multiply by two, and use the resulting number (in feet) for joint spacing. In other words, for a 4" slab, the joints should be about 8' to 10' apart. Like I said, that's a pretty conservative approach, and it's geared towards paving, which usually gets different loads than building slabs. Guys like Doug Sinjem who do building slabs day and night can probably give better advice for building slabs. And you're absolutely right about any slab penetrations like pipes and conduits, as well as inside corners and jogs. Crack magnets is what they are.

    Quote Originally Posted by Steve Ash View Post
    Don't worry...I doubt it is going anywhere. It is just a cosmetic thing you hate to see. I have one crack in my 32 x 48 floor, been there since the first week I poured it and I ain't tripped over it yet no matter how many brewskis I might have swallerd.
    Yeah, Larry's slab doesn't sound like it's going anywhere for a looooong time. And let me guess...your 32 x 48 floor either cracked right down the middle (to become two 32 x 24 slabs) or about 1/3 of the way across (to become 32 x 32 and 32 x 16 slabs).

    Benny's #4 Law of Concrete: It wants to be square.
    When the going gets weird, the weird turn pro. - Hunter S. Thompson
    When the weird get going, they start their own forum. - Vaughn McMillan

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  7. #7
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    Answers!!!!

    first of all the control jints is the proper term in my case, plastic strip inserted in during the pour, not the thick tar based celotex kind of stuff vaughn, and they have the control joints going from my pipes to the other side and i would understand that aspect of it. 2 of these cracks are now wheres close to a pipe underneath or sticking out threw the crete.. and i havnt checked to see how deep they are but i looked at a floor that i had poured in 87, and its 28x 40 with no cracks yet this is just alitte to soon for my idea but will wait and see what the crete guy has to say... the control joints are no more than 9 ft apart. and they are doing what here supposed to do... i understand that the bearing walls shouldnt go any where with 2 rebar in them but i can just see the inspector seeing these and then saying i need to replace the slab i wont be a happy camper... the shift isnt there as of now steve or vaughn and i wont stumble on them even after i get watered down.
    If in Doubt, Build it Stout!
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  8. #8
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    A couple of over things that can lead to cracks are, the ground under the concrete was not compacted enough or the conrete dreid to quick. A good concrete job should not crack.

  9. #9
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    Larry, I don't know what to tell you at this point. I inspected a lot of concrete in my early years for Tx, DOT, (Bridges, etc), and did prep work for Contractors on the same type projects and one thing that always seemed to be the "Most Determining Factors" in eliminating cracking was Proper Compaction, and adequate Steel. (Note; this assumes the right "slump" concrete is used, drier being stronger and less crack prone)

    Since my contractor was so slow getting anything done on my shop, it was a plus for my slab, since it sat for 3 1/2 months after it was ready, before the pour was actually made, and was rained on a few times but completely dry when the pour was made. This "Set Time" allowed all the "Base" dirt to resettle and compact with no hard and soft spots during that time. I personally added rebar tied to their 6" steel mesh every two feet in both directions across the slab, giving 2' squares between the rebar. I also monitored and directed the vibration of the pour which was a 5" thick slab (plus the footings). I was told about two months later that of the 11 projects those folks poured in a 3 week period, that mine was the ONLY one that did not have even one crack in it and all the others were cracked pretty badly. It has been over 3 years now, and there has been only one 8" long hairline surface crack develop close one end of my DC/Electrical trench that angles over to the edge of the slab. We also used the fiberglass fibers mixed in the concrete for the pour, (which was a first for me, and I don't know if it had any effect or not).

    Note: There is not One Expansion Joint anywhere in the slab. The shed room slab was poured later, and there is no cracks anyplace where the two slabs join, or anywhere in that slab either. I also have conduit, water, and sewer lines coming up through the slab in 4 different locations in the building, and no cracks have developed around any of those.

    I sure hope yours settles down and the cracks don't open up any further. (I hate Concrete Cracks).
    Last edited by Norman Hitt; 06-15-2007 at 09:43 PM.

  10. #10
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    well i wish it were the case

    but i found two more tonight and no word from the guy that did it i had rewire in it norman but the ground wasnt comacted other than the 3 weeks of rain here and ther on it.. looks like i will be havin more as time goes on.... i think i will regroup next time...the slab was 4" and i was told that i was in good shape... wrong
    If in Doubt, Build it Stout!
    One hand washes the other!
    Don't put off today till tomorrow!

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