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Concrete expansion joints needed?

KBenj | Posted in Construction Techniques on November 7, 2008 03:32am

On a new addition to my home the mason when pouring the garage floor slab (at basement level approximately 4′ below grade on three sides) did not use any control or expansion joints.

The lack of control joints doesnt bother me nearly as much as the lack of expansion joints as I can handle some cracking of the concrete surface rather than along a control joint.

My real question regarding the expansion joint is whether or not the slab which is 27 feet by 20 feet will have the possibility of expanding enough to crack and/or push the block foundation walls out of place (or possibly off the footer?)   I can say this is the first time I have seen a slab in this condition poured without an expansion joint and I am unsure of the magnitude of the issue and below is the only solution I can think of other than a jackhammer.

To cut in an expansion joint at the wall is going to be nearly impossible as the slab turns down and is up to 12″ thick in places so I was wondering if a + cut in the slab with an expainsion joint installed may take some of the strain off the walls when the slab expands. Joint would be saw cut through the slab creating a cross in the middle of the slab.

I appreciate anyone who would be able to help me with this issue and thank you in advance.

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  1. jrnbj | Nov 10, 2008 04:55am | #1

    bump.....

  2. caseyr | Nov 10, 2008 05:40am | #2

    I am not an expert in this area, but my parents house was built in 1960 and the garage, which is slightly larger than what you have cited, has neither control joints or expansion joints. There is very little cracking and no apparent displacement of any part of the foundation that abuts the garage slab. This is in north central Oregon with a frost depth of about two feet. You might indicate your region and/or climate situation so someone can better evaluate your situation. Someone will probably ask you to fill in your profile information.

    1. KBenj | Nov 10, 2008 04:00pm | #5

      This slab is poured in the Northern Virginia area with a frost depth of about 3 feet.

       

       

  3. User avater
    Jeff_Clarke | Nov 10, 2008 07:27am | #3

    "My real question regarding the expansion joint is whether or not the slab which is 27 feet by 20 feet will have the possibility of expanding enough to crack and/or push the block foundation walls out of place (or possibly off the footer?)"

    Concrete doesn't expand, it shrinks as it cures, thus the need for contraction joints.  Generally speaking, while there certainly is a certain amount of expansion and contraction with temperature changes, inside a structure (as opposed to pavement exposed to the elements) the chances of a slab expanding much beyond its original volume is pretty small.

    The perimeter joint that you describe is called an isolation joint the minimum joint is 1/4" (1/2" better).   The intent isn't so much to allow 'room for expansion' but rather to isolate (separate) the concrete slab from the foundation wall.  Without it, I don't think you'll have any problems with the slab pushing the wall around - but you may have portions of the slab adhering to the foundation wall - and cracking as the slab moves and the wall doesn't.

    Jeff   



    Edited 11/10/2008 8:14 am ET by Jeff_Clarke

    1. brownbagg | Nov 10, 2008 02:42pm | #4

      expansion joint every eighty feet, control joint 10 x 10

  4. DaveRicheson | Nov 14, 2008 10:56pm | #6

    I'm with CaseyR on this one. You don't need to cut expansion joints, but you could go ahead and cut your + as control joints. You need only cut them about 1/4 to 1/3  of the thickness of the slab deep. You will be close to brownbags 10x10 recomendation.

    Clean out the cuts well, add backer rod and caulk with a self leveling caulk. Do not leave the control joints open. they will fill up with debris, freeze when wet, and jack the slab apart. Trust me on this, I know :-(

    1. fingersandtoes | Nov 15, 2008 05:50am | #9

      Filling the joints with colored grout rather than caulk makes for a nicer looking slab.

      1. User avater
        popawheelie | Nov 15, 2008 10:13am | #10

        I used a self leveling caulk made for filling cracks and it is just about the same color as concrete.

        But I didn't want the caulking to show so I filled the joints just shy of full and spinkled sand on top of  the caulking.

        They virtually dissappeared. Especially since I acid etched the slab to expose the sand in it.

        I'm very happy with it.

        1. DaveRicheson | Nov 15, 2008 05:05pm | #13

          I have used the sand sprinkle technique on joints in sidewalks and driveways with a broom finish. We even use it on expansion joints in brick walls. It blends so well it is hard to see it is not a mortar joint or part of the finished surface. For the best results we use a graded masonry sand.

          Never tried it on a steel trowel finished concrete floor. How does it look?

          1. User avater
            popawheelie | Nov 15, 2008 05:49pm | #14

            I don't think it would look right on a smooth troweled floor. I'd just use the self leveling caulking for that purpose. Backing rod is a good idea. I cut some in half because I had the wrong size. There's no advantage to filling the whole joint with caulking. The caulking I used recomended it be at least a 1/4" thick.

      2. DaveRicheson | Nov 15, 2008 04:59pm | #12

        Never tried that. I've always used backer rod and the a caulk like SL-1.

        I think keeping the moisture and debri out of the saw cut is more important than a slight color difference between a gray caulk and gray concrete. I'm not sure motar would keep the moisture out and could take the place of debri in the jacking process, but could be wrong on that.

        I didn't do the joints in my shop floor for a couple years after I finished it. Just went merrily along thinking I'll get to it when I get some of this extra stuff moved out. No point in moving all those boxes more than once, right? Well you pointed out what would happen. Enough junk, moisture and no heat combined to jack everyone of those control joints apart. They went from a diamond blade kerf width to a good 3/8". Butt ugly by the time I got around to caulking them.

        1. fingersandtoes | Nov 15, 2008 08:01pm | #15

          I don't know about outside but I do it all the time with interior slabs. The main floor of my own house is a slab saw cut into 3'x3' squares with a dark green grout. I find that whatever grout you use brings out that color in the concrete. When my house was just finished the appraiser came by and asked how we had got the granite floor slabs so level (ok so he was a bit slow).

          1. brownbagg | Nov 15, 2008 08:34pm | #16

            mine are 2x2 with dark brown stain and black grout, they ask the same question

          2. fingersandtoes | Nov 16, 2008 06:45am | #19

            My big problem now is that while most of the floor looks like new, after 12 years the high traffic areas need re-sealing. I'm not sure how to prep it. A light sand of the whole floor and clean with de-greaser where it has worn away? Any advice?

          3. brownbagg | Nov 16, 2008 05:04pm | #21

            same problem, sand with a floor buffer and apply a coat of clear epoxy

          4. fingersandtoes | Nov 16, 2008 07:46pm | #22

            Is that what you have on your floor now? I've got a nasty Zylene based sealer. To re-coat means everyone and pets leaving until it off-gasses.

          5. DaveRicheson | Nov 15, 2008 08:41pm | #17

            Got some pictures?

            Sounds like something we might like to do with the basement floor in our house. With lots of south facing windows, the slab is suppose to act as a heat sink, durring the winter months, to re-radiate the trapped heat at night. DW wants to do something decorative with it.

          6. fingersandtoes | Nov 16, 2008 06:47am | #20

            Sure. This weekend is crazy. I'll try and get something together for early in the week.

          7. fingersandtoes | Nov 26, 2008 04:50am | #23

            Dave, Sorry for the delay. I haven't forgotten the pictures I'm just snowed under with various things right now. I'll try and get them up on the weekend.

          8. DaveRicheson | Dec 01, 2008 08:51pm | #24

            No sweat.

            I took a few days vacation before turkey day. I don't use the pc at home much, cause it is dial up, and Tauton has blocked acces to the site cause we won't let them set cookies.

            Going to broadband this week, so may change our security settings and try to look in occassionally from home.

  5. jigs_n_fixtures@icloud.com | Nov 14, 2008 11:55pm | #7

    The thermal movement coefficient for concrete is roughly 0.0000066-in/in/degree F.

    So the 27 foot direction will expand/contract 0.0021 inches/ degree F.   

    Given a 100 degree fluctuation in temperature, you will have 0.2 inches of change in length of the 27-foot section. 

    The problem occurs, when the slab contracts, and sand/crud falls in the crack.  This will start a jacking cycle that may eventually cause problem. 

    The expansion joint material around the edges is elastic and keeps the sand/crud from filling the joint over time.  Plus as it is compressible, it keeps the slab from exerting much force on the perimeter wall, if the crud falls in. 

    You might want to install a bead of elastic caulk between the perimeter wall and the slab after the curing shrinkage has occurred to seal things up. 

  6. BoJangles | Nov 15, 2008 02:12am | #8

    The lack of control joints doesnt bother me nearly as much as the lack of expansion joints as I can handle some cracking of the concrete surface rather than along a control joint.

    You don't need any expansion joints but you should cut in crack control joints immediately if not sooner.  It doesn't take long for concrete to start shrinking and cracking.

    Divide your slab in to 4 pieces and you should be OK.

  7. JeffinPA | Nov 15, 2008 03:41pm | #11

    You should not need expansion joints.

    With exp joints, it makes  it harder to seal for radon, if you have concern for that as well.

    Been involved with hundreds and hundreds of homes, similar size and larger, no exp joints, just control joints.

    I would consider cutting in control joints if it were me.

  8. User avater
    rjw | Nov 16, 2008 12:48am | #18

    concrete expands/contracts something like 1/10,000 per degree temp change.

    The temp of the ground under the slab probably doesn't change a whole lot, depending on siting and usage.


    "Ask not what the world needs. Ask what makes you come alive... then go do it. Because what the world needs is people who have come alive."

    Howard Thurman

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