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Thread: Waterproof Sealing a 1/4" crack in concrete

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    Mod Squad jblizzle's Avatar
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    Waterproof Sealing a 1/4" crack in concrete

    Not related to a pool, but I know we have concrete experts that might be able to help.

    Summary:
    What can I use to waterproof seal a ~1/4" gap between 2 concrete slabs in my house to prevent water from seeping up onto the floor?

    Details:
    Our house has a ~20' x ~50' room, part of which was once a carport but in the 80s expanded and enclosed for a TV repair workshop. We use it as a play room and it had nasty carpet in it that the wife unilaterally decided to rip out so that she could stain the concrete. Discovered a nasty black tar like substance on the concrete that is not easy to remove and a layer of mortar(?) between the 2 concrete slabs that attempted to smooth the transition between the two. We have aborted on the staining and now I am having to install laminate floors.

    We had to grind down the slab interface to make it more flat and found that the mortar had also been used to fill the gap between the 2 slabs. I can just flip it out with a screw driver so it clearly did not bond with either slab which explains why on occasion when we get a LOT of rain and water pools outside against the slabs (I need to also try to reduce this) the carpet would sometimes feel wet along the crack.

    Now that we are putting in laminate, even with using a vapor barrier, I want to do a better job sealing this crack so that the wood floor is not damaged by water that might wick up between the slabs.

    So, any thoughts on the best product to fill the gap? I am wondering if using some type of cement product would work or if I would be better off with a caulk or epoxy product. Seems like a little flex might be good, but I am not sure.

    Surprisingly, the 30x20 slab that was added has no relief seems (wrong terminology), but there are no cracks anywhere in the room, so things must be pretty well settled.

    Thanks for any info anyone can provide.
    Jason, TFP Moderator
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    Melt In The Sun's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    I don't know the answer, but if you don't get one here, the folks over at johnbridge.com can surely help you.
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    Mod Squad jblizzle's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    Quote Originally Posted by Melt In The Sun
    I don't know the answer, but if you don't get one here, the folks over at johnbridge.com can surely help you.
    Thanks. I will go check it out.
    Jason, TFP Moderator
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    UnderWaterVanya's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    My mother's house had a half basement added after the fact and there was a foundation error where they had to extend back another 2' - leaving a seam. The seam was a nightmare. They eventually had to put an interior weeping tile / french drain all around the perimeter of the basement in order to capture the water and remove it before it got under the slab. They made a huge mistake and tied their gutters to this system at one point - which resulted it a massive flood inside the basement - imagine dumping all the water from your roof into your french drain and think what would happen.

    In any case after setup of the french drain and disconnection of the gutters from that system they have had good luck. But they still on occasion have a slight moisture infiltration so they run dehumidifiers from time to time. I would not suggest laminate in this situation. If you do go ahead - do something like what my mom did and use a very thick moisture barrier.
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    Mod Squad jblizzle's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    Being in Tucson, the water problem is maybe a handful of days per year and actually we did not find any moisture the last 2 years (and when we had large rains, I was specifically feeling for moisture). Just noticed a moist carpet the first year we were there.

    Our front yard slopes slightly to the house and then half the roof drains to the front also, so we can get a lot of water up front with no where to go. Planning on adding gutters to the front to divert a lot of that water away from the front, so that will likely help a lot. I just also want to do the best I can to seal the crack.

    Thanks for the response.
    Jason, TFP Moderator
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    Re: Waterproof Sealing a 1/4" crack in concrete

    Since there is no visible movement between slabs and the water problem is not an active leak situation I would opt for foam backer rod and self leveling concrete sealant. If you think it has the potential to be worse than that, you can always get a low pressure crack injection kit or bring in a pro.

    The first option is much cheaper and will offer plenty of flexibility. Quikrete makes both and Lowe's carries Quikrete products.

    I'm not a concrete expert but I did play one on television.

    Just my $.02

    Note: Quikrete makes both meaning the backer rod and polyurethane sealant not the
    injection kit.
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    Melt In The Sun's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    It's worth fixing the drainage too...much better to tackle the problem at the source rather than the end location. Assuming you have somewhere lower you can run a drain pipe, it wouldn't be too much work to reroute that water with a trench and some gravel and drain pipe.
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    Re: Waterproof Sealing a 1/4" crack in concrete

    MITS hit the best idea, I think. Fix the source.

    Water never goes up.....gravity is always pulling it down.

    However, water outside your slab that is higher than that crack will migrate under the slab and come up at the crack........not because it is going up but because the water outside (and higher than the top surface of the crack) is simply trying to seek equilibrium.

    Give that water a path (french drain) to travel away and down from the slab and the crack will have no moisture.

    Being only a 1/4 inch, I would fill it with polyurethane caulk after I fixed the drainage problem.
    Dave S.
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    Mod Squad jblizzle's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    @ UN1017: pretty much exactly what I was starting to think. Use a foam back and then a sealant. I had not decided between the self-leveling sealant or just a silicone concrete sealant (happened to be looking at the Dap product line, but had seen the Quikcrete options). Although maybe the self-leveling would better flow into any nooks, but the sides of the slabs are pretty smooth. It is a big enough crack that I am not sure an injection kit would be needed.

    @ MITS easier said than done ... thus the reason I have not done it in the 3+ years we have been there There is another carport slab near the intersection, so my ditch would have to go uphill and around that and then it would lead into the gravel driveway. It would be nice to run a drain all the way to the wash across the driveway, but now we are talking some distance. If I added the gutters, then I could get the water over to the driveway again, although would be nicer to have have it all running the dirt/gravel driveway.

    EDIT .... unless you want to come over and help me dig

    @ Dave, I thought I had heard that the water could in fact wick up the crack? Not true? I guess I find it hard to believe that the water got that deep ... although maybe that is why it only happened the once and since then I cleaned up all the leaves and stupid mesquite pods that maybe were allowing it to pool higher.
    Jason, TFP Moderator
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    Mod Squad jblizzle's Avatar
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    Re: Waterproof Sealing a 1/4" crack in concrete

    I inquired with the Dap "ask an expert" and they recommended a polyurethane concrete sealant over the self-leveling or the silicone concrete sealant ... crazy how many products they have that seem to do the same thing.

    So, I think I am going to cram some foam backer rod in there and top it with 1-2" of the polyurethane sealant (probably will put this on the outside of the house as well. Slap some tape down over the crack and top it all with the vapor barrier sheet. Then see what I can do to get the water to run away from the house and maybe add gutters as well.

    Luckily I likely have until next July before we get any significant amount of rain that would potentially cause a problem.

    Thanks for the sounding board
    Jason, TFP Moderator
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    Re: Waterproof Sealing a 1/4" crack in concrete

    When I finished the basement we used Delta fl underlayment. Put the laminate on top of it buit the walls on it also. One of the boxs stores sells it. They don't know they do, showed up with the skew number and ordered it.

    http://www.cosella-dorken.com/bvf-ca-en ... cts/fl.php
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