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Herbicide that is compatible with sealant

Discussion in 'Pesticide & Herbicide Application' started by Devon Nullman, Apr 12, 2012.

  1. Devon Nullman

    Devon Nullman LawnSite Member
    Messages: 3

    My back yard is mostly a pool and a concrete deck with brick strips. It has been in place since 1994 and over the years the concrete has shifted ever so slightly so there are small gaps (1/16") between the bricks and the concrete. Weeds grow out of these gaps. I need to re-caulk the pool and I thought it would be a good time to seal the gaps also. I plan to use SikaFlex Self Leveling polyurethane sealant and I also plan to saw each gap to about 1/8" wide. My question is this:

    1-what herbicide can I use that will not leave any oily/hydrocarbon residue? Any contamination will prevent the sealant from adhering properly.

    2-what herbicides last the longest ?

    I am assuming that these weeds only grow because the wind gets a seed in the gap. The sealant should stop that from happening.
  2. rlitman

    rlitman LawnSite Bronze Member
    Messages: 1,732

    I did the cracks in my sidewalk last summer like this.
    I sprayed them all with roundup, and a week later everything was brown. I ran my edger in the cracks to pull out the roots and dirt, and finished cleaning them out with a screwdriver.
    I then pushed a foam caulk backing rod into the cracks to set the depth to 3/4", and used a scrap of the foam to cap the ends of the crack (or else the self leveling sealant will slowly pour out the ends).
    Finally, I filled it all in with Sikaflex SL from the quart size tube. It was really quite easy, it looks great, and I haven't had any adhesion issues, or plants growing back.

    The way I see it, once every weed in the crack is killed, and you've sealed it up, you won't get new ones, so you don't have to worry about how long the herbicide acts.
    Roundup doesn't have a residual action, but it's also so diluted with water that I didn't notice any residue on the concrete. It's just not something to worry about (it probably is though, with some "ground sterilent" herbicides).

    My only comments on your plan are that 1/8" is a bit narrow to apply that stuff out of the tube. I'd shoot for 1/4". It self-levels, but it's not runny and thin, and you really want to control the depth of the filler with a backer. Adhesion isn't the issue, but it can tear from the concrete if things move at all (and a crack is proof that things moved). If you keep the cross section of the sealant within the parameters in the instructions, you can rely on it's elasticity to stretch and squish if the concrete moves. If you fill to deep, it can't stretch the same way.
  3. Devon Nullman

    Devon Nullman LawnSite Member
    Messages: 3

    Thanks much for the reply. I agree that the sealant should be wider, in fact Sika Tech Support told me a good rule is twice as wide as it is deep. I just am not sure I can cut 1/4" without it getting ugly and making an incredible brick and concrete dust mess.
  4. rlitman

    rlitman LawnSite Bronze Member
    Messages: 1,732

    Ok, here's a question for the paver experts here (so it seems like this is kind of on the wrong forum to me, to get the best answer, but lets see who's reading this).

    Why grind open the spaces at all. If they're only 1/16" gaps, and are stable, why not just clean out the gaps (maybe blow out everything with a power washer after killing the weeds), and fill them in with polymeric sand?

    I mean, opening up the gaps is opening a can of worms you probably don't want to get into, and 1/8" is really too narrow to do with Sikaflex SL. I'm not knocking the product (I found it to be great), but it's consistency is only a little runnier than ketchup, and I just don't think you can get it in there, even if you just opened the tip of the tube by poking it with an awl instead of cutting the tip off.

    Besides, polymeric sand should last just as long as the polyurethane, block weeds just as well, and will look much better. Hopefully someone with more experience with it will chime in about how it will work in your situation (and maybe some application tips).
    Last edited: Apr 12, 2012
  5. Devon Nullman

    Devon Nullman LawnSite Member
    Messages: 3

    I bought a 40 lb pail of Sakrete Paver Set and sifted it through an ordinary kitchen sifter to get out the biggest pieces. I replaced those with an equivalent amount of fine sand. After cutting all joints to around 1/8" x 2" deep I used a squeeze bottle to get the stuff in the joints, then misted it as instructed. It blends in with the mortar very nicely, looks great. I may even use what's left to repair a few divots in the pool coping grout where I think acid splashed on it and ate away a bowl shaped depression.

    Thanks so much to everyone for your suggestions.

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