this post was submitted on 25 May 2026
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What is this thing?

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What's the name of this concrete separator material? (lemmy.world)
submitted 1 month ago by lettruthout@lemmy.world to c/whatisthisthing@lemmy.world
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Located between patio slabs at a house in Southern California. Am looking to possibly replace it during the cosmetic repairs planned for the area. Even if I don't replace it, am wondering what this stuff is called and what it's supposed to do.

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[–] finalarbiter@lemmy.dbzer0.com 18 points 1 month ago (2 children)

Looks like a zip strip, not sure if that's a brand name. Basically, they're put in when pouring slabs to control cracking behavior in the concrete when it shrinks/grows.

The alternative would be saw-cut control joints, which generally look and perform better but take a lot more effort.

[–] Tar_alcaran@sh.itjust.works 3 points 1 month ago* (last edited 1 month ago) (2 children)

They're less tall than the slab though, so either they were pushed into it from the top, after pouring, or it was cut and then filled with these things.

Edit: they were pushed in before hardening, you can see the seam on the side, you couldn't get that any other way

[–] finalarbiter@lemmy.dbzer0.com 3 points 1 month ago* (last edited 1 month ago) (1 children)

Yes, that's how they work. Neither zip strips nor control joints go all the way through the slab.

image of an installed zip strip

This creates an area that is weaker than the surrounding slab, acting kind of like a perforation. As the concrete cures and shrinks, it will crack along that line and not be visible from the top of the slab. Concrete of any decent volume will almost always crack as it cures, so this keeps the top of the slab looking nice.

If I had to guess, the slabs here shifted due to the soil underneath settling and creating a void underneath the slab. The increased stress in the unsupported region led to a crack along the strip, and allowed the two parts of the slab to move relative to each other. Since OP mentioned they're in California, it's possible this was earthquake-induced.

[–] lettruthout@lemmy.world 3 points 1 month ago (1 children)

Yeah the stress possibly was helped along by the earth dancing around here, but settling caused by the irrigated lawn may be the main culprit.

[–] finalarbiter@lemmy.dbzer0.com 1 points 1 month ago* (last edited 1 month ago)

Oh, yeah I'd be willing to bet the irrigation is the primary culprit here. When you go to fix this, you should consider addressing the drainage path of your irrigation. Otherwise, the soil may continue to shift underneath the new slab and cause the same issue in a few years.

[–] lettruthout@lemmy.world 1 points 1 month ago

Yeah, I can see that now. Good eyes!

[–] over_clox@lemmy.world 9 points 1 month ago* (last edited 1 month ago) (1 children)

I'm not sure if that's any sort of specialty material, but around my area in south Mississippi, they tend to just use thin strips of wood. Its there to help accomodate for thermal expansion and contraction between hot and cold seasons and try to reduce stress and cracks in the concrete between slabs.

[–] Tar_alcaran@sh.itjust.works 2 points 1 month ago

Plastic, especially non UV resistant plastic, seems like a terrible material for an expansion joint. It also looks very short.

[–] wesker 7 points 1 month ago (1 children)

Jason.

[–] IAmYouButYouDontKnowYet@reddthat.com 5 points 1 month ago

Who doesn't recognize Jason!?

[–] WaterWaiver@aussie.zone 3 points 1 month ago* (last edited 1 month ago) (1 children)

That looks like a hard plastic, which is weird.

Usually they use something flexible for "expansion joint filler", like flat foam https://www.bunnings.com.au/sika-10-x-100mm-x-25m-abelflex-expansion-joint-filler-foam_p1060067

Also there are types designed simply to reduce the volume of a flexible filler (black polyurethane?) needed to fill the gap with caulking guns. https://www.bunnings.com.au/sika-13mm-x-50m-abelrod-gap-filler_p1060406

[–] lettruthout@lemmy.world 1 points 1 month ago

Both of those suggestions seem to agree with the planned maintenance: grind down the "peaks", remove that zip strip, use filler materials (like those and caulk over the gap filler), then float in the "valleys". After all that, paint the entire thing to help cover up the minor imperfections.

Yeah, not the very best solution, but the house is going to be sold and the next owners might tear the whole patio out in order to build an ADU (Accessory Dwelling Unit, aka: Mother-in-law Apartment, aka: Dower/Dowager House).

[–] zelahdieliekeis@piefed.blahaj.zone 1 points 1 month ago

Looks like landscape edging, maybe used as a form. You could just pull it out and not replace it, but you might fill it with sand or dirt to keep other things from piling in or prevent mosquito water.