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.
this post was submitted on 25 May 2026
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What is this thing?
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Yes, that's how they work. Neither zip strips nor control joints go all the way through the slab.
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.
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.
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.