Small concrete expansion gap – normal or problem?
Hey everyone,
I’m working on a residential project where we just poured a new basement slab against an existing foundation wall. There’s a tiny gap (maybe 1/8”) showing up along part of the edge where the concrete meets the wall. The crew says it’s just normal shrinkage, but this being my first slab project, I want to be sure.
I understand concrete shrinks as it cures, but how do you know when a small gap like this is harmless versus when it might cause issues later? The space is conditioned and has proper vapor barriers underneath.
Would love to hear from others’ experiences – is this something you typically address or leave alone? Any simple rules of thumb for when to intervene?
Thanks in advance for sharing your knowledge!
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Replies
That amount of shrinkage is totally normal. Best practice, in my opinion, is to seal the vapor retarder to the wall, to maintain airtightness and reduce the risk of radon infiltration, if that's a potential issue in your area. You can get flexible, gray sealant made for concrete to seal the gap.
Gaps up to 1/8"-3/16" anywhere in the slab should be expected, unless a robust reinforcing schedule was used. If the gaps are larger, or the slab on each side of the gap is not coplanar, that may indicate a problem.
Zara,
It's completely to be expected. Slabs will also move seasonally if their moisture level changes, but in a conditioned basement, once the initial shrinkage has occurred it should be pretty stable.
There is no way to address it beyond keeping your mix as dry as possible, and no way to intervene after the fact. But as it has no real consequences, that doesn't matter much.
Probably not a big deal. If you have a proper footing drain, you shouldn't have water ingress problems. About the only potential "issue" I could see here would be if you need to install a radon mitigation system, in which case it would be standard practice to seal that gap with caulk -- something easy to do in the future if needed. Aside from that, it's not something to worry about.
Bill