Why Does Concrete Sink?

Why Does Concrete Sink?

After 42 years of repositioning concrete, I’ve come to the conclusion that 99% of the reason concrete settles is that it has been poured on ground that has been recently excavated or dug. This is usually the case around a building foundation. The excavator purposely over-dug the site, giving the masons room to trowel the joints, waterproof the exterior foundation wall, or install the storm sewer or gutter drains. Look around almost any home foundation, and it’s evident the ground by the house has sunken, and the concrete against the foundation along with it.

Concrete Sink
Concrete Sink

 Unstable Ground

Also, consider all the buried utilities around the home: water, electric and gas trenches, storm, and sanitary sewer trenches. This doesn’t include tree stumps that were uprooted or valleys that were filled in when the foundation was dug. Many trenches should be 3-4 feet below the surface to ensure that they don’t freeze or could be accidentally dug up in the future. The builder could backfill the trenches with a premium fill like sand or gravel to ensure minimal settling, but right next to the trench is a mountain of fill that came out of the hole. Available, and free, so it goes right back into the ground, and it takes about 10 years to finish settling. This is what keeps all concrete levelers in business.

Creating Concrete Sink

Northern Ohio is full of clay, and that trench that was filled with recently dug earth is now full of air pockets. It will take 10 years before that ground will be considered again like “virgin” land. (This is, of course, only true if your builder never buried his scraps in the trench because it is cheaper than a dumpster.) This is the reason why our limited warranty mainly protects concrete that is over 10 years of age (2-year warranty on concrete that is under 10 years old). Just remember, anytime the ground is disturbed, it takes time to settle. This is especially true with clay. Areas that have very sandy soil settle much faster.

New Construction

creating concrete sink
creating concrete sink

With new construction, there is usually a one year warranty against concrete sink from the builder. In situations where the front walkway meets the porch, or where the porch slab meet meets the house, or the attached garage meets the foundation, the builder will install a piece of re-bar into the wall. This helps when the concrete is poured around the re-bar pins, it will hold up the concrete from settling for a brief period. Remember, it is freshly dug ground beneath that is settling, allowing the builder to “get out of town” before a problem is noticed. Most building departments go along with this practice. Don’t ask me why – that’s a question for your building department!

Concrete Sink Now?

You might ask why your concrete is still settling even though it is over 10 years of age. Because there is still a void or hollow area beneath the concrete, so until that slab is sitting solidly on the ground, you will still see concrete sink, or even worse – break.

If you notice chipmunks or other creepy crawlers running into holes under the concrete, it is because they found a ready-made hole. They didn’t create the problem, they are just taking advantage of it. That is unless you have seen them hauling dirt out the holes with their little wheelbarrows

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