Concrete 101: Frost Heave and the Collapsing Sidewalk!
Ever wonder why that last block of your sidewalk that leads into your house dropped? The problem and solution are easier than you think!
The main reason sidewalks, porches, driveways, etc. “drop” is due to frost in the ground. Assuming you live in an area where you experience the four seasons, the ground will change elevation from winter to spring time (especially if your house is new construction)! When the ground freezes in the winter time it will expand because water expands when frozen. Then, the ground will thaw in the spring time and this is when the elevation will shift which leads to a sidewalk “dropping.” Think about it like this, if the ground was an air mattress the winter time would be when it would blow up and the spring time would be when the air is completely let out. This will act as a push/pull affect on concrete, which will lead to cracks, complete breaks, and even “heaving” and “dropping” of the concrete.
You’re probably wondering why new construction is more prone to this than a building that has been around for quite some time. New construction often means there is loose, new soil that is not compacted. No matter how much tamping/compaction there is, the ground will just require time to compact completely. Fear not! There are methods to avoid having the sidewalk collapse from underneath your feet.
Usually the best plan to avoid “frost heave” is to only pour concrete outside during the warm portions of the year (the ground must be free of frost, so there should be a few weeks between snow/ice/below freezing temps and 40 deg+ temps when pouring outside), compaction (I know what I said earlier, but the more you can do beforehand the less the ground will push/pull on the concrete overtime), and the use of rebar and expansion joints.
Rebar can help keep concrete together, but its main purpose is to help keep concrete in place. If you’re doing a sidewalk from your house to the driveway, then you’ll use rebar to tie the concrete together and into the house (not the driveway). In order to avoid cracking and ruining the walk it needs to be able to move with the ground elevations which is why it will not be tied into the driveway, this is where an expansion joint would go in order to allow the walk to move freely from the driveway. The end that is tied into the house should stay at its elevation and not raise or fall through the years.
Concrete, the weather, and dumb luck can prevent this proven method from working 100%, but often this should go a long way in maintaining the integrity of your project for years. If your sidewalk, porch, steps, etc. have fallen against the house allowing for water, leaves, ice, snow, etc. to build up this could be a reason why. Meaning that it wasn’t properly done in the first place and that this is what it will take to fix the problem.
Here are a couple links to some other helpful articles that explain the mechanics behind Frost Heave: