Why is My Concrete Slab Sinking?

The earliest forms of concrete-like mixtures date back thousands of years. Concrete is one of those materials that has withstood the test of time. We use as much of it today as many of the civilizations that came before us. With that in mind, concrete is subject to one particularly unsettling problem: it can sink.

Concrete is a very heavy material. As such, even the best planned project completed to the highest standards can still be subject to sinking. We have all seen it. From concrete patio slabs to sidewalks, concrete can sink anywhere from a few inches to more than a foot.

So why do slabs and sidewalks seem to sink in some spots and not in others? The Concrete Raising Company, a concrete slab repair company in Salt Lake City, says sinking concrete is generally due to one of three things.

1. Poor Soil Compaction

An important part of any concrete installation is soil compaction. Soil compaction is the process of using weight and pressure to force dirt molecules to fit more closely together. If you have ever made a snowball, the process is similar. You use the force and pressure of your hands to press snowflakes together into a tight, compact ball. A tightly compacted snowball gives you something you can easily throw.

Soil is compacted in order to improve its density and increase its weight bearing properties. If soil is not compacted properly, it can eventually give way to heavy concrete. The weight of the concrete bears down and forces the compaction that was not achieved prior to installation. Sinking is the natural result.

2. Soil Washout

Another reason concrete slabs sink is soil washout. This is a more common problem in environments with sandy soil. For example, Florida homeowners have to pay particular attention to any signs of sinking slabs or sidewalks because they could indicate washout underneath.

Soil washout occurs when water underneath the surface washes away loosely compacted soil, leaving a void in its wake. When that happens, gravity takes over. The soil above the washed-out area sinks to fill the void left by the washout. Everything above that sinks as well.

3. Changes in Water Content

Constantly changing water volumes in underlying soil can lead to sinking problems even when a complete washout doesn’t occur. This is more of a problem in northern environments where rain is problematic in the spring and summer followed by snow and ice over the winter.

Constantly changing water volumes can leave some portions of soil quite wet while others are dry. Add to that the freeze/thaw cycle and maintaining a consistent water level becomes more difficult. This can lead to shifting soil and sinking concrete.

Addressing Sinking Concrete

Until a few years ago, the typical way to address sinking concrete was to remove it, level and compact the soil, and pour new concrete. However, the process is expensive and time-consuming. It also doesn’t guarantee that future sinking will not occur.

Another way to address sinking concrete is to fill the area underneath the soil with a durable filler substance. This type of slab repair is safe, efficient, and very easy to deploy. Best of all, it can be done in mere minutes.

Affecting this type of repair requires drilling holes in the soil around the affected area. Filler material is injected into the holes to fill in empty spaces and raise the concrete to its original position. The substance hardens and becomes permanent. That’s it.

Now you know why your concrete slab is sinking. The question is, how are you going to have it repaired?

By Cole Harris

Sawyer Cole Harris: Sawyer, a DIY enthusiast, shares home project tutorials, woodworking tips, and creative ways to personalize your space.