Why does concrete sink?

Concrete around our home, like driveways, patios, and sidewalks, should enhance our quality of life.

But when these areas start to crack and settle. Suddenly, you can be dealing with issues like pooling water, uneven surfaces, and dangerous trip hazards. Before repairing your sinking concrete, it is essential to understand why the concrete has sunk to make sure it does not happen again. Most of the time, the culprit is why concrete falls and becomes uneven because of three issues with the soil below the concrete slab.

First, when the home was first built, the contractor moved dirt around the building to level the site before construction. This created fill soils underneath the concrete slabs that are never as stable as the original soil. The concrete is heavy, and when the fill soil settles, the concrete above goes with it.

Secondly, during drought conditions or even times of warm weather and little rain, the soil under your slab will dry out and shrink. Soil that contains large amounts of clay and silt, like here in Nebraska, will shrink substantially when it gets dry. The concrete will eventually crack and sink into these voids causing uneven surfaces and tripping hazards.

Finally, when it rains, the moisture can wash away the soil or leave it so saturated and soft that it cannot support the concrete above it. Sometimes this is due to improper drainage, or other times it is a matter of unusual amounts of water getting to the soil underneath the concrete. Lines in the concrete are called control joints because they are supposed to control the cracking patterns when the concrete was initially poured. These control joints will eventually crack through the slab after the concrete is poured in place. This is a perfect path for water to flow down and under the concrete slab. This creates more erosion and more wet, weak soil. When this happens, the soil shifts, and there becomes unsupported areas of concrete. An easy concept to think about is stepping in the mud. Now, imagine not only the concrete's weight on the unsupported soil but driving a car on a driveway add tremendous added weight. If part of your concrete slab is supported in one section and not in others, the unsupported pieces may crack away from the other section, which will result in additional cracks in the concrete or unlevel concrete. Another movement you get from concrete is concrete heave. If you get too much moisture underneath your slab, that soil will expand and cause the concrete to become uneven in portions, in parts.

All sorts of concrete settles, residentially and commercially. What you'll see on your residential home, you could see different levels of your concrete settlement throughout your sidewalk and your driveway, causing uneven and trip hazards. On commercial concrete, what you'll see is on roadways like on a bridge. The bridge is a very sturdy structure that will not settle. The highway slabs themselves are not on an underlying support system. So, they will have a potential settlement, which can cause unevenness in the road. And the big bumps you feel before you hit the bridges.

Of course, there are times that concrete sinking is due to poor craftsmanship, whether the contractor didn't take into consideration the soil type, potential exposure to water, and the compaction of the soil. Although concrete sinks for a variety of reasons, the common problem is always the soil. Different types of soil have various load-bearing capacities. When a soil's load-bearing capacity is exceeded, the earth will compact and settle. Unfortunately, when the concrete starts to sink, the contractor that poured the concrete is long gone with a taillight warranty.

Regardless of why the concrete sinks, there are services that can resolve the problem without replacing the entire concrete area of concern with the same problem soil. What we use a Thrasher to fix settled concrete is a two-part polyurethane foam that we come in and inject underneath the slab. We can use that to lift and level the slabs and fill any additional voids underneath the slab. Once the slabs have been lifted level, we can also provide a silicone polyurethane sealant that will keep water from going through any joints and causing future erosion or possible heave situations.

One of our concrete repair experts, Brent, explains what causes concrete slabs to sink, become uneven and crack. Brent also explains how polyurethane foam injection is used to lift and level uneven concrete slabs for homeowners and highways, as well as the benefits of sealing concrete joints to help prevent future concrete settlement and damage.