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Sinking Concrete Slabs

The signs of slab settling can get you ahead of the structural issues on your property. To know when it’s time for concrete lifting repairs, it’s smart to watch out for these tell-tale signs.

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Concrete settling can happen anywhere on your property. Once you spot a settling slab, it’s important to call in an expert to avoid expensive repairs down the road. Settling concrete can be fixed with concrete lifting solutions, but repairs are easier when the slab is intact. 

The more a slab settles, the more likely it is to crack and break. Certain concrete lifting solutions, like PolyRenewal™, can fix a settling slab despite the crack. However, there are limits to how broken a slab can be before repairs are no longer a workable solution. Replacing a concrete slab is a lot more expensive than repairing one, and, ultimately, the new slab has a chance of settling in the future as well. 

Before reaching the point of no return, it’s important that you’re able to identify when a slab is settling. Being one step ahead of the structural failure can save you time, money, and a big headache. 

Signs That Your Concrete Is Sinking 

Not all concrete slabs settle the same way. A lot of it depends on where the slab is located around the property, how it was made, how much moisture it’s exposed to, and how much weight it must bear daily. If you see signs of settling on a slab but don’t see any on another, then take a closer look: the signs may be subtle, but the state of the concrete can be very indicative of problems to come. 

If you suspect that your slab is settling but you’re having trouble finding clear evidence, you can always contact your local experts at Dry Pro for a free, professional inspection. 

Here are some of the problem signs of concrete slab settling:  

  • Cracked Concrete 

Concrete cracks are a clear sign of a settling slab. There’s a specific way that a concrete slab cracks when it’s going to settle, which makes it easy to predict how exactly the concrete will settle. This is because of the poor tensile strength of concrete. 

Tensile strength refers to a material’s ability to withstand pressure. Concrete cracks and crumbles when too much pressure is concentrated on a specific part of its frame, which is why concrete breaks apart when hit with a hammer, despite being an otherwise strong building material. Concrete is better able to withstand pressure when it’s on an even base. 

Because settling concrete is on an uneven layer of soil, when it’s time for the slab to bear a heavy load, the concrete is unable to handle it, and it cracks. When the soil is only slightly uneven, the cracks will appear, but the slab will remain level with the rest of the area. When the settling is advanced, the part of the slab that is above the biggest void in the concrete will have sunk low to the ground, either before or after the crack. This usually happens around the part of the slab that is weakest—the corners. 

Typically, this problem sign is most likely to occur on concrete slabs that need to support a lot of weight daily, such as the driveway. The weight of the car can become too much and cause cracks.  

  • Unevenness 

When an intact slab begins to settle, it’s because the soil underneath it is uneven. This will result in a lopsided slab. Slab settling is a slow process, but it can quickly accelerate and get out of hand before you even realize it. If you notice that your slab is uneven when it wasn’t before, then contact an expert for an inspection before the settling gets worse.  

  • Wobbling 

One of the earlier signs of concrete settling is flimsiness. If you feel the slab wobbling whenever you step on it, then it’s probably starting to settle. Don’t ignore this wobble since it can be a tripping hazard. Some slabs remain even and don’t sink despite there being a massive void underneath the slab. For those who have poor balance, especially children and the elderly, this can result in an injury.  

  • Voids 

Check the slabs that are right next to the soil in your yard because there may just be a void that lets you see under the slab. When the soil in your yard has eroded, a void sometimes forms right alongside the concrete slab. This void allows water to flow under the slab and to erode the soil underneath even further. Typically, once a void is there, settling occurs rapidly. 

If you do see a void, then do not try to lift the slab to cover it with soil. The only way to fix the slab is by using a professional concrete lifting method.  

  • Potholes  

Potholes aren’t exclusive to the road. They can also happen on residential slabs, especially those that are heavily exposed to moisture. With potholes, a circular depression forms somewhere on the slab. This hole is caused by a slab having to bear a heavy load when the soil underneath has a void. 

The pothole typically occurs in the middle of the slab itself. Once the pothole is there, repairing the slab is difficult, but not impossible. Because potholes are more likely to form after winter because of frost heave, keep an eye on your slabs during the spring for signs of a pothole forming. 

Why Does Concrete Sink? 

Concrete settling is not a result of poorly made concrete slabs. It all has to do with the soil under the slab. Before a concrete slab is poured, the soil that will go under it is meant to be compacted and even. Soil compaction is when the soil is pressed together to make it denser. 

However, as the soil erodes over the years, it becomes looser, and the layer is no longer even. Because the slab no longer has an even base to rest on, the slab settles against the eroded layer of soil. 

What Causes Concrete Settling? 

Concrete settling is caused by the erosion of the soil underneath the slab. Soil can erode for varied reasons, especially in a place with four distinct seasons like Charlotte, NC. The soil in the region can have layers that are both sandy and filled with clay, which also attributes to settling.  

  • Shrinking and Swelling 

Cecil soil is the soil that spans the area in Charlotte, NC. The subsoil is brown sandy loam, but just eight inches down, in the subsoil, the soil turns red and clay can be found. Clay soils are a problem because they expand when saturated with water. This loosens up the layer and makes it easier for the soil to move around. When the moisture leaves the soil, it compacts together, and the soil shrinks considerably. 

Because of the shrinkage, a gap is formed between the soil layer and the concrete slab. The slab eventually settles against the soil layer since it cannot stay even if there is nothing underneath it.  

  • Soil Washout 

Soil washout is when a stream of water washes away a layer of soil. The soil in Charlotte has a top layer that is sandy, so washout is a usual form of erosion. This usually happens during the summer when precipitation is highest in the city. 

  • Freeze-Thaw and Frost Heave 

The freeze-thaw cycle is a term used to describe the way ice pushes against the soil as it expands. When water turns into ice, it expands by 9% and displaces the things surrounding it. If the soil under your slab has a lot of water in it during the winter, then that water will push against the soil as it freezes and turns into ice. Once the ice melts, only the gaps in the soil are left from where the solid ice once was. 

As for frost heave, this refers to the upwards pressure that is caused by the expanding ice. It may not seem like it, but expanding ice is strong enough to push up against concrete slabs and cause them to warp to the point where they have a convex shape. When the ice melts, not only is there a gap under the slab, but also the concrete is now warped and weakened because of the pressure. When placing a heavy load on the slab, the weakened part of it most affected by frost heave will give way, and a pothole will form.  

  • Heavy Loads 

As time goes on, the soil underneath your concrete slabs becomes loose and shifty. The looser the soil becomes, the easier it is to get displaced by pressure from a heavy object. Heavy loads of concrete accelerate the soil erosion process, which is why concrete slabs that are used heavily (like driveway slabs and walkway slabs) settle more often than others. 

How Can a Settling Slab Be Fixed? 

When your slab is settling, you may wonder if the slab needs to be torn apart and then re-poured. However, unless the slab is completely broken, repairing it is the best solution. There are a few concrete repair methods to choose from, but the one that is the most permanent solution is PolyRenewal™. 

When doing your research, you may come across the term mudjacking or grout leveling, but these are older solutions that have a lot of downsides. PolyRenewal™ is the most advantageous form of concrete lifting, and once you understand how it works, you’ll know why. 

Sinking Concrete Slabs


When choosing which concrete lifting solution to go for, it’s not enough to simply choose whichever is cheapest. Between PolyRenewal™ and other methods like mudjacking, mudjacking is the cheapest concrete lifting method. At least, that’s what it seems like on the surface. 

Because of all the downsides of mudjacking, the repair job isn’t reliable, and you could end up with a broken slab later down the line. Although you save up money with mudjacking for the actual repairs, you’ll end up spending more on slab replacement later. A more permanent solution like PolyRenewal™ is a better investment for homeowners who wish to only repair their slab once. 

  •  The Disadvantages of Mudjacking 

Mudjacking is one of the oldest forms of concrete lifting. It involves drilling holes and pumping in a cement mixture under the slab. Enough cement is pumped in that the pressure lifts the slab and evens it out. There are multiple problems with this method, the first one being the potential blowout from the pressure of the slurry being pumped in. This is when the slurry basically “explodes” outwards and leaves a huge mess. Another problem with mudjacking is that the hardened cement under the slab can still erode and crumble since it’s permeable and vulnerable to water and the freeze-thaw effect. 

Mudjacking does not protect the soil from further erosion. It still allows water to pass through to the soil since cement is porous. The soil then softens and loosens up as it’s exposed to water, and the weight of the cement only displaces the soil further. When it comes down to it, mudjacking only lasts around ten years or so, and that’s only when the slab is heavily protected from weather-related factors.  

  • The Advantages of PolyRenewal™ 

With PolyRenewal™, contractors drill holes into the slab, but these are much smaller than the holes drilled for mudjacking. This allows contractors to work with slabs that are weak, since the drilling is minimally invasive. Instead of a cement slurry, polyurethane foam is injected into the slab. The foam expands and completely fills out any gaps in the soil; it’s the expanding polyurethane foam that lifts the slab, not pressure, so there’s no risk of blowout. The entire process is done in a few hours, and the polyurethane cures in just 15 to 30 minutes. 

Polyurethane is a synthetic plastic that has no harmful additives, so it’s safe to inject underground. Because it’s plastic and impermeable, it doesn’t get affected by the freeze-thaw cycle and it never washes away once cured. Since it’s waterproof, it blocks moisture from reaching the soil, slowing down the erosion process and keeping your slab even for decades. Polyurethane is strong and can support heavy loads while still being lightweight, so soil displacement is minimal. 

Polyurethane foam can lift cracked slabs and level the broken piece with the rest of the concrete. That said, polyurethane is not glue, so it cannot lift slabs that are severely cracked. Past a certain point of damage, you’ll need to replace the slab instead of repairing it, which costs more money. 

Besides, with a repair method like PolyRenewal™, you are able to ensure that the erosion process is slowed, and your slab stays even for a very long time. If you get a new slab, however, then the soil erosion process is not addressed, and the new slab has the potential to settle as well. To avoid this, it’s in your best interest to learn how to keep your settling concrete from cracking.  

  • Exposure to Moisture 

The bigger the gap under the slab, the more the concrete settles and the more likely it is to break. Keeping the slab as even as possible and preventing rapid soil loss is the best way to avoid a cracked slab. This might mean making some alterations, such as not using the pool until repairs are done or using driveway covers, at least during the winter to protect from snow. 

Moisture is something that not only erodes soil; it also erodes concrete. Water can displace concrete particles over time, which is especially damaging to concrete that is already weak due to a poor curing process or an incorrect recipe. Concrete damage is most severe during the winter when the moisture in the slab expands as it turns into ice. Charlotte, NC has 59 nights a year where the temperature drops to or below freezing. This is nearly two months of micro-tears from freeze-thaw, so you need to limit how much exposure to snow the slab gets if you wish to avoid cracks.  

  • Weight Bearing 

Because concrete has poor tensile strength, it is most likely to crack when it begins settling. If you notice the tell-tale signs of settling in a slab, then you need to hold off on putting any weight on it until repairs are done. Not only does too much weight on a settling slab cause cracking, but also it can accelerate the soil erosion process. 

If one of your slabs is settling, then it may not be the only one that has soil problems. Slab settling can happen to multiple slabs at once, and the signs usually appear shortly after the first slab begins to settle. To prevent multiple slabs from settling, there are a few things you need to consider doing.  

  • Drainage 

Soil erosion can never fully be prevented, but it can be slowed down considerably if you limit the soil’s exposure to moisture. This can be done by improving your yard drainage system. It’s important that as little moisture as possible goes near the even slabs, especially if there’s one that is already settling. 

Because soil washout is so common in Charlotte thanks to the characteristics of the city’s soil, having a positive yard grade helps immensely. A positive yard grade means that water flows away from the foundation every time it rains. You can always speak to a landscaping expert to examine your yard to see if any re-grading needs to be done. 

  •  Seasonal Care 

Every winter, you should try to shovel as much snow from your yard as you can, especially during the early evening. The temperature falls below freezing at night, but even if it snows while you’re asleep, at least the snow will not melt and saturate the water like it does during the day. Early morning and evening are the best times to remove snow to protect your slab. 

During the summer, when it rains, some protection might also be good for your concrete, particularly your driveway, which is the easiest to protect. A simple, collapsible driveway cover can block the excessive amount of rain that falls between May and August in Charlotte. It’s not recommended that you cover the slabs year-round because of potential mold growth, so focus on doing so when it rains the most. 

Call Dry Pro for Concrete Lifting Solutions 

Dry Pro has been repairing homes since 1999. We value our customers’ time and understand that certain home repairs should only be done once. Therefore, we are dedicated to bringing the homeowners of Charlotte, NC the most reliable, permanent solutions in the industry. 

If you need your settled concrete lifted, then call us, or use our online contact form to schedule a free inspection. We’ll send a certified field expert to look and give you a rundown of the repair cost. It’s that easy, so don’t hesitate to get in contact with us! 

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