Why Does Concrete Sink and How Can Concrete Lifting Fix the Problem?

Concrete sinks when the soil beneath it shifts, washes away, or compresses, leaving the slab without uniform support. Over time, that lack of support causes sections of concrete to drop, crack, and create uneven surfaces that are both hazardous and structurally damaging. Concrete lifting, specifically through polyurethane foam injection, reverses this problem by filling voids beneath the slab and raising it back to a level position without the need for full replacement.

TLDR / Key Takeaways

• Concrete slabs sink primarily due to poor soil compaction, water erosion, expansive clay soils, and natural soil consolidation over time
• Soil erosion from improper drainage and plumbing leaks washes away supporting material beneath slabs, creating voids that lead to settlement
• Expansive clay soils, common across Texas, Wisconsin, and the central United States, swell and shrink with seasonal moisture changes, causing differential movement
• Concrete lifting uses polyurethane foam injection to fill voids, compact loose soil, and hydraulically raise sunken slabs back to level
• Foam leveling offers smaller drill holes, faster cure times, less cleanup, and resistance to future erosion compared to mudjacking
• Slabjacking is significantly less disruptive and less expensive than full concrete replacement, with surfaces usable within hours
• Concrete must be in reasonably sound structural condition for lifting to work; heavily shattered slabs may require replacement

What Causes Concrete to Sink?

Understanding why concrete sinks requires looking at the relationship between the slab and the ground beneath it. A concrete slab is only as stable as the soil supporting it. When that support system fails, settlement follows.

Poor Soil Compaction During Construction

The most preventable cause of sinking concrete is inadequate soil compaction before the slab is poured. When soil is not properly compacted to at least 95% of its maximum dry density, it naturally settles over time under the weight of the concrete and everything placed on it [source]. This creates voids and uneven support, leading to differential settlement where one section of a slab sinks more than another. In homes with basements, the backfill around the foundation is especially vulnerable because it is frequently not compacted properly during construction. According to engineering standards, backfill should be compacted in layers no more than six inches thick to prevent subsequent settlement issues.

Soil Erosion Beneath the Slab

Soil erosion is a process that displaces the upper layer of soil through the action of water, wind, and other forces. Beneath concrete slabs, water is the primary culprit. When rain, flooding, or plumbing leaks channel water under a slab, the soil supporting it gets washed away. This creates empty spaces, or voids, where the concrete has no support at all. Over time, the slab settles into those voids under its own weight and the weight of vehicles, equipment, or structures on top of it. Surface runoff that concentrates along the edges of slabs due to poor grading or improper drainage patterns accelerates this process significantly.

Expansive Clay Soils and Moisture Fluctuations

Expansive clay soils present a unique and persistent problem for concrete slabs. These clay soils contain minerals like montmorillonite and smectite that undergo large volume changes in response to moisture. During wet seasons, the clay absorbs water and swells, exerting upward pressure on the slab. During dry periods, the clay shrinks as it loses moisture, leaving gaps beneath the concrete. This repeated swelling and shrinking cycle causes concrete to move up and down over time, leading to cracking, uneven surfaces, and structural stress. Soils with a plasticity index over 35 are classified as having high expansion potential, and liquid limits over 50 indicate significant swelling characteristics.

Soil Consolidation

Soil consolidation is the gradual process by which saturated soils decrease in volume under applied pressure. When a heavy concrete slab is placed on soil saturated with water, the weight is initially borne by the water trapped in the soil pores rather than by the soil particles themselves. Over time, as that water is squeezed out and drains away, the soil particles pack more tightly together and the ground surface drops. This process is most pronounced in clay-rich soils with low permeability, where consolidation can continue for years or even decades after construction. The settlement occurs gradually and is often uneven, making it particularly damaging to large, continuous slabs.

Additional Contributing Factors

Other factors can accelerate or worsen concrete settlement. Freeze-thaw cycles in colder climates cause soil to heave when water in the ground freezes and expands, then settle when it thaws. Tree roots growing beneath or near slabs can displace soil and create voids. Organic material buried during construction, such as tree stumps or root systems, decomposes over time and leaves behind cavities. Earthquakes and seismic activity can also shift the ground supporting concrete, leading to sudden and uneven settlement.

How Concrete Lifting Works

Concrete lifting, also known as slabjacking, is a repair technique that raises sunken concrete back to its original level without removing and replacing the slab. The process has been used since the early 20th century and has evolved from simple soil-cement mixtures to advanced polymer-based systems.

The Polyurethane Foam Injection Process

The most widely used modern method for concrete lifting involves injecting expanding polyurethane foam beneath the slab. Here is how the process works:

1. Strategic hole drilling: Small holes, typically less than one inch in diameter, are drilled through the sunken concrete slab at carefully planned locations to maximize lift efficiency.

2. Injection port installation: A tapered delivery port is inserted into each drilled hole to create a sealed connection for the injection gun.

3. Foam injection: A two-part polyurethane polymer is injected through the ports beneath the slab. The material follows the path of least resistance, first finding and filling voids, then expanding into weaker soils and compacting them.

4. Controlled lifting: The foam expands within seconds, generating upward hydraulic pressure that gradually raises the concrete. Skilled technicians monitor the lift carefully and can control the process to achieve precise elevation.

5. Curing and patching: The foam reaches a hydrophobic state within about 30 minutes and cures fully shortly after. The injection holes are then patched with a color-matching compound, leaving the surface clean and usable.

Closed-cell polyurethane foams used in lifting applications can provide baseline lifting capabilities of approximately 6,000 pounds per square foot, and some formulations achieve compressive strengths of 50 to 100 psi in a free-rise state. That equates to 7,200 to 14,000 pounds of support per square foot.

Why Polyurethane Foam Outperforms Traditional Mudjacking

Traditional mudjacking uses a mixture of soil, sand, water, and cement pumped under the slab to create hydraulic lift. While mudjacking has been used for decades, polyurethane foam offers several distinct advantages for concrete lifting applications.

One of the most significant differences is that polyurethane foam does not retain moisture and will not erode when subjected to rainwater once cured. Mudjacking material, by contrast, is a cement-based slurry that can be susceptible to erosion if the underlying soil continues to wash away. Additionally, the lightweight nature of polyurethane means it does not add significant weight to already compromised soil, reducing the risk of further settlement.

Real-World Scenarios Where Concrete Lifting Solves the Problem

Residential Scenarios

Commercial and Municipal Applications

Concrete lifting is not limited to residential properties. Sidewalks, warehouse floors, airport runways, highways, and municipal infrastructure all experience settlement problems and benefit from foam leveling. The Illinois Tollway, for example, has recognized polyurethane foam as a method that meets compressive strength requirements for supporting highway slabs. The ability to complete repairs rapidly, with the concrete usable within hours rather than days, makes foam leveling especially valuable for commercial and public infrastructure where downtime carries real costs.

Actionable Strategies for Preventing Future Settlement After Concrete Lifting

Lifting sunken concrete addresses the immediate problem, but preventing future settlement requires attention to the conditions that caused it in the first place.

1. Fix Drainage Issues Before Lifting

Water is the primary driver of soil erosion and settlement beneath concrete. Before any lifting project, identify and correct drainage problems directing water under or along the edges of slabs. This includes gutters, downspouts, grading, and surface water flow patterns.

2. Stabilize Expansive Soils

In areas with expansive clay soils, consider chemical soil stabilization or moisture barrier installation to reduce the moisture fluctuations that cause repeated swelling and shrinking. Maintaining consistent soil moisture around foundations and slabs is one of the most effective long-term prevention strategies.

3. Monitor Early Warning Signs

Watch for cracks forming in concrete, especially those that appear to be widening or growing. Pay attention to doors and windows that begin sticking, which can indicate foundation movement. Address small settlement issues early before they become major structural problems.

4. Address Plumbing Leaks Immediately

Underground plumbing leaks are a common and often hidden cause of soil erosion beneath concrete slabs. Even small, slow leaks can wash away significant amounts of supporting soil over time. If you notice unexplained dampness, reduced water pressure, or higher-than-expected water bills, investigate promptly.

5. Choose Professional Assessment Over DIY Solutions

Concrete lifting requires specialized equipment, trained technicians, and precise material handling. Improper installation of polyurethane foam can result in uneven lifting, incomplete void filling, or excessive heat buildup. A professional assessment will determine whether lifting is even the right solution, as slabs with excessive cracking or structural deterioration may need replacement instead.

Factors That Affect Concrete Lifting Performance

Several variables influence how well and how permanently a concrete lifting repair performs.

• Slab condition: Concrete must be structurally sound enough to withstand the lifting process. Slabs that are heavily cracked, crumbling, or deteriorated may not hold together during the lift.
• Soil type and stability: The underlying soil must be capable of supporting the slab once voids are filled. Organic soils, extremely soft clays, or active landslide areas may continue to move even after lifting.
• Void size and distribution: Larger, more extensive voids require more material and can be more challenging to fill completely and uniformly.
• Drainage conditions: If water continues to erode soil after the lift, settlement will eventually recur regardless of how well the initial repair was performed.
• Climate and freeze-thaw cycling: In cold climates like Wisconsin, repeated freeze-thaw cycles place additional stress on both the concrete slab and the supporting soil, making proper water management even more important for long-term stability.
• Age and weight of the slab: Older slabs that have been settling for many years may have developed internal structural fatigue that limits their ability to be lifted safely.

Get a Professional Concrete Assessment from Proseal Spray Foam

Proseal Spray Foam serves homeowners and property owners in Waupaca, Wisconsin and the surrounding areas with professional concrete lifting solutions using advanced polyurethane foam technology. Our experienced team evaluates each project individually, determining the root cause of settlement and recommending the most effective repair strategy. Whether you are dealing with a sunken driveway, an uneven garage floor, or a settled sidewalk, we have the expertise and equipment to restore your concrete to its proper position.

Contact us today to discuss your concrete leveling needs. Call us at (715) 227-6295 or email [email protected] to request a quote or schedule an on-site assessment.

Request a Quote  |  Schedule an On-Site Assessment

Frequently Asked Questions

How long does concrete lifting take to complete?

Most residential concrete lifting projects are completed within a few hours, and the concrete is typically ready for use the same day. The polyurethane foam cures within approximately 30 minutes, so there is minimal downtime compared to traditional concrete replacement, which can take several days to cure.

Can all sunken concrete be lifted back to level?

Not always. Concrete that is severely cracked, crumbling, or structurally compromised may not hold together during the lifting process. A professional assessment is necessary to determine whether lifting is feasible or if replacement is the better option. Generally, concrete in fair to good structural condition responds well to foam leveling.

Will concrete lifting damage my lawn or landscaping?

Polyurethane foam injection is a minimally invasive process. The small injection holes, typically less than one inch in diameter, create far less disruption than tearing out and replacing concrete. There is minimal mess, no heavy equipment tracking across your yard, and the surrounding landscaping remains largely undisturbed.

How long will a concrete lifting repair last?

When the underlying cause of settlement is addressed, such as correcting drainage issues or stabilizing expansive soils, concrete lifting repairs can last decades. The polyurethane foam itself is a permanent material that does not degrade, erode, or wash away. The key to long-term success is fixing the conditions that caused the settlement in the first place.

Is polyurethane foam lifting safe for the environment?

Polyurethane foam is a plastic-based product, and there are environmental considerations with any synthetic material. The foam cures quickly and becomes inert once hardened. The small volume of material used in most residential projects, combined with the elimination of concrete waste that would otherwise go to landfills, makes lifting a repair option with less overall environmental impact than full slab replacement.

Sources

• Concrete Leveling – Wikipedia – Comprehensive reference covering causes of concrete settlement and slabjacking methods including mudjacking, limestone grout leveling, and expanding structural foam leveling.
• Settlement (Structural) – Wikipedia – Engineering reference on the downward movement of building foundations due to changes in underlying soil conditions.
• Expansive Clay – Wikipedia – Scientific overview of clay soils that undergo large volume changes with moisture fluctuations, including identification criteria and risk assessment.
• Soil Erosion – Wikipedia – Detailed reference on the displacement of soil by water and other erosive agents, including factors affecting erosion rates and physical processes.