Effective Solutions for Repairing Concrete Slab Voids

Concrete slabs are essential for driveways, sidewalks, patios, and foundations, but over time, voids can form beneath them, leading to cracks, uneven settling, and structural issues. These gaps often develop due to soil erosion, shifting, or poor drainage. If left unaddressed, they can cause major problems, including trip hazards, water damage, and further concrete deterioration.

For homeowners looking to fill voids under concrete slabs, there are temporary solutions available, but for long-term stability, professional repairs like polyurethane foam injection are the most effective. Below, we discuss what causes these voids, explore DIY and professional repair options, and outline the best prevention strategies.

What Causes Voids Beneath Concrete Slabs?

Understanding why voids form is crucial before attempting a DIY fix or professional repair. Several factors can contribute to gaps under concrete:

1. Soil Erosion

Excessive water flow can wash away soil beneath a slab, creating an empty space. Common causes include:

• Poor drainage systems
• Leaking underground pipes
• Heavy rain or improper landscape grading

2. Soil Settlement

If the ground was not properly compacted before pouring concrete, it may shift or compress over time, leading to void formation.

3. Freeze-Thaw Cycles

Cold weather can cause water in the soil to freeze and expand, lifting the concrete. When the ice melts, the soil contracts, leaving behind a void.

4. Poor Water Management

Short downspouts, clogged gutters, and improperly sloped landscapes allow water to accumulate around slabs, accelerating soil displacement.

5. Drought & Expansive Soil Shrinkage

Clay-heavy soil expands when wet and shrinks when dry. Over time, repeated cycles of expansion and contraction can cause gaps under concrete.

6. Hydrostatic Pressure

When water builds up beneath a slab without an escape route, it can weaken the soil and create pressure that shifts the concrete above.

7. Animal Burrowing

Rodents and small animals like moles can create tunnels under concrete, displacing soil and causing sections of the slab to sink.

8. Lack of Maintenance

Neglecting small cracks or gaps allows water to seep beneath the slab, further eroding the supporting soil and worsening void formation.

How to Fill Void Under Concrete Slab: DIY vs. Professional Repairs

Once voids form beneath a slab, they must be filled to restore stability. While some homeowners may attempt to fill the void themselves, it’s important to understand their limitations compared to professional repairs. To fill the void themselves

DIY Methods to Fill Voids Under Concrete Slabs

Homeowners looking for a temporary fix may try one of these common DIY approaches:

1. Sand or Gravel Fill

• Process: Dry sand or gravel is shoveled or washed into the void using water pressure.
• Pros: Inexpensive and simple for small voids.
• Cons: Does not lift the slab and may require repeated applications as materials settle.

2. Expanding Spray Foam

• Process: Hardware store spray foam (not polyurethane foam injection) is applied through drilled holes in the slab.
• Pros: Readily available and provides some expansion.
• Cons: Foam may break down over time, and it lacks the strength of professional-grade polyurethane.

3. Cement Slurry Injection (Basic Mudjacking Alternative)

• Process: A mixture of cement and water is poured into holes to fill voids.
• Pros: Can offer a firmer fill than sand.
• Cons: The mixture can shrink, crack, and require reapplication.

Limitations of DIY Repairs

While the DIY options may work temporarily, they don’t provide a lasting solution. DIY methods lack the strength, expansion capability, and durability of professional-grade polyurethane foam injection.

Professional Solutions for Filling Voids Under Concrete

For a permanent and structurally sound solution, professional repair methods provide the best results.

1. Polyurethane Foam Injection (Most Effective Solution)

Polyurethane foam injection is the best method for filling voids and lifting sunken slabs. The process involves drilling small holes into the concrete and injecting a high-density expanding foam beneath the slab. As the foam expands, it fills voids, raises the slab, and stabilizes the underlying soil.

Advantages of Polyurethane Foam Injection:

• Expands Fully – Fills every gap and void beneath the slab.
• Water-Resistant – Prevents further soil erosion.
• Lightweight Yet Strong – Won’t add excessive weight that could cause further settling.
• Fast & Non-Invasive – Small injection holes and same-day use.

2. Mudjacking (Traditional, Less Effective Alternative)

Mudjacking involves injecting a cement-based slurry beneath the slab to lift it. While this method has been used for decades, it has several disadvantages compared to polyurethane foam injection.

Drawbacks of Mudjacking:

• Heavier Material – Adds weight to already unstable soil.
• Limited Void Filling – May not fully expand into all spaces.
• Long Curing Time – Can take weeks before the surface is usable.

Many modern contractors prefer polyurethane foam injection due to its durability and long-term reliability.

Preventing Voids Under Concrete Slabs

Once repairs are completed, homeowners can take steps to prevent void formation in the future:

• Improve Drainage – Keep gutters clean, extend downspouts, and slope the yard away from concrete surfaces.
• Install French Drains – These underground drainage systems redirect excess water from concrete areas.
• Seal Concrete Cracks – Regularly inspect and seal small cracks to prevent water infiltration.
• Control Soil Moisture – Water dry soil during droughts to prevent excessive shrinkage.
• Monitor Tree Roots – Large tree roots can shift and displace soil, leading to slab instability.

Schedule a free inspection today to learn how we can help restore stability to your property!