Concrete slabs and sidewalks are sturdy, but they’re not invincible. Over time, voids can form underneath, threatening their stability and potentially leading to costly repairs. Whether it’s a driveway, patio, or basement floor, this guide shows homeowners how to identify, assess, and fill voids under concrete.
*Costs in this article sourced from contractor estimates used by Angi.
What are Concrete Voids?
Voids under concrete slabs or sidewalks are empty spaces that form when the soil beneath settles or erodes.
Several factors can cause these voids, including:
- Soil erosion due to poor drainage
- Uncompacted substrate during initial construction
- Underground water leaks from pipes
- Natural soil settling over time
- Poor soil composition or quality
If left unaddressed, these cavities can lead to serious structural issues. They can cause the concrete to crack or sink, resulting in tripping hazards, or lead to water accumulation, which worsens erosion. Concrete voids can also increase the likelihood of freeze-thaw damage in colder climates.
Identifying Voids Under Concrete
Before attempting to fill a void, homeowners need to confirm its presence and assess its extent. Here are some obvious signs that may indicate a void:
- Cracks in the concrete surface
- Sinking or uneven sections of the slab
- Hollow sound when tapping on the concrete
- Water pooling in certain areas after rain
Laying a straightedge or level across a concrete surface can highlight sunken areas. Professional inspection tools, such as ground-penetrating radar, offer a more accurate assessment of cavity locations and sizes.Â
How to Fill Voids Under Concrete Slabs
Several methods exist for filling a void under a concrete slab, including DIY and professional solutions. The solution you use could depend on the cause of the cavity, but here are some common tactics.
Mudjacking
Mudjacking is a traditional method that involves pumping a slurry mixture under the concrete to fill voids and lift the slab. It’s cost-effective for larger areas, uses natural materials, and is fast. However, it can be messy and isn’t suitable for all holes. If mudjacking isn’t done correctly, there’s also a potential for future settling.
Polyurethane Foam Injection
Foam injection is ideal for broken corners caused by erosion. This more modern technique uses expanding lightweight foam that won’t burden the soil. The process is quick and clean while offering long-lasting results. However, it’s more expensive than mudjacking and requires specialized equipment and expertise.
Grout Injection
Similar to mudjacking, grout injection uses a cement-based mixture. It provides excellent compressive strength and is ideal for larger voids. Although durable and long-lasting, grout injection is more expensive and time-consuming than mudjacking.
Sand-Cement Topping Mixture
Sand-cement topping mixtures, also known as sand-based cement, are a simple and cost-effective solution for small voids. Materials are easily accessible at local hardware stores. However, it doesn’t always offer the same durability as other methods and will require some manual labor.
DIY vs. Professional
Homeowners should only attempt to fill minor voids under concrete slabs. Here’s a step-by-step DIY guide:
- Clean the area around the void thoroughly.
- Drill small holes through the concrete slab to access the void.
- Mix a sand and cement slurry according to package instructions.
- Pour or pump the mixture into the holes until the void is filled.
- Allow the mixture to cure completely before using the area.
While DIY methods can work for minor voids, larger or more complex issues often require professional intervention. Consider calling a concrete specialist if:
- The void is extensive or under a large portion of the slab.
- There are signs of significant structural damage.
- The cause of the void is unclear or related to plumbing issues.
- DIY attempts have been unsuccessful.
- The slab supports heavy loads.
Cost Considerations
The cost of filling voids under concrete can vary widely depending on the method used and the extent of the problem. Here’s a general overview:
- Mudjacking: $3 to $8 per square foot
- Polyurethane foam injection: $6 to $25 per square foot
- Sand-based cement: $3 to $10 per square foot
Be aware of potential hidden costs, such as:
- Additional repairs if the problem is not fully addressed
- Long-term damage from improper void filling
- Increased insurance premiums if the structural integrity is compromised
Preventing Future Voids
Prevention is key to maintaining a stable concrete surface. Here are some strategies to keep voids from forming:
- Address any plumbing leaks promptly.
- Maintain gutters and downspouts.
- Direct water away from foundations and slabs by installing french drains or extending downspouts.
- Create slopes so water moves away from the structure
- Use gravel or rock beds around the perimeter to promote better drainage.
- Use quality soil and proper compaction techniques during initial construction.
- Perform regular inspections to catch potential issues before they become major problems.
- Seal any cracks in the concrete to prevent water ingress.
Safety Precautions
When dealing with concrete repairs, safety should be a top priority. Take these precautions:
- Wear protective gear, including gloves and safety glasses.
- When using chemical products, work in a well-ventilated space.
- Follow manufacturer instructions for any materials used.
- Be cautious of electrical lines or plumbing that may be hidden beneath the concrete.
- Keep children and pets away from the work area.
- Consult with a professional if you’re unsure about any aspect of the repair.
Final Thoughts About Filling Voids Under Concrete
Homeowners can take proactive steps to maintain the integrity of their concrete surfaces. If a void forms under the concrete, it’s often best to consult a professional. Licensed contractors can determine the size of the cavity, how it affects the structure, and the most suitable methods for resolving the issue.
Would you rather leave the problem-solving to a pro? Fill out the form below to connect with local concrete experts to help:
