How to Find Voids Under Concrete: A Pro's Guide

That hairline crack in your warehouse floor or the slightly uneven section of a concrete walkway might seem like a minor issue, but it’s often a warning sign of a much bigger problem lurking just out of sight. Underneath the surface, voids—empty spaces between the slab and the soil—can form from erosion or ground settlement, compromising the structural integrity of your entire foundation. Ignoring them is a gamble that can lead to costly repairs and serious safety hazards. This guide is your complete resource on how to find voids under concrete, covering everything from simple manual checks to advanced GPR scanning, so you can diagnose the problem accurately and choose the right solution.

Key Takeaways

Identify Voids Before They Become a Crisis: Waiting for cracks to appear is a reactive and costly strategy. Using non-destructive tools like Ground Penetrating Radar (GPR) allows you to find hidden voids early, preventing dangerous structural failures and expensive emergency repairs.
Choose the Right Tool for a Clear Picture: Manual checks only reveal surface-level problems. GPR provides a complete map of subsurface conditions, but its accuracy depends on expertise. A skilled technician can handle challenges like moisture or dense rebar to deliver clear data for confident decision-making.

Focus on Prevention for a Permanent Solution: Simply filling a void doesn't solve the underlying issue. To prevent voids from returning, you must address the root cause by managing water with proper drainage and sticking to a regular maintenance and inspection schedule.

What's Happening Beneath Your Concrete Slab?

That solid concrete slab you walk or drive on every day might not be as stable as it looks. Underneath the surface, hidden problems can develop that compromise the integrity of your entire structure. These issues often start as small, unseen gaps called voids. Understanding what causes these voids and the risks they pose is the first step toward protecting your property from costly damage. Before you can fix the problem, you need to know what you’re dealing with.

What Causes Voids to Form?

Voids are simply empty spaces that form between the bottom of a concrete slab and the ground that’s supposed to be supporting it. They don’t just appear out of nowhere; they’re usually the result of a specific issue with the soil underneath. Voids can happen when the ground wasn't packed down well before the concrete was poured, a process known as poor compaction. Over time, water can wash away soil through erosion, or the ground can settle naturally. In other cases, buried organic materials like old tree roots or construction debris can rot away, leaving an empty space behind.

Spot the Warning Signs and Structural Risks

A void under your concrete is more than just an empty space—it’s a weak spot. Without solid ground to rest on, the unsupported slab can begin to settle unevenly, crack, or even break completely. This creates serious safety issues, like tripping hazards where different sections of the slab no longer line up. Water often makes the problem worse. It can seep into existing voids, wash away more soil, and make the gaps even bigger. This cycle further weakens the slab's support, accelerating the damage and leading to more significant structural problems if left unaddressed.

Why Ignoring Voids Costs You More

It’s easy to overlook what you can’t see, but ignoring potential voids is a gamble. Even if a foundation looks perfectly fine on the surface, hidden voids can be incredibly dangerous, especially when heavy equipment like cranes or loaded trucks are used on site. A slab that seems stable can fail suddenly under pressure. The cost of an emergency repair or a full slab replacement is always higher than the cost of a proactive inspection. Using professional subsurface utility mapping services to identify voids early helps you avoid project delays, prevent accidents, and protect your budget from unexpected expenses.

How to Find Voids: The Manual Approach

Before bringing in advanced equipment, you can perform a few manual checks to get a preliminary idea of what’s happening under your concrete. These methods are a good starting point for any site assessment and can help you spot obvious problems. Think of them as the first line of defense in diagnosing subsurface issues. While they have their limitations, a quick physical inspection can give you valuable clues and help you determine if a more detailed investigation is necessary. These simple, hands-on techniques can often confirm your suspicions and build the case for a professional scan.

Start with a Visual Surface Inspection

The easiest way to begin is by simply walking the site and looking closely at the concrete surface. Your eyes are a powerful first tool. Look for tell-tale signs like spiderweb cracks, uneven or sunken slabs, and areas where water seems to pool after it rains. These surface-level issues are often the first indicators of hidden voids forming below. If one section of a concrete path is lower than the others, or if you see cracks radiating from a specific area, it’s a strong signal that the ground underneath has shifted or washed away, leaving an empty space. Document these areas with photos and notes to build a clear picture of the problem zones.

Listen for Trouble with Manual Sounding

Once you’ve completed a visual check, you can use a simple technique called manual sounding. This involves tapping the concrete surface with a hammer or dragging a heavy chain across it. As you move across the slab, listen carefully for any changes in the sound. A solid, well-supported section of concrete will produce a dull thud. However, if you hear a distinctly hollow or drum-like sound, you’ve likely found a void. This change in acoustics is a reliable sign that there’s an air gap between the concrete and the subgrade, and it helps you pinpoint potential problem areas for further investigation.

Analyze Core Samples

If a visual inspection and sounding suggest a void, drilling for core samples is a more direct (and destructive) way to confirm it. This involves drilling a small hole through the concrete slab to physically inspect the conditions underneath. Coring allows you to measure the thickness of the slab, check for the presence of rebar, and see if there is a gap between the concrete and the soil. While it provides definitive proof of a void in that specific spot, it’s an invasive process that leaves a hole to be patched. It gives you a very localized piece of information but doesn’t reveal the full extent or shape of the void.

Know the Limits of Manual Checks

While these manual methods are useful for initial assessments, it’s important to understand their limitations. A visual inspection can’t spot a void that hasn’t caused surface damage yet, and sounding can miss deep or small voids. Core samples only confirm what’s happening in one tiny spot, leaving the rest of the area a mystery. These techniques can give you a false sense of security, as they often fail to detect all subsurface voids, especially those that are just beginning to form. For a complete and accurate picture of what’s happening underground, you need technology that can see through the concrete.

Go Deeper: Advanced Void Detection Technology

When manual checks aren't enough, or you need a more comprehensive picture of what’s happening underground, it’s time to bring in the technology. Non-destructive testing (NDT) methods let us see beneath the surface without drilling or digging, giving you a clear map of hidden voids, delamination, and other potential issues. These advanced techniques use different types of energy—like radar waves, stress waves, or sound waves—to create a detailed image of the subsurface. Each method has its strengths, and the right choice depends on your specific site conditions and project goals. Let's look at a few of the most effective options.

Ground Penetrating Radar (GPR): See Through Concrete

Ground Penetrating Radar, or GPR, is one of the most powerful tools for finding hidden voids. Think of it like an ultrasound for your concrete. A GPR unit sends high-frequency radio waves into the ground. When these waves hit a boundary between two different materials—like the shift from solid concrete to an air-filled void—they bounce back to the receiver. The strong signal reflection clearly marks the void's location and size. This technology is incredibly effective because the difference in electrical properties between air and concrete creates a sharp contrast in the radar imaging. Our GPR scanning services can quickly map out what’s happening under your slab, giving you a clear picture without any damage.

The Impulse-Response Method

The impulse-response method is another non-destructive technique that works by listening to how your concrete responds to a small impact. A technician uses a special hammer to create a low-strain impact on the slab's surface, generating stress waves that travel through the concrete. A sensor placed nearby measures how the slab vibrates in response. Solid, well-supported concrete will have a distinct response, while concrete over a void will sound and vibrate differently. This method is a quick and affordable way to assess the structural integrity of large areas like pavements, bridge decks, and warehouse floors. It’s particularly useful for evaluating the quality of concrete and identifying areas that need repair.

Ultrasonic Pulse Echo Testing

Similar to the impulse-response method, ultrasonic pulse echo (UPE) testing uses sound waves to inspect concrete. Instead of a hammer, this technique uses a transducer to send high-frequency sound pulses into the material. These sound waves travel through the concrete and reflect off any internal flaws, boundaries, or objects. By analyzing the time it takes for the echo to return, technicians can identify the location of voids, cracks, or delamination. UPE is great for getting detailed information about the condition of the concrete itself, helping to pinpoint weak spots and determine the extent of damage hidden from view. It’s a precise tool for a close-up look at what’s happening inside your slab.

How to Choose the Right Technology

So, which technology is right for your project? The best choice often depends on the depth you need to scan, the size of the area, and your budget. For most applications, GPR is a fast and highly effective way to identify voids, especially those closer to the surface. Its ability to produce a clear, map-like image of the subsurface makes it a favorite for planning repairs and avoiding hazards. Other methods like impulse-response or ultrasonic testing can provide complementary data about the concrete's quality. The key is to match the tool to the task. A professional subsurface mapping team can help you assess your site and recommend the most efficient approach to get the clear, actionable data you need.

What Affects Void Detection Accuracy?

Ground Penetrating Radar is an incredibly powerful tool for finding voids, but it’s not an x-ray machine. The clarity of the results depends heavily on site conditions. An experienced technician knows how to read the data in context, but several key factors can influence the accuracy of a scan. Understanding these variables helps you know what to expect and how to prepare your site for the best possible outcome.

How Concrete and Soil Properties Impact Scans

The composition of your concrete and the soil beneath it are the biggest factors in scan quality. GPR works by detecting differences in subsurface materials, so if the soil has a similar density to the concrete, voids can be harder to distinguish. Heavily reinforced concrete with a dense rebar grid can also create interference, scattering the radar signal and making the data more complex to interpret. Similarly, certain soil types, like dense, wet clay, absorb GPR signals more than sandy or gravelly soils. A professional subsurface utility mapping team can adjust their equipment and analysis techniques to account for these challenging conditions.

The Role of Moisture and Temperature

Water is one of the biggest challenges for GPR. High moisture content in the ground or standing water on the concrete surface can absorb the radar energy, preventing the signal from penetrating deeply. This is known as signal attenuation. Essentially, the water "dampens" the signal, making it difficult to get a clear picture of what lies beneath. While scans can often be performed in damp conditions, saturated ground significantly reduces the depth and clarity of the results. Extreme heat can also sometimes affect equipment performance, so scheduling scans during optimal weather conditions can contribute to more reliable data collection.

Scanning Deep vs. Shallow Voids

Not all voids are created equal, and their depth plays a major role in detection. GPR equipment uses different antennas that operate at various frequencies. High-frequency antennas provide high-resolution images but have shallow penetration depth, making them perfect for locating voids just beneath the slab. Low-frequency antennas can penetrate much deeper into the ground but offer lower resolution. An expert technician will select the right antenna based on the suspected depth of the voids and the overall site goals. This is why a one-size-fits-all approach to GPR scanning rarely works; the equipment must be matched to the specific problem you’re trying to solve.

Why Surface Conditions Matter

The condition of the concrete surface itself has a direct impact on data quality. For the GPR antenna to work effectively, it needs to maintain good contact with the ground. Rough, uneven surfaces, loose debris, or significant cracks can create air gaps between the antenna and the concrete, distorting the signal. Thick surface coatings or layers of asphalt over the concrete can also obstruct the radar waves. For the most accurate results, the scanning area should be as clean, dry, and smooth as possible. Taking the time to sweep or clear the surface before a scan is a simple step that can make a big difference.

What to Expect from a Professional Scan

Hiring a professional crew to scan for voids takes the guesswork out of assessing your concrete slab. Instead of relying on manual checks alone, you get a clear picture of what’s happening underground. A professional service follows a structured process to deliver accurate, reliable data you can use to plan repairs and prevent future problems. From the initial site walk to the final report, here’s a step-by-step look at what a professional void detection scan involves.

Step 1: Assess and Prep the Site

The first thing a professional crew will do is perform a thorough site assessment. This involves walking the area, understanding your project goals, and identifying any potential challenges or surface obstructions. They’ll ask about the history of the slab, known utility locations, and any warning signs you’ve noticed. This initial evaluation is key to planning an effective scan. Preparing the surface by clearing away debris or equipment ensures the GPR equipment can make direct contact with the concrete, which is essential for gathering clean data. This prep work provides a safe and cost-effective way to look beneath the surface and confirm you have a solid foundation.

Step 2: Select the Right Equipment

Not all subsurface scanning technology is the same. Based on the initial assessment, the crew will select the best equipment for your specific situation. While there are several non-destructive testing methods available, Ground Penetrating Radar (GPR) is the most common and versatile tool for detecting voids under concrete. The technician will choose a GPR antenna with the right frequency for your project—lower frequencies can penetrate deeper, while higher frequencies provide greater detail for shallower targets. Using the correct equipment is the difference between getting a fuzzy picture and a high-resolution map of the subsurface. This is where expertise in GPR scanning becomes critical for accurate results.

Step 3: Collect Accurate Data

Once the site is prepped and the equipment is selected, the technician begins the scan. They will move the GPR unit in a systematic grid pattern across the concrete surface. As the unit moves, it sends high-frequency radio waves into the ground. These signals bounce back to the receiver when they encounter a change in material. A strong signal reflection typically indicates the boundary between the concrete and the air within a void, creating a clear anomaly in the data. This process generates a real-time, cross-sectional image of the subsurface, allowing the technician to "see" what's happening beneath the slab without any destructive digging or drilling.

Step 4: Interpret the Scan Results

The data collected by the GPR unit is just one piece of the puzzle. The real skill lies in interpreting it correctly. An experienced technician analyzes the GPR data to distinguish between voids, rebar, conduits, and other subsurface features. Different materials cause the radio waves to bounce back at different speeds and strengths, which helps the analyst identify what they’re seeing. For example, the reflection from a plastic pipe looks different from an air-filled void. This expert analysis turns raw data into a meaningful map of the subsurface, ensuring you can tell the difference between a potential problem and a standard structural element.

Step 5: Get Clear, Actionable Reports

The final step is receiving a report that makes sense. A professional firm won’t just hand you a confusing set of GPR images. Instead, they’ll provide a clear, comprehensive report that details their findings. This typically includes a map showing the location and estimated size of any detected voids, along with photos and descriptions. The report gives you the actionable information you need to move forward with confidence, whether you’re planning soil borings, trenching, or scheduling repairs. This focus on delivering clean, engineer-ready results ensures your entire project team is on the same page.

How to Fix Voids Under Concrete

Once you’ve confirmed that there are voids beneath your concrete, the next step is choosing the right repair method. Leaving them unaddressed can lead to serious structural problems, from cracked slabs to foundation failure. Fortunately, there are several effective techniques to fill these gaps and restore stability to your concrete. Each method has its own strengths, and the best choice depends entirely on your site’s specific conditions, including the size of the void and the state of the underlying soil. Think of it less as a simple patch job and more as a way to give your slab the solid, stable ground it needs to perform for years to come.

Slab Jacking (Mudjacking)

Slab jacking, often called mudjacking, is a trusted and traditional method for concrete repair. The process is straightforward: a special cement-based slurry is pumped through small holes drilled into the concrete slab. This mixture flows into the empty spaces, filling the voids completely and lifting the slab back to its original, level position. It’s a great option for stabilizing settled concrete because the material is strong and durable. The main goal of slab jacking is to provide a solid, cementitious foundation that prevents further movement and restores the structural integrity of your concrete surface.

Polyurethane Foam Injection (Polyjacking)

Polyurethane foam injection, or polyjacking, is a more modern approach to void filling. This technique involves injecting a high-density, two-part polyurethane foam beneath the slab. Once injected, the foam undergoes a chemical reaction, causing it to expand rapidly and fill every part of the void. As it expands and cures, it gently lifts the concrete. The foam is incredibly lightweight, which means it won’t add significant weight to already unstable soil. It’s also a very fast process—the foam cures in minutes, allowing the area to be used almost immediately. This makes polyjacking an excellent choice for projects where minimizing downtime is critical.

Compaction Grouting

When the problem isn't just an empty space but also weak or loose soil, compaction grouting is often the best solution. This method involves injecting a very thick, low-mobility grout into the ground at high pressure. Unlike the fluid mixtures used in mudjacking, this stiff grout doesn’t flow into the void. Instead, it forms a dense bulb of material that displaces and compacts the surrounding soil, increasing its density and load-bearing capacity. This technique effectively strengthens the ground beneath your slab, providing a stable base and preventing future settlement. It’s a powerful solution for improving poor soil conditions.

Choose the Best Fix for Your Site

Selecting the right repair method is crucial for a long-lasting solution. There’s no one-size-fits-all answer, so you’ll need to consider several factors. Think about the size and depth of the voids, the type of soil on your site, and the weight of the concrete slab itself. The cost-effectiveness of each solution is also a key consideration. For example, polyjacking might be faster but could have a different price point than mudjacking. To make the best decision, it’s wise to consult with a geotechnical engineer or a concrete repair specialist. They can help you evaluate your options and recommend the most appropriate fix for your specific situation.

How to Prevent Voids from Coming Back

Fixing a void under your concrete is a great first step, but it doesn't guarantee the problem is gone for good. Without addressing the root cause, you're just setting yourself up for a repeat performance down the road. The real goal is to create a stable environment where voids are far less likely to form in the first place. This proactive approach saves you from the cycle of costly repairs, structural risks, and unexpected downtime. It’s about shifting from a reactive mindset—fixing problems as they appear—to a strategic one that protects your assets for the long haul.

Think of it like this: filling a void is like patching a tire, but preventing one is like making sure your tires are properly inflated and aligned to avoid blowouts. By implementing a few key preventative measures, you can ensure the ground beneath your slabs remains solid and supportive. This not only gives you peace of mind but also protects your investment and maintains the safety and integrity of your site. A little prevention now can save you a massive headache later. Let's walk through the essential, actionable steps you can take to keep those underground voids from making a comeback.

Install and Maintain Proper Drainage

Water is the number one enemy of stable subgrade. When it's not managed correctly, it erodes the soil beneath your concrete, washing away fine particles and creating the empty spaces that become voids. Proper drainage correction is your first line of defense. Start by checking that gutters and downspouts are clear and directing water far away from your foundations and slabs. Ensure the ground around your structures is graded to slope away, preventing water from pooling. If you have subsurface drains, make sure they are functioning correctly and aren't clogged with debris. A well-designed drainage system is fundamental to preventing soil erosion and keeping your concrete properly supported.

Create a Regular Maintenance Plan

Once your drainage is in order, don't just set it and forget it. Consistent maintenance is what keeps small issues from turning into major void problems. Create a simple, routine checklist for your property. This should include walking the site to look for early warning signs like soil erosion, new cracks in the concrete, or areas where water is pooling after it rains. Catching these issues early allows you to take action before significant damage occurs. A regular maintenance schedule helps you stay ahead of problems and ensures the soil beneath your concrete remains stable and compacted, providing the support your slabs need to perform.

Set Up a Monitoring Strategy

For critical areas or sites with a history of soil issues, a visual inspection might not be enough. This is where a proactive monitoring strategy comes in. Using advanced tools like Ground Penetrating Radar (GPR) for periodic checks allows you to see what’s happening beneath the surface without any destructive testing. GPR scans can identify developing voids or areas of soil disturbance long before they cause visible surface damage. This non-invasive approach gives you the data you need to intervene early, often with less expensive and disruptive repair methods. Think of it as a regular health check-up for the ground beneath your facility.

Schedule Professional Inspections

While your own maintenance plan is crucial, there's no substitute for an expert eye. Scheduling periodic professional inspections can help you catch subtle issues that are easy to miss. A trained technician can use specialized equipment like GPR to get a clear picture of subsurface conditions and assess the health of your concrete slabs. These professional inspections provide a comprehensive evaluation, identifying not just existing voids but also the conditions that could lead to new ones. Partnering with a team of experts for utility locating services gives you actionable data and clear recommendations, helping you prioritize repairs and maintain a safe, stable site for years to come.

Frequently Asked Questions

My concrete looks fine, but I'm planning a heavy construction project. Should I still get it scanned? Absolutely. A slab can look perfectly stable on the surface while hiding significant voids underneath. The immense weight of heavy equipment like cranes or loaded trucks can cause that seemingly solid slab to fail without warning. A proactive GPR scan is a smart investment to confirm the ground can support your project, helping you prevent accidents, costly damage, and major delays. It’s all about verifying your site conditions before you put your crew and equipment at risk.

How quickly do I need to fix a void once I find one? It’s best to address a void as soon as you can. Even a small void is a weak spot, and it will almost certainly get worse over time as water seeps in and erodes more soil. This puts increasing stress on the unsupported concrete, leading to cracking and settling. Acting quickly usually means a simpler and less expensive repair. Waiting often turns a small issue into a major structural problem that could require a full slab replacement.

What's the main difference between mudjacking and polyjacking for repairs? Think of it as a choice between a traditional, heavy-duty material and a modern, lightweight one. Mudjacking uses a cement-based slurry to fill the void and lift the slab, creating a very solid, concrete-like base. Polyjacking uses an expanding polyurethane foam that is incredibly light and cures in minutes. Polyjacking is often preferred when you need to minimize downtime and avoid adding extra weight to unstable soil, while mudjacking is a time-tested and effective solution for creating a dense, strong foundation.

Can GPR see everything under my concrete? GPR is an amazing tool, but it does have limits. It excels at identifying changes in material, which is why it's so good at spotting air gaps (voids), rebar, and utilities. However, its accuracy can be affected by things like very wet clay soil, which absorbs the radar signal, or extremely dense rebar grids that scatter it. An experienced technician knows how to interpret this complex data and adjust for site conditions, but it's important to remember that it provides a detailed map, not a perfect photograph.

Is a hollow sound always a sign of a dangerous void? A hollow sound is definitely a red flag that deserves a closer look. While it's a very reliable indicator that a void is present, it doesn't tell you the whole story. The void could be small and shallow or large and deep. Manual sounding helps you pinpoint suspicious areas, but it can't reveal the size or shape of the gap. That's why it's a great first step, but a professional GPR scan is needed to understand the true extent of the problem and determine if it poses a structural risk.