Concrete scanning, also known as concrete imaging, uses ground penetrating radar (GPR) to detect embedded utilities, rebar, and structural elements embedded in concrete before cutting, coring, or drilling a structure. It is a nondestructive method of assessing concrete structures. Because GPR uses reflected radio waves, this concrete imaging method is safe, non-invasive, and effective for concrete assessment of columns, slabs, and walls.
Concrete scanning identifies and locates objects within the concrete, which is vital for job site safety. Concrete scanning prevents injuries, damage, and additional costs during construction. Concrete scanning technicians can use ground penetrating radar scans to know the concrete depth, objects within the concrete, and any air pockets that cannot be seen from the surface.
During construction, a concrete scan is vital to:
• Maintain the structural integrity of the concrete
• Avoid costly repairs during the project
• Reducing personal injury risk
• Reduce the risk of damage to utilities, rebar, post-tension cables, pipes, and conduits
It is recommended to plan for concrete scanning if existing concrete has structural or utility elements, such as concrete floors. Imagine you need to cut, core, or saw concrete. Concrete scanning is recommended in these circumstances to mitigate potential damages, such as the accidental strike of unknown or unmarked rebar, conduits, or utilities during construction, remodeling, renovation, or concrete demolition activities. Concrete sawing, cutting, and drilling is such a forceful process that if anything is struck during the process, it’s often too late. Obstructions cannot be felt or navigated during the process and have to be marked prior to sawing, cutting, or drilling, due to the power of the tools being used.
The two main non-destructive options of finding out what is inside a concrete slab are x-rays or ground penetrating radar (GPR) to “see into” the concrete slab or structure. Both provide a closer look at what’s inside, including rebar, conduit, post-tension cables, and more. GPR scanning is the #1 choice for concrete scanning and imaging due to its speed, efficiency, accuracy and cost effectiveness. X-ray imaging requires access to both sides of the concrete making it difficult to use in many instances. The use of x-ray imaging also emits harmful radiation to technicians and those in the area- special precautions must be taken to ensure site safety. Ground penetrating radar is considered the go-to technology for concrete scanning, though accurate interpretation of results is dependent upon properly trained technicians. Ground penetrating radar has fewer limitations that x-ray imaging and the quality of the results is generally more than sufficient for most applications. Use of x-ray may be advisable when GPR scanning is unable to pick up necessary detail, or in an instance where tight tolerances call for absolute precision.
Beams, Bridges, Ceilings, Columns, Decks, Floors, Pylons, Roofs, Slabs, Tunnels, Walls
In most cases, concrete is scanned with GPR technology. Ground Penetrating Radar (GPR) is a geophysical method of identifying what lies beneath the ground using electromagnetic waves. Finding out what's underneath the surface without breaking the ground is essential because it provides crucial information about subsurface infrastructure in a non-invasive manner.
Ground penetrating radar consists of three primary components. They include a control unit, a transmitter, and a receiver.
Even though GPR equipment comes in many forms, its function remains the same. A transmitter sends electromagnetic waves of a specific frequency into the ground through an antenna. The waves are reflected up to the antenna's receiver with varying travel times and signal intensities (or amplitudes). In turn, this information is encoded and sent to the control unit to generate a visual display for the operator. This data is displayed graphically.
The effective range of ground penetrating radar is determined by the electrical conductivity of the earth or concrete, the transmitting frequency, and the radiated power. While higher frequencies offer higher resolution, they cannot penetrate the ground as far as lower frequencies, and vice versa. Electromagnetic waves have a more difficult time penetrating conductive ground. This is due to attenuation. Consequently, electromagnetic waves often generate images with a compromised depth or resolution. Scanning ice can produce GPR results as deep as hundreds of feet, while dry sand and concrete may have images of a maximum of 20 feet, and wet soils like clay may only allow penetration of a few inches. Typically, GPR is used to make images of the ground within five to ten feet of the surface.
Due to its complexity, GPR requires highly qualified experts to operate effectively. This data is challenging to analyze because the collected information creates layers or hyperbola that requires expert-level understanding. Trained experts can distinguish the difference between clear versus obstructed areas. Additionally, GPR experts can identify the differences between subsurface obstructions like tree roots, rocks, USTs, pipes, conduits, and more.
Ground penetrating radar is an excellent tool to find the locations of obstructions but it’s possible that some conditions, operations, or desired results, may not be suited for GPR’s capabilities.
Limiting Factors of Concrete Scanning
Concrete scanning is generally reliable but some conditions may interfere. These include:
Modern construction technologies and documentation practices pose these issues less frequently, but they can and do arise from time to time.
How Much Time Does Concrete Scanning Take?
GPR can identify the position and depth of embedded objects such as rebar, conduits, post-tension cables, and more. If there is no interference or unusual conditions at the site, GPR can detect embedded structures quickly, sometimes within a few seconds. But typically the time required for a project depends on various factors such as the time frame of the project, the client’s needs, the type of slab and the amount of reinforcement in the slab, and more.
Can Concrete Scanning Identify the Differences Between Subsurface Obstructions?
No, not by itself. Using GPR, we can tell that an obstruction is at a given location. We can also determine the depth of an object. Translating information into useful utility and other material markings requires a human mind and eye to translate the image on the control unit display to show the client what obstructions lie below the surface. Skilled concrete scanning technicians can almost definitively identify what obstructions are beneath the ground by using their expertise and training.
The GPRS Green Box Guarantee is an industry-leading, proprietary concrete scanning program that provides contractors the necessary assurance of safety when drilling, cutting, or coring through an elevated concrete deck. GPRS uses green boxes to indicate areas clear of obstruction.
The equipment operator and nearby workers are at high risk of electric shock from electrical conduit strikes. In addition, work and operations near the damaged conduit can be disrupted. By cutting, coring, or drilling through a post-tension cable, severe structural damage can take place. A severed post-tension can compromise structural integrity, posing severe safety concerns for everyone involved.
The cost of repairing damaged wires and conduits after an electrical strike can be thousands of dollars. Tendon repair and restressing can cost up to several thousand dollars per tendon. A job site can be shut down if rebar, post-tension cables, or electrical conduit are cut through, which increases costs.
GPRS Project Managers can clear dozens of cutting/coring areas daily with an exemplary level of accuracy. Demolition can be greatly accelerated by cutting, coring, and drilling through concrete that is known to be clear of obstructions. By preventing delays caused by line strikes and damaged reinforcement, contractors can keep their schedules on track and reduce the need for change orders and budget overruns.
The Green Box Guarantee information can be found directly on the coring location. Green Box Guarantee locations and parameters are explained in GPRS walkthroughs after jobs are completed. GPRS has the highest accuracy in the industry; we consistently perform above 99% accuracy.
Cutting or coring poses the inherent risk of striking rebar, conduits, and post-tension cables–a strike results in higher repair costs, schedule delays, and safety risks for your employees. GPRS's Green Box Guarantee allows you to keep your team safe while cutting and coring concrete.
Can Ground Penetrating Radar Scan Concrete Slab-On-Grade?
Yes, we can. Usually, this question comes from someone familiar with concrete x-rays. However, the use of x-rays to determine the location of reinforced steel is limited to elevated slabs because it requires access to both sides of a slab. The radioactive isotope is on one side of the slab, and the film is on the other side of the slab.
This is an entirely different technology from ground penetrating radar. GPR only requires one side of the concrete to be accessible. As a result, slab-on-grade applications are ideal for GPR.
Can Ground Penetrating Radar Differentiate Between Rebar, Post Tension Cables, Electrical Conduits, And Other Embedded Materials?
GPR does not determine the type of object being located; however, skilled concrete scanning technicians use all available data to determine the type of reinforcing steel or electrical conduit present.
By combining GPR with other site data and experience, technicians can accurately identify all subsurface obstructions. Findings are then marked on the concrete. For example, in a square layout, an obstruction on an angle will typically be a conduit. Conduits may have subtle differences in the GPR reflection than rebar, different depths, spacing, patterns, etc.
How Accurate Is Ground Penetrating Radar with Marking Obstructions in Concrete?
Our horizontal accuracy is typically +/- ¼” to the object's center in concrete. Using GPR, we can also identify the depth of each object in concrete to within +/- 10-15%.
While GPR is very accurate, we always recommend clients cut, drill, or core one to two inches from any line marked as an obstruction. While GPR typically cannot provide the exact diameter of a subsurface obstruction, a skilled technician can provide a very accurate estimate of diameter.
Is Ground Penetrating Radar A Risk to Health?
This concern usually arises from someone with experience in x-raying concrete because radiation exposure from x-rays is known to pose health risks. The use of x-rays to identify items embedded in concrete remains a viable and accurate method, but x-ray companies must ensure safety by clearing people from an area around the x-ray location. They must create a safe and controlled environment.
In contrast, the typical power output of a GPR antenna is less than the power output of most cell phones. Consequently, there are no health concerns for our project managers, construction workers, or tenants in the building where the work is taking place.
Additionally, GPR is a soundless process, so noise pollution is not an issue.
How Long Does It Take to Scan an Area for Core Drilling?
Ground penetrating radar is a highly efficient and fast method of scanning. GPR is very versatile as it can do anything from determining the slab thickness in an entire warehouse or pinpointing rebar for a core drilling location.
GPRS typically uses a 2-foot by 2-foot layout for core drilling locations. Generally, scanning and marking a location of this size takes about 10 minutes.
When speculating about the future of concrete scanning, one must also consider legislation. It's possible that hiring a concrete imaging company could be a legal mandate because of the massive amounts of damage and the safety risk posed when striking post-tension cables.
Furthermore, with the cloud becoming an increasingly significant part of our lives, we can predict that databases will eventually become the primary means of storing concrete scanning and location data. As of now, data is siloed between areas and companies, making the exchange of information hard. In the future, a centralized service could store and collect concrete location data. This data will most likely be acquired and managed by private companies through a shared database or individual methods.
Facilities owners and managers could greatly benefit from these databases.
Even though no one can accurately predict what the future holds for concrete imaging, it is a fact that methods will continue to advance as technology advances.