GPR Concrete Scanning: Identifying What Lies Beneath

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Jul/12/18

Similar to sonar or the radar that police use, GPR scanning sends radar waves into the ground, concrete or other surfaces in a specific frequency or frequency range. Typically, polarized, high-frequency radio waves between 10 MHz and 2.6 GHz are emitted. This energy may be reflected or scattered back to the surface.

If there is similar material between the scanner and the end of the wave, it dissipates and then disappears as it passes through the various soil layers. The conductivity of the subsurface materials, the frequency and the power of the antenna can affect the depth of penetration.

 

Penetration Distance

 

The depth a GPR scanning system can reach is determined by the antenna’s frequency strength and the surface it is moving through. Concrete, limestone, and granite are resistive materials. Low-frequency signals, typically up to 200 MHz, can travel approximately 100 feet in this type of substrate. Clay, shale, and saltwater are high-conductivity materials. They absorb the waves transmitted by GPR, making it difficult for them to penetrate. The average penetration depth is 3 feet.

 

Although the low frequency only produces a low-resolution scan, it is ideal for deep site investigations such as locating sinkholes, delineating landfills or locating large objects. High-frequency antennas may go to 1,500 MHz and penetrate less than 20 feet. These shallow scans are in high definition, which enables operators to analyze objects close to the surface, such as utility lines and conduits.

 

GPR scanning systems with dual-frequency antennas typically utilize 300 and 800 MHz, allowing them to locate shallow and deep targets. This eliminates the need to scan an area twice, and the system can display both sets of results on one screen. Antennas that are in contact with the ground typically have the most robust signal strength, although there are air-launch systems used above the ground.

 

What Does It Reveal

 

If there is something substantial underground, or a distinct boundary between materials depending on the time and distance, the waves bounce off objects or features and return to the equipment. The reflections produce an image filled with peaks and hyperbolas. Depending on the frequency used, soil changes may also be detected and recorded. It is also possible to identify voids and cracks. Borehole radars that use GPR scanning are utilized in underground mining applications.

 

Array of Applications

 

Ground penetrating radar is versatile. The non-destructive electromagnetic radiation operates in the microwave band. It is effective in a variety of media, from ice, rock, and soil to freshwater, structures, and concrete. When integrated with calibration techniques, GPS technology and the 3D capabilities of a sophisticated imaging system, it is appropriate for a wide range of applications.

 

Concrete Scanning

 

Concrete is one of the most common materials scanned. It is in a variety of structures, from buildings to roadways and runways. Construction professionals know that there are risks when working with concrete due to the inability to see what lies beneath. In some cases, the equipment operator only needs to know the concrete depth. However, more often he or she will use GPR scanning to pinpoint particular objects.

 

Identifying rebar, embedded utilities and post-tension cable locations are critical to maintaining safety for equipment operators as well as the public. For those involved in concrete cutting, structural integrity is paramount. GPR offers a safe, reliable option for working in an unstable environment. Real-time results allow accurate inspection of concrete slabs and structures, from columns, balconies, bridges, and tunnels to decks, roofs and ceilings.

 

Airport Runways and Taxiways

 

Due to its noninvasive nature, GPR can be used to check the structural condition of asphalt and pavement structures at airports. Completing forensic and environmental surveys at night reduce or eliminate flight traffic or safety impacts. If rehabilitation or new construction is needed, GPR scanning can identify the best locations for the asphalt, concrete or pavement to be cut, which reduces downtime.

 

Utility Locations

 

Finding underground pipes and conduits can be challenging. Rerouting and replacing them can be costly and time-consuming. The disturbance to the environment and disruption of typical traffic patterns is inconvenient and frustrating for everyone involved. GPR utilizes resources more efficiently than conventional methods as it’s less invasive. As the operator can see the pipe locations, there is no unnecessary digging or drilling required.

 

Unlike electromagnetic locators, which are unable to detect nonmetallic objects, the effective GPR scanning depth is not limited to 10 to 15 feet. Ground-penetrating radar can be used in identifying the following:

 

  • Abandoned lines
  • Previously unknown or illegal connections
  • Conduit
  • Clay pipes
  • Plastic and PVC
  • Gas
  • Telecommunication wires
  • Septic tanks
  • Water boxes
  • Power lines
  • Fiber-optic lines
  • Transite pipe
  • Metal pipe

 

Structural Assessments

 

Due to their differing materials, structures need a wide GPR scanning frequency range to assess integrity and damage at a variety of resolutions. Large depths require lower frequencies than shallow distances. Higher resolutions are also achieved with higher MHz, delivering an increased level of detail. Each frequency offers one set of features. Multiple frequencies provide detailed information, enabling you to collect more data for a more accurate subsurface image.

 

Transportation Maintenance

 

Maintaining the structural reliability of roadways and bridges, extending their usable life, monitoring their condition and detecting issues early on can reduce overall damage and maintenance costs. Spalling, cracks, rebar corrosion and voids are common types of damage on bridges. Conventional assessment methods are labor intensive, invasive and detrimental to regular traffic patterns. Nondestructive testing equipment, such as GPR, enables subsurface testing for identifying and evaluating flaws and weak points.

 

Benefits of GPR

 

Real estate owners and managers are driven to renovate their properties to net the largest return on their investment. State and local governments must maintain their infrastructure to ensure the safety of citizens on roadways, bridges, and tunnels. The nondestructive nature of GPR scanning provides accurate imaging that is efficient and cost-effective when compared with conventional methods. The sensors can be used in tight spaces so that a full subsurface image can be created for in-depth analysis of potential damage. Pinpointing the exact location that needs attention, from cracks and voids to identifying rebar and other obstacles, improves the ability to keep projects on track and within budget.

 

 

Sources

 

https://en.wikipedia.org/wiki/Ground-penetrating_radar

http://www.usradar.com/ground-penetrating-radar-applications/

http://vtrans.vermont.gov/sites/aot/files/highway/documents/materialsandresearch/completedprojects/Final%20Report%20-%20738%20-%20High%20Speed%20GPR%2010-17-2016.pdf

https://www.edoxx.com/single-post/2017/05/22/How-Does-GPR-Scanning-Work

https://geomodel.com/methods/ground-penetrating-radar/

https://www.canadianundergroundinfrastructure.com/article/20013/gpr-using-electromagnetic-pulses-to-find-what-lies-beneath