Non-destructive investigation and damage prevention using Ground Penetrating Radar

Radar's evolution from research tool to mainstream applications

Linda Wilson
Senior Consultant
Holland & Davis LLC
Houston, Texas

Like many technologies, Ground Penetrating Radar (GPR) is evolving rapidly as an investigative tool due to advances in computers and software and an ever-increasing number of commercial applications.

It's not your father's Oldsmobile
General Motors ran a memorable ad campaign in the 1980s featuring children of popular television actors happily driving dad in a newly-redesigned, dramatically-updated automobile. The punch line summed up the advantages and appeal of the new model for the new generation in a simple phrase, "It's not your father's Oldsmobile."

Similarly the new generation of GPR technology, aided by advanced programmable computer chips, improved analysis software and a larger body of knowledge, provides vastly better information compared with GPR systems available five and 10 years ago. "Radar is no longer just a tool for researchers, but a practical tool for mainstream maintenance and inspection," says Dennis Johnson, president of Geophysical Survey Systems Inc., a leading manufacturer and the first company to develop commercial systems for GPR. "GPR had been relegated to the world of academic research and very specialized applications. Certainly, single survey line interpretations required an experienced operator and contributed to radar's reputation for being too complex for most practical applications. Now, by surveying multiple lines, data is much more robust, and using the 3D imaging software that's available, interpretation is much faster and easier to do...and easier to learn."

A brief primer on the technology
GPR systems transmit ultra-wideband pulses to map subsurface conditions nondestructively. A GPR system imparts a tiny pulse of electromagnetic energy into a surface or structure and measures the reflected energy from any buried targets within that structure. The reflected signal is digitized and the image output is a continuous profile of the scanned line. Multiple parallel survey lines allow the creation of 3D images that make interpretation much easier. The total power of transmitted pulses during system operation is only about 1/1,000th of a mobile phone transmitter; therefore, GPR poses no problem to users' health or communications systems.

This article focuses on four major applications of GPR in the public sector that save both time and money...and provide robust data without destruction to the surfaces being investigated:

  • Highways
  • Bridges
  • Concrete Structures
  • Utility Mapping
GPR data aids public officials and engineers in determining how to most effectively allocate the limited funding available for maintenance of transportation assets. Dimitri Grivas, Professor of Civil Engineering at Rensselear Polytechnic Institute and co-leader of the civil infrastructure and geotechnical applications areas for CenSSIS (Center for Subsurfacing Sensing & Imaging Systems), comments, "GPR technology works. Using radar, road agencies can collect key information on what exists below the surface. It helps determine quickly and accurately layer thicknesses, locations of rebars and utility conduits, leaking pipes and concentrations of voids. Such information enables better management decisions and results in considerable savings."

GPR applications for highway inspection
Radar systems are able to collect roadway information at posted speeds up to 65 mph so that traffic flow is minimally affected. There is no need to block off sections of highway during inspection. The fastest GPR systems can now scan at rates up to 800 scans per second. This translates into 10 scans per foot at 55 mph. The high scan rate permits imaging of small voids and pipes. Both concrete pavement (up to 15 inches) and bituminous/asphaltic pavement (up to 3 feet) can be surveyed with a high degree of accuracy.

On new projects, accurate pavement thickness profiles collected continuously along roadways provide a better method for verifying adherence to QC/QA specifications. Pavement thickness accuracy is critical in establishing valid pavement load capacities—a factor which can greatly enhance the life of any roadway network. The digital "infrastructure profiles" can be archived for immediate access, and they facilitate planning of long-term maintenance programs.

Radar is ideal for identification and inventory of older pavement systems whose construction history is often unknown. Lack of good data presents an expensive challenge in determining whether it is more cost effective to spot-repair a stretch of roadway or to partially or fully reconstruct it. Argentina is using GPR to map and assess roadway conditions. Data on over 3,000 km of highway has been collected, much of it in the Andes along the Argentina-Chile border.

GPR applications for bridge deck inspection
Radar data collection can be done on overlaid and bare concrete decks reinforced with longitudinal and transverse steel (upper and lower mesh) to identify deterioration quantities (delamination, punky concrete, corroded rebar) and their locations on the deck structure.

As in other applications, radar can be used for QC/QA verification on new bridges. Accurate measurement of concrete cover (depth of rebar from exposed concrete or asphalt surface) on new construction is a major criterion used by most bridge design engineers to assess its resistance or susceptibility to future corrosion. Shallow cover causes reinforcement to be attacked by chloride ions at an earlier age. Excessively deep cover causes cracking in the deck, providing a direct pathway for chlorides to reach the steel. Accurate QA verification of concrete cover allows engineers to better predict the service life of a bridge deck. Two states now require GPR surveys on new bridges to determine contractor pay for quality of concrete cover.

In cases of bridge maintenance, radar complements and enhances traditional engineering evaluations. Radar-imaged deterioration maps can be used to guide the optimal use of corrosion potential testing, chloride testing and coring—minimizing the deck area required for sampling. The resulting data provides an accurate overall deterioration threshold, which separates critical maintenance zones from areas not requiring immediate attention.

According to Glen Roberts of New Hampshire DOT Bureau of Materials & Research, "When ground-truth information from maintenance operations or dismantling is available, comparison of deterioration predicted by radar images and actual deterioration is very good. Six bridge decks have been compared with ground truth, all with positive results."

GPR applications for concrete structure inspection
Radar quickly and accurately locates (in 3D) rebar, tension cables, plastic and fiber optic conduits, and voids to a depth of 18 inches in existing concrete structures (e.g., tunnels, walls, floors, slabs, decks, balconies and garages). In addition, it is used to detect and map the relative condition of concrete so that rehab projects can be prioritized and planned more effectively. For QA on new concrete structures, radar is being used to inspect and measure slab thickness.

In addition, radar is being used extensively to determine deterioration of reinforced concrete balconies in hotels and condominiums on the Florida coast, where salt water corrosion erodes load-carrying capacity. Visual inspections alone are not sufficient to assess structural integrity since most corrosion remains hidden.

GPR applications for utility detection
Speed, accuracy and minimal additional equipment are important issues on active construction sites. "Wearable" computer-controlled GPR systems are in use today, reducing interference with ongoing work. GPR systems detect utilities of all material types, including PVC, and will map closely-spaced utilities in 3D.

In another situation at the Denver International Airport, Delta Airlines wanted to install new conduit in concrete floor in their gate area without disturbing existing lines or the traveling public. Critical communications links between gate areas already existed. Delta could not take the chance that they would be severed during either the detection or drilling phase. Radar was successfully used to find the PVC pipe containing the critical fiber optic cables. Coring, cutting and drilling proceeded without interrupting existing utilities or communications.

In summary, GPR technology offers several benefits over traditional methods of surveying and testing products and materials:

  • Minimal set-up time, with little or no disruption to other activities at the site
  • Faster data collection speed
  • Nondestructive and continuous large area data collection—10,000 square feet of surface can be thoroughly covered in a few hours
  • Real-time target analysis, with x-y and z axis location
  • Not labor intensive—one person could do the surveying and analysis
  • Original data is digitally-processed so results can be archived for future reference
Each inspection technology offers its own merits and has pluses and minuses. The ultimate answer will come from a combination of methods available in a single system, with multiple sensor software producing a single set of data that can be manipulated to suit the application. Considering the rate of progress in radar over the past five years, that day may not be too far away.

To reach Linda Wilson, call (603) 898-5112 or send e-mail to lwilson@hdinc.com.