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Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) is a high-frequency electromagnetic method commonly applied to a number of engineering problems associated with both new and aging concrete structures. In the transportation sector, GPR surveys are routinely and successfully used for quality assurance (QA) verification of new construction. This includes periodic condition evaluations, beginning with a baseline survey. GPR is currently used to measure pavement thickness, determine pavement structure and condition, or categorize and inventory existing pavement types on roadways-for both hot-mix asphalt (HMA) and Portland cement concrete (PCC) pavements. It is also used in evaluating existing concrete bridge decks to determine extent of deterioration caused by corrosion.

Basic Concept: GPR is an electromagnetic (EM) method based on the rapid generation of well timed, "bursts" of wide-band radio emissions, with high-resolution signals being generated in the MHz to GHz frequency range. Impulse radar penetrates materials that are relatively resistive to electrical current, like asphalt and concrete.

The electromagnetic energy used by the GPR system depends primarily on two physical properties of the ground; the electrical conductivity and the dielectric constant. The dielectric constant determines the speed with which the wave travels and the electrical conductivity determines the attenuation of the electromagnetic signals. Through air, GPR travels at light speed; through another material, its speed is inversely proportional to the square root of its dielectric constant.

Most concrete in-service, has a dielectric of roughly 9 (GPR moves through the medium three times slower than the speed of light), and water slows GPR to only 1/9 its speed in air, because water has the highest dielectric constant (80) of any (dielectric) medium. Asphalt overlays typically have a dielectric constant of about 6 to 6.5. GPR moves faster in asphalt than in most concrete, except when concrete is extremely well cured and dry.

The dielectric contrast — absolute difference in magnitude between two adjacent materials through which a GPR signal will propagate — controls whether there will be a measurable reflection of energy transmitted from the antenna back to the receiver at the surface. The greater the dielectric contrast, the higher the amplitude of this reflection will be. Also, the polarity of the signal will depend on whether the GPR is propagating through a higher, then lower dielectric material or the reverse scenario exists. These and many other subtle GPR signal characteristics help determine accurate depth to rebar, concrete deterioration (condition), overlay thickness, deck thickness, and other structural properties of interest.

Electrical conductivity, the inverse of resistivity, is a property that measures how well a material transports or disperses an electrical current through that given medium. Highly conductive materials, such as metal or seawater, effectively impede all or most of a GPR signal from penetrating them. Similarly, conductive materials that do not fully impede signal penetration, but significantly impair the ability to penetrate them, typically attenuate or disperse the GPR signal so that very little of it can return to the receiver to be measured. Chloride-contaminated concrete is a relatively conductive medium, causing some relative increase in GPR signal amplitude to be measured at a wet surface where chlorides have intruded, and, more importantly, causing a marked decrease in amplitude at the top mat reinforcing level. Much of this amplitude decrease is attributed to greater signal attenuation of the GPR signal through the contaminated concrete, allowing less energy to return and be measured at the surface. These combined conditions are what allow GPR to be effective in identifying conditions that are dominant when rebar is corroding, corrosion products are causing cracking, delamination and spalling, and concrete is otherwise deteriorating.

Data Acquisition: GPR can be used as soon as the pavement is hard enough to drive on. Using the vehicle-mounted horn antenna scans of the pavement can be made in a very short time. Man portable units are more accurate for testing rebar locations since they move slower and get more accurate relative spatial data.

Data Processing: Standard geophysical processing as described in the earlier section is used to make the GPR image. Since the image gives signals as a function of travel in the pavement, velocity must be measured or assumed in order to obtain a depth image; therefore, local calibration of the velocity is recommended.

Data Interpretation: The combined effects of dielectric and conductive properties of subsurface materials, particularly at interfaces of dissimilar material, provide the basis for analyzing GPR data in the time domain, where two-way travel time and amplitude characteristics of the signal are used to make intuitive interpretations about concrete condition. GPR is very effective not only at QA assessments that verify uniformity of the concrete material to some degree and proper placement of the reinforcing elements in a structure, but also at investigations that contribute significantly toward identifying the degree and extent of deterioration beneath the surface of a pavement.

Advantages: Because it is the most rapid data collection technique among geophysical methods in terms of both wave propagation and sampling rates (scans/second), a GPR survey can be performed at walking or slow driving speeds (for QA assessments and condition assessments of concrete pavement). With more sophisticated sensors and data collection methods, a GPR survey can be performed at speeds requiring little or no traffic control, with essentially full coverage of the deck at extremely close spatial sampling.

Since data are collected to provide "real-time" images that can be quickly evaluated and processed in the field to provide some simple answers to problems, GPR is the most valuable geophysical instrument to accurately and quickly determine reinforcement position, placement, and density (PPD), which includes both layout and spatial position (including cover depth). Yet GPR data can also be processed in the office to obtain much more sophisticated condition assessments on older concrete structures with a great deal of detail and accuracy in pinpointing deteriorated zones.

Limitations: Interpreted GPR thickness can sometimes be distorted by changes in conditions or calibration of velocities.