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Nondestructive Evaluation (NDE) Web Manual

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Ground Penetrating Radar (GPR) for Detecting Voids

Application

GPR is a rapid, nondestructive testing method that can be used for a wide range of applications. It can be used to identify and map areas with a high likelihood of corrosion-based deterioration, assess construction quality, determine structural reinforcement layout, and estimate thickness of the deck, overlays, or reinforcement cover. It can also be used for bridge deck and tunnel linings condition assessment, pavement layer thickness measurements, mine detection, archeological investigations, geophysical investigations, borehole inspection, building inspection, etc. Typical GPR applications for bridge elements include the following:(1)

  • Measuring concrete cover and mapping rebar configuration.
  • Assessing potential for concrete deterioration and delamination.
  • Measuring deck thickness and mapping cables, conduits, other embedded objects.
  • Identifying heterogeneity in concrete such as rock pockets, honeycombing, and voids.

Description

A GPR antenna transmits high-frequency EM waves into the structure and detects the reflected signals. GPR is an accepted technology to diagnose existing structures with apparent deteriorations. GPR is best utilized as a complementary tool in the engineer’s toolbox, rather than as a technology that provides a complete solution to every problem as a stand-alone application. It can be implemented as ground-coupled (figure 1) or air-coupled (figure 2) GPR.

Figure 1. Photo. Data Collection on a Bridge Deck Using Ground-Coupled GPR.
Figure 1. Photo. Data Collection on a Bridge Deck Using Ground-Coupled GPR.

Figure 2. Photo. Data Collection on a Bridge Deck Using Air-Coupled GPR.
Figure 2. Photo. Data Collection on a Bridge Deck Using Air-Coupled GPR.
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Physical Principle

GPR uses electromagnetic (EM) wave pulses in the microwave band of the radio spectrum to image the subsurface. GPR detects the reflected signals from reflectors (boundaries related to a mismatch of electric properties) in subsurface structures.

A GPR antenna transmits high-frequency EM waves into the structure. A portion of the energy reflected back to the surface from any reflector, such as a rebar or an anomaly, is received by the antenna (figure 3). The remainder of the energy penetrates beneath this interface and is reflected back to the receiver from other interfaces until the transmitted energy diminishes. In the case of an air-coupled antenna, typically suspended 1 to 2 ft above ground, the part of the trace before the first strong reflection represents the travel of EM waves through air.

Figure 3. Illustration. Physical Principle of GPR.
Figure 3. Illustration. Physical Principle of GPR.

The bridge deck condition is defined by the signals returned to the receiver; the speed at which they return; and other properties of the signal, including amplitude, attenuation, signal polarity, and wavelength. The signal responses are different for various interfaces due to the contrast in the electrical properties of adjacent layers. The electrical properties of primary interest are the electrical conductivity (inverse of resistivity) and relative dielectric permittivity (dielectric constant). These two properties govern the ability of the EM energy to penetrate that particular medium and the speed at which EM waves propagate through the medium, respectively. The contrast in the dielectric constant between adjacent materials dictates the strength of the reflected energy to the surface.

EM waves cannot penetrate into metals (e.g., rebar or metallic expansion joints). Even dense wire screens and thin foils are impermeable to EM waves. The penetration of the GPR’s EM waves is fair to good in most construction materials (concrete, hot mix asphalt, or engineered pavement soils). GPR signal’s penetration in concrete that is moist and high in free chloride ions (e.g., fresh concrete) or other conductive materials can be adversely affected. For example, the GPR signal will be highly attenuated in a reinforced concrete deck that has undergone severe corrosion induced deterioration, and limit the depth of assessment. Dielectric constant for typical construction materials, including concrete, are shown in the following table 1.

Table 1. Relative Dielectric Permittivity for Different Materials.
Material Dielectric Constant*
Air1
Clay25-40
Concrete8-10
Crashed base6-8
Gravel4-7
Hot mix asphalt 4-8
Ice 4
Insulation board 2-2.5
Sand 4-6
Silt 16-30
Silty Sand 7-10
Water (fresh) 81
*Moisture can significantly increase the dielectric values
(e.g., for gravel it may go from 5 to above 20)

Data Acquisition

Data collection with GPR should be carried out by an experienced and qualified operator. The following list comprises the general steps required to achieve quality results on bridge decks:

  1. Turn on the GPR system, and allow it to stabilize (warm up) while adjusting settings and calibrating survey wheel/distance measuring instrument.
  2. Calibrate survey wheel/distance measurement instrument (DMI) on a fixed distance over the deck’s surface, roughly one-third to one-half of the deck length.
  3. Set up signal position for top of scan, gain, vertical high-pass (HP), and low-pass (LP) filters based on recommendations from the manufacturer.
  4. Set up GPR sampling window for sufficient time range (8 to 12 ns for a ground-coupled, and up to 40 ns for an air-coupled antenna), vertical sampling (512 samples/scan, but ranging from 256 to 1024), scan rate (100 to 400 scans/second), and spatial scan density (24 to 60 scans/ft for a ground-coupled, and about 12 scans/ft for an air-coupled antenna).
  5. Collect data and perform quality control review of onscreen output that is sent to permanent storage (hard drive, flash drive, memory stick, or other medium).

Simple data collection on a bridge deck can be accomplished on a series of lines spaced about 2 ft apart, using a hand-pushed or pulled cart with an encoder-based wheel or a vehicle mounted air-coupled antenna. Smaller carts allow access into tighter confines (e.g., where deck meets curb, parapet, jersey barriers, etc.). The GPR survey lines should be perpendicular to the direction of top reinforcement. If the survey is conducted in the direction parallel to the orientation of the top reinforcement, the survey should be conducted with two antennas of dual polarization. For concrete girders (precast prestressed, post-tensioned, cast-in-place concrete girders), GPR field data are typically collected at a spacing equal to or less than the rebar spacing within the girder. A 1-ft by 1-ft (or more closely spaced) grid is usually used to map voids and delamination, and to image, locate, and map reinforcement within the girder and the layout of any embedded conduits, post-tensioned cables, or prestressing strands. Position and depth can be determined reasonably accurately if the measurements are performed properly.

Data Processing

Most software used for identifying and mapping layers, embedded targets (such as voids), or reinforcement provide algorithms that automatically tag, or “pick,” the maximum signal reflections from the targets or features of interest, based on user input (negative or positive peak, etc). Interactive software allows modification of the “picks” or cleanup of the analysis so that errant data are not selected for analysis or output. Two-way travel time, velocity, and x- and y- coordinates for each “pick” are output, all information needed for the mapping software.

3D imaging programs are also used to identify the relative location of suspect voids within the mapped area, the layout, relative location, and depth of individual or groups of bars, conduits, or cables. These 3D software packages generally allow migration, a process that refocuses GPR energy to its point of origination when the targets are of small size and appear as arch-shaped hyperbolas (figure 4). Migration can also enhance the visibility of volumetric targets such as suspect voids, heterogeneities, or other indications of defects within concrete. The sign of the reflection from a void will be an indication whether it is “dry” or filled with water or other material.

Figure 4. Composite Graph. Map of Measured Two-way Travel Time, x- and y-coordinates, and Identified Positions of Rebars and Horizontal Cracks at and above the Top Rebar Level.
Figure 4. Composite Graph. Map of Measured Two-way Travel Time, x- and y-coordinates, and Identified Positions of Rebars and Horizontal Cracks at and above the Top Rebar Level.

Data Interpretation

Using dedicated software, x- and y-coordinates from each hyperbolic maxima are matched with a measure of two-way travel time and an estimated or indirectly measured (migrated) velocity. Velocity calibration is performed by using cores or drill holes and measuring directly from the concrete surface to the top of a bar selected for calibration or a void selected for verification. The migrated or calibrated velocity (generally in inches or centimeters per nanosecond) is then used to calculate the depth to a target, such as a suspect void. When imaging software is used, 2D plan view maps or 3D transparency can also be used to better illustrate how features such as suspect voids, rebars, conduits, and cables are laid out within the surveyed element (figure 5).

Figure 5. Contour Map. Processed GPR Data Collected on a section of Cast-in-place (CIP) Reinforced Concrete Girder, Showing Voided Areas Beneath Transverse Rebar.
Figure 5. Contour Map. Processed GPR Data Collected on a section of Cast-in-place (CIP) Reinforced Concrete Girder, Showing Voided Areas Beneath Transverse Rebar.

Advantages

  • Well-established field data collection processes.
  • Much more rapid than other methods.
  • Reliable and repeatable.
  • Correlates well with electrical resistivity to describe corrosive environment.
  • Correlates well with other NDE methods when the defects are severe.

Limitations

  • Requires advanced expertise and training for data collection, processing, and interpretation.
  • Primarily sensitive to identifying corrosive environment within the upper half of the deck when measurements are made from the deck surface.
  • Data negatively influenced by extremely cold conditions.
  • De-icing agents can limit the signal penetration.
  • Does not directly assess deck delamination, except under very restrictive conditions (e.g., epoxy-injected repairs that are separating two deck sections or delaminated areas filled with water).
  • Requires calibration with other NDE methods and other ground truth activities.
  • Less cost-effective than other methods, particularly when applied to smaller individual structures.
  • Accuracy is limited to position and depth of bars in the upper half of the deck. Lower half of the deck can be difficult because of the rebar mat in the upper half.

References

  1. ASTM, Standard Test Method for Evaluating Asphalt-Covered Concrete Bridge Decks Using Ground Penetrating Radar, D6087-08, ASTM International, West Conshohocken, PA, 2008.

Page last modified on November 3, 2015.