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Ground Penetrating Radar Imaging of Concrete at a Nuclear Power Plant

by Peter Giamou*

 

Introduction and Background

Having regularly conducted investigations of concrete structures, I've found that many challenging projects stretch the capability of concrete imaging technology. In this paper, I highlight the use of ground penetrating radar imaging at a nuclear plant.

Our company was retained to complete a ground penetrating radar concrete imaging investigation at the Clinton Power Station, south of Chicago, Illinois. The objective of the investigation was to map the spatial location and depth of all embedded reinforcement bars and conduits that existed within a 1.8 by 3 m (6 by 10 ft) area prior to the installation of a new crane hoist. It was important to be able to use an investigative tool that would allow the intelligent placement of anchor bolts for the crane hoist. The data acquisition and interpretation of the results had to be delivered on site.

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The entire job was completed in one day,
with the results produced on the spot.

The site engineer required a nondestructive approach to be used for this project, since the structural integrity of the building could not be compromised in any way. X-rays were ruled out for health and safety reasons, since the building operated constantly and had an open concept. Other concerns with X-rays involved the speed and simplicity of data acquisition. The exposure times for conventional radiography would have been too long given the size of the work area and thickness of the concrete floor. The ground penetrating radar scanning technique was selected as the method of choice since it provided rapid scanning of large areas with immediate onsite results.

High frequency radar scans were used to image a 350 mm (14 in.) thick suspended concrete slab. Complications included the thicker than average slab, which was due to the presence of two beams, and having to operate in an extremely noisy and high security area. High resolution data of excellent quality enabled definition of embedded structural elements. The maintenance engineers were able to use the results to effectively plan the placement of anchor points for the hoist without damaging the integrity of the structure.

Ground Penetrating Radar Imaging

Measurements were made with a readily available commercial concrete scanning system. This is a high frequency (1 GHz) ground penetrating radar system. The system, shown in Figure 1, consists of an antenna transducer, handle, wheel odometer, battery and digital video logger. The video logger provided a hard disk for data storage and a display screen for viewing data during acquisition. The unit is self-contained in a portable, wheeled carrying case that doubles as a display stand on site.

Figure 1 — Concrete imaging system in operation.

Ground penetrating radar uses echo-sounding principles. The radar system produces a short duration pulse of radio wave energy that is transmitted into the concrete. Changes in material composition (which can change the electrical character) cause some of the energy to be reflected back. The reflected signals are detected and amplified at the receiving antenna and stored on the data logger. Objects (reinforcement bars and conduits) and subsurface voids embedded in the concrete can be detected because these objects will have markedly different electrical properties than the host concrete.

In the last few years, ground penetrating radar imaging has become widely used. Data, carefully acquired over a grid area, are processed to produce depth slice maps of the subsurface. The resulting images are similar to X-ray images, but with lower resolution.

Survey Procedures

The work site was inside a five story, cast in place, concrete reinforced structure adjacent to one of the nuclear reactors. This building houses generators, support equipment and supplies for the operation of the plant. The area of interest was on the fifth (top) level, near the edge of the mezzanine floor overlooking the central portion of the building. The floor consisted of a 350 mm (14 in.) thick suspended concrete slab with #4 and #5 reinforcing bars placed in a bidirectional pattern at 300 mm (12 in.) centers on upper and lower mats. The floor was covered with a polyurethane coating. The work area was open, very smooth, flat, well lit and clean, but noisy due to all the operating machinery.

The area of interest was intentionally placed over intersecting beams so that the bulk of the load could be directly transferred to the beams. A total of six scan grids were collected, each 1.2 by 1.2 m (4 by 4 ft), covering a total area of 2.4 by 3.6 m (7.9 by 11.8 ft; Figure 2). The initial equipment setup and calibration, and the establishment of the grid layout, took about 2 h to complete.

Figure 2 - Plan map of survey grid.

The data were acquired in a bidirectional pattern at 100 mm (4 in.) line spacing intervals along a total of 39 lines in one direction and 26 lines in the orthogonal direction. Several velocity calibrations were completed at various locations on the slab to provide an accurate estimate of the radar wave velocity within the slab - a necessary step to properly process and generate a depth scale for the data. The radar wave velocity in the concrete at this location of the building was 115 mm/ns (4.5 in./ns), which is considered moderately fast for most concrete types. Data acquisition for all six grids took about 1 h to complete.

Data Processing and Interpretation

The six grids of data were processed to produce depth slices and were interpreted individually on site. Small tick marks were made on the floor on all four sides of each grid, based on measurements read from the ground penetrating radar screen view. The tick marks were made with pencil, since it was requested by the site engineer that no permanent markings be left. Site contacts were warned to protect the work area from any water, cleaning or traffic, in order to preserve the markings until construction was complete. The ticks were connected using duct tape to create a final interpreted plan view of the embedded features within the concrete floor. This process took about 3 h to complete.

A line-by-line review of all the individual radar profiles was carried out to confirm the interpretation. This should be a standard quality assurance procedure with any concrete scanning project. There are some situations where the eye of an experienced user can pick out an unusual feature that the processed plan map depth slices do not easily reveal. Reading radar profiles is somewhat different than interpreting depth slices. It requires a greater understanding of how ground penetrating radar systems work, how electromagnetic signals propagate in a medium, and the characteristic signatures of reflections from various features. Nonetheless, any technician can be taught these tricks, enabling a one-person crew to complete radar scanning surveys on most sites.

On site, it was possible to see the slab bottom reflection and beam boundaries in the ground penetrating radar profiles. The operator sees each radar section as it is acquired, providing good quality control. Figure 3 shows a sample radar cross-section from grid 3 (as shown on the map in Figure 2). The section shows two upper reinforcement bar mats, one bottom reinforcement bar mat (on the right side only), the slab bottom (on the right side only), the beam edge, and the signal response from a stirrup parallel to the profile line direction.

The presence of the beams made the interpretation more challenging because the amount of reinforcement bar loading in the beams was significantly greater than in the other locations. The thicker concrete, tighter bar spacing and additional mats of reinforcement limited signals from penetrating deeper than the first layer of reinforcement bar in the beam locations. Areas not underlain by a beam exhibited both a top and bottom set of reinforcing bars at fairly consistent spacing. The processed radar data for the six grids are shown, pieced together, in Figure 4a, for a depth slice from 75 to 100 mm (3 to 4 in.) below the floor surface. The matching from grid to grid indicates accurate grid registration and careful data acquisition. This composite image of all six grids together made interpreting the location of the intersecting beams quite easy. The additional reinforcement bars (inferred to be stirrup cages) stand out in the image. A photo of a typical cage is shown in Figure 4b for reference.

More detailed views of the top-right grid are shown in Figure 5. The ground penetrating radar image shown in Figure 5a shows a beam on the left half of the image. The stirrups can be counted and the unevenness of their placement can be measured. The deeper section (Figure 5b) indicates a diagonal feature within the slab that carried current (as measured by a power line signal detection device) and was interpreted to be an electrical cable conduit.

Figure 3 - Sample cross-section showing typical ground penetrating

Figure 4 - Radar and visible images: (a) composite plan map formed from the six grids representing a depth slice between 75 and 100 mm (3 and 4 in.), with the beam cages crossing through the center of the area; (b) example of a prefabricated stirrup cage used in concrete beam construction.

Figure 5 - Grid 6: (a) display presenting the details of the beam structure at 75 to 100 mm (3 to 4 in.); (b) detailed view showing a weak diagonally-trending feature between 300 and 325 mm (12 and 13 in.) depth, later determined to be an embedded electrical conduit.

Summary and Conclusion

Ground penetrating radar was successfully used in this project with minimal intrusion and disruption to the operation of the nuclear plant. The entire job was completed in one day, with the results produced on the spot. Given the size of the area that needed to be scanned and the thickness of the concrete, ground penetrating radar proved to be the best technology available to meet the stringent requirements of working at a nuclear plant.

The current case history demonstrates the utility of ground penetrating radar for rapid noninvasive testing of a concrete structure. The whole project was completed on site and satisfied the project engineer's needs. Some key benefits of using ground penetrating radar at this site were:

• it presented no safety hazards that would require the work area to be cleared of personnel
• the ability to provide rapid measurements with immediate results
• the ease of adapting to site conditions
• that access needed to be only from one side of the floor
• it gave an accurate estimate of structure depth
• it provided results that were readily understandable to customers
• it recorded digital results for future reference
• it caused no disruption or damage to the structure.

We regularly use ground penetrating radar on many construction projects. With growing industrial experience and understanding of construction practices, ground penetrating radar has become an essential component of building forensics services.

 

* multiVIEW Locates, Inc., 1091 Brevik Place, Mississauga, ON L4W 3R7, Canada; (905) 629-8959; fax (905) 629-7379; e-mail pgiamou@multiview.ca.

Copyright © 2006 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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