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91 a p p e N D I x K air-Coupled Gpr Operating principles The Texas A&M Transportation Institute (TTI) air-coupled ground-penetrating radar (GPR) antenna transmits pulses of radar energy with a central frequency of 1 GHz into a tun- nel lining. These waves are reflected at significant layer inter- faces in the lining. The reflected waves are captured by the system and displayed as a plot of reflection amplitude (volt- age) versus arrival time. As shown in Figure K.1, the largest peak is the reflection from the surface. The amplitudes before the surface reflection are internally generated noise and, if significant, should be removed from the trace before signal processing. The reflections that can also be of significance to tunnel personnel are those that occur after the surface echo. These represent significant interfaces within the lining, and the measured travel time is related to the depth to another layer or to a defect. For example, in Figure K.1 the time between the surface echo A1 and A2 is related to the depth to another layer or to a defect. The software developed at TTI automatically measures the amplitudes of reflection and time delays between peaks. Using these measurements, the operator can calculate layer dielectrics and depths to another layer or defect. The equations used are summarized in Equations K.1 through K.3: A A A A a m m 1 1 (K.1) 1 1 2 ε = + ï£«ï£ ï£¶ï£¸ â ï£«ï£ ï£¶ï£¸       where ea = dielectric of lining surface, A1 = amplitude of reflection from the surface in volts (peak A1 in Figure K.1), and Am = amplitude of reflection from a large metal plate in volts (this represents the 100% reflection case). h c t a 1 1 (K.2) i( ) = â ε where h1 = depth to another interface (such as to another layer, void, or other defect), c = constant (speed of the radar wave in air as measured by the system), and Dt1 = time delay between peaks A1 and A2 in Figure K.1. A A A A A A A A b a m m m m 1 1 (K.3) 1 2 2 1 2 2 { } { } { } { }ε = ε â +  ï£ï£¬   â â  ï£ï£¬         where eb = dielectric of the lower layer, void, or other defect, and A2 = amplitude of reflection from the top of the lower layer or defect in volts (peak A2 in Figure K.1). Dielectric values and depths can be readily determined from two software packages developed by TTI: COLORMAP and Pavecheck. Both software packages are relatively easy to use for production-level purposes. air-Coupled Gpr results for the ttI test Specimens TTI personnel collected air-coupled GPR data on concrete and shotcrete specimens that contained delaminations or voids. The TTI team determined that the equipment could detect only three simulated voids, all located in the shotcrete sections: ⢠Specimen D, an air-filled void placed 7.625 in. from the surface; ⢠Specimen F, an air-filled void placed 3 in. from the surface; and ⢠Specimen G, a water-filled void placed 3 in. from the surface. Air-Coupled Ground-Penetrating Radar Field Tests
92 The equipment could not detect delaminations or voids in the other sections. Figure K.2 shows the analysis of the GPR data on Speci- men D using the COLORMAP program. The program indi- cated that the depth to the defect was 7.7 in. The program calculated a surface dielectric of 8.2 and a void dielectric of 6.6. If an air-filled void exists, the calculated dielectric of the void is less than the surface dielectric. Figure K.3 shows the analysis of the GPR data on Specimen F using the COLORMAP program. The program indicated that the depth to the defect was 2.6 in. The program calculated a surface dielectric of 9.1 and a void dielectric of 7.3. If an air- filled void exists, the calculated dielectric of the void is less than the surface dielectric. Figure K.4 shows the analysis of the GPR data on Specimen G using the COLORMAP program. The program indicated that Figure K.1. Air-coupled GPR operation. Figure K.2. Analysis of air-coupled GPR data on Specimen D.
93 Figure K.3. Analysis of air-coupled GPR data on Specimen F. Figure K.4. Analysis of air-coupled GPR data on Specimen G.
94 Figure K.5. Air-coupled GPR data collected for the Washburn Tunnel. Figure K.6. Air-coupled GPR data collected for the Chesapeake Channel Tunnel roof. the depth to the defect was 2.7 in. The program calculated a surface dielectric of 8.5 and a void dielectric of 12.4. If a water-filled void exists, the calculated dielectric of the void is greater than the surface dielectric. Air-Coupled GpR Results from Tunnel Testing Washburn Tunnel In the Washburn Tunnel, which is completely lined with tiles, the air-coupled GPR data were collected every foot and indi- cated changes in the surface dielectric along the length of the tunnel. An example of air-coupled GPR data collected in the Washburn Tunnel is shown in Figure K.5. This figure was generated by the Pavecheck program developed by TTI to analyze air-coupled GPR data. The dielectric values shown in Figure K.5 have not been corrected for changes in the dis- tance between the antenna and the tunnel lining. As can be inferred in Figure K.5, the distance between the antenna and the tunnel surface did vary because of the difficulty of keep- ing the vehicle moving in a straight line. However, the TTI team believes the data are useful in their current form. The unusually large peak on the left side of the figure is associated with a steel plate installed in the tunnel lining. Chesapeake Channel Tunnel At the top of the Chesapeake Channel Tunnel lining, the team used air-coupled GPR data to locate one area with no surface distress for in-depth testing. The data were collected every foot with the antenna aimed directly at the top of the tunnel lining. Data could not be collected from the top sides of the tunnel because of the cables and utilities installed there. The area chosen for testing had a surface dielectric value of 18.7, which is unusually high for concrete, at Station 486+67. Fig- ure K.6 shows the air-coupled GPR data for this area. As can be inferred from Figure K.6, the distance between the antenna and the tunnel lining surface was kept relatively constant (the antenna was mounted on a pushcart and pointed directly at the top of the tunnel lining). The results of the in-depth test- ing in this area showed that a shallow delamination existed at that location. The team tested other locations at the top of the tunnel and on the tiled tunnel wall. The team used infrared data from the SPACETEC equip- ment to determine testing locations on the tiled tunnel wall
95 at the Chesapeake Bay. The team could not collect air-coupled GPR data at that location because construction equipment blocked access to the wall at the time of the air-coupled GPR data collection. The in-depth evaluation devices were able to detect defects in the areas tested. The TTI team also collected handheld infrared camera images in the Chesapeake Channel Tunnel roof and roadway; selected images are shown in Appendix L. The team found few changes in temperature in the tunnel roof. The team found that collecting images on tiled tunnel linings with this equipment was problematic because the tile reflected heat from any heat- generating source, including construction equipment, lights, and people. In addition, the team was not able to effectively compare the SPACETEC results along the area tested by the team because that would have required a lane closure on the other side to effectively obtain images with the handheld device. The vehicle-mounted thermal camera scans were also affected by construction equipment operations during the scans, so the team could not generate comparisons between the SPACETEC results and that device. Figure K.7 shows an example of the air-coupled GPR data taken along the tiled tunnel wall. The dielectric values shown in Figure K.7 have not been corrected for changes in the distance between the antenna and the tunnel lining (a version of this software will be developed soon with this capability). However, the TTI team believes the data are useful in their current form. The unusually large peaks are associated with steel plates or fixtures installed on the tun- nel surface. Eisenhower Memorial Tunnel In the Eisenhower Memorial Tunnel, the in-depth evaluation devices were able to detect defects in the areas tested. The loca- tions were selected for testing with the in-depth devices based on observed surface distress and feedback from the tunnel operator. The team encountered problems with collecting air-coupled GPR in the top portion of the Eisenhower Memorial Tunnel with an exposed concrete surface, mainly because cables and other obstructions were in the way. In addition, the team could not collect data at the top of the tunnel because of the distance between the ceiling and the roof. Figure K.8 shows an example of the data collected (the antenna was mounted on a pushcart). Figure K.7. Air-coupled GPR data collected for the Chesapeake Channel Tunnel tiled wall. Figure K.8. Air-coupled GPR data collected for the Eisenhower Memorial Tunnel, top portion.
96 Figure K.9. Air-coupled GPR data collected for the Hanging Lake Tunnel roof. Figure K.10. Air-coupled GPR data collected for the Hanging Lake Tunnel tiled wall. The antenna was pointed at the side of the tunnel; the team had difficulty keeping the pushcart moving in a straight line, so the distance between the antenna and the lining surface varied. Although GPR data were collected on the tiled roadway sec- tion, the data proved not to be usable because the tiles were mounted on steel panels, and the panels were apparently not attached directly to the concrete. The TTI team also collected handheld infrared camera images in the top section of the Eisenhower Memorial Tunnel and found significant temperature changes. Appendix L con- tains selected images from the handheld device and the ther- mal scan. Hanging Lake Tunnel In the Hanging Lake Tunnel, the in-depth evaluation devices were able to detect defects in the areas tested on the tunnel roof. The locations selected for testing with the in-depth devices were requested by the tunnel operator. Figure K.9 shows an example of the air-coupled data taken on the Hanging Lake Tunnel roof (the antenna was mounted on a pushcart and pointed directly at the top of the lining). As can be inferred from Figure K.9, the distance between the antenna and the tunnel lining surface was kept relatively constant. Also, Figure K.9 shows two distinct interfaces. Figure K.10 shows an example of the air-coupled GPR data taken on the Hanging Lake Tunnel tiled wall. The dielectric values shown in Figure K.10 have not been cor- rected for changes in the distance between the antenna and the tunnel lining. However, the TTI team believes the data are useful in their current form. The unusually large peaks are associated with steel plates or fixtures installed on the tunnel surface.
97 No Name Tunnel In the No Name Tunnel, the team collected only air-coupled GPR data. Figure K.11 shows an example of the air-coupled GPR data taken in that tunnel. As can be seen in Figure K.11, the dielectric values are unusually low. The team did not encounter this issue in the other tunnels tested. One explana- tion is that the antenna was inadvertently set to a lower power output mode, which resulted in lower reflection amplitudes from the tunnel lining. In any case, the air-coupled GPR data indicated possible layer interfaces in this tunnel. Figure K.11. Air-coupled GPR data collected for the No Name Tunnel.
