Mapping Voids, Debonding, Delaminations, Moisture, and Other Defects Behind or Within Tunnel Linings (2013)

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30 C h a p t e r 4 Chapter 2 reports the advisory expert panel’s findings on performance criteria, which indicated that NDT should be able to detect any defect within or immediately behind tunnel linings that have a minimum surface area of 1 sq ft, and any defect needs to be located within 1 ft of the actual location on the tunnel lining. The panel also noted that NDT should identify delaminated areas and voids up to 4 in. deep as mea- sured from the lining surface with an accuracy within 0.25 in. According to the results reported in Chapter 3, the follow- ing techniques can detect defects with minimum surface areas of 1 sq ft up to 4 in. deep (and in some cases even deeper): • Air-coupled ground-penetrating radar; • Thermography (handheld thermal camera); • SPACETEC scanner; • Ground-coupled ground-penetrating radar; • Ultrasonic tomography; • Ultrasonic echo; and • Portable seismic property analyzer ultrasonic surface waves and impact echo. All these techniques appear to provide useful information for evaluating tunnel linings and should be considered for implementation; but the limitations of each technology need to be considered and are outlined in individual appendices. None of the devices are able to detect a 1-sq-ft void in a steel lining behind concrete. In addition, the 0.25-in. accuracy cri- terion for defects up to 4 in. deep can be problematic for the in-depth devices. A 0.5-in. accuracy appears to be more realistic. Table 4.1 summarizes the accuracy, detection depth, deterioration mechanisms detected, tunnel lining types, and other information for these technologies. The following sequence of testing is suggested for evalu- ating tunnel linings based on the research conducted under this study: • Collect thermal images and air-coupled GPR data on the tunnel lining. Air-coupled GPR data should be collected every foot along the tunnel lining. Thermal images can be collected every foot as well; however, the equipment covered in this report can collect data at a spacing determined by the camera operator or tunnel inspector. Ideally, the data should be collected on the same day; however, they can be collected separately. The thermal images should be collected when the air temperature is rising or falling; areas of possible defects may show up better in the thermal images. The data from any of these devices can be obtained at a walking pace (around 1 mph, or 1.61 km/h). Air-coupled GPR data can be obtained at much higher speeds, but the geometry and fea- tures in tunnels may hinder operation of the equipment at speeds much greater than 1 mph. • Analyze the data from the scanning devices listed in Table 4.1. Select areas for in-depth testing based on the GPR surface dielectric results, thermal images, and observed surface dis- tresses that are of concern to tunnel inspectors. • Conduct in-depth testing with the ground-coupled GPR and either the ultrasonic tomography, ultrasonic echo, or PSPA device. The choice of equipment can be based on the cost and the type of defect to be detected (tile debonding, delamination, or voids). The ultrasonic tomography and ultrasonic echo devices may be more appropriate for mea- suring and mapping defects greater than 2 in. from the tun- nel lining surface. The ultrasonic tomography device is more expensive than the other two devices; however, it has the capability to provide more information in the field about such defects. The PSPA may be more appropriate for deter- mining the limits of shallow defects. • Evaluate the data collected from these devices. The SPACETEC scanner is available only through a service provider. Service providers can also perform NDT by using the actual or similar devices or techniques described in this report. However, all but the SPACETEC equipment can be operated by tunnel owner personnel. The equipment and essential data processing software are commercially available. To implement Conclusions and Recommended Research

31 Table 4.1. Summary of Nondestructive Testing (NDT) Devices Device Accuracy Detection Depth Deterioration Mechanisms Detected Tunnel Lining Types Other Information Air-coupled GPR Locates defect within 1 foot of its actual location Does not measure depth, but indicates areas of high moisture or low density (high air voids). Such areas may represent prob- lems within or behind the tunnel lining. Tile debonding, delaminations, air-filled voids, water-filled voids, moisture intrusion Concrete, tile-lined concrete, and shotcrete This is a scanning tool that can indicate where to conduct testing with in-depth devices. Thermography (handheld thermal camera) Locates defect within 1 ft of its actual location Does not measure depth, but can indicate tile debonding, delami- nations up to 1 in. and voids up to 3 in. Tile debonding, delaminations, air-filled voids, water-filled voids, moisture intrusion Concrete, tile-lined concrete, and shotcrete This is a scanning tool that can indicate where to conduct testing with in-depth devices. SPACETEC scanner Locates defect within 1 ft of its actual location Does not measure depth, but can indicate tile debonding, possibly delaminations up to 1 in. and possibly voids up to 3 in. Tile debonding, delaminations, air-filled voids, water-filled voids, moisture intrusion Concrete, tile-lined concrete, and shotcrete This is a scanning tool that can indicate where to conduct testing with in-depth devices. Testing can only be conducted through a service contract. Ground- coupled GPR Can determine defect depth within 10% of the actual depth with- out reference cores—5% if cores are available Can possibly detect defects at any depth within or immediately behind tunnel linings. However, specimen testing indicates it cannot locate 1-sq-ft voids in steel plates behind tunnel linings. Delaminations, air-filled voids, water-filled voids, moisture intrusion Concrete, tile-lined concrete, and shotcrete Experienced personnel are needed to interpret defect locations and depths from the GPR scans. Specimen testing indicates it cannot locate 1-sq-ft voids in steel plates behind tunnel linings. Ultrasonic tomography In concrete, can detect voids within 0.5 in., shallow delaminations within 0.75 in. In shotcrete, can detect air-filled voids within 0.7 in., water-filled voids within 1.21 in., shallow delamina- tions within 1.88 in. Can detect defects up to 8 in. deep accord- ing to specimen tests. Tunnel tests indicate it can detect possible defects up to 20 in. deep. Delaminations and voids Concrete, tile-lined concrete, and shotcrete This device may not be effective for measuring defects that are 2 in. or less from the lining sur- face. It may not be accu- rate enough for measuring defect depths in shotcrete. Ultrasonic echo Comparable to the ultrasonic tomog- raphy system according to tunnel testing with both devices. Can measure tunnel lining thickness within 3% of the actual thickness Comparable to the ultrasonic tomography system according to tunnel testing with both devices Delaminations and voids Concrete and shotcrete This device may not be effective for measuring defects that are 2 in. or less from the lining surface. It may not be accurate enough for measuring defect depths in shotcrete. Tunnel tests indicate problems with using this device on tiles. Portable seismic property analyzer (PSPA) ultrasonic surface waves and impact echo Ultrasonic surface waves: about 15% of the actual depth for defects up to 6 in. deep Impact echo: 10% for deep delami- nations greater than 6 in. deep Ultrasonic surface waves: up to 6 in. deep Impact echo: up to 18-in. deep Delaminations and voids Concrete, tile-lined concrete, and shotcrete Quantifying the depth of defects that are shallow or extensive may be difficult with this device. It may not get good results when testing on very rough concrete surfaces, oily surfaces, and severely curved surfaces.

32 each of these methods, however, the personnel in charge need to be sufficiently trained in data collection, reduction, and interpretation. Of the devices tested under this study, the handheld ther- mal camera appears to be the easiest to use and can be effec- tively used by tunnel owner personnel. Data collection and analysis of the images can be conducted in the field. Con- versely, the air-coupled and ground-coupled GPR equipment require considerably more training and experience for data collection and operation. These devices involve the use of integrated systems containing a data collection module, com- puter, antenna, and distance-measuring indicator. Data analy- sis of the air-coupled GPR data will generally be simpler than the ground-coupled GPR data. The researchers recommend that the surface dielectric data from the air-coupled GPR be used for determining where to conduct more in-depth tests; these data are easily generated by GPR analysis programs. The training and experience needed to effectively collect and ana- lyze data by using the ultrasonic tomography, ultrasonic echo, and PSPA equipment are expected to be less than that for the GPR equipment. For rapid scanning of tunnel linings, data from the SPACETEC scanner, the air-coupled GPR, and thermal cam- era images can indicate areas where further inspection by tun- nel personnel may be warranted. All devices were able to detect problems within 1 ft of the actual location on the tunnel lining. However, the SPACETEC scanner is not for sale. Data collec- tion and analysis are provided by SPACETEC through a service contract. The 1-GHz air-coupled GPR antennae, such as the one used in this study, are no longer for sale in the United States because of Federal Communications Commission regulations, though several service providers still own these antennae. Antennae available for sale in the United States should be effec- tive for collecting data if they meet the radar specifications contained in Appendix T. According to this study, thermal cameras have the ability to detect 1-sq-ft voids 3 in. deep when significant concrete thermal gradients exist, and the lit- erature suggests they can detect even deeper voids. However, the research team believes that vehicle-mounted thermal cam- era systems are not quite ready for implementation; further software development is needed. Ground-coupled GPR, ultrasonic tomography, ultrasonic echo, and the PSPA are all able to detect defects up to a depth of 4 in. However, for ground-coupled GPR, the defects can be detected only if they contain significant air pockets or sig- nificant moisture. Ultrasonic tomography can detect deeper defects but cannot directly detect defects if they are less than 2 in. from the surface. All of these devices will require a combination of class- room and hands-on training for collecting and analyzing data. Although beyond the scope of this study, laser scanning and digital photogrammetry techniques can also provide informa- tion about tunnel lining profile and surface distress that may be useful to tunnel inspectors. Finally, service providers can collect and analyze data for clients using the devices listed above. However, clients should consider the limitations for each device before selecting a ser- vice provider. references Gucunski, N., and A. Maher. 1998. Bridge Deck Condition Monitoring by Impact Echo Method. Proc., International Conference MATEST ’98—Life Extension, Brijuni, Croatia, pp. 39–45. Shokouhi, P., N. Gucunski, A. Maher, and S. Zaghloul, 2005. “Wavelet- Based Multiresolution Analysis of Pavement Profiles as a Diagnostic Tool,” Transportation Research Record: Journal of the Transportation Research Board, No. 1940, Transportation Research Board of the National Academies, Washington, D.C.