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46 Dr. Hegeon Kwun of SwRI indicated that they have tested was exposed masked any potential defects in the PE. It should anchorage areas of main suspension cables where wires are be noted that the best results occurred immediately after the separated. SwRI researchers believe that the MsS technology specimens were moved from the climate-controlled laboratory can be applied to stay cables; however, they as yet have not to the outside in a warm and sunny environment. This thermal had an opportunity to test stay cables. Dr. Kwun believes that, gradient may not be representative to normal environmental in the anchorage zone, small defects (some broken wires) heating and cooling except in extreme conditions. It is likely would likely not be detectable using these guided waves. that the effectiveness of the thermography would be limited to early morning or late evening. They also noted that: Laser Ultrasound The infrared thermography was very effective in detecting unfilled splits in the HDPE under certain environmental conditions. This A guided ultrasonic wave for NDT and evaluation can be condition requires sudden variation in the ambient temperature to applied to a test structure (e.g., a strand) in different ways, result in temperature gradient in the cable material. It is believed that to keep the effectiveness of thermography for unfilled split such as coupled (contact) ultrasonic transducer, MsS, or laser detection, perhaps also for filled split detection, thermography ultrasound. In ultrasonic testing of strands in stay cable an- should be combined with heat generation source. chorages, the ends of seven-wire strands are typically ground smooth to allow perfect coupling with the ultrasonic trans- Figure 65 shows a thermographic image from this test series. ducer. However, typically larger transducers that are used cannot be practically coupled to individual wires. The same Finally, another method that has been discussed in the liter- applies to the MsS technique. However, the laser ultrasound ature for detection of corrosion in steel cables is Time Domain can be applied as a point load anywhere at the end of the Reflectometry (TDR). This method has been referred to as strand or wire. It can also be applied eccentrically to gener- "closed-loop" radar (Ciolko and Tabatabai 1999). It has been ate both longitudinal and flexural modes (Rizzo and Lanza di widely used in identifying problems in transmission lines. The Scalea 2004). In this article the authors discuss the dispersive process involves sending a high-frequency signal through the and attenuating behavior of guided ultrasonic waves in multi- sensing cable and monitoring the reflections. The reflections wire strands. The use of laser ultrasound may potentially offer come about as a result of impendence changes along the length a way to improve the basic ultrasonic technique for inspec- of the cable. There have been a number of research efforts tion of stay cable anchorage, either on its own or in combi- aimed at using strands as sensing wires in the TDR setup. nation with the MsS technique. Ciolko and Tabatabai (1999) reported that the results of labo- ratory and field studies on this method were not encouraging. Other Methods Liu et al. (2002) discussed using TDR in a manner slightly Telang et al. (2004) performed a number of tests on two mock- different from the earlier studies. In this research, an external up stay cable specimens to evaluate various NDT techniques wire is used in conjunction with the strand to form the "trans- including impulse response, impulse radar, infrared thermo- mission line" for TDR tests. This method is sensitive to the graphy, and radiography. These specimens contained parallel presence of or variations in moisture. At the present time, the steel wires enclosed within PE sheathing and grouted. The available data do not indicate a potential for successful field objective was to find methods that could be used to identify applications to stay cables. deficiencies in PE sheathing (cracking and previously epoxy- repaired cracks), damage to ultraviolet (UV)-resistant wrap- ping over the sheathing, and grout defects. The sheathing LONG-TERM EVALUATION AND MONITORING defects were hidden under a UV-resistant wrapping tape. The This section includes methods that could be used for long-term sheathing was cut in different directions to represent cracks monitoring and inspections of stay cables. before wrapping. Telang et al. (2004) reported that the impulse response method was found not to be effective. The impulse radar method (involving high-frequency EM energy) was Acoustic Monitoring reportedly successful in detecting grout voids or damage. Fig- ure 64 shows a radar survey identifying grout voids. Acoustic monitoring is a passive method for detection of wire breaks in stay cables. It "listens" for shock waves emanating Telang et al. (2004) summarized their results as shown in from wire breaks. It is called "passive" because it cannot Table 8. They suggested that splits in PE sheathing (under the detect existing wire breaks. It has to be there and be "on" if it tape) can best be identified with infrared thermography or low- is to detect a break. energy X-rays. The authors explained that the thermographic method was not able to discern filled voids or voids in the Acoustic monitoring for stay cables probably began when shade on the bottom of the specimen. In addition, the method a method for detecting wire breaks during qualification tests of was not able to see defects beneath areas with damage to the stay cables was needed. Various test laboratories that per- UV tape. The solar heating of the black pipe in areas where it formed such tests needed to count the number of wire breaks
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47 FIGURE 64 Sample impulse radar survey (Telang et al. 2004). TABLE 8 COMPARISONS OF SOME NDT METHODS FOR DEFECTS IN PE SHEATHING, GROUT, AND TAPE Effectiveness Epoxy- Relative Adaptability to Unfilled Filled Damage Grout Relative Inspection Environmental Split in Split in to UV Void or NDT Method Cost Rate Conditions Sheathing Sheathing Tape Damage Impulse low high high none none none none response Impulse radar low medium high none none none good Infrared low medium low good none good none thermography Low energy X-ray high low high good fair good good Source: Telang et al. (2004).
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48 FIGURE 65 Infrared thermography image for detection of HDPE splits under tape (Telang et al. 2004). during fatigue tests on the cable specimens, and therefore University of TexasAustin prepared a write-up of the test developed their own acoustic monitoring techniques. The plan and a summary of their findings based on two Master's basic system essentially consists of accelerometers located at theses. The research team's conclusions for the acoustic mon- selected points and the anchorages. The location of a break itoring system are given here: could be determined by comparing the arrival time of the shock wave at different sensors. It should be noted that the [The system] provides an accurate method for monitoring wire attenuation of acoustic waves in grouted cables is much higher breaks due to fatigue damage in grouted stay cables. The system was able to identify the number of wire breaks accurately. The than in ungrouted cables. locations of the estimated wire breaks along the free length of the cable were typically within 6 in. of the actual breaks. The Tabatabai et al. (1995) performed tests on a one-tenth-scale accuracy of the system was less near the anchor heads, but the model of a nuclear containment structure containing unbonded geometry of the specimen is much more complex in this region. The locations of the estimated wire breaks near the ends of the post-tensioning strands. Wires were cut, and the wire breaks cable tended to be within 18 in. of the actual breaks. were detected by accelerometers. A commercial acoustic monitoring system based on piezo- Long-Term Sensor-Based Monitoring electric sensors and proprietary software is available. The sys- tem was initially developed for post-tensioned buildings and A number of parameters can be measured on stay cables using parking garages, and was then extended to bridges and other sensors. In previous sections of this report, examples of long- structures. According to the company, this system has been term vibration monitoring were given. Uniaxial or biaxial installed on the following cable-stayed bridges: accelerometers are generally used for vibration monitoring. Uniaxial accelerometers are used to capture in-plane vibra- · Fred Hartman Bridge (Texas)--acoustic monitoring tions. Biaxial accelerometers can measure both in-plane and system was installed in March 2002 on all 192 cables out-of-plane vibrations. The accelerometers are attached to (grouted seven-wire strands). the cable with suitable clamps or other hardware that could · Quincy Bridge (Illinois)--system was installed in June withstand long-term exposure. The positions of the sensors 2002 on 14 of 56 stays (grouted seven-wire strands). are selected to maximize the desired sensor response for the · Seyssel Bridge (France)--system was installed in May vibration modes of interest, and considering access limita- 2003 on 4 of 36 stays (grouted seven-wire strands). tions. In conjunction with acceleration measurements, weather · Penang Bridge (Malaysia)--system was installed in data are also typically collected including wind speed, direc- December 2003 on 120 of 148 stay cables (grouted bars). tion, rain, and so forth. The sensors are connected to a high- speed data acquisition system at a secure location on the A research program involving the acoustic monitoring sys- bridge. The system is typically powered with AC (alternat- tem has been in progress at the University of TexasAustin. ing current) power (if available) or solar panels. The system On request, Prof. Sharon L. Wood and the research team at should be designed in such a way as to protect against dam-
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49 age resulting from lightning, vandalism, moisture, extreme drain holes are placed in the bottom of the area between the heat, and extreme cold. Typically, data transfer to the office guide pipe and the transition pipe near the threaded anchor can be accomplished through wireless or landline modems. heads to prevent accumulation of water. Considering that pen- Data can also be stored on-site for manual retrieval. etration of moisture is an important issue, humidity measure- ments or moisture sensing inside the guide pipe, anchorage In addition to vibration measurements, stay cable sensing caps, or other cable components could be made in the future. could also include cable tension measurements through load cells or other force sensors. Bronnimann et al. (1998) reported It appears that in at least two cable-stayed bridges that were on the testing of distributed fiber optic strain sensors for stay in the path of hurricanes, the idea of monitoring cable vibra- cables. Continuous (nondiscrete) strain or even acceleration tions with security cameras mounted on the tower, the deck, or sensing along the length of cable, if practical and reasonable on the shores was explored, but it is believed not to have been from a cost standpoint, can be important in condition assess- implemented. There is also no indication as to whether any ments. In such cases, the localized strain changes, or changes monitoring of rainwind vibrations using security cameras has in mode shapes indicative of damage, could potentially be occurred. determined. In large-scale monitoring systems, an appropriate method A search of the literature and the survey results did not for analyzing and interpreting the large amounts of data that identify any bridges where moisture or humidity sensing is are collected must be designed. This has been an important performed. In the James River Bridge in Richmond, Virginia, issue in all large-scale monitoring systems.