Risk-Based Inspection and Strength Evaluation of Suspension Bridge Main Cable Systems (2023)

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91 C H A P T E R 6 Review of NDE/SHM Methods General Discussion A variety of NDE technologies has been considered in this investigation. The primary challenge is finding technologies and sensors that are suitable for applications on main cables of suspension bridges taking into consideration the limitations imposed by working a few hundred feet above the roadway, with limited or no traffic disruption, in harsh environments, and with limited power sources. Because of the complex structure of a main cable of a suspension bridge (thousands of high-strength wires, tightly compacted together in a cylindrical shape), there are many nondestructive evaluation techniques that work either in principle or on smaller cables, e.g. suspenders, but, when brought to a main cable, they fail or provide unsatisfactory results. The conclusion of a thorough literature search on the topic is that at this time there is no publication that demonstrates unequivocally the success of a particular nondestructive evaluation technology or SHM methodology in assessing the conditions of suspension bridge main cables. Many attempts have been and are made by various bridge owners, both in the US and abroad, but the findings are inconclusive and often end up in proprietary technical reports. However, thanks to continuous technological advances in materials and electronics, new technologies are being developed and tested which will hopefully provide reliable results in the near future. In the following sections, we will discuss some of the technologies that have been tried on cable systems of suspension bridges highlighting the pros and cons of each technology. NDE Inspection Techniques Nondestructive evaluation (NDE) provides information about conditions inside a cable without the investigator having to remove the covering or otherwise alter the condition of the cable. The nonremoval of the exterior wrapping and avoiding the cable wedging operation would be extremely advantageous for the bridge owners as well as for the overall “health” of the cable. However, at this time, it is reasonable to say that the amount of information obtained through NDE technologies will be much more limited than that obtained by cable wedging: In fact, while in only a few specific cases current NDE methods can determine the loss of cross-sectional area in the cable, they will not necessarily identify cracks in wires, and distinguish among corrosion stages. Magnetic Flux Leakage Some of the most promising technologies are those that rely on the application of a magnetic field around the cable. According to the Magnetic Flux Method (MFM) theory, when a cable is longitudinally saturated by a strong magnetic field, the magnetic flux along the cable is proportional to the cable’s cross-sectional area. If the magnetic field is applied at locations with different cross- sectional areas, the magnetic flux at these locations will show a similar variation. Hence, the idea behind the MFM is to have a magnet that generates a magnetic field in the cable and to have this magnet move along the length of the cable. The variations in the magnetic flux will be representative of the

92 variations in the cable’s cross-section, providing information on the loss of cross-sectional area induced by the corrosion process. No information on cracked wires and on the distribution of the different corrosion stages will be provided. This methodology was thoroughly tested on a full-scale cable mock-up at Columbia University (see Figure 32, FHWA Report 2014). The cable specimen is 20 inches in diameter and 20 feet long, comprised of more than 9000 0.196-inch diameter high-strength steel wires. A large, stationary electromagnet (almost 4 feet long) was built to generate the strong magnetic field (55Ka/M) that saturated the cable and, since the magnet was not moving, the variation of the cross-section was simulated by removing wires from the surface of the cable for the entire length of the magnet. Subsequently, a field test was also conducted on the main cable of the Manhattan Bridge in NY (Figure 33), where the same large electromagnet was used in stationary conditions at two locations along a cable panel. The results of both the experimental and field tests were inconclusive. The magnet and the MFM data acquisition system were provided by Tokyorope, Inc. (Japan). Source: FHWA (2014) Figure 32. MMFM Bobbin Mounted on Cable Mock-up Figure 33. MMFM Bobbin Mounted on Main Cable of Manhattan Bridge

93 The limitations of this technology for successful suspension bridge main cable applications are multiple, and are mainly linked to the size of the cable to inspect and to the logistics of running such a test in the field. First of all, in order to be picked up by the sensor, the reduction of the cable’s cross- section must extend over the entire length of the magnetic field sensor (almost the entire length of the magnet): while this could be a possible scenario for ungalvanized wire cables, it is not realistic for cables with galvanized wires, where broken wires show gaps between the two fracture surfaces of the order of 2-3 inches and the breaks are not at the same identical location. Hence, the sensitivity of such measurements is quite low. In addition, the condition to create such a strong magnetic field that saturates also the core of the cable imposes a large demand of electric power that is not always easy to acquire, especially in the field. With regard to field applications, in addition to the demand of electric power, there are multiple difficulties related to moving such a large magnet along the cable length, with the magnet that needs to be kept at a constant distance from the cable surface so not to affect the measurements. However, such magnetic field-based methods have been successfully applied to cables of small diameter (e.g. the vertical suspender cables of suspension bridges) and are currently used on many bridges worldwide. Magnetostriction The phenomenon known as MS is based on the fact that the presence of a magnetic field can induce a small change in the physical dimensions of ferromagnetic materials and, conversely, a physical deformation or strain (or stress which causes strain) produces a change of magnetization in the material. In an NDE technology that uses the MS effect, the system consists of two parts: one part is used for applying a time varying magnetic field or detecting a magnetization change in the material while the other is a means for providing DC bias magnetic fields to the component (a permanent magnet to enhance the efficiency of the energy transduction between electric and mechanical energies). When a pulse of electrical current is applied to the coil in the transmitting part of the system, a time varying magnetic field is applied to the component under inspection. This field in turn generates a pulse of elastic waves in the component via the magnetostrictive effect, waves that propagate in both directions along the length of the component. The presence of damage and corrosion damage (in the form of reduction of cross-section) results in part of the propagating wave being reflected while part is transmitted through the damaged area. When the propagating elastic pulse reaches the receiving component of the magnetostrictive system, it causes a change in the magnetic induction of the material via the inverse magnetostrictive effect. This change induces an electric voltage in the receiving sensor that is subsequently conditioned and processed. By looking at the pattern of the reflected and transmitted waves in terms of amplitude, spectral dispersion and arrival time, it is possible to estimate the location and the extension of the damaged areas. This technology, having been tested experimentally as well as in the field, has the same limitation as the MFM technology: it provides satisfactory results only when applied to small strands (a couple of hundreds of wires) but fails when applied to a main cable. As shown in a test program done at Columbia University (Figure 34, FHWA Report 2014), the MS technology (provided by Southwest Research Institute) was successful in detecting a cross-section loss of 3% in a 127-wire strand but did not provide accurate measurements for larger bundles of wires (in the order of 600 wires). The reasons for such a limitation in detecting damage in large cables are linked to the difficulty in generating a strong enough magnetic pulse that penetrates deep into the cable cross-section (e.g. a complete saturation of the cable is needed) as well as in reading the returned signal from the cable core. In fact, in dealing with a

94 relatively large number of wires, the increase in attenuation and loss of the signal is due to transmission of the wave energy into adjacent wire bundles, resulting in energy loss from the main pulse. However, although the inability to deal with main cables, magnetostrictive technologies are currently used in assessing the conditions of vertical suspender ropes and of wire strands at anchorages. Source: FHWA (2014) Figure 34. MS System Tested on a Coiled 127-wire Strand Health Monitoring Techniques In this section, we will review systems that can be installed at various locations along the cable and that record some specific quantities, like wire breaks, relative humidity, temperature, etc. It is noteworthy that such systems do not provide any information on the condition of the cable at the time of the installation and so the condition assessment has to be interpreted relative to the beginning of the monitoring practice. Acoustic Emission Monitoring systems based on AE are commonly deployed on main cables of suspension bridge for the purpose of monitoring wire breaks. AE testing is a powerful method for examining the behavior of materials deforming under stress. AE may be defined as a transient elastic wave generated by the rapid release of energy within a material when the local stresses around a discontinuity reach levels close to yielding. The high level of stresses can result in cracks growing, fibers breaking and many other modes of active damage in the stressed material. The elastic wave generated by the rapid release of energy at a crack tip will travel through the material and will be picked up by a sensor that will process the data and determine the entity and location of damage. An AE system is basically a “passive” system in the sense that sensors are waiting for something to happen and, when something happens, they will record and process the signals. However, the accuracy of such a measurement depends on a variety of factors such as the relative

95 position and distance between the sensor(s) and the crack opening, the type of material, the interference with other components, etc. A typical AE system consists of a piezoelectric or fiber optic (newly developed) sensor that detects the acoustic wave traveling through the material being tested and converts it into an electric signal, a preamplifier that adds gain to the electric signal and an A/D converter board where the signal is digitized and analyzed in both the time and frequency domains. By using several sensors distributed along a cable, the sources of AE, such as wire breaks can be identified, detected and located. Practical limitations on monitoring acoustic waves resulting from low energy events such as corrosion or localized yielding are that the AE sensors must discern this signal from background noise and that AE signals must be correctly interpreted. Although this technology is currently being used on main cables of many suspension bridges, caution should be used in interpreting the data. One of the main factors of concern is linked to the attenuation of the wave energy traveling from the wire break to the sensor(s). In order to avoid the problem of attenuation of the wave energy through the insulating paste and wrapping wire, AE sensors are placed on the solid block of the cable bands which are in direct contact with the wires. When a wire breaks in the interior core of the cable, the released energy will disperse through the surrounding wires and, depending on the position of the wire break relative to the sensor, it might not be detected by the sensor(s). If a wire on the outer layers of the cross-section breaks, the energy dissipation is much more contained and the signal read by the sensor(s) much stronger. Using triangulation between sensors at adjacent cable bands, it could be possible to locate the position of the wire break along the length as well as in the cross-section: an attempt was made in a study conducted at Columbia University (FHWA Report 2014) where a prenotched wire was pulled to failure in a full-scale cable mock-up, instrumented with an AE system, provided by Mistras, with sensors at both ends of the cable specimen. The system successfully identified the location of the break, although the wire break was in one of the outer layers and relatively close to one of the cable bands and the test was conducted in ideal laboratory conditions. Another source of concern in using AE systems is the effects of environmental and other factors on the accuracy of the measurements. Rain, thermal effects (expansion/contraction), wire slipping, etc. can generate disturbances that are picked up by the AE sensor(s) and that need to be filtered out from the data analysis. This requires a careful selection of the AE sensor used in the application, sensor that has to be sensitive enough to pick up wire breaks from the inner core but not sensitive to environmental noise. The challenge for this technology is the confidence level that some owners have regarding the capability of such a technology to solely detect wire breakings in the outer as well as in the inner core of the cable. Combination of AE and Magnetostrictive Technologies In dealing with cables of small dimensions, e.g. suspender ropes, a potential future development in assessing their condition is represented by a system that could combine the advantages of both AE and MS technologies. The idea behind this AE-MS system is that a baseline measurement of the state of the suspender can be first established and then, by interrogating the system periodically, any variation of the wave propagation characteristics of the system can be related to a certain type of damage. In this type of system, the MS component will generate a traveling elastic wave that propagates along the rope and its propagation characteristics are established by analyzing the measurements from the AE sensors. If damage appears in the rope, the propagation characteristics of the rope change in the vicinity of the

96 damaged area and thus the characteristics of the signal propagating along the rope. In this way, an array of AE sensors will serve as continuous online monitoring detectors of wire breaks or other type of damage and help evaluating the damage in combination with the MS component. This AE-MS system was tested on a 61-wire strand (Figure 35, FHWA Report 2014) and the results were promising Source: FHWA (2014) Figure 35. MS System Tested on a Coiled 127-wire Strand Sensors in the Interior of a Main Cable There have been numerous efforts to install sensors in the interior of a suspension bridge main cable to monitor its conditions. These sensors can monitor environmental variables like temperature and humidity as well as the sensor’s corrosion rate: even though this is not the corrosion rate of the bridge wire, these measurements indicate the likelihood of corrosion taking place and so they can be used to infer the conditions of the cable itself. In 2014, an extensive study on corrosion monitoring in main cables of suspension bridges (FHWA Report, 2014) surveyed all the available sensing technologies that could be used for such an application. Various sensors based on different principles (e.g. Linear Polarization Resistance, Electrochemical Impedance Spectroscopy, etc.), some commercially available and some developed for the specific application, were tested by embedding them in a full-scale cable mock and by subjecting the cable specimen to cyclic environmental conditions for more than a year. Selected sensors were then

97 embedded, through cable wedging, in a panel of the main cable of the Manhattan Bridge in NY and measurements were collected for almost one year. In Figure 36, the measurements of the temperature and relative humidity recorded at one sensor in one of the cables of the Manhattan bridge are shown for three different days (January 23, March 12 and August 1, 2011) Figure 36. Temperature and Relative Humidity Readings over 24 Hour Period from Winter to Summer From the literature review performed for this project, it appears that no advancement in sensor technologies for this type of application has been made since the publication of the FHWA Report (2014). There are many challenges, some theoretical and some logistical, in the development of an internal monitoring system for a main cable of a suspension bridge. As far as the sensors available today, the measurements they provide are not direct measurements of the corrosion of bridge wires: the sensors monitor either their own corrosion rate or quantities such as temperature and relative humidity that can be used to infer the corrosivity of the internal environment. From here, the difficult task to calibrate sensor readings with the actual conditions of the interior wires. In addition, there are a variety of challenges due more to the specific logistics of the installation, e.g. sensor miniaturization, sensor survival to large compaction forces, electrical wired connections from embedded sensors to the outside of the cable, sensor service life expectancy, sensor malfunctioning and replacement, etc. It is expected that, in the near future, advancements in new materials and electronics will allow for development of such a monitoring system that could be installed either during construction (for new cables) or during a cable wedging operation (for existing cables) and could provide valuable information of the internal conditions of the cable Preventive Measures Rather than developing tools to in essence watch the elements deteriorate, means are being developed to prevent the corrosion from taking place. The most promising approach to date is the use of dehumidification systems that keep the humidity level in the interior of the cable low. It has long been

98 known that there is a correlation between humidity and ferrous corrosion. In the case of main cables, dehumidification is accomplished by injecting dry air into the interior of the cable and by removing humid air from the cable at some injection ports along the cable length. By keeping the relative humidity of the air inside the cable below 40%, it is possible to virtually cease any corrosion activity on the wires and prolong the life of the cable. As early as the 1980’s some bridge owners started installing dehumidification systems in the cable anchorages. The installation of a well-maintained dehumidification system proved to be very effective. This technology was then applied to main cables of new and existing bridges in the early 2000’s. As a result, there are several cases where existing bridges with acoustic monitoring systems registered a virtual stopping of recording AE events after the installation of the system. It is now becoming more and more common practice to install such a dehumidification system on bridge cables. The beneficial effect due to the presence of a dehumidification system has a positive effect on the remaining strength of the cable and on the recurrence of inspections.