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19 Capabilities and Limitations of NDE Methods Introduction This chapter discusses the capabilities and the limitations of each of the NDE methods that were used for the condition assessment of the mock-up full-scale PT girder specimen and the large-scale stay cable specimens. The methods that are discussed include: ⢠Electromagnetic methods: GPR, IRT, and ECT; ⢠Magnetic methods: MFL, MMFM-Permanent and MMFM-Solenoid; ⢠Mechanical wave and vibration methods: IE, UST, USE, SPV-UPV, LFUT, and sounding; ⢠Visual inspection; ⢠Electrochemical methods: EIS; and ⢠Combinations of methods: GPR/USE, GPR/IE, MFL/ sounding, MFL/IE, and IRT/USE. All the NDE methods are discussed in terms of their capabil- ity of identifying tendon and grout defects, their applicability based on the duct location and material, influencing param- eters such as effect of concrete cover, reinforcement congestion, and layered ducts, and their accessibility requirements. Electromagnetic Methods Ground Penetrating Radar GPR technique involves emitting electromagnetic pulses from an antenna and receiving the reflected pulses from inter- nal reflectors. A variation in the materialâs electrical conduc- tivity and dielectric permittivity results in a difference in the reflected pulses. By analyzing the reflected pulses, it may be possible to identify anomalies in the structure. Consequently, GPR seems ideal for identifying grout defects in internal ducts of PT girders. The capabilities and limitations of the GPR inspection technique are discussed in the following. ⢠Capability of identifying defects: GPR cannot detect strand defects in external HDPE ducts. It can detect voids in exter- nal HDPE ducts with moderate accuracy, however cannot quantify the volume of the void. It can also detect com- promised grout, and water infiltration defects in external HDPE ducts with low accuracy. GPR cannot detect strand or grout defects in external metal ducts, within the internal metal or plastic ducts, or the anchorage zones. However, GPR can be used to locate internal metal and plastic ducts. ⢠Duct location: While acceptable predominantly for internal ducts (testing on a concrete surface), with a proper setup, GPR may be used to identify voids within external ducts. ⢠Duct type: If it is desirable to detect conditions within the duct, GPR is only applicable to nonmetallic ducts. How- ever, if it is desirable to locate the internal ducts, then GPR is applicable to both metal and nonmetal ducts. ⢠Effect of concrete cover: The effect of concrete cover is dependent on the scanning frequency. For high frequen- cies (~500â3000 MHz) penetration depth can typically exceed 24 in. ⢠Effect of layered ducts: GPR is unable to even locate the far duct due to the large reflections from the steel strands in the near duct. ⢠Effect of reinforcement congestion: The presence of steel highly reflects the electromagnetic waves, thereby strongly affecting GPRâs capability of locating ducts, especially in the anchorage regions. ⢠Accessibility requirements: The area required for GPR scanning is device dependent. For ground-coupled GPR inspection, it is required that the wheels of the device be in physical contact with the structure to ensure turning of the wheels which also acts as a distance meter. The cre- ation of a 3D image requires either a 2 ft à 2 ft or 2 ft à 4 ft manually accessible testing surface. However, these requirements could vary if an air-coupled GPR device is used. Testing within the anchorage region typically does not provide useful information due to the large volume C h a p t e r 4
20 of the highly reflective reinforcement cage present in the anchorage zones. Infrared Thermography The primary basis for IRT inspection lies in the emissiv- ity of individual materials within the object being examined. Depending on the emissivity of the different materials, each material within the object may release or absorb heat at different rates, and the differential temperatures during this transition period can provide valuable information about the object. The uneven cooling or heating of the metal or nonmetal ducts, the surrounding concrete, the good grout, and the various defects, should be identifiable in a temperature profile. Therefore, it is important to perform IRT during times of the day when atmo- spheric temperature gradients are high, thus forcing the object being inspected to heat or cool in order to reach equilibrium with the surrounding environment. The capabilities and limitations of the IRT inspection technique are discussed below. ⢠Capability of identifying defects: IRT cannot locate strand defects in external HDPE ducts. IRT has high accuracy in locating void and water infiltration defects and low accuracy in detecting compromised grout in external HDPE ducts. However, it cannot differentiate between these defects. It is also possible to make rough estimates on the size of the void and water infiltration defects. IRT does not detect strand or grout defects in external metal ducts, and also does not locate internal metal or plastic ducts if they are buried deep within concrete, let alone identify defects. While IRT cannot be used to locate defects within the ducts embedded in the anchorage zones, it can detect the void and water infiltration defects in the end caps of the anchorage regions with moder- ate to high accuracy. As in the case of external HDPE ducts, IRT cannot differentiate between void and water infiltration defects, and it is also possible to make rough estimates on the size of these defects in the end caps. ⢠Duct location: IRT is mainly applicable to external ducts. Applicability of IRT to internal ducts largely depends on the depth within concrete where the ducts are located. ⢠Duct type: Applicable only to nonmetallic ducts. ⢠Effect of concrete cover: The effect of concrete cover has a significant effect for investigating and even locating internal ducts. ⢠Effect of layered ducts: Investigation of layered ducts is not possible with IRT. ⢠Effect of reinforcement congestion: It is expected that surrounding reinforcement will strongly affect any inves- tigation using IRT. ⢠Accessibility requirements: The only requirement for IRT is the ability for the infrared camera to have a good field of view of the subject being inspected. It is also important to avoid any uneven heating in the region under investigation. Electrical Capacitance Tomography ECT obtains capacitance data from multi-electrode sen- sors and by several iterations makes permittivity images of sections. ECT has the advantages of being a safe, fast, and relatively inexpensive technique. However, there are inherent problems with the image resolution, which is a major road- block in the application of this method. The capabilities and limitations of the ECT inspection technique are discussed here. ⢠Capability of identifying defects: ECT cannot locate strand defects in external HDPE ducts. While this method has moderate accuracy in detecting voids in external HDPE ducts, it has low accuracy in detecting water infiltration and compromised grout in external HDPE ducts. ⢠Duct location: ECT is applicable for external ducts. ⢠Duct type: ECT is applicable to HDPE ducts. ⢠Accessibility requirements: ECT requires moderate acces- sibility around the external ducts (less than 12 in. radial clearance from center of duct). Magnetic Methods Magnetic Flux Leakage In the MFL inspection technique, a strong permanent mag- net is used to directly magnetize the ferrous material (steel) within the ducts. This induces flux paths in the material between the two poles of the magnet. Where section loss is present, the magnetic field in the material âleaksâ from its typical path of least resistance. A magnetic field detector (comprised of Hall- effect sensors) between the poles of the magnet is sensitive to this change in magnetic field and indicates the leak. The capabilities and limitations of the MFL inspection technique are discussed below. ⢠Capability of identifying defects: MFL can locate strand defects in both metal and nonmetal external ducts with moderate to high accuracy, however it cannot differentiate between corrosion, section loss, and breakage. It consistently locates corrosion, section loss, and breakage with a loss in metallic area greater than 5%. However, in some cases loss in metallic area as low as 1% may also be detected. It can be used to estimate the loss of metallic area, although these estimates may not have high accuracy. MFL cannot detect grout defects in ducts. The effects from the magnetization of the metallic end pipe embedded within the anchorage zone, which is also called âend effect,â can make the inter- pretation of results challenging.
21 ⢠Duct location: Applicable mostly to external ducts. ⢠Duct type: Applicable to both metal and nonmetal ducts. ⢠Accessibility requirements: For the investigation of external ducts, a clearance of approximately 12 in. radius is required from the center of the duct. MMFM-Permanent Magnet In this method, a magnetizer installed on the sensor head is guided along the free span of the ducts. The controller unit gathers the data and this data is transmitted to a personal laptop. The permanent magnet type measurements give the signal search coil measurements (direct signal from the search coil) and the magnetic flux (integrated signal of the search coil). Since the integrated signal of the search coil correlates with cross-sectional area of the cable, any valleys in these signals indicate a loss of metallic area. The perma- nent magnet also gives measurements from the Hall-effect sensors, which detect the MFL that supplements the detec- tion of defects. The capabilities and limitations of the MMFM-permanent magnet inspection technique are discussed here. ⢠Capability of identifying defects: MMFM-permanent mag- net can locate the strand defects in both metal and nonmetal external ducts with moderate to high accuracy. However, it cannot differentiate between corrosion, section loss, and breakage. MMFM-permanent magnet consistently locates corrosion, section loss, and breakage with a loss in metal- lic area greater than 5%. However, in some cases loss in metallic area as low as 1.5% may also be detected. MMFM- permanent magnet inspection can be used to obtain esti- mates in the loss of metallic area, although these estimates may not be accurate. MMFM-permanent magnet does not detect grout defects in ducts. ⢠Duct location: Applicable to external ducts. ⢠Duct type: Applicable to both metal and nonmetal ducts. ⢠Accessibility requirements: For the investigation of exter- nal ducts, a clearance of approximately 12 in. radius is required from the center of the duct. MMFM-Solenoid The solenoid type measurements are used to identify and quantify the loss of metallic area in external ducts. Electric current is passed through the wire that is wound around a drum that encases the tendon. This is then guided along the length of the free span of the external ducts. The con- troller unit gathers the data and this data is transmitted to a computer. These measurements, known as the scan mea- surements locate the metal defects in the external ducts. In regions of interest the solenoid may be held stationary at that location and point measurements may be made to quantify the defects in the tendons. The capabilities and limitations of the MMFM-solenoid inspection technique are discussed here. ⢠Capability of identifying defects: MMFM-solenoid can locate the strand defects in both metal and nonmetal exter- nal ducts with moderate to high accuracy. However, it cannot differentiate between corrosion, section loss, and breakage. MMFM-solenoid consistently locates corrosion, section loss, and breakage with a loss in metallic area greater than 5%. However, in some cases loss in metallic area as low as 1.5% may also be detected. MMFM-solenoid inspection can be used to obtain estimates in the loss of metallic area, although these estimates may not be accurate. MMFM- solenoid does not detect grout defects in ducts. ⢠Duct location: Applicable to external ducts. ⢠Duct type: Applicable for both metal and nonmetal ducts. ⢠Accessibility requirements: For the investigation of external ducts, a clearance of approximately 12 in. radius is required from the center of the duct. Mechanical Wave and Vibration Methods Impact Echo The IE method involves hitting the surface of the area of interest with a small impactor or impulse hammer and iden- tifying the reflected wave energy with a displacement or accel- erometer receiver mounted on the surface near the impact point. The reflections of the stress wave from internal defects, material interfaces, or other anomalies are captured by trans- ducers on the testing surface. Because the impact generates a high energy pulse and can penetrate deep into concrete, the IE method is particularly promising for identifying defects in concrete structures. It produces a better signal to noise ratio than other ultrasonic techniques because of its low attenuation in composite materials such as concrete. The capabilities and limitations of the IE inspection tech- nique are discussed here. ⢠Capability of identifying defects: IE cannot locate strand defects in internal or external ducts. IE can locate voids and water infiltration in internal metal ducts with moderate accuracy, and compromised grout and water infiltration in internal nonmetal ducts with low to moderate accuracy. IE can also locate compromised grout, voids, and water infiltration in external HDPE ducts with moderate accu- racy. While IE can make rough estimates on the size of the defects, there may be large errors. ⢠Duct location: Applicable to both internal and external ducts.
22 ⢠Duct type: Applicable to both metal and nonmetal ducts. ⢠Effect of concrete cover: The effect of concrete cover is dependent on the impact. Very thick concrete cover may prevent successful measurement. ⢠Effect of layered ducts: Layered ducts do not yield meaning- ful results due to the large reflections from the near duct. ⢠Effect of reinforcement congestion: Presence of steel highly reflects acoustic waves, thereby negatively affecting inves- tigation using IE. ⢠Accessibility requirements: For nonautomated scanning systems, accessibility required is typically a 2 ft à 2 ft area. The area required for an automated scanning system is dependent on the system. Testing within the anchorage region is generally not possible using IE due to the physi- cal structure of the region and the highly reflective metal used in the anchorages. Ultrasonic Tomography The UST technique uses a linear array of dry-point-contact transducers that generate shear waves at a center frequency of 55 kHz. A sensor or group of sensors emits a stress pulse (shear wave or S-wave) into the specimen. As the waves prop- agate, areas with changes of impedance reflect portions of the wave, and these reflections are captured by another sensor or group of sensors. Through time-of-flight measurements and frequency/amplitude characteristics, defects and/or disconti- nuities can be determined. The capabilities and limitations of the UST inspection technique are: ⢠Capability of identifying defects: UST did not locate strand or grout defects in internal metal or nonmetal ducts, or in the anchorage regions. UST can locate internal ducts when the scanning is done perpendicular to the length of the ducts. Because of the configuration of the device, it is not useful in detecting defects in external ducts. ⢠Duct location: Linear array UST testing is limited to test- ing on smooth concrete surfaces, and therefore applicable to internal ducts. ⢠Duct type: As long as sufficient bonding between the duct lining and surrounding grout is maintained (no shrinkage cracks or air gaps present), this method is applicable to any duct type. Metal ducts do tend to reflect acoustic waves more than nonmetal, therefore inspection within metal ducts may not be possible. ⢠Effect of concrete cover: UST devices perform better when the concrete cover is between 2 to 12 in. Deeper cover may be acceptable provided there is no heavy reinforcement congestion. ⢠Effect of layered ducts: Ducts behind other ducts cannot be discerned using UST. ⢠Effect of reinforcement congestion: Surrounding reinforce- ment strongly affects investigation using UST, since the pres- ence of steel highly reflects acoustic waves. This makes any object directly beneath the reinforcement undiscernible and areas between reinforcement visible. Densely spaced rein- forcement will limit investigation beyond the location of the reinforcement bars. ⢠Accessibility requirements: An accessible area of about 2 ft à 2 ft or larger is required depending on scanning incre- ments. Testing within the anchorage regions generally does not provide useful information due to the large amount of reflective steel used in the anchorages. Ultrasonic Echo In the USE method, the structural element is mechanically excited by a pulse in the inaudible ultrasonic range and the reflected portions of the pulse are received. Reflections occur at interfaces with metal (e.g., reinforcement, tendon duct) and with air (e.g., back-wall, air-filled void). In contrast to radar, since no total reflection occurs at the reinforcement, ultra- sound may also be used to study components with high rein- forcement density. In the USE method it has to be noted that a single measurement allows no conclusion about the position of a single rebar or duct. Only measurements along a mea- surement grid with a constant measuring point distance allow carrying out a reconstruction calculation with a subsequent imaging of individual reinforcement bars or ducts. Compared to the radar method, the resolution here is often coarse due to the diffusion of signals at the aggregate. The capabilities and limitations of the USE inspection technique are discussed here. ⢠Capability of identifying defects: USE can locate grout defects in internal nonmetal and metal ducts with low to moderate accuracy. USE does not detect strand defects in internal ducts. This method has low accuracy in detecting voids and water infiltration in anchorage regions. ⢠Duct location: Applicable to internal ducts. ⢠Duct type: As long as sufficient bonding between the duct lining and surrounding grout is maintained (no shrinkage cracks or air gaps present), this method is applicable to metal and nonmetal internal ducts. However, it requires calibration based on the duct material. ⢠Effect of concrete cover: USE devices perform better when the concrete cover is between 2 in. to 12 in. Deeper cover may be acceptable provided there is no heavy reinforce- ment congestion. ⢠Effect of layered ducts: Ducts behind other ducts cannot be discerned using USE. ⢠Effect of reinforcement congestion: Surrounding reinforce- ment will strongly affect investigation since the presence of
23 steel highly reflects acoustic waves. This makes any object directly beneath the reinforcement undiscernible and areas in between reinforcement visible. Densely spaced reinforce- ment will limit investigation beyond the location of the reinforcement bars. ⢠Accessibility requirements: The automated scanner with dual probe transducers require approximately 3 ft à 3 ft clearance around the inspected region. Testing within the anchorage regions generally does not provide useful infor- mation due to the large amount of reflective steel used in the anchorages. Sonic/UPV The UPV method is based on the speed and amplitude of a compressional wave pulse and is used for determining material velocity and integrity conditions. Ultrasonic source and receiver transducers are placed on opposite ends of a given test path, and a signal is sent between them. The signal travel time and attenu- ation provide information as to the concrete integrity along that signal path. Since this method uses a source and receiver to pass wave energy through a test member, it requires access to two sides of a member for evaluation of interior material condi- tions. A faster measured velocity in a given concrete material generally correlates with greater strength and better integrity. The SPV method is similar to UPV test method and is often used instead of UPV when more signal energy from the source is required, such as over greater distances or through some materi- als. Test equipment typically include an instrumented hammer and an ultrasonic receiver transducer. It is typically necessary to improve contact between the receiving transducer and the concrete surface by coupling the transducer to the concrete with grease. The signal is then amplified and filtered and recorded by the computer where the signal can be viewed and the travel time and voltage amplitude recorded. The pulse velocity of the concrete is calculated by dividing the pulse travel distance by the travel time (difference in arrival times of the impact force and received compressional wave). The SPV velocity and ampli- tude are reduced by voids, cracks, honeycomb, and other flaws between the source and receiver such as weakly bonded surface patches. Generally, the faster the velocity the better the concrete quality. A complete air-filled void inside a column or beam may result in zero signal transmission or a significant time delay as the signal travels around the void. The capabilities and limitations of the SPV-UPV inspec- tion technique are discussed here. ⢠Capability of identifying defects: SPV-UPV does not detect strand or grout defects within the ducts in the anchorage regions. ⢠Duct location: This is applicable to internal ducts as long as there is access from both sides of the structure. ⢠Duct type: This method is applicable to both metal and nonmetal internal ducts. ⢠Effect of concrete cover: Typical concrete cover is not an issue for SPV-UPV inspection. ⢠Effect of layered ducts: Ducts behind other ducts can be discerned using UPV. The position of the two transducers can be varied such that direct, semi-direct, and indirect tests can be performed, which aids in mapping out the volume of the defect. ⢠Effect of reinforcement congestion: Surrounding reinforce- ment will strongly affect any investigation since the pres- ence of steel highly reflects acoustic waves. This makes any object directly beneath the reinforcement undiscernible and areas in between reinforcement visible. Densely spaced reinforcement will therefore hide any investigation beyond the barsâ location. ⢠Accessibility requirements: For UPV devices, the area required for scanning is about 12 in. on both sides of the structure. UPV technique can be used for anchorage defects. Con- ventional UPV testing requires access to two surfaces, pref- erably two parallel surfaces such as the top and bottom surfaces of a slab or the inside and outside surfaces of a wall. However, this test can be performed using the indirect methods, which does not require access to two surfaces. ⢠Effect of Large Voids: The compressional wave velocity in areas with defects is slower than in areas with sound con- crete, and the signal amplitude is often lower. For struc- tural members containing large voids, signal transmission may be completely lost. In some defect areas, such as hon- eycombs, the compressional wave velocity may almost be the same as in sound areas, but distortion of the signal (fil- tering of high frequencies) may be used as an indication of a honeycomb defect. Low Frequency Ultrasound The LFUT system is designed to generate and receive low frequency ultrasonic waves in a pitch-catch fashion that prop- agate across the cross-section. LFUT uses very low frequency ultrasound to extend the penetration capability of conven- tional ultrasonic testing. The LFUT technique uses contact transducers with couplant between transducers and the tested medium. A sensor emits a stress pulse into the specimen. As the waves propagate, areas with changes of impedance reflect portions of the wave, and these reflections are captured by the receiving sensor. Through time-of-flight measurements and frequency/amplitude characteristics, defects and/or disconti- nuities can be determined. The capabilities and limitations of the LFUT inspection technique are discussed here. ⢠Capability of identifying defects: LFUT detects grout defects in external HDPE ducts with low to moderate accuracy.
24 However, LFUT does not provide an estimate for the size of the grout defects. LFUT cannot detect strand defects. This method also does not detect defects in external metal ducts. ⢠Duct location: Applicable for external ducts. ⢠Duct type: Applicable for external HDPE ducts. ⢠Accessibility requirements: For the investigation of exter- nal ducts, LFUT needs physical access for the placement of the transducers on the ducts and the transducers need approximately 6 in. clearance. Sounding Sounding is a simple acoustic NDE technique. Sounding is performed by tapping an impactor or other tapping device along the surface of inspection, and listening for a change in the acoustic response produced by the tapping. The loca- tions that indicate a change in acoustic response typically imply the presence of a void or water infiltration, requiring further examination of the location. Sounding is a subjec- tive inspection technique, but it is easy and can be quickly performed in the field. This method does not have the capa- bility of detecting compromised grout, or metallic defects. Additionally, this method can be difficult to be performed on bridges without closing the bridge to traffic due to traffic noise interfering with testing. The capabilities and limitations of the sounding inspection technique are discussed here. ⢠Capability of identifying defects: Sounding cannot locate strand defects in external ducts. It can detect void and water infiltration defects in external ducts with high accu- racy, although application of sounding inspection on metal ducts could be slightly more challenging compared to HDPE ducts. Sounding inspection cannot differentiate between void and water infiltration defects. While sound- ing cannot be used to identify strand and grout defects in the internal tendons and anchorage regions, it can be used to detect voids, compromised grout, and water infiltration defects within the end caps of the anchorage regions with high accuracy. ⢠Duct location: Applicable for external ducts. ⢠Duct type: Acceptable for both metal and nonmetal exter- nal ducts. ⢠Accessibility requirements: Sounding requires physical access to allow manual tapping on the external ducts. Visual Inspection VT method is extensively used for preliminary bridge inspection. VT is often the first step during inspection and can be used to quickly identify areas of major deterioration that warrant immediate attention and further more detailed evaluation. VT can also serve to identify deterioration of the ducts, possibly indicating locations where water and atmospheric gases have been able to infiltrate into the inte- rior of the duct. Although very useful for external PT and stay cable ducts, VT of internal PT ducts has limited appli- cation because of accessibility issues. For all applications, a borescope and/or videoscope can greatly aid visual inspec- tion by providing a means to inspect the interior of the duct or conduct VT of hard to reach areas. In these cases, a borescope and/or videoscope can also be carried out to further inspect an area already identified by using another NDE method. The capabilities and limitations of the VT inspection tech- nique are discussed here. ⢠Capability of identifying defects: VT can be used as a pre- liminary method to detect flaws in the external ducts and for signs of flaws in internal tendons on the surface of the structure. VT is very effective in investigating the end caps of the anchorage regions, provided the end caps can be removed. VT can also provide good quantitative esti- mate about the defect. However, VT is usually limited by the lack of accessibility to the inspection area. An investi- gation using borescope needs access to insert the camera of the equipment into the region of interest. Provided there is sufficient access, this method can detect grout defects in both internal and external metal and nonmetal ducts with high accuracy. Borescope can also be used to inspect anchorage zones through the grout ports. Large strand defects and corrosion can be detected by VT, while identifying smaller defects could be challenging. ⢠Duct location: VT is mainly applicable to external ducts. However, borescope is applicable to both internal and external ducts. ⢠Duct type: Applicable to both metal and nonmetal ducts. ⢠Effect of concrete cover: Concrete cover is not an issue to the borescope inspection itself, but can cause a hindrance in accessing internal tendons. ⢠Effect of layered ducts: Layered duct can cause accessibility issues for borescope investigation, but is not an issue for the inspection itself. ⢠Effect of reinforcement congestion: Can cause accessibility issues for borescope investigation, but is not a hindrance to the inspection technique itself. ⢠Accessibility requirements: VT requires physical access to the duct being inspected. Borescope and/or videoscope devices require accessibility into the ducts of the internal and external tendons.
25 Electrochemical Methods Electrochemical Impedance Spectroscopy EIS is an impedance technique that applies a low-amplitude voltage (alternating current) to the steel under inspection over a wide range of frequencies. By measuring the changes in phase shift and signal amplitude, the impedance of the concrete-steel interface can be calculated. The capabilities and limitations of the EIS inspection technique are discussed here. ⢠Capability of identifying defects: EIS inspection can identify corrosion in external HDPE ducts with moderate accuracy. EIS cannot detect grout defects. ⢠Duct location: EIS is applicable for external ducts. ⢠Duct type: It can be used with external HDPE or other nonconductive ducts. ⢠Accessibility requirements: EIS requires physical access to the duct that is being inspected, and the ability to drill small holes into the external duct. The holes must be sealed after the EIS testing. ⢠As EIS inspection generates detailed information, sophis- ticated approaches are required to interpret the data and extract meaningful results. Combinations of Methods GPR/USE USE by itself is a slow NDE technique for the inspection of large areas, whereas GPR by itself does not collect enough detailed information. A combination of GPR and USE meth- ods may be best used to measure the depth of subsurface layer interfaces such as voids. When combined, GPR and USE can provide faster evaluation than the methods used alone. GPR can be used to highlight potential areas of interest or locate the internal ducts, which can then be followed up with the USE method for a more detailed inspection. Interpretation of individual technology inspection results may require a high level of experience and education about the methods. The capabilities and limitations of the individual methods presented previously apply here. GPR/IE IE by itself is a slow NDE technique for the inspection of large areas, whereas GPR by itself does not collect detailed enough information. A combination of GPR and IE meth- ods may be used to measure the depth of shallow subsurface layer interfaces. When combined, GPR and IE can be used for a fast and detailed evaluation of the area of interest. GPR can be used to identify potential areas of interest or to locate the internal ducts. IE can then be used to follow-up on these areas for a more detailed inspection. Interpretation of indi- vidual technology inspection results may require a high level of experience and education about the methods. The capabilities and limitations of the individual methods presented previously apply here. MFL/Sounding MFL is best used to detect strand defects such as corrosion, breakage, and section loss. While sounding cannot identify strand defects, sounding can isolate voided regions accurately and at low costs. In most cases, primary tendon corrosion, breakage, and section loss conditions occur where there are voided conditions. The void locations identified by the sound- ing technique can then be inspected for strand defects using MFL. The combination of sounding and MFL can likely reduce the overall inspection cost. While interpretation of results from sounding is quite simple, MFL inspection may require a high level of experience and education about the method. The capabilities and limitations of the individual methods presented previously apply here. MFL/IE IE is used as a high speed method to localize potential grout or void conditions in external tendons. While IE cannot iden- tify strand defects, IE can isolate voided and water infiltration regions accurately and at low costs. In most cases, primary strand defects such as corrosion, breakage, and section loss conditions occur where there are voided conditions. The loca tion of void and water infiltration defects isolated by IE inspection can then be inspected using MFL to detect strand defects. Therefore, a combination of MFL and IE methods may be used to more quickly identify regions of grout defects and strand defects. However, the interpretation of the indi- vidual technology inspection results require a high level of experience and education about the methods. The capabilities and limitations of the individual methods presented previously apply here. IRT/USE USE is a time-consuming method and too slow for the inspection of large areas. However, IRT could be effective in locating void defects in ducts that are embedded within shal- low concrete. In this case, IRT can be used to locate the regions of interest, and USE can be used to perform a detailed inspec- tion of this region. However, IRT typically does not identify defects in the internal ducts, if the ducts are embedded deep within the concrete, and in this case this combination would not offer any additional advantage. While interpretation of
26 results from IRT are straightforward, interpretation of USE inspection results may require a high level of experience and education about the method. The capabilities and limitations of the individual methods presented previously apply here. Closing Remarks The capabilities and limitations of each of the NDE meth- ods used in this study for the condition assessment of post- tensioning and stay cable systems were presented in this chapter. Very few methods were capable of identifying grout defects in internal tendons and in the anchorage regions. None of the NDE technologies that were investigated is capa- ble of detecting strand defects (corrosion/section loss/breakage) within the ducts buried in concrete, such as the webs, flanges, deviators and anchorage regions of the post-tensioning system, and deck and pylon anchorages of the stay cable system. None of the NDE technologies that were investigated is capa- ble of identifying grout defects (compromised grout/void/ water infiltration) with high accuracy for the ducts buried in the concrete, such as the webs, flanges, deviators and anchor- age regions of the post-tensioning system, and deck and pylon anchorages of stay cable system. However two NDE methods, USE and IE, could identify grout defects in internal ducts with low to medium accuracy.
