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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
×
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Suggested Citation:"Appendix B - Test Protocols." National Academies of Sciences, Engineering, and Medicine. 2011. LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14497.
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56 Proper implementation of test procedures and interpreta- tion of results from condition assessment require information on reinforcement type and geometry, as well as backfill and site conditions. The subsurface environment surrounding the elements must be characterized in terms of soil or rock types, moisture conditions, presence of organics, and electrochemi- cal parameters known to contribute to corrosiveness. Installa- tion details include reinforcement type, metal type, and degree of corrosion protection. Quantitative guidelines are available for assessing the potential aggression posed by an underground environment relative to corrosion (FHWA, 1993). Generally, moisture content, chloride and sulfate ion concentration, resistivity and pH are identified as the factors that most affect corrosion potential of metals underground. Details for collect- ing, testing, and evaluating soil and groundwater samples are described in the recommended practices prepared by Withiam et al. (2002) and Elias et al. (2009). In what follows sampling and testing protocols for condition assessment and corrosion monitoring reinforcements are described. Sampling Selected sites for evaluating the overall performance of earth reinforcements should encompass different reinforce- ment types, loading, environmental and drainage conditions, backfill, and in-situ soil or rock characteristics representative of installations and construction practices that have been used within the United States over the past 30 to 40 years. Sampling protocols are described for both Type I and Type II reinforcements. Type I Reinforcements (MSE) In general, approximately 20 to 30 in-service reinforcements, and 20 to 30 coupons should be monitored at each site. These elements are distributed amongst three or four monitoring sta- tions. The number of monitoring stations depends on the length and geometry of the wall. As a rule of thumb, two loca- tions spaced at least 200 ft (60 m) apart should be considered for mechanically stabilized earth (MSE) structures 800 ft (250 m) or less in length and three locations for longer struc- tures. At each location, corrosion should be monitored at a minimum of two depths from the surface, or preferably, at depth intervals of 10 to 13 ft (3 to 4 m) because differences in oxygen content, moisture content, and salt concentration can produce different corrosion behavior. One critical location (center of structure) should be selected for establishing test locations at both shallow and deep positions. Higher oxygen and salt content are anticipated near the surface, and higher moisture contents or free water near the base of a structure. Prior field programs have indicated that where groundwater intrudes at the base of the structure, higher corrosion rates should be anticipated. Where this condition is not likely, repre- sentative estimates may be obtained from shallow-depth mon- itoring. The shallow-depth stations should be approximately 5 ft (1.5 m) in depth, and the deep position should be approxi- mately at one-fourth of the structure height from base level. Each monitoring station incorporates two to three sam- pling points generally located near the base, middle, and top of the walls. Sampling points include at least two reinforce- ments wired for monitoring, one steel coupon, one galvanized coupon, possibly a zinc coupon, and an access hole for place- ment of a reference electrode in contact with the wall fill. Spe- cial C-clamps are used to facilitate electrical connection and wiring to existing in-service reinforcements. Soldered con- nections are preferred for new installations. Photographs 1-8 in Figure B-1, depict installation of a typical corrosion mon- itoring station for in-service reinforcements. Ideally, three types of coupons should be placed at each location and depth; zinc, steel, and galvanized. In the case of galvanized reinforcements both plain steel and galvanized coupons, and in some instances pure zinc coupons are installed. For monitoring, it is desirable to have one-zinc, one-steel, and up to four galvanized coupons at each depth. The multiple A P P E N D I X B Test Protocols

57 1. Coring hole through precast panel to access backfill and soil reinforcements 2. Access holes advanced at station 3. Galvanized coupons prepared for installation 4. C-clamps for wiring reinforcements 5. C-clamp attached to reinforcement 6. C-clamp connection sealed with epoxy Figure B-1. Typical installation of a corrosion monitoring station. (continued on next page)

Figure B-1. (Continued). 7. Station with junction box 8. Typical junction box 58 galvanized coupons can provide opportunities for periodic removal. Coupons each have two leads to provide backup in case one connection fails. Coupons are made from the same or similar material as the in-service reinforcements and are placed within the wall fill to provide baseline measurements during monitoring. In general, more monitoring locations should be established for structures where poor performance is anticipated or known to exist (Withiam et al, 2002; Hegazy et al, 2003). Particular attention should be given to monitoring near drainage inlets or other areas that may be subject to fluctuations in moisture content, high moisture content, or inundation. However, mon- itoring at locations with “normal” conditions is still necessary to serve as a baseline and to ensure that the sample statistics are not skewed. Practices vary among state departments of transportation (DOTs) and not all establish corrosion monitoring stations in the same manner including all the details as described in the section. In particular, Caltrans installs a cluster of 18 inspec- tion rods in a grid pattern that includes six columns and three rows. The inspection rods are spaced at 10-foot intervals ver- tically and are approximately 25 feet apart in the lateral direc- tion. The inspection rods are made from the same material as the in-service reinforcements. In North Carolina, often only a single monitoring point is established near the base of the wall that includes between two and four in-service reinforcement wires for monitoring and zinc and steel coupons (i.e., galva- nized coupons are not installed). Details of the practices from California and North Carolina are described in Appendix C. Type II Reinforcements Nondestructive testing (NDT) and condition assessment requires a sampling strategy whereby the appropriate sample size is selected to provide a statistical basis for the test results. Withiam et al. (2002) and Hegazy et al. (2003) describe a sim- plified sampling criteria based on the probability that the sam- pled population will represent conditions throughout the site. The recommended sample size is based upon the total num- ber of elements at the site, the importance of the facility rela- tive to the consequences of failure, and a reference, or baseline, condition for comparison to observations. Generally, for a population consisting of 10 to 200 metal-tensioned elements, between 10 and 40 randomly distributed samples are required. Corrosion Monitoring and Condition Assessment Type I Reinforcements Visual Observations Visual observations can be made on the exposed portions of the earth reinforcements, and readings of half-cell potential and corrosion rate are collected from in-service reinforcements

that are wired for monitoring, and from coupons installed within the wall fill. For older walls that are retrofitted for cor- rosion monitoring, the condition of the reinforcing strips near the tie-strip may be observed where they are connected to the precast concrete wall-facing after advancing the access holes and exposing the reinforcements. For walls where core holes are not advanced through the wall face, reinforcements may be examined from shallow excavations near the surface along the top of the wall. Half-Cell Potential Measurement The half-cell potential, Ecorr, is the difference in potential between the metal element and a reference electrode. Equip- ment required for performing measurement of half-cell poten- tial includes a half cell, a high impedance voltmeter, and a set of lead wires. A copper/copper sulfate reference electrode (CSE) was used for this study. Lead wires are attached to the end of the test element and the half cell. The lead from the half-cell is con- nected to the negative terminal of the voltmeter, and the test element lead is connected to the positive terminal. Results from the test can provide a comparison between metallic elements at different locations at the same site, as well as identify the pres- ence of different metals, (e.g., zinc or iron). Half-cell potentials may be correlated with zinc loss and used to monitor the con- dition of galvanized reinforcements. Coupons or dummy reinforcements assist in interpretation of half-cell potential measurements. Plain steel, galvanized steel, and zinc coupons may provide baseline measurements for comparison. Half-cell potentials are useful to assess the condition along the surface of the reinforcements/coupons. Half-cell poten- tials are affected by the environment, including soil moisture and salt content, as well as by conditions on the surface of the test element, including the presence of a passive film layer and metal oxides. Therefore, care should be taken when interpret- ing measurements to identify when effects other than corrosion or presence of zinc on the surface are affecting measurements of half-cell potential. Multiple measurements of half-cell poten- tial are necessary (i.e., numerous samples) and reference val- ues for steel and zinc potentials need to be obtained under site-specific conditions (i.e., nominal values for zinc and steel potentials may not reflect site conditions). Linear Polarization Resistance Linear polarization resistance (LPR) measurements are used to observe instantaneous corrosion rates. Lawson et al. (1993), Elias et al. (2009), and Berkovitz and Healy (1997) describe the application of the LPR technique to MSE reinforcements. Polarization resistance measurements require an instrument to generate a plot of potential versus applied current (E versus iapp) for a range of approximately E ± 20 mV relative to the free corrosion potential of the reinforcement being monitored. Three electrodes are required to perform the test including working, counter, and reference electrodes. The working elec- trode is the reinforcement being monitored and a nearby rein- forcement is used as a counter electrode. The potential at the interface of the working electrode is controlled through current impressed between the working and counter electrodes. A CSE serves as a reference electrode to monitor the changing poten- tial of the working electrode. The measured resistance, PR′, is actually the sum of the interface and soil resistance (PR′ = PR + Rs) and a correction for soil resistance is often necessary. The LPR uses polarization resistance measurements to esti- mate the corrosion rate at an instant in time. The measure- ment represents an average of the corrosion occurring over the surface area of the test element. LPR measurements are made with the FHWA PR Monitor supplied by CC Technolo- gies (Model # PR 4500) following the protocol described by Elias (1990) and Berkovitz and Healy (1997). A few parame- ters, including an environmental constant, the surface area of the test element, and the density and valence of the metal species must be known, or assumed, to relate the measured polarization resistance to corrosion rate. Also, the measured polarization resistance is corrected for uncompensated soil resistance inherent to testing within the underground envi- ronment. The PR Monitor measures the soil resistance (Rs) via the AC impedance technique and subtracts this from the total polarization resistance to render the corrected polariza- tion resistance. The soil resistance is a function of the specific resistance (ρ), which is related to wall fill properties includ- ing moisture and salt content, as well as the geometry of the system, including the surface area of the reinforcement and the distance between the reinforcement and the reference cell. LPR measurements represent the corrosion rate at the instant of measurement. Corrosion rates may vary, and measurements with respect to time are needed. Thus, initial measurements are often taken after installation of corrosion monitoring stations, followed by measurements at 6-month intervals and thereafter for a 2-year duration, and then measurements at 5- or 10-year intervals. Type II Reinforcements Details of the recommended practice for condition assess- ment of Type II reinforcements are described in Withiam et al. (2002) and Fishman et al. (2005). In general, the protocol is described as follows: • Collect preliminary information including installation details and site conditions. • Identify appropriate mathematical models of service life and use these models to estimate metal loss from corrosion and remaining service life. • Probe the elements with nondestructive tests, supplemented with invasive testing as appropriate, to assess the existing con- dition of selected elements comprising the metal-tensioned system. 59

60 • Compare results of the condition assessment to expecta- tions based on site conditions and estimated metal loss. • Recommend an action plan based on results from the con- dition assessment. Installation details have an effect on the vulnerability of the system to corrosion and on our ability to probe the elements and interpret data from NDT. Relevant details include steel type, corrosion protection measures, drill hole dimensions, bond length, free length, total length, date of installation, level of prestress, grout type, and use of couplings. If the sys- tem is protected with an adequate, well constructed corrosion protection system [e.g., meeting the requirements of PTI Class I (PTI, 2004)], then corrosion has not been found to be a problem. However, construction details, element durabil- ity, and workmanship associated with the corrosion protec- tion system may affect the service life. Nondestructive test techniques are used to probe the ele- ments, and the results are analyzed for condition assessment. Four NDTs are commonly applied for condition assessment including measurement of half-cell potential, polarization current, impact, and ultrasonic testing. Half-cell potential and polarization measurements are electrochemical tests and the impact and ultrasonic techniques are mechanical tests involving observations of wave propagation. In general, these NDTs are useful indicators of the following aspects of the condition assessment: • Half-cell potential tests serve as an indicator of corrosion activity. • Results from the polarization test are correlated with the sur- face area of steel that may be in contact with the surround- ing rock mass (i.e., indicator of grout quality and degree of corrosion protection) and may be used to estimate an aver- age corrosion rate. • Impact test results are useful to diagnose loss of prestress, assess grout quality, and indicate if the cross section is compromised from corrosion or from a bend or kink in the element. • Ultrasonic test results are useful for obtaining more detailed information about the condition of elements within the first meter from the proximal end of the element. Withiam et al. (2002) and Fishman et al. (2002 and 2005) describe details of NDT including test procedures. Half-cell potential and LPR measurements are similar to those described for Type I reinforcements. Sonic Echo Measurements The sonic echo method (impact test) is used for evaluating cracking of grouts, fracture of tendons, and loss of element section. For this test method, the specimen is impacted using a hammer or ball device, which generates elastic compression waves with relatively low frequency content. Equipment required for the impact echo test method includes an impact device, an accelerometer, velocity or displacement transducer for measuring the specimen response, and a data acquisition system. Components of the test are connected with standard coaxial cables and Bayonet Neill-Concelman (BNC) connec- tors. Generally, an accelerometer is attached to the free end of the element and the impact is also applied to the free end. Ultrasonic Test The ultrasonic test method is a good technique for evaluat- ing grout condition, fracture of elements, and abrupt changes in the element cross section. The method has many of the fea- tures of the sonic echo technique except that the transmitted signal contains relatively higher frequencies. Ultrasonic waves are radiated when an ultrasonic transducer applies periodic strains on the surface of the test object that propagate as stress waves. Compression waves consisting of alternating regions of compression and dilatation propagate along the axial direc- tion of a rock bolt. Equipment required for the test includes a pulse source/receiver unit, an ultrasonic transducer, and a data acquisition system. The ultrasonic transducer is acoustically coupled to the exposed end of the anchor rod. Grease is used as an acoustic couplant. The time taken for sound pulses, generated at reg- ular intervals, to pass through the specimen and return, is measured. Return pulses may be either from a single reflec- tion at a discontinuity or from multiple reflections between a discontinuity and the end of the specimen. The patterns of the received pulses can provide valuable information about the nature of a defect, and of the structure of the material being tested. The advantage of the pulse-echo method is that only one side of the specimen needs to be accessed for transducer placement. Data Interpretation Impact Tests Impact (sonic echo) test results are interpreted by plotting time-histories of the responses measured by the accelerometer for each impact test. The maximum responses correspond to the impact, and the responses are normalized with respect to these maximum values. Figure B-2 presents typical time histories designated as Bolt Numbers 1, 3, and 10. The decay of the initial response is shown on the left-hand side of Fig- ure B-2 and is useful to assess the relative level of prestress car- ried by the elements. A relatively high rate of decay (i.e., highly damped system due to more dispersion) is indicative of high

remaining prestress, and a low rate of decay is associated with a loss of prestress. Based on past experience, a high rate of decay is indicated if the signal strength decays to less than 20% of the original signal strength within a millisecond. As shown in Figure B-2, Bolt #10 and Bolt #1 are examples of a high rate of decay. A reflection at approximately 0.5 ms is evident from the response of Bolt #1, but the rate of decay subsequent to this reflection is high. The response of Bolt #3 is an example of a low rate of decay. Responses from impact testing are recognized in terms of relatively strong, versus relatively weak, signal attenuation. If the surrounding grout is very high quality, then strong reflec- tions are not expected beyond a distance of approximately 10 to 15 feet. The plots on the right-hand side of Figure B-2 depict the time history of the response over 5 milliseconds. Based on installation records, and observations from bolts that have been exhumed from this site, we expect that lengths are between 10 and 20 feet, and the elements (rock bolts in this case) are surrounded by grout for approximately 5 feet at the distal end. Assuming that the compression wave velocity of the steel bolt is approximately 18,000 ft/s, the corresponding arrival times for reflected waves is 0.5 to 1.7 milliseconds from the beginning of the grout column and approximately 1 to 2.2 ms from the end of the bolt. Evaluation of these reflec- tions serves two purposes. First, we may assess the difference in grouted lengths from these results to compare with the assumed lengths (corresponding to surface areas) used to interpret the LPR measurements. Second, the strengths of the reflected signals are useful to access grout quality. Good grout quality corresponds to a weak reflected signal from the distal end. If a strong reflection is recognized, then grout quality is considered poor, and the reinforcements may not be com- pletely surrounded by grout in the bonded zone, or the grout may be highly fractured. Based on past experience, strong reflections correspond to reflected signal amplitudes greater than 20%, moderate is between 10% and 20%, and the ampli- tudes of weak reflections are less than 10% of the maximum response. Using these criteria, the time histories shown in Figure B-2 depict weak reflections for Bolt #10 and strong reflections for Bolts #1 and #3. -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 0 0.0002 0.0004 0.0006 0.0008 N or m al iz ed R es po ns e Time (sec) Bolt #10 -1 -0.5 0 0.5 1 0 0.001 0.002 0.003 0.004 N or m al iz ed R es po ns e Time (sec) Bolt #10 -1 -0.5 0 0.5 1 N or m al iz ed R es po ns e Time (sec) Bolt #1 -1 -0.5 0 0.5 1 0 0.001 0.002 0.003 0.004 N or m al iz ed R es po ns e Time (sec) Bolt #1 -1 -0.5 0 0.5 1 1.5 0 0.0002 0.0004 0.0006 0.0008 N or m al iz ed R es po ns e Time (sec) Bolt #3 -1 -0.5 0 0.5 1 1.5 -0.001 N or m al iz ed R es po ns e Time (sec) Bolt #3 tR1 tR2 tR1 tR2 tR1 tR2 0.00020 0.0004 0.0006 0.0008 0.001 0.003 Figure B-2. Typical time histories of responses from impact tests. 61

62 Ultrasonic Tests Due to the higher frequency content of the sound waves compared to the sonic echo test, results from ultrasonic test- ing provide more resolution and are better suited to detect reflection sources located within the first few feet from the backside of the anchor plate. This region is often associated with a relatively high concentration of oxygen, and subject to cycles of wetting and drying, that promotes corrosion. Often the reinforcement is not in contact with the surrounding rock- mass near the anchorage, so corrosion at this location cannot be captured by LPR measurements, and the ultrasonic test is an alternative method to detect a potential loss of cross section from behind the anchorage assembly. Half-Cell Potentials The primary purpose of half-cell potential measurements is to establish when significant portions of the galvanized steel reinforcements have lost zinc and steel is exposed to the wall fill. For a given material in a given environment, the potential is an indication of the corrosion activity. The more positive the potential, the greater, in general, is the corrosion. Poten- tial measurements are therefore only qualitative indications of corrosion activity and should only be used to determine the composition of the surface. Galvanized and plain steel coupons provide baseline mea- surements for comparison with half-cell potentials of galva- nized in-service reinforcements. Typical values of Ecorr with respect to a CSE are between −1,000 mV to −800 mV for pris- tine galvanized steel or zinc, and −700 mV to −400 mV for plain carbon steel. If the potential of the reinforcing element is close to that of a recently placed galvanized coupon, it is inferred that the zinc is still present along the length of the reinforcement. As the potential becomes more positive and begins to approach that of the steel coupons, the zinc coating is being lost as steel is exposed on the surface. The interpretation of potential measurements for galvanized reinforcements considers that four distinguishable layers of zinc coating are formed as a result of the hot-dip process used to gal- vanize MSE reinforcements. The outside layer is nearly pure zinc, and the succeeding inner layers are essentially zinc-iron alloys. Progressively higher iron contents prevail as the interface with the base steel is approached. Therefore, as zinc consump- tion progresses towards the base steel interface, the half-cell potential is consistently shifted toward values inherent to iron. Ultimately, measurements of the half-cell potential reflect the presence of steel after all four layers of the zinc coating are exhausted and bare steel is exposed, at least in some areas. For Type II reinforcements, or soil nails that may be sur- rounded by grout, half-cell potentials can indicate if an ele- ment is effectively passivated or can indicate if the grout is a resin type. Elements passivated by portland cement grout will have half-cell potential greater than −200 mV. Limits recom- mended by ASTM C-876 suggest that half-cell potentials more positive than −200 mV indicate a low likelihood that corrosion is occurring, while values more negative than −300 mV indi- cate a high likelihood that corrosion is occurring. For resin- grouted systems and steel reinforcements, half-cell potentials are generally more negative than −500 mV. Corrosion Rates from LPR Measurements The corrosion current density is the current within the cor- rosion cell in the absence of any external sources. Stern and Geary (1957) showed that for small deviations from the free corrosion potential (±20 mV), the corrosion current density is inversely proportional to polarization resistance as: where  = the shift of the half-cell potential from the open cir- cuit potential (volts); iapp = applied current (amperes/cm2); icorr = corrosion current density (amperes/cm2); βa = anodic Tafel constant (volts/decade); βc = cathodic Tafel constant (volts/decade); B = environmental constant (B ≈ 0.035 V for galvanized steel and B ≈ 0.026 V for steel); and Rp = polarization resistance normalized for area that involves multiplying the polarization resistance (PR) by the reinforcement surface area (As) in contact with backfill; that is, Rp (Ω-cm2) = PR × As. The LPR measurement technique involves scanning or stepping the potential from (−5 to −20 mV) to (+5 to +20 mV) around the free corrosion potential, while simultaneously measuring the applied current. Polarization resistance is determined from the slope of this plot (i.e., Rp = /iapp). If the surface area of the working electrode is known, corrosion current density may be determined from the measured polar- ization resistance and, ultimately, related to corrosion rate. Elias (1990) and Lawson et al. (1993) discussed the need to correct the measurement of Rp for soil resistance. If the soil resistance is relatively large, the measured PR′ can be much greater than the true value for PR, and the estimated corrosion rate may be significantly less than the actual corrosion rate occurring at the surface. To correct for the effect of soil resis- tance, an AC signal is applied to the working electrode at the end of the standard polarization measurement cycle. During a high frequency measurement, the AC voltages reverse mag- nitude and polarity so rapidly that the interface capacitance does not impede polarization, and PR is short-circuited. This R d di i p app app a c = ⎡ ⎣⎢ ⎤ ⎦⎥ = ⎡ ⎣⎢ ⎤ ⎦⎥ =→ →    0 0 2 Δ Δ β β .3× +( ) =i B icorr a c corrβ β ( )B-1

permits independent measurement of Rs, allowing PR to be calculated as PR′ − Rs. Based on Faraday’s Law, corrosion rate (CR) can be esti- mated from icorr as follows: where W = atomic weight (e.g., 55.84 for steel and 65.37 for zinc), n = valence (e.g., 2 for steel or zinc), and ρ = density in g/cm3 (e.g., 7.87 for steel and 7.14 for zinc). Quantification, or estimation, of errors inherent to measure- ment of corrosion rate involves an assessment of (1) param- eters that are often assumed and used to relate polarization resistance (measured) to corrosion rate and (2) the accuracy of the polarization resistance measurement. Errors in measure- ment include those associated with measuring polarization resistance and solution resistance, and errors computing the corrosion rate arise from the selection or assumption of the parameters and constants used for Equations (B-1) and (B-2). Parameters for Computing Corrosion Rate from LPR Measurements An environmental constant (B) relating polarization resist- ance to corrosion current density (icorr), and the constants relating icorr to corrosion rate need to be known or assumed to compute corrosion rate from measurement of polarization resistance. These inputs depend upon metal type and the physicochemical properties of the backfill. In general, the B parameter is assumed as 0.026 V for steel elements and 0.05 V CR m B-2 μ ρyr i W n corr⎛⎝⎜ ⎞ ⎠⎟ = ×( )× × × 3 27 106. ( ) for zinc elements embedded in soil. The selection of B for gal- vanized elements is more ambiguous because it is not known a priori if zinc, steel, or a mixture of zinc and iron is exposed on the surface of the element. However, a value of B equal to 0.035 V is often used to consider galvanized elements. Simi- larly, the constant relating icorr to corrosion rate may vary by a factor of approximately 1.3, which can be realized by com- paring the atomic weights, densities, and valances of steel and zinc for use in Equations (B-1) and (B-2). The environmental constant is related to the tafel slopes, which define the slopes of the anodic and cathodic branches of the overpotential where they become linear in a plot of over- potential versus the logarithm of applied current. Tafel slopes were measured at 11 sites, included as part of Task 6, using spe- cial equipment that applies overpotential (±250 mV), which exceeds what is needed for LPR measurements (±20 mV). Results from these measurements are presented in Table B-1. Direct measurement of tafel slopes is limited because of the need for special equipment and because imparting this level of overpotential can have a lasting effect on the electrochemical properties on the surface of the reinforcement (i.e., future measurements of corrosion rate may be affected by test history). The means and ranges of the environmental constant, B, that were measured at selected sites are: Material Mean (V) Range (V) Steel 0.024 0.010–0.030 Galvanized 0.035 0.010–0.058 Zinc 0.040 0.030–0.050 The means of the measurements are very close to the assumed values used in Equations (B-1) and (B-2) to reconcile corrosion rate from LPR measurements. B (V) State Site Element Steel Galvanized Zinc NH I-93 Southbound, Barron Mtn. A 0.033 - - B 0.024 - - C check - - D 0.023 - - NC I540 Exit 26B - 0.010 0.040 0.030 I540 & TTC - 0.020 0.050 0.030 I64 MP 423 - 0.020 0.020 0.050 I77 at Tyvola - 0.023 0.034 0.050 NY SHR Northwest Abutment 1 - 0.042 - SHR Southwest Abutment 1 - 0.056 - SHR Southwest Abutment 2 - 0.058 - CA Site No. 532819 R 15 - 0.010 - Site No. 532819 R 8 - 0.010 - Site No. 532822 R 1 - 0.037 - Site No. 532823 R 12 - 0.010 - Site No. 541093 12 - - NY MMCE Lab - 0.020 0.054 - N OTE : TTC = Triangle Town Center, SHR = Sweet Home Road, MMCE = McMahon & Mann Consulting Engineers, and - = not applicable. Table B-1. Summary of observed environmental constants. 63

64 Comparison of Device Performance Results obtained with a commercially available general- purpose, corrosion monitoring device (GAMRY G600) are compared with those from a unit built specifically for the FHWA (PR Monitor) for monitoring the performance of MSE reinforcements. The hardware (Potentiostat/Galvanostat/ Zero Resistance Ammeter) incorporated into each unit is similar. However, the general-purpose equipment allows user flexibility in terms of data processing and interpretation, and, in contrast, the user cannot alter the protocols programmed into the FHWA unit. Both the GAMRY G600 and the FHWA PR Monitor correct for uncompensated solution resistance (soil resistance) as part of the LPR measurement. Both units measure the soil resistance via an AC input and subtract this from the measured polarization resistance to render the cor- rected polarization resistance. The PR Monitor supplied by CC Technologies, Inc. utilizes a potential control stepping sequence that is completely flexi- ble and programmable by the operator. The PR Monitor also presents the coefficient of linear regression used to calculate the value of PR′ from the  vs. iapp plot. A regression coefficient of 0.9 or greater indicates a reasonably good fit of the data. The corrosion current density is determined from Equation (B-1) using the measured value of PR, As, and the appropriate envi- ronmental constant. Finally, Equation (B-2) is used to estimate corrosion rate. McMahon & Mann Consulting Engineers (MMCE) and Caltrans performed redundant tests with the GAMRY and FHWA PR monitors. Data were collected from sites in north- ern and southern California during the periods from July 14, 2007 to August 24, 2007 and April 9, 2008 to May 1, 2008. The data set includes 61 individual measurements of polarization resistance from 10 different locations. Corrosion rates are computed using the polarization resistance with the correc- tion for Rs. Once the uncompensated solution resistance is obtained, different operators will compute the same corro- sion rate using Equations (B-1) and (B-2); assuming they use the same parameters for surface area, environmental constant (B) and metal valance, density, and atomic weight. Therefore, differences in results from these devices are with respect to the manner in which PR′ and Rs are rendered. Results of LPR measurements performed with the GAMRY equipment and operated by Caltrans, and measurements made by MMCE using the FHWA PR Monitor are compared in Figure B-3. Figure B-3(a) depicts measurements of PR′ that are not corrected for the uncompensated solution resistance (Rs), and Figure B-3(b) is the independent measurement of Rs. Measurements of PR′ correlate very well (ρxy = 0.98). One data point falls outside the trend line corresponding to a measurement with the GAMRY G600 that is approximately one half of that obtained with the PR Monitor. Some of the GAMRY data were processed and analyzed by both Caltrans and MMCE using a different protocol. Caltrans determined the polarization resistance from the slope of the overpotential versus impressed current for a selected linear region in the vicinity and symmetric with zero applied current (i.e., at the open circuit potential). MMCE determined the polarization resistance at the slope within a region ±10 mV from the open circuit potential. The latter is similar to the protocol employed by the FHWA PR Monitor. Figure B-3(a) shows that the GAMRY data as reconciled by MMCE are closer to the mea- surements from the FHWA PR Monitor. This is expected, but the comparison serves to demonstrate that there is a small com- ponent of measurement variability that is operator dependent, and related to data processing. Figure B-3(b) demonstrates that measurements of Rs include more variation compared to measurements of PR′. The coeffi- cient of correlation between measurements using the PR Mon- itor and the GAMRY G600 is 0.87 when considering the entire data set. However, five of the data points that lie above the trend line in Figure B-3(b) are from the same site located in San Bernardino, California. One of these data points also corresponds to the one outlying data point identified in Fig- ure B-3(a). If the five data points from San Bernardino are removed, a coefficient of correlation equal to 0.94 is obtained. Measurements of Rs from the GAMRY G600 (with the San Bernardino data points removed) are on average 15% higher than those measured with the PR Monitor. The reason for the difference in measurements may be related to the manner in which the measurements are made. Both units use an impressed AC current to make the measure- ments. The PR Monitor measures Rs with a square wave signal at a frequency of 270 Hz. The GAMRY G600 considers a broad spectrum of response using electrochemical impedance spec- trometry (EIS) and renders the value of Rs by plotting the total impedance versus frequency (Bode plot). The latter measure- ments are theoretically more robust, but EIS measurements are more sensitive to noise and interference and may become unstable; and are also more difficult to interpret if different metals (e.g. zinc and iron) are present on the surface and/or if oxide film layers are present on the surface. LPR Compared to Tensile Strength Loss Caltrans tested specimens identified with pitting from inspection rods retrieved from sites in Northern California during the period from July 14, 2007 to August 24, 2007. Each data point presented in Table B-2 involves measurement of ten- sile strength at two locations: one including the pitted cross sec- tion (Tpitted) and another from a nearby intact location (Tintact) that serves as a reference measurement. The strength loss expressed as a percentage of the intact strength is computed as: % (Strength Loss B= − × T T T intact pitted intact 100 -3)

Strength losses less than 4% are not considered to be sig- nificant and may reflect strength variation inherent to the material, rather than from loss of section. The average of the strength variation shown in Table B-2 is 5.2%, with a maxi- mum of 15.8% and minimum 0.3%. The maximum of 15.8% was recorded for plain steel reinforcements (i.e., not galva- nized) and all of the measurements were from specimens that had been exposed to the backfill for approximately 20 years and are for rod shaped specimens made from cold-drawn steel wire (ASTM A-82). The observed loss of tensile strength is compared to the loss anticipated based on metal loss models proposed by Darbin et al. (1988) and the Caltrans-Interim design strength loss model (Jackura et al., 1987). The AASHTO metal loss model was not considered here because the backfills do not meet corresponding AASHTO criteria. The loss of strength of the Darbin and Caltrans-Interim metal loss models is computed as follows: For the Darbin model: where X is the loss of base steel in inches after the zinc (assumed to be 86 μm thick) is consumed, and t is time in years. This equation includes a factor of 2 to consider the effect of local- ized corrosion activity (i.e., pitting). The diameter after t years of service is then computed as: where df is the diameter after t years and di is the initial diameter of the specimen prior to being galvanized; both d d Xf i= − ×( )2 ( )B-5 X tin. m m in. m ( ) = × −( )× × ⎛⎝⎜ ⎞ 25 86 2 1 25 400 0 65μ μ μ . , ⎠⎟ ( )B-4 (a) measured (PR′ = PR + Rs) polarization resistance (b) backfill resistance (Rs) 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 900.0800.0 GAMRY (Ω) FH W A P R M on ito r ( Ω ) GAMRY/Caltrans GAMRY/MMCE GAMRY (Ω) FH W A P R M on ito r ( Ω ) 0 50 100 150 200 250 0 50 150 250100 200 Figure B-3. Polarization resistance measurements with the FHWA PR Monitor and the GAMRY G600. 65

66 diameters are in units of inches. The corresponding loss of strength is: Alternatively, according to the Caltrans-Interim design guidelines: where A is the percentage of the original diameter that remains after t years, C is the life of the zinc protecting the surface of the steel (assumed by Caltrans as 10 years for these backfill condi- tions), and k is the metal loss per year considering the effect of localized corrosion (assumed by Caltrans as 0.0011 in./yr for these backfill conditions). The corresponding loss of strength is: The equations for estimating loss of strength appear to be an upper bound with respect to the observed strength loss. With one exception, the Caltrans-Interim design equation estimates greater loss of strength compared to the observed strength loss displayed in Table B-2. The strength losses esti- mated with the Darbin model are approximately 65% to 70% of those estimated with the Caltrans-Interim design equa- tions, but, in general, these estimates still represent an upper bound to the observed strength losses. The highest measured corrosion rate of 5.7 μm/yr com- pares well with the rate predicted via the Darbin model con- % ( )Loss B-8= −100 A A d k t C d i i = − × × −( )[ ] × 2 100 2 2 ( )B-7 % ( )Loss B-6= − × ⎛ ⎝⎜ ⎞ ⎠⎟1 100 2 2 d d f i sidering 20-year-old reinforcements. However, corrosion rates measured with the LPR technique do not always corre- late well with respect to pitting (i.e., the specimen with the deepest pit is not the same specimen with the highest corro- sion rate measured via the LPR technique). LPR Compared to Weight Loss/Thickness Loss Mild to moderate corrosion was observed from sites in northern California. Caltrans measured the pit depth at sixteen locations along selected inspection elements as summarized in Table B-3. Appendix C includes photographs depicting the locations where pitting was observed. All of these data are from rod-type elements and, generally, pitting does not result in a uniform loss of radius but rather affects a limited portion of the cross section. Pit depths were measured by subtracting the remaining thickness (remaining diameter) from the initial diameter determined from measurements of a nearby section that appeared to be intact. Table B-3 also includes the estimated uniform loss based on the Darbin model (Darbin, 1988) for gal- vanized elements and Elias (1990) for plain steel elements. The ratio of maximum section loss to estimated uniform loss ranges from 1.2 to 4.8 with an average loss ratio of 2.4. The estimated uniform rate of metal loss is compared to the corrosion rate measured at an instant in time via the LPR technique. The corrosion rate computed via measurement of LPR may be in error by a factor of 2 considering the selection of parameters needed to relate the measurement of polariza- tion resistance to corrosion rate. Moderate to severe corrosion was observed from sites in southern California. Table B-4 is a summary (Caltrans Site Details Sample Details Observed Strength Anticipated Strength Bridge # Locale Age ρ(yr) (Ω-cm) Sample1 Location Specimen Location (ft)2 di (in.) % Loss Darbin % Loss Caltrans Interim % Loss 10-0284 Redwood Valley 18 2527 B/13 6-10 0.371 4.5 6.5 9.3 10-0284 Redwood Valley 18 2527 B/14 6-10 0.369 7.8 6.5 9.3 10-0284 Redwood Valley 18 2527 M/8 6-10 0.372 8.8 6.5 9.2 20-0269 Preston 20 2821 T/2 6-10 0.373 2.0 7.4 11.4 20-0269 Preston 20 2821 M/8 5-9 0.374 5.5 7.4 11.4 28-0303 Richmond/Castro 18 1434 T/2 6-10 0.498 13.3 4.8 6.9 28-0303 Richmond/Castro 18 1434 M/8 6-10 0.499 0.3 4.8 6.9 28-0303 Richmond/Castro 18 1434 B/15 6-10 0.499 2.5 4.8 6.9 28-0303 Richmond/Castro 18 1434 M/7 5-9 0.499 1.2 4.8 6.9 28-02943 Richmond/Regatta 19 1600 B/17 6-9 0.421 15.8 29.13 18.9 10-0279 Hopland 18 NA T/3 6-10 0.375 4.3 6.4 9.2 10-0279 Hopland 18 NA T/2 6-10 0.375 1.5 6.4 9.2 10-0277 Hopland 18 NA T/1 6-10 0.374 1.1 6.4 9.2 1 T, M, and B = top, middle, & bottom; ## inspection rod number location in cluster (elements of 1–18) based on field identification form. 2 Distance into fill from wall face. 3 Plain steel specimen (i.e., not galvanized) and )in.( 400,25 12)8640()in.( 8.0 m mtmX μ μμ ××−×= . Table B-2. Results from tensile strength testing of pitted specimens from sites in northern California.

Bridge # Age Years ρ Backfill Ω-cm Elev.2 Distance from face (ft) Pit Depth (μm) Est. Uniform Loss (μm) Loss Ratio Est. Uniform Rate (μm/yr) LPR Meas. Rate (μm/yr) 28- 0297 19 NA3 T 6 647.7 169.5 3.8 5.8 4.9 28- 0306 17 533 T 2.5 304.8 157.7 1.9 6.0 2.3 28- 0306 17 533 B 1 419.1 157.7 2.7 6.0 1.3 10- 0286 18 2522 M 1 254.0 163.6 1.6 5.9 1.4 10- 0284 18 2522 B 8 342.9 163.6 2.1 5.9 1.4 10- 0284 18 2522 B 4 139.7 163.6 - 5.9 2.9 10- 0284 18 2522 B 7 584.2 163.6 3.6 5.9 2.9 20- 0269 20 2821 M 8 317.5 175.2 1.8 5.7 2.3 28- 0303 18 1434 B 4 203.2 163.6 1.2 5.9 2.9 28- 0303 18 1434 T 9 520.7 163.6 3.2 5.9 1.9 28- 0303 18 1434 M 5 406.4 163.6 2.5 5.9 2.3 28- 0303 18 1434 M 6.5 355.6 163.6 2.2 5.9 2.8 28- 02941 19 1600 B 5.5 304.8 421.8 - 17.8 25.0 28- 02941 19 1600 B 7.7 1155.7 421.8 2.7 17.8 25.0 28- 02941 19 1600 M 2.5 508 421.8 1.2 17.8 28.0 10- 0278 18 NA T 4.5 787.4 163.6 4.8 5.9 1.7 1Bare steel, i.e. not galvanized 2B=bottom, M=middle, T=top 3NA = not available Table B-3. Summary of section loss observed from inspection rods exhumed by Caltrans. Inspection Element Backfill Condition CR (μm/yr) Bridge No. Locale Location, Type w ρ % min (Ω-cm) Ecorr (mV) Zinc (oz/ft2) Pitting Loss LPR 53-2819 07-LA-47 R13, Rod - 1610 -793 1.5 N - 0.4 53-2821 07-LA-47 L14, Rod 8.4 1763 -474 - Y 47 179 53-2821 07-LA-47 L16, Rod 8.1 1389 -484 - Y 26 32 53-2822 07-LA-47 L13, Rod 8.5 3580 -740 1.4 N - 0.9 53-2822 07-LA-47 L14, Rod 9.1 2072 -537 - Y 26 104 53-2822 07-LA-47 L15, Rod 10.7 5849 -714 1.5 N - 1.0 53-2823 07-LA-47 L17, Rod 8.8 1763 -511 Y 99 42 53-2823 07-LA-47 L15, Rod 9.1 2223 -475 Y 99 25 54-1093 08-SBD-30/215 L2, Strip 2.9 6223 -540 5.5 Y 9.51 0.7 54-1093 08-SBD-30/215 L12, Strip 2.1 12705 -594 4.7 N - 0.7 54-1094 08-SBD-30/215 L8, Strip - - -581 4.8 Y 6.01 1.2 54-1094 08-SBD-30/215 L14, Strip - - -610 1.3 Y 7.51 1.3 56-0794 08-Riv-10 L7, Strip - - -356 Y NA 80 56-0794 08-Riv-10 L 11, Strip - - -567 Y 40 50 56-0794 08-Riv-10 L 13, Strip 0.4 377 -550 5.2 Y 281 3.7 10-0279 01-Men-101 L15, Rod 3.2 - -612 4.7 N - 0.30 10-0279 01-Men-101 L17, Rod 1.8 8414 - - - - - LV 09-Men-395 L9, Strip 0.5 11375 - 7.0 N - - 1 pit involves a small surface area on strip High moisture content, low ρmin, and corresponding higher corrosion NOTE: NA = data not available. Corroded strip broke during extraction. Table B-4. Summary of laboratory data from Caltrans and comparison with field observations.

68 southern sites) that provides a comparison of backfill and inspection rod conditions, and corrosion rates measured via direct observation and from LPR measurements. Corrosion rates computed from observations of remaining diameter/pit depth shown in Table B-4 compare qualitatively with measurements from LPR. In cases where relatively high corrosion rates were measured via LPR (> 20 μm/yr), pitting and corresponding loss of section were also observed along the inspection rods. The corrosion rates at the point of maximum section loss may be four times higher than the average rates measured via LPR, which is consistent with expectations con- sidering the geometry of the rod shaped inspections elements (Smith et al., 1996). In a couple of instances, corrosion rates measured via LPR are higher than direct observations, however, these LPR measurements are anomalous, and when repeated with different equipment (GAMRY vs FHWA PR Monitor) such high values of corrosion rate are not consistently observed. Pitting observed for strip-type reinforcements covered small areas that did not have a significant impact on tensile strength, and relatively low corrosion rates are indicated via LPR. Correlations of corrosion rate and loss of zinc are particularly interesting because the backfill samples were retrieved from the same locations as the inspection elements. This is not usu- ally the case, and most often backfill data is derived from sam- ples taken at stockpiles or from random locations within the backfill. Higher corrosion rates and lower and remaining zinc (<2 oz/ft2) measurements are consistently correlated with back- fill samples that simultaneously exhibit relatively low minimum resistivity (ρmin) and high moisture content. This trend is illus- trated in Figures B-4 and B-5. Higher corrosion rates are not always observed in Figure B-4 when ρmin is low, or in Figure B-5 when moisture contents are higher. However, a comparison of points with CR > 20 μm/yr reveals that both of these conditions are met in these instances. This comparison demonstrates the value of obtaining backfill samples and corrosion rate measure- ments at the same location and at similar times. The data shown in Figures B-4 and B-5 help to explain why higher corrosion rates are not always observed from sites with poor quality back- fill (e.g., low ρmin) and can be useful to reconcile some of the variation apparent from our performance database. Inspection elements that exhibited high corrosion rates appeared to break at a reduced cross section during extraction. Therefore, a lot of the data on remaining tensile strength do not correspond to the locations with the most severe section loss. Tensile strength data are useful to document the remaining strength of less corroded sections (that did not break upon extraction),and to study inherent variation of material strength. Data obtained from extraction of inspection elements dur- ing field work for Task 6, performed in cooperation with Cal- trans, demonstrate that the ratio of maximum metal loss (i.e., loss of tensile strength) to average corrosion rate or metal loss ranges from 1.2 to 4.8 with a mean of 2.4. This factor appears to be inversely proportional to severity of corrosion and tends to range between 2 and 3 when more severe loss of cross section is observed. LPR measurements are particularly effective to discern the occurrence of relatively mild, moderate, or severe cor- rosion. For galvanized elements, corrosion rates via LPR correlate best with the percentage of zinc remaining on the surface. When more than 70% of the surface is covered by zinc, corrosion rates measured via LPR reflect the rate of zinc loss. However, there may be instances where localized corrosion of steel may not be reflected in the LPR measure- ment of corrosion rate. This is more of an issue at sites with relatively poor or marginal quality fill materials where metal loss is less uniform and localized loss of zinc is observed. In general, corrosion rates from LPR measurements are consis- tent with observations of maximum metal loss considering a factor between 2 and 3 relating the average to the maximum metal loss. 0 50 100 150 0 5000 10000 15000 ρmin (Ω-cm) CR ( μ m /y r) 0 50 100 150 0 5 10 15 w% CR ( μ m /y r) Figure B-4. Minimum resistivity (min) vs. corrosion rate (CR) for correspond- ing backfill and inspection rod locations. Figure B-5. Moisture content (w%) vs. corrosion rate (CR) for corresponding backfill and inspection rod locations.

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TRB’s National Cooperative Highway Research Program (NCHRP) Report 675: LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems explores the development of metal loss models for metal-reinforced systems that are compatible with the American Association of State Highway and Transportation Officials' Load and Resistance Factor Design Bridge Design Specifications.

NCHRP Research Results Digest 364: Validation of LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems summarizes the results of research to further validate some key results of a project that resulted in publication of NCHRP Report 675.

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