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56 APPENDIX B Test Protocols Proper implementation of test procedures and interpreta- length and geometry of the wall. As a rule of thumb, two loca- tion of results from condition assessment require information tions spaced at least 200 ft (60 m) apart should be considered on reinforcement type and geometry, as well as backfill and site for mechanically stabilized earth (MSE) structures 800 ft conditions. The subsurface environment surrounding the (250 m) or less in length and three locations for longer struc- elements must be characterized in terms of soil or rock types, tures. At each location, corrosion should be monitored at a moisture conditions, presence of organics, and electrochemi- minimum of two depths from the surface, or preferably, at cal parameters known to contribute to corrosiveness. Installa- depth intervals of 10 to 13 ft (3 to 4 m) because differences in tion details include reinforcement type, metal type, and degree oxygen content, moisture content, and salt concentration can of corrosion protection. Quantitative guidelines are available produce different corrosion behavior. One critical location for assessing the potential aggression posed by an underground (center of structure) should be selected for establishing test environment relative to corrosion (FHWA, 1993). Generally, locations at both shallow and deep positions. Higher oxygen moisture content, chloride and sulfate ion concentration, and salt content are anticipated near the surface, and higher resistivity and pH are identified as the factors that most affect moisture contents or free water near the base of a structure. corrosion potential of metals underground. Details for collect- Prior field programs have indicated that where groundwater ing, testing, and evaluating soil and groundwater samples are intrudes at the base of the structure, higher corrosion rates described in the recommended practices prepared by Withiam should be anticipated. Where this condition is not likely, repre- et al. (2002) and Elias et al. (2009). In what follows sampling sentative estimates may be obtained from shallow-depth mon- and testing protocols for condition assessment and corrosion itoring. The shallow-depth stations should be approximately monitoring reinforcements are described. 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- Sampling pling points generally located near the base, middle, and top Selected sites for evaluating the overall performance of of the walls. Sampling points include at least two reinforce- earth reinforcements should encompass different reinforce- ments wired for monitoring, one steel coupon, one galvanized ment types, loading, environmental and drainage conditions, coupon, possibly a zinc coupon, and an access hole for place- backfill, and in-situ soil or rock characteristics representative ment of a reference electrode in contact with the wall fill. Spe- of installations and construction practices that have been cial C-clamps are used to facilitate electrical connection and used within the United States over the past 30 to 40 years. wiring to existing in-service reinforcements. Soldered con- Sampling protocols are described for both Type I and Type II nections are preferred for new installations. Photographs 1-8 reinforcements. 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 Type I Reinforcements (MSE) location and depth; zinc, steel, and galvanized. In the case of In general, approximately 20 to 30 in-service reinforcements, galvanized reinforcements both plain steel and galvanized and 20 to 30 coupons should be monitored at each site. These coupons, and in some instances pure zinc coupons are installed. elements are distributed amongst three or four monitoring sta- For monitoring, it is desirable to have one-zinc, one-steel, and tions. The number of monitoring stations depends on the up to four galvanized coupons at each depth. The multiple
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57 1. Coring hole through precast panel to 2. Access holes advanced at station access backfill and soil reinforcements 3. Galvanized coupons prepared for 4. C-clamps for wiring reinforcements installation 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)
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58 7. Station with junction box 8. Typical junction box Figure B-1. (Continued). galvanized coupons can provide opportunities for periodic nized coupons are not installed). Details of the practices from removal. Coupons each have two leads to provide backup in California and North Carolina are described in Appendix C. case one connection fails. Coupons are made from the same or similar material as the in-service reinforcements and are Type II Reinforcements placed within the wall fill to provide baseline measurements during monitoring. Nondestructive testing (NDT) and condition assessment In general, more monitoring locations should be established requires a sampling strategy whereby the appropriate sample for structures where poor performance is anticipated or known size is selected to provide a statistical basis for the test results. to exist (Withiam et al, 2002; Hegazy et al, 2003). Particular Withiam et al. (2002) and Hegazy et al. (2003) describe a sim- attention should be given to monitoring near drainage inlets plified sampling criteria based on the probability that the sam- or other areas that may be subject to fluctuations in moisture pled population will represent conditions throughout the site. content, high moisture content, or inundation. However, mon- The recommended sample size is based upon the total num- itoring at locations with "normal" conditions is still necessary ber of elements at the site, the importance of the facility rela- to serve as a baseline and to ensure that the sample statistics tive to the consequences of failure, and a reference, or baseline, are not skewed. condition for comparison to observations. Generally, for a Practices vary among state departments of transportation population consisting of 10 to 200 metal-tensioned elements, (DOTs) and not all establish corrosion monitoring stations in between 10 and 40 randomly distributed samples are required. the same manner including all the details as described in the section. In particular, Caltrans installs a cluster of 18 inspec- Corrosion Monitoring and tion rods in a grid pattern that includes six columns and three Condition Assessment rows. The inspection rods are spaced at 10-foot intervals ver- tically and are approximately 25 feet apart in the lateral direc- Type I Reinforcements tion. The inspection rods are made from the same material as Visual Observations the in-service reinforcements. In North Carolina, often only a single monitoring point is established near the base of the wall Visual observations can be made on the exposed portions that includes between two and four in-service reinforcement of the earth reinforcements, and readings of half-cell potential wires for monitoring and zinc and steel coupons (i.e., galva- and corrosion rate are collected from in-service reinforcements
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59 that are wired for monitoring, and from coupons installed working, counter, and reference electrodes. The working elec- within the wall fill. For older walls that are retrofitted for cor- trode is the reinforcement being monitored and a nearby rein- rosion monitoring, the condition of the reinforcing strips near forcement is used as a counter electrode. The potential at the the tie-strip may be observed where they are connected to the interface of the working electrode is controlled through current precast concrete wall-facing after advancing the access holes impressed between the working and counter electrodes. A CSE and exposing the reinforcements. For walls where core holes serves as a reference electrode to monitor the changing poten- are not advanced through the wall face, reinforcements may tial of the working electrode. The measured resistance, PR, is be examined from shallow excavations near the surface along actually the sum of the interface and soil resistance (PR = PR + the top of the wall. Rs) and a correction for soil resistance is often necessary. The LPR uses polarization resistance measurements to esti- Half-Cell Potential Measurement mate the corrosion rate at an instant in time. The measure- ment represents an average of the corrosion occurring over The half-cell potential, Ecorr, is the difference in potential the surface area of the test element. LPR measurements are between the metal element and a reference electrode. Equip- made with the FHWA PR Monitor supplied by CC Technolo- ment required for performing measurement of half-cell poten- gies (Model # PR 4500) following the protocol described by tial includes a half cell, a high impedance voltmeter, and a set of Elias (1990) and Berkovitz and Healy (1997). A few parame- lead wires. A copper/copper sulfate reference electrode (CSE) ters, including an environmental constant, the surface area was used for this study. Lead wires are attached to the end of the of the test element, and the density and valence of the metal test element and the half cell. The lead from the half-cell is con- species must be known, or assumed, to relate the measured nected to the negative terminal of the voltmeter, and the test polarization resistance to corrosion rate. Also, the measured element lead is connected to the positive terminal. Results from polarization resistance is corrected for uncompensated soil the test can provide a comparison between metallic elements at resistance inherent to testing within the underground envi- different locations at the same site, as well as identify the pres- ronment. The PR Monitor measures the soil resistance (Rs) ence of different metals, (e.g., zinc or iron). Half-cell potentials via the AC impedance technique and subtracts this from the may be correlated with zinc loss and used to monitor the con- total polarization resistance to render the corrected polariza- dition of galvanized reinforcements. Coupons or dummy tion resistance. The soil resistance is a function of the specific reinforcements assist in interpretation of half-cell potential resistance (), which is related to wall fill properties includ- measurements. Plain steel, galvanized steel, and zinc coupons ing moisture and salt content, as well as the geometry of the may provide baseline measurements for comparison. system, including the surface area of the reinforcement and Half-cell potentials are useful to assess the condition along the distance between the reinforcement and the reference cell. the surface of the reinforcements/coupons. Half-cell poten- LPR measurements represent the corrosion rate at the instant tials are affected by the environment, including soil moisture of measurement. Corrosion rates may vary, and measurements and salt content, as well as by conditions on the surface of the with respect to time are needed. Thus, initial measurements are test element, including the presence of a passive film layer and often taken after installation of corrosion monitoring stations, metal oxides. Therefore, care should be taken when interpret- followed by measurements at 6-month intervals and thereafter ing measurements to identify when effects other than corrosion for a 2-year duration, and then measurements at 5- or 10-year or presence of zinc on the surface are affecting measurements intervals. of half-cell potential. Multiple measurements of half-cell poten- tial are necessary (i.e., numerous samples) and reference val- Type II Reinforcements ues for steel and zinc potentials need to be obtained under site-specific conditions (i.e., nominal values for zinc and steel Details of the recommended practice for condition assess- potentials may not reflect site conditions). ment of Type II reinforcements are described in Withiam et al. (2002) and Fishman et al. (2005). In general, the protocol is Linear Polarization Resistance described as follows: Linear polarization resistance (LPR) measurements are used · Collect preliminary information including installation to observe instantaneous corrosion rates. Lawson et al. (1993), details and site conditions. Elias et al. (2009), and Berkovitz and Healy (1997) describe the · Identify appropriate mathematical models of service life application of the LPR technique to MSE reinforcements. and use these models to estimate metal loss from corrosion Polarization resistance measurements require an instrument to and remaining service life. generate a plot of potential versus applied current (E versus · Probe the elements with nondestructive tests, supplemented iapp) for a range of approximately E ± 20 mV relative to the free with invasive testing as appropriate, to assess the existing con- corrosion potential of the reinforcement being monitored. dition of selected elements comprising the metal-tensioned Three electrodes are required to perform the test including system.
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60 · Compare results of the condition assessment to expecta- section. For this test method, the specimen is impacted using tions based on site conditions and estimated metal loss. a hammer or ball device, which generates elastic compression · Recommend an action plan based on results from the con- waves with relatively low frequency content. Equipment dition assessment. required for the impact echo test method includes an impact device, an accelerometer, velocity or displacement transducer Installation details have an effect on the vulnerability of the for measuring the specimen response, and a data acquisition system to corrosion and on our ability to probe the elements system. Components of the test are connected with standard and interpret data from NDT. Relevant details include steel coaxial cables and Bayonet Neill-Concelman (BNC) connec- type, corrosion protection measures, drill hole dimensions, tors. Generally, an accelerometer is attached to the free end of bond length, free length, total length, date of installation, the element and the impact is also applied to the free end. level of prestress, grout type, and use of couplings. If the sys- tem is protected with an adequate, well constructed corrosion Ultrasonic Test protection system [e.g., meeting the requirements of PTI Class I (PTI, 2004)], then corrosion has not been found to be The ultrasonic test method is a good technique for evaluat- a problem. However, construction details, element durabil- ing grout condition, fracture of elements, and abrupt changes ity, and workmanship associated with the corrosion protec- in the element cross section. The method has many of the fea- tion system may affect the service life. tures of the sonic echo technique except that the transmitted Nondestructive test techniques are used to probe the ele- signal contains relatively higher frequencies. Ultrasonic waves ments, and the results are analyzed for condition assessment. are radiated when an ultrasonic transducer applies periodic Four NDTs are commonly applied for condition assessment strains on the surface of the test object that propagate as stress including measurement of half-cell potential, polarization waves. Compression waves consisting of alternating regions of current, impact, and ultrasonic testing. Half-cell potential compression and dilatation propagate along the axial direc- and polarization measurements are electrochemical tests and tion of a rock bolt. Equipment required for the test includes a the impact and ultrasonic techniques are mechanical tests pulse source/receiver unit, an ultrasonic transducer, and a involving observations of wave propagation. In general, these data acquisition system. NDTs are useful indicators of the following aspects of the The ultrasonic transducer is acoustically coupled to the condition assessment: exposed end of the anchor rod. Grease is used as an acoustic couplant. The time taken for sound pulses, generated at reg- · Half-cell potential tests serve as an indicator of corrosion ular intervals, to pass through the specimen and return, is activity. measured. Return pulses may be either from a single reflec- · Results from the polarization test are correlated with the sur- tion at a discontinuity or from multiple reflections between a face area of steel that may be in contact with the surround- discontinuity and the end of the specimen. The patterns of ing rock mass (i.e., indicator of grout quality and degree of the received pulses can provide valuable information about corrosion protection) and may be used to estimate an aver- the nature of a defect, and of the structure of the material being age corrosion rate. tested. The advantage of the pulse-echo method is that only · Impact test results are useful to diagnose loss of prestress, one side of the specimen needs to be accessed for transducer assess grout quality, and indicate if the cross section is placement. compromised from corrosion or from a bend or kink in the element. · Ultrasonic test results are useful for obtaining more detailed Data Interpretation information about the condition of elements within the first Impact Tests meter from the proximal end of the element. Impact (sonic echo) test results are interpreted by plotting Withiam et al. (2002) and Fishman et al. (2002 and 2005) time-histories of the responses measured by the accelerometer describe details of NDT including test procedures. Half-cell for each impact test. The maximum responses correspond to potential and LPR measurements are similar to those described the impact, and the responses are normalized with respect for Type I reinforcements. 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- Sonic Echo Measurements ure B-2 and is useful to assess the relative level of prestress car- The sonic echo method (impact test) is used for evaluating ried by the elements. A relatively high rate of decay (i.e., highly cracking of grouts, fracture of tendons, and loss of element damped system due to more dispersion) is indicative of high
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61 Bolt #10 Bolt #10 1 1 tR2 Normalized Response Normalized Response 0.8 0.6 0.5 0.4 0.2 0 0 -0.2 0 tR1 0.001 0.002 0.003 0.004 -0.4 0 0.0002 0.0004 0.0006 0.0008 -0.5 -0.6 -0.8 -1 -1 Time (sec) Time (sec) Bolt #1 Bolt #1 tR1 1 Normalized Response 1 tR2 Normalized Response 0.5 0.5 0 0 0 0.0002 0.0004 0.0006 0.0008 0 0.001 0.002 0.003 0.004 -0.5 -0.5 -1 -1 Time (sec) Time (sec) Bolt #3 tR1 tR2 Bolt #3 1.5 Normalized Response 1.5 Normalized Response 1 1 0.5 0.5 0 0 -0.001 -0.5 0.001 0.003 -0.5 0 0.0002 0.0004 0.0006 0.0008 -1 -1 Time (sec) Time (sec) Figure B-2. Typical time histories of responses from impact tests. remaining prestress, and a low rate of decay is associated with the steel bolt is approximately 18,000 ft/s, the corresponding a loss of prestress. Based on past experience, a high rate of decay arrival times for reflected waves is 0.5 to 1.7 milliseconds is indicated if the signal strength decays to less than 20% of from the beginning of the grout column and approximately the original signal strength within a millisecond. As shown in 1 to 2.2 ms from the end of the bolt. Evaluation of these reflec- Figure B-2, Bolt #10 and Bolt #1 are examples of a high rate tions serves two purposes. First, we may assess the difference of decay. A reflection at approximately 0.5 ms is evident from in grouted lengths from these results to compare with the the response of Bolt #1, but the rate of decay subsequent to assumed lengths (corresponding to surface areas) used to this reflection is high. The response of Bolt #3 is an example interpret the LPR measurements. Second, the strengths of the of a low rate of decay. reflected signals are useful to access grout quality. Good grout Responses from impact testing are recognized in terms of quality corresponds to a weak reflected signal from the distal relatively strong, versus relatively weak, signal attenuation. If end. If a strong reflection is recognized, then grout quality is the surrounding grout is very high quality, then strong reflec- considered poor, and the reinforcements may not be com- tions are not expected beyond a distance of approximately 10 to pletely surrounded by grout in the bonded zone, or the grout 15 feet. The plots on the right-hand side of Figure B-2 depict may be highly fractured. Based on past experience, strong the time history of the response over 5 milliseconds. Based on reflections correspond to reflected signal amplitudes greater installation records, and observations from bolts that have than 20%, moderate is between 10% and 20%, and the ampli- been exhumed from this site, we expect that lengths are tudes of weak reflections are less than 10% of the maximum between 10 and 20 feet, and the elements (rock bolts in this response. Using these criteria, the time histories shown in case) are surrounded by grout for approximately 5 feet at the Figure B-2 depict weak reflections for Bolt #10 and strong distal end. Assuming that the compression wave velocity of reflections for Bolts #1 and #3.
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62 Ultrasonic Tests have half-cell potential greater than -200 mV. Limits recom- mended by ASTM C-876 suggest that half-cell potentials more Due to the higher frequency content of the sound waves positive than -200 mV indicate a low likelihood that corrosion compared to the sonic echo test, results from ultrasonic test- is occurring, while values more negative than -300 mV indi- ing provide more resolution and are better suited to detect cate a high likelihood that corrosion is occurring. For resin- reflection sources located within the first few feet from the grouted systems and steel reinforcements, half-cell potentials backside of the anchor plate. This region is often associated are generally more negative than -500 mV. 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- Corrosion Rates from LPR Measurements mass near the anchorage, so corrosion at this location cannot The corrosion current density is the current within the cor- be captured by LPR measurements, and the ultrasonic test is rosion cell in the absence of any external sources. Stern and an alternative method to detect a potential loss of cross section Geary (1957) showed that for small deviations from the free from behind the anchorage assembly. corrosion potential (±20 mV), the corrosion current density is inversely proportional to polarization resistance as: Half-Cell Potentials d ac B Rp = = = = The primary purpose of half-cell potential measurements app 0 app 0 di i 2.3 3 × icorr ( a + c ) icorr is to establish when significant portions of the galvanized steel (B-1) reinforcements have lost zinc and steel is exposed to the wall fill. For a given material in a given environment, the potential where is an indication of the corrosion activity. The more positive = the shift of the half-cell potential from the open cir- the potential, the greater, in general, is the corrosion. Poten- cuit potential (volts); tial measurements are therefore only qualitative indications of iapp = applied current (amperes/cm2); corrosion activity and should only be used to determine the icorr = corrosion current density (amperes/cm2); composition of the surface. a = anodic Tafel constant (volts/decade); Galvanized and plain steel coupons provide baseline mea- c = cathodic Tafel constant (volts/decade); surements for comparison with half-cell potentials of galva- B = environmental constant (B 0.035 V for galvanized nized in-service reinforcements. Typical values of Ecorr with steel and B 0.026 V for steel); and respect to a CSE are between -1,000 mV to -800 mV for pris- Rp = polarization resistance normalized for area that tine galvanized steel or zinc, and -700 mV to -400 mV for involves multiplying the polarization resistance (PR) plain carbon steel. If the potential of the reinforcing element by the reinforcement surface area (As) in contact with is close to that of a recently placed galvanized coupon, it is backfill; that is, Rp (-cm2) = PR × As. inferred that the zinc is still present along the length of the reinforcement. As the potential becomes more positive and The LPR measurement technique involves scanning or begins to approach that of the steel coupons, the zinc coating stepping the potential from (-5 to -20 mV) to (+5 to +20 mV) is being lost as steel is exposed on the surface. around the free corrosion potential, while simultaneously The interpretation of potential measurements for galvanized measuring the applied current. Polarization resistance is reinforcements considers that four distinguishable layers of zinc determined from the slope of this plot (i.e., Rp = /iapp). If the coating are formed as a result of the hot-dip process used to gal- surface area of the working electrode is known, corrosion vanize MSE reinforcements. The outside layer is nearly pure current density may be determined from the measured polar- zinc, and the succeeding inner layers are essentially zinc-iron ization resistance and, ultimately, related to corrosion rate. alloys. Progressively higher iron contents prevail as the interface Elias (1990) and Lawson et al. (1993) discussed the need to with the base steel is approached. Therefore, as zinc consump- correct the measurement of Rp for soil resistance. If the soil tion progresses towards the base steel interface, the half-cell resistance is relatively large, the measured PR can be much potential is consistently shifted toward values inherent to iron. greater than the true value for PR, and the estimated corrosion Ultimately, measurements of the half-cell potential reflect the rate may be significantly less than the actual corrosion rate presence of steel after all four layers of the zinc coating are occurring at the surface. To correct for the effect of soil resis- exhausted and bare steel is exposed, at least in some areas. tance, an AC signal is applied to the working electrode at the For Type II reinforcements, or soil nails that may be sur- end of the standard polarization measurement cycle. During rounded by grout, half-cell potentials can indicate if an ele- a high frequency measurement, the AC voltages reverse mag- ment is effectively passivated or can indicate if the grout is a nitude and polarity so rapidly that the interface capacitance resin type. Elements passivated by portland cement grout will does not impede polarization, and PR is short-circuited. This
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63 permits independent measurement of Rs, allowing PR to be for zinc elements embedded in soil. The selection of B for gal- calculated as PR - Rs. vanized elements is more ambiguous because it is not known Based on Faraday's Law, corrosion rate (CR) can be esti- a priori if zinc, steel, or a mixture of zinc and iron is exposed mated from icorr as follows: on the surface of the element. However, a value of B equal to 0.035 V is often used to consider galvanized elements. Simi- m i ×W larly, the constant relating icorr to corrosion rate may vary by CR = ( 3.27 × 106 ) × corr (B-2) yr ×n a factor of approximately 1.3, which can be realized by com- paring the atomic weights, densities, and valances of steel and where zinc for use in Equations (B-1) and (B-2). W = atomic weight (e.g., 55.84 for steel and 65.37 for zinc), The environmental constant is related to the tafel slopes, n = valence (e.g., 2 for steel or zinc), and which define the slopes of the anodic and cathodic branches of = density in g/cm3 (e.g., 7.87 for steel and 7.14 for zinc). the overpotential where they become linear in a plot of over- potential versus the logarithm of applied current. Tafel slopes Quantification, or estimation, of errors inherent to measure- were measured at 11 sites, included as part of Task 6, using spe- ment of corrosion rate involves an assessment of (1) param- cial equipment that applies overpotential (±250 mV), which eters that are often assumed and used to relate polarization exceeds what is needed for LPR measurements (±20 mV). resistance (measured) to corrosion rate and (2) the accuracy of Results from these measurements are presented in Table B-1. the polarization resistance measurement. Errors in measure- Direct measurement of tafel slopes is limited because of the ment include those associated with measuring polarization need for special equipment and because imparting this level of resistance and solution resistance, and errors computing the overpotential can have a lasting effect on the electrochemical corrosion rate arise from the selection or assumption of the properties on the surface of the reinforcement (i.e., future parameters and constants used for Equations (B-1) and (B-2). measurements of corrosion rate may be affected by test history). The means and ranges of the environmental constant, B, Parameters for Computing Corrosion Rate that were measured at selected sites are: from LPR Measurements Material Mean (V) Range (V) An environmental constant (B) relating polarization resist- Steel 0.024 0.0100.030 ance to corrosion current density (icorr), and the constants Galvanized 0.035 0.0100.058 relating icorr to corrosion rate need to be known or assumed to Zinc 0.040 0.0300.050 compute corrosion rate from measurement of polarization resistance. These inputs depend upon metal type and the The means of the measurements are very close to the physicochemical properties of the backfill. In general, the B assumed values used in Equations (B-1) and (B-2) to reconcile parameter is assumed as 0.026 V for steel elements and 0.05 V corrosion rate from LPR measurements. Table B-1. Summary of observed environmental constants. 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 R8 - 0.010 - Site No. 532822 R1 - 0.037 - Site No. 532823 R 12 - 0.010 - Site No. 541093 12 - - NY MMCE Lab - 0.020 0.054 - NOTE: TTC = Triangle Town Center, SHR = Sweet Home Road, MMCE = McMahon & Mann Consulting Engineers, and - = not applicable.
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64 Comparison of Device Performance and MMCE using a different protocol. Caltrans determined the polarization resistance from the slope of the overpotential Results obtained with a commercially available general- versus impressed current for a selected linear region in the purpose, corrosion monitoring device (GAMRY G600) are vicinity and symmetric with zero applied current (i.e., at the compared with those from a unit built specifically for the open circuit potential). MMCE determined the polarization FHWA (PR Monitor) for monitoring the performance of resistance at the slope within a region ±10 mV from the open MSE reinforcements. The hardware (Potentiostat/Galvanostat/ circuit potential. The latter is similar to the protocol employed Zero Resistance Ammeter) incorporated into each unit is by the FHWA PR Monitor. Figure B-3(a) shows that the similar. However, the general-purpose equipment allows user GAMRY data as reconciled by MMCE are closer to the mea- flexibility in terms of data processing and interpretation, and, surements from the FHWA PR Monitor. This is expected, but in contrast, the user cannot alter the protocols programmed the comparison serves to demonstrate that there is a small com- into the FHWA unit. Both the GAMRY G600 and the FHWA ponent of measurement variability that is operator dependent, PR Monitor correct for uncompensated solution resistance (soil resistance) as part of the LPR measurement. Both units and related to data processing. measure the soil resistance via an AC input and subtract this Figure B-3(b) demonstrates that measurements of Rs include more variation compared to measurements of PR. The coeffi- from the measured polarization resistance to render the cor- cient of correlation between measurements using the PR Mon- rected polarization resistance. itor and the GAMRY G600 is 0.87 when considering the entire The PR Monitor supplied by CC Technologies, Inc. utilizes data set. However, five of the data points that lie above the a potential control stepping sequence that is completely flexi- trend line in Figure B-3(b) are from the same site located in ble and programmable by the operator. The PR Monitor also San Bernardino, California. One of these data points also presents the coefficient of linear regression used to calculate the corresponds to the one outlying data point identified in Fig- value of PR from the vs. iapp plot. A regression coefficient of ure B-3(a). If the five data points from San Bernardino are 0.9 or greater indicates a reasonably good fit of the data. The removed, a coefficient of correlation equal to 0.94 is obtained. corrosion current density is determined from Equation (B-1) Measurements of Rs from the GAMRY G600 (with the San using the measured value of PR, As, and the appropriate envi- Bernardino data points removed) are on average 15% higher ronmental constant. Finally, Equation (B-2) is used to estimate than those measured with the PR Monitor. corrosion rate. The reason for the difference in measurements may be McMahon & Mann Consulting Engineers (MMCE) and related to the manner in which the measurements are made. Caltrans performed redundant tests with the GAMRY and Both units use an impressed AC current to make the measure- FHWA PR monitors. Data were collected from sites in north- ments. The PR Monitor measures Rs with a square wave signal ern and southern California during the periods from July 14, at a frequency of 270 Hz. The GAMRY G600 considers a broad 2007 to August 24, 2007 and April 9, 2008 to May 1, 2008. The spectrum of response using electrochemical impedance spec- data set includes 61 individual measurements of polarization trometry (EIS) and renders the value of Rs by plotting the total resistance from 10 different locations. Corrosion rates are impedance versus frequency (Bode plot). The latter measure- computed using the polarization resistance with the correc- ments are theoretically more robust, but EIS measurements tion for Rs. Once the uncompensated solution resistance is are more sensitive to noise and interference and may become obtained, different operators will compute the same corro- unstable; and are also more difficult to interpret if different sion rate using Equations (B-1) and (B-2); assuming they use metals (e.g. zinc and iron) are present on the surface and/or if the same parameters for surface area, environmental constant oxide film layers are present on the surface. (B) and metal valance, density, and atomic weight. Therefore, differences in results from these devices are with respect to LPR Compared to Tensile Strength Loss the manner in which PR and Rs are rendered. Results of LPR measurements performed with the GAMRY Caltrans tested specimens identified with pitting from equipment and operated by Caltrans, and measurements made inspection rods retrieved from sites in Northern California by MMCE using the FHWA PR Monitor are compared in during the period from July 14, 2007 to August 24, 2007. Each Figure B-3. Figure B-3(a) depicts measurements of PR that data point presented in Table B-2 involves measurement of ten- are not corrected for the uncompensated solution resistance sile strength at two locations: one including the pitted cross sec- (Rs), and Figure B-3(b) is the independent measurement of tion (Tpitted) and another from a nearby intact location (Tintact) Rs. Measurements of PR correlate very well (xy = 0.98). One that serves as a reference measurement. The strength loss data point falls outside the trend line corresponding to a expressed as a percentage of the intact strength is computed as: measurement with the GAMRY G600 that is approximately one half of that obtained with the PR Monitor. Some of the Tintact - Tpitted % Strength Loss = × 100 (B-3) GAMRY data were processed and analyzed by both Caltrans Tintact
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65 900.0 800.0 FHWA PR Monitor () 700.0 600.0 500.0 400.0 GAMRY/Caltrans 300.0 200.0 GAMRY/MMCE 100.0 0.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 GAMRY () (a) measured (PR = PR + Rs) polarization resistance 250 200 FHWA PR Monitor () 150 100 50 0 0 50 100 150 200 250 GAMRY () (b) backfill resistance (Rs) Figure B-3. Polarization resistance measurements with the FHWA PR Monitor and the GAMRY G600. Strength losses less than 4% are not considered to be sig- Darbin and Caltrans-Interim metal loss models is computed nificant and may reflect strength variation inherent to the as follows: material, rather than from loss of section. The average of the strength variation shown in Table B-2 is 5.2%, with a maxi- For the Darbin model: mum of 15.8% and minimum 0.3%. The maximum of 15.8% 1 in. X ( in.) = ( 25 m × t 0.65 - 86 m ) × 2 × (B-4) was recorded for plain steel reinforcements (i.e., not galva- 25, 400 m nized) and all of the measurements were from specimens that had been exposed to the backfill for approximately 20 years where X is the loss of base steel in inches after the zinc (assumed and are for rod shaped specimens made from cold-drawn to be 86 m thick) is consumed, and t is time in years. This steel wire (ASTM A-82). equation includes a factor of 2 to consider the effect of local- The observed loss of tensile strength is compared to the loss ized corrosion activity (i.e., pitting). anticipated based on metal loss models proposed by Darbin The diameter after t years of service is then computed as: et al. (1988) and the Caltrans-Interim design strength loss d f = di - 2 × ( X ) (B-5) model (Jackura et al., 1987). The AASHTO metal loss model was not considered here because the backfills do not meet where df is the diameter after t years and di is the initial corresponding AASHTO criteria. The loss of strength of the diameter of the specimen prior to being galvanized; both
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66 Table B-2. Results from tensile strength testing of pitted specimens from sites in northern California. Observed Anticipated Site Details Sample Details Strength Strength Specimen Darbin Caltrans Age Sample1 di Bridge # Locale Location % Loss Interim (yr) (-cm) Location (in.) (ft)2 % Loss % 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 118) based on field identification form. 2 Distance into fill from wall face. 3 Plain steel specimen (i.e., not galvanized) and 1 in. X (in.) = (40 m × t 0.8 - 86 m) × 2 × ( ). 25,400 m diameters are in units of inches. The corresponding loss of sidering 20-year-old reinforcements. However, corrosion strength is: rates measured with the LPR technique do not always corre- late well with respect to pitting (i.e., the specimen with the d2 f deepest pit is not the same specimen with the highest corro- % Loss = 1 - 2 × 100 (B-6) di sion rate measured via the LPR technique). Alternatively, according to the Caltrans-Interim design guidelines: LPR Compared to Weight Loss/Thickness Loss Mild to moderate corrosion was observed from sites in A= [di - 2 × k × (t - C )]2 × 100 (B-7) northern California. Caltrans measured the pit depth at sixteen di2 locations along selected inspection elements as summarized in where A is the percentage of the original diameter that remains Table B-3. Appendix C includes photographs depicting the after t years, C is the life of the zinc protecting the surface of the locations where pitting was observed. All of these data are from steel (assumed by Caltrans as 10 years for these backfill condi- rod-type elements and, generally, pitting does not result in a tions), and k is the metal loss per year considering the effect of uniform loss of radius but rather affects a limited portion of the localized corrosion (assumed by Caltrans as 0.0011 in./yr for cross section. Pit depths were measured by subtracting the these backfill conditions). The corresponding loss of strength is: remaining thickness (remaining diameter) from the initial diameter determined from measurements of a nearby section % Loss = 100 - A (B-8) that appeared to be intact. Table B-3 also includes the estimated uniform loss based on the Darbin model (Darbin, 1988) for gal- The equations for estimating loss of strength appear to be vanized elements and Elias (1990) for plain steel elements. The an upper bound with respect to the observed strength loss. ratio of maximum section loss to estimated uniform loss ranges With one exception, the Caltrans-Interim design equation from 1.2 to 4.8 with an average loss ratio of 2.4. estimates greater loss of strength compared to the observed The estimated uniform rate of metal loss is compared to strength loss displayed in Table B-2. The strength losses esti- the corrosion rate measured at an instant in time via the LPR mated with the Darbin model are approximately 65% to 70% technique. The corrosion rate computed via measurement of of those estimated with the Caltrans-Interim design equa- LPR may be in error by a factor of 2 considering the selection tions, but, in general, these estimates still represent an upper of parameters needed to relate the measurement of polariza- bound to the observed strength losses. tion resistance to corrosion rate. The highest measured corrosion rate of 5.7 m/yr com- Moderate to severe corrosion was observed from sites pares well with the rate predicted via the Darbin model con- in southern California. Table B-4 is a summary (Caltrans
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Table B-3. Summary of section loss observed from inspection rods exhumed by Caltrans. Bridge Age Backfill Elev.2 Distance Pit Est. Loss Est. LPR # Years from Depth Uniform Ratio Uniform Meas. -cm face (m) Loss Rate Rate (ft) (m) (m/yr) (m/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 1 Bare steel, i.e. not galvanized 2 B=bottom, M=middle, T=top 3 NA = not available Table B-4. Summary of laboratory data from Caltrans and comparison with field observations. Inspection Element Backfill Condition CR (m/yr) Bridge Locale Location, w min Ecorr Zinc Pitting Loss LPR No. Type % (-cm) (mV) (oz/ft2) 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 min, and corresponding higher corrosion NOTE: NA = data not available. Corroded strip broke during extraction.
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68 150 150 CR (m/yr) CR (m/yr) 100 100 50 50 0 0 0 5000 10000 15000 0 5 10 15 min ( -cm) w% Figure B-4. Minimum resistivity (min) Figure B-5. Moisture content (w%) vs. vs. corrosion rate (CR) for correspond- corrosion rate (CR) for corresponding ing backfill and inspection rod backfill and inspection rod locations. locations. southern sites) that provides a comparison of backfill and ments at the same location and at similar times. The data shown inspection rod conditions, and corrosion rates measured via in Figures B-4 and B-5 help to explain why higher corrosion direct observation and from LPR measurements. rates are not always observed from sites with poor quality back- Corrosion rates computed from observations of remaining fill (e.g., low min) and can be useful to reconcile some of the diameter/pit depth shown in Table B-4 compare qualitatively variation apparent from our performance database. with measurements from LPR. In cases where relatively high Inspection elements that exhibited high corrosion rates corrosion rates were measured via LPR (> 20 m/yr), pitting appeared to break at a reduced cross section during extraction. and corresponding loss of section were also observed along the Therefore, a lot of the data on remaining tensile strength do not inspection rods. The corrosion rates at the point of maximum correspond to the locations with the most severe section loss. section loss may be four times higher than the average rates Tensile strength data are useful to document the remaining measured via LPR, which is consistent with expectations con- strength of less corroded sections (that did not break upon sidering the geometry of the rod shaped inspections elements extraction),and to study inherent variation of material strength. (Smith et al., 1996). In a couple of instances, corrosion rates Data obtained from extraction of inspection elements dur- measured via LPR are higher than direct observations, however, ing field work for Task 6, performed in cooperation with Cal- these LPR measurements are anomalous, and when repeated trans, demonstrate that the ratio of maximum metal loss (i.e., with different equipment (GAMRY vs FHWA PR Monitor) loss of tensile strength) to average corrosion rate or metal loss such high values of corrosion rate are not consistently observed. ranges from 1.2 to 4.8 with a mean of 2.4. This factor appears Pitting observed for strip-type reinforcements covered small to be inversely proportional to severity of corrosion and tends areas that did not have a significant impact on tensile strength, to range between 2 and 3 when more severe loss of cross section and relatively low corrosion rates are indicated via LPR. is observed. Correlations of corrosion rate and loss of zinc are particularly LPR measurements are particularly effective to discern interesting because the backfill samples were retrieved from the occurrence of relatively mild, moderate, or severe cor- the same locations as the inspection elements. This is not usu- rosion. For galvanized elements, corrosion rates via LPR ally the case, and most often backfill data is derived from sam- correlate best with the percentage of zinc remaining on the ples taken at stockpiles or from random locations within the surface. When more than 70% of the surface is covered by backfill. Higher corrosion rates and lower and remaining zinc zinc, corrosion rates measured via LPR reflect the rate of (<2 oz/ft2) measurements are consistently correlated with back- zinc loss. However, there may be instances where localized fill samples that simultaneously exhibit relatively low minimum corrosion of steel may not be reflected in the LPR measure- resistivity (min) and high moisture content. This trend is illus- ment of corrosion rate. This is more of an issue at sites with trated in Figures B-4 and B-5. Higher corrosion rates are not relatively poor or marginal quality fill materials where metal always observed in Figure B-4 when min is low, or in Figure B-5 loss is less uniform and localized loss of zinc is observed. In when moisture contents are higher. However, a comparison of general, corrosion rates from LPR measurements are consis- points with CR > 20 m/yr reveals that both of these conditions tent with observations of maximum metal loss considering a are met in these instances. This comparison demonstrates the factor between 2 and 3 relating the average to the maximum value of obtaining backfill samples and corrosion rate measure- metal loss.