LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems (2011)

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42 The main focus of this study is reliability of metal loss modeling and service life estimates for earth reinforcements, including reinforcements for MSE, soil nails, rock bolts, and ground anchors. Reliability analysis is useful for the following: 1. Describing reliability of metal loss models for use in design, 2. Describing effect of deviations in electrochemical proper- ties and site conditions on service life, 3. Calibrating resistance factors for use in LRFD, and 4. Providing tools for asset management that can be used to estimate vulnerability and remaining service life of exist- ing systems. During Phase II of this research, fieldwork was undertaken to broaden the database describing in situ performance of earth reinforcements compared to what was available at the conclu- sion of Phase I. Additional data were collected to enhance the geographic distribution of sites included in the monitoring effort and to obtain more information representative of a range of fill conditions including high, good and marginal quality, more sites with LPR measurements providing a better spatial and temporal distribution of measurements at given sites, and more sites with older reinforcements (i.e., older than 25 years). Data were also obtained to further verify the use of LPR mea- surements to estimate corrosion rate, thus providing a sound basis to use these measurements for statistical analysis of mea- surements and for reliability analysis of service-life estimates and calibration of resistance factors for use in LRFD. For MSE reinforcements (Type I), electrochemical proper- ties of the fill were observed to have a significant impact on performance, and the effect of time on corrosion rate is clearly indicated by these data. The spatial distribution of corrosion rates appears to be random, although spatial trends are appar- ent from data obtained with respect to several of the sites in the database. No significant differences are observed between different climates for galvanized elements, however, marine environments had a detrimental effect on corrosion rates for plain steel (i.e., not galvanized) reinforcements. Also, it was found that seasonal variations affect measured corrosion rates, and considering the climate in the northeastern United States, measurements may vary by a factor of approximately 1.5 throughout a given year. Data were partitioned considering different fill conditions, reinforcement type and time frames rendering COVs between approximately 40% and 60% within each category. In gen- eral, metal loss models available from the existing literature, including the AASHTO model, were found to be conserva- tive. Use of the AASHTO metal loss model is evaluated within the framework of reliability-based design and calibration of resistance factors for LRFD. Results from LRFD calibrations rendered resistance factors corresponding to a target reliability index of 2.3 and pf ≈ 0.01. The following conclusions apply to the resistance factor cali- brations for LRFD of MSE walls: • Computed resistance factors vary depending on the method used to compute reinforcement load, that is, simplified or coherent gravity method. • Considering galvanized reinforcements in good backfill con- ditions (i.e., meeting AASHTO criteria for electrochemical parameters) the computed resistance factor is slightly less than what is recommended in the current AASHTO specifications. • Considering galvanized reinforcements in high quality fill renders resistance factors that are slightly higher than those currently specified by AASHTO for design of MSE walls. • Data were generated to consider plain steel (i.e., not galva- nized) reinforcements with fill materials that meet the AASHTO requirements for electrochemical parameters, a conservative metal loss model, and maximum design life of less than 50 years. • Data were generated to consider marginal quality fill (i.e., not quite meeting AASHTO criteria) with galvanized reinforcements, a very conservative metal loss model, and maximum design life of less than 50 years. Type II reinforcements include rock bolts and ground anchors, and their performance is related to the degree of cor- C H A P T E R 4 Conclusions and Recommendations

43 rosion protection included in the installation. Many rock bolts are only protected by grout and a lack of coverage may occur as a gap behind the anchor plate where grout is lost to the surrounding rock mass during installation, or from rem- nants of plastic cartridges inherent to resin-grouted installa- tions. However, the design load is often based on the pullout resistance rather than yield of the reinforcement, and metal loss does not appear to be significant for design lives of 50 or 75 years. For a 100-year design life, the rupture limit state likely controls the performance and resistance factors appro- priate to this design have been calibrated. The calibration uses approximately 70 observations of metal loss from sites located in the United States, Scandinavia, and the United Kingdom. Compared to MSE reinforcements (Type I), rock bolts are not necessarily redundant so a target reliability index of approx- imately 3.1 rather than 2.3 was used for the calibration, cor- responding to pf ≈ 0.001. Ground anchors for permanent installations generally have a Class I, double corrosion protection system including a trum- pet head assembly to protect the area behind the anchor head. There have not been any observations of poor performance when Class I corrosion protection measures are incorporated with proper detailing and workmanship during installation. If the anchor head does not include a trumpet head assembly that is filled with grease, the area near the anchor head may be vul- nerable to corrosion. If acidic conditions or high chloride concentrations prevail, then the service life may be severely compromised from hydrogen embrittlement or SCC along exposed portions of the anchor. Recommended Resistance Factors for LRFD Current practice (LRFD) for the design of metallic rein- forcements for MSE applications is to ensure that reinforce- ments maintain enough yield resistance to keep the probability that overstress occurs (i.e., probability of occurrence, pf) below acceptable limits throughout the design life of the facil- ity. Galvanized reinforcements are recommended and sacri- ficial steel is included in the cross section to compensate for the expected loss of steel subsequent to depletion of the zinc coating along the surface. Provided that the reinforced fill material meets AASHTO criteria for electrochemical param- eters, the metal loss model for galvanized metallic MSE rein- forcements described in the current AASHTO specifications is recommended for computing the nominal amount of sac- rificial steel. This recommendation is based upon approxi- mately 1,000 measurements of metal loss and corrosion rate from samples of galvanized reinforcements and coupons, cal- culation of the resistance bias and corresponding statistics, and reliability-based calibration of the resistance factor for LRFD considering the yield limit state. The current AASHTO model is necessarily conservative, and corresponding resis- tance factors correlate to an acceptably low probability of occurrence for a 75- or 100-year design life. The resistance fac- tors listed in Table 27 are recommended for use with LRFD considering the yield limit state, and using the AASHTO metal loss model to compute the nominal resistance at the end of the design life. Different resistance factors are recommended for good versus high quality fill, strip or grid reinforcements. The protocol for sampling and electrochemical testing of wall fill, described in Table 4, is recommended to assess fill quality and if the materials meet the criteria for good or high quality fills. Although the calibrations resulted in different resistance factors depending on use of the simplified or coherent gravity methods for computing nominal load, use of the same resis- tance factor is recommended for either method. This is based on the fact that the difference is relatively small, and similar designs are rendered when the same resistance factors are applied as described from the results of the example problem described in Appendix F. Also, the simplified method was originally calibrated to render similar results compared to the coherent gravity approach with the traditional allowable stress methods of design. Furthermore, the load bias associated with the coherent gravity method (D’Appolonia, 2007) may not consider the effect of reinforcement depth, and the cali- bration appears to favor reinforcements placed within the top 20 feet of the wall. The bias factor for reinforcements located below this depth is expected to be less, corresponding to cal- ibrated resistance factors that are higher and closer to those obtained with respect to the simplified method. The factors recommended for use with good quality fill and galvanized reinforcements are based on results presented in Table 17 from calibrations performed with respect to the simplified method of analysis. The recommended value of 0.65 corresponds well with values of φ computed considering a 75-year design life. Designs considering a 100-year design life are likely to result in relatively thicker reinforcements, Resistance Factors ( )Metal Type Backfill Quality Metal Loss Model Design Life (Years) Strip Grid Galvanized Good AASHTO 75 or 100 0.65 0.55 Galvanized High AASHTO 75 or 100 0.80 0.70  Table 27. Summary of recommended LRFD strength reduction factors for galvanized reinforcements.

44 therefore, the lower resistance factors of 0.55 and 0.60 com- puted for thinner reinforcements are not recommended; φ = 0.65 corresponds to thicker reinforcements with S = 6 mm. For galvanized grid-type reinforcements embedded within good quality fill the recommended value of φ = 0.55 corre- sponds to most of the values presented in Table 17, and is lower than the maximum of φ = 0.6 computed for W7 and W9 size wires and a 75-year design life. The factors recommended for use with high quality fill and galvanized reinforcements are based on results presented in Table 18 from calibrations performed with respect to the sim- plified method of analysis. The recommended value of 0.80 for strip-type reinforcements corresponds well with the value com- puted for 4-mm-thick reinforcements and a 75-year design life, and is generally less than the values computed considering a 100-year design life, although it is a little higher (by 0.05) than the value of 0.75 computed for the thicker reinforcements (S = 6 mm). The recommended value of φ = 0.8 is considered rea- sonable because using higher quality fill results in designs with relatively thinner reinforcements, and will most likely be con- sidered in conjunction with longer design lives. For galvanized grid-type reinforcements embedded within high quality fill, the recommended value of φ = 0.70 is equal to or lower than most of the values presented in Table 18, but is 0.05 higher than the values of 0.65 computed for W11 and W14 size wires and design lives of 75 years. However, this is not considered to be a signif- icant difference, so use of φ = 0.70 is considered to be a reason- able representation of the results presented in Table 18. Current AASHTO specifications prescribe resistance factors of 0.75 for strip-type reinforcements and 0.65 for grid-type reinforcements with rigid facing units (see Chapter 1, Table 7). These resistance factors apply with respect to use of either the simplified or coherent gravity methods to compute reinforce- ment loads. The resistance factors recommended in Table 27 are similar to these in the current AASHTO specifications for walls constructed with higher quality reinforced fill materials. Based on the information shown in Table 27, resistance factors should be reduced by 0.15 for fills that meet current AASHTO criteria for electrochemical parameters, but not by a very wide margin (i.e., good fill). Alternatively, the same resistance fac- tors could be used with the understanding that the probability of occurrence for the case of good fill conditions is greater com- pared to when high quality fills are used in construction. Use of plain steel (i.e., not galvanized) reinforcements is not recommended. However, data on the performance of plain steel reinforcements were analyzed, statistics on metal loss were generated, and resistance factors for use in LRFD were calibrated. The objectives of the study are to present dif- ferences with respect to design with galvanized reinforce- ments. Only fill materials that meet current AASHTO criteria for electrochemical parameters were considered, however, the performance of steel reinforcements depends on whether good or high quality fills are used during construction. The following equations are recommended to estimate nominal sacrificial steel requirements for good and high quality fills: For good quality fill: For high quality fill: For good quality fill, only a 50-year design life is considered corresponding to a nominal sacrificial steel requirement of 1,829 µm per side according to Equation (27). Thus, a total of 3.66 mm of sacrificial steel is required considering metal loss from all surfaces (additional diameter for round elements and additional thickness for strip-type elements). This is consider- ably higher than the current AASHTO requirement of 0.82 mm for a 50-year design life with galvanized reinforcements. A 75-year design life is considered when high quality fill is used in construction corresponding to a nominal sacrificial steel requirement of 975 µm per side according to Equation (28), or approximately 1.95 mm added to the diameter or thickness of an element. This is also higher than the current AASHTO requirement of 1.42 mm for a 75-year design life with galva- nized reinforcements. For both cases, calibrated resistance fac- tors of approximately 0.35 were computed considering use of the simplified or coherent gravity methods. Design efficiency factors for the case of plain steel reinforcements are less than half of those realized for the case of galvanized steel reinforce- ments (efficiency factor 0.2 compared to 0.5). Use of materials for reinforced fill that do not meet current AASHTO requirements is not recommended. However, data exist in the literature from several sites in which special studies were conducted to access the condition and remaining service life at sites where marginal quality fill was used, often inadver- tently. These data are used to assess the conservatism inherent to existing models for computing nominal sacrificial steel requirements and to calibrate resistance factors for the yield limit state. Marginal quality fills are described herein as hav- ing 5 < pH < 10 and 1,000 Ω-cm < ρmin < 3,000 Ω-cm. Only the use of galvanized reinforcements and design lives (tf) less than 50 years are considered with respect to use of marginal fills. The following equation is recommended for computing nominal sacrificial steel requirements: Application of Equation (29) presumes that zinc coating with a minimum required thickness of 86 µm per side will be X side t yrs yr side f μ μm m⎛⎝⎜ ⎞⎠⎟ = −( )× ⎛ ⎝⎜ ⎞ ⎠⎟10 28 2( 9) X side yr side t yr μ μm m⎛⎝⎜ ⎞⎠⎟ = × ( )13 28( ) X side yr side t yr μ μm m⎛⎝⎜ ⎞⎠⎟ = × ( )80 270 8. ( )

consumed within 10 years, and the base steel will be consumed at a rate of 28 µm/yr per side thereafter. Although the zinc life is relatively short, the main purpose of the zinc is to mitigate the development of macrocells and promote more uniform corrosion. For tf of 50 years, the nominal sacrificial steel requirement according to Equation (29) is 2.24 mm (i.e., X = 1120 µm/side). If Equation (29) is the basis for computing the nominal sacrificial steel requirements, a resistance factor of 0.30 is recommended for LRFD and the yield limit state. Type II reinforcements include rock bolts and ground anchors. Due to the fact that these reinforcements are often surrounded by grout or protected via a single (Class II) or dou- ble (Class I) corrosion protection system, only the portions of the assembly that are exposed and in contact with the surrounding environment are vulnerable to corrosion. Due to fundamental differences in the materials, installation details, and workmanship applied to rock bolts versus ground anchors, the reliability inherent to service life estimates of these installa- tions is described separately. For rock-bolt installations, the most vulnerable locations are behind the bearing plate, which often includes a gap, or other locations where the reinforcement is not completely surrounded by grout or is otherwise left unprotected. Metal loss is a concern at these locations, and previous design guid- ance has not directly considered metal loss in the considera- tion of service life. However, resistance to pullout, rather than rupture resistance, often controls the lock-off load for rock- bolt installations; therefore the resistance of the reinforce- ment section may not be fully mobilized at any time during the service life. Chapter 3 includes an example from a site where pullout resistance controls the lock-off loads and data on metal loss of Type II reinforcements, available from the lit- erature, are used to assess the resistance bias at the end of the design life. The example demonstrates that for the selected site, metal loss is not a significant concern for service lives less than 75 years. A resistance factor for the rupture limit state and a 100-year design life is computed as 0.55, corresponding to a target reliability index of 3.12 and pf ≈ 0.001. This exam- ple demonstrates how the statistics generated from metal loss measurements can be used to calibrate resistance factors for LRFD of Type II reinforcements. However, the computed resistance factor is sensitive to the lock-off load, and depends on the sizes and steel types of the reinforcements. If lock-off loads are controlled by rupture (rather than pull- out resistance), then sacrificial steel requirements must be con- sidered explicitly. Equation (27) is recommended to compute nominal sacrificial steel requirements. Resistance factors can then be calibrated using the statistics describing metal loss measurements from Type II reinforcements cited in Chapter 3. Ground anchor systems that use high-strength steels with GUTS in excess of 150 ksi are vulnerable to other forms of cor- rosion that may include hydrogen embrittlement and SCC. Use of Equation (27) does not apply to degradation from hydrogen embrittlement or SCC. In these cases, service lives are severely compromised if the reinforcements are exposed and in contact with the surrounding soil or rock mass, particularly for environments that are acidic or high in chlorides. Therefore, high-strength steel reinforcements must be isolated from the environment via a corrosion protection system. In these cases, a double corrosion protection system (Class I) is recommended and the service life is governed by the quality and detailing inherent to the double corrosion protection system. Data were collected during this research from one site with high-strength steel reinforcements and a double corrosion protection system. These data indicate that the corrosion protection system at this site is intact and performing well; a grease-filled trumpet head is included with the anchor head assembly. Recommendations for Asset Management Asset management is an important issue facing highway operations, and forecasting the needs for maintenance, retrofit, or replacement of existing facilities is an important component of transportation asset management (TAM). Earth-retaining structures should be included in a TAM program along with pavements, bridges, ancillary structures, and so on, to help ensure optimal usage of limited available funding (FHWA, 2008). Properly defining the existing inventory and the devel- opment of a performance database are important components of asset management. Relatively rapid, nonintrusive, and non- destructive test techniques are needed to collect data necessary for corrosion monitoring and condition assessment of earth- retaining structures. Results from condition assessment and corrosion monitoring indicate when, or if, accelerated corro- sion is occurring and can help transportation agencies decide on the most appropriate course of action when subsurface con- ditions are unfavorable and service life is uncertain. Agencies can also use these data to evaluate the variance associated with the performance of an inventory; this is valuable information for those with an interest in making reliability-based decisions. This report describes the framework of a performance database useful for asset management, test techniques and protocols that are being employed to collect performance data for earth rein- forcements, data interpretation, and preliminary information available from data that has been collected to date. Performance Data The performance database includes thousands of mea- surements of element conditions and corrosion rates from more than 150 sites distributed throughout the United States and Europe. The large sample domain allows evaluation of sample statistics, distributions of element conditions and 45

46 corrosion rates, and corresponding probability and reliabil- ity analyses. These issues are related to reliability of metal loss modeling, quantification of the effect of construction practice on performance, and understanding the cost bene- fits of using different materials. All of these are important components of asset management. For example, the data- base can be used to • Study the mean and variance of corrosion rates for data sets grouped according to different climate, site conditions, and reinforced fill conditions; • Quantify performance for marginal reinforced fills; and • Evaluate the performance of different materials (e.g., steel vs. zinc, other forms of metallization, and the use of poly- meric coatings). These applications will lead to better estimates of service life and can help to quantity the benefits of selecting higher quality backfill for construction or the costs associated with using mar- ginal quality fill. Performance data can also facilitate evaluation of alternative materials, including use of galvanized versus plain steel, or other corrosion protection measures that may include epoxy or polymer coatings. Practices that may lead to poor per- formance may also be identified and quantified, including the impact that poorly maintained drainage inlets may have on service life, or the effect of fill contamination during service. The database needs to be continuously updated and should include performance data from sites where good practice has been followed as well as from sites with questionable conditions. Maintenance, Rehabilitation, and Replacement Issues that can address future needs for maintenance, reha- bilitation, retrofit, or replacement include • Spatial variations of element condition and corrosion rate (e.g., top vs. bottom of wall), • Special areas that may deserve increased maintenance (e.g., in proximity to drainage inlets), and • Effects of different climates, use of deicing agents, and so forth. Improved knowledge of spatial variations and special prob- lems can lead to improved allocation of resources. For exam- ple, in some cases, extended service life may be best achieved by retrofitting areas surrounding drainage inlets, or the ben- efits of improved maintenance of drainage inlets may be real- ized in terms of increased service life. In areas where deicing salts are used, corrosion monitoring can demonstrate the need to maintain pavements, improve drainage, or install and maintain impervious barriers. Update Experience with Different Reinforced Fills An example of the experience gained from collecting and analyzing data relates to the use of reinforced fills that may or may not meet AASHTO specifications for electrochemical properties. The database was divided into two primary groups including data from reinforced fill conforming to AASHTO criteria and from reinforced fill not conforming to AASHTO criteria. The AASHTO corrosion model was applied to esti- mate reinforcement corrosion rates and to compare them to measured corrosion rates. The observations below were made from the existing database. These observations may be updated as more data become available. • For reinforced fills conforming to AASHTO criteria, the AASHTO corrosion model overestimates steel corrosion rates for 98% of the data. It should be noted that most of the data in this group are associated with reinforced fills that meet AASHTO requirements by a wide margin. • For reinforced fills conforming to AASHTO criteria, marine environments have minor to no effect on measured cor- rosion rates of galvanized reinforcements, but marine environments accelerate corrosion rates of plain steel reinforcements. • For reinforced fills that do not satisfy AASHTO criteria, marine environments are associated with relatively high corrosion rates. • Reinforced fills that do not meet AASHTO criteria (i.e., soil resistivity values ρ < 3,000 Ω-cm and pH values < 5) can significantly affect steel corrosion rates, which tend to dra- matically increase beyond rates estimated by the AASHTO corrosion model. • Based on available data, organics content, chlorides, sul- fates, and relatively high values of pH have much less effect on measured corrosion than do relatively low resistivity and low pH. • Review of the latest research information confirms the safety of the electrochemical requirements for fill and associated metal loss rates in the current AASHTO standards. Recommendations for Future Research NCHRP Project 24-28 assessed and improved the predictive capabilities of existing computational models for corrosion potential, metal loss, and service life of metal-reinforced systems used in retaining walls and highway cuts and fills. Methodology was developed that incorporates the improved predictive mod- els into an LRFD approach for the design of metal-reinforced systems. Recommended additions and revisions were prepared to incorporate the improved models and methodology in the AASHTO (2009) LRFD Bridge Design Specifications.

Additional research is recommended to further validate the predictive models for corrosion potential, metal loss, and service life of metal-reinforced systems from Project 24-28. The validation will require measurements at an independent set of field sites across the United States, supplemented with results from laboratory measurements. Field sites should include rock-bolt installations and MSE walls. Testing of the metal-reinforced systems at each field site will require both (1) NDT techniques (e.g., ultrasonic testing, sonic echo, impulse response, and electrochemical testing) and (2) direct measurement after exhumation of in situ reinforcements or installation of dummy elements by state DOTs or other agen- cies. The direct measurements will validate the NDT methods as well as the predictive models based upon these methods. Type I Reinforcements The following objectives apply to Type I reinforcements and the need for data to validate the performance models and address limitations inherent to the database compiled as part of NCHRP Project 24-28: • Evaluate effect of marginal fills on performance and service life, • Study bias inherent to LPR measurements of corrosion rate, and • Assess the corrosion rate of steel after zinc has been con- sumed from galvanized elements. To accomplish these objectives: 1. Develop a relationship between fill resistance (measured as part of the LPR test) and fill resistivity. This relationship depends upon the geometries of the test electrodes, elec- trode spacing, fill characteristics, and method of measur- ing fill resistance. If this relationship can be established, it will then be possible to develop much better correlations between measured corrosion rates and the electrochemi- cal properties of the fill. 2. Compare measurements of corrosion rates with direct observations of metal loss from reinforcements that have been exhumed subsequent to LPR measurements. These data will be very useful, particularly to relate loss of tensile strength to LPR measurements, as loss of strength is often from metal loss that has occurred over localized areas. 3. Collect data from sites with galvanized reinforcements where base steel is corroding after zinc has been consumed. Different assumptions regarding corrosion of the base steel have a significant impact on resistance factor calibrations for LRFD. Further research could also be pursued to further demon- strate applications of performance data. In particular, if the need for rehabilitation or retrofit is identified, cost-effective methods for rehabilitation and retrofit should be selected. Guidance will target improvements to areas where they are most needed, which may be close to sources of fill contam- ination or otherwise based on the spatial distribution of corrosion or loss of service observed at a particular site. A well-maintained and populated database will facilitate devel- opment of site-specific guidance based on the experiences that have been documented from other sites. Guidance needs to be developed for sampling and evaluating backfill and per- formance of in-service reinforcements. The recommended sampling is likely a hybrid between stratified and random sampling. Representative sample locations are stratified with respect to the vertical direction and stations are randomly located along the length of the wall. A cluster of measure- ments at each sample point should be averaged to render the corrosion rate at that location. The sensitivity of designs generated from the recommen- dations provided in the report need to be evaluated. Typical designs should be executed using recommended resistance factors for LRFD considering use of galvanized or plain steel reinforcements and various fill materials (i.e., high quality, good, and marginal). In this way the impact of these factors on design parameters, including the size and spacing of rein- forcements, can be evaluated. Type II Reinforcements The following objectives apply to Type II reinforcements and the need to (1) substantiate use of electrochemical test techniques for corrosion monitoring and integrity testing of corrosion protection systems and (2) extract more information on existing conditions from the results of dynamic testing (e.g., sonic echo and impulse response). More data from sites with double corrosion protection systems are required to generate statistics describing the reliability of these installations. • Study application of corrosion monitoring with LPR tech- niques. Seek measurements and observations that can char- acterize the surface area in contact with the surrounding earth material, and knowledge of the influence of grout and other components of the corrosion protection system. • Refine data analysis techniques for dynamic tests (wave propagation techniques). Verify results obtained with these techniques, and evaluate the limitations of these NDTs for probing earth reinforcements. • Collect more data to document the performance of corro- sion protection systems. 47