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Page 1
Suggested Citation:"Chapter 1 - Background." 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:"Chapter 1 - Background." 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.
×
Page 2
Page 3
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 3
Page 4
Suggested Citation:"Chapter 1 - Background." 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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Page 5
Suggested Citation:"Chapter 1 - Background." 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:"Chapter 1 - Background." 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:"Chapter 1 - Background." 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:"Chapter 1 - Background." 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.
×
Page 8
Page 9
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 9
Page 10
Suggested Citation:"Chapter 1 - Background." 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.
×
Page 10
Page 11
Suggested Citation:"Chapter 1 - Background." 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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1Transportation agencies use a variety of metal-reinforced systems in geotechnical applications, including soil and rock reinforcements, ground anchors, and tiebacks. These systems support retaining walls, bridge abutments, approaches, and highway embankments, and they also stabilize roadway cuts and fills. Corrosion is known to have an impact on service life, and engineers, faced with the task of allocating budgets to reha- bilitate aging facilities, need reliable techniques to estimate that remaining service life. Service life estimates for new systems need to be evaluated, and consideration of metal loss in the design needs to be consistent with the reliability-based approach adopted in the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications. NCHRP Project 24-28 addresses these needs by developing a database to document the performance of earth reinforcement systems, and by perform- ing the statistical and reliability analyses necessary to consider service life within the context of reliability-based design. The objectives of NCHRP Project 24-28 are to (1) assess and improve the predictive capabilities of existing computational models for corrosion potential, metal loss, and service life of metal-reinforced systems used in geotechnical engineering applications; (2) develop methodology that incorporates the improved predictive models into an LRFD approach for the design of metal-reinforced systems; and (3) recommend addi- tions and revisions to the AASHTO LRFD specifications to incorporate the improved models and methodology. Current design specifications (AASHTO, 2009) incorporate metal loss models, but these models have limited application with respect to reinforcement type and fill conditions. Results from this study serve to broaden the recommendations for metal loss modeling, and describe effects of fill quality and reinforce- ment type on performance and service life. Earth Reinforcements For the purpose of this study, metal-reinforced systems are broadly categorized into two types. Type I reinforcements are passive elements used in the construction of metallically reinforced earth structures [i.e., mechanically stabilized earth (MSE)] that may consist of steel strips, welded wire fabric, wire mesh, or soil nails. Type I elements are not prestressed, and load is transferred to the elements as the structure deforms during construction, and throughout its service life. Type II reinforcements are active elements that are prestressed dur- ing installation and include ground anchors (strands and bars) and rock bolts. Due to the application of prestress, the loads in active systems are controlled and involve more certainty com- pared to passive systems. In general, Type II reinforcements consist of relatively high-strength steel and a higher level of corrosion protection compared to Type I reinforcements. A significant difference between element types is that Type I elements are often designed by including sacrificial steel to account for metal loss due to corrosion, whereas Type II ele- ments do not include sacrificial material and the durability of the corrosion protection system controls the design life. Details of Type I Reinforcements Most steel reinforcements for MSE structures are hot-rolled steel strips, welded wire grids, or bar-mat grids manufactured from cold-drawn wire as depicted in Figure 1. Standard sizes of reinforcements, steel grades, and details of galvanization have evolved, and current practices differ from those employed prior to approximately 1978. Type I reinforcements are man- ufactured from mild steel and, currently, steel strips are man- ufactured from ASTM A-572, Grade 65 steel. Prior to 1978 steel strips were manufactured from Grade 36 steel, described by ASTM A-446. Grids are manufactured from Grade 80 cold- drawn wire in accordance with ASTM A-82, and deformed welded wire is sometimes used, as described by ASTM A-496. Bar mats are often configured with between two and five lon- gitudinal wires, whereas welded wire mesh may have 10 or more longitudinal wires per unit. Most often the reinforcements include hot-dip galvanizing for corrosion protection that is applied in accordance with ASTM A-123 for wire- or strip-type reinforcements. Minimum C H A P T E R 1 Background

requirements for thickness of zinc coating depend on the thick- ness and size of the reinforcements. AASHTO specifications require a minimum of 86 µm per side for MSE reinforcements, as described in ASTM A-123. However, for reinforcements installed prior to 1978 and galvanized in accordance with ASTM A-525 or A-641, the specified initial thickness of zinc coating was less than the current specifications of 86 µm and ranged between 17 µm and 30 µm. The hot-dip process provides good coverage of zinc along the surface, but the distribution of thickness is difficult to control. Thus, the mean thickness of the zinc coating neces- sarily exceeds the AASHTO minimum requirements of 86 µm. This ensures a low probable occurrence of a spot with less than the minimum requirement. From the standpoint of reliability analyses it is important to recognize that zinc thickness is a variable. Sagues et al. (1998) and Rossi (1996) report measurements of zinc coating thickness from frequent intervals along the lengths of a limited number of strip-type reinforcement samples. These data reflect a mean zinc coat- ing thickness of approximately 150 µm. However, these data are very limited and more data describing the variation of the thickness of zinc coating are needed to obtain a reasonable distribution of measurements. Soil nails are steel bars with diameters ranging between 1 and 1.5 inches that are inserted into a 4-inch to 12-inch diameter drill hole and surrounded by grout. Often Grade 60 deformed, reinforcing steel bars are employed for soil nails but high strength prestressing steel bars (e.g., Grade 150) are sometimes used. Soil nails may be galvanized or epoxy coated and sometimes are encapsulated similar to the corrosion pro- tection systems described for Type II reinforcements in the next section. However, there are several new developments in soil nails that may not be fully encapsulated, if at all, includ- ing self-drilling and self-grouted nails, screwed-in nails, and dynamically inserted nails (i.e., inserted using a nail gun or sonic method). 2 (a) (b) (c) Welded Wire Mesh Ribbed Strips Figure 1. Examples of metallic reinforcements used in MSE construction: (a) bar-mat grid reinforcements, (b) hot-rolled steel strip reinforcements, and (c) welded-wire grid reinforcements.

Details of Type II Reinforcements Type II reinforcements include ground anchors and rock bolts. Key features of these systems, summarized in Table 1, are described in this section. More complete details of these rein- forcements, including descriptions of components, materials, installation details, and performance issues can be found in USACOE (1980), International Federation for Prestressing (FIP) (1986), NCHRP Web Document 27 (D’Appolonia et al., 2001), Kendorski (2003), and Sabatini et al. (1999). Tensioned elements of the system include bar and strand components. The steel grade and level of prestress employed in these systems are relevant to the type of corrosion problems that may occur, and prediction of service life. Bar elements are available in a variety of steel grades ranging from Grade 60 to 160. Strand elements are manufactured from Grade 250 and 270 high-strength steel and generally consist of seven wire strands with six wires wrapped around a seventh wire called the “king wire.” Wire tension systems using the button head anchorage of BBRV (Birkinmaier, Brandistini, Ros, and Vogt) and Prescon have also been used, but are not as popular as strands. These systems use a set of parallel wires, rather than strands, as reinforcing elements. Ground anchors include an anchored or “bonded” zone and a free length or “unbonded” zone. The bonded zone is anchored to the soil or rock with cement grout. Current guidance documents [Post Tensioning Institute (PTI), 2004; Sabatini et al., 1999] recommend incorporating corrosion protection measures into the design of ground anchors. Cor- rosion protection measures include the use of coatings, protec- tive sheaths, grouting, encapsulation, and electrical isolation. Use of portland cement-based grout provides limited corro- sion protection as a barrier, and by fostering a passive film layer due to its high alkalinity. Recent installations employ Class I or Class II corrosion pro- tection systems as recommended by PTI (2004). For Class I protection the anchor is encapsulated (often referred to as double corrosion protection) and, for Class II, the anchor is protected by grout (often referred to as single corrosion pro- tection). The free lengths of the anchors are protected by grease and plastic sheaths, and a trumpet head assembly surrounds the reinforcements behind the bearing plate. Double corrosion protection is recommended for ground anchors in aggressive ground conditions and permanent installations. Products on the market today offer systems that comply with the current standards. However, many of the older installations do not incorporate details that meet today’s standards, or may have been installed without any corrosion protection beyond the passivation of the grouted portion of the tensioned elements. Rock bolts are installed with either mechanical anchorages (e.g., expansion shell, split wedge), or are grouted into rock using portland cement or resin grout. The anchorage may either be concentrated near the end point of a mechanical device or by the short length of grout near the end of the bolt; or the bolt may be fully grouted with the pullout resistance distributed along the length of the bonded zone. Older style rock bolts with mechanical anchorages may have no corrosion protection. Portland cement or resin grouted rock bolts are surrounded by grout, but the bolts heads are often not encapsulated. There is also the possibility of voids along the grouted length. Rock bolt installations may also be similar to ground anchors with a free length and a bonded zone, but trumpet head assemblies are not always installed, leaving the area behind the head of the rock bolt exposed. Durability and Performance Issues for Earth Reinforcements Durability of earth reinforcements is controlled by backfill characteristics, site conditions, climate, steel type (galvanized or not), and details of project construction and in-service operations. Weatherby (1982), FIP (1986), Briaud et al. (1998), D’Appolonia et al. (2001), Withiam et al. (2002), and Elias et al. (2009) describe factors that contribute to corrosion poten- tial of earth reinforcements and measurement of the relevant electrochemical parameters for soils and groundwater. In general, “minimum” resistivity (ρmin), pH, chemical compo- sition including the presence of organics, porosity, and ground- water level are the factors that most affect the corrosiveness of the underground environment. Generally, ground conditions 3 Table 1. Summary of Type II reinforcements. Type of Metal Tensioned Systems Tendon Type Anchorage Type Corrosion Protection Ground anchors Strands or bars Cement grout in bonded zone More recent permanent installations use Class I or Class II Protection (PTI, 2004); older systems may have no protection other than grout cover. Rock bolts Usually bars, but could be strands Mechanical, resin grout, or cement grout Epoxy coating, galvanized, grout cover, older installations may have none

are considered aggressive if one or more of the following con- ditions are detected (PTI, 2004; Elias et al., 2009): • pH < 4.5 for steel; or • 4.5 > pH > 10 for galvanized elements; • ρmin < 2000 Ω-cm; • presence of sulfides, sulfates, or chlorides; and • presence of organics. Metal loss and service life models are correlated with under- ground conditions, particularly with respect to electrochem- ical properties of soil and groundwater. The National Bureau of Standards (NBS) commissioned a study to observe metal loss from steel and galvanized specimens that were buried under a variety of soil conditions for more than 50 years. Based on the results from the NBS study, Romanoff (1957) proposed the following power law to predict rates of corrosion of buried metal elements: where x is loss of thickness per side or loss of radius, k and n are constants, and t is time (years). Equation (1) applies to uniform-type corrosion and may also consider localized- or pitting-types of corrosion, but does not consider more complex forms of corrosion, including hydrogen embrittlement or stress corrosion cracking (SCC) as described by Fontana (1986), FIP (1986), and Sabatini et al. (1999). Equation (1) reflects observations that the corrosion rate is generally higher during the first few years and attenuates with respect to time (i.e., n is < 1). This is due to the steel sur- face becoming “passivated” from stable corrosion by-products that adhere to the surface and formation of a passive film layer. Equation (1) serves as the basis for several models used to esti- mate service life and associated sacrificial steel requirements. These models differ in terms of the data sets (e.g., fill condi- tions) used to regress the model parameters, and the time frame over which metal loss is considered. Note that in Equa- tion (1) “x” describes metal loss that may include zinc and steel for galvanized elements. However, other metal loss equations presented in this report use “X” to denote loss of steel sub- sequent to zinc deletion for galvanized elements. Metal loss models for Type I and Type II reinforcements are discussed separately with due consideration given to differences in site conditions, construction details, and metal type (e.g., use of galvanized reinforcements for Type 1 reinforcements). Type I Reinforcements Based on research conducted over the past several decades (e.g., King, 1978; Darbin et al., 1988; Elias, 1990) values for the x kt n= ( )1 constants k and n are identified depending on metal type (e.g., galvanized or plain steel) and for fill conditions representative of MSE construction. Linearized versions of Equation (1) have been adopted as a conservative approach to extrapolate obser- vations of metal loss over limited time frames and for design recommendations (Rehm, 1980; Jackura et al., 1987; Elias, 1990; AASHTO, 2009). Darbin et al. (1988) and Elias (1990) proposed equations, having the same form as Equation (1), to estimate steel loss for plain steel and galvanized elements, respectively. These models are developed using measurements of corrosion from elements buried in fill representative of MSE construction. The following models apply to galvanized and plain steel rein- forcements, respectively: For galvanized elements: For plain steel elements: where X is loss of steel (base metal) in units of µm, and tf is service life in years. For Equation (2) loss of base steel occurs subsequent to depletion of the zinc coating, and zi is the ini- tial zinc thickness. Equation (2) is applicable to the range of fill conditions representative of MSE wall construction that exhibit ρmin greater than 1000 Ω-cm. Data reviewed for Equa- tion (3) are based on the NBS data set for plain steel and include a wider range of fill conditions. The service life of earth reinforcements is related to the remaining tensile capacity and not necessarily the maximum pit depth. This is because pit penetrations have a limited impact on the overall remaining cross section. Rather than measuring and modeling pit depths, metal loss models and measurements of corrosion rates for earth reinforcements are averaged over the surface area of the reinforcement. Thus, metal loss is idealized as uniformly distributed over the surface. A factor of 2 is commonly applied to this uni- form corrosion rate to consider the actual loss of tensile strength capacity (Elias, 1990; Jackura et al., 1987); in other words, the loss of tensile strength is twice that anticipated based on the average loss of section. Although the factor of 2 is often taken as a constant, Smith et al. (1996) describes how the local factor may vary with respect to reinforcement shape. Equations (2) and (3) include a factor of 2 to consider the maximum metal loss and associated loss of tensile strength. X yr t fμ μ m m( ) = ×80 30 8. ( ) if then m m t z X yr tf i> ⎛⎝⎜ ⎞⎠⎟ ( ) = ⎛ ⎝⎜ ⎞ ⎠⎟ ×25 50 1 54. μ μ f i f i yr z t z X 0 65 1 54 2 25 . . ( ) − × ( ) ≤ ⎛⎝⎜ ⎞⎠⎟ μm if then μm( ) = 0 2( ) 4

Equation (2) considers that loss of zinc is uniform with x = 25t0.65, but a factor of 2 is applied to the steel corrosion rate to consider the maximum metal loss after zinc is con- sumed as X = 50t0.65. Although corrosion rates for both galvanized and plain steel clearly vary exponentially with respect to time, a num- ber of models (including the AASHTO model) approximate loss of steel using linear extrapolation for the purpose of design. These models assume that the rate of zinc consump- tion is higher in the first few years and then levels off to a steady but significantly lower rate. Once the galvanized zinc coating is depleted, it is assumed that the base carbon steel corrodes at the carbon steel rate. This is a conservative assump- tion that does not consider that the insoluble by-product of zinc corrosion continues to protect the underlying steel (Rehm, 1980). Calibration of LRFD resistance factors for galvanized reinforcements assumes that the steel cross section is not con- sumed before the zinc coating, which serves as the sacrificial anode protecting the base steel. Since the zinc layers do not con- tribute to the tensile strength of the reinforcements, strength loss is also delayed until the zinc is consumed, and loss of steel section is described according to Equation (4). In general the thickness of steel, X, consumed per side over the design life, tf, may be computed as where C is the time for zinc depletion which is computed based on the initial zinc thickness, zi, the initial corrosion rate for zinc, rz1, the subsequent zinc corrosion rate, rz2, and the duration for which rz1 prevails (t1–usually taken as 2 to 3 years). The corrosion rate of the base steel sub- sequent to zinc depletion is rs. C t z r t r i z z = + − ×( )⎛ ⎝⎜ ⎞ ⎠⎟1 1 12 , X t yrs C yrs r yr f sμ μ m m( ) = ( ) − ( )( )×. . ( )4 Rehm (1980), Jackura et al. (1987) and Elias (1990) propose models for estimating metal loss of galvanized reinforcements in fill conditions applicable to MSE construction. Details of these other models are described in Appendix A and are referred to as the Stuttgart or Caltrans Interim models. These models consider fill conditions that are more severe relative to the potential for corrosion compared to the AASHTO require- ments, and are sometimes, but not always, associated with correspondingly higher corrosion rates. Table 2 is a summary of the different fill conditions, model parameters and cor- responding estimates of zinc life, and estimates of steel loss considering a 75-year service life. Considering a service life of 75 years and an initial zinc thickness zi, equal to 86 µm per side, the steel loss computed with the Darbin equation [Equation (2)] is 655 µm per side. For a 75-year design life, and zi equal to 86 µm, the AASHTO, Stuttgart-high salt, and Caltrans Interim-select models yield estimates of steel loss close to that computed with the Darbin model. Differences between these models include the data sets that are used to regress model parameters (i.e., corrosion rates). This demonstrates that the current AASHTO model uses corrosion rates that are applicable to fill conditions that are more severe relative to those allowed by the specifications; in other words, the AASHTO model is conservative. For fill conditions that are considered marginal by AASHTO stan- dards, due to higher salts contents (models below the double line in Table 2), considerably higher steel losses are estimated. A complete comparison of steel losses computed with the models above the double line in Table 2 as a function of ser- vice life is presented in Figure 2. Comparisons with the Darbin model shown in Figure 2 imply that the factor of 2 described by Elias (1990) is implicit in the piecewise linear models from Stuttgart, AASHTO, and Caltrans. Table 3 summarizes the AASHTO-recommended metal loss model for design of MSE structures (AASHTO, 2009) and 5 Table 2. Summary of piecewise linear metal loss models for galvanized reinforcements. Electrochemical Parameters Considered 1 Metal Loss Parameters Steel Loss/Side t f = 75 yrs. Model pH ρ mi n Ω -cm Cl - ppm SO 4 ppm r z 1 2 μ m/yr r z 2 μ m/yr r s μ m/yr C 3 yrs. X 4 μ m Stuttgart- mi ldly corrosive 4.5 to 9 >1,000 <20 <50 6 2 9 39 324 Caltrans Interim- select 5 >7 >1,000 <500 <2,000 NA NA 13 20 715 AASHTO- mildly corrosive 5 to 10 >3,000 <100 <200 15 4 12 16 708 Stuttgart- hi gh salt, saturated 4.5 to 9 >1,000 <50 <500 17 2 2 12 20 654 Caltrans Interi m -neutral >7 >1,000 <500 <2,000 NA NA 28 10 1820 Caltrans Interi m -acidic <7 >1,000 <500 <2,000 NA NA 33 10 2145 Caltrans Interi m -corrosive >7 < 1,000 <500 <2,000 NA NA 71 6 4899 1 Electrochem ical parameters considered for design by Caltrans have been updated since the Interim models were proposed in 1987. See Appendix A for details. 2 Applies to the first 2 years except for the Stuttgart high salt model where r z 1 applies to the first 3 years. 3 C is the tim e to zinc deletion (i.e ., initiation of steel loss) assuming z i is 86 μ m. 4X is the steel loss per side for a 75-year service life. 5 Caltrans select fill is clean gravel with less than 25% passing the No. 4 sieve and less than 5% fines.

the corresponding fill material requirements. The fill require- ments are intended to control corrosion potential with fills that are between noncorrosive and “mildly” corrosive. Based on the information in Table 3, the steel loss per side (X) in µm/yr for a given service life, tf, and initial thickness of zinc coating, zi, is computed as The AASHTO model does not give any guidance for corro- sion rates or metal loss modeling of plain steel (i.e., not galva- nized) reinforcements or for fills that do not meet the stringent electrochemical requirements. A significant effort was devoted in this project to documenting the performance of in-service reinforcements and to verifying the reliability of the AASHTO (and other) models used in MSE structure design. The frequency and distribution of samples for assess- ment of electrochemical parameters need to be given careful X yr t yr z yr f iμ μ μμm m m m ( ) = × − − −( ) ⎛ ⎝ ⎜⎜⎜ ⎞ ⎠ ⎟⎟12 2 30 4 ⎟ yr ( )5 consideration. The number of samples required increases when evaluating more aggressive or marginal backfill materi- als, and when more confidence is needed for design (With- iam et al., 2002; Hegazy et al., 2003). Existing data involving frequent sample intervals at sites with poor conditions depict a wide scatter in results (Whiting, 1986; Fishman et al., 2006). For moderate to large sized projects, with fill sources that are expected to be relatively nonaggressive relative to corrosion (i.e., mildly corrosive soils meeting AASHTO criteria), Table 4, taken from Elias et al. (2009), can be used to determine the number of samples that should be taken from each source and evaluated for electrochemical parameters. More samples should be retrieved if marginal quality reinforced fills are being contemplated for construction (not recommended), or when undertaking performance evaluations at sites with poor reinforced fill conditions. In addition to the mean values used for design [i.e., the mean of the minimum resistivity (ρmin) values obtained from each test], the distribution and variabil- ity of the measurements is of significant interest from the standpoint of reliability-based design (LRFD). Table 4 places restrictions on the allowable standard devi- ations (σ) of the resistivity and salt content (see comment 3) measurements. If these standard deviations are exceeded, then the sampling should be repeated. If the standard devia- tion, computed using the total numbers of samples, is still outside the limits of Table 4, then the backfill source should not be used for MSE wall fill. If resistivity less than 3,000 Ω-cm is obtained from any test, obtain additional samples in the vicinity of this sample location to identify if there are specific areas wherein the material is unsuitable. Stockpiles should be sampled from the top, middle, and bottom portions and an excavator with a bucket should be 6 0 200 400 600 800 1000 0 20 40 60 80 100 St ee l L os s pe r s id e (μm ) Time (years) Darbin Caltrans-select AASHTO Stuttgart-salt Figure 2. Comparison of metal loss models. Table 3. AASHTO metal loss model and backfill requirements. Metal Loss Model Backfill Requirements pH 5 to 10 Component type (age) Loss (µm/yr) Minimum resistivity ≥3,000 Ω-cm Zinc (<2 yrs), rz1 15 Chlorides <100 ppm Zinc (>2 yrs), rz2 4 Sulfates <200 ppm Steel (after zinc), rs 12 Organic content <1%

used to remove material from approximately 2 feet beyond the edge of the stockpile. Particular emphasis on sampling needs to be placed at sites where different reinforced fill sources and/or types are being considered, and each source should be sampled as described in Table 4. Differences in the electrochemical properties of the soil fill can adversely affect corrosion rates and contribute to more severe and localized occurrences of metal loss. In instances where more easily compacted (e.g., open-graded) material is placed adjacent to the wall face, significant differences in the soil fill conditions may exist with respect to position along the reinforcements. For cases where reinforcements are not elec- trically isolated (e.g., metallic facing), variations of backfill types along the height of the wall may also have a significant effect on corrosion rates of metallic reinforcements. Type II Reinforcements Since the integrity of the corrosion protection system is known to have a significant effect on service life, condition assessment must focus on obtaining information on the sys- tem’s integrity. Properly installed grease and sheathing, and protection at the anchor head assembly, can provide sub- stantial benefits on service life. Equation (1) and correspond- ing parameters from Table 5 should be applied to those systems where protection is questionable; otherwise corrosion cannot occur. For high strength steel reinforcements, corro- sion processes may also include hydrogen embrittlement or SCC. Equation (1) does not apply to these types of corrosion processes and, for these cases, the end of service is considered to be when the corrosion protection is compromised. Equation (1) is applied to estimate metal loss of Type II reinforcements assuming that attack from the surrounding environment is immediate and unaffected by the presence of a corrosion protection system or grout cover surrounding the reinforcements. Corrosion protection measures include the use of coatings, protective sheaths, passivation with grout, and encapsulation. Thus, the estimated metal loss is applica- ble to unprotected portions of the installation and is a con- servative estimate for portions of the reinforcements that are passivated by grout or otherwise protected from corrosion. The appropriate parameters for use in estimating metal loss are based on the corrosiveness index of the surrounding earth. According to the recommendations described in With- iam et al. (2002), the parameters “k” and “n” for use in Equa- tion (1) are adjusted relative to soil/rockmass conditions as summarized in Table 5. The constant “n” is taken as one for simplicity and considering the relatively short time frame (<20 years) inherent to most of the observations used to develop the table. Ground conditions in Table 5 are described as average, corrosive, or highly corrosive based on electro- chemical characteristics of the surrounding material that may be soil, rock joint infill, or groundwater. Average conditions and corrosive conditions refer to relatively neutral (pH > 5) and ρmin greater than 2,000 Ω-cm, or 700 Ω-cm < ρmin < 2,000 Ω-cm, respectively. Highly corrosive conditions are acidic (pH < 4), 7 Preconstruction DuringConstructionRange of ρmin (Ω-cm) General Description No. Samples σresistivity (Ω-cm) Sample Interval (yd3) Comments >10,000 Crushed rock and gravel, <10% passing No. 10 sieve 1 / 31 NA NA 5,000 to 10,000 Sandy gravel and sands 3 / 61 <2,000 4,000 / 2,0001 <5,000 Silty sands and clayey sand, screenings 5 / 101 <1,000 2,000 / 1,0001 1. pH outside the specified limits is not allowed for any sample. 2. Backfill sources shall be rejected if ρmin measured for any sample is less than 700 Ω-cm, Cl- - > 500 ppm or SO4 > 1,000 ppm. 3. For materials with ρmin < 5,000 Ω-cm, σ for Cl and SO4 shall be less than 100 ppm and 200 ppm, respectively. 1 Number of resistivity tests / number of tests for pH, Cl-, and SO4. Table 4. Recommended sampling protocol for electrochemical testing of MSE wall fill (Elias et al., 2009). Ground Conditions Parameter Average Corrosive HighlyCorrosive k (μm) 35 50 340 n 1.0 1.0 1.0 Table 5. Recommended parameters for service life prediction model for Type II reinforcements (Withiam et al., 2002).

correspond to ρmin < 700 Ω-cm, or have very high chloride content (> 500 ppm). The estimated service life of unprotected rock reinforce- ment systems in moderately aggressive ground conditions is approximately 50 years (Kendorski, 2003), but may be much lower for very aggressive ground conditions, and particularly for high strength steel subject to low pH environments (Withiam et al., 2002). Test Protocol and Measurement Techniques Recommended practices for corrosion monitoring and con- dition assessment of Type I and Type II reinforcements were developed from previous studies including FHWA Demonstra- tion Project 82, Durability/Corrosion of Soil Reinforced Struc- tures (Elias, 1990), and NCHRP Report 477: Recommended Practice for Evaluation of Metal-Tensioned Systems in Geotech- nical Applications, (Withiam et al., 2002). These studies evalu- ated application of test techniques at a number of selected field sites and developed recommended practices for corrosion monitoring and condition assessment. Measurement Techniques Early corrosion monitoring practices involved exhuming and examining samples of reinforcements for evidence of cor- rosion, including loss of cross section. The practice of exhum- ing in-service reinforcements is limited to reinforcements that are accessible and usually near the surface of the structure. However, special inspection elements may be placed and later extracted from various positions along the wall face including the top, middle, and bottom of the wall (Jackura et al., 1987). Corrosion rate may be estimated from weight loss and thick- ness measurements, provided the original thickness or weight and composition (e.g., zinc thickness) of the reinforcements are known. Corrosion rate decays with time (Romanoff, 1957), and a catalog of measurements made at different times is required to assess the rate of metal loss with respect to time. Less invasive techniques employing nondestructive electro- chemical tests such as measurement of half-cell potential and linear polarization resistance (LPR) were implemented for cor- rosion monitoring of MSE walls beginning in the later 1980s (Lawson et al., 1993; Elias, 1990) and are also applied to Type II reinforcements (Withiam et al., 2002). With these tech- niques, a large number of samples are monitored and frequent measurements may be collected. Half-Cell Potential The half-cell potential, Ecorr, is the difference in potential between the metal element and a reference electrode. A copper/ copper sulfate reference electrode (CSE) is commonly used to monitor earth reinforcements. Results from the test can provide a comparison between metallic elements at different locations at the same site and identify the presence of differ- ent metals, for example, zinc or iron. 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. Linear Polarization Resistance LPR measurements are used to observe the instanta- neous corrosion rate. Lawson et al. (1993), Elias (1990, 1997), Berkovitz and Healy (1997), and Elias et al. (2009) describe that application of the LPR technique to MSE reinforcements and application to Type II reinforcements is similar. Polariza- tion resistance is measured from the response to an impressed current and the corrosion rate is computed via the Stern- Geary equation (Stern and Geary, 1957). The surface area of the test element must be known since the Stern-Geary equa- tion is based on the corrosion current density, which is mea- sured in terms of current per unit surface area. The measured resistance, PR′, is actually the sum of the interface and soil resistance (PR′ = PR + Rs) with a correction for soil resistance often necessary (Elias, 1990). Sonic Echo Test Measurements The sonic echo method (impact test) is used for evaluating cracking of grouts, fracture of tendons, and loss of section or loss of prestress for Type II reinforcements (Rodger et al., 1997). The end of the reinforcement is impacted using a ham- mer or air gun, which generates elastic compression waves with relatively low frequency content. The waves are reflected by changes in geometry or conditions along the length of the reinforcement, including the ends of the elements, transitions from free to bonded zones, and irregularities that may be encountered along the length. Gong et al. (2005) and Liao et al. (2008) describe application of the sonic echo test to eval- uate the length and integrity of soil nail installations. Ultrasonic Test Measurements The ultrasonic test method is a good technique for evalu- ating grout condition, fracture of elements, and abrupt changes in the element cross section for Type II reinforce- ments. The method has many of the features of the sonic echo technique, except that the transmitted signal contains rela- tively higher frequencies. An ultrasonic transducer is acoustically coupled to the exposed end of the test element. Grease is used as an acoustic couplant. The time taken for sound pulses, generated at regular 8

intervals, to pass through the specimen and return, is mea- sured. Return pulses may be either from a single reflection at a discontinuity, or from multiple reflections between a dis- continuity and the end of the specimen. The patterns of the received pulses and the arrival times can provide valuable information about the nature of a defect, and of the integrity of the material being tested. Performance Database An important component of this research is to organize and incorporate performance data from earth reinforcements into a database. The database is analyzed to assess the reliability of current models for estimating metal loss and service life. The New York State Department of Transportation (NYSDOT) (Wheeler, 2002b), the Colorado Department of Transportation (CDOT) (Hearn et al., 2004), the Association for Metallically Stabilized Earth (AMSE, 2006), the Kentucky Transportation Research Cabinet (Beckham et al., 2005), the Ohio Department of Transportation (Timmerman, 1990), and the National Park Service (Anderson et al., 2009) have all developed databases for retaining walls. In general, these databases follow a format and protocol consistent with that employed by the FHWA mandated Bridge Management System (Hearn et al., 2004). These databases were considered and used as a basis to develop the framework for the performance database developed as part of NCHRP Project 24-28. The database developed for this proj- ect provides input necessary for statistical analysis of perfor- mance data, reliability analysis, and calibration of resistance factors for reliability-based design (i.e., LRFD). The AMSE has compiled an inventory documenting details of MSE walls constructed in the United States over the past 35 years (AMSE, 2006). The majority of walls con- structed with grid reinforcements serve as retaining walls, but approximately one-third of the walls with strip rein- forcements serve as part of a bridge structure (abutment or wing walls). Approximately half of the walls in the AMSE inventory are located in the western region of the United States, within an arid climate where backfill sources are alka- line. Approximately 80% of the fill materials included in the AMSE database have a pH of between 6.5 and 8 (slightly acidic to slightly alkaline) and ρmin > 10,000 Ω-cm. This is similar to data collected in France [Terre Armée Interna- tional (TAI), 1977] indicating that approximately half of the walls included in the French survey had ρmin > 10,000 Ω-cm and 90% had pH values between 6 and 8.5. Thus, a large por- tion of the inventory is constructed with fill material that meets AASHTO requirements by a wide margin, and may be considered “high quality fill.” Compared to steel grid-type reinforcements, which are used predominantly within the western region of the United States, use of strip reinforcements is more uniformly dis- tributed geographically. Approximately 40% of the walls constructed with strip reinforcements are located in the more temperate southern climates, where soils are normally slightly acidic. Load and Resistance Factor Design (LRFD) LRFD is a reliability-based design method by which loads and resistances are factored such that: where Qni are nominal (i.e., computed) loads from sources that may include earth loads, surcharge loads, impact loads, or live loads; γi is the load factor for the ith load source; Rn is the nominal (i.e., computed) resistance; and φ is the resistance factor and is usually less than 1. Load and resistance factors are applied such that the asso- ciated probability of the load exceeding the resistance is low. The limit state equation corresponding to Equation (6) is where g is a random variable representing the safety margin; R is a random variable representing “measured or actual” resistance; Q is a random variable representing “measured or actual” load; Qi are random variables for “measured or actual” loads from various sources that may include earth loads, sur- charge loads, impact loads, or live loads; and λR and λQi are bias factors defined as the ratio of measured (actual) to nominal (computed) values of resistance and load, respectively. Figure 3 depicts the limit state equation described by Equa- tion (7) and the area beneath the tail to the left of g = 0 is the probability that g < 0 will occur, pf (i.e., pf = P[g⎟ R, Q] < 0). This area is related to the reliability index, β, which is defined as the number of standard deviations between the mean value of g(R, Q) and the origin of the g(R, Q) function. Table 6 describes the relationship between β and pf. In general, β = 0 corresponds to a 50% probability of occur- rence and the probability of occurrence is inversely propor- tional to β. The objective of LRFD is to find values for load and resistance factors, γi and φ, to achieve a target reliability index, βT, corresponding to an acceptable probability of occurrence, pf. g R Q R Q R Qi R n Qi ni, ( )( ) = − = − >∑λ λ 0 7 γ φi ni nQ R≤∑ ( )6 9

Table 6. Relationship between  and pf. Resistance Factors for Design of Earth Reinforcements Reliability-based calibration of the strength reduction fac- tor for LRFD modeling is focused on the design of MSE wall systems, since the AASHTO LRFD specifications for MSE walls include metal loss as an explicit part of the design. Ground anchor systems described in the AASHTO specifica- tions incorporate a Class I corrosion protection system, therefore metal loss is not incorporated into the design calcu- lations. Current AASHTO specifications include resistance factors for the structural resistance of ground anchors that consider variations inherent to steel manufacturing and fab- rication. The value of φ varies depending on steel type as 0.9 for mild steel (ASTM A-615) and 0.8 for high-strength steel tendons (ASTM A-722). The AASHTO specifications do not specifically address design calculations in support of rock-bolt installations. To address this need, service life estimates and example calibrations of resistance factors for rock bolts are also included in this report. The current AASHTO (2009) LRFD Bridge Design Specifica- tions for design of MSE walls include resistance factors for the yield limit state that are calibrated with respect to safety factors that prevailed for the former allowable stress-based design (ASD). Table 7 is a summary of resistance factors for the yield limit state as presented in the current AASHTO specifications. The ASD employed safety factors of 1.8 (i.e., 1/0.55) or 2.1 (i.e., 1/0.48) relative to yield of strip-type reinforcements or grid- type reinforcements, respectively. The higher safety factor for grid reinforcing members corresponds to a lower resistance fac- tor and is intended to ensure that no individual wire is stressed to more than 0.55Fy. This compensates for interior longitudinal elements that carry higher load compared to exterior elements due to load transfer through the transverse members of the bar mat. The safety factor of 2.1, and corresponding resistance fac- tor of 0.65, is appropriate for bar mats with four or more longi- tudinal elements but should be higher for elements with only three longitudinal elements. However, this point is not addressed in the current AASHTO specifications. D’Appolonia (2007) assessed strength reduction factors for the yield limit state via reliability-based calibration, but did not consider metal loss from corrosion as a variable. This project extends these studies to consider variability of metal loss and the impact that this has on computed levels of reliability using existing design methodologies and methods for computing the load transferred to the reinforcements. Calibration of the resistance factors uses load factors from the AASHTO LRFD specifications and calibration methodology recommended by Allen et al. (2005). The resistance factor is calibrated with respect to a target reliability index, βT, (i.e., probability of occurrence), which accounts for the redundancy of the system and load redistribution inherent to the yield limit state. Probability of Occurrence (Exceeding Yield) for Existing Construction Generally, MSE wall systems are prefabricated, resulting in distinct reinforcement and reinforcement spacing. Thus, reinforcement yield resistance is available in discrete incre- ments determined by the distinct size of the reinforcement and reinforcement spacing selected for the project. Reinforce- ment sizes and spacings are selected based on particular design locations, often near the base of the wall; and unless the wall is very tall, these dimensions are held constant throughout. Therefore, yield resistance is not optimized with respect to the yield limit state, and for many reinforcement locations, there is a large disparity between reinforcement loads and resistance. D’Appolonia (2007) studied this case using data that included measurements of reinforcement load that could be compared with the available yield resistance. Essentially, the results reported by D’Appolonia describe the probability of occur- rence for as-built conditions, rather than for a conceptual design for which yield resistance is optimized with respect to the limit state. 10 Figure 3. Statistical model of limit state equation. Reliability Index (β) Probability of occurrence (pf) 2.0 2.275 x 10-2 2.5 6.210 x 10-3 3.0 1.350 x 10-3 3.5 2.326 x 10-4 4.0 3.167 x 10-5 4.5 3.398 x 10-6 5.0 2.867 x 10-7

Results from Monte Carlo simulations of the limit state function and comparison with closed form solutions as reported by D’Appolonia indicate that the probability of occurrence for as-built conditions is very low, corresponding to β > 3.5 and pf < 0.0001. These results are insensitive to metal loss and do not depend on the choice of resistance fac- tor. This leads to the conclusion that reinforcement yield is very unlikely given the as-built conditions of MSE walls, and the yield limit state does not appear to have a significant impact on performance. The D’Appolonia model assumes that the difference between yield resistance and reinforcement load is randomly distrib- uted. In reality this is not the case. For example, the difference may be much smaller for reinforcements located near the base of the wall or other locations that may govern the required yield resistance. Furthermore, for tall walls there may be a number of locations where yield resistance is selected to meet a given load. Thus, locally, the probability of occurrence may be much higher than that predicted by D’Appolonia. Alternatively, this report describes reliability-based cali- bration for resistance factors considering that the yield limit state function is explicitly applied at every reinforcement location. Thus, the potential for overdesign is not directly included in the analysis; however, a target reliability index, βT of 2.3 corresponding to pf = 0.01, is adopted considering the large redundancy inherent to the system (Allen et al., 2005). Considering as-built conditions, the resistance factors computed by this technique are conservative, although they are in the range of those incorporated into AASHTO (2009) as shown in Table 7. 11 Table 7. Resistance factors for yield resistance for MSE walls with metallic reinforcement and connectors from Table 11.5.6-1, AASHTO (2009). 1 Apply to gross cross section less sacrificial area. For sections with holes, reduce gross area in accordance with AASHTO (2009) Article 6.8.3 and apply to net section less sacrificial area. 2 Apply to grid reinforcements connected to rigid facing element, for example, a concrete panel or block. For grid reinforcements connected to a flexible facing mat or that are continuous with the facing mat, use the resistance factor for strip reinforcements. Reinforcement Type Loading Condition Resistance Factor Static loading 0.75 Strip reinforcements1 Combined static/earthquake loading 1.00 Static loading 0.65 Grid reinforcements1,2 Combined static/earthquake loading 0.85

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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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