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75 Appendix A. Test Protocols â Recommended Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials 1. Introduction 1.1. This protocol is focused on characterizing the steel corrosion potential of earthen materials. 1.2. Corrosion performance of black and galvanized steel elements embedded in earthen materials are considered. 1.3. Corrosion potential considers the electrochemical properties of earthen materials, site conditions, metal type, and applications that may include mechanically stabilized earth (MSE), soil nails (SN), steel (and reinforced concrete) piles and culverts and drainage pipes. 1.4. Earthen materials incorporate soils and aggregates that may include crushed rock or native soils that are excavated and processed to meet specified gradations. 1.5. Earthen materials may be native (in situ), existing fills or newly placed fills. Fills may be from native materials excavated and used âas-isâ or may be native materials that are excavated and processed. 1.6. Earthen materials are characterized in terms of maximum particle size, gradation, fines content, plasticity index, and mineralogy. 1.7. Native soils are earthen materials that are in their natural state and have not been disturbed by excavation and processed to satisfy a specific gradation. 1.8. Newly placed materials are fills that will be, or are being, placed during construction. 1.9. Existing materials are earthen materials that are in place prior to construction and may include old fills or native (in situ) materials. 1.10. Although many test techniques and standards exist for the measurement of electrochemical properties of earthen materials (including resistivity, pH, and chloride and sulfate contents), not all test methods are suitable for all types of earthen materials and applications. Test methods for measurement of electrochemical properties must be selected based on the nature and character of the earthen material. In addition, practicality of each test considering the site conditions and accuracy of the results should be considered before selecting any test methods/standards. 1.11. Site conditions include topography, details of constructed facilities, the location of the water table, drainage conditions, climate (temperature, humidity, etc.). 1.12. The protocol describes different applications (MSE, SN, piles, culverts, and drainage pipe) and the factors that may affect steel corrosion in these applications. 1.13. The protocol describes sampling procedures and requirements for characterizing corrosion potential of earthen materials. 1.14. The protocol refers to the most common test standards for the measurement of electrochemical properties including resistivity, pH, and concentration of chloride, and sulfate. 1.15. The protocol describes different schemes for characterizing the corrosion potential of earthen materials.
76 2. Referenced Documents 2.1. AASHTO Standards T-27 Sieve Analysis of Fine and Coarse Aggregates; T-88 Particle Size Analysis of Soils; T-89 Determining the Liquid Limit of Soils; T-90 Determining the Plastic Limit and Plasticity Index of Soils; T-288 Determining Minimum Laboratory Soil Resistivity; T-289 Determining pH of Soil for Use in Corrosion Testing; T-290 Determining Water-Soluble Sulfate Ion Content in Soil; T-291 Determining Water-Soluble Chloride Ion Content in Soil; R026 Standard Practice for Assessment of Corrosion of Steel Piling for Non-Marine Applications; and LRFD Bridge Design Specifications 2.2. ASTM Standards D2419 Standard Test Method for Sand Equivalent Value of Soils and Fine Aggregate; D4327 Standard Test Method for Anions in Water by Suppressed Ion Chromatography; G57 Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four- Electrode Method; and D4220 Standard Practices for Preserving and Transporting Soil Samples 2.3. TXDOT Tex-129-M Test Procedure for Measuring the Resistivity of Soils and Aggregates; and Tex-620-M Test Procedure for Determining the Conductivity, pH, Sulfate Content and Chloride Content of Soil and Coarse Aggregate 2.4. NCHRP NCHRP Report 477 Appendix A, âRecommended Practice for Evaluating Metal Tensioned Systems Used in Geotechnical Applications.â NCHRP Report 474, âService Life of Culverts.â
77 2.5. FHWA/NHI FHWA-NHI-09-087, âCorrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Soil Slopes.â November 2009, 155 pp. FHWA-NHI-16-009, âGeotechnical Engineering Circular No. 12 â Design and Construction of Driven Pile Foundations,â September 2016, 563 pp. FHWA-NHI-14-007, âGeotechnical Engineering Circular No. 7 â Soil Nail Walls Reference Manual,â February 2015, 425 pp. French National Project Clouterre (1991), âRecommendations Clouterre 1991 (English Translation),â Federal Highway Administration, Washington, D.C., 321pp. Samtani, N. C. and Nowatzki, E. A. (2006), âHollow-Core Soil Nails State-of-the-Practice,â FHWA Report Unassigned, Office of Bridge Technology, Federal Highway Administration, Washington. D.C., 51pp. Samtani, N. C. and Nowatzki, E. A. (2010), âHollow Bar Soil Nails Review of Corrosion Factors and Mitigation Practice,â Report FHWA-CFL/TD-10-002, Federal Highway Administration, Central Federal Lands Highway Division, Lakewood, CO., 82pp. 2.6. Post-Tensioning Institute (PTI) Recommendations for Prestressed Rock and Soil Anchors 3. Identify Application 3.1. Mechanically stabilized earth walls (MSEW) are fill walls (i.e. bottom-up construction) consisting of alternating layers of compacted fill and soil reinforcement elements fixed to a wall facing forming a composite soil structure. Soil reinforcements may be metallic or geosynthetic and fill sources include mined sands and gravels, or processed aggregates, that should not be corrosive when metallic reinforcements are used. Fills commonly used in MSE construction have textures that range between fine silty sand and coarse aggregate, are relatively homogenous and often free draining. Fill materials are compacted during construction at near optimum moisture contents and maximum density as determined by laboratory and in situ testing, or until sufficient interlock is achieved as indicated by proof rolling in the case of coarse aggregate fills. 3.2. Soil nails (SN) are closely spaced reinforcements that are drilled and grouted into existing earthen materials via top-down construction to form a composite structure that includes a shotcrete facing. Soil nails may be solid steel bars or hollow bar soil nails (HBSN). Solid bar soil nails (SBSN) may include Class I or II corrosion protection (PTI) depending upon the corrosivity of the existing soils and whether the installation is considered permanent or temporary. HBSNs employ sacrificial steel and are surrounded by grout. However, because of the brittleness of the grout and the inevitable formation of microcracks in tension, and the possibility of soil intrusion into the hole, the degree to which the grout protects the HBSN is uncertain. Thus, the electrochemical properties of the existing soils are important factors affecting the service-life design of the SBSN and HBSN.
78 3.3. Piles are deep foundation elements that may include steel piles that are hammered or predrilled into the existing soils. They may penetrate through different soil layers (and the water table) with different electrochemical properties. Where grades have been raised there may be industrial fill near the surface. Corrosion problems have been observed near the surface where industrial fills have been placed and the groundwater table fluctuates near the interface between the fill and native ground. In addition, different soil layers have different oxygen contents (above and below the water table), which provides the electromotive force required for corrosion. 3.4. Culverts/drainage pipes are conduits that collect or direct stormwater, provide drainage to embankments and roadways, and allow embankments and roadways to cross streams or creek beds. They are installed within existing or newly placed soils and may be plastic, reinforced concrete, or metal sections. Although bedding and structural backfill are usually placed during construction, often the surrounding existing soil determines the corrosion levels in culverts/drainage pipes. For culverts, corrosion may occur on the both the water, and soil sides. 4. Relevant Characteristics of Earthen Materials 4.1. Nature â Best practices vary depending upon the nature of earthen materials. Sampling and laboratory testing are recommended from potential sources of newly placed materials. Field tests (in-place testing) along with sampling and laboratory testing are recommended for existing materials and during construction with newly placed materials. 4.2. Unconventional fills â May incorporate lightweight or recycled materials. These may include recycled concrete, slag, cinder ash, or lightweight materials such as foamed glass, expanded clay, expanded slate or expanded shale. There is not much known about the factors affecting the behavior of unconventional fills and additional studies are needed to investigate and adapt test procedures for the measurement of electrochemical properties and for characterizing corrosion potential of these materials. This protocol does not address testing and characterizing corrosion potential for unconventional fills. 4.3. Size/gradation â The texture of earthen materials may be coarse or fine. Also, depending on the gradation, the material may be free draining or not free draining. For laboratory testing, the maximum particle size and the gradation of the sample affect the sample size requirements and the appropriate testing conditions. For tests on leachate, the maximum particle size and the gradation affect the required dilution ratios and mixing procedure. Best practices are to select test standards including sample treatments and specimen preparations that are best suited to the texture of the sample. 4.4. Minerology â Carbonates, pyritic minerals, siliceous minerals, and clay may be present and affect the performance of metal elements and the characterization of corrosion potential. Descriptions of the parent rock types and knowledge of the corresponding mineralogy are useful to discern the presence of these constituents. Quarries identify their parent rock types, which can also be identified from bedrock geology maps, provided the location of the source is known. Best practices are to characterize mineralogy based on the knowledge of the source. 4.4. Anaerobic conditions and the presence of sulfate-reducing bacteria may contribute to microbial induced corrosion (MIC). For example, soils underlying marsh, bogs, and swamps or exposed to marine environments are highly corrosive for this reason. Best practices are to identify
79 conditions where there will be a lack of oxygen within materials that incorporate organics and sulfate and to consider the possibility of MIC, i.e. recognize that corrosion can occur in the absence of oxygen. We recommend in situ measurements of the oxygen reduction potential (ORD), when possible for susceptible sites, to identify if aerobic or anaerobic conditions prevail in situ. 4.5 Contaminants â Earthen materials may become contaminated from polluted groundwater, runoff from fertilized fields, infiltration of deicing salts, or contaminated stormwater during the service life of the metal elements. Best practices for characterizing corrosion potential should consider that properties of earthen materials may be altered over time due to the presence of contaminants. 5. Requirements for Sampling and Testing 5.1. The number of samples. Sampling requirements depend on the homogeneity of the source and the precision of the test procedure. Recommended sampling intervals for MSE fill are described in FHWA-NHI-09-087, Table 2-2, âRecommended Sampling Protocol for Electrochemical Testing of MSE Wall Fill.â These recommendations may also apply to other applications involving newly placed materials. 5.1.1. For existing soils see the in-situ sampling techniques described in Section 5.5. 5.1.2. Best practices for sampling âin placeâ materials will be identified. This will include the application of the Wenner 4-probe test as described in ASTM G57 with limited sampling for laboratory testing and comparisons between in situ and laboratory test results. 5.2. Sample preparations. Sample treatments and specimen preparations including separations into different sized fractions, additions and mixing with water, and curing period vary with respect to different test procedures. Best practices depend upon the nature of the material as described in Section 6. 5.3. Storage. Samples may be stored prior to testing. Best practices for storing samples including storage temperatures, storage times, and requirements for storage containers are specified in the test standards. 5.4. Temperature. Temperatures of the sample maintained during the test may have an effect on the test results. Best practices are described in the test standards for making temperature measurements and correcting measurements to a given reference temperature. 5.5. Samples may be obtained from the source, stockpiles, or after placement. Existing materials may be sampled in situ with a split spoon sampler, from auger cuttings, test pits, or other means of retrieving soil samples from the subsurface. 5.5.1. Soil or groundwater samples should be retrieved that are representative of materials surrounding the metal elements of interest. Several representative soil samples may need to be obtained if conditions vary along the lengths of the elements. 5.5.2. Care should be taken during sampling to avoid contamination of the soil samples and the loss of moisture during storage and transportation to the laboratory. The intent, precautions, and procedures of ASTM D 4220 (Group B) are applicable to this protocol.
80 5.5.3. Collect soil samples from depths of at least 3 ft. below the water table if not to the end of the element unless organics are present. If organics are present, MIC is a concern, however, MIC has not been observed from depths below 50 feet. Therefore, if MIC is a concern, retrieve samples from depths down to 50 feet if not to the end of the element. 6. Methods of Testing Figure A-1, attached to the end of this protocol, is a flowchart describing the characterization of earthen materials and the selection of the appropriate test procedures and standards for measurements of electrochemical properties as described in Sections 6.1 through 6.4. 6.1. Material Characterization. Identify the physical characteristics of the material including gradation (AASHTO T-27 or T-88), and Atterberg limits (AASHTO T-89 and T-90). 6.1.1. Determine the grading number (GN) and the percent passing a number 10 sieve, where GN = 1/100 *(PP1 in + PP3/4 in + PP3/8 in + PP#4 + PP#10+PP#40+PP#200) and PP signifies percent passing. The GN is often used to characterize the coarseness of aggregates used for highway construction. 6.1.2. For materials with more than 15% passing the number 10 sieve, determine the Atterberg limits, which are useful to limit the clay content of the finer fraction (PI < 6). Best practices also include the use of the sand equivalent test (SE) to evaluate if the fine particles included in an earthen material are clay-like or inert (e.g. SE > 12). 6.2. pH â Materials should be sampled and tested in the laboratory via AASHTO T-289 or Tex- 620-M to determine the pH. Details of storage, transport, temperature control, and sample treatments need to be followed as described by these procedures. 6.2.1. Measure pH in accordance with AASHTO T-289 if the GN >3, or if the percent passing the No. 10 Sieve is greater than 25%. 6.2.2. Measure pH in accordance with Tex-620-M if the GN < 3, and if the percent passing a No. 10 Sieve is less than 25%. 6.3. Chloride and sulfate - Ion exchange chromatography (IC) (e.g. ASTM D4327) is the preferred test technique for measuring chloride and sulfate ion concentrations. This test has the advantage that measurements of sulfate and chloride concentrations are performed with a single sample and the method includes a means to assess interferences that may affect the measurements. Measurements made on the aliquot are relatively rapid compared to the methods employed for AASHTO T-290 and T-291. ASTM D4327 only includes testing of anions in water. Other test procedures need to be followed to extract soluble chloride and sulfate from the surfaces of soil particles and preparation of the aliquot for testing. 6.3.1. AASHTO T-290 and T-291 describe procedures for preparing an extract and then ASTM D4327 may be followed for measurements of sulfate and chloride ion contents from the aliquot. The best practice is to express the salt contents with respect to the total weight of the soil sample, rather than the total volume of the aliquot.
81 6.3.2. Tex-620-M describes procedures for leaching extractions and also specifies chloride and sulfate measurements via IC. 6.3.3. Measure salt contents (chloride and sulfate) in accordance with AASHTO T-290 and T-291 if the GN >3, or if the percent passing the No. 10 Sieve is greater than 25%. 6.3.4. Measure salt contents in accordance with Tex-620-M if the GN < 3, and if the percent passing a No. 10 Sieve is less than 25%. 6.4. Resistivity â Both laboratory and field measurements are recommended. Moisture, compaction conditions, and treatments can be controlled for laboratory testing. Field tests provide measurements under in situ conditions and a larger volume of soil is included from field measurements compared to laboratory tests. Field tests are useful to characterize performance at different times (seasons) during service. 6.4.1. Newly placed material â Laboratory tests 220.127.116.11. AASHTO T 288 applies to sands, fine sands, silty sands, clayey sands, silt, clay, silty clay, and clayey silts. The method describes testing at increasing moisture contents until reaching a âminimumâ resistivity. Testing soils at moisture contents beyond saturation may relate to conditions for the waterside of culverts but is not relevant to other applications (e.g. MSE fills). Thus, in many applications the moisture content at 100% saturation should be considered the end- point for AASHTO T288. Density and degree of saturation are important parameters relating to soil resistivity, but compaction is not a variable for the current test standard. Dry density should be determined based on measurements of water content and the weight of material in the box after compaction. Dry density and moisture content are used to verify when the degree of saturation has reached 100%. 18.104.22.168. Tex-129-M measures the resistivity of soil materials in a two-electrode soil box, which is sized according to the nominal maximum particle size of the material being tested. Thus, this test applies to soils with any gradation, up to a maximum particle size of 1 3/4 inches. Increments of moisture are applied during the test, similar to AASHTO T 288, however, particle sizes up to 1 3/4 inches are included in the specimen. The material is not separated prior to testing and the entire gradation is included in the test. 22.214.171.124. Measure resistivity in accordance with AASHTO T 288 if the GN >3 or if the percent passing the No. 10 Sieve is greater than 25%. 126.96.36.199. Measure resistivity in accordance with Tex-129-M if the GN < 3, and if the percent passing a No. 10 Sieve is less than 25%. 6.4.2. In situ field test 188.8.131.52. ASTM G57 Wenner/Schlumberger array is recommended for in situ testing of materials surrounding culvert locations, piles, and soil nails at different depths due to its simplicity and efficiency. This test may also be applied to measure resistivity of MSE wall fill during or after construction. Best practice is to compare the results from the Wenner test with those obtained from the laboratory tests (soil boxes) at the same moisture contents. In situ resistivity measurements should be collected using lines orientated perpendicular to the longitudinal axis of the reinforcements and should include approximately five tests per lift using electrode spacing that are approximately equal to the lift thickness.
82 7. Screening/Characterization The goal of sampling and testing earthen materials described in this protocol is for proper characterization of corrosion potential. A number of schemes exist for screening and characterizing corrosion potential of earthen materials, and these are often developed for specific applications that may include aspects of the installations, site conditions and electrochemical properties. These schemes may (1) apply statistics to the data to forecast corrosivity, (2) set threshold values for electrochemical measurements to identify corrosive conditions, or (3) consider a number of variables that incorporate site conditions and electrochemical properties of earthen materials in the assessment of corrosion potential. 7.1. Percentiles â Define the threshold for the percentile value (e.g. 90th percentile) for electrochemical parameters such as resistivity. This approach is applied by the US Bureau of Land Reclamation to assess the level of corrosion protection needed for pipeline installations. 7.2. Parameter thresholds â Limits on electrochemical parameters including resistivity, pH, chloride and sulfate contents. If any one of the parameters is not within prescribed thresholds the material is considered corrosive. The AASHTO LRFD Bridge Design Specifications specify thresholds and limits for resistivity, pH, and salt contents to identify MSE wall fills that will not be corrosive towards galvanized steel reinforcements. 7.3. Rating/multi-variant â e.g. German Gas and Water Works Standard DVGW GW9. The German Gas and Water Works Engineersâ Association Standard (DVGW GW9) is one of the earliest corrosion assessment methods and has been applied to pipeline construction in Europe. A number of categories are included in the assessment that incorporates physical and electrochemical properties of the earthen material (soil) and site conditions that include groundwater levels and the presence of industrial fills. Points/marks are assigned for each category depending on values for the relevant parameters and the marks are summed to render an overall score. The score is used to assess corrosivity with lower (more negative) scores correlated with increasing levels of corrosivity. The scheme considers the benefits from the presence of carbonates on the corrosion behavior of buried metals, which is unique compared to other schemes.