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Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials (2021)

Chapter: Chapter 5 - Conclusions and Needs for Future Research

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Page 64
Suggested Citation:"Chapter 5 - Conclusions and Needs for Future Research." National Academies of Sciences, Engineering, and Medicine. 2021. Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials. Washington, DC: The National Academies Press. doi: 10.17226/26076.
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Page 65
Suggested Citation:"Chapter 5 - Conclusions and Needs for Future Research." National Academies of Sciences, Engineering, and Medicine. 2021. Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials. Washington, DC: The National Academies Press. doi: 10.17226/26076.
×
Page 65
Page 66
Suggested Citation:"Chapter 5 - Conclusions and Needs for Future Research." National Academies of Sciences, Engineering, and Medicine. 2021. Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials. Washington, DC: The National Academies Press. doi: 10.17226/26076.
×
Page 66

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64 5.1 Main Conclusions This report proposes a protocol describing best practices for sampling, testing, and charac- terizing the steel corrosion potential of earthen materials. The protocol incorporates alterna- tives to the current AASHTO test standards for measuring electrochemical properties, including resistivity, pH, and chloride and sulfate ion content. The protocol was developed from a review of current test procedures and practices, a program of laboratory testing that included a broad range of materials and test alternatives, and observations of corrosion rates from galvanized and plain steel elements subject to corrosion. The current AASHTO test procedures are limited to testing materials that incorporate a sig- nificant amount of material passing the No. 10 sieve. Modified test standards considered as alternatives to the AASHTO tests include Tex-129-M and Tex-620-M for measurements of resis- tivity and salt content, respectively. Unlike the AASHTO tests, these alternatives incorporate larger-sized particles within the test specimens. Tex-129-M and Tex-620-M were selected from a suite of candidates on the basis of the precision and repeatability of the results observed from the laboratory test program, the utility of the test results, and the observed performance of plain and galvanized steel elements subjected to corrosion within these materials. Conceptually, there is a threshold (i.e., PP#10) beyond which the portion of the material finer than the No. 10 sieve controls the performance and the ability of corrosion currents to flow through the materials. (Corrosion current is the electrical current produced in the medium— in this case, soil—during the corrosion process.) The finer particles have higher salt content and lower resistivity as compared with the measurements obtained from the bulk samples. If there is sufficient material passing the No. 10 sieve, then the corrosion currents will be concentrated along paths where the finer materials are concentrated (i.e., current follows the path of least resis- tance). Hence, corrosivity is affected more by the properties of the finer portions of the material than of the bulk of the material that includes both fine and coarse portions. The research team concluded that results from Tex-129-M apply well to materials with less than approximately 22% of particles passing the No. 10 sieve. For materials with more than 22% passing the No. 10 sieve, AASHTO T 288 is appropriate for the measurement of resistivity. These observations were used to develop the proposed protocol presented in the appendix, in which the 22% threshold is rounded up to 25% passing the No. 10 sieve. In general, the proposed protocol describes the application of the current AASHTO test series for samples with a grading number (GN) > 3 or when the percentage passing the No. 10 sieve is greater than 25%. Otherwise, if GN < 3 and the percentage passing the No. 10 sieve is less than 25%, the Texas modified procedures are recommended (i.e., Tex-129-M and Tex-620-M). The grading number is included with the screening to restrict the use of Tex-129-M to coarse- textured samples with a relatively high gravel content. C H A P T E R 5 Conclusions and Needs for Future Research

Conclusions and Needs for Future Research 65 The materials included in this study were grouped into clusters on the basis of ranges of resistivity and according to corrosion indices determined from the German method for charac- terizing corrosivity (DVGW GW 9). The German method is a multivariate approach for classi- fying corrosivity that considers the electrochemical properties of the relevant earthen materials, site conditions, and the presence of carbonates or industrial by-products. Ranges of resistivity and corrosion indices that corresponded to noncorrosive, mildly corrosive, and moderately to severely corrosive conditions were defined. Corrosion rates measured from plain and galva- nized steel elements were compared with the rates cited in the literature corresponding to the given corrosivity descriptions. Relatively good comparisons were observed when test results were applied according to the proposed protocol. The research team cooperated with selected transportation agencies to implement the recommended protocol as a shadow specification. The data included characterization of dif- ferent sample sources (e.g., maximum particle size and gradation) along with the measure- ments of geochemical and electrochemical properties of the samples, including resistivity, pH, and chloride and sulfate content. A program of in situ testing that included the Wenner four-probe technique [according to ASTM G57 and Wenner (1915)] was used in the field for measurement of electrical resistivity. This was followed by collecting the representative material samples from the site and from the source to perform electrochemical tests in the laboratory according to modified and current AASHTO test procedures. The practicality and implementation of the suggested protocol were evaluated through interaction with lab- oratories engaged in electrochemical testing and suppliers/owners in different states, including the University of Texas at El Paso, McMahon & Mann, the New York State DOT, and the Texas DOT (AASHTO tests only). The experience and data collected from implementing the proposed protocol on active con- struction projects indicate that the modified test procedures and the test protocol for improved characterization of corrosion potential are easy to implement as compared with the traditional methods. The owners/contractors were able to perform the modified test procedures and, with few exceptions, could acquire the equipment needed to perform these tests. Recommendations as to which test procedures should be applied to the characterizations of corrosivity were found to be clear and easy to implement. 5.2 Recommendations for Future Research 1. The test program described in this study included materials that may be described as sands and gravels that did not incorporate more than 5% passing the No. 200 sieve. This sample domain was relevant to practices in MSE wall construction; however, it was not representa- tive of earthen materials encountered in other applications, which may include installations of piles, culverts, and drainage pipes. Also, in some states, materials with more than 15% of particles passing the No. 200 sieve are common in MSE wall construction. Thus, more data are needed to evaluate the characterization of steel corrosion potential for earthen materials with significant fine content (material passing the No. 200 sieve, i.e., silt and clay fractions). Additionally, more data are needed to distinguish between the effects of silty fines and those of clayey fines on corrosion potential. In some cases, the endpoints for resistivity tests may need to be modified, depending on the nature and the application of the material. For some materials and applications, the proper test endpoint is when the material is saturated, and in other cases, the endpoint needs to be when the lowest (minimum) resistivity is reached. The latter definition for endpoint means that the specimen may be in a slurry state at the end of the test—that is, not a compacted specimen. In this study the endpoint for resistivity testing was at the point of saturation.

66 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials 2. Further research is needed to consider the use of nonconventional materials in construction that may include industrial by-products, recycled materials, or lightweight fills. The com- position and chemical and electrochemical properties of these materials are significantly different from soils. Hence, special test considerations and sample preparations need to be developed for proper measurement of electrochemical properties. 3. This study used observations of the performance of metal elements embedded within earthen materials from laboratory measurements of corrosion rates in addition to in situ measurements from in-service soil reinforcements. Laboratory tests are an efficient means of obtaining data from a number of different fill types. More data are needed to expand the performance database and to refine the characterizations of corrosivity and capabilities for service life modeling. The research team suggests that an extensive program of laboratory tests verified with field measurements be undertaken to expand the performance database. 4. More data from field testing using the Wenner four-probe technique are necessary. Results from field testing may be useful in deciding whether collecting more samples from the site and laboratory testing is necessary. If the results from field testing match the laboratory measurements from Tex-129-M at similar levels of moisture content, then the materials that were sampled and tested prior to construction are similar to those that were delivered to the site and placed during construction. If the results do not match, then more samples should be selected from the source and the site for further laboratory testing and confirmation that materials that are not corrosive are being placed at the site. At this point, only data from four sites are available, and more data and experiences are needed to validate this approach. 5. Additional implementation activities are needed to promote the recommended protocol, transfer information about the sampling and testing described in the protocol, and inter- act with AASHTO committees and the state DOTs to consider the modified practices. The following activities for promoting the implementation of the proposed protocol are suggested: – Combine all of the required information, including the protocol and the specifications, into one modified test standard. This may include modifications and enhancements to the current AASHTO standards. – Prepare course materials, guidebooks, and laboratory demonstrations for training. – Conduct interlab tests to obtain more data on the precision and repeatability of the tests included in the proposed protocol. – Attend and make presentations at AASHTO committee and FHWA regional meetings to engage the state DOTs and transfer knowledge that was gained in pursuit of NCHRP Project 21-11 and the proposed protocol. – Engage the construction industry, including contractors, who often select fills for con- struction, and suppliers of MSE wall systems who promote the use of good construc- tion practices and specifications. This can be accomplished via interaction with various industry groups, including the Association for Mechanically Stabilized Earth (AMSE) and the Associated General Contractors of America (AGC). – Engage FHWA in assisting in deployment of these practices. This may include updates to existing documents published by FHWA that describe corrosion and degradation of soil reinforcements, culverts, and so forth and sampling and testing of fill materials—for example, FHWA-NHI-09-087 (Elias et al. 2009).

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There is a need to identify new or improved laboratory and field test methods to measure the electrochemical properties of earthen materials surrounding buried or embedded steel elements.

The TRB National Cooperative Highway Research Program's NCHRP Research Report 958: Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials presents a protocol for evaluating the corrosion potential of earthen materials in contact with steel highway structures.

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