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1 SUMMARY Study Purpose NCHRP Project 21-11 aims to assess and improve the current methods for characterizing the steel corrosion potential of earthen materials. Electrochemical properties of earthen materials such as electrical resistivity, pH, salt concentrations, and organic contents are commonly used to characterize the corrosion potential of buried metal elements that are in direct contact with the surrounding soil. AASHTO test standards, adopted in the early 1990s, are among the most common practices in the United States to determine the electrochemical properties of earthen materials. However, these methods do not consider the vastly different characteristics of earthen materials used in infrastructure construction, nor do they distinguish issues inherent to particular applications. AASHTO T-288 (2016) requires a portion of the fill finer than the No. 10 sieve to determine the resistivity of specimens compacted within a relatively small soil box. This gradation affects the conductivity of the soil by altering the soil texture and may lead to resistivity results which are different than the original soil (i.e., resistivity of a fine grain soil is generally lower than a coarse grain soil). Hence, the limitations associated with the current AASHTO test standards must be recognized and alternatives need to be considered to address these limitations. The main product from this research is a test protocol for sampling, testing, and charactering the steel corrosion potential of earthen materials. The specific research objectives are: (1) to identify, sample, and characterize representative earthen materials; (2) to determine the effects of different electrochemical measurement techniques and different specimen preparation procedures (e.g., aggregate size) on the measured electrochemical properties of the soil and leachate samples; (3) to establish links between laboratory and field measurements for proper interpretation of laboratory test results; and (4) to develop a test protocol and corresponding characterization of corrosion potential that more accurately reflects the corrosivity of earthen materials compared to the conventional methods. Approach We performed this study in three phases. Phase I (Literature review) includes a search and review of existing information, synthesis of national and international practices, the identification and prioritization of knowledge gaps, and preparation of a draft protocol for sampling, testing, and evaluating steel corrosivity of earthen materials. Phase II (Evaluation) includes a systematic study of alternative test methods for measuring electrochemical properties in the laboratory, algorithms for assessment of the corrosion potential of earthen materials, and further development of the protocol. Phase III (Validations), evaluates the practicality of the proposed protocol and alternative laboratory test methods (investigated in Phase II). We studied alternative laboratory test procedures for measuring electrochemical properties of soils applied to a sampling domain incorporating a broad range of materials (mostly those commonly used in MSE wall constructions). The data included characterization of different sample sources (e.g., maximum particle size and gradation) along with the measurements of geochemical and electrochemical properties of the samples including resistivity, pH, chloride, and sulfate contents.
2 We documented performance data (i.e., corrosion rates) of plain and galvanized steel specimens, embedded in 19 of these sources. While electrochemical test results were used to characterize the corrosion potential of each source, the performance data were used to correlate these characterizations to the corrosion rates. Alternatives to AASHTO tests to measure soil resistivity include ASTM G-187 (2018), Tex-129- E (1999), Tex-129-M, ASTM WK24261, SCT 143 (2008) and Tex-620-M. Resistivity test methods are of two general types that include (1) measurements of voltage drop in response to an applied current passing through the compacted soil sample in a soil box (galvanostatic test), or (2) conductivity measurements on aqueous solutions extracted from soil samples (leachates). Other differences between the tests are in terms of sample treatments that may include sieving, air drying, heating, methods of mixing, time of settling/curing, and filtering. Test methods ASTM WK24261 and Tex-129-M are new test methods (under development) that are currently being considered for implementation by ASTM and TxDOT. Tex-129-E (1999) is the current TxDOT standard that may be superseded by Tex-129-M. In general, tests for pH and salt content are performed on extracts obtained after diluting a small soil sample with deionized water. Specific details of specimen preparation such as the size of the soil sample, fraction of soil included in the sample (e.g., portion finer than sieve No. 10), dilution ratio, soaking period, method and time of mixing, and filtration of solids vary amongst the different test procedures. These factors can significantly affect the obtained electrochemical results. Alternatives to AASHTO tests for measurement of pH include ASTM D4972 (2019), SCT 143 (2008), Tex-128-E (1999), Tex-620-M, a procedure developed by CorrTest and described as part of NCHRP Project 21-06 (2009), and a new test method which is currently under consideration by ASTM Committee D18. The latter two test methods and Tex-620-M are applicable to measure the pH for relatively coarse-grained materials, while the other tests are more applicable to measure pH for finer materials. Alternatives to AASHTO tests to measure soluble salt contents include Tex-620-J (2005) and Tex- 620-M. In addition, ASTM D4327 (2017) provides a more robust technique which uses ion- exchange chromatography (IC) to determine soluble salt contents including chloride and sulfate ion content as well as other anions that are more applicable to drinking and wastewaters. This technique can be applied to the samples that are prepared in accordance with AASHTO T-290 (2016) and AASHTO T-291 (2013). In addition, the sulfate and chloride contents can be determined from the same specimen when using IC. We compared results obtained from different test procedures in terms of (a) precision/repeatability of the results, (b) bias of the results compared to those obtained from the current AASHTO tests, and (c) trends we identified from the data. We made these comparisons to check whether any of the procedures perform better than others in terms of repeatability, precision, and bias. We also identify cases where the results from different test methods are similar, and where the results are different. For cases where differences in results are observed, we performed further analyses to identify the best result for characterizing the steel corrosion potential.
3 Precision and Bias Resistivity â¢ The best precisions are observed from Tex-620-J (2005), Tex-129-E (1999), Tex-129-M, and Tex-620-M with repeatability ranging from 6.8% to 7.6%. â¢ The precisions from the other test methods are less with repeatability ranging between 9.1% and 13.2%. Results from ASTM WK24261, with a repeatability of 13.2%, have the poorest repeatability compared to the other test methods for resistivity. â¢ Results from tests performed on leachates extracted from soil-water mixtures (Tex-620-J (2005), Tex-620-M, and SCT 143 (2008)) have repeatability comparable to what is achieved from the soil box tests (Tex-129-M, Tex-129-E (1999), ASTM G-187 (2018), AASHTO T-288 (2016), and ASTM WK24261). â¢ Results obtained from the Texas modified procedures for the measurement of resistivity/conductivity (Tex-129-M and Tex-620-M) have improved repeatability compared to those obtained from AASHTO T-288 (2016), SCT 143 (2008), ASTM G-187 (2018), and ASTM WK24261. â¢ The mean bias is approximately one for Tex-129-E (1999) with a coefficient of variation (COV) of 22%. The results from Tex-129-E (1999) and AASHTO T-288 (2016) were close because of the similarities between these test methods. The two tests differ in terms of the sieve size used to separate the specimen from the sample (No. 8 vs. No. 10) and the 12- hour curing period prescribed by AASHTO T-288 (2016) for the first moisture increment. For the sand and gravel materials that were tested in this study, these differences did not have a significant impact on the results. Other test procedures show mean bias values that are noticeably higher than 1.00 (as high as 5.22 in Tex-620-M) with COVs generally higher than 50%. â¢ The mean bias is greater for test procedures that involve coarser gradations (i.e., ASTM G- 187 (2018), Tex-129-M, and ASTM WK24261). ASTM G-187 (2018) includes particle sizes up to 1/4â, but Tex-129-M and ASTM WK 24261 both include particle sizes up to 1 3/4â. This is reflected in the mean bias values, which are higher for results obtained from ASTM WK 24261 and Tex-129-M compared to those from ASTM G-187 (2018). The bias from ASTM WK24261 is higher than that from Tex-129-M due to the manner in which measurements are taken after the sample is drained for ASTM WK24261. â¢ We grouped the data from each test method according to the fineness of the samples (fine sand, coarse sand, and gravel). We observed that, as the coarseness of the sample increases, the mean bias and the COV increase. Considering materials characterized as fine sand, and results from the soil-box tests, the average mean bias is close to one with a relatively low COV (average COV = 8%). On the other hand, the biases for coarse sand and gravel are 1.6 and 3.1, respectively considering results from Tex-129-M which includes coarse particles within the test specimen. Also, the COVbias increase incrementally for materials characterized as coarse sands and gravels, where COVs in excess of 30% are observed. â¢ The mean biases for the tests on compacted soil specimens and tests performed with leachates are 2.13 and 2.95, respectively. We expected the observed differences because we cannot include the effects from tortuosity using conductivity measurements from leachate.
4 â¢ Biases from tests on the leachates are all greater than one, even for samples that are separated into finer components (e.g., for Tex-620-J (2005) the sample is separated on a No. 40 sieve). This is due to the different dilution ratios and methods of mixing and extracting leachates used in different leaching tests compared to soil box tests. Salt Content â¢ Precision/repeatability is similar among test methods for measurements of salt contents. â¢ Higher salt contents are measured via Tex-620-J (2005) compared to the AASHTO tests; salt contents from Tex-620-M are generally lower than other test methods. â¢ The best correlations between salt contents and resistivity measurements are obtained from the AASHTO test standards. pH â¢ Measurements of pH from Tex-620-M are less repeatable compared to measurements from other test methods investigated in this study. â¢ In general, Tex-620-M renders pH values that are higher compared to those obtained from the other test methods investigated in the study. â¢ Results from NCHRP 21-06 (2009) are more repeatable and do not have a significant bias compared to AASHTO T-289 (2018). Correlation with CR We used the coefficient of determination, R2, between the corrosion rate and resistivity measurements as an index to rank the accuracy of the obtained results from each of the resistivity tests that were included in the test program. Resistivity is often considered to be an indicator of corrosivity as this single parameter is correlated with numerous factors that play roles in corrosion reactions, including salt and moisture contents (King 1977; Romanoff 1957). The data set for the regression analysis included measurements from 19 sample sources incorporating 28 measurements of corrosion rates. Observed corrosion rates included 18 data points from galvanized steel specimens and 10 data points from plain steel specimens. Measurements presented herein are the maximum observed from each site/source. We use the maximums to consider the durability of the most vulnerable elements. The data set included the in-situ measurements of corrosion rates and corrosion rates measured from laboratory tests. We conclude that results from Tex-129-M apply well to materials with less than approximately 22% passing the No.10 sieve. For materials with more than 22 percent passing a No.10 sieve, AASHTO T-288 is appropriate for the measurement of resistivity. We used these observations to develop the proposed protocol presented in Appendix A. Recommended Protocol We incorporated recommendations into the proposed protocol that are based upon results from our analyses of the data collected in Phase II. We summarized the proposed protocol in the form of a flowchart shown in Figure 3-14. The characteristics of the materials are described in terms of
5 grading number (GN) and the percentage passing the No. 10 sieve. In general, the proposed protocol describes the application of the current AASHTO test series for samples with GN > 3, or if the percent passing the No.10 sieve is greater than 25%. Otherwise, if the GN < 3, and the percent passing the No.10 sieve is less than 25%, the Texas modified procedures are recommended (i.e., Tex-129-M and Tex-620-M). Implementation Study During Phase III of NCHRP Project 21-11, we cooperated with selected transportation agencies whereby the recommended protocol was implemented as a âshadow specification.â The data included characterization of different sample sources (e.g., maximum particle size and gradation) along with the measurements of geochemical and electrochemical properties of the samples including resistivity, pH, chloride, and sulfate contents (i.e., the total salt content). The Wenner 4- probe technique (according to ASTM G57 (2012) and Wenner (1915)) was used in the field for measurement of electrical resistivity. Representative samples of fill were collected from the site or from the source. The samples were subject to electrochemical tests in the laboratory using Texas modified and AASHTO test procedures. We also evaluated the practicality and implementation of the suggested protocol through the interaction with laboratories engaged in electrochemical testing, and suppliers/owners in different states. Data from the implementation study allowed us to compare the results obtained by different labs using the same test standards. We also performed in-situ measurements of resistivity for comparison with laboratory measurements that were obtained from specimens with the same gradation as the material placed in the field and measured at similar moisture contents. The information and the data we have obtained from the implementation of Phase III of NCHRP Project 21-11 show the effects of reinforcements on the measurements of fill resistivity, and the benefits of orientating the lines for the Wenner 4-probe test perpendicular to the reinforcements. These data also showed the variability that is inherent to the measurements and the effects of reinforcements on these variations. We have also observed a correspondence between laboratory and field measurements of resistivity. The experience and data collected from implementing the proposed protocol on active construction projects indicate that the modified test procedures and the test protocol for improved characterization of corrosion potential are easier to implement compared to 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.
6 Organization of the Report NCHRP Project 21-11 aims to assess and improve the current methods for characterizing the steel corrosion potential of earthen materials. The main goal of this research is to develop test protocols to promote characterization of corrosion potential for earthen materials consistent with in-service conditions and observations of field performance. This report describes the results, data collection and interpretation, conclusions and recommendations from NCHRP Project 21-11. Chapter 1 is a summary of the advantages and shortcomings associated with the current test methods and practices for assessing the corrosivity of earthen materials (Phase I). Chapter 2 summarizes the research approaches used to achieve the main goal of the study. That chapter also summarizes the objectives and research tasks included in each phase of the project. In Chapter 3, we describe the sample domain used in the laboratory program and the results obtained from the laboratory measurements. That chapter includes discussions and comparisons between the measurements of electrochemical properties obtained from different test methods with a focus on development of recommendations and protocols for sampling and testing earthen materials and characterizing steel corrosion potential. In Chapter 4, we describe cooperation with owners/contactors and implementation of our recommendations and proposed protocol as a âshadow specificationâ on active construction projects. This is followed by the comparison of the laboratory test results obtained from different labs and samples from the same source. We also propose improvements to current practices for in-situ measurements of resistivity and comparisons between results from laboratory and field measurements. Chapter 5 includes conclusions and recommendations for future research.