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1 NCHRP Project 21-11 aimed to assess and improve the current methods for character- izing the steel corrosion potential of earthen materials. The main goal of this research was to develop a test protocol to promote characterization of corrosion potential for earthen materials consistent with in-service conditions and observations of field performance. NCHRP Research Report 958: Electrochemical Test Methods to Evaluate the Corrosion Poten- tial of Earthen Materials describes the data collection and interpretation, conclusions, and recommendations from NCHRP 21-11. Electrochemical properties of earthen materials, such as electrical resistivity, pH, salt con- centrations, and organic content 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 for determining 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 applica- tions. AASHTO T 288 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 that are different from those of the original soil (i.e., the resistivity of a fine-grained soil is generally lower than that of a coarse-grained 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 charac- terizing the steel corrosion potential of earthen materials. The specific research objectives were as follows: 1. Identify, sample, and characterize representative earthen materials; 2. Determine the effects of different techniques for electrochemical measurement and dif- ferent procedures for specimen preparation (e.g., aggregate size) on the measured electro- chemical properties of compacted soil or leachates extracted from the solid samples; 3. Establish links between laboratory and field measurements for proper interpretation of laboratory test results; and 4. Develop a test protocol and corresponding characterization of corrosion potential that more accurately reflects the corrosivity of earthen materials as compared with the con- ventional methods. S U M M A R Y Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials
2 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials Approach The study was performed in three phases. Phase I (literature review) included a search and review of existing information, a synthesis of national and international practices, the identification and prioritization of knowledge gaps, and preparation of a draft protocol for sampling, testing, and evaluating the steel corrosivity of earthen materials. Phase II (evalua- tion) included 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 (validation) evaluated the practicality of the proposed protocol and alternative laboratory test methods (investigated in Phase II). The research team studied alternative laboratory test procedures for measuring electro- chemical properties of soils applied to a sampling domain incorporating a broad range of materials (mostly those commonly used in the construction of mechanically stabilized earth walls). The data included characterization of different sample sources (e.g., maximum par- ticle size and gradation) along with measurements of the geochemical and electrochemical properties of the samples, including resistivity, pH, and chloride and sulfate content. Perfor- mance data (i.e., corrosion rates) of plain and galvanized steel specimens embedded in 19 of these sources were documented. While electrochemical test results were used to characterize the corrosion potential of each source, the performance data were used to correlate these characterizations with the corrosion rates. Alternatives to AASHTO tests to measure soil resistivity include ASTM G187, Tex-129-E, Tex-129-M, ASTM WK24621, SC-T-143, and Tex-620-M. Resistivity test methods are of two general types: ⢠Measurement of voltage drop in response to an applied current passing through a compacted soil sample in a soil box (galvanostatic test) and ⢠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 WK24621 and Tex-129-M are new methods (under development) that are currently being considered for implementation by ASTM and the Texas Department of Transportation (DOT). Tex-129-E is the current Texas DOT standard that may be super- seded 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., the portion finer than the No. 10 sieve), dilution ratio, soaking period, method and time of mixing, and filtration of solids, vary among the different test procedures. These factors can significantly affect the obtained electrochemical results. Alternatives to AASHTO tests for measurement of pH include ASTM D4972, SC-T-143, Tex-128-E, Tex-620-M, a procedure developed by CorrTest and described as part of NCHRP Project 21-06 (Vilda 2009), and a new test method currently under consideration by ASTM Committee D18. The latter two test methods and Tex-620-M are applicable to measuring pH for relatively coarse-grained materials, while the other tests are more applicable to measuring pH for finer materials. Alternatives to AASHTO tests for measuring soluble salt content include Tex-620-J and Tex-620-M. In addition, ASTM D4327 provides a more robust technique that uses ion exchange chromatography (IEC) to determine soluble salt content, including chloride
Summary 3 and sulfate ion content as well as other anions that are more applicable to drinking water and wastewater. This technique can be applied to samples prepared in accordance with AASHTO T 290 and AASHTO T 291. In addition, the sulfate and chloride content can be determined from the same specimen when using IEC. Results obtained from the different test procedures were compared in terms of ⢠Precision and repeatability, ⢠Bias relative to those obtained from the current AASHTO tests, and ⢠Trends identified in the data. These comparisons were made to check whether any of the procedures performed better than others in terms of repeatability, precision, and bias. The research team also identified cases in which the results from different test methods were similar and in which the results were different. For cases in which differences in results were observed, further analyses were performed to identify the best result for characterizing the steel corrosion potential. Precision and Bias Resistivity ⢠The best precision was observed in the results from Tex-620-J, Tex-129-E, Tex-129-M, and Tex-620-M, with repeatability ranging from 6.8% to 7.6%. ⢠The precision of the other test methods was less, with repeatability ranging between 9.1% and 13.2%. The repeatability of the results from ASTM WK24621, at 13.2%, was the poorest as compared with the other test methods for resistivity. ⢠The repeatability of the results from tests performed on leachates extracted from soilâ water mixtures (Tex-620-J, Tex-620-M, and SC-T-143) was comparable to what was achieved in the soil box tests (Tex-129-M, Tex-129-E, ASTM G187, AASHTO T 288, and ASTM WK24621). ⢠The repeatability of the results obtained from the Texas modified procedures for the measurement of resistivity/conductivity (Tex-129-M and Tex-620-M) was improved as compared with that obtained from AASHTO T 288, SC-T-143, ASTM G187, and ASTM WK24621. ⢠The mean bias was approximately 1.00 for Tex-129-E, with a coefficient of variation (CV) of 22%. The results from Tex-129-E and AASHTO T 288 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 versus No. 10) and the 12-hour curing period prescribed by AASHTO T 288 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 showed mean bias values that were noticeably higher than 1.00 (as high as 5.22 in Tex-620-M), with CV values generally higher than 50%. ⢠The mean bias was greater for test procedures that involved coarser gradations (i.e., ASTM G187, Tex-129-M, and ASTM WK24621). ASTM G187 includes particle sizes up to ¼ in., but Tex-129-M and ASTM WK24621 both include particle sizes up to 1¾ in. This is reflected in the mean bias values, which were higher for results obtained from ASTM WK24621 and Tex-129-M as compared with those from ASTM G187. The bias from ASTM WK24621 was higher than that from Tex-129-M due to the manner in which measurements for ASTM WK24621 are taken after the sample is drained. ⢠The data from each test method were grouped according to the fineness of the samples (fine sand, coarse sand, and gravel). It was observed that, as the coarseness of the sample
4 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials increased, the mean bias and the CV increased. When the materials characterized as fine sand and the results from the soil box tests were considered, the average mean bias was close to 1.00, with a relatively low CV (average CV = 8%). However, when the results from Tex-129-M, which includes coarse particles within the test specimen, were con- sidered, the biases for coarse sand and gravel were 1.6 and 3.1, respectively. Also, the CVbias increased incrementally for materials characterized as coarse sands and gravels, for which CVs in excess of 30% were observed. ⢠The mean biases for the tests on compacted soil specimens and tests performed with leachates were 2.13 and 2.95, respectively. The observed differences were expected because the effects from tortuosity using conductivity measurements from leachate could not be included. ⢠The biases from the tests on the leachates were all greater than 1.00, even for samples separated into finer components (e.g., for Tex-620-J, the sample is separated on a No. 40 sieve). This was because of the different dilution ratios and methods of mixing and extracting leachates used in the different leaching tests as compared with soil box tests. Salt Content ⢠Precision/repeatability was similar among test methods for measurements of salt content. ⢠Salt content measured according to Tex-620-J was higher than that measured according to the AASHTO tests; salt content determined with Tex-620-M was generally lower than that obtained with other test methods. ⢠The best correlations between salt content and resistivity measurement were obtained with the AASHTO test standards. pH ⢠Measurements of pH from Tex-620-M were less repeatable as compared with measure- ments from other test methods investigated in this study. ⢠In general, Tex-620-M rendered pH values that were higher than those obtained with the other test methods investigated in the study. ⢠Results from NCHRP 21-06 (Vilda 2009) were more repeatable and did not have a sig- nificant bias as compared with AASHTO T 289. Correlation with Corrosion Rate The coefficient of determination, R2, between the corrosion rate and resistivity measure- ments was used as an index to rank the accuracy of the results obtained 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 content (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. The measurements presented herein are the maximum observed from each site or source. The maximums were used 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.
Summary 5 The report concludes 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% of particles passing the No. 10 sieve, AASHTO T 288 is appropriate for the measure- ment of resistivity. These observations were used to develop the proposed protocol presented in the appendix. Recommended Protocol The proposed protocol incorporates recommendations based on results from analyses of the data collected in Phase II of the study. The characteristics of the materials are described in terms of 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 in which the percentage passing the No. 10 sieve is greater than 25%. 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). Implementation Study During Phase III of NCHRP 21-11, the research team cooperated with selected transpor- tation agencies to implement the recommended protocol as a shadow specification. The data included characterization of different sample sources (e.g., maximum particle size and gradation) along with measurements of the geochemical and electrochemical properties of the samples, including resistivity, pH, and chloride and sulfate content (i.e., the total salt content). The Wenner four-probe techniqueâaccording to ASTM G57 and Wenner (1915)âwas 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 that used Texas modified procedures and AASHTO test procedures. The practicality and implementation of the suggested protocol were also evaluated through interaction with laboratories engaged in electrochemical testing and suppliers and owners in different states. Data from the implementation study allowed comparison of the results obtained by dif- ferent labs using the same test standards. In situ measurements of resistivity were also per- formed 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 content. The information and the data obtained from the implementation of Phase III of NCHRP 21-11 show the effects of reinforcements on measurements of fill resistivity and the benefits of orienting the lines for the Wenner four-probe test perpendicular to the reinforcements. These data also showed the variability that is inherent in the measure- ments and the effects of reinforcements on these variations. A correspondence between laboratory and field measurements of resistivity was also observed. 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 as compared with the traditional methods. The owners and contractors were able to perform the modi- fied test procedures and, with few exceptions, could acquire the equipment needed to per- form these tests. Recommendations as to which test procedures should be applied to the characterization of corrosivity were found to be clear and easy to implement.
6 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials Organization of the Report Chapter 1 summarizes 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 as well as the objectives and research tasks included in each phase of the project. Chapter 3 describes the sample domain used in the laboratory program and the results obtained from the laboratory measurements (Phase II). It 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. Chapter 4 describes cooperation with owners and contractors and implementation of the research teamâs recommendations and proposed protocol as a shadow specification on active con- struction projects (Phase III). Laboratory test results obtained from different labs are compared with samples from the same source. Improvements to current practices for in situ measurements of resistivity and comparisons between results from laboratory and field measurements are also proposed. Chapter 5 presents conclusions and recommenda- tions for future research.
