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68 Chapter 5. Conclusions and Needs for Future Research 5.1. Main Conclusions We have proposed a protocol describing best practices for sampling, testing, and characterizing the steel corrosion potential of earthen materials. The protocol incorporates alternatives to the current AASHTO test standards for measuring electrochemical properties including resistivity, pH, chloride and sulfate ion contents. We developed the protocol 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 significant amount of material passing a No.10 sieve. Modified test standards considered as alternatives to the AASHTO tests include Tex-129-M and Tex-620-M for measurements of resistivity and salt contents, respectively. Unlike the AASHTO tests, these alternatives incorporate larger sized particles within the test specimens. We selected Tex-129-M and Tex-620-M from a suite of candidates based on the precision and repeatability of the results observed from the laboratory test program, the utility of the test results, and the observed performances 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 currents1 to flow through the materials. The finer particles have higher salt contents and lower resistivities compared to 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 resistance). Hence, the corrosivity is affected more by the properties of the finer portions compared to the bulk properties of the material. We concluded that results from Tex-129-M apply well to the materials with less than approximately 22 percent 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, where the 22 percent threshold is rounded up to 25 percent passing a No. 10 sieve. In general, the proposed protocol describes the application of the current AASHTO test series for samples with grading number (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). The GN is included with the screening to restrict the use of Tex-129-M to coarse-textured samples with a relatively high gravel content. We grouped the materials included in this study into clusters based on ranges of resistivity and according to corrosion indices determined from the German Method (DVGW-GW9) for characterizing corrosivity. The German Method is a multivariate approach for classifying 1 Corrosion current is the electrical current produced in the medium (in this case soil) during the corrosion process.
69 corrosivity that considers the electrochemical properties of the relevant earthen materials, site conditions, and the presence of carbonates or industrial by-products. We selected ranges of resistivity and corrosion indices corresponding to noncorrosive, mildly corrosive, and moderately to severely corrosive conditions. We compared corrosion rates measured from plain and galvanized steel elements to the rates cited in the literature corresponding to the given corrosivity descriptions. We observed relatively good comparisons, when test results were applied according to the proposed protocol. 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. A program of in-situ testing that included the Wenner 4-probe technique (according to ASTM G57 (2012) 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 using modified and current 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 including UTEP, McMahon & Mann, NYSDOT and TXDOT (AASHTO Tests only). 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 easy 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. 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 percent passing a No. 200 sieve. This sample domain was relevant to MSE wall construction practices. However, this sample domain was not representative of earthen materials encountered in other applications that may include installations of piles, culverts, and drainage pipes. Also, in some states materials with more than 15% passing a No. 200 sieve are common in MSE wall constructions. Thus, more data are needed to evaluate the characterization of steel corrosion potential for earthen materials with significant fine contents (material passing a No. 200 sieve, i.e., silt and clay fractions). Additionally, more data are needed to distinguish between the effects of silty fines compared to clayey fines on the 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
70 test, which is not a âcompactedâ specimen. In this study the endpoint for resistivity testing was at the point of saturation. 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 compositions, 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 measurements of electrochemical properties. 3. For this study, we 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. We suggest 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 4-probe technique are necessary. Results from field testing may be useful to decide if collecting more samples from the site and laboratory testing are necessary. If the results from field testing match the laboratory measurements from Tex-129-M, at similar levels of moisture content, then this indicates that 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, we only have data from four sites, 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 interact with AASHTO committees and the State DOTs to consider the modified practices. We suggest the following activities for promoting the implementation of the proposed protocol: â¢ 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. â¢ Prepare course materials, guidebooks, and laboratory demonstrations for training. â¢ Conduct inter-lab tests to obtain more data on the precision and repeatability of the tests included in the proposed protocol. â¢ 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 21-11 and the proposed protocol. â¢ Engage the construction industry including contractors that often select fills for construction, and suppliers of MSE wall systems that promote the use of good construction practices and specifications. This can be accomplished via interactions
71 with various industry groups including the Association for Mechanically Stabilized Earth (AMSE) and the Associated General Contractors of America (AGC). â¢ Engage the FHWA to assist in deployment of these practices. This may include updates to existing documents published by FHWA that describe corrosion and degradation of soil reinforcements, culverts, etc. and sampling and testing of fill materials. (e.g. FHWA-NHI-09-087, Elias et al., 2009).