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47 Additional research is recommended to further validate Further research could also be pursued to further demon- the predictive models for corrosion potential, metal loss, and strate applications of performance data. In particular, if the service life of metal-reinforced systems from Project 24-28. need for rehabilitation or retrofit is identified, cost-effective The validation will require measurements at an independent methods for rehabilitation and retrofit should be selected. set of field sites across the United States, supplemented with Guidance will target improvements to areas where they are results from laboratory measurements. Field sites should most needed, which may be close to sources of fill contam- include rock-bolt installations and MSE walls. Testing of the ination or otherwise based on the spatial distribution of metal-reinforced systems at each field site will require both corrosion or loss of service observed at a particular site. A (1) NDT techniques (e.g., ultrasonic testing, sonic echo, well-maintained and populated database will facilitate devel- impulse response, and electrochemical testing) and (2) direct opment of site-specific guidance based on the experiences measurement after exhumation of in situ reinforcements or that have been documented from other sites. Guidance needs installation of dummy elements by state DOTs or other agen- to be developed for sampling and evaluating backfill and per- cies. The direct measurements will validate the NDT methods formance of in-service reinforcements. The recommended as well as the predictive models based upon these methods. sampling is likely a hybrid between stratified and random sampling. Representative sample locations are stratified with respect to the vertical direction and stations are randomly Type I Reinforcements located along the length of the wall. A cluster of measure- The following objectives apply to Type I reinforcements ments at each sample point should be averaged to render the and the need for data to validate the performance models and corrosion rate at that location. address limitations inherent to the database compiled as part The sensitivity of designs generated from the recommen- of NCHRP Project 24-28: dations provided in the report need to be evaluated. Typical designs should be executed using recommended resistance Evaluate effect of marginal fills on performance and service factors for LRFD considering use of galvanized or plain steel life, reinforcements and various fill materials (i.e., high quality, Study bias inherent to LPR measurements of corrosion rate, good, and marginal). In this way the impact of these factors and on design parameters, including the size and spacing of rein- Assess the corrosion rate of steel after zinc has been con- forcements, can be evaluated. sumed from galvanized elements. Type II Reinforcements To accomplish these objectives: The following objectives apply to Type II reinforcements 1. Develop a relationship between fill resistance (measured and the need to (1) substantiate use of electrochemical test as part of the LPR test) and fill resistivity. This relationship techniques for corrosion monitoring and integrity testing of depends upon the geometries of the test electrodes, elec- corrosion protection systems and (2) extract more information trode spacing, fill characteristics, and method of measur- on existing conditions from the results of dynamic testing (e.g., ing fill resistance. If this relationship can be established, it sonic echo and impulse response). More data from sites with will then be possible to develop much better correlations double corrosion protection systems are required to generate between measured corrosion rates and the electrochemi- statistics describing the reliability of these installations. cal properties of the fill. 2. Compare measurements of corrosion rates with direct Study application of corrosion monitoring with LPR tech- observations of metal loss from reinforcements that have niques. Seek measurements and observations that can char- been exhumed subsequent to LPR measurements. These acterize the surface area in contact with the surrounding data will be very useful, particularly to relate loss of tensile earth material, and knowledge of the influence of grout strength to LPR measurements, as loss of strength is often and other components of the corrosion protection system. from metal loss that has occurred over localized areas. Refine data analysis techniques for dynamic tests (wave 3. Collect data from sites with galvanized reinforcements propagation techniques). Verify results obtained with these where base steel is corroding after zinc has been consumed. techniques, and evaluate the limitations of these NDTs for Different assumptions regarding corrosion of the base steel probing earth reinforcements. have a significant impact on resistance factor calibrations Collect more data to document the performance of corro- for LRFD. sion protection systems.