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responding reductions in the propagation velocities for compression waves traveling within the grout. 4. The cross-sectional areas of the elements at the sources of reflections were observed from results of IR testing, which are obtained from the mobility at resonance. CONCLUSIONS This project addressed proposals made in NCHRP Project 24-28 to (1) obtain more reliable data and reduce uncertainty with respect to the per- formance of MSES constructed with marginal quality fills; (2) obtain additional performance data from older installations of MSES reinforcements; and (3) implement more robust test techniques to evaluate the existing conditions of rock bolts, soil nails, and ground anchors. Results presented herein serve to further validate results from NCHRP 24-28 and the predictive models for corrosion potential, metal loss, and service life of metal-reinforced sys- tems developed therein. 1. The variability of the performance of mar- ginal fills observed in NCHRP Project 24-28 is due to uncertainty with respect to the fill properties, which may also be inherently variable. Measurements of fill resistivity obtained at the location and time of corrosion rate measurements reduce this uncertainty and improve the ability to model the perfor- mance of MSES constructed with marginal quality fills. Results presented in this report demonstrate that the resistivity of in-situ materials surrounding earth reinforcements can be determined at the time and location of corrosion rate measurements. The technique employs measurements of resistance via the three-electrode technique, and a simple rela- tionship between the measured resistance and resistivity. Measurements of corrosion rate and resistivity in the three-electrode configu- ration are most useful, since the majority of measurements in the database compiled as part of NCHRP Project 24-28 utilize this technique. Thus, resistivity is computed using the measured resistance, and the width and length of the reinforcement. This tech- nique was verified via measurements under known conditions within a test embankment, and data collected from five different sites in California with access to MSE reinforce- ments and fill materials for sampling and testing. 2. The fieldwork included the oldest MSE wall in the United States, which was constructed in 1971, along Route 39 through the San Gabriel Mountains near Los Angeles, and was 39 years old at the time when observations were made. Measurements of corrosion rate obtained from this MSE wall are extremely interesting because they are some of the first confirmed observations of the rate of steel consumption subsequent to depletion of zinc coating of galva- nized reinforcements used in MSE construction in the United States. These observed rates are much less than those used in design and appear to be closer to the corrosion rates used for zinc, suggesting that a discrete change in corrosion rates as zinc is depletedâas is implicit in the current AASHTO model for metal loss of MSE reinforcementsâdoes not necessarily occur. 3. The impulse response test was implemented for probing the lengths of rock bolts to access details of the installation and condi- tions surrounding the structural elements (i.e., steel rods). The IR test is useful to locate and identify the size and shape of anomalies along rock bolt installations. This test is considered an improvement over the sonic echo technique, which is the existing practice for condition assessment of rock bolts. Results from the IR test were verified through tests performed with dummy rock bolts that included both typical and deliber- ately distressed installations. Results from this study indicate that the IR test is more robust and renders additional information for condition assessment of rock bolts compared to what is achievable from the SE test. How- ever, the SE test is adequate to assess remain- ing levels of pre-stress, and to identify the free lengths of rock bolt installations. Knowl- edge of the free length is useful if the total length is known, and general details of the installation are needed for correlating data from further electrochemical testing (e.g., lengths are needed to assess surface areas of the test elements and correlate corrosion rates from LPR measurements). 5
