LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems (2011) / Chapter Skim
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Type I reinforcements are passive elements used in the construction of metallically reinforced earth structures [i.e., mechanically stabilized earth (MSE)
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(c) Welded Wire Mesh Ribbed Strips Figure 1.
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, or are grouted into rock using portland cement or resin grout. The anchorage may either be concentrated near the end point of a mechanical device or by the short length of grout near the end of the bolt; or the bolt may be fully grouted with the pullout resistance distributed along the length of the bonded zone.
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This is because pit penetrations have a limited impact on the overall remaining cross section. Rather than measuring and modeling pit depths, metal loss models and measurements of corrosion rates for earth reinforcements are averaged over the surface area of the reinforcement.
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For a 75-year design life, and zi equal to 86 µm, the AASHTO, Stuttgart-high salt, and Caltrans Interim-select models yield estimates of steel loss close to that computed with the Darbin model. Differences between these models include the data sets that are used to regress model parameters (i.e., corrosion rates)
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Based on the information in Table 3, the steel loss per side (X) in µm/yr for a given service life, tf, and initial thickness of zinc coating, zi, is computed as The AASHTO model does not give any guidance for corrosion rates or metal loss modeling of plain steel (i.e., not galvanized)
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Differences in the electrochemical properties of the soil fill can adversely affect corrosion rates and contribute to more severe and localized occurrences of metal loss. In instances where more easily compacted (e.g., open-graded)
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Linear Polarization Resistance LPR measurements are used to observe the instantaneous corrosion rate. Lawson et al.
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, the Kentucky Transportation Research Cabinet (Beckham et al., 2005) , the Ohio Department of Transportation (Timmerman, 1990)
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This project extends these studies to consider variability of metal loss and the impact that this has on computed levels of reliability using existing design methodologies and methods for computing the load transferred to the reinforcements. Calibration of the resistance factors uses load factors from the AASHTO LRFD specifications and calibration methodology recommended by Allen et al.
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Alternatively, this report describes reliability-based calibration for resistance factors considering that the yield limit state function is explicitly applied at every reinforcement location. Thus, the potential for overdesign is not directly included in the analysis; however, a target reliability index, βT of 2.3 corresponding to pf = 0.01, is adopted considering the large redundancy inherent to the system (Allen et al., 2005)
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