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20 comparing measured corrosion rates to resistivity mea- forcement type does not appear to have a significant impact surements include a lot of scatter. Some of the scatter may on corrosion rates, but lower COVs are realized when data are be due to spatial and temporal differences between mea- partitioned into groups defined by reinforcement type. surement of corrosion rate and sampling and testing of The best results in terms of lower COV are from galvanized reinforced fill materials. However, the study is useful to reinforcements between 2 and 16 years old, where the COVs demarcate threshold levels of resistivity wherein corrosion range between approximately 30% and 60%. Higher COVs rates may be significantly affected and to define ranges are realized for younger reinforcements (<2 years old) and within which particular metal loss models may apply. reinforcements that are older than 16 years. This may be due 3. Study of the effect of climate/region on measured corrosion to variations in the time it takes for the zinc surface to become rates considering data from different geographic regions passivated for younger reinforcements, and the variation of associated with different climates, and construction and remaining zinc on the surface of older reinforcements. Data maintenance practices. The purpose of this study is to fur- are more scattered (i.e., have higher COVs) considering fill ther evaluate if data should be partitioned into regions for materials that do not meet AASHTO requirements (min the purpose of reliability analysis. < 3,000 -cm), and this is may be because, although a low 4. Partitioning the data into sites that incorporate reinforced value of min is indicative of the potential for higher corrosion fill materials meeting AASHTO requirements, and consid- rates, this potential may not be realized if the moisture con- ering metal loss or corrosion rate as a function of time. tent is kept low, and moisture content and degree of satura- The purpose of this study is to evaluate the robustness of tion exhibit significant variability. available metal loss models and the probability of exceed- More scatter is evident for plain steel reinforcements. ing metal loss rates used in design. This may be due to the tendency for galvanized surfaces to 5. Observation of trends for marginal fills that do not meet undergo more uniform corrosion compared to plain steel; AASHTO criteria for reinforced fills. The purpose of this also, steel may be more sensitive to changes in environment study is to make recommendations on the appropriate over the range of conditions for which measurements were parameters for modeling metal loss and the reliability of obtained. metal loss estimates for a selected range of resistivity; for example, between 1,000 -cm and 3,000 -cm. Bias of LPR Measurements Detailed results from these studies are included in Appen- Figure 9 depicts observations of corrosion and metal loss dix D. Data are grouped by quality of reinforced fill, age of with respect to age of the reinforcements for fill conditions sample, and reinforcement type. Figures 7 and 8 summarize meeting the AASHTO criteria described in Table 3. Observa- the statistics (mean and COV) from these data groups for gal- tions included in Figure 9 are via LPR measurements from vanized and plain steel reinforcements, respectively. Rein- sites located in the northeastern, mid-Atlantic, southeastern, Figure 7. Summary of statistics for galvanized reinforcements (* limited samples).
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21 Figure 8. Summary of statistics for plain steel reinforcements (NA indicates data are not available). southwestern, and western United States, and from weight- Figure 9 includes approximately 404 data points from LPR loss measurements from reinforcements that were exhumed measurements and 50 weight-loss measurements. Weight- from sites in Europe (Darbin et al., 1988). Since LPR mea- loss and LPR measurements are not from the same samples, surements render corrosion rate at an instant in time, these and the samples are from different sites. However, all fills meet data must be extrapolated to estimate metal loss. Metal loss is electrochemical requirements similar to AASHTO. These data computed as the product of the measured corrosion times the are useful to demonstrate that metal loss extrapolated from age of the reinforcement, adjusted for higher corrosion rates LPR measurements are in the same range as those observed assumed to occur during the first 2 years of service. Except for directly via weight-loss measurements. Metal losses com- younger reinforcements that are less than 2 years old, it is puted from LPR appear to be equal to or higher than those assumed that 30 m of zinc per side is lost during the first from weight-loss measurements. Thus, the methodology of 2 years, and the measured corrosion rate is considered to be using LPR measurements to estimate metal loss appears to be constant thereafter. This assumption is less significant con- conservative (at least for the range of corrosion rates depicted sidering older reinforcements. in Figure 9). 200 LPR Measurements Weight Loss Measurements 150 AASHTO Model Metal Loss, m/side 100 50 0 0 5 10 15 20 25 30 Age of Element, years Figure 9. Comparison of LPR and weight loss measurements for galvanized elements in fill materials that meet AASHTO criteria.