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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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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.
