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35
Table 21. Summary of considering plain steel Table 22. Summary of considering plain steel
reinforcements and good quality fill material. reinforcements and high quality fill material.
Reinforcement Design Reinforcement Reinforcement Design Reinforcement
Type Life Thickness/Size Simple/Coherent Type Life Thickness/Size Simple/Coherent
W7 0.25/0.20 W7 0.20/0.20
Grid 50 years W9 0.30/0.25 Grid 75 years W9 0.30/0.20
W11 0.35/0.25 W11 0.35/0.25
W14 0.40/0.35 W14 0.35/0.30
nized reinforcements, therefore, the nominal metal loss model 0.35. The efficiency ratio (/R) for this case is approxi-
used in the denominator of Equation (15) is based on data col- mately 0.25.
lected by the National Bureau of Standards for plain steel in
fill materials similar to those typically used in the construction
Marginal Fill Quality
of MSE and described by Eq. (3). The analysis is limited to a
50-year service life since the sacrificial steel requirements Resistance factors are calibrated considering the use of fill
considering 75- and 100-year service lives are considered to that does not meet AASHTO criteria for electrochemical
be impractical. Thus, a shorter service life is considered parameters as described in Table 3. Fill with pH in the range of
appropriate when using plain steel as opposed to galvanized five to seven, but with min between 1,000 -cm and 3,000
reinforcements. The mean of the resistance bias, R, tends to -cm is referred to as marginal quality fill. This calibration
decrease with respect to increase in reinforcement size and is performed considering the use of galvanized reinforce-
ranges between 1.4 and 1.9 with COV between 30% and 40%, ments and a 50-year service life.
and a distribution that is approximately normal. Based on the analysis of the observed corrosion rates for
Table 21 summarizes the results of the resistance factor cal- marginal fill, and the paucity of data for reinforcements less
ibration. The resistance factors tend to increase with respect than 10 years old, extrapolations of metal loss assume that the
to reinforcement size and are approximately 0.1 to 0.15 lower zinc coating will survive 10 years. Corrosion rate measure-
than those computed for galvanized reinforcements with the ments are available from six sites located in California that
conservative steel model and longer service lives as depicted appear to reflect corrosion rates of base steel subsequent to
in Table 15. The efficiency ratio (/R) for this case is approx- depletion of the zinc coating. A mean corrosion rate and stan-
imately 0.2, which is also lower than the efficiency ratio com- dard deviation of 32 m/yr and 21 m/yr, respectively, and a
puted for galvanized reinforcements lognormal distribution are used to describe the statistics of
these measurements. These statistics appear to be conserva-
tive compared to corrosion rates observed from plain steel
High Quality Fill ( > 10,000 -cm)
elements that are more than 10 years old at the time of mea-
Based on the summary of statistics from corrosion rate surement as described in Appendix E.
measurements depicted in Figure 8, a mean corrosion rate and Computations of resistance bias and corresponding calibra-
standard deviation of 12 m/yr and 9.6 m/yr, respectively, tions of resistance factors are performed considering nominal
represent the statistics for plain steel grid-type reinforcements requirements for sacrificial steel computed with Models I and
within high quality fill, and the distribution can be approxi- II as described by Equations (17a) and (17b). These calcula-
mated as lognormal. The resistance bias is computed for differ- tions consider zi = 86 m, a design life (tdesign) equal to 50 years,
ent sizes of grid-type reinforcements (W7, W9, W11, and and grid reinforcements with W20 size longitudinal wires.
W14). For this case, the nominal metal loss model used in the Results from these computations are presented in Table 23 in
denominator of Eq. (15) is based on the Caltrans-Select model terms of the statistics of the resistance bias and corresponding
(Jackura et al., 1987) described in Table 2, corresponding to rs calibrations of resistance factors.
= 13 m/yr. Given the more favorable sacrificial steel require- As expected, the bias associated with Model I is less than
ments compared to the previous case, the analysis considers Model II, but the COVs are nearly the same. Due to the differ-
service lives of 75 years. The mean of the resistance bias, R, ences in the bias, the resistance factor calibrated for Model I is
tends to decrease with respect to increase in reinforcement size also less than that associated with Model II. However, because
and ranges between 1.1 and 1.2 with COV between 30% and the COVs of the bias are similar the calibrated resistance fac-
35%, and a distribution that is approximately normal. tors render the same design efficiency, /R, for each case. Sim-
Table 22 summarizes the results of the resistance factor ilar design efficiencies result in similar design details for a given
calibration. The resistance factors tend to increase with MSE geometry, load case, design life, and so on. An example
respect to reinforcement size and range between 0.25 and problem is presented in Appendix F that demonstrates that