LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems (2011) / Chapter Skim
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Pages 18-41
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From page 18... ...
Also, direct physical measurement of section loss is performed on 18 samples retrieved from six of the monitoring stations (i.e., three reinforcements exhumed from six of the stations)
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From page 19... ...
19 data with geographic location and view all of the performance data and pertinent information associated with that location. Type I -- Measured Corrosion Rates Consistent with the data needs for reliability analysis and calibration of strength reduction factors for LRFD, the following studies were performed: 1.
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From page 20... ...
, and this is may be because, although a low value of ρmin is indicative of the potential for higher corrosion rates, this potential may not be realized if the moisture content is kept low, and moisture content and degree of saturation exhibit significant variability. More scatter is evident for plain steel reinforcements.
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From page 21... ...
Thus, the methodology of using LPR measurements to estimate metal loss appears to be conservative (at least for the range of corrosion rates depicted in Figure 9)
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From page 22... ...
In general, corrosion rates from LPR measurements are consistent with observations of maximum metal loss considering a factor between 2 and 3 relates the average to the maximum metal loss. This is consistent with the factor of 2 commonly used to relate loss of tensile strength to uniform corrosion losses, as discussed by Elias (1990)
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From page 23... ...
Because there does not appear to be a significant effect of climate on measured corrosion rates, measurements from different regions are combined to evaluate the effects of backfill character, time, and reinforcement type on corrosion rates and observations of metal loss for galvanized reinforcements. Thus, data from all the regions were used to generate statistics for galvanized reinforcements used in the reliability analysis and calibration of resistance factor.
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From page 24... ...
and 11(b) compare corrosion rates measured via the LPR technique to the AASHTO metal loss model (see CR ≈ −1400 160 75ρ .
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and 12(b) compare corrosion rates measured via the LPR technique to the Elias and Stuttgart metal loss models proposed for design.
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(Elias model) the differences between the mean of the observed corrosion rates and the Elias model, depicted in Figure 12(a)
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From page 27... ...
. These statistics demonstrate that the corrosion rates for marginal quality fill are approximately two to three times higher than those observed from good quality fills.
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From page 28... ...
and the observed corrosion rate of steel subsequent to zinc depletion is taken as 32 μm/yr with standard deviation of 21 μm/yr and a lognormal distribution. The corrosion rate of steel is based on observations from galvanized reinforcements after 8 years of service with the lower corrosion rates (i.e., < 4 μm/yr)
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From page 29... ...
This approximation is considered conservative due to the likely attenuation of corrosion rate with respect to time. The corrosion rates used to extrapolate metal loss are considered constants over prescribed time intervals, and are higher than those expected to prevail at the end of service.
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From page 30... ...
Probability grids similar to Figure 14 are generated for each variable used to describe corrosion rates and metal loss. In most cases lognormal distributions were found to fit well with the observed corrosion rates.
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From page 31... ...
. In addition, "measured" corrosion rates for steel were multiplied by 2 to render loss of tensile strength from LPR measurements.
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From page 32... ...
The bias distribution is approximately normal considering a 75-year service life, but is better represented by a Weibull distribution considering a 100-year service life. Table 18 is a summary of the resistance factors calibrated with metal loss measurements from sites with high quality reinforced fill.
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From page 33... ...
This formulation is based on the following assumptions, which are the same as those employed in the Monte Carlo simulations: • The distribution of corrosion loss over all elements in the structure mirrors the overall distribution of corrosion measured in the field; • During the early life of the structure, the corrosion rate distribution reflects that of the galvanized elements; • Loss of base steel is initiated after the zinc coating is consumed; • The highest rate of metal loss takes place in the region of maximum reinforcement stress, and the service life of a given element is over when the sacrificial steel in the highest stressed region is consumed; and • Corrosion rates are constant with time. The resulting formula to compute the probability that metal loss, X, exceeds a given threshold, X′, is given by where P is probability; X is loss of steel defined by tf, zi, rz, σz, rs, σs; X ′ is a given amount of steel loss; tf is service life; zi is the initial zinc thickness; rz is the mean zinc corrosion rate; σz is the standard deviation of zinc corrosion rate; rs is the mean steel corrosion rate; σs is the standard deviation of steel corrosion rate; r0 = zi/tf and is the lowest rate of zinc corrosion for which base steel will be consumed within tf; fz(rz)
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From page 34... ...
The calibration is performed considering reinforced fill quality that meets AASHTO criteria for electrochemical properties, and both good and high quality fill are considered. Good Quality Fill Based on the summary of statistics from corrosion rate measurements depicted in Figure 8, a mean corrosion rate and standard deviation of 25 μm/yr and 14 μm/yr, respectively, represent the statistics for plain steel grid-type reinforcements within good quality fill, and the distribution can be approximated as lognormal.
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From page 35... ...
Based on the analysis of the observed corrosion rates for marginal fill, and the paucity of data for reinforcements less than 10 years old, extrapolations of metal loss assume that the zinc coating will survive 10 years. Corrosion rate measurements are available from six sites located in California that appear to reflect corrosion rates of base steel subsequent to depletion of the zinc coating.
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From page 36... ...
Generally speaking, rock bolts are more susceptible to metal loss from corrosion compared to ground anchorages. For these reasons, results from condition assessment and analysis of data relative to rock bolt and ground anchor installations are distinct.
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From page 37... ...
Corrosion rates for resin-grouted rock bolts at the Barron Mountain and Beaucatcher Rock Cuts are relatively low. However, the LPR measurements only reflect corrosion rates in areas that are in direct contact with the surrounding earth and may not include areas where there is a gap or void separating the steel reinforcement surface from the 37 Site Highway State Reinforcement Type Year Installed Anchorage Type Prestress (kips)
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From page 38... ...
Also, roof bolts installed at the SRCM may not have a gap behind the anchor plate similar to the rock cut installations such that LPR measurements reflect corrosion rates near the proximal end where moisture and oxygen are more prevalent. Data in Figure 17 also confirm that the non-restressable anchors tested at the I-99 17th Street Exit ramp in Altoona, PA, may not be adequately protected by portland cement grout behind the anchor plate and localized corrosion is occurring at an average rate of 5 μm/yr.
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From page 39... ...
However, another utility of sonic echo testing is the confirming or obtaining of missing information about the geometry of the installation. Data from sonic echo tests have been used in this study to verify bolt lengths and bond lengths that may then be used to estimate the surface area of the rock bolts in contact with the surrounding rock in order to reconcile corrosion rates from LPR measurements.
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From page 40... ...
Thus, the statistics rendered from the data in Table 25 appear to be reasonable. The statistical variation of metal loss represented by these parameters can be used to calibrate resistance factors for LRFD similar to that for Type I (MSE)
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From page 41... ...
. LPR measurements indicate very low average rates of metal loss and significant metal loss was not observed from several elements that were exhumed for inspection (Fishman, 2005)
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