Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 13
13
and calibration methodology recommended by Allen et al. (4) comparing results with expectations for service life mod-
(2005). The resistance factors consider the nominal metal loss els (Fishman et al., 2005).
used in design and the redundancy of the design and load NDT applied to Type I reinforcements includes measure-
redistribution inherent to the identified limit states. ment of half-cell potential and LPR. Half-cell potential mea-
The reliability index () for other systems that are vulnera- surements are useful to probe the surface and assess if corrosion
ble to metal loss (unprotected soil nails and rock bolts) will also has occurred and whether or not zinc coating remains on the
be considered and compared to the T values used in design. surface of galvanized reinforcements. LPR is useful to estimate
corrosion rate at an instant in time. Single measurements do
not provide enough information and a sampling strategy is
Task 8--Recommend Revisions
incorporated into the test protocol to consider random, spa-
to AASHTO LRFD Specifications
tial, and temporal variations in measurements.
Based on the results from Task 7, revisions to the current Additional NDT applied to Type II reinforcements includes
AASHTO LRFD specifications used in the design of metal- impact and ultrasonic tests. Impact test results are useful to
tensioned systems were reviewed and recommended. In diagnose loss of prestress, assess grout quality, and indicate if the
particular, resistance factors for design of MSE walls are rec- cross section is compromised from corrosion or from a bend or
ommended that take into account the estimated metal loss kink in the element. Ultrasonic test results are useful for obtain-
over the service life of the installation. Metal loss parameters ing more detailed information about the condition of elements
will be updated as appropriate for galvanized and plain steel within the first few feet from the proximal end of the element.
reinforcements, while taking into consideration different back-
fill characteristics. Calibration of Resistance Factors
for LRFD
Task 9--Submit Final Report The procedure for reliability-based calibration of resistance
This final report summarizes the findings of, draws conclu- factors for LRFD is as follows (Allen et al., 2005):
sions from, and documents the research products, including
1. Consider limit state equation for yield of reinforcements.
· A performance database documenting the attributes and 2. Statistically characterize the data upon which the calibration
metal loss observed for a variety of metal-tensioned systems is based.
used in geotechnical applications, including the additional 3. Select a target reliability index.
results from field studies conducted in Task 6. 4. Use reliability theory to compute resistance factors.
· Updated metal loss models that consider targeted levels of
Factors that impact the extent to which variability of metal
confidence, sources of error, and different types of elements loss affects probability of occurrence need to be included in
and site conditions. the reliability-based calibration. To help identify these factors,
· Recommended revisions to the current AASHTO LRFD
Figure 4 illustrates how the steel incorporated into the design
specifications, including updated resistance factors for the of a reinforcement cross section can be interpreted to include
design of MSE walls and other earth reinforcements.
· Discussion of deficiencies in present knowledge and rec-
ommendations for future work.
Test Protocol
Berkovitz and Healey (1997) and Elias et al. (2009) describe
test protocols and procedures for sampling and testing Type I
reinforcements. Withiam et al. (2002) present a recommended
practice resulting from NCHRP Project 24-13 for condition
assessment and service life modeling of Type II reinforcements.
These procedures, protocols, and recommended practices were
followed in the course of this research. Appendix B describes
salient details of test procedures, sampling, data analysis, and
interpretation for Type I and Type II reinforcements. In gen-
eral, the protocols include (1) assessing the site and installation
conditions; (2) sampling and testing backfill, groundwater,
and in situ earth materials; (3) performing nondestructive Figure 4. Idealized
testing (NDT) supplemented with visual observations; and reinforcement cross section.
