Click for next page ( 14

The National Academies of Sciences, Engineering, and Medicine
500 Fifth St. N.W. | Washington, D.C. 20001

Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

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.