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36 Table 23. Resistance bias and calibration of considering construction with marginal quality fill. Model R1 R1 COV R1 pf / R I [Eq. (17a)] 1.01 0.29 0.29 0.30 2.37 0.009 0.297 II [Eq. (17b)] 1.63 0.46 0.28 0.50 2.26 0.011 0.301 1 Normally distributed. this is indeed the case. The results from this exercise demon- Research Coal Mine (SRCM), which presents a more aggres- strate how changing the metal loss model used in design will sive environment relative to corrosion. not effect a change in design if the resistance factors are prop- Grout type is an especially important detail as the extent erly calibrated. The best way to achieve a more efficient design and type of grout surrounding an element affects the vulner- is to improve the COV of the bias. This may be achieved by ability of the system to corrosion. Portland cement-based using models that do a better job of capturing the behavior grout is alkaline and protects the steel reinforcement by pas- (e.g., capture trends that may be related to space, time, fill sivating the surface as well as providing a barrier to moisture characteristics, and site conditions) and by improving the and oxygen. Half-cell potential measurements, depicted in quality and quantity of performance data. Figure 16, are useful to assess if the steel surface is passi- vated, or if corrosion is occurring. The alkalinity of the port- Type II--Condition Assessment land cement grout tends to shift the potential at the surface of a steel reinforcement to a more positive value. A half-cell For Type II reinforcements, installation details have an potential greater than -200 mV relative to a CSE indicates effect on the vulnerability of the system to the surrounding the surface of the steel reinforcement is passivated. Figure 16 environment and corresponding susceptibility to corrosion, depicts the means and ranges of half-cell potential measure- and on our ability to probe the elements and interpret data ments from sites listed in Table 24. In general, sites with resin- from NDT. Relevant details include steel type, corrosion pro- grouted reinforcements exhibit half-cell potentials less than tection measures, drill hole dimensions, bond length, free/ -200 mV, which on average range between approximately stressing length, total length, date of installation, level of pre- -400 mV and -700 mV. Reinforcements surrounded with stress, grout type, and use of couplings. For rock bolts, the portland cement grout exhibit half-cell potentials greater grout surrounding the reinforcement is often the only corro- sion protection afforded to the reinforcements. More complex than -200 mV (maximum values). Although there are some installation details are incorporated into ground anchorages notable exceptions, in general, these data serve to demonstrate that include elaborate corrosion protection measures, as the effectiveness of portland cement grout to protect steel earth described by PTI (2004). Construction details, durability of reinforcements. different material components, and workmanship associated The best demonstration of the effectiveness of portland with the corrosion protection system affect the service life and cement grout to passivate the steel reinforcements is with durability of ground anchorages. Generally speaking, rock respect to the dam tie-downs wherein the steel wires are sur- bolts are more susceptible to metal loss from corrosion com- rounded by portland cement grout within a concrete gravity pared to ground anchorages. For these reasons, results from dam. The fully grouted steel bar tendons at the Barron Moun- condition assessment and analysis of data relative to rock bolt tain Rock Cut are generally passivated, but there are some and ground anchor installations are distinct. elements of the population for which the grout protection Table 24 is a summary of sites with Type II reinforcements appears to be compromised. The degree of protection afforded that were included in the fieldwork conducted as part of Task 6, to the strands behind the anchor plate at the reaction blocks and where measurements of corrosion rate and information along the I-99 17th Street exit ramp in Altoona, PA, do not on the condition of the reinforcements were obtained. Six of appear to be protected by grout and this will be discussed later the installations described in Table 24 are rock bolts, and in this section when describing the integrity of ground anchor three are ground anchorages. Reinforcement age ranges from installations. The half-cell potentials with respect to the restress- 8 to 43 years when monitoring was conducted (20072008). able anchors at the same site in Altoona, PA, are lower because A variety of site conditions prevail, but in general, the sites measurements reflect conditions along the surface of the gal- provide an environment that is slightly acidic with pH ranging vanized trumpet head, and, in contrast to steel, zinc is not between four and six, and fairly conductive with resistivities passivated by alkalinity. less than 10,000 -cm. One exception to this is the National For resin grout installations, relatively high half-cell poten- Institute for Occupational Safety and Health (NIOSH) Safety tials indicate that corrosion may have occurred, but these
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37 Table 24. Summary of sites with Type II reinforcements evaluated during Phase II. Site Highway State Reinforcement Year Anchorage Prestress Corrosion Comments Type Installed Type (kips) Protection Barron I-93 NB NH Rock bolts-- 1974 Polyester 40 None Grouted in Mountain Grade 150 resin grout bond zone Rock Cut prestressing steel only--bare rods and Grade stressing length 80 smooth steel rods Barron I-93 SB NH Rock bolts-- 1974 Polyester 40 None Grouted in Mountain. Grade 150 resin grout bond zone Rock Cut prestressing steel only--bare rods and Grade stressing length 80 smooth steel rods Beaucatcher I-240 W NC Rock bolts-- 1982 Epoxy 40 Grout Grouted full- Rock Cut Grade 150 resin grout length prestressing steel rods Safety NIOSH PA Roof bolts-- 1988 Resin grout, N/A Grout/none Fully-grouted, Research Pittsburgh Grade 60 steel expansion or nongrouted Coalmine Research rods shell or slot roof bolts (SRCM) Laboratory and Wedge Safety NIOSH PA Roof bolts-- 2000 Resin grout N/A Grout Fully-grouted Research Pittsburgh Grade 60 steel roof bolts Coalmine Research rods Laboratory Barron Mtn. I-93 NB NH Rock bolts-- 1974 Portland 0 Grout Fully-grouted Rock Cut Grade 150 cement passive prestressing steel grout elements rods New Dams in N/A Dam tie- 1964 Portland 125 Grout Fully-grouted Brunswick, the downs--Grade cement parallel wire, Canada Musquash 270 cold drawn grout buttonhead River steel wire anchorages Basin grouted into rock 17th Street I-99 PA Reaction 1992 Portland 100 Class I-- Restressable exit ramp blocks--Grade cement double strands 270 seven wire grout corrosion surrounded by strand ground protection grease in anchors grouted system trumpet head into rock. 17th Street I-99 PA Reaction 1992 Portland 100 Class I-- Nonrestressable exit ramp blocks--Grade cement double strands 270 seven wire grout corrosion surrounded by strand ground protection grout behind anchors grouted system the bearing into rock plate NOTE: NIOSH = National Institute for Occupational Safety and Health. NH I-93 (tendons) measurements are also affected by salt concentrations and PA Anchors (NR) Dam Tie-Downs PA Anchors (R) Bruceton Mine moisture content of the surrounding rock mass. Higher salt NH I-93 (NB) NH I-93 (SB) concentrations and dry conditions tend to shift half-cell NC I-240 potentials to more negative values. This is evident from mea- surements taken at several locations in western New York 0 where monitoring was performed at regular intervals over a -100 -200 2-year duration (see Appendix C). -300 Figure 17 presents the means and ranges of corrosion rate Ecorr (mV) -400 -500 measured via the LPR technique at some of the sites listed -600 in Table 24. Corrosion rates for resin-grouted rock bolts at the -700 Barron Mountain and Beaucatcher Rock Cuts are relatively -800 -900 Resin Grout Portland Cement Grout low. However, the LPR measurements only reflect corrosion -1000 rates in areas that are in direct contact with the surround- Figure 16. Ranges of half-cell potential measurements ing earth and may not include areas where there is a gap or for Type II reinforcements. void separating the steel reinforcement surface from the