Click for next page ( 42

The National Academies | 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 41
41 but with = 0.8 renders pf 0.0001. This implies that the cur- bolts is with respect to a gap behind the anchor plate. This rent AASHTO specifications imply a service life of approxi- may be remedied with a trumpet head assembly filled with mately 75 years for the selected example. grease. Alternatively, metal losses need to be considered using Equation (23), similar to those for resin-grouted installations. Portland Cement Grout Based on results from NDT and direct observations, port- Ground Anchors land cement grout quality generally appears to be good for the If the ground anchor system is protected with an adequate rock bolts inspected at the sites listed in Table 24 and depicted corrosion protection system [e.g., meeting the requirements of in Figure 16. Half-cell potential measurements indicate that PTI Class I (PTI, 2004)], then corrosion is generally not a prob- the presence of a passive film layer protects the rock bolts from lem. Observations from NDT indicate that, generally, grout corrosion. However, passivation of the steel may be compro- quality along the bonded zone appears to be good and no mised by the presence of chlorides or acidic conditions. Chlo- defects or anomalies were encountered along the stressing rides may be present along the rock face as a residue from salt lengths. The main concern for ground anchorages is near the spray produced from deicing of the highway. There is some anchor head assembly and the fact that high-strength, pre- evidence of this from the testing of fully grouted tendons at the stressed steel elements may be vulnerable to hydrogen embrit- Barron Mountain Rock Cut (Fishman, 2004). LPR measure- ments indicate very low average rates of metal loss and sig- tlement and SCC. Hydrogen embrittlement and SCC tend to nificant metal loss was not observed from several elements occur in acidic or chloride-rich environments, and, without that were exhumed for inspection (Fishman, 2005). In gen- proper detailing and workmanship at the anchor head assem- eral, for installations where lower half-cell potentials (less than bly, the service life of the elements is severely compromised by -200 mV) are realized corrosion monitoring should be per- these environments. Time to failure for hydrogen embrittle- formed to assess the rate of metal loss. Very high rates of ment and SCC can be relatively short, and the previously cited corrosion are possible when the passive film layer is locally models describing rate of metal loss for uniform corrosion or compromised. Grout cracking may also occur at the beginning incursion of pit depths are not applicable to assess service life. of the bonded zone due to the application of prestress. How- Measurements of half-cell potential and corrosion rate for ever, the surface is still protected via the alkaline environment ground anchors indicate that portland cement grout does not until crack widths exceed a minimum value (e.g., 1 mm). always serve to passivate steel, and the area behind the bearing Grout and return tubes are used to install portland cement- plate is vulnerable to corrosion. Use of a trumpet head filled grouted rock bolts such that the grout quantity can be adjusted with grease appears to be a more effective measure to protect as needed when grout is lost from the drill hole. This is in con- the reinforcements as it is isolated from the surrounding envi- trast to resin-grouted bolts for which a fixed quantity of grout ronment by virtue of the dielectric properties of the grease. is inserted into the drill hole. Thus, the coverage from port- Loss of prestress may also affect service life. Current obser- land cement grout is expected to be better when compared vations in the database do not include sites where this has to resin-grouted installations. Also, prestressed rock bolts been observed to be a problem. However, if the anchorage require a stressing length that is often protected by grease and zone is within soil or rock types that creep, then loss of pre- a plastic sheath. However, a gap behind the anchor plate is stress could affect service life. Regular maintenance of lock- still possible unless a trumpet head assembly is employed. off loads could be implemented to achieve a given service life Thus, the main concern with portland cement-grouted rock for these conditions.