NCHRP-RRD-364.pdf
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1、Research Results Digest 364 October 2011 INTRODUCTION This digest summarizes the results of research conducted as a follow-up to NCHRP Project 24-28, “LRFD Metal Loss and Service-Life Strength Reduction Fac- tors for Metal Reinforcements in Geotech- nical Applications.” NCHRP Project 24-28 assessed
2、and improved models for corro- sion potential, metal loss, and service life of metal-reinforced systems used in retaining walls, and highway cuts and fi lls. The proj- ect compiled a performance database for earth reinforcements that describes the exist- ing conditions and measured corrosion rates o
3、f in-service reinforcements at approxi- mately 170 sites across the United States and Europe. Data were obtained from rein- forcement types typical of mechanically stabilized earth systems (MSES) as well as installations of rock bolts and ground anchors. These data were used to identify and group tr
4、ends related to the character of the fi ll or in-situ earth materials, reinforce- ment type, and various site conditions. Based on these trends, the reliability of metal-loss modeling and service-life pre- dictions were computed and the corre- sponding biases with respect to nominal tensile strength
5、s used in design were evalu- ated and then used to calibrate appropriate resistance factors for use in load and resis- tance factor design (LRFD). The results, fi ndings, and conclusions of NCHRP Proj- ect 24-28 were published as NCHRP Report 675: LRFD Metal Loss and Service-Life Strength Reduction
6、Factors for Metal- Reinforced Systems.1 MSES reinforcements consist of galva- nized steel strips, mats, or grids. The possi- bility of metal loss is considered in design through selection of fi ll materials that are relatively nonaggressive, and the use of sacrifi cial steel in the reinforcement cro
7、ss section. NCHRP Project 24-28 found that estimates of sacrifi cial steel requirements and remaining strength were reliable for reinforcements surrounded by high- or good-quality fi ll materials. However, the performance estimates for reinforcements in marginal-quality fi lls were highly uncer- tai
8、n. Also, the estimated performance of galvanized reinforcements after the base steel was exposed by depletion of the zinc coating was subjective due to a lack of data from older installations. This situation could VALIDATION OF LRFD METAL LOSS AND SERVICE-LIFE STRENGTH REDUCTION FACTORS FOR METAL-RE
9、INFORCED SYSTEMS This digest summarizes key fi ndings of NCHRP Project 24-28A, “Validate the Results of NCHRP Project 24-28,” conducted by McMahon the other half had known defects including reduced cross-sec- tions, breached sheathing along the free length, or voids in the grout along the bonded len
10、gths. The SE and IR tests were applied to the dummy rock bolts, and the validity of the results was con- fi rmed based on comparisons with known condi- tions. Results from both the IR test and the SE test rendered useful information for condition assessment. Levels of pre-stress were interpreted fro
11、m SE test results in qualitative terms as high, moderate, or rela- tively low, based on a comparison of results between samples. This capability has been recognized previ- ously, and the results from the IR do not offer any improvements in this regard. Clear refl ections were apparent from the inter
12、faces between the free lengths and the bonded zones, such that results from the SE test were useful to identify the free lengths of the test elements. However, details of anomalies and condi- tions within the bonded zones were difficult to discern, or not discernable, from results of SE testing. Imp
13、or- tant features of the installations were more apparent from the IR test results, compared with the SE test results, including: 1. Interfaces between bonded and unbonded zones are more distinct. 2. Details and conditions within the bonded zone are apparent in the results from the IR test, which ca
14、nnot be discerned from the results of SE testing. 3. Mobility plots are affected by anomalies in terms of distinct reductions in the energy (peaks) and frequencies at resonance. This is due to the energy loss from additional wave refl ections caused by the anomalies, and cor- 4 responding reductions
15、 in the propagation velocities for compression waves traveling within the grout. 4. The cross-sectional areas of the elements at the sources of refl ections were observed from results of IR testing, which are obtained from the mobility at resonance. CONCLUSIONS This project addressed proposals made
16、in NCHRP Project 24-28 to (1) obtain more reliable data and reduce uncertainty with respect to the per- formance of MSES constructed with marginal quality fills; (2) obtain additional performance data from older installations of MSES reinforcements; and (3) implement more robust test techniques to e
17、valuate the existing conditions of rock bolts, soil nails, and ground anchors. Results presented herein serve to further validate results from NCHRP 24-28 and the predictive models for corrosion potential, metal loss, and service life of metal-reinforced sys- tems developed therein. 1. The variabili
18、ty of the performance of mar- ginal fi lls observed in NCHRP Project 24-28 is due to uncertainty with respect to the fi ll properties, which may also be inherently variable. Measurements of fi ll resistivity obtained at the location and time of corrosion rate measurements reduce this uncertainty and
19、 improve the ability to model the perfor- mance of MSES constructed with marginal quality fi lls. Results presented in this report demonstrate that the resistivity of in-situ materials surrounding earth reinforcements can be determined at the time and location of corrosion rate measurements. The tec
20、hnique employs measurements of resistance via the three-electrode technique, and a simple rela- tionship between the measured resistance and resistivity. Measurements of corrosion rate and resistivity in the three-electrode confi gu- ration are most useful, since the majority of measurements in the
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