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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Validation of LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14587.
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Suggested Citation:"Findings ." National Academies of Sciences, Engineering, and Medicine. 2011. Validation of LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems. Washington, DC: The National Academies Press. doi: 10.17226/14587.
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3test results with those found under known conditions. 3. Further verify the test methods and relation- ships developed in Task 2 through field test- ing. This was accomplished through mea- surements of corrosion rates and resistivity from sites with marginal fills where fill sam- ples could be retrieved and tested for resistiv- ity using a standard test box as a basis of comparison. 4. Obtain field data from older sites to evaluate the long-term performance of base steel subse- quent to depletion of zinc coating from older in-service reinforcements. FINDINGS Resistivity Measurements The linear polarization resistance (LPR) technique has been used to determine the corrosion rate of in- service earth reinforcements. A three-electrode con- figuration is commonly employed whereby a current is impressed between two in-service reinforcements that serve as the working and counter electrodes, and the surface potential of the working electrode is mea- sured with respect to a half-cell that serves as the ref- erence electrode. The earth material surrounding the working electrode (earth reinforcement under test) serves as an electrolyte and measurements of polariza- tion resistance must be corrected to consider the effects of this additional impedance. Current practice is to measure the resistance of the earth material via an AC impedance technique immediately following the DC polarization measurements. Results from this project demonstrate that earth resistivity (ρ) can be computed from measurements of earth resistance as: where L and D are the electrode length and diame- ter, respectively, and R is the measured earth resis- tance. This very simple expression appears to render good results in most cases. Thus, measurements of earth resistivity can be made at the same time and location as corrosion rate measurements for earth reinforcements. A test embankment was constructed to verify the use of Equation 1 for relating the measured fill (earth) resistance to resistivity. The embankment ρ π= × × × ×( ) −⎡⎣⎢ ⎤⎦⎥ 2 8 1 1L RL D ln ( ) was split into two sections, each with a different fill material of known resistivity. Different reinforce- ment types, sizes, and shapes were installed and the spacing between the reinforcements was varied. Measurements from the test embankment and use of Equation 1 yielded values of ρ that were in reason- able agreement with the known values of resistivity as long as the reinforcements were located at least 2 feet away from the surface of the embankment. On average, resistivities computed with Equation 1 were within 10 to 20 percent of the known values. Field Monitoring of MSES Equation 1 was further verified via data from testing in-service MSES reinforcements at sites located in California under the jurisdiction of Cal- trans. Five sites were included in the field investi- gation with ages of reinforcements that ranged between 5 and 39 years old. In-situ measurements of resistivity were obtained from the three-electrode, AC impedance technique and use of Equation 1. Additionally, samples of fill material were retrieved through access holes penetrating the precast con- crete wall face units and tested for resistivity via a soil box similar to that described in AASHTO T 288. The latter test results served as a basis for comparison with in-situ measurements of resistivity. Reasonable results were obtained wherein in-situ testing rendered a range of results that was consis- tent with baseline values obtained from samples tested in the soil box. Furthermore, corrosion rate measurements were negatively correlated with resis- tivity (i.e., higher corrosion rates were associated with lower resistivity). Thus, results from corrosion rate measurements correlated very well with in-situ measurements of resistivity. This fieldwork included the oldest MSE wall in the United States, which was constructed in 1971, along Route 39 through the San Gabriel Mountains near Los Angeles, and was 39 years old at the time when observations were made. Reinforcements unearthed from this site appeared to be in excellent condition with no significant metal loss, although base steel was exposed in some locations. Observed corrosion rates were less than those anticipated from the current metal-loss rates suggested by AASHTO for MSES design. These data suggest that the bene- fits of galvanization are realized for much longer than the 16 years implied by the current edition of the AASHTO LRFD Bridge Design Specifications.

Thus, measurements of corrosion rate obtained from the Route 39 site are extremely interesting and are some of the first confirmed observations of the rate of steel consumption subsequent to depletion of zinc for galvanized reinforcements used in MSE con- struction in the United States. These rates are much less than those used in design and appear to be closer to the corrosion rates used for zinc, suggesting that a discrete change in corrosion rates as zinc is depleted, as implied by the current AASHTO model for metal loss of MSE reinforcements, does not necessarily occur. Implementation of Impulse Response Test for NDT of Rock Bolts The impulse response test (IR) is used for probing the lengths of rock bolts to access details of the instal- lation and conditions surrounding the structural ele- ments (i.e., steel rods). The IR test involves applica- tion of an impact to the free end of the rock bolts with an instrumented hammer and measurement of the response of the element with a transducer attached near the point of impact. Measurements of the impact force and the resulting response are processed in the frequency domain. A mobility curve is obtained by dividing the velocity response spectrum by the force spectrum, and incorporates a range of frequencies corresponding to the energy content of the impact (0–4000 Hz). The initial slope of the mobility plot yields the dynamic stiffness of the system, and the fre- quency of the peaks at resonance describes the geom- etry. By comparison, the SE test does not require an instrumented hammer, and only involves measuring the response of the rock bolt after impact, which is presented in the time domain. The mobility curve yields the impedance of the rock bolt corresponding to the material properties and geometry of the installation. The impedance of the rock bolt is related to the geometric mean of the heights of the resonant peaks in the portion of the mobility curve where the shaft response is in reso- nance. Mobility is related to impedance as (Davis and Robertson, 1975) where ρ is mass density = 4.66 (lb*s2/ft4) for grouted rock bolts, Vp is compression wave velocity = 12,000 ft/s for grouted rock bolts, and A is the cross- N V Ap = × × 1 2 ρ ( ) sectional area corresponding to the resonant portion of the mobility plot. Thus, the cross-sectional area corresponding to the resonant portion of the mobil- ity plot can be determined from measurements of cyclic mobility and knowledge of the density and compression wave velocity of the grout surrounding the earth reinforcement. The utility of the IR test was evaluated by testing six dummy rock bolts installed in cooperation with the New Hampshire Department of Transportation at the site of the Barron Mountain Rock Cut along I-93 near Woodstock, NH. Dummy rock bolts were installed using materials and techniques similar to actual rock bolt installations. Installations varied with respect to geometry including the free lengths, bonded lengths, and total lengths of the rock bolts. One-half of the installations were “normal”; 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 lengths. The SE and IR tests were applied to the dummy rock bolts, and the validity of the results was con- firmed 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 from 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 reflections were apparent from the interfaces 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. Impor- 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 cannot 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 reflections caused by the anomalies, and cor- 4

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Validation of LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Research Results Digest 364: Validation of LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems summarizes the results of research to further validate some key results of a project that resulted in publication of NCHRP Report 675: LRFD Metal Loss and Service-Life Strength Reduction Factors for Metal-Reinforced Systems.

NCHRP Report 675 explores the development of metal loss models for metal-reinforced systems that are compatible with the American Association of State Highway and Transportation Officials' Load and Resistance Factor Design (LRFD) Bridge Design Specifications.

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