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87 2000 Truck 1 Truck 2 1500 Compressive Strength (kPa) 1000 500 0 0 5 10 15 20 25 30 Time (days) Figure 4.30. Compressive strength of laboratory-cured cylinders from TAMU field test. Long-Term Corrosion Testing The research team will continue to monitor this unique long-term corrosion site and hope that these data will prove As mentioned earlier in this section, a major thrust of this useful in developing information about the service life of met- field test is to generate field data on the corrosion of metals als embedded in CLSM. Long-term excavatability studies also imbedded in CLSM in the field. As was expected, the rate of will be performed as part of these ongoing efforts. corrosion under these field conditions has been quite low, and after more than 2 years of monitoring, little active corrosion has been measured. For completeness, a brief summary of the Summary of Key Findings corrosion data (half-cell potential) is provided in Tables 4.19 from Field Tests and 4.20. These tables show the average half-cell potential measurement against the Cu-CuSO4 reference electrode of the This chapter has summarized the findings from six CLSM four galvanized steel or the four ductile iron pipes exposed to field tests performed throughout the United States. As previ- the same conditions. The half-cell potentials shown are time- ously mentioned, the main goals of this field testing program weighted averages for the conditions shown. were to fill in the gaps in understanding CLSM behavior and Table 4.19. Time-weighted average half-cell Table 4.20. Time-weighted average half-cell potentials for the clay site. potentials for the sand site. Weighted Weighted Soil Soil Environment Condition Pipe Type Half-Cell Environment Condition Pipe Type Half-Cell Type Type Potential (V) Potential (V) Galvanized 0.7672 Galvanized 0.7006 CLSM CLSM Ductile 1.0719 Ductile 0.6169 Galvanized 0.7018 Galvanized 0.6340 Chloride CLSM/Soil Chloride CLSM/Soil Ductile 1.0030 Ductile 0.9711 Galvanized 0.6850 Galvanized 0.4242 Soil Soil Ductile 1.0135 Ductile 0.5911 Clay Sand Galvanized 0.6997 Galvanized 0.8330 CLSM CLSM Ductile 0.8849 Ductile 0.8708 Galvanized 0.6165 Galvanized 0.4646 Non-chloride CLSM/Soil Non-chloride CLSM/Soil Ductile 0.8923 Ductile 0.9200 Galvanized 0.5585 Galvanized 0.3537 Soil Soil Ductile 0.9236 Ductile 0.9078
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88 performance and to validate the test methods, specifications, makes full reliance on laboratory strengths when predicting and guidelines developed in the earlier stages of this project. excavatability difficult. Testing cylinders in the laboratory In general, the field tests proved to be quite successful and under conditions expected in field installations is recom- enlightening. For the most part, the test methods, specifications, mended when excavatability is a concern. and guidelines developed under this project were found to be · The removability modulus, originally developed by Hamil- appropriate and effective. Several specific technical issues were ton County (Ohio) engineers, is a useful tool in attempt- addressed in the course of these field tests, with an emphasis ing to predict excavatability. This method is an empirical on aspects of CLSM behavior that could not be adequately approach to predicting excavatability using an equation evaluated in the laboratory, such issues as excavatability and featuring the unit weight and 28-day strength value of corrosion. Although the relevant data from some long-term CLSM. This approach can be further improved upon by corrosion field tests were not collected under this project using field-cured strengths, thereby minimizing the dis- (because of the slow rate of corrosion in field installations), connect with laboratory-cured cylinders, in the equation. it is anticipated that these data will be collected in the future The inclusion of unit weight is actually quite helpful as a and presented to the appropriate AASHTO committees for parameter used in predicting excavatability because it tends consideration. to pick up the aggregate-related effects associated with Some of the specific findings from this field testing pro- excavatability. Specifically, in some field tests, the lack of gram are briefly summarized below: aggregates in CLSM (e.g., mixtures with 95 percent Class F fly ash, 5 percent portland cement, and water added for · The basic tests for CLSM, such as flow, air content, and unit desired fluidity) resulted in a mixture that was easier to weight, were found to be effective and easy to implement, excavate than would have been expected, based solely on and most jurisdictions involved in the field tests were already the strength of the mixture. routinely using the tests in practice. · The DCP was found to be a particularly useful tool in mon- · The compressive strength of CLSM was measured in each itoring early-strength gain of CLSM, as well as the long- field test using the testing methodology developed under term strength and excavatability of installations. This this project. The approach for proper handling, curing, cap- method is unique in that it allows for measuring the prop- ping, and testing was validated throughout the process. erties of CLSM as a function of depth, thereby avoiding a · The tests showed that strength measured on standard-cured shortcoming of surface penetration tests (needle pen- cylinders can vary significantly from actual CLSM strength etrometer, soil penetrometer) that only assess the near- in field applications, mainly because of differences in time- surface behavior. temperature histories. This disconnect appears to be great- · Due to the high fluidity of CLSM mixtures, floating of est when fly ash is used, and as such, users should be aware pipes or unintentional shifting of utilities may result, and of this issue when considering long-term excavatability. users should take precautions to avoid this behavior. Such · There is no single property of CLSM (e.g., compressive precautions are addressed in the specifications and guide- strength) that can be used as a definitive index for excavata- lines developed under this project for backfill applications. bility. Compressive strength is the most commonly mea- · More long-term monitoring is essential for a true assess- sured and reported CLSM property, and can be a reasonable ment of corrosion of metals in CLSM. Tests initiated under index of excavatability in some cases. However, the discon- this project will continue to be monitored, and the relevant nect between the strength of laboratory-cured cylinders and findings will be communicated to the appropriate AASHTO the actual long-term strength of CLSM in trenches, etc. committees.