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30 Engineers in Hamilton County, Ohio, and the city of condition was assumed to represent saturation. Because the Cincinnati have found this methodology to be effective in lim- samples were moist cured prior to testing, the samples were es- iting long-term strength gain and ensuring future excavatabil- sentially already saturated prior to sample conditioning. Thus, ity. The research team used the same approach in calculating the requirement that the B-value (ratio of pore water pressure RE values for the C-series mixtures and comparing the results to confining stress) be greater than or equal to 0.95 was waived to other direct or indirect indices of CLSM strength gain. for these tests. A confining pressure of 173 kPa was applied In preliminary investigations within this project, the split- during the tests. ting tensile strength of CLSM was identified as a potential in- dicator of excavatability. Splitting tensile strength is also a Triaxial Shear Strength very simple property to measure (without the need to cap the cylinders). The stress conditions of CLSM specimens under A commonly used soil triaxial test method (USACE EM splitting tensile testing may be quite similar to stress condi- 1110-2-1906) was used for testing the shear strength of tions of CLSM mixtures under digging conditions with a CLSM. The samples were cast in Shelby tubes (approximately shovel or backhoe. The E-series mixtures (Table 3.11) were 70 mm diameter) and were stripped after 7 days. Testing was used to evaluate various capping materials and methods, with performed under consolidated and drained conditions at this some of the key capping-related parameters shown in Table time, and additional specimens were tested at an age of 28 days 3.19. In addition, mixtures E1 through E8 were tested exten- (note that the specimens were moist cured in a fog room from sively to evaluate the effect of durometer value (neoprene pad the time of stripping until the time of testing). The pore water hardness) on CLSM strength, as described later in this report. pressure was maintained at 34.5 kPa, and the confining pressures were 69, 103.5, and 172.5 kPa. The loading rate was California Bearing Ratio 0.38 mm/min, the same loading rate used for most standard unconfined compression tests. The test was terminated when Moist-cured specimens were tested at an age of 28 days the residual strength was reached or the stress-strain curve using a slightly modified version of AASHTO T 193. The became essentially flat. By curve fitting, the effective internal only modification was that the CLSM was placed into the friction angle, , and the effective cohesion, c, were deter- molds without compaction, as is required for testing soils. mined. Various other shear strength test methods could also After 7 days of curing, the collar of the mold was removed, be used for evaluating CLSM; the method selected for this and the surface of the CLSM specimen was trimmed level study should not be considered as the only viable approach. using a straight edge. Drying Shrinkage Resilient Modulus No standard methods exist to measure the drying shrink- Moist-cured specimens were tested at an age of 28 days age of CLSM. A method commonly used in Germany for self- using a slightly modified version of AASHTO T 292, with the leveling floor screeds was modified and used in this study. modification relating to the deviator stresses. In trial testing, CLSM was cast into an 87.5 × 26.3 × 1000 mm steel channel. the deviator stresses listed in Table 4 of AASHTO T 292 were The channel had one fixed end plate with an anchor and one not found to be sufficiently high to introduce deformations. movable end plate with an anchor. Before the CLSM was cast, The selection of deviator stresses was based on previous re- wax paper was placed on the inside of the channel to reduce search performed at Texas A&M University. Load condi- friction. CLSM was then placed in the channel forms. The tioning of 41 kPa was used for the 1000 repetitions. Since the amount of shrinkage was measured using a linear variable dif- completion of the laboratory component of this project, ferential transformer (LVDT) that measured the displacement AASHTO T 292 has been replaced by AASHTO T 307. Re- of the movable end plate. Shrinkage measurements were taken search should be conducted using this new test method in daily for the first week and once a week thereafter. the future to ensure that it is a viable test method for evalu- ating CLSM. Durability Test Methods Water Permeability Corrosion The water permeability of six CLSM mixtures, moist cured A comprehensive laboratory corrosion program was per- for 28 days, was measured using ASTM D 5084. A back pres- formed, with the objective to characterize the corrosion per- sure of 69 kPa was applied and maintained until no additional formance of ductile iron and galvanized steel embedded in water entered the sample (approximately 30 minutes). This CLSM and to identify key parameters that significantly influ-
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31 ence the corrosion performance of these materials. This re- Resistor search was performed in two phases: the first phase was a CLSM Soil smaller scale study (using the 38 initial CLSM mixtures), and the second phase was a more significant follow-up study aimed at confirming the findings from the first phase and de- veloping a thorough understanding of the corrosion of met- als in CLSM. Metallic coupons machined from ductile iron and galva- nized steel pipes were tested in two conditions: uncoupled Solution and coupled. Figures 3.2 and 3.3 show the samples for the un- level coupled and coupled conditions, respectively. In the uncoupled state, metallic coupons were embedded in 75 × 150 mm plastic cylindrical molds containing CLSM. The center of the metallic coupon was placed at the center of Metallic Coupons the cylinder, 50 mm from the top surface. Because CLSM is a low-strength material, care was taken not to damage the sam- ples after casting. Precutting the plastic molds longitudinally and taping these cuts closed prior to casting minimized the damage for the uncoupled specimens. After casting, the tape was removed and the plastic mold was separated (not re- moved) from the CLSM sample surface. Figure 3.3. Corrosion test setup for compar- Coupled samples were prepared to address the issue of ing corrosion performance of galvanic metals not being completely embedded in CLSM in the field coupled coupons in CLSM and sand. applications. For the coupled conditions, pairs of ductile iron or galvanized steel coupons were embedded in 100 × paper that would allow the exposure solution to enter into 200 mm plastic molds that were half-filled with CLSM and the cylinders while preventing the soils from being washed half with soil. In this condition, one of the metallic coupons from the molds. was completely embedded in CLSM and the other coupon Control samples were similar to the uncoupled samples, was completely embedded in soil and they were connected but metallic coupons were completely embedded in sand. with a 10 ohm resistor at the top as shown in Figure 3.3. The Ductile iron coupons, 13 × 24 × 4 mm in size, were machined metallic coupons were secured such that both were approx- from a 300 mm diameter commercially available ductile iron imately 5 mm from the CLSM/soil interface. Six holes pipe (AWWA C151, Grade 60-42-10) and zinc galvanized steel (4 mm diameter) were drilled at 15 mm above the bottom coupons, 13 × 24 × 3.5 mm in size, were machined from a of each cylinder and the holes were wrapped with a filter 300 mm diameter zinc galvanized steel culvert (uncoated thick- ness approximately 3.40 mm). All CLSM samples were cured for 28 days at 23 ± 2°C and a relative humidity greater than 98 percent. Later, samples 75 x 150 mm cylinder were exposed to a 3.0 percent sodium chloride solution or Coupon distilled water. The liquid level was maintained at a level of 90 mm throughout the test program. As previously stated, the corrosion study was performed in two phases. In the first phase, a large number of CLSM mix- tures were evaluated with a low number of samples per CLSM 100 mm mixture. In the second phase, a lower number of CLSM mix- CLSM tures were evaluated with a higher number of samples com- or pared to the first phase. The number of samples was increased sand in the second phase for a better statistical analysis. In both phases, uncoupled and coupled samples were prepared and tested. Figure 3.2. Corrosion test setup for comparing corro- In the Phase I investigation, the initial thirty-eight CLSM sion performance of coupons in CLSM and sand mixtures (thirty mixtures and eight duplicates) were evaluated (uncoupled condition). to determine the influence of CLSM constituent materials and
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32 proportioning on the corrosion of metals embedded in batch for both sample types). In the Phase II study, the resis- CLSM. The mixture proportions and fresh CLSM character- tivity of CLSM and soils were not measured from separately istics are shown in Tables 3.3 through 3.5. In this first phase, cast samples, but from each of the actual exposed uncoupled only ductile iron coupons were evaluated. Three coupled and and coupled samples following ASTM G 57. uncoupled samples for each of the thirty-eight CLSM mix- In the Phase I study, two 50 × 100 mm cylinders were cast tures and five control samples were fabricated. All of the sam- for each CLSM mixture at the same time as the corrosion ples were exposed to 3.0 percent sodium chloride solution for samples were cast to evaluate their pH. At 182 days after cast- 18 months. The control samples and the soil section of cou- ing, the CLSM cylinders were removed from the curing room, pled samples were filled with a sand meeting the "graded and pore solution was extracted from the samples and im- sand" requirements of ASTM C 778, "Standard Sand." mediately evaluated for pH. In the Phase II study, CLSM and In the Phase II investigation, a total of 13 CLSM mixtures distilled water solutions (1:1 by weight) were prepared from were selected and cast to evaluate the corrosion of metals em- each exposed uncoupled and coupled sample to evaluate for bedded in CLSM. The mixture proportions and fresh CLSM pH. In both phases, a pH combination electrode connected characteristics are shown in Table 3.16. Lower case letters to a bench top multimeter with a precision of 0.01 was used added to the mixture designation indicate separate batches. to measure the pH. In the second phase, the pH of soil sam- Ductile iron and galvanized steel coupons were evaluated for ples used in the coupled samples was also determined using corrosion activity. A minimum of five coupled and five un- 1:1 by weight distilled water solutions. Because only one type coupled samples were prepared for each of the thirteen CLSM of clay and only one type of sand was used in the samples, mixtures and exposure conditions. More than 1000 samples only randomly selected soil samples from coupled samples were evaluated in the Phase II study. Half of the samples were exposed to the chloride and distilled water environments exposed to 3.0 percent sodium chloride solution and the re- were collected and tested. One soil pH value was determined maining samples were exposed to distilled water. All samples for each type of soil exposed to each type of environment in were exposed for 26 months. Sand and clay were used to fill a coupled sample. the soil section of each coupled sample. The sand met the Chloride contents were determined using a test method "graded sand" requirements of ASTM C 778. The clay used developed under the Strategic Highway Research Program. was obtained from the National Geotechnical Experimenta- This method rapidly determines the chloride content in ce- tion Site located at the Texas A&M University Riverside Cam- mentitious materials (Cady and Gannon 1992). pus. The plastic and liquid limits of the clay were 20.9 percent and 53.7 percent, respectively, and the hydraulic conductivity Freezing and Thawing coefficient was 5 × 10-4 m/year. In both phases, metallic coupons were removed from the ASTM D 560, a method designed to measure the freeze- samples at the end of the exposure period and were evalu- thaw resistance of soil-cement mixtures, was used with one ated for mass loss following ASTM G 1, "Preparing, Clean- modification: thawed specimens were not brushed because of ing, and Evaluating Corrosion Test Specimens." Ductile the low strength of CLSM. CLSM samples were exposed to a iron coupons were cleaned using cleaning procedure C.3.5 temperature change from -18°C (a freezer) to 23°C (the fog and galvanized steel coupons were cleaned using cleaning room) in each cycle. Samples were exposed to 12 cycles, un- procedure C.9.5. In the case of the coupled samples, only less they suffered severe damage at an earlier time. Mass loss the coupons embedded in the sand were evaluated for mass was monitored as an indicator of damage. In the initial study, loss as they were determined early in this study to be the six cylinders from each mixture were exposed to freeze-thaw anode. The coupon embedded in the CLSM section of these cycles. Three of these cylinders were moist cured for 7 days samples exhibited limited corrosion, if any. Evaluation and the other three were moist cured for 28 days prior to of the corrosion performance of coupons was based on freeze-thaw cycling. Because the tests typically used for con- the percent mass loss due to corrosion (amount of mass crete, such as the ASTM C 666, were found to be too severe in loss resulting from corrosion divided by the original mass preliminary trials for CLSM, the modified soil cement method of coupons). was found to be a more suitable approach. In the Phase I study, the resistivity of the CLSM and sand In addition to the original six mixtures, a follow-up study were evaluated using a resistivity box (or soil box) as de- was conducted to specifically investigate the effects of freeze- scribed in ASTM G 57, "Field Measurement of Soil Resistiv- thaw damage on CLSM permeability. Eleven mixtures (D-series ity Using the Wenner Four-Electrode Method." Resistivity in Table 3.10) were used to study the freezing and thawing measurements were obtained from saturated samples 182 days effects on permeability. The specimens were 100 × 125 mm after casting. These samples were cast at the same time with and were subjected to freezing and thawing cycles at an age of the corrosion samples (i.e., the CLSM came from the same 28 days (as per the modified version of ASTM D 560 described