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73 Table 4.10. Mixture proportions of three CLSM mixtures provided by local producers. Cement Fly Ash Type Fly Ash Coarse Air Fine Aggregate Water Mixture Content (ASTM C Content Aggregate 3 3 Content (kg/m ) (kg/m ) (kg/m3) 618) (kg/m3) (kg/m3) (%) A 18 Class F 178 909 908 168 7 B 18 Class F 178 909 906 168 7 C 18 Class F 178 771 1008 197 5 material. These issues include flowability, compressive strength directly from the chute of a ready-mix truck, which was the development, setting time, subsidence, and excavatability of method used for filling all the trenches. CLSM. The field version of the needle penetrometer (ASTM C 403) was used to characterize the setting and hardening of CLSM mixtures in the trenches. Cylinders (75 × 150 mm) were also Materials and Mixture Proportions cast for compressive strength testing. The cylinders were capped For the field test, mixtures from three CLSM producers in to prevent moisture loss and were left at the site until the test the Oakland, California, area were selected by EBMUD engi- date. Three days after casting the other samples were trans- neers. The proportions for the three mixtures (one per pro- ported to a curing room at EBMUD. Samples were tested at 4, ducer) are shown in Table 4.10. The raw materials (cement, fly 7, 28, and 63 days by EBMUD technicians using neoprene pads ash, aggregates) varied from producer to producer, but the (as per the recommendations provided in Appendix B) general mixture proportions were quite similar. The excavatability of the CLSM mixtures was investi- gated 63 days after their placement into the trenches by EBMUD engineers. Qualitative assessments were performed to Experimental Program determine the excavatability of the CLSM mixture with a hand Six trenches (referred to as trenches 1 through 6), approxi- shovel, a solid steel bar, and a backhoe. mately 1.2 m wide, 1.5 m deep, and 2.7 m long, were laid out by EBMUD staff. Each CLSM mixture was used to backfill two Test Results trenches. Flow and air content of each CLSM mixture were measured before placement, and adjustments were made to Fresh Properties achieve the target flow of approximately 150 to 225 mm. The flow and air content of the three CLSM mixtures are Trenches 1 and 2 were filled with CLSM mixture A. The mix- shown in Table 4.11, along with the ambient temperature and ture was determined to be too stiff and 322 kg of water was relative humidity at time of placement. The flow values for the added to increase its flow to 150 mm. Trenches 3 and 4 were three mixtures were adequate for the trench filling (some water filled with CLSM mixture B. The mixture had a good consis- was added to mixture A to obtain the desired flow). The air tency and no water was added. Trenches 5 and 6 were filled contents were less than expected (based on the mixture pro- with mixture C, with no additional water needed to achieve the portions provided by the three suppliers), but no adjustments target flow. Figure 4.11 shows one of the trenches being filled were made to the air content of the field mixtures. Setting and Hardening Figure 4.12 shows the setting time and hardening data for the three CLSM mixtures used to backfill the six trenches dur- ing the field test. The data are based on field penetrometer Table 4.11. Fresh properties for CLSM mixtures used in EBMUD field test. Flow Air Ambient Relative Mixture (mm) Content (%) Temperature (°C) Humidity (%) A 152 1.0 30 31 B 216 0.7 30 36 Figure 4.11. Backfilling trenches with CLSM. C 191 0.4 30 31
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74 10000 8000 measured by the penetrometer test Penetration resistance (kPa) 6000 4000 A-1 A-2 B-3 2000 B-4 C-5 C-6 0 0 2 4 6 8 10 12 Time (hr) Figure 4.12. Field penetrometer data from EBMUD field test. values taken from each of the trenches. Note that the legend term in the fog room. The strength of mixture A, especially after denotes the mixture, followed by the trench number (for 63 days, was substantially higher than the other mixtures. example, A-1 denotes mixture A placed in trench 1). The set- ting times for a given mixture were quite similar from one Excavatability trench to another. Mixture A hardened quicker than the other two mixtures, which may be caused by one or a combi- The excavatability of one trench from each of the CLSM nation of several factors, including lower water content (as mixtures (trenches 2, 4, and 6) was evaluated 63 days after evidenced by stiffer consistency of as-received CLSM) and placement using manual methods (shovel and steel bar) and different cement and fly ash sources. mechanical methods (backhoe). The difficulty of excavating the trenches was evaluated based on whether the power and time required was low, moderate, or high; the results are sum- Compressive Strength marized in Table 4.13. As expected, trench 2 was the most Compressive strength test results of the three CLSM mixtures difficult of the three to excavate, and when excavated with a are shown in Table 4.12. For each mixture, tests were conducted backhoe, the chunks removed were quite large, which could on cylinders stored adjacent to the trenches and in a fog room be problematic when excavating around pipes. (standard curing). Cylinders were cured under two different Interestingly, trench 4 was fairly difficult to remove with a conditions: field and moisture room. In virtually every case, the shovel, even though the compressive strength was less than laboratory-cured cylinders exhibited lower strengths than the 0.5 MPa at the time of removal. This difficulty in removing field-cured cylinders. This phenomenon is most likely due to the CLSM with a shovel is likely attributed to the coarse ag- the higher temperatures on site and perhaps also due to the gregates contained in this mixture. This result illustrates that leaching of hydration compounds from cylinders stored long- strength alone is not an adequate indicator of excavatability; Table 4.12. Compressive strength of field- and laboratory-cured cylinders from EBMUD field test. 4-Day 7-Day 28-Day 63-Day Mixture Strength (kPa) Strength (kPa) Strength (kPa) Strength (kPa) Field-Cured 323 348 890 >1950 A Lab-Cured 310 410 779 >1950 Field-Cured 241 241 504 497 B Lab-Cured 212 263 351 381 Field-Cured 224 280 459 602 C Lab-Cured 221 246 358 314