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Soil-cement pipe embedment has been used by the Bureau of Reclamation for about 25 years. The ingredients of the soil-cement can vary, but typically it is a combination of soil, portland cement, and water. In most cases, the pipe trench is trimmed so that a semicircular excavation is created that is only slightly larger than the pipe diameter. The soil-cement is used to fill the gap between the pipe and the in situ soil. Thus, the native trench material must be able to provide adequate supporting strength to the pipe. The consistency of the soil-cement can vary from a fluid (slurry) to a mixture of about 25-cm (10-in.) slump, depending on the placement requirements. The consistency, ingredients, and placement dimensions can all vary as long as two basic requirements are met: 1) The material must be placed so that there is complete contact between the pipe and the in situ soil; and 2) The unconfined compressive strength of the hardened material is at least 700 kPa (100 lb/in. 2) at 7 days. The most suitable soil to use is a silty sand with the fines content not exceeding about 30 percent. This allows native soils from the trench excavation or from nearby the construction site to be used. Fly ash has been used in place of cement, and bentonite has been added to improve pumping characteristics. The versatility and consistent mixing and placement characteristics of soil-cement slurry have made it a popular choice for contractors.

The new Denver International Airport (DIA) has used the world's greatest volume of flowable backfill, estimated at 450,000 yd 3. For one of the initial projects, more than 70,000 yd 3 of flowable backfill were utilized as a bedding material for a concrete pipe drainage system located beneath the taxiways at DIA. The project is unique due to the large volume of flowable backfill required for a single contract, use of on-site sand, and the efficiency of the mixing and placing operation. Paper presents a case study of the project, including mix designs, cost, mixing operation, placing, quality control, and properties of the in-place material.

The effect of mix proportions on the compressive strength of flowable fill is investigated in this study. Flowable fill, composed of Type I cement, Class C fly ash, sand, and water, meets the requirements of ACI Committee 229 as a controlled low-strength material (CLSM) if the 28-day compressive strength is 1200 psi (8.3 MPa) or less. Extensive laboratory tests were conducted to determine the effects on the engineering properties of varying the water/cement plus fly ash, sand/cement plus fly ash, and fly ash/cement ratios. A wide range of ratios was evaluated to provide a cost-effective mix design for various material costs. Compressive strengths were determined at 1, 7, 14, and 28 days, providing an indication of the rate of strength development and ultimate compressive strength.

Paper covers various aspects of controlled low-strength material (CLSM) and discusses CLSM's durability factors. It reviews CLSM's early history, listing possible uses and applications. Quality assurance and quality control compaisons are made between conventional portland cement concrete and CLSM. CLSM durability factors are referenced for each possible use and application, and tests are listed for CLSM's durability and methods to insure this durability.

Most CLSM applications are designed not to resist freeze and thaw. However, in some applications, CLSM is susceptible to freeze-thaw deterioration. The failure of CLSM specimens in freeze and thaw is due mostly to surface scaling. Using a foam generator air-entraining admixture to distribute the voids throughout the mix yields good results. While the permeable void content of a typical CLSM mixture at 28 days is about 27 percent, low-strength CLSM has increasing pulse velocity but does not disintegrate into pieces. However, a typical higher strength CLSM (about 1.103 MPa or 160 psi at 28 days) with the same permeable voids has an almost constant pulse velocity during the freeze-thaw test with a slow rate of cooling. Typical 28-day compressive strength of CLSM ranges from 0.345 to 1.379 Mpa (50 to 200 psi), which is higher than that of most compacted soil or granular fills. The most important property of CLSM at early age is strength development, so that several hours after placement, the structure or facility can resume operation. The results show that there are two important stages in CLSM strength development: the stiffening stage, which indicates that the CLSM is beginning to develop cohesion; and the hardening stage, which indicates that the CLSM has hardened to the point at which it can sustain some load. Presently, only the hardening stage is recognized. CLSM can sustain foot traffic without surface depression 2 to 3 hr after the cement is in contact with the water. However, every CLSM develops strength differently, so that the hardening stage based on time in hours is inappropriate. Using penetration-resistance members is more appropriate for CLSM.