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.

Controlled Low-Strength Material (CLSM) is a self-compacted cementitious material used primarily as a substitute for compacted backfill. The material is known by many names including flowable fill, controlled density fill, unshrinkable fill and soil-cement slurry. The American Concrete Institute defines CLSM as a material with a maximum unconfined compressive strength of 1200 psi. For most applications, however, the compressive strength of CLSM does not exceed about 300 psi. This makes it possible for the material to be removed should future excavation be necessary. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP150

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.

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.

Presents results of research performed to identify optimum mix proportions for production of controlled low-strength materials (CLSM) with high fly ash content. CLSM is defined by ACI Committee 229 as a cementitious material that is in a flowable state at the time of placement, with a specified compressive strength of 1200 psi (8.3 MPa or 172,800 psf) or less at 28 days. The fly ash used in this study met the requirements of ASTM C 618 for Class F material. Tests were carried out on concrete designed for 500 to 1500 psi compressive strength at 28 days, with fly ash contents of approximately 500 lb/yd 3. Slump was held at 8 ¦1 in. for all mixes produced. Compressive strengths at 28 days were found to range from 290 to 1640 psi. Construction experience and other planned applications are also discussed. 141-494