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7 have successfully used bottom ash in CLSM (Naik et al. 1998; chemical admixtures can be used in CLSM to obtain specific Karim et al. 1996). target properties. Gravel and Crushed Stone Other Materials Used in CLSM CLSM has mostly evolved using only sand as aggregate. One advantage of CLSM technology is its capacity to include However, in the Pacific Northwest, many CLSM mixtures use constituent materials outside the field of conventional concrete. gravels up to 25 mm top size (Fox 1989). The reasons for the In addition to the aggregate materials previously described, use of gravel center on availability of sand, economy, and per- there are other materials used in CLSM as aggregates. Colored formance. Concrete technology demonstrates that if the largest glass that cannot be recycled by local bottle manufacturers has top-size aggregate is used, the lowest void content in the com- been crushed to pass a 12.5 mm (0.5 in.) sieve and was success- bined aggregates will be achieved. Reduced voids result in a fully used in CLSM as an aggregate (Ohlheiser 1998). A special lower paste requirement, which correspondingly reduces the process was utilized so that the crushed glass could be handled cost of cementitious materials. Gravel can be a viable material with bare hands. Phosphogypsum is a by-product of the pro- as aggregate in CLSM proportions. Economics are likely to duction of phosphoric acid and has been shown to be a viable determine whether gravel is used or not. Performance of CLSM aggregate for CLSM (Gandham et al. 1996). mixtures with gravel may be expected to be similar to those Crushed limestone is a favorite coarse aggregate for con- with sand only. crete. However, the leftover screening fines (about 15 to 20 per- cent of total aggregates) during rock processing are piled up. CLSM is a potential way to bring value to this by-product ma- Water terial (Crouch et al. 1998). Higher air content was found to There are no special requirements for water to be used in be important for these mixtures. Another source of high-fines CLSM. As a general rule, any water that is suitable for con- aggregate is recycled concrete. Current practice usually only crete will work well for CLSM, including recycled wash water involves using recycled concrete as coarse aggregate, leaving for ready-mix concrete trucks. an abundance of fines (passing 300 m sieve), which may be well suited for use in CLSM. Cement kiln dust (CKD) is a powder by-product of port- Chemical Admixtures land cement manufacturing in rotary kilns. It is used to treat Air-Entraining Agents and Foaming Agents or stabilize soft or contaminated soil or sludge. Pierce et al. (2003) examined its use as the replacement for cement in Air-entraining agents (AEAs) are the most commonly used CLSM. Various contents of CKD were found to produce chemical admixture in CLSM. AEAs are typically added as excavatable CLSM mixtures. Katz et al. (2002) found that use part of the batching process, with air contents in the 20 to of finer CKD particles results in higher water demand. The 30 percent range being common. These AEAs are formulated durability aspects of using CKD in CLSM have not been specifically for use in CLSM to obtain higher air contents than studied in detail and further work may be needed. conventional concrete. For even higher air contents, a foam- ing gun can apply a foaming agent to CLSM to produce a fluid, lightweight product. The advantages of CLSM with Mixture Proportions relatively high air contents include low density, improved Currently no standard mixture proportioning method for insulation properties, reduced segregation and bleeding, de- CLSM has been widely adopted. There has been considerable creased water and/or cement content, improved frost resis- research done on factors affecting proportioning (Janardhanam tance, and lower material cost. Also, high air contents may et al. 1992; Bhat and Lovell 1996), but there is no single, unified be used to limit long-term strength gain to assure future method (such as ACI 211 for conventional concrete). The wide excavatability. range of materials used in CLSM, including various off-spec or by-product materials, makes it quite difficult for standard Other Chemical Admixtures mixture proportioning techniques to be widely applicable. However, several fairly typical approaches to designing CLSM Set accelerators have been used to a lesser extent to increase mixtures have emerged and can be grouped in broad classes. the speed of construction (e.g., earlier opening of traffic) and Regardless of the approach to mixture proportioning, key to minimize subsidence of CLSM. Dyes can be incorporated properties sought are fluidity with minimal segregation, ac- in the mixture to distinguish CLSM from the surrounding ceptable setting times, and adequate strength gain (also a func- soils, which facilitates identifying the CLSM backfill. Other tion of whether excavatability may be needed in the future).