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14 Gandham et al. (1996) used the TCLP (EPA SW-846, Method Bridge Approaches 1311) to test CLSM containing phosphogypsum. The toxic contents of the mixtures were found to be well below the EPA A common problem associated with conventional com- leachate standards. pacted fill is the consolidation of the fill material with time. The so-called "bump at the end of the bridge" syndrome is common on many bridge approaches and is caused by the CLSM Applications settlement of soil at the interface of the bridge and the ap- CLSM is used as an alternative to compacted fill mainly for proach slab. CLSM can serve as a desirable alternative to backfill, utility bedding, void fill, and bridge approach appli- conventional compacted fill for bridge approaches because cations. Before summarizing the current practice of using of its low compressibility and ease of application. CLSM can CLSM for these applications, a brief overview of the general be used either in the initial construction to prevent long- benefits of using CLSM in each application is provided. term settlement or as a replacement option for existing bridge approaches. Backfill Other CLSM Applications The fluidity of CLSM makes it a rapid and efficient back- filling material, compared to conventional compaction. In addition to the four major applications previously dis- Time-consuming compaction is not needed and the quality cussed, CLSM has been utilized in various applications and of backfill depends on only the mixture specified. The effi- new applications are expected to surface as the construc- ciency of using CLSM is especially evident when limited tion community gets more familiar with this material. Cur- space prevents or hinders the use of compaction machinery. rent applications embrace bridge replacement (Iowa DOT), The backfilling rate of CLSM (by volume) is about 50 times structural fill, insulation and isolation fill, erosion control, that of manual compaction by a laborer. RSMeans (1995) es- and others. timated that five common laborers could backfill at a rate of 46 m3/day including compaction of the soil, which makes the Summary of 1998 Questionnaire average rate per laborer approximately 9 m3/day. Sullivan (1997) noted that CLSM can be placed at a rate of approxi- In the early stages of this NCHRP project, a survey was dis- mately 60 m3/h, significantly higher than conventional back- tributed to all state DOTs, with the majority of the states re- fill. As such, CLSM can improve productivity and decrease sponding. Detailed information on this survey can be found construction costs. In addition, the use of CLSM provides a in Folliard et al. (1999). For conciseness, only limited infor- safe working environment. mation is provided in this section. Utility Bedding CLSM Usage by State DOTs Proper bedding for pipes and utilities is critical for pipe Even though CLSM was proven to be flexible for many ap- performance. However, preparation of pipe or utility bed- plications and most state DOTs had specifications for its use, ding is a time consuming process, with either compacted soil the quantity of CLSM used was relatively low in 1998. Figure 2.2 or hardened concrete. Proper compaction in the haunch zone shows the state DOT survey results on the estimated quantity is a particular challenge. CLSM can be an effective alternative of CLSM used annually. The survey results indicate that the to both concrete and granular materials because of its flow- relatively high cost of CLSM and lack of knowledge on the ability and strength characteristics. Its use for bedding appli- use, testing, and performance of CLSM were hampering its cations can be of high quality and cost effective. widespread use. CLSM is used by state DOTs mainly for backfill, utility bedding, void fill, and bridge approach applications. Other Void Fill applications for CLSM include bedding for granite curbs, en- Underground structures or other voids that have been gineered fill, and as a lightweight fill to cover swamp areas. Of taken out of service have the potential to fail and cause addi- the forty-four states that responded to the survey, only two tional damage to surrounding structures. Because of its flu- states were not then specifying the use of CLSM. Figure 2.3 idity, CLSM is an ideal material for void fill applications. The shows the 1998 applications of CLSM for each state agency. strength of CLSM can be adjusted to meet the excavation re- The use of CLSM was quite new to some state DOTs, as quirement. In addition, CLSM costs less than conventional shown in Figure 2.4. The dominant applications were back- concrete for such applications. fill and bedding material. The majority of CLSM was pro-

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15 * * > 5,000 m3 * * (3 States) * * * * < 100 m3 (4 States) * * * * 1,000 m3 to 5,000 m3 (3 States) a) utility bedding b) void fill 100 m3 to 1,000 m3 (7 States) * * * * * * * * * * * * Seventeen states provided estimated quantities; other states did not respond or quantity was unknown. c) bridge approaches d) backfill (* = no response) Figure 2.2. Annual quantity of CLSM used by state DOTs. Figure 2.3. CLSM applications by state in 1998. duced at ready-mixed concrete plants. According to a 1995 tions a "bump" may form due to the settlement of compacted survey, 90 percent of the 3,000 ready-mixed concrete pro- fill. However, for utility bedding, the advantages of CLSM were ducers in the United States produce some type of flowable fill recognized not only by state DOTs but also by city agencies. (U.S. EPA 1998). The benefits of using CLSM as a backfill ma- Void fill is another common application of CLSM products. terial were then recognized by at least forty-two state DOTs. Although the majority of states use CLSM for void fill appli- However, the survey found that CLSM was not a problem- cations (70 percent), only seven of the forty-four states stated free product as it seemed to be. Because of the mismatch be- that using CLSM for void fill was their dominant application. tween CLSM and compacted fill, in certain backfill applica- This situation is most likely because the majority of states NR NR RI NR NR NR NR NR NR No Response 0-1 years of use 1-4 years of use +4 years of use Source: After Folliard et al. (1999) Figure 2.4. Duration of CLSM use by state DOTs (up to 1998).