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3 CHAPTER 1 Introduction and Scope Introduction the cracks of concrete materials and geotechnical engineer- ing. Although considerable literature is available on the topic, CLSM is a relatively new technology whose use has grown CLSM is often not given the level of attention it deserves by in recent years. CLSM, often referred to as flowable fill, is a either group. highly flowable material typically composed of water, ce- Many states have developed specifications (in some cases, ment, fine aggregates, and, often times, fly ash. Other by- provisional) that govern the use of CLSM. However, these product materials--such as foundry sand and bottom ash-- specifications differ from state to state and, moreover, a vari- and chemical admixtures--including air-entraining agents, ety of different test methods are currently being used to de- foaming agents, and accelerators--also have been used suc- fine the same intended properties. This lack of conformity, cessfully in CLSM. both on specifications and testing methods, has also hindered CLSM is typically specified and used as an alternative to the proliferation of CLSM applications. compacted fill in various applications, especially for backfill, There are also technical challenges that have served as ob- utility bedding, void fill, and bridge approaches. Backfill in- stacles to widespread CLSM use. For instance, it is often ob- cludes applications such as backfilling walls (e.g., retaining served in the field that excessive long-term strength gain may walls) or trenches. Utility bedding applications involve the make it difficult to excavate CLSM at later ages. This strength use of CLSM as a bedding material for pipe, electrical, and gain can be a significant problem that translates to added cost other types of utilities and conduits. Void-filling applications and labor. Other technical issues deserving attention are the include the filling of sewers, tunnel shafts, basements, and compatibility of CLSM with different types of utilities and other underground structures. CLSM is also used in bridge pipes, the potential leaching of constituent materials and ele- approaches, either as a subbase for the bridge approach slab ments, and the durability of CLSM subjected to freezing and or as backfill against wingwalls or other elements. thawing cycles. There are various inherent advantages of using CLSM in- In summary, CLSM represents a significant and important stead of compacted fill in these applications. These benefits in- technology that will likely continue to grow in popularity and clude reduced labor and equipment costs (due to self-leveling usage. However, because of the challenges described in previ- properties and no need for compaction), faster construction, ous paragraphs, research is needed to better understand the and the ability to place material in confined spaces. The rela- behavior of CLSM and to apply this knowledge to appropri- tively low strength of CLSM is advantageous because it allows ate test methods, specifications, and design criteria. This re- for future excavation, if required. Another advantage of CLSM port summarizes research performed under NCHRP Project is that it often contains by-product materials, such as fly ash 24-12(01) that aimed at filling these gaps and developing stan- and foundry sand, thereby reducing the demand on landfills, dard test methods, specifications, and guidelines for using where these materials may otherwise be deposited. CLSM in backfill, utility bedding, void fill, and bridge approach Despite these benefits and advantages over compacted fill, applications. the use of CLSM is not currently as widespread as its poten- tial might predict. One reason is that CLSM is somewhat a hy- Research Objectives brid material; that is, it is a cementitious material that behaves more like a compacted fill. As such, much of the information The objectives of the research were (1) to define the prop- and discussions on its uses and benefits have fallen between erties of CLSM necessary for its use as wall backfill, utility