Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction (2008)

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

4bedding and backfill, void fill, and bridge approaches; (2) for these applications, to define test methods and develop crite- ria for the necessary properties of CLSM, including its corro- sion potential and possible environmental impact; (3) to de- fine the relationships between the properties of CLSM and its constituents; (4) to define field methods to monitor in-place properties of CLSM for construction acceptance; and (5) to prepare design criteria and construction guidelines for CLSM to take advantage of its properties for backfill, utility bedding, void fill, and bridge approaches. Overview of Report The report is organized as follows: • Chapter 2, State of the Art and Current Practice – Provides synthesis of current practice and available lit- erature on CLSM. – Describes materials, mixture proportions, applications, relevant properties of CLSM, and research needs • Chapter 3, Laboratory Testing Program – Describes materials and mixture proportions used in laboratory program – Summarizes results of tests on fresh properties, hard- ened properties, and durability aspects of CLSM • Chapter 4, Field Evaluations of CLSM – Describes six field tests conducted throughout the United States – Summarizes efficacy of test methods and specifications in field applications • Chapter 5, Conclusions and Suggested Research – Summarizes key findings and conclusions from project – Identifies topics and issues that deserve further atten- tion in future research • Appendices – Appendix A, Corrosion Study (available in NCHRP Web-Only Document 116) – Appendix B, Recommended Test Methods for CLSM – Appendix C, Recommended Specifications for CLSM – Appendix D, Recommended Practice for CLSM – Appendix E, Implementation Plan (available in NCHRP Web-Only Document 116)