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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
×
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Suggested Citation:"Chapter 2 - State of the Art and Current Practice." National Academies of Sciences, Engineering, and Medicine. 2008. Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction. Washington, DC: The National Academies Press. doi: 10.17226/13900.
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5Introduction Research performed under this project included a com- prehensive literature review and a survey of state Department of Transportation (DOT) practice regarding CLSM use for backfill, utility bedding, void fill, and bridge approaches. A more detailed review of literature and information related to CLSM was included in the Phase I Interim Report for NCHRP Project 24-12 (Folliard et al. 1999); only a brief syn- thesis is provided in this chapter. The remainder of this chapter presents a brief summary of information gathered on CLSM, focusing mainly on labora- tory and field research projects. It is based on a comprehensive literature search and interactions with various state DOTs, the American Concrete Institute (ACI), the Portland Cement Association (PCA), the National Ready Mixed Concrete As- sociation (NRMCA), the American Public Works Association (APWA), and other agencies and organizations. Much of the information on current state DOT practice was obtained through the use of a written survey distributed in 1998 as part of the aforementioned Phase I Interim Report for NCHRP Project 24-12. Historical Background The development of CLSM can be viewed as a natural evo- lution of plastic soil-cement, with the main improvements re- lated to increased flowability and improved quality control. One of the earliest records of the use of CLSM was in 1964 by the U.S. Bureau of Reclamation as the bedding of a 515-km long pipeline in the Canadian River Aqueduct Project (Adaska 1997). Since then, CLSM has been used on many projects for backfill (Brewer 1992; Sullivan 1997), utility and pipe bedding (Adaska and Krell 1992; Larsen 1993), void fill (Gray et al. 1998; Hook and Clem 1998), and bridge approach applications (Snethen and Bensen 1998). Other applications include using CLSM for structural fill (ACI Committee 229 1999; Clem et al. 1995; Buss 1989), encapsulation of contaminated soils (Gardner 1998), soil stabilization (Green et al. 1998), and erosion control (Larsen 1988, 1993). Over the past 40 years, various terms have been used to de- scribe what is currently known as CLSM, including flowable fill, unshrinkable fill, controlled density fill, flowable mortar, plastic soil-cement, soil-cement slurry, and K-Krete®. In 1984, ACI Committee 229 was formed, and the ACI-approved term “controlled low-strength material or CLSM” was adopted. In its 1999 committee report, ACI Committee 229 defined CLSM as a self-compacted, cementitious material used primarily as a backfill alternative to compacted fill (ACI Committee 229 1999). Today, CLSM has been used throughout the United States for a wide range of applications, using a spectrum of different materials. Materials This section describes the most common constituent ma- terials used in CLSM, including portland cement, fly ash, ag- gregates (including foundry sand), chemical admixtures, and other by-product materials. A significant benefit of CLSM is the ability to use a wide range of local materials, including by- product materials. Because of the relatively high material cost of CLSM (compared to compacted fill), the ability to specify and use by-products such as fly ash and foundry sand will be critical to the continued growth of CLSM usage. Portland Cement Although any type of portland cement can be used in CLSM, ASTM C 150 Type I is the most commonly used. The prevail- ing criteria are the local availability and cost of cement, and as such, Type II or Type I/II cements may be more common in some regions of the United States. Because of the compara- tively low cement contents found in CLSM, common concrete durability problems, such as alkali-aggregate reaction and C H A P T E R 2 State of the Art and Current Practice

6internal sulfate attack, appear quite unlikely. Type III port- land cement has been successfully used in CLSM to achieve higher early strengths and to reduce subsidence. Supplementary Cementitious Materials Fly Ash About 62 million tons of fly ash, a by-product of coal com- bustion, were estimated to have been generated in 2001. Fly ash is used mostly in portland cement concrete, but its use in CLSM has grown considerably in recent years. Although fly ash has become an important construction material, approx- imately 70 to 75 percent of the fly ash generated annually is still disposed in landfills (FHWA 1997). Much of this unused fly ash does not meet specifications for use in portland cement concrete (ASTM C 618), sometimes because of high percent- ages of unburned carbon, as measured by the loss on ignition (LOI) test. Fortunately, it has been demonstrated that CLSM can be successfully produced using a wide variety of fly ash types and sources, including high-carbon fly ash that is not typically permitted in concrete. Both Class F and Class C fly ash (according to ASTM C 618) are commonly used in CLSM, as well as ashes that do not conform to ASTM C 618. The use of fly ash in CLSM provides for excellent flowabil- ity and helps to minimize segregation, as well as reduces the cost of the mixture (as fly ash is typically less costly than port- land cement). Fly ash is used in higher dosages in CLSM than in conventional concrete mixtures; typically fly ash composes more than half the binder, and in the case of rapid-setting CLSM, fly ash is used as the only binder, without portland ce- ment. Based on the 1998 survey of current practice (Folliard et al. 1999), of the forty-two states specifying CLSM, twenty- seven states had specifications for CLSM containing fly ash, and eleven states allow fly ash that did not meet ASTM C 618 specifications to be used in CLSM. More specific information on types and dosages of fly ash used in typical CLSM mixtures is provided later in this chap- ter, and the laboratory and field evaluations described in Chap- ters 3 and 4 included the use of a range of different fly ashes. Other Supplementary Cementitious Materials Although fly ash is the most commonly used supplemen- tary cementitious material (SCM) in CLSM, other SCMs can and have been used. Materials such as slag, metakaolin, silica fume, and rice husk ash are all suitable for use in CLSM. Aggregates Various aggregate types have been used successfully in CLSM. With the exception of CLSM paste mixtures (typically containing just fly ash, portland cement, and water), most CLSM contains fine aggregate (most commonly concrete sand). Only a small percentage of CLSM used in practice contains coarse aggregate. Concrete Sand A wide range of fine aggregates may be used successfully in CLSM, but conventional concrete sand (ASTM C 33) is the most common, especially for CLSM produced at ready-mixed concrete plants (the dominant source of CLSM). Sand that does not meet ASTM C 33 requirements (e.g., gradation) can be and often times has been used in CLSM production, provided that the specified flowability and constructability requirements are satisfied. Foundry Sand Foundry sand, a by-product of the metal-casting industry, has been studied and used successfully in CLSM and its use has increased in recent years (Bhat and Lovell 1996; Tikalsky et al. 1998). Foundry sand is becoming a more viable candi- date for use in CLSM because of its lower cost, increasing availability, and satisfactory performance. It is estimated that for every ton of metal castings produced and shipped that a typical foundry generates approximately one ton of waste sand (Kennedy and Linne 1987). A concern with using foundry sand in CLSM is the poten- tial for environmental impact caused by leaching of heavy metals present in the foundry sand. Therefore, ferrous foundry sands are more commonly used in CLSM because of the concerns about the heavy metals content of nonferrous foundry sands. The most commonly used waste foundry sand in CLSM is “green sand,” a term applied when the original sand is treated with a bonding agent (usually clay) to optimize the efficiency of the sand in the molding process. Bottom Ash Bottom ash and fly ash are both by-product materials of coal combustion. Bottom ash is formed by large noncombustible particles that cannot be carried by the hot gases. These parti- cles descend on hoppers or conveyors, at the bottom of the furnace, in a solid or partially molten condition. Then, the particles gradually cool to form bottom ash. Bottom ash parti- cles are typically porous and angular in shape. As a by-product material, bottom ash is commonly disposed of in ponds. In this process, bottom ash is passed through a crusher to reduce the size of large particles and is transported hydraulically through pipelines to the pond shore. The typical range of particle sizes falls between 75 µm and 25 mm. Researchers and practitioners

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

Unconfined compressive strength of CLSM is always an im- portant design parameter, and the vast majorities of applica- tions are designed for future excavatability and typically have strengths of 1.0 MPa or less. For these mixtures, low cement contents are used (e.g., 30 to 60 kg/m3), with or without fly ash. In general, fluidity is achieved by high water contents (and low cement contents), and segregation is addressed through the use of AEAs and high fines contents (from fly ash, sand, etc.). Table 2.1 summarizes four of the more common CLSM mixture types that have been widely used. Note that this table does not implicitly include CLSM modified with a foaming agent/gun, but any of the mixtures can be treated by this process to increase air content. The mixture types are referred to herein as Groups A through D, with the mixtures described in Groups A and B adopted from FHWA (1997). The ma- terials typically used in any of these CLSM mixture designs are portland cement, sand, fly ash, water, and AEA, but the spec- trum of mixtures used in the field can be highlighted by con- sidering that some mixtures have no portland cement (Group C: only fly ash as binder in rapid-setting CLSM, typically pro- duced in volumetric mixer on site), and some have no aggre- gates (Group B: a paste composed of about 95 percent fly ash and 5 percent portland cement, with water added as needed for fluidity). Mixtures in Group A typically include relatively small amounts of portland cement and moderate levels of fly ash, combined with sand and water. Lastly, Group D is a typ- ical mixture that relies upon portland cement as the only binder, with AEAs used to generate air contents in the 15 to 30 percent range. Batching, Mixing, and Transporting CLSM is typically batched, mixed, and transported in sim- ilar fashions as concrete. Most flowable fill is batched at ready- mixed plants and mixed in truck mixers. The high fluidity of CLSM may create difficulties in transporting full or near-full loads in ready mixed trucks. To address this potential prob- lem, some producers hold back part of their mixing water for on-site addition, and many will add liquid AEAs (or use pneumatic guns to generate air) at the job site, rather than at the plant, thereby reducing the mixture volume in the truck en route to the site. Some CLSM mixtures are produced using volumetric, mobile-type mixers. Rapid-setting CLSM mix- tures, which typically contain high–calcium oxide (CaO) fly ash as the only binder, are almost always produced on site using volumetric mixers because of the short handling time of such mixtures before setting. Properties of CLSM This section provides information on the properties of CLSM that most affect its performance in key applications. The most important fresh, hardened, and durability-related properties are briefly described next. Fresh CLSM Properties Flowability One of the most important attributes of CLSM is its ability to flow easily into confined areas, without the need for conven- tional placing and compacting equipment. The self-leveling properties of CLSM significantly reduce labor and increase construction speed. Because the enhanced flow properties of CLSM are critical to successful placement and performance, flowability is measured routinely and is an important quality control parameter. ASTM D 6103, “Flow Consistency of Controlled Low Strength Material,” has gained some acceptance since its adop- tion by ASTM. The test method uses a 75 × 150 mm cylinder, which is lifted, allowing the CLSM to slump and increase in diameter. The final diameter is typically used to differentiate between various degrees of flowability. A final diameter of 200 mm or higher is typical of a highly flowable mixture. ASTM C 939, “Flow of Grout for Preplaced-Aggregate Con- crete,” measures the efflux time of CLSM as it passes through a flow cone. Several state DOTs have, over the years, specified this test method for CLSM, and the Florida and Indiana DOTs required or require an efflux time of 30 seconds ± 5 seconds (ACI Committee 229 1999). 8 Table 2.1. Typical CLSM materials and proportions. CLSM Mixture Types Fly Ash (kg/m3) Sand (kg/m3) Cement (kg/m3) Water (kg/m3) Air (%) Range 119 - 297 1483 - 1780 30 - 119 198 - 494 0.5 - 4.0 Aa Typical 178 1542 59 297 Range 949 - 1542 None 47 - 74 222 - 371 1 - 5 Ba Typical 1234 None 62 247 Range C Typical 275 1500 165 1 Range 1200 - 1500 30 - 60 130 - 300 15 - 30 D Typical aAfter FHWA (1997)

Segregation and Bleeding Because of the high fluidity of CLSM mixtures, the poten- tial for excessive segregation and bleeding exists, especially with very high water contents. Generally, the use of fly ash and AEAs is beneficial in minimizing the potential for segregation and excessive bleeding. The use of low-density CLSM with high air contents (e.g., 15 to 35 percent by volume) allows for reductions in water content and bleeding (Hoopes 1998). ASTM C 940, “Expansion and Bleeding of Freshly Mixed Grouts for Pre-Placed Aggregate Concrete in the Laboratory,” is a simple, but effective method of measuring the total vol- ume and accumulation of bleed water on the top of CLSM. Although there are no commonly used methods available to measure the segregation of CLSM, visual observations during mixing and placing serve as good, practical indicators. Hardening Time Hardening time is the approximate period of time required for CLSM to gain sufficient strength to support the weight of a person (ACI Committee 229 1999). The hardening time can be as short as 1 hour, but generally takes 3 to 5 hours (Smith 1991). The early hardening characteristics of CLSM are af- fected by several parameters, including mixture proportions, climatic conditions, and the surrounding environment, espe- cially drainage conditions. Because measuring the early age compressive strength of CLSM is not practical, test methods for penetration resistance are most commonly used to quan- tify setting and hardening time. Laboratory penetrometers (e.g., ASTM C 403, “Time of Setting of Concrete Mixtures by Penetration Resistance”), as well as soil pocket penetrome- ters, are commonly used to measure the setting and hard- ening of CLSM. Design penetration values are sometimes specified to schedule construction practices and the time to opening of traffic. Other techniques sometimes used for CLSM include the dynamic cone penetrometer and Kelly ball. Subsidence Subsidence occurs when CLSM loses water (through bleed- ing and absorption into surrounding soil) and entrapped air, resulting in a reduction in volume. CLSM with high water content has been found to exhibit a subsidence depth equal to approximately 1 to 2 percent of the trench depth (McLaren and Balsamo 1986). The actual amount of subsidence that occurs for a given placement depends on the materials and mixture proportions used, as well as placement heights, the environmental conditions and permeability of surrounding soil. Subsidence generally only occurs during CLSM place- ment and up until the mixture hardens. Using sufficient fines (e.g., fly ash), accelerating admixtures, or high early-strength cement may be effective in limiting subsidence by minimizing the propensity for subsidence or decreasing the window of vulnerability of CLSM. Hardened CLSM Properties Compressive Strength The compressive strength (or unconfined compressive strength to be consistent with geotechnical terminology) of CLSM is the most common hardened property measured, and the one most commonly found in state DOT specifica- tions. Compressive strength and flowability were the two most commonly specified CLSM properties in the 1998 survey of current practice (Folliard et al. 1999); these and other CLSM properties and tests are highlighted in Table 2.2. CLSM compressive strength values are often used as an index for excavatability or digibility (e.g., maximum allowed values of 0.35 to 1.0 MPa), when future excavation may be re- quired. Materials and mixture proportions must be selected to ensure that these strength values are not exceeded in the long term. Also, for some applications, early-age compressive strength may be specified for constructability reasons (e.g., 9 Table 2.2. CLSM properties typically specified and measured by state DOTs. Property Number of States Testing Common Test Method(s) Flow 18 ASTM D 6103 (or similar) and ASTM C 143 Compressive strength 17 AASHTO T 22 and ASTM D 4832 Unit weight 14 AASHTO T 121 Air content 10 AASHTO T 152 Set time 7 ASTM C 403 Durability 2 pH and resistivity Shrinkage 1 Visual Geotechnical 1 Direct shear Temperature 1 Modified ASTM C 1064 Chlorides/sulfates 1 Determination of ion contents Permeability 0 None Source: Folliard et al. (1999)

for subsequent paving or opening to traffic). Some applica- tions (e.g., void fill) may not necessarily demand specific strength values, and in these cases, strength may not need to be measured. More information on applications of CLSM is discussed later in this chapter. The development of CLSM compressive strength is differ- ent from conventional concrete in that it is thought to have two components of strength: particulate and nonparticulate (Bhat and Lovell 1996). The nonparticulate component of strength results from the cementitious (and pozzolanic) re- action of cement and fly ash with water, whereas the particu- late component of strength is similar in nature to that of granular soil. Water-cement ratio plays an important role in the development of unconfined compressive strength (Bhat and Lovell 1996), but in some instances, cement content may be more influential (Brewer 1992) or easier to control. The type and amount of fly ash (if used) also has a major effect on compressive strength, especially on long-term compressive strength. ASTM D 4832, “Preparation and Testing of Controlled Low-Strength Material (CLSM) Test Cylinders” is the most common method used by state DOTs for evaluating CLSM strength. The most critical potential problem with this and related compression test methods for CLSM is the relatively low strength of CLSM. This characteristic low strength cre- ates difficulties in handling CLSM test specimens (e.g., strip- ping cylinders) and in testing cylinders, where large-capacity concrete compression machines have poor accuracy in the required low load range. Many load frames used by research laboratories for testing CLSM are in the 1,300 to 2,220 kN capacity range (Folliard et al. 1999). For a 150 × 300 mm cylinder with a compressive strength of 1.0 MPa, the maxi- mum load at failure is only about 18 kN, or approximately 1 percent of the load frame capacity. The precision of these larger load frames in the lower compressive load range is not sufficient in most cases to produce an accurate measure of compressive strength. This problem becomes exacerbated when smaller diameter cylinders are used or lower strength CLSM is used, especially at early ages. Concerns regarding machine capacity and accuracy, as well as curing conditions, cylinder mold types, and other aspects of compression testing were evident in the 1998 survey conducted under this project, and significant emphasis was placed in the laboratory phase of this project (Chapter 3) on improving the test method. A revised version of this test is recommended in Appendix B. Excavatability Easy removal of CLSM from trenches is essential when util- ities fail or require repair. Undesired long-term strength gain may prohibit the removal of CLSM using conventional means of shovels or backhoes. Prior studies have been performed using actual excavation equipment to assess the ease of excavating CLSM in trenches, and correlations were made with other CLSM properties, such as unconfined compressive strength (Landwermeyer and Rice 1997). Similar efforts were also part of the current project, as discussed in Chapters 3 (laboratory evaluations) and 4 (field studies). Many CLSM users have specified maximum unconfined compressive strength values to ensure that CLSM can be ex- cavated at later ages. Another approach, outlined in the Hamil- ton County (Ohio) Performance Specification for CLSM, is to specify a removability modulus, which is both a function of 28-day unconfined compressive strength and density of CLSM in the field. If the calculated value of the removabil- ity modulus is less than 1.0, the specific CLSM is considered to be removable. The majority of states require that CLSM compressive strengths not exceed some pre-defined early strength in order for the material to meet excavatability requirements. Performance-based specifications based on locally available materials in some cases have proven to be acceptable in limit- ing the long-term strength gain. An alternative approach is to limit the cement content of the CLSM mixes. About 20 per- cent of the states place limits on the amount of cement that can be added to CLSM, thus limiting the ultimate strength of the mixture (Folliard et al. 1999). The ability to predict long-term strength gain is paramount to assuring that CLSM will remain excavatable. Thus, methods of predicting strength gain for various combinations of con- stituent materials were a prime focus on research conducted under this project, and correlations between excavatability and various CLSM properties (e.g., compressive strength, tensile strength, etc.) and test values (e.g., dynamic cone penetrom- eter) were attempted (see Chapter 3). Permeability The permeability of CLSM to both liquids and gases has a significant impact on performance of CLSM in various appli- cations. The permeability of CLSM affects several important properties, including drainage characteristics, durability, and leaching potential. An advantage that CLSM has, compared to conventional concrete, is that actual water permeability tests can be conducted (conventional concrete is too impermeable for practical measurements of water permeability). The most common method of assessing CLSM permeability is ASTM D 5084, “Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Perme- ameter.” Typical values for CLSM obtained from this test method are in the range of 10−4 to 10−5 cm/s, but higher strength mixtures may reduce the permeability to as low as 10−7 cm/s (ACI Committee 229 1999). Low-density, air-entrained CLSM mixtures tend to have significantly higher permeability. CLSM 10

mixtures with 30 and 21 percent air were found to yield per- meability values of 1.7 × 10−2 and 1.2 × 10−3 cm/s, respectively (Hoopes 1998). Shear Strength As the use of CLSM continues to spread into more engi- neered applications as an alternative to conventional com- pacted fill, it is becoming more important to quantify CLSM properties in terms of geotechnical engineering parameters by either direct measurement or by developing correlations between geotechnical and concrete test results. The shear properties of CLSM are particularly important and can be as- sessed using both a direct shear test (ASTM D 3080) and a tri- axial shear–consolidated drained test (U.S. Army Corps of Engineers [USACE] 1986). The equipment required for both test methods is standard in most state DOT soils laboratories. Some studies have focused on the shear properties of CLSM (Bhat and Lovell 1996; Dolen and Benavidez 1998; Hoopes 1998). The shear properties of CLSM are quite high and often exceed typical compacted fill shear strengths, especially at later ages as hydration proceeds (Hoopes 1998). In triaxial shear testing, CLSM showed an internal friction angle ranging from 20 to 30 degrees (FHWA 1997). California Bearing Ratio and Resilient Modulus California bearing ratio (CBR) testing is used to determine the strength of subbase and subgrade materials. The resilient modulus assists in providing design coefficients for multi- layered pavements by defining the relationship between stress and the deformation of granular base and subbase layers. This is especially important when considering CLSM for use in bridge approaches or whenever CLSM will serve as a func- tional base or subbase material. Common soil test methods that could potentially be ap- plied to CLSM include AASHTO T 193, “Standard Method of Test for the California Bearing Ratio,” AASHTO T 274, “Resilient Modulus of Unbound Granular Base/Subbase Ma- terials and Subgrade Soils,” AASHTO T 292, “Resilient Mod- ulus of Subgrade Soils and Untreated Base/Subbase Materials” and AASHTO T 307, “Determining the Resilient Modulus of Soils and Aggregate Materials.” Consolidation The consolidation of CLSM can be measured using ASTM D 2435, “One-Dimensional Consolidation Properties of Soil.” This method is easy to perform, requires minimal equipment, and can be used to estimate both the rate and total amount of settlement for CLSM used in various applications. In addition, values obtained from consolidation testing are used to derive bedding factors and soil stiffness values needed for pipe bed- ding design (Hoopes 1998). Drying Shrinkage Compared to conventional concrete, CLSM typically has a very high water-cement ratio and water content, two factors that are known to cause excessive drying shrinkage in con- crete. However, the limited studies that have focused on CLSM shrinkage have not found it to be a significant factor. Typical linear shrinkages have been reported in the range of 0.02 to 0.05 percent (ACI Committee 229 1999). Gandham et al. (1996) also found the drying shrinkage of CLSM to be minimal. Katz et al. (2002) found the drying shrinkage of CLSM mixtures is affected by the water content and the mix- tures’ ability to hold the water during drying conditions. The standard concrete method to measure shrinkage, AASHTO T 160 may not be appropriate for CLSM. This method requires embedding gage studs at both ends of the specimen, and the method also requires significant handling of the shrinkage prisms during form removal and subsequent measurements. Because of the low strength and fragile nature of CLSM specimens, the gage studs may not bond sufficiently, and the specimens may be damaged because of the handling. Limited testing of drying shrinkage properties was performed under this project, as described in the next chapter. These ef- forts focused on in-situ measurements of shrinkage in specially designed molds that allowed for length-change measurements immediately after casting, without the need to remove the specimen from the formwork. Thermal Conductivity The transport of high-temperature fluids through pipes is common. Due to the nature of CLSM and because one of its major uses is for pipe backfill, CLSM can be used as an in- sulating material to prevent heat loss from the pipe. Low- density, air-entrained CLSM is particularly well suited for pipe backfill because of its enhanced insulating properties. Though rarely measured, the thermal and insulating properties of CLSM are important parameters. Methods that may be applied to CLSM include ASTM D 5334, “Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure,” and ASTM C 177, “Steady-State Heat Flux Mea- surements and Thermal Transmission Properties by Means of the Guarded-Hot-Plate Apparatus.” Durability and Environmental Issues Related to CLSM At the time that this research project was initiated, no major problems had been reported related to inadequate 11

durability of CLSM in field applications. Similarly, no signif- icant problems were cited regarding the leaching of CLSM constituent materials (e.g., heavy metals from fly ash or foundry sand) into the surrounding environment. However, because durability problems typically take years to manifest, there may be some concerns over the long-term durability of CLSM. This section provides a brief overview of relevant issues related to durability and leaching. Freezing and Thawing Resistance Several studies have focused on the resistance of CLSM to freezing and thawing (Bernard and Tansley 1981; Krell 1989; Burns 1990; Nantung 1993; Gress 1996). The unique struc- ture of CLSM creates some intriguing challenges when its freezing and thawing resistance is being assessed. First, CLSM may be damaged by both internal hydraulic pressure and frost heave when exposed to freezing and thawing cycles. Sec- ond, test methods that have been developed for conventional concrete have been found to be too severe for testing CLSM. In particular, Nantung (1993) found that AASHTO T 161, the most common method used for concrete, was far too severe for testing CLSM. He proposed modifications to the method to provide for less severe freezing conditions that bet- ter simulate field conditions. Gress (1996) performed laboratory and field testing of CLSM and found that CLSM can survive freezing and thaw- ing damage, but proposed that the top 50 to 150 mm of CLSM trenches be removed after set and backfilled with a frost heave–compatible base material to ensure uniform heaving of pavement and trench. When laboratory test meth- ods to assess frost resistance of CLSM are being considered, the potential for frost heave damage can not be overlooked. ASTM D 560, “Freezing and Thawing of Compacted Soil- Cement Mixtures,” has been used to measure the freeze-thaw resistance of CLSM (Janardhanam et al. 1992). This method is much less severe than AASHTO T 161 and may be a more viable test method for CLSM. Corrosion Corrosion deterioration of metal pipes placed in CLSM has not yet surfaced as a serious problem in field applications. But, because of the long-term nature of corrosion and other durability problems, it could prove to be an important aspect of CLSM durability. Very few studies have focused on the cor- rosion of metals in CLSM (Abelleira et al. 1998; Brewer 1991), but considerable information and data exist on the corrosion of metals in soils. The following section summarizes studies on steel corrosion in CLSM, as well as in conventional com- pacted fill, with particular emphasis on the mechanisms of corrosion likely to occur in CLSM. Before initiation of the NCHRP research described in this report, there were no available guidelines on the corrosion performance of metallic materials embedded in CLSM. Exist- ing guidelines on the corrosivity of soils around metallic ma- terials, which do not consider the characteristics of a cemen- titious material (i.e., CLSM), often indicate that CLSM could be detrimental to the corrosion performance of pipes embed- ded in CLSM. Probably one of the most common methods used to determine the corrosivity of soils around ductile iron pipes is the ANSI/AWWA C105/A21.5, “American National Standard for Polyethylene Encasement for Ductile-Iron Pipe Systems.” This standard, shown in Appendix B, assigns points for various soil backfill characteristics (such as pH, resistivity, moisture content, etc.), and, if the sum of the points from all characteristics is more than 10, the soil is assumed to be cor- rosive. For soils with pH values greater than 8.5, the standard notes that these soils are generally quite high in dissolved salts, resulting in lower resistivity values and higher assigned point values. However, the high pH of the CLSM results from the hydroxyl ions and alkalis present in the pore solution and not from dissolved salts. High-pH pore solutions have been well documented to result in stable, protective, passivating oxide films on iron products (Broomfield 1997). Information on other CLSM properties that may impact corrosion are de- scribed briefly in the following paragraphs. Several key CLSM parameters affect the likelihood of cor- rosion, including permeability, pH, resistivity, buffering ca- pacity, presence of chlorides, and exposure conditions (i.e., type and nature of native soil, etc.). The permeability of CLSM to water and oxygen is critical because both water and oxygen are required for the corrosion process to occur. The migration rate of chloride is critical because these ions can significantly increase localized corrosion. Water permeability tests (ASTM D 5084), air permeability tests, and chloride diffusion data can be used to design CLSM to protect metals from corroding. In addition, the absorption capacity of CLSM also may be meas- ured using ASTM C 642, “Density, Absorption, and Voids in Hardened Concrete,” to determine the degree of moisture available for corrosion in CLSM mixtures. The effects of pH on corrosion rate are shown in Figure 2.1. At high values of pH, iron is passivated, with a very low cor- rosion rate, but as the pH decreases, the corrosion rate in- creases rapidly. Because CLSM typically exhibits a pH (from extracted pore water) of greater than 11.5, corrosion is not ex- pected to be a severe problem. However, the pH of CLSM has been measured to drop when high dosages of fly ash are used, and when some types of foundry sand are used (FHWA 1997). ASTM G 51, “Measuring the pH of Soil for Use in Cor- rosion Testing,” has been used to assess the pH of CLSM. However, pH values by themselves are not sufficient to pre- dict or design against corrosion, but can be very effective in conjunction with other basic test results. 12

Resistivity measurements indicate the relative ability of an electrolytic material to carry electrical currents. When metal- lic samples are placed in a medium, the ability of the medium to conduct electrical currents will influence the degree of corrosion activity. For soils, resistivity is one parameter used to determine the “corrosivity.” Table 2.3 shows typical corrosivity classifications for different soil resistivities. The Wenner four-electrode method (ASTM G 57) is typically used to determine soil resistivity and can be easily used to measure CLSM resistivity. The rate of chloride diffusion through CLSM is an im- portant parameter that can provide important information about CLSM applications in saline environments. Although this type of testing has not been reported in the literature for CLSM applications, it is widely recognized for concrete ap- plications. This test could be accomplished by following the typical approach for concrete, in which chloride profile data can be used with Fick’s Second Law to predict the rate of chlo- ride penetration through CLSM. Because CLSM is used in a range of applications, the expo- sure conditions and corrosion resistance will vary widely. For trench backfill and bedding applications, the corrosion activ- ity of embedded metallic piping systems can be increased by the development of galvanic cells. Galvanic cells can develop when the metallic pipe is embedded in two different material types. For trench backfill applications, a typical scenario in- cludes a lateral pipe across the trench. For pipe bedding ap- plications, galvanic cells can develop when the metallic pipe displaces the CLSM bedding material and rests on the origi- nal soil. Because the CLSM is often significantly different than the original soil conditions, the potential for high corrosion rates may exist. Test methods typically used to measure corrosion in con- crete may be applied to CLSM, including ASTM G 109, “De- termining the Effects of Chemical Admixtures on the Corro- sion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments”; ASTM G 59, “Conducting Potentio- dynamic Polarization Resistance Measurements”; and ASTM G 1, “Preparing, Cleaning, and Evaluating Corrosion Test Specimens.” In addition, Abeleirra et al. (1998) have proposed a simple test method that measures the corrosion of metal coupons immersed in CLSM. With this test method, CLSM, as compared to a conventional fill, was shown to reduce the cor- rosion of embedded metals. The method, however, did not study the galvanic effects of metals embedded in both CLSM and soil. Significant research, including both laboratory and field evaluations, was performed under this NCHRP project to evaluate the potential for corrosion of metals in CLSM. In- formation on these efforts is provided primarily in Chapters 3 and 4, and information gleaned from these efforts was ulti- mately integrated into recommended test methods and spec- ifications for CLSM. Leaching and Environmental Impact The tendency for leaching and subsequent environmental impact appears more critical in the case of CLSM (compared to conventional concrete) because of its higher permeability and also because of the common use of by-product materials, such as fly ash and foundry sand, which may contain heavy metals. Leaching is a relatively slow process and because CLSM is a relatively new technology, sufficient long-term field data and observations are not available to make an in- formed assessment of CLSM leaching effects. Research at Purdue University focused on the effects of foundry sands on CLSM leachate and environmental impact (Bhat and Lovell 1996). Tests to determine pH and leachate characteristics (using a bioassay method) found that only one of eleven mixtures showed unusually high concentrations of heavy metals in the expressed pore solution. Naik et al. (1998) found relatively high concentrations of total dissolved solids in leachate extracted from CLSM containing clean coal ash. 13 Table 2.3. Classification of corrosivity of soils. Soil Resistivity (ohm-cm) Corrosivity Classificationa 0 to 1000 Very severe 1000 to 2500 Severe 2500 to 5000 Moderate 5000 to 10000 Mild Greater than 10000 Very mild aGeneral classifications from industry and published data. Figure 2.1. Influence of pH on corrosion rate. 0 0.002 0.004 0.006 0.008 0.01 0.012 8 9 10 11 12 13 14 Co rro sio n Ra te (m m/ ye ar) pH Source: After Whitman et al. (1924)

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

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

have more pipe installation work than void fill work, and not necessarily a result of CLSM being more applicable for bedding/backfill applications than void fill applications. A fairly new application for CLSM is for use as a subbase/ base under bridge approaches. For example, the Delaware Department of Transportation (DelDOT) favors the use of CLSM for many of its bridge approaches. Research by Oklahoma DOT and Oklahoma State University indicated satisfactory results (Snethen and Bensen 1998). Based on these applications, CLSM appears to be effective, compared to compacted fill, at reducing settlement and minimizing the “bump at the end of the bridge.” CLSM has been used in significant volumes by most states for only a few years (up to 1998); therefore, there are still chal- lenges that must be overcome to further increase usage. Fig- ure 2.4 shows the number of years each state DOT has been using CLSM. Almost half of the states that responded to the survey have used CLSM for less than 4 years (as of 1998). Quality Assurance and Quality Control Quality assurance and quality control are essential for the successful long-term performance of materials and struc- tures. CLSM is a unique material with a variety of applica- tions. Quality assurance serves as a management tool, is generally developed within the owner’s organization, and en- compasses quality control and independent assurance pro- grams. Results from the survey indicate that approximately half of the states have quality assurance programs for CLSM within their materials department. Almost all DOTs have some type of quality assurance program in place. Quality control is generally a contractor’s tool to ensure that a product meets specification requirements. For CLSM applications, this could include material handling (in the field and in the laboratory); construction practices; and material sampling, testing, and inspection. Interestingly, approximately 40 percent of responding state DOTs perform quality control within their organization for CLSM applications. Nearly all other state DOTs hold the contractor or supplier responsible for quality control. Responses from the 1998 survey found that CLSM is spec- ified by a variety of state DOT sections, including materials, geotechnical, roadway design, bridge design, utility design (also known as pipe design or hydraulics), and construction. Because so many different parties are involved in the specify- ing and testing of CLSM, logistical and management difficul- ties may occur. A more standardized quality assurance pro- gram where CLSM specifications, testing procedures, and construction methods are clearly organized and managed should lead to a better understanding of CLSM performance and lead to more widespread use. Summary This chapter briefly described the history and background of CLSM, including information on relevant materials, mix- ture proportions, properties, and applications. A review of this information highlights some of the key research needs that existed prior to conducting the research described in the remainder of this report: • Lack of standardized test methods, specifications, and con- struction guidelines for CLSM • Concerns over long-term strength gain (and impact on excavatability) • Potential concerns over long-term durability of CLSM, especially related to corrosion of utilities It is hoped that the findings from this project (highlighted in Chapters 3 and 4) will help fill some of the gaps in under- standing related to CLSM and will lead to an increase in CLSM usage in a range of transportation applications. 16

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Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 597: Development of a Recommended Practice for Use of Controlled Low-Strength Material in Highway Construction explores the use of controlled low-strength material (CLSM) in highway construction applications, in particular, as backfill, utility bedding, and void fill and in bridge approaches. The report also examines a recommended practice for the use of CLSM that was developed through a series of full-scale field experiments.

This report presents the full text of the contractor’s final report of the project and three of the five appendices, which present the test methods (Appendix B), specifications (Appendix C), and practice (Appendix D) recommended for implementation. The corrosion study (Appendix A) and implementation plan (Appendix E) are available online as NCHRP Web-Only Document 116.

There is a summary document, Paths to Practice, available.

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