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11 mixtures with 30 and 21 percent air were found to yield per- bedding factors and soil stiffness values needed for pipe bed- meability values of 1.7 × 10-2 and 1.2 × 10-3 cm/s, respectively ding design (Hoopes 1998). (Hoopes 1998). Drying Shrinkage Shear Strength Compared to conventional concrete, CLSM typically has a As the use of CLSM continues to spread into more engi- very high water-cement ratio and water content, two factors neered applications as an alternative to conventional com- that are known to cause excessive drying shrinkage in con- pacted fill, it is becoming more important to quantify CLSM crete. However, the limited studies that have focused on properties in terms of geotechnical engineering parameters CLSM shrinkage have not found it to be a significant factor. by either direct measurement or by developing correlations Typical linear shrinkages have been reported in the range of between geotechnical and concrete test results. The shear 0.02 to 0.05 percent (ACI Committee 229 1999). Gandham properties of CLSM are particularly important and can be as- et al. (1996) also found the drying shrinkage of CLSM to be sessed using both a direct shear test (ASTM D 3080) and a tri- minimal. Katz et al. (2002) found the drying shrinkage of axial shearconsolidated drained test (U.S. Army Corps of CLSM mixtures is affected by the water content and the mix- Engineers [USACE] 1986). The equipment required for both tures' ability to hold the water during drying conditions. test methods is standard in most state DOT soils laboratories. The standard concrete method to measure shrinkage, Some studies have focused on the shear properties of CLSM AASHTO T 160 may not be appropriate for CLSM. This (Bhat and Lovell 1996; Dolen and Benavidez 1998; Hoopes method requires embedding gage studs at both ends of the 1998). The shear properties of CLSM are quite high and often specimen, and the method also requires significant handling exceed typical compacted fill shear strengths, especially at of the shrinkage prisms during form removal and subsequent later ages as hydration proceeds (Hoopes 1998). In triaxial measurements. Because of the low strength and fragile nature shear testing, CLSM showed an internal friction angle ranging of CLSM specimens, the gage studs may not bond sufficiently, from 20 to 30 degrees (FHWA 1997). 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- California Bearing Ratio and Resilient Modulus forts focused on in-situ measurements of shrinkage in specially California bearing ratio (CBR) testing is used to determine designed molds that allowed for length-change measurements the strength of subbase and subgrade materials. The resilient immediately after casting, without the need to remove the modulus assists in providing design coefficients for multi- specimen from the formwork. layered pavements by defining the relationship between stress and the deformation of granular base and subbase layers. This Thermal Conductivity is especially important when considering CLSM for use in bridge approaches or whenever CLSM will serve as a func- The transport of high-temperature fluids through pipes is tional base or subbase material. common. Due to the nature of CLSM and because one of Common soil test methods that could potentially be ap- its major uses is for pipe backfill, CLSM can be used as an in- plied to CLSM include AASHTO T 193, "Standard Method sulating material to prevent heat loss from the pipe. Low- of Test for the California Bearing Ratio," AASHTO T 274, density, air-entrained CLSM is particularly well suited for pipe "Resilient Modulus of Unbound Granular Base/Subbase Ma- backfill because of its enhanced insulating properties. Though terials and Subgrade Soils," AASHTO T 292, "Resilient Mod- rarely measured, the thermal and insulating properties of CLSM ulus of Subgrade Soils and Untreated Base/Subbase Materials" are important parameters. Methods that may be applied to and AASHTO T 307, "Determining the Resilient Modulus of CLSM include ASTM D 5334, "Determination of Thermal Soils and Aggregate Materials." 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 Consolidation of the Guarded-Hot-Plate Apparatus." The consolidation of CLSM can be measured using ASTM D 2435, "One-Dimensional Consolidation Properties of Soil." Durability and Environmental Issues This method is easy to perform, requires minimal equipment, Related to CLSM and can be used to estimate both the rate and total amount of settlement for CLSM used in various applications. In addition, At the time that this research project was initiated, no values obtained from consolidation testing are used to derive major problems had been reported related to inadequate
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12 durability of CLSM in field applications. Similarly, no signif- Before initiation of the NCHRP research described in this icant problems were cited regarding the leaching of CLSM report, there were no available guidelines on the corrosion constituent materials (e.g., heavy metals from fly ash or performance of metallic materials embedded in CLSM. Exist- foundry sand) into the surrounding environment. However, ing guidelines on the corrosivity of soils around metallic ma- because durability problems typically take years to manifest, terials, which do not consider the characteristics of a cemen- there may be some concerns over the long-term durability titious material (i.e., CLSM), often indicate that CLSM could of CLSM. This section provides a brief overview of relevant be detrimental to the corrosion performance of pipes embed- issues related to durability and leaching. 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 Freezing and Thawing Resistance Standard for Polyethylene Encasement for Ductile-Iron Pipe Several studies have focused on the resistance of CLSM to Systems." This standard, shown in Appendix B, assigns points freezing and thawing (Bernard and Tansley 1981; Krell 1989; for various soil backfill characteristics (such as pH, resistivity, Burns 1990; Nantung 1993; Gress 1996). The unique struc- moisture content, etc.), and, if the sum of the points from all ture of CLSM creates some intriguing challenges when its characteristics is more than 10, the soil is assumed to be cor- freezing and thawing resistance is being assessed. First, CLSM rosive. For soils with pH values greater than 8.5, the standard may be damaged by both internal hydraulic pressure and notes that these soils are generally quite high in dissolved salts, frost heave when exposed to freezing and thawing cycles. Sec- resulting in lower resistivity values and higher assigned point ond, test methods that have been developed for conventional values. However, the high pH of the CLSM results from the concrete have been found to be too severe for testing CLSM. hydroxyl ions and alkalis present in the pore solution and not In particular, Nantung (1993) found that AASHTO T 161, from dissolved salts. High-pH pore solutions have been well the most common method used for concrete, was far too documented to result in stable, protective, passivating oxide severe for testing CLSM. He proposed modifications to the films on iron products (Broomfield 1997). Information on method to provide for less severe freezing conditions that bet- other CLSM properties that may impact corrosion are de- ter simulate field conditions. scribed briefly in the following paragraphs. Gress (1996) performed laboratory and field testing of Several key CLSM parameters affect the likelihood of cor- CLSM and found that CLSM can survive freezing and thaw- rosion, including permeability, pH, resistivity, buffering ca- ing damage, but proposed that the top 50 to 150 mm of pacity, presence of chlorides, and exposure conditions (i.e., CLSM trenches be removed after set and backfilled with a type and nature of native soil, etc.). The permeability of CLSM frost heavecompatible base material to ensure uniform to water and oxygen is critical because both water and oxygen heaving of pavement and trench. When laboratory test meth- are required for the corrosion process to occur. The migration ods to assess frost resistance of CLSM are being considered, rate of chloride is critical because these ions can significantly the potential for frost heave damage can not be overlooked. increase localized corrosion. Water permeability tests (ASTM ASTM D 560, "Freezing and Thawing of Compacted Soil- D 5084), air permeability tests, and chloride diffusion data can Cement Mixtures," has been used to measure the freeze-thaw be used to design CLSM to protect metals from corroding. In resistance of CLSM (Janardhanam et al. 1992). This method addition, the absorption capacity of CLSM also may be meas- is much less severe than AASHTO T 161 and may be a more ured using ASTM C 642, "Density, Absorption, and Voids in viable test method for CLSM. 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. Corrosion At high values of pH, iron is passivated, with a very low cor- Corrosion deterioration of metal pipes placed in CLSM has rosion rate, but as the pH decreases, the corrosion rate in- not yet surfaced as a serious problem in field applications. creases rapidly. Because CLSM typically exhibits a pH (from But, because of the long-term nature of corrosion and other extracted pore water) of greater than 11.5, corrosion is not ex- durability problems, it could prove to be an important aspect pected to be a severe problem. However, the pH of CLSM has of CLSM durability. Very few studies have focused on the cor- been measured to drop when high dosages of fly ash are used, rosion of metals in CLSM (Abelleira et al. 1998; Brewer 1991), and when some types of foundry sand are used (FHWA but considerable information and data exist on the corrosion 1997). ASTM G 51, "Measuring the pH of Soil for Use in Cor- of metals in soils. The following section summarizes studies rosion Testing," has been used to assess the pH of CLSM. on steel corrosion in CLSM, as well as in conventional com- However, pH values by themselves are not sufficient to pre- pacted fill, with particular emphasis on the mechanisms of dict or design against corrosion, but can be very effective in corrosion likely to occur in CLSM. conjunction with other basic test results.
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13 0.012 trench backfill and bedding applications, the corrosion activ- ity of embedded metallic piping systems can be increased by 0.01 the development of galvanic cells. Galvanic cells can develop when the metallic pipe is embedded in two different material Corrosion Rate (mm/year) 0.008 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 0.006 displaces the CLSM bedding material and rests on the origi- nal soil. Because the CLSM is often significantly different than 0.004 the original soil conditions, the potential for high corrosion rates may exist. 0.002 Test methods typically used to measure corrosion in con- crete may be applied to CLSM, including ASTM G 109, "De- 0 termining the Effects of Chemical Admixtures on the Corro- 8 9 10 11 12 13 14 pH sion of Embedded Steel Reinforcement in Concrete Exposed to Source: After Whitman et al. (1924) Chloride Environments"; ASTM G 59, "Conducting Potentio- dynamic Polarization Resistance Measurements"; and ASTM Figure 2.1. Influence of pH on corrosion rate. 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 Resistivity measurements indicate the relative ability of an coupons immersed in CLSM. With this test method, CLSM, as electrolytic material to carry electrical currents. When metal- compared to a conventional fill, was shown to reduce the cor- lic samples are placed in a medium, the ability of the medium rosion of embedded metals. The method, however, did not to conduct electrical currents will influence the degree of study the galvanic effects of metals embedded in both CLSM corrosion activity. For soils, resistivity is one parameter and soil. used to determine the "corrosivity." Table 2.3 shows typical Significant research, including both laboratory and field corrosivity classifications for different soil resistivities. The evaluations, was performed under this NCHRP project to Wenner four-electrode method (ASTM G 57) is typically evaluate the potential for corrosion of metals in CLSM. In- used to determine soil resistivity and can be easily used to formation on these efforts is provided primarily in Chapters measure CLSM resistivity. 3 and 4, and information gleaned from these efforts was ulti- The rate of chloride diffusion through CLSM is an im- mately integrated into recommended test methods and spec- portant parameter that can provide important information ifications for CLSM. about CLSM applications in saline environments. Although this type of testing has not been reported in the literature for Leaching and Environmental Impact CLSM applications, it is widely recognized for concrete ap- plications. This test could be accomplished by following the The tendency for leaching and subsequent environmental typical approach for concrete, in which chloride profile data impact appears more critical in the case of CLSM (compared can be used with Fick's Second Law to predict the rate of chlo- to conventional concrete) because of its higher permeability ride penetration through CLSM. and also because of the common use of by-product materials, Because CLSM is used in a range of applications, the expo- such as fly ash and foundry sand, which may contain heavy sure conditions and corrosion resistance will vary widely. For 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- Table 2.3. Classification of formed assessment of CLSM leaching effects. corrosivity of soils. Research at Purdue University focused on the effects of foundry sands on CLSM leachate and environmental impact Soil Resistivity Corrosivity (Bhat and Lovell 1996). Tests to determine pH and leachate (ohm-cm) Classificationa 0 to 1000 Very severe characteristics (using a bioassay method) found that only one 1000 to 2500 Severe of eleven mixtures showed unusually high concentrations of 2500 to 5000 Moderate heavy metals in the expressed pore solution. Naik et al. (1998) 5000 to 10000 Mild Greater than 10000 Very mild found relatively high concentrations of total dissolved solids a General classifications from industry and published data. in leachate extracted from CLSM containing clean coal ash.