Geofoam Applications in the Design and Construction of Highway Embankments (2004) / Chapter Skim
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From page 38... ...
2-1 CHAPTER 2 RELEVANT ENGINEERING PROPERTIES OF BLOCK-MOLDED EPS Contents Background...................................................................................................................................2-3 Introduction...............................................................................................................................2-3 Manufacturing (Molding) EPS .................................................................................................2-3 Overview...............................................................................................................................2-3 Block Molding ......................................................................................................................2-5 Physical Properties and Issues ......................................................................................................2-8 Introduction...............................................................................................................................2-8 Density ......................................................................................................................................2-9 Fusion......................................................................................................................................2-11 Block Dimensions...................................................................................................................2-11 Color .......................................................................................................................................2-13 Flammability ...........................................................................................................................2-13 Durability ................................................................................................................................2-15 Environmental Effects ............................................................................................................2-17 Mechanical Properties.................................................................................................................2-18 Introduction.............................................................................................................................2-18 Compression ...........................................................................................................................2-19 Introduction.........................................................................................................................2-19 Rapid Loading Testing........................................................................................................2-20 Compatibility with MQC/MQA Testing.........................................................................2-26 Monotonic.
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From page 39... ...
2-2 Cyclic ..............................................................................................................................2-32 Poisson's Ratio................................................................................................................2-33 Time-Dependent Behavior (Creep and Relaxation) ................................................................2-34 Introduction.........................................................................................................................2-34 Testing ................................................................................................................................2-35 Constitutive Modeling of the Stress-Strain-Time Behavior of EPS ...................................2-38 Introduction.................................................................................................................2-38 Laboratory Creep Tests...............................................................................................2-40 Full-Scale Model Creep Test ......................................................................................2-43 Full-Scale Field Monitoring........................................................................................2-44 Summary of Comparison of Measured and Calculated Values of Total Strain.
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2-3 Comparison of Large-Scale Direct Shear and Torsional Ring Shear Tests ................2-59 Geofoam/GC Geomembrane Interface Test Results...................................................2-60 Geofoam/Nonwoven Geotextile Interface Test Results..............................................2-61 Summary of EPS Interface Strengths..........................................................................2-62 Thermal Properties......................................................................................................................2-62 References...................................................................................................................................2-63 Figures ........................................................................................................................................2-68 Tables..........................................................................................................................................2-89 ______________________________________________________________________________ BACKGROUND Introduction This chapter presents an overview of the engineering properties of EPS. A knowledge of the physical, mechanical (stress-strain-time-temperature)
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From page 41... ...
2-4 Overview There are two distinct steps involved in manufacturing (molding) EPS: • A manufacturer called a resin supplier produces the raw material that is formally called expandable polystyrene but colloquially referred to as beads or resin.
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2-5 geofoam through a distributor generally results in a greater unit cost for the product because of distributor markup for their overhead and profit. In many cases, there is no value added by a distributor.
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2-6 dries during this seasoning period as a relatively significant amount of water vapor and liquid (which can artificially increase the apparent density of the EPS) that is condensed steam from molding remains in the block at the end of molding.
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2-7 to colloquially as beadboard, a term that the EPS industry in the U.S.A. appears to deprecate because of an-often negative connotation associated with this term.
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2-8 way that virgin prepuff does so that the overall bead fusion of the finished EPS is poorer than if all virgin prepuff were used. The resulting new EPS with regrind has a more crumbly texture.
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2-9 correlates with a significant engineering property of soils (6)
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2-10 Second, there will be density variations (called density gradients in the industry) within every block, also a result of inherent variability in the EPS manufacturing process.
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2-11 Therefore, the date of molding for the block from which a test specimen is obtained should always be recorded as part of the test data. • Density can be affected by absorbed atmospheric moisture.
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2-12 meet certain dimensional quality criteria for straightness, etc. Although it is possible to factory cut a seasoned block into a smaller size, such cutting can add significantly to the unit cost of the final EPS-block geofoam product.
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2-13 The first alternative is generally not feasible for government projects such as road construction. In addition, experience in the latter part of the 1990s has indicated that more and more EPS block molders in the U.S.A.
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2-14 will burn freely in air until all the material is consumed provided there is an initial ignition source. If the OI of the material is greater than 21 percent, it will not support continuous combustion after initial ignition (this is generally referred to as being self extinguishing)
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2-15 There is another flammability issue separate from the inherent flammability of the EPS block. It is related to the outgassing of the blowing agent used in the manufacturing process.
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2-16 effect of EPS-block geofoam on the in situ environment is discussed separately in the following section. In general, EPS-block geofoam has proven to be a very robust geosynthetic product, much more problem-free on the whole compared to many other types of geosynthetics where there is a potential for significant physical damage to and detrimental chemical changes within the geosynthetic during and after construction.
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2-17 with Timbor for lightweight fill applications. The additive does not affect any of the geotechnical relevant physical, mechanical or thermal properties of the EPS.
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2-18 ozone layer that are associated with manufacturing EPS. Furthermore, because the cells within EPS are completely filled with air within a few days after molding, there is no concern about long-term outgassing of potentially toxic and hazardous gases as has been a problem with other types of plastic foam.
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2-19 • The interface shear properties, both between EPS blocks as well as between EPS and dissimilar materials (both soil and non-soil)
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2-20 Loading in unconfined uniaxial compression has been and remains the primary mode of loading for tests performed on EPS-block geofoam for both quality control and research purposes. This is because compression is by far the predominant mode of loading for EPS in load-bearing applications, including when used as lightweight fill.
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2-21 • confining stress (if any) on the test specimen, and • ambient temperature and relative humidity in the laboratory where the test is performed.
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2-22 Observation: There has been a trend since about the 1980s toward occasional use of specimens that are right circular cylindrical in shape with dimensions similar to soil specimens used in triaxial tests, i.e. approximately 150 mm (5.9 in.)
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2-23 cubes indicate that the larger specimens are approximately 50 percent stiffer compared to the smaller specimens at small strains (17)
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2-24 measurement (as most test apparatus would be expected to have)
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2-25 Recommendation: A minimum specimen age is desirable to ensure that the test specimen has undergone at least most of its seasoning and facilitate comparison of test results. Based on informal discussions with EPS block molders in the U.S.A., it appears that for the pentane blowing agent content and mold sizes in common use, a minimum age of three days is sufficient.
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2-26 includes a radial confining stress provides a more accurate estimate of EPS mechanical behavior. Recommendation: There appears to be no compelling reason not to use unconfined compression tests to measure the compressive strength and initial tangent Young's modulus.
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2-27 Monotonic. The most commonly performed test on EPS specimens involves straincontrolled compression loading at a relatively rapid rate, typically 10 percent per minute, with the load applied in a monotonically increasing fashion until a desired strain level is reached.
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2-28 been observed within Zone 1 prior to the linear portion (18)
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2-29 From Hooke's law relation, σ = (Eti)
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2-30 of EPS is given the notation σc10. This point on the stress-strain curve is shown in Figure 2.1.
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2-31 yσ 6.83ρ 48.4= − (2.6) where σy = yield stress in kPa and ρ = EPS density in kg/m3.
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2-32 percent regrind (33)
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2-33 section of this chapter, the pre-puff changes from spherical shape to a more polyhedral shape after the pre-puff is further expanded in the second step of the manufacturing process. Each face of a polyhedron represents a contact plane with an adjacent polyhedron.
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2-34 ν = 0.0056 ρ + 0.0024 (2.7) where ρ = EPS density in kg/m3.
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2-35 is discussed here. The temperature dependent behavior is discussed separately.
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2-36 • test duration, and • ambient temperature in the laboratory where the test is performed. Using the above list of variables for reference, the observed variations in test variables and recommendations for creep testing in practice are as follows: • Specimen shape and dimensions.
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2-37 • Applied stress level. This will be dependent on the stress that the EPS-block will be subjected to in the particular load-bearing application.
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2-38 • Temperature. Little creep testing has been conducted at temperatures other than de facto standard laboratory conditions.
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2-39 include the general power-law equation and the Findley equation (39)
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2-40 ρ = EPS-block geofoam density in kg/m3. However, it was found in (39)
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2-41 published laboratory creep test results and the calculated strain values derived from the general power-law and modified Findley equations to assess the accuracy of these equations. Figures 2.7 and 2.11 provide a qualitative comparison between various size EPS specimens with a density of 20 kg/m3 (1.25 lbf/ft³)
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2-42 the same test sample. It is also recommended that the elastic-limit stress be determined from compressive strength tests because, as will be discussed later, the elastic-limit stress may be a useful guide for estimating the onset of significant creep effects (18)
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2-43 and the modified Findley equation underestimates the measured strains. However, the modified Findley equation provides the best agreement with the measured values especially as the time, t, increases.
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2-44 (horizontal) to 2 (vertical)
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2-45 blocks is 0.6 m (2 ft)
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2-46 At larger compressive stresses of 67, 70, and 79 kPa (1,399, 1,462 and 1,650 lbs/ft²) , the total strains determined by the power-law equation and the modified Findley equation significantly overestimate and underestimate, respectively, the measured full-size block and fullscale test fill values.
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2-47 and tests of longer duration. The modified Findley equation should be refined to include test results from compressive stresses outside the 30 to 50 kPa (627 to 1,044 lbs/ft²)
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2-48 currently unavailable. Such a model would be useful in practice to estimate creep behavior beyond the duration of creep tests.
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2-49 • If the applied stress produces an immediate strain greater than 1 percent, creep strains can rapidly increase and become excessive for lightweight fill geofoam applications. The stress level for significant creep strain corresponds to the yield stress which is approximately 75 percent of the compressive strength.
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2-50 hourglass-shaped test specimens required for tensile testing per the ASTM C 1623 standard test method (51) and the availability of other types of tests (most notably flexure which is discussed subsequently)
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2-51 specimen dimensions and strain rates used to obtain the data in Figure 2.15. No long-term test data is known to exist for the flexure mode of loading (18)
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2-52 assess interface friction between the surface of EPS blocks and a variety of other materials is of interest in projects where significant horizontal design loads or internal sliding can occur. Two types of interfaces that are of interest for EPS-block geofoam in lightweight fill applications include an EPS/EPS interface and an EPS/dissimilar material interface.
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2-53 post-peak strength loss and thus a residual interface friction angle is not reported. Previous testing also indicates that the value of δ is independent of EPS density because shearing occurs on the surface of the specimen, although the normal stress is assumed to be low enough that excessive deformation of the EPS did not occur (19)
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2-54 EPS/Dissimilar Material Interfaces. A significant gap in the published literature exists for interface friction values between EPS block and other materials likely to be encountered in lightweight fills such as planar geosynthetics (chiefly geotextiles and geomembranes)
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2-55 suggested that the friction coefficient between EPS blocks and soil is approximately 0.5 (δ=27 degrees)
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2-56 • Geofoam: EPS-block geofoam with a unit weight of 20 kg/m3 (1.25 lbf/ft3)
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2-57 were used for the direct shear tests to avoid displacement rate-related discrepancies in the test results. The successful use of a torsional ring shear apparatus to measure the shear strength of geosynthetic/geosynthetic and geosynthetic/soil interfaces is described in (61-63)
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2-58 The geomembrane specimens were cut from a geomembrane sheet using a hydraulic jack and steel die that is the same size and shape as the required specimen. An adhesive was used to secure the geomembrane specimen to a plastic insert that fits into the top platen of the direct shear box.
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2-59 geomembrane ring was cut so that eight wedges or flaps of geotextile that were equal in size and spacing remained. Epoxy was applied to the back of the smooth geomembrane and the eight geotextile wedges were folded over and adhered to the backside of the smooth geomembrane.
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2-60 21 kPa (436 lbs/ft²)
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2-61 interface exhibits a peak and residual strength at each of the three normal stresses. These three shear stress-displacement relationships were used to develop the peak and residual failure envelopes for this interface that are presented in Figure 2.18.
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2-62 normal stresses of 12, 20, and 26 kPa (250, 426, 550 lbs/ft2)
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2-63 problems are discussed in Chapter 4. The key aspects of the thermal behavior of EPS-block geofoam are: • The coefficient of thermal conductivity of EPS block in the as-molded (dry)
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2-64 4. Horvath, J
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2-65 23. Preber, T., Bang, S., Chung, Y., and Cho, Y., "Behavior of Expanded Polystyrene Blocks." Transportation Research Record No.
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2-66 42. Duskov, M., and Scarpas, A., "Three-Dimensional Finite Element Analysis of Flexible Pavements with an (Open Joint in the)
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2-67 61. Stark, T
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FIGURE 2.1 PROJ 24-11.doc 2-68
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FIGURE 2.2 PROJ 24-11.doc EPS Density (kg/m3)
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FIGURE 2.3 PROJ 24-11.doc 2-70
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FIGURE 2.4 PROJ 24-11.doc EPS Density (kg/m3)
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FIGURE 2.5 PROJ 24-11.doc 2-72
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FIGURE 2.6 PROJ 24-11.doc Compressive strain (%)
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FIGURE 2.7 PROJ 24-11.doc Time, t (days)
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FIGURE 2.8 PROJ 24-11.doc t ε Primary Secondary Tertiary σ x Rupture 2-75
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FIGURE 2.9 PROJ 24-11.doc Density = 14.5 kg/m3 Time, t (days)
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FIGURE 2.10 PROJ 24-11.doc 2-77
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FIGURE 2.11 PROJ 24-11.doc Time, t (days)
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FIGURE 2.12 PROJ 24-11.doc Time, t (days)
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FIGURE 2.13 PROJ 24-12.doc Time, t (days)
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FIGURE 2.14 PROJ 24-11.doc Time, t (days)
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FIGURE 2.15 PROJ 24-12.doc EPS density (kg/m3)
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FIGURE 2.16 PROJ 24-12.doc 2-83
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FIGURE 2.17 PROJ 24-12.doc 2-84
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FIGURE 2.18 PROJ 24-12.doc 2-85
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FIGURE 2.19 PROJ 24-12.doc 2-86
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FIGURE 2.20 PROJ 24-12.doc 2-87
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FIGURE 2.21 PROJ 24-12.doc EPS density (kg/m3)
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TABLE 2.1 PROJ 24-11.doc Temperature Range Rate of Change Comment less than 0°C (+32°F) 0% Remains approximately constant +23°C (+73°F)
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TABLE 2.2 PROJ 24-11.doc Density Temperature Stress Duration Strain Strain Increase 20 kg/m3 (1.25 lbs/ft3)
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