3. Material Properties

3.1 Density

EPS geofoam density can be considered the main index in most of its proper-ties. Compression strength, shear strength, tension strength, flexural strength, stiff-ness, creep behavior and other mechanical properties depend on the density. The cost of manufacturing an EPS geofoam block is considered linearly proportional to its density. Non-mechanical properties like insulation coefficients are also density de-pendent.

EPS densities for practical civil applications range between 11 and 30 kg/m3. For other applications like insulation higher densities are more efficient (van Dorp, 1988). With its lightweight property, geofoam blocks can be easily handled after manufacturing, during curing, transportation or placement in the field. Two workers can handle a 0.6m X 1.2m X 2.4 m half size block of an average weight of 35 kg for 20 kg/m3 density EPS geofoam.

In the United States, manufacturers and designers working with EPS geofoam are familiar with density classification used by thermal insulation standards, C 578-95. Table 2-2 shows 5 EPS types, which are categorized by ASTM C 578-95. Tables 2-3 and 2-4 show EPS densities used in two other countries.

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3.2 Compression
3.2.1 Compressive Strength and Stress Strain Curve

Figure 2-3 shows the uniaxial compression stress strain curve of EPS geofoam for two different densities. The two densities shown are considered the extreme val-ues for most engineering applications done so far. Specimens are 0.05m cubes tested at a displacement rate of 0.005m/min. From the figure the stress strain curve can be simply divided into two main straight lines connected with a curved portion. The slope of the straight-line portions increase with density. The stress at any strain level increases also with the density. The bead size has no important effect on the com-pressibility of cut specimens (BASF Corp., 1968)

Table 2-2 ASTM C 578-95 EPS Densities

Table 2-3 EPS Types in Japan (after Miki, G., 1996)

Table 2-4 EPS Types in United Kingdom (after Sanders, 1996)

There is no defined shear rupture for EPS geofoam under compression. As will be shown later in chapter six, more than 70 % strains are reached without any break point and the tests were stopped because the maximum travel of the machine head was reached. The 1%, the 5%, and the 10% strains are common reference strain level, at which the stress is considered as the strength of the material. Tables 2-5, shows the compressive strength of EPS geofoam as given by ASTM C578-95.

EPS geofoam under confining compression Sun (1997) reported that with in-crease in confining stress the strength and initial tangent modulus decrease. Sun con-cluded these results based on axial deviator stress strain curves, which are important for submerged EPS geofoam.

Figure 2-3 EPS Uniaxial Compression Stress Strain Curves
(after Ne-gussey and Elragi, 2000b)

Table 2-5 EPS Types in United Kingdom (after Sanders, 1996)

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The stress strain curve of EPS geofoam has an initial linear portion. The value of the slope of this initial portion is defined as the initial tangent modulus. Also it is known as Young’s Modulus as well as the modulus of elasticity. EPS geofoam initial modulus is a function of the density as shown from figure 2-4. For EPS geofoam, as shown from the same figure, there is no agreement from the researchers on a constant value for each density. For a 20kg/m3 density the initial modulus ranges between 5Mpa and 7.75Mpa, which means a 55% difference. The relation is linear for some researchers (Horvath, 1995b and Miki, H., 1996) while it’s nonlinear for others (Dus-kov, 1997 and Eriksson and Trank, 1991). The researchers used specimens with vary dimensions.

Duskov, (1990) reported that the back calculated moduli of elasticity of EPS geofoam were found to be between 13 MPa and 34 MPa under pulse force. These values were observed to be much higher than the value of the modulus of elasticity (5MPa) obtained under the semi static loading. Duskov (1997) after testing 20kg/m3 EPS geofoam, reported that low temperatures, water absorption level, and exposure to freeze-thaw cycles, separately or combined, seem to have no negative influence on the mechanical behavior of the EPS geofoam that he had tested. Elragi et al. (2000) showed the effect of sample size on the initial Modulus. For larger specimens, the ini-tial modulus is higher.

Figure 2-4 Initial Tangent Modulus for EPS geofoam

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Poisson’s ratio is an index of the lateral pressure of EPS geofoam on adjacent structural elements, in contact, for a certain applied vertical load on the EPS geofoam mass. Value range between 0.05 and 0.5 are found in the literature for EPS geofoam as shown in table 2-6. These values range from material like water (Poisson’s ratio equals to 0.5) to rigid materials like concrete (Poisson’s Ratio equals to 0.15) Chapter three present a solution to this discrepancy.

Table 2-6 EPS Types in United Kingdom (after Sanders, 1996)

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The compression behavior of EPS geofoam is strain rate dependent (Ne-gussey, 1997). Higher strain rates result in higher initial modulus and higher com-pression strength. A more extensive work is shown in chapter three.

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EPS geofoam may experience cyclic loading in a number of situations. This can include traffic loading and dynamic loading. The majority of laboratory testing and field observations suggest that the cyclic load behavior of block molded EPS geo-foam is linear elastic provided that the strains are no greater than approximately 1%. For three loading cycle tests, the initial tangent modulus in the second and third cy-cles is much less than that for the first cycle, when the three cycles are loaded to 10% strain (Eriksson and Trank, 1991). Flaate (1987) reported that cyclic load tests show that EPS geofoam will stand up to an unlimited number of load cycles provided the repetitive loads are kept below 80% of the compressive strength. More cyclic loading testing results are shown in chapter three.

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3.3 Tension

Tensile strength of EPS material can be an indication of the quality of fusion of the prepuffs and any recycled EPS geofoam used in the process (Horvath, 1995b). From figure 2-5 it can be seen that the tension strength increases with the density.

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3.4 Flexural

Flexural strength tests are widely used as a quality control test in EPS geo-foam manufacturing plants. The maximum stress is calculated assuming the material is linear elastic up to failure. Although this is not an accurate assumption, the calcu-lated values are widely used in quality control. The material fails in tension as a crack on the tension side appears at the moment of failure.

The flexural strength increases with density of the material as shown in table 2-7. From table 2-7 and figure 2-5 it can be seen that the values of the flexural strength are almost the same as the tension strength since the mode of failure is ten-sion in the outer points

Figure 2-5 EPS Geofoam Tensile Strength (after BASF, Corp., 1997)

Table 2-7 ASTM C 578-95 EPS Flexural Strength

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3.5 Creep

EPS geofoam is susceptible to time dependent creep deformation when a con-stant stress level is applied. A number of parameters affect the creep behavior of EPS geofoam, among which is the density. Creep deformations decrease with density in-crease (Sun, 1997).

Figure 2-6 represents the results of three 0.05m cube specimens each are sub-jected to an unconfined axial stress for a period of over 500 days. The stresses are 30%, 50% and 70% of the strength of the material. The three specimens are of type VIII and minimum density of 18kg/m3. It can be seen from the figure that the creep behavior is stress level dependent. For the lower stresses, very little creep deforma-tion occurred after 500 days.

Both full scale and laboratory creep tests have been performed (Aab?e, 2000) A test was done with 2m height of geofoam loaded to 52.5% of its compressive strength. Results observed in a three year period show continuous deformation with time. The strain after the three years was about 1% and slightly increasing with time. The full-scale test was for an EPS bridge abutment. Stresses in the geofoam abutment ranged between 25 and 60% of EPS strength at 5% strain. Observed deformation after 10 years in operation shows negligible creep. The effect of specimen size on the creep behavior and further creep results and observations are shown in chapters three and five.

Figure 2-6 EPS Creep Behavior for Different Stress Levels (after Sheeley, 2000)

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3.6 Interface Friction

Sheeley (2000) did a comprehensive study of geofoam interface shear behav-ior for small and large samples. Normal stresses in the range of practical interest were used and different interfaces were investigated. Geofoam to geofoam interface shearing developed peak and residual strengths are shown in figure 2-7. The effect of density on interface strength of geofoam was negligible. There was difference be-tween wet and dry interface strengths in the range of normal stresses used in practice and for short-term exposure to water. A strong adhesive bond developed between geofoam and cast in place concrete interfaces and both peak and residual interface strengths were high. The interface strength between geofoam and geomembrane sur-faces was low. Substitution of a concrete load distribution slab with a geomembrane may therefore result in a much weaker interface. Values of both peak and residual friction factor are shown in table 2-8. Although values of 0.65 were reported for EPS geofoam to EPS geofoam interface, 0.5 can be considered a conservative coefficient of friction as Nomaguchi (1996) obtained from both static and dynamic tests.

Figure 2-7 EPS Interface Friction (after Sheeley, 2000)

In practice, metal binder plates are sometimes used to attach foam layers to each other. Sheeley and Negussey (2000) reported that binder plates did not provide increased shear resistance in one directional loading and had reduced resistance in reverse loading and reloading.

Kurose and Tanaka (1996) have proposed a new technique in which H and C shaped EPS blocks could be successfully used in embankment construction. The main idea is to have interlocked EPS blocks to act as one unit.

Table 2-8 EPS Geofoam Interface Friction Factors (after Sheeley and Ne-gussey, 2000)

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3.7 Thermal Resistance

EPS geofoam consists of approximately 98% air and 2% polystyrene (BASF Corp., 1997). The air entrapped within the geofoam is a poor heat conductor, there-fore, making EPS geofoam excellent for heat insulation. The R-value measures ther-mal resistance of substances. R-values for typical soil and concrete in general are much less than 0.1 m3 ?C/W. Figure 2-8 shows the R-values of EPS geofoam as given in ASTM C 578-95 and the range of 0.5- 0.8 m3 ?C/W for EPS geofoam is much higher than the R-values of typical soil and concrete. The R-value of geofoam increases with the density. The curves tend to level horizontally with increasing the density. van Dorp, 1988 mentioned that it reaches its maximum around a foam den-sity of 35kg/m3. The R-value tends to decrease with temperature increase as shown in the same figure. Another factor that will affect the thermal resistance of EPS geofoam is the amount of moisture absorption (Negussey, 1997). R-value degrades or de-creases with moisture absorption while aging has no effect on the R-value (Hunts-man, 1999i). This is because the closed cell structure of EPS contains only air.

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3.8 Ultraviolet Effect

Exposing the expanded polystyrene geofoam to ultraviolet will yellow the sur-face and a powder like texture will appear. Sheeley, (2000) tested the effect of this new surface on the interface friction between foam blocks and concluded that ultra-violet degradation diminished the peak interface strength between geofoam and cast in place concrete. Power washing removed with commercially available equipment effectively the degradation and improved the adhesion bond strength.

Figure 2-8 ASTM C 578-95 EPS R-Values

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3.9 Flammability

Expanded polystyrene geofoam is combustible and should not be exposed to open flame or other ignition sources. Combustion products are carbon monoxide, car-bon dioxide, water and soot. The manufacturer can include fire-retarding additives during production, which will increase cost by 5 – 10% if procedures generating heat and flame are required near geofoam (Sun, 1997). The fire retardant is mainly to de-crease the potential of fire spread from a small flame source. The melting temperature of polystyrene is 150?C (Mandal, 1995).

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3.10 Water Absorption

The water absorption of expanded polystyrene is low. Although water absorp-tion decreases as density increases as shown in table 2-9, fusion is the most important factor influencing the moisture resistance of expanded polystyrene. Good fusion re-duces the amount of water absorption. For 9 –12 years of service, equilibrium values of 8-9 % volume have been found in EPS fills below the ground water table (van Dorp, 1988).

Table 2-9 % Volume of Water Absorption (German Specifications, after van Dorp, 1988)

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3.11 Resistance to Attacks

Chemical resistance of thermoplastic expanded polystyrene is dependent on time, temperature, and applied stress in functional use (Huntsman, 1999f). Chemical attack usually results in the softening and cracking of the plastics. Expanded Polysty-rene has the same resistance to chemical reagents as general-purpose polystyrene. Most acids and their water solutions do not attack polystyrene; however strong oxi-dizing acids do. The thin cell walls and large exposed surface of expanded polysty-rene make it sensitive to attack by solvents. Table 2-10 shows some of the chemical reagents and solvents and the corresponding EPS resistant.

Since it has no nutritional value, expanded polystyrene does not attract ants, termites, or rodents; however it is not resistant to them. Habitation by insects can be a problem for geofoam, as they can burrow through geofoam to reach food or to estab-lish a comfortable home. Marine borers can also attack polystyrene as they do wood.

Fungal attack has not been observed on expanded polystyrene. EPS does not support bacterial growth as well. The main reason is that expanded polystyrene can-not supply nutrients for fungal or bacterial growth.

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3.12 Energy Absorption

EPS geofoam is utilized in packaging, as its energy absorption characteristics provide good protection. From the stress strain curves of EPS geofoam shown in fig-ure 2-3 it can be seen that the area under the curve, which represents the strain energy absorbed by the material increases with density. Sliding between the EPS geofoam blocks is another way to produce damping (Kuroda, et al., 1996).

Table 2-10 Selected EPS Resistant Behavior
(after BASF Corp., 1997 and van Dorp, 1988)

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3.13 Inertness Properties

Polystyrene is completely inert. Any corrosion of metals in contact with ex-panded polystyrene will be caused by other factors (R-control, 1999a).

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3.14 Acoustical properties

Expanded polystyrene, when used in combination with other building materi-als effectively reduces the transmission of airborne sound through partitioned walls, ceilings and floors (Huntsman, 1999g). EPS has the advantage of being lightweight and effective in thicknesses as low as 0.625 cm It can replace thicker, heavier materi-als.

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3.15 Durability

No deficiency effects are to be expected from EPS fills placed in the ground for a normal life cycle of 100 years, Aab?e (2000). Aab?e added that this should hold true provided possible buoyancy forces resulting from fluctuating water levels are properly accounted for, the blocks are properly protected from accidental spills of dissolving agents and the applied stress level from dead loads is kept below 30-50% of the material strength.

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3.16 Environmental Effect

EPS geofoam is made of polystyrene beads and polystyrene is not biodegrad-able and chemically inert in both soil and water. Therefore EPS geofoam will not contaminate the ground or ground water. Adding to this, EPS is widely used for food containers. If EPS is burned either accidentally or intentionally as a part of a waste to energy program, the products of composition are primarily carbon dioxide and water. EPS is a recyclable material and 5% of volume of the manufactured geofoam is tradi-tionally recycled geofoam. Raw beads, prepuffs, regrind and small molded parts can obstruct sewers and waterways. They have been found in the digestive tracts of fish (Huntsman, 1999a).

The plastic foam insulation producers faced a major set back issue for their industry because of ozone depletion caused by the use of low thermal conductivity blowing gases such as chlorofluorocarbon 11 and 12 (CFC-11 and CFC-12). Industry has obtained an acceptable immediate solution by using hydro chlorofluorocarbon (HCFCs) 141b and 142b for polyisocyanurate and polystyrene, respectively. The phase out of HCFCs may be required within the next 10 years, but other chemicals, such as hydro fluorocarbons (HFC-134a and HFC-245fa) exist and could become the next generation of blowing agents (McElroy, 1998).

An important benefit of EPS geofoam is that it now does not use CFC or HCFC in its manufacture as most other polymeric foam does. (Horvath, 1993 and GeoTech, 1999b). Extruded polystyrene, a member of the geofoam family, and the main competitor to EPS in insulation applications does have ozone-depleting gases as the blowing gas in its manufacturing. Huntsman, (1999a, 1999c, 1999e) provides a guide to clean air permitting for expandable polystyrene processors.

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