Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 189 (2017) 239 – 246
Transportation Geotechnics and Geoecology, TGG 2017, 17-19 May 2017, Saint Petersburg, Russia
Expanded polystyrene (EPS) geofoam: preliminary characteristic evaluation Y. Z. Bejua,*, J. N. Mandalb a
Research Scholar, Civil Engineering Department, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India b Professor, Civil Engineering Department, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076, India
Abstract Expanded polystyrene (EPS) geofoam is a fascinating area in the field of geotechnical engineering and its service life is comparable to other construction materials. It can be used for backfilling embankments and highways both vertical and sloped sides. Moreover, it has been used as a compressible inclusion behind retaining walls, above pipelines, tunnels or culverts and below slabs or beams at a foundation. In the present study, an attempt has been made to comprehend the behavior of EPS geofoam using different laboratory tests such as water absorption test, compressive strength test, flexural strength test and triaxial test. The shear strength behavior of EPS geofoam was examined through a series of triaxial tests conducted for confining pressure of 50, 100 and 150 kPa. The tests have been carried out on EPS geofoam samples of three different densities, 12, 15 and 20 kg/m3. The behavior of EPS geofoam in compression is a function of density, strain rate and sample size. The higher density of EPS geofoam develops high compressive strength and the size of prismatic specimen of EPS geofoam affects the modulus values. The flexural strength of EPS geofoam increases with the increase in density. Higher density test specimen failed at lower deformation due to increase in the stiffness with an increase in density. The triaxial test results indicated that the cohesion and angle of internal friction of EPS geofoam increase with an increase in density. However, the cohesion is found to be the major parameter which contributes the shear strength of EPS geofoam. The water absorption capacity of EPS geofoam was found very less and also decreased with an increase in density. 2017The The Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license ©©2017 Authors. Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology Geoecology. Keywords: EPS geofoam; Deformation; Density; Cohesion; Angle of internal friction
* Corresponding author. Tel.: +91-916-776-9109; fax: +91-222-576-7302. E-mail address:
[email protected]
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology
doi:10.1016/j.proeng.2017.05.038
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1. Introduction Expanded polystyrene (EPS) geofoam is super-lightweight a rigid cellular plastic foamed polymeric geosynthetic material and its density is only about 1% of the density of soil [1]. The extensive popularity of EPS geofoam material is due to its many outstanding characteristics, e.g. water resistant, non-biodegradable, eco-friendly, easy to transport and install easily without special equipment, compressible, etc. The ultra-lightweight properties of EPS geofoam can be used for the construction of embankments and pavements over poor soil [2,3]. Moreover, EPS geofoam can be used as a compressible inclusion behind earth retaining structures, beneath a grade beam, above culverts [4-7]. Different researchers have been studied the material behavior of EPS geofoam under various loading conditions: tensile strength and elastic modulus [8]; compression creep behavior [9-11]; uniaxial and compression behavior [12-14]). The influence of confining stress on EPS geofoam water absorption capability checked by [15]. The mechanical behavior of EPS geofoam affected by density, strain rate, confining stress and temperature [16,17]. The properties of EPS geofoam have been studied experimentally for many years by several researchers who are involved in the design and use of geofoam product. However, no studies have been carried out so far for water absorption, compressive and flexural strength, and shear strength parameters on low densities of EPS geofoam. The aim of the present study is to understand the behavior of different low densities of EPS geofoam using different test methods. The effect of density, sample size and applied stress on the behavior of EPS geofoam are reported. 2. Laboratory investigation A series of water absorption, compressive strength, flexural strength and triaxial tests were performed to study the behavior of EPS geofoam. The EPS geofoam densities used in this study are 12, 15 and 20 kg/m 3. The materials were available in cylindrical, rectangular and cube form which different sizes of the specimens were sampled as per ASTM D7557-09 [18] specification for different tests. The specimen cutting work was carried out at Packshield Industries limited, which is a manufacturer and supplier of EPS geofoam in Mumbai, India. 2.1. Water absorption tests The water absorption test was performed according to ASTM C272-12 [19] standard test method. The test specimens with dimension 75 mm length, 75 mm width and 13 mm thickness were used. The specimens were marked according to their densities. The dry mass of the specimens was taken with sensitive balance an accuracy of 0.001g. The test was performed by immersing the test specimen in a container horizontally under 150 mm head of water for 24 hours. After that, the specimens removed from the conditioning and wipe off all water surface with a dry clean cloth until no visible water was present and immediately the wet mass of the specimens were weighed and recorded with the same accuracy. The water absorption of EPS geofoam in percent is calculated as the mass of water absorbed by EPS geofoam specimen to its initial dry mass. Three specimens from each density sample were tested and their average value was calculated. 2.2. Compressive strength tests This test method provides information regarding the behavior of EPS geofoam materials under compressive loads. The compressive strength test was performed as per ASTM D1621-10 [20] standard test method on digital compressive strength test machine. The test setup consists of two flat plates, one attached to the stationary base of the testing instrument and the other attached to the moving crosshead to deliver the load to the test specimen. The sizes of these plates were larger than the specimen loading surface to ensure that the specimen loading is uniform. The test specimens were cubical in cross section with sizes 50 mm, 100 mm and 150 mm for each density of 12, 15, and 20 kg/m3 were tested. The specimens placed between the compressions platens and their center line were aligned at the center line of the compression platens and the load was distributed uniformly over the entire loading surface of the specimen. The compression platen displacement and the corresponding load data were recorded until the specimen has been compressed 15% of its original thickness at a constant strain rate of 2.5 mm/min.
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2.3. Flexural strength tests The flexural strength test was conducted on EPS geofoam in accordance with ASTM C203-12 [21] on a computerized universal testing machine using three point test method. The test setup consists of typical span arrangement with a span length 250 mm. The EPS geofoam test specimen was considered as a beam on two supports of the span and loaded by means of a load fitting midway between the supports. The test specimen with dimension 300 mm × 100 mm × 25 mm was used. The specimen was loaded with a deformation rate of 4.2 mm/min. The deformation rate was calculated from the empirical equation given in the test specification. Load with the corresponding deformation of the test specimen was recorded till its failure. Four test specimens with each density were tested and the average value is calculated. 2.4. Triaxial tests Unconsolidated undrained (UU) triaxial tests were performed on the EPS geofoam samples with dimensions 75 mm diameter and 150 mm height, for different densities under 50 kPa, 100 kPa and 150 kPa cell pressures. The influence of applied cell pressure and EPS density on the behavior of EPS geofoam is investigated in the test. The test specimen was covered with rubber membrane and fitted with an upper loading plate by using membrane stretcher. Transparent chamber was then fitted properly and the test specimen was mounted centrally on the pedestal of triaxial cell with upper loading plate. The UU triaxial test setup with test specimen is shown in Fig. 1. Water was filled inside the chamber which acts as an operating fluid. The axial load was applied with a constant strain rate of 1.5 mm/min as per ASTM D2850-15 [22] testing method. Each test was conducted up to a maximum axial stain of 15%. The average required loading time for each sample was about 15 minutes. Deviator load was measured by 5 kN capacity load cell and vertical displacement was measured by linear variable differential transformer (LVDT) connected to a digital indicator. The measuring devices were calibrated before their use.
Fig. 1. Triaxial tests (a) line sketch view of EPS (b) placement of test specimen (c) loading of EPS sample in triaxial cell.
3. Results and discussion The water absorption test results were found to be 4.41%, 3.6% and 2.88% for 12, 15 and 20 kg/m3 densities of EPS geofoam respectively. The test result shows that the water absorption property of EPS geofoam is very less and decrease as its density increase. The compressive stresses of EPS geofoam are a function of density. Fig. 2 shows the compressive behavior of EPS geofoam material under compressive loading condition. From which compressive strength of EPS geofoam material has determined at, 1%, 5% and 10% strain levels which are, in general, a range of compressive strength. The stress-strain curves show that the behavior of EPS geofoam under compression loading system depends on its densities; the higher density of EPS geofoam develops high compressive strength. The nature of the stress-strain curves are analogous for all densities and sizes tested. The stress-strain behaviors of EPS geofoam are found to be
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nonlinear. Meanwhile, it is directly proportional up to 1.5% of the strain level and between 1.5 to 4% of strain level the yield points were developed.
Fig. 2. Stress-strain behavior of EPS geofoam for different densities under compressive loading specimen size of (a) 50 x 50 x 50 mm (b) 100 x 100 x 100 mm (c) 150 x 150 x 150 mm.
Beyond the yield point, the compressive stress increases slightly with increased strain with linear variation. The size of prismatic specimen geofoam affects the modulus values that is modulus value increased with increasing of specimen size. The average compressive strength values were dogged. The compressive strength test results at 1%, 5% and 10% strain levels, yield strength and initial tangent modulus for each density of EPS geofoam are depicted in Table 1. The density of EPS geofoam is a normal index property for quality and modulus as design parameters. A direct relationship found between compressive strength and density of EPS geofoam. Fig. 3(a) indicates the relation between compressive stresses and sample sizes at10% strain level. It is observed that there exists very nearly a direct connection between these two parameters. Initial tangent moduli were determined from the linear portion of the stress-strain curve. The EPS geofoam increasing initial tangent modulus with increasing density and sample sizes as displayed in Fig. 3(b). The yield stress of EPS geofoam increased with density portrayed in Fig. 3(c). The compressive strength values of EPS geofoam specimen measured in the laboratory agreed with the value given by ASTM D6817-13 [23]. Table 1. Compressive strength test results of EPS geofoam. Density of geofoam, ρ (kg/m3)
Properties Compressive strength, σc (kPa)
Initial tangent modulus, Ei (kPa)
Yield strength, σy (kPa)
At 1%
At 5%
At 10%
12
16
32
48
1611
29
15
44
62
71
2769
52
20
72
97
112
5086
96
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Fig. 3. (a) compressive stress gain at 10% strain level with increasing sample sizes (b) comparisons of initial tangent moduli (c) relationship between yield stresses with density of EPS geofoam.
The flexural strength test results are illustrated in Table 2. The test setup and the load-deformation behavior of EPS geofoam under flexural loading condition are depicted in Fig. 4. From the test results, it is observed that the flexural strength of EPS geofoam increases with the increase in density. Higher density test specimen failed at lower deformation due to increase in the stiffness with an increase in density. Table 2. Flexural strength test results. Density of geofoam,
Flexural strength,
Deformation at failure,
ρ (kg/m3)
σf (kPa)
∆f (mm)
12
88
25.53
15
169
23.4
20
219
21.36
Fig. 4. Flexural strength test on EPS geofoam (a) test setup (b) load-deformation behavior of EPS geofoam under flexural loading condition.
3.1. Stress-strain and shear strength characteristics In UUT test, no definite failure pattern was observed when EPS geofoam specimens were tested under different cell pressures. The samples were compressed with an increase in the deviator load and very small bulging was observed on their surfaces. Similar patterns were observed for all densities of test specimens under each cell pressure. The deformed different densities of EPS geofoam test specimens under cell pressure of 50 kPa, 100 kPa,
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and 150 kPa are shown in Fig. 5. For all densities of EPS geofoam, the behavior of stress-strain curves is observed to be nearly similar and linear up to 1.5% of axial strain and thereafter a very little increase in deviator stress was observed with increase in axial strain. Fig. 6 shows the non-linear stress-strain behavior for different densities of EPS geofoam under different cell pressures. As shown from this figure, after yielding point the slope decreases and closes rapidly to the plastic stage.
Fig. 5. Observed deformation of EPS geofoam test specimens for densities (a) 12 kg/m3 (b) 15 kg/m3 and (c) 20 kg/m3.
Applied cell pressure and density of EPS are found to be the influential parameters of stress-strain behavior under triaxial loading condition. The stress-strain behavior of EPS geofoam is considerably affected by density, but the effect of applied cell pressure is minor. With the increase in cell pressure, no significant increase in deviator stress is observed for same density EPS geofoam. Within 0-1.5% strain range, the stress-strain behavior of EPS geofoam with same density is almost similar even though different cell pressures. Higher density EPS involves closed packing of the cellular structure and therefore shows a higher value of compressive strength and deviator stress. Also, initial tangent modulus (Ei) is determined as a slope of tangent line to the origin of the stress-strain curve. The initial tangent modulus of EPS geofoam increasing with an increasing in density and cell pressures as depicted in Fig. 6(d). The initial tangent modulus found in the triaxial test is not much different with the value determined by compressive strength test method. The initial tangent modulus values calculated for a particular density of EPS geofoam under different cell pressures are found to be nearly equal. Fig. 7 shows the Mohr’s circle for different densities EPS geofoam. The values of cohesion and angle of internal friction for different densities EPS geofoam obtained from the triaxial test results are given in Table 3. From this test results, it is observed that the cohesion is a major parameter which contributes the shear strength of EPS geofoam and it is a function of density. Meanwhile, the value of angle of internal friction increases very less with increase in density of EPS geofoam.
Fig. 6. Stress-strain behavior of EPS geofoam under triaxial loading condition for densities (a) 12 kg/m3 (b) 15 kg/m3 (c) 20 kg/m3 (d) comparison of initial tangent modulus.
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Fig. 7. Mohr’s circle for different densities of EPS geofoam (a) 12 kg/m3 (b) 15 kg/m3 and (c) 20 kg/m3. Table 3. Shear strength parameters of EPS geofoam using direct and triaxial test methods. Density of geofoam,
Cohesion,
Angle of internal friction,
ρ (kg/m3)
c (kPa)
ϕ (°)
12
28.25
1.25
15
32.75
1.38
20
37.5
2.25
4. Summary and conclusions The physical and mechanical characteristics of EPS geofoam are determined using standard test methods. The following conclusions are drawn from the present study: x Density is a good index property for classification of EPS geofoam. Both compressive strength and modulus of EPS geofoam increased with density. The size of specimen geofoam affects the modulus values i.e. modulus value increased with increasing of specimen size. x The water absorption capacity of EPS geofoam was found very less and also decreased with an increase in density. x The quality and strength of geofoam material rely on upon its density since Young's modulus diminishes with diminishing density. x Compressive strength characteristics of EPS specimens are appreciably influenced by its density and specimen size.
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x
It is observed that the cohesion is a major parameter which contributes the shear strength of EPS geofoam and it is a function of density. Meanwhile, the value of the angle of internal friction increases very less with an increase in density of EPS geofoam.
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