Experimental Study of the Mechanical Behavior of a Bentonite with the Insertion of Crushed Polyethylene Terephthalate

Transcription

1 Experimental Study of the Mechanical Behavior of a Bentonite with the Insertion of Crushed Polyethylene Terephthalate N. S. L. Louzada 1,a, L. M. Repsold 2,b and M. D. T. Casagrande 3,c 1,2,3 Rua Marquês de São Vicente, 225, Gávea - Rio de Janeiro, RJ - Brazil a nathaliallouzada@gmail.com, b lucasrepsold@globo.com, c michele_casagrande@pucrio.br Keywords: direct shear tests, crushed PET, waste, reinforced soil, bentonite. Abstract. Every year millions of PET bottles are discarded into the environment. In order to reduce the disposal of this polymer in nature, this study aims to evaluate the mechanical behavior of a bentonite mixed with crushed PET. The potential use of this waste material in geotechnical applications may ultimately reduce the problem of wrong disposal, and this material can improve the strength and deformation characteristics of the soil. This paper presents an experimental study to evaluate the mechanical behavior of bentonite and mixtures with different contents of PET waste by direct shear tests, in order to obtain the strength parameters of the Bentonite-PET mixtures. The bentonite used was mixed with 0.3 and 0.5% of crushed PET by dry weight. Characterization tests were performed on the bentonite. Direct shear tests at normal stresses of 50 and 100 kpa were done on the bentonite and mixtures. The results show that the strength parameters are influenced by the addition of the crushed PET, thus improving characteristics such as friction angle and cohesion of the mixtures. These mixtures may be used in landfill and other geotechnical works, so this paper proposes to contribute to a better understanding and interpretation of the behavior of soil reinforced with PET waste. Introduction Soil reinforcement is defined as a technique to improve the geotechnical characteristics of the soil [1]. Reinforcement consists of inserting certain material in another, in order to increase its properties. Therefore, many different kinds of materials have been studied with the aim of improving the strength properties of some soils. PET bottles production are increasing annually and they are still wrongly discharged. This research aims to evaluate the use of a fine crushed PET as a soil reinforcement, through experimental laboratory tests, giving an environmentally friendly end to this material. Finally, the use of fine crushed PET would decrease the demand of natural resources, and add value to this material; it also could reduce the impact in nature such as, air pollution, river s aggradation, and eliminating current problems of waste disposal in dumps and landfills. Materials Bentonite. The sodic bentonite used in this studied (Fig.1) was commercially purchased in Rio de Janeiro, and according to the ABNT NRB 6502:1995, this bentonite can be classified as clay. The results of physics index and particle size distribution are presented in the results section.

2 Figure 1 - Bentonite Sample used in this research. PET Flakes. The PET flakes came from the crushing process made in the material crusher at the Laboratory of Structures and Materials of PUC-Rio. The hole crushing process can be divided in four steps described below. 1st step: the PET bottles were cleaned and the top and bottom parts and the label were removed (Fig 2). Figure 2 -PET bottles used The PET pass through a high speed crusher machine, which crushes the material using five very sharp blades. The PET bottles are introduced on the top of the machine, the blades cut them until they pass through the sieve, and finally the crushed material is collected in a box. This crushing process is repeated for each sieve and until the material achieve the desired size, for this studied the PET size used was 1 mm (Fig.3). Figure 3 - the PET flakes with 1mm size Mixtures. The bentonite was mixture with two different contents of PET flakes, 3 and 5% on dry weight. The bentonite samples were prepared with 170% of humidity, which correspond a void ratio of 4,93. To make sure that the material stayed with the right properties, it was calculated the right amount of dry material that should be placed into the shear box. Table 1 summarizes the materials used end the abbreviations adopted for them.

3 Table 1 - Abbreviations used for the soil and mixtures Material/ Mixture Soil (%) Fine Crushed PET (%) Abbreviation Bentonite B100 Mixture B97F03 Mixture B95F05 Experimental Procedures. With the aim to determinate the index properties of the samples of the bentonite and mixtures, characterization tests were performed in each sample. The soil was prepared according to the Brazilian technical standard (Brazilian Association of Technical Standards ABNT). The performed tests followed next standards: [2] NBR 6457/1986 Soil Sample Preparation for compaction test and characterization; [3] NBR 7181/1984 Soil Particle Size Analysis; [4] NBR 6508/1984 Soil Determination of the specific gravity of the soil solids; [5] NBR 6459/1984 Soil Determination of the liquid limit; [6] NBR 7180/1984 Soil Determination of the plastic limit. The specific gravity of soil solids, particle size analysis, liquid limit and plastic limit test were carried out using the bentonite. The results will be present next. Direct Shear Test. The direct shear tests were carried on with the bentonite and the mixtures, with the aim to determinate the shear strength of the soil. This strength is obtained through the parameters of cohesion (c) and friction angle (ϕ). These tests were performed following the methods described by the standard ASTM D 3080/2004. To perform this test, the specimen was placed inside the metallic shear box. The box is divided horizontally in two and the normal force is applied on the top of the specimen in the shear box. The shear force is applied by moving one half of the box relative to the other in order to cause failure. The upper and lower boxes are separated in 1,0 mm before initiates the shear test, so it is possible to have the relative displacement between them. Above and below the specimen are placed plates with grooves, which provide friction to the soil preventing it to slide when a horizontal force is applied. Filter papers to prevent the particles from being carried, and porous stones so the drainage can occur freely, even if the specimen was partially or totally saturated, are placed in the right position. The shear test was performed with strain control, where a constant rate of shear displacement is applied to the upper half of the box by an engine, which operates with cogs with a velocity that is determinate by a factor, according to the load applied vertically. This velocity is calculated through the data that comes from the initial phase of the test called the consolidation phase, where the specimen is submitted to vertical stress and it is measured the variation of the specimen height with the time, until this height stabilize. Through the graphic of vertical displacement versus square root of time (t), is obtained the value of t100, that corresponds to 100% of consolidation, and the velocity to be used in the shear phase is calculated. The consolidation time was estimated in 24 hours. In the shearing phase, the failure of the specimen occur along the division plane of the box. The horizontal displacement of the upper half of the box was measured by a horizontal LVDT (Linear Variable Differential Transformer), which works as a sensor to measure the linear displacement. The variation of the specimen height, in other words, the variation of the volume of the specimen throughout the test are obtained by the vertical LVDT readings. The load ring measures the horizontal force applied on the specimen. Results and Analysis. Specific Weight. The value of specific weight (Gs) for the Bentonite was obtained through the arithmetic average of four determinations, and the maximum variation was 1,1%. The value of Gs found was 2,90. Particle Size Test. The particle size curve of the bentonite was reached through the sedimentation test and it is presented in the Fig.4.

4 Figure 4 - Particle size curve of the bentonite Atterberg Limits. Through the results achieved in the laboratory for the bentonite, it was found a Liquid Limit of 368,4% and a Plastic Limit of 53,7%, resulting in a Plastic Index (IP = LL PL) equal to 314,7%. Direct Shear Test. The tests were carried out in similar specimen, for the bentonite and the mixtures. It was carried out tests in three different confining stresses, 50 and 100kPa. Next, will be presented the graphics of deviator stress and vertical displacement versus axial displacement, which correspond to the direct shear tests performed in the bentonite and in the mixtures. In Figure 5, the curves of deviator stress and vertical displacement versus axial displacement are presented, which correspond to the direct shear tests performed in the bentonite and in the mixture B97F03 with confining stress of 50 and 100kPa. Figure 5 - Curves of shear strength and vertical displacement versus axial displacement for the bentonite and the mixture B97F03 in the direct shear test.

5 In Figure 6, the curves of deviator stress and vertical displacement versus axial displacement are presented, which correspond to the direct shear tests performed in the bentonite and in the mixture B95F05 with confining stress of 50 and 100kPa. Figure 6 - Curves of shear strength and vertical displacement versus axial displacement for the bentonite and the mixture B95F05 in the direct shear test. Table 2 summarizes the Mohr-Coulomb strength parameters for peak and residual strength of the bentonite and mixtures. Table 2 - Results of shear strength parameters for the bentonite and the mixtures. Material/Mixture Peak Strength Cohesion (kpa) Friction Angle ( ) Residual Strength Cohesion (kpa) Friction Angle ( ) B100 2,5 4,0 4,4 0,7 B97F03 5,0 2,5 3,8 1,0 B95F05 12,7 0,5 7,9 1,3 It can be inferred from the graphics that an improvement in the cohesion related with the peak strength is observed in both mixtures, however, the friction angles get smaller. This phenomenon can be explained because the interaction between the particles of bentonite and PET flakes are higher than the interaction of the particle of the pure bentonite, so the cohesion increases. In contrast, the friction

6 angle reduces because the difference among the peak strength of 50 and 100 kpa remains smaller than the difference found in the pure bentonite. In the case of the residual strength for the mixture B97F03, the cohesion decreases, whereas with the mixture B95F05 it has a better improvement. For both mixtures the friction angle increase with the addition of the fine crushed PET. This phenomenon also highlights that PET flakes change the mechanical behavior of the material in relation with the peak and residual strength. The mixture B95F05 caused a major improvement in the strength parameter, where, for peak strength the cohesion became five times higher than the pure soil and for the residual strength the friction angle raised 70% and the cohesion raised 79%, compared with the pure bentonite. The graphic vertical displacement versus horizontal displacement of the mixture B97F03, for 50 and 100 kpa of vertical stress were higher than the values found for the bentonite. For the mixture B95F05the insertion of fine crushed PET, caused a swelling in the specimen just at a vertical stress of 100kPa. References [1] Ling I, Leshchinsky D, Tatsuoka F (2003). Reinforced soil engineering: advances in research and practice. Marcel Dekker Inc.A.N. Author, Article title, Journal Title 66 (1993), [2] Associação Brasileira de Normas Técnicas (1984). ABNT NBR 7181: SOLO Análise granulométrica (In Portuguese). Rio de Janeiro, Brazil. [3] Associação Brasileira de Normas Técnicas (1984). ABNT NBR 6459: SOLO Determinação do limite de liquidez (In Portuguese). Rio de Janeiro, Brazil [4] Associação Brasileira de Normas Técnicas (1984). ABNT NBR 7180: SOLO Determinação do limite de plasticidade (In Portuguese). Rio de Janeiro, Brazil. [5] Associação Brasileira De Normas Técnicas (1984). ABNT NBR 6508: SOLO Determinação da densidade real dos grãos (In Portuguese). Rio de Janeiro, Brazil. [6] Associação Brasileira de Normas Técnicas (1986). ABNT NBR 6457: AMOSTRAS DE SOLOS Preparação para ensaios de compactação e caracterização (In Portuguese). Rio de Janeiro, Brazil. [7] Associação Brasileira de Normas Técnicas (1986). ABNT NBR 7182: SOLO Ensaio de Compactação (In Portuguese). Rio de Janeiro, Brazil.