Monotonic shear behaviour of sand-tyre chips mixtures

Transcription

1 Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) Proc. 18 th NZGS Geotechnical Symposium on Soil-Structure Interaction. Ed. CY Chin, Auckland Maki Okamoto Connell Wagner, Auckland, NZ Rolando P. Orense Faculty of Engineering, University of Auckland, NZ Masayuki Hyodo and Jiro Kuwata Department of Civil Engineering, Yamaguchi University, Japan Keywords: tyre chips, composite materials, laboratory tests, monotonic shear behaviour ABSTRACT It is estimated that about 88% of 14 million scrap tyres generated in Japan in 26 were either reused or recycled. Because of this, attention has been paid on scrap tyres as new ground material, and recent researches have been moving towards this direction. One method of recycling scrap tyres is by processing them into tyre chips. This paper introduces the results of undrained and drained triaxial compression tests performed to investigate the monotonic shear characteristics of tyre chips-sand mixtures of various combinations. Test results showed that if a small quantity of tyre chips is mixed with sand, the static strength is influenced greatly. 1 INTRODUCTION Currently, about 14 million scrap tyres are generated in Japan every year as the automobile society develops, and the recycling of scrap tyres has become significant. About half of the 88% of the recycled tyres is used as fuel because it is cheaper than coal. However, the process generates large amount of carbon dioxide and incineration ashes. If the use of waste tyres as fuel is continued, it will lead to environmental problem in the near future. On the other hand, scrap tyres provide numerous advantages from the viewpoint of civil engineering practices. They have light weight, high elastic compressibility, high vibration-absorption capacity, high hydraulic conductivity, and temperature-isolation potential. Therefore, scrap tyre is gaining attention as new geomaterials. Scrap tyres can be used in several ways - either as whole, halved or even shredded. The effective use of waste tyres as geomaterial has advanced greatly in the United States during the first half of the 199s. For example, waste tyres were used as road embankments (Bosscher et al., 1997) and as lightweight backfill in retaining walls (Lee et al., 1999). Furthermore, a standard has been provided regarding the use of old tyres for engineering works through the ASTM Standards (ASTM, 1998). Various researches were also performed along this line in other parts of the world. In recent years, the use of tyres in bridge approaches was examined (Youwai and Bergado, 24), as well as in embankments and retaining walls (Humphrey et al., 26) and their environmental impact (Tuncer et al., 27). Although technological advancement in Japan regarding the effective use of old tyres as new geomaterial is still in its early stage, researches on their application as tyre chip-mixed solidified soil (Kikuchi et al., 26), fill improvement (Mitarai et al., 26) and as earthquake-resistant reinforcement (Hazarika et al., 26) have been actively conducted. In the future, it is necessary to accumulate information on the mechanical characteristics of tyre chips for use as construction materials in soil structures, as well as to address stability concerns of tyre chip-sand mixtures. Considering this background, a series of undrained and drained triaxial compression tests was conducted to understand the monotonic shear characteristics of composite materials containing 1

2 Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) tyre chips and sand mixed at various proportions. Based on the results of the triaxial compression tests, a discussion is presented on the shear characteristics of tyre chip-sand mixtures under various confining pressures. 2 MATERIAL USED AND EXPERIMENTAL METHOD 2.1 Physical properties of materials In this research, the test specimen was made of two types of materials: (1) Souma silica sand No.5 with revised grain size distribution, and (2) tyre chips. The tyre chips were derived from used tyres, with metals and fibres removed beforehand, and processed into smaller pieces measuring 1 mm in diameter. The tyre chips and Soma silica sand were mixed at various proportions, i.e., the mix ratios of sand to tyre chips by volume were set at 1:, 9:1, 8:2, 7:3, 5:5, 3:7 and :1. Table 1 summarizes the physical properties of the soil mixture, including density of soil particles (ρ s ), minimum and maximum dry densities (ρ dmin and ρ dmax ), maximum and minimum void ratios (e max and e min ), mean diameter (D 5 ) and coefficient of curvature (U c ), respectively, of the samples used in the experiments. In the table, sf (sand fraction) indicates the proportion by volume occupied by Soma silica sand in the tyre chip-sand mixture. Thus, sf = 1 indicates samples consisting of sand only, while sf = represents sample with tyre chips only. The density of the particles of tyre chips is 1.15 g/cm 3, which is relatively light compared to conventional geomaterials and represents only 2/5 of the particle density of the Soma silica sand. The maximum and minimum dry densities and maximum and minimum void ratios for the composite materials shown in Table 1 are summarized in Figure 1(a) in terms of their relation with sf. It is evident from the figure that the values of e max and e min show almost the same values within the range of sf = ~.3, and when sf =.3~1, the values of e max and e min decrease with increase in sf. On the other hand, both ρ min and ρ max increase in value with increase in sand fraction from sf = (tyre chip only) to sf = 1 (Soma Silica Sand No. 5 only). It is apparent that the lower the dry density of the mixture, the higher is the void ratio because the densities of particles of tyre chips and sand differ widely; and this is one of the features of this soil mixture. Figure 1(b) illustrates the grain size distribution curve for each sample type. Due to the difference in particle density between Soma silica sand No. 5 and tyre chips, the soil mixtures with sand : tyre chip mix ratio by volume of 1:, 9:1, 8:2, 7:3, 5:5, 3:7 and :1, have the corresponding sand : tyre chip mix ratio by dry unit weight as 1:, 95:5, 9:, 84:16, 7:3, 5:5, and :1, respectively. Therefore, even if 7% of the entire volume of the sample with sf =.3 consists of tyre chips, the particle size characteristics (D 5, U c ) of the soil mixtures as shown in Figure 1(b) with percent finer by weight in the vertical axis, are much closer to those of pure Soma silica sand than for pure tyre chips. Table 1: Physical Properties of soil samples Sand fraction ρ s (g/cm 3 ) ρ dmin (g/cm 3 ) ρ dmax (g/cm 3 ) e max e min D 5 (mm) U c sf=1(soma sand) sf= sf= sf= sf= sf= sf= sf= (Tire chips)

3 B Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) Void ratio, e emin ρdmax TireChips-sand mixture emax ρdmin Sand fraction, sf Dry density, ρd (g/cm 3 ) Percent finer by weight (%) : sf=1 (Soma sand) : sf=.9 : sf=.8 : sf=.7 : sf=.6 : sf=.5 : sf=.3 : sf= (Tire chips) Grain size (mm) (a) (b) Figure 1: (a) Relations between void ratio, dry density and sand fraction; and (b) Grain size distribution curves of various soil mixtures 2.2 Specimen preparation and experimental method The tyre chip-sand mixture specimen used for undrained and drained triaxial compression tests were prepared by moist tamping method. First, the tyre chips were washed with a detergent to remove impurities that adhered to their surfaces, and then exposed to warm air to dry for two days. The dried tyre chips and Soma silica sand No. 5 were mixed at the prescribed mix ratio by volume. Water was added to the mixture to obtain a sample with initial water content w = 1% after which the sample was thoroughly mixed again. Membrane was installed in the pedestal of the triaxial apparatus, and the mold 1 cm high and 5 cm in diameter was set up. The test specimen was prepared by placing the soil mixture inside the mold in five layers, with each layer compacted at a prescribed number of times by dropping an iron rammer from a prescribed height to control the compaction energy, Ec, which is given by the following expression. WR H NL NB Ec = (1) V In the above expression, W R is the rammer weight (=.116kN), H is the drop height (m), N L is the number of layers (= 5), N B is the number of drops per layer, and V is the volume of mould 3 (m ). In the experiments, the test specimens were prepared by adjusting the height of drop H and the number of drops N in order to obtain two levels of compaction energy, Ec = 51 kj/m 3 and BB 166 kj/m 3. These compaction energies were chosen such that the relative density of Soma silica sand No. 5 specimen (sf=1) was Dr=2% (for Ec=51 kj/cm 3 ) and Dr=5% (for Ec=166kJ/cm 3 ), respectively. Because water content has a large effect on the compaction characteristics of soils, a constant initial water content and compaction energy were adopted, and test specimens of tyre chip-sand mixtures were prepared with different sf. To saturate the specimens, the voids in the specimens were first filled with CO 2 and de-aired water was allowed to percolate, after which back pressure of 1 kpa was applied for two hours. As a result of this procedure, all test specimens were confirmed to have B-value >.95. The saturated specimens prepared as outlined above were then isotropically consolidated at three levels of confining pressure of σ c = 5, 1, 2 kpa, and undrained and drained triaxial compression tests were conducted. 3 BEHAVIOUR IN UNDRAINED AND DRAINED CONDITIONS 3.1 Behaviour in undrained condition Firstly, the results of undrained triaxial compression tests are discussed. Figures 2(a) and 2(b) show the relationships between deviator stress and axial strain for specimens with Ec=166kJ/m 3 and Ec=51kJ/m 3, respectively, while the corresponding effective stress paths are shown in 3

4 9 k q ( 6 s e s tr r to 3 σ c'=1kpa Ec=166kJ/m 3 sf=.7 sf=.9 sf=.5 sf=.3 Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) sf=1. 15 k 12 q ( s 9 e s tr r 6 to 3 σ c'=1kpa Ec=51kJ/m 3 sf=1. sf=.3 sf=.5 sf=.9 sf=.7 sf= Axial strain ε a (%) sf= Axial strain ε a (%) (a) Ec=166 kj/m 3 (b) Ec=51 kj/m 3 Figure 2: Undrained triaxial test results showing the relationships between deviator stress and axial strain at confining pressure σ c =1kPa. 9 k q ( s6 e s tr r to 3 σ c'=1kpa Ec=166kJ/m sf=.5 sf=.7 sf=.9 sf= sf=.3 sf=1. k12 q ( s 9 s tres r to 6 Figures 3(a) and 3(b), respectively. Figure 2(a) shows that for compaction energy Ec=166kJ/m 3, the specimen which includes tyre chips shows decreased strength as compared with specimen with sf=1. (sand only). The mixtures with sf=~.7 indicate nearly the same strengths. For the specimen containing 1% tyre chips (i.e., sf=.9), its shear strength is about half of that of specimen with sf=1.. It is noted that the shear strength seem to decrease greatly by adding tyre chips to sand, even with small quantity. On the other hand, Figure 2(b) showed that for compaction energy Ec=51kJ/m 3, specimen with sf=1. (sand only) indicated strain softening behaviour similar to that of loose sand. Comparing the mix ratios, it is seen that the higher the value of sf, the more the strength increases. When the figures are compared, the shear strengths when the axial strain reaches 2% are almost equal regardless of compaction energy σ c'=1kpa Ec=51kJ/m 3 sf=.5 sf=.7 sf=.9 sf=.3 sf= sf= Effective mean principal stress p' (kpa) Effective mean principal stress p' (kpa) (a) Ec=166 kj/m 3 (b) Ec=51 kj/m 3 Figure 3: Undrained triaxial test results showing the relationships between deviator stress and effective mean principal stress at confining pressure σ c =1 kpa. From Figure 3(a), it is observed that when sf=.7, contraction appeared because of negative dilatancy. This is because a part of the link of the grains of sand was cut off by the mixed tyre chips, and it is believed that this caused the formation of weak sand structure. However, the contractive tendency deteriorated with the decrease in sand content and it seems that there is no volume shrinkage in the sample of pure tyre chips (sf=). 4

5 Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) 3.2 Behaviour in drained condition Next, the results of drained monotonic triaxial tests are discussed. Figures 4(a) and 4(b) show the deviator stress-axial strain and volumetric strain-axial strain relations for specimens with Ec=166 kj/m 3 and Ec=51 kj/m 3, respectively. Figure 4(a) shows that when sf=1., the peak deviator stress appears at an early stage of shearing followed by strain softening and the volumetric strain showed dilative tendency. However, when sf= (tyre chip only), the deviator stress - axial strain relation is virtually linear. Moreover, there is neither peak nor failure, even at 2% axial strain. Similarly, the plots for sf=.3~.5 show strain hardening behavior. On the other hand, the inherent behavior of sand slowly appeared as the value of sf rises to.7. However, it is obvious from the figure that the behavior of specimen with sf=.9 is different from that of pure sand. It also noticed that when the axial strain reaches 2%, the shear strength of specimens with sf=.7~1. are almost equal. The volumetric strains of specimens which include tyre chips develop a tendency to increase monotonously toward compression and a steady state condition was not reached when sf <.7. Deviator stress, q (kpa) TireChips-sand Mixture ƒð c'=1kpa :sf=1. Ec=166kJ/m 3 :sf=.9 :sf=.8 :sf= Axial strain, εa (%) :sf=.5 :sf=.3 :sf= Volumetric strain, εv (%) Deviator stress, q (kpa) TireChips-sand Mixture ƒð c'=1kpa :sf=1. Ec=51kJ/m 3 :sf=.9 :sf=.8 :sf= Axial strain, εa (%) :sf=.5 :sf=.3 :sf= (a) (b) Figure 4: Drained triaxial test results at confining pressure σ c =1 kpa: (a) Ec=166 kj/m 3 ; and (b) Ec=51 kj/m 3 Volumetric strain, εv (%) Figure 4(b) shows the relationships between deviator stress and axial strain for specimens with Ec=51 kj/m 3. It is noted that similar results were obtained as those with Ec=166 kj/m 3, indicating that compaction energy has no effect on specimens which include tyre chips. Figure 5 illustrates the relation between secant friction angle (at ε a =15%) and confining pressure. From this figure, it can be seen that the secant angle increases as the sand fraction increases. % = a 15 φ ε g l e n an o t i n tfric S eca : sf=1. : sf=.9 : sf=.8 : sf=.7 2 : sf=.5 : sf=.3 TireChips-sand Mixture : sf= Confining pressure σ c '(kpa) Figure 5: Influence of confining pressure on the secant friction angle 5

6 Okamoto, M., Orense, R., Hyodo, M. & Kuwata, J. (28) When the specimen contained even a small amount of tyre chips, there is a decrease in secant angle. Moreover, the secant angle decreases further with the increase in confining pressure when sf <.9. 4 CONCLUDING REMARKS In this research, undrained and drained triaxial tests were performed on tyre chip-sand mixtures in order to examine their monotonic shear characteristics. The main findings obtained from the test results are as follows. 1. The specimen made of pure tyre chips showed linear stress-strain relation and the volumetric strain showed compressive behavior. 2. In the undrained tests, samples containing tyre chips showed decrease in strength. Moreover, negative dilatancy was maximum when sf = The drained test results showed that as sf increased, the rigidity of the specimen also increased. As the volume of tyre chips in the sand mixture increased, the secant friction angle showed large decrease with increase in confining pressure. REFERENCES ASTM (1998) Standard practice for use of scrap tires in civil engineering applications, Annual Book of ASTM Standards, ASTM, 22 (6), Bosscher, P. J., Edil, T. B. and Kuraoka, S. (1997) Design of highway embankments using tire chips, Journal of Geotechnical and Geoenvironmental Engineering, 123 (4), Edil, T.B. (27) A review of Environmental Impacts and Environmental Applications of Shredded Scrap Tires, University of Wisconsin-Madison, Madison, Wisconsin, USA Humphrey, D. N. (24) Effectiveness of design guidelines for use of tire derived aggregate as lightweight embankment fill, Recycled Materials in Geotechnics, A. H. Aydilek and J. Wartman, eds., Geotechnical Special Publication No. 127, ASCE, Baltimore, Hazarika, H., Sugano, T., Kikuchi, Y., Yasuhara, K., Murakami. S., Takeichi, H., Ashoke, K.K., Kishida T., Mitarai, Y. (26) Evaluation of a recycled waste as smart geomaterial for earthquake reinforcement of structure, Proc., 41st Japan National Conference on Geotechnical Engineering, Kikuchi, Y., Nagatome, T. and Mitarai, Y. (26) Failure mechanism during shear of rubber chip-mixed solidified soil, Report of the Port and Airport Institute, 45 (2), Lee, J. H., Salgado, R., Bernal, A. and Lovell, C. W. (1999) Shredded tires and rubber-sand as lightweight backfill, Journal of Geotechnical and Geoenvironmental Engineering, 125 (2), Mitarai, Y., Kawai, H., Kishida, T., Nagatome, T., Yasuhara, K., Murakami, S., Sugano, T., Hazarika, H., Kikuchi, Y., Tatarazako, N., Takeichi, H. and Ashoke, K.K. (26) Damping capability of impact load by used tire chips, Proc., 41st Japan National Conference on Geotechnical Engineering, Youwai, S. and Bergado, D.T., (24) Numerical analysis of reinforced wall using rubber tire chips-sand mixture as backfill material, Computers and Geotechnics, Vol. 31,