Dynamic shear modulus and damping ratio of clay mixed with waste rubber using cyclic triaxial apparatus
Introduction
Developments in modern-day mechanical life have led to a strong upsurge in the production of various types of wastes including rubber [[1], [2], [3]]. Insufficient control over such wastes can thus bring about many environmental problems in future. As an alternative to incineration or landfilling of wastes, with their own adverse environmental effects, the reuse of wastes is of both technical and environmental importance in a range of industries. So far, numerous studies have reflected on the use of waste tires of different shapes in civil and geotechnical engineering. Depending on their size, such wastes can be consumed in place of non-renewable gravel and sand materials to reduce use of natural items. Over and above their advantages connected with environmental protection and less contamination, waste tires also offer a high damping capacity and are of great technical importance [2]. Such wastes have been further exploited for improving properties of concrete, asphalt, and sandy soil. Moreover, there has been much interest in excess waste tires of different shapes to add to strength and strain properties [[4], [5], [6], [7]]. Clay or sandy soils may be subjected to multiple dynamic loads such as machinery, piling, and traffic or seismic loads [8]. Depending on their applications, dynamic loads in soil dynamics can be applied at different frequencies. In this respect, damping ratio, D, and shear modulus, denoted by G, are among the extensively used dynamic parameters in geotechnical analysis and design [8]. A number of tests including cyclic simple shear test, cyclic triaxial test, as well as resonant column experiments and cyclic torsional shear are also available to evaluate different dynamic parameters [[8], [9], [10]]. Generically, normalized damping ratio and shear modulus for dry mixtures of rubber-gravel and rubber-sand vs. shear strain amplitude were accordingly proposed by Ref. [11]. In another research, a vibration barrier containing sand-rubber materials were investigated, which could reduce the vibration amplitude [12]. Rubber waste and sand mixture can be additionally used as a low-cost material in seismic isolation applications [13]. In a recent study, the dynamic properties of unbound granular materials (UGM) containing recycled tire were explored and a simple approach was consequently proposed to estimate the UGM properties [14].
Numerous studies have been so far conducted on dynamic properties of soils [[15], [16], [17], [18]]. The effects of saturation on damping ratio and shear modulus of clay soil have been accordingly investigated using resonant column experiments, confirming that damping ratio and shear modulus decrease and increase, respectively, as the saturation degree is augmented [19]. The impact of plasticity on the dynamic properties of silty sand has been further analyzed at low strains using the torsional resonant column test apparatus [20]. had also evaluated the effects of cyclic loading on saturated clay through cyclic triaxial tests, reporting the insignificant impact of confining stress on damping ratio and its significant effect on shear modulus [21]. had similarly investigated the dynamic behavior of clay comprising organic materials using the bender element and resonant column tests. According to their results, the main factors affecting the stiffness of CSs containing organic compounds were organic matter content and porosity ratio. Many studies are also available on the effects of granular rubber added to sandy soils. The effects of granular rubber content on dynamic properties and liquefaction of sandy soils has been accordingly examined in the work of [22]. Based on their findings, the use of granular rubber may reduce the risk of liquefaction, leading to a growth in damping ratio. To assess damping ratio and shear modulus in sandy soil-granular tire mixtures at different shear strain amplitudes, various experimental studies have been conducted on the effects of grain size and saturation conditions, and several correlations have been established for damping ratio and shear modulus in sandy soils [11]. The effects of stress rotation and intermediate stress ratio on the properties of rubber-sand mixtures have been correspondingly investigated in the work of [24]; wherein boosting a and/or b would increase pore water pressure (PWP) and diminish shear strength.
The effect of granular rubber on physical and mechanical properties of CSs has been also examined in the related literature. The results on the Atterberg limit of CS-rubber powder mixtures in this respect have indicated the lower plasticity of this mixture compared with pure soil, which improve the performance of CS in executive projects [2]. This reduction in the plasticity index (PI) can also affect the dynamic parameters of the soil. According to the studies carried out by Ref. [25] on clays with different degrees of plasticity, the shear modulus of the clay would change based on their PI. Therefore, modifying the amount of rubber added to clay could change the dynamic parameters.
The OMC and the MDD of mixtures of CRM are of practical importance, as evaluated in several studies [4,[26], [27], [28]].
The strength of clay-rubber mixtures (CRMs) indicates that the uniaxial strength of CS having low granular rubber contents (up to 5%) can be improved [2,5,26,29,30]. The strength of CS-granular tire waste mixtures additionally suggests an increase in the angle of internal friction by adding 10% tire scraps [31]. The results on consolidation of sand-bentonite mixture have further shown a decline in the soil compaction coefficient by adding about 10% tire chips [32]. Through compaction, uniaxial, and the Californian Bearing Ratio tests, the geotechnical properties of cement-stabilized CS-tire fibers have been also evaluated to use them as fill materials, materials behind retaining walls, and sub-base layers in light-traffic roads. The results have demonstrated that low tire fiber contents up to 7.5% can be mixed with cement [33]. Some studies on CRMs are summarized in Table 1.
Despite extensive studies on the effects of granular rubber on dynamic properties of sandy soils, little research has been conducted on the impact of granular rubber on the geotechnical properties of CSs. Furthermore, no research has thus far investigated damping ratio and shear modulus of CRMs in literature, to the best of authors’ knowledge.
On the other hand, local research determining geotechnical properties of soil in certain regions may be of engineering importance [5,40]. Rubber powder and granular rubber produced from waste rubber were abundantly available in a large number of markets near the research area, forming a homogeneous mixture with CSs and as a result providing uniform geotechnical properties in the mixture. Given that low-plasticity CS is found in the south of Tehran, Iran [28], clarifying the dynamic parameters of CRMs used in projects can pave the way for sustainable development from environmental, technical, and economic perspectives. In this experimental study, the geotechnical properties of CRMs were evaluated using monotonic and cyclic triaxial tests. As well, shear strength and strain, shear modulus, and damping ratio of the mixtures were determined. Moreover, the effects of two different rubber sizes including rubber powder and granular rubber were investigated at different shear strain amplitudes. To improve the interpretation of the results, the microstructures of pure rubber and CRMs were observed under a binocular microscope.
Section snippets
Low plasticity clay
In this study, clay soil (CS) collected from the south of Tehran, Iran, was used in the experiments. The tests were also conducted based on the American Society for Testing and Materials (ASTM) standards. The parameters obtained from the tests are listed in Table 2. Fig. 1a shows particle size distribution of the soil. Accordingly, the soil is mainly composed of fine-grained CS. Fig. 1b illustrates the ranges of the Atterberg limits in Tehran clay. As observed, the soil is categorized in the
Monotonic triaxial tests
The initial conditions of the samples in terms of maximum dry density (MDD) and optimum moisture content (OMC) are given in Table 4. It can be seen that growth in the rubber content of the samples leads to a fall in their dry density and a rise in their OMC. The reason for these trends has been discussed in various studies, but in general, they can be attributed to differences in the specific gravity of soil and rubber [3].
Fig. 6 shows the results of the monotonic triaxial tests including the
Conclusions
The geotechnical properties of the CRM including strength, volumetric strain, shear modulus, and damping ratio were evaluated in this study under the influence of different parameters, i.e., rubber size, rubber content, and shear strain amplitude to identify the mixture behavior. The main findings are summarized below:
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According to the high-resolution binocular microscopic images, the rubber grains were not spherical but longitudinal in shape. Moreover, the longitudinal elements positively
CRediT author contribution statement
Davood Akbarimehr: Conceptualization, Methodology, Software, Writing - original draft, Visualization, Investigation, Interpretation, Supervision, Software, Validation, Writting. Kazem Fakharian: Equipment Supply and Preparations, Interpretation, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the contributions of Dr. Esmail Aflaki for his valuable advices in initial planing of the laboratory work.
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