Investigation of the Behaviour of Sand-rubber Composite Interfaces at Different Scales

不同尺度下橡膠-砂顆粒混合物的行為研究

Student thesis: Doctoral Thesis

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Award date25 Apr 2022

Abstract

Recycled rubber is an elastomer type of shredded tyre product with very low specific gravity and high energy dissipation properties. It has very attractive properties to be mixed with geomaterials and used in various geotechnical engineering applications such as light-weight composite, alternative and low-cost vibration isolation earth material, or earthen drainage system in landfills. However, several practical and theoretical issues are to be addressed in the interest of engineering applications and research.

In terms of engineering applications, many studies on geosynthetics indicate that composite materials have greater time-dependent deformations (i.e., more significant influence of creep) than that of natural geomaterials, highlighting the importance of examining the deformation and stiffness when the composites are subjected to service loads (i.e., influences of time of constant confinement application). To date, there are some preliminary works focusing only on the creep for sand-rubber mixtures with non-crushable host sand, whereas the studies on composites involving crushable grains subjected to complex loading conditions are very limited. With respect to the multi-scale analysis of sand-rubber mixtures, the bulk behaviour is directly related to the contact stiffness and interface friction angle. Insights into the behaviour of these complex (composite) materials have also been obtained through discrete-based numerical simulations, such as the discrete element method (DEM). Those numerical modellings, which require benchmark contact properties at the composite interfaces as input parameters, also necessitate experiment-based investigations at the grain-scale.

The thesis attempted to fill the gap in the literature regarding the preceding issues for engineering applications and research, respectively. For this purpose, this study carried out experiments focusing on the long-term deformation and stiffness of element-size sand-rubber mixtures and some fundamental grain-scale tests that look into the problem of rigid-soft grains interactions. At the element scale, 1-D compression tests were carried out on two types of sand-rubber mixtures with different rubber fractions to investigate the potential link between the immediate and the long-term deformations. The bender element tests were performed, focusing on the time-dependent small-strain stiffness (Gmax) subjected to long-term and complex loading conditions. These investigations are important as a benchmark to evaluate the post-construction behaviour of composites involving shredded rubber products. At the grain-scale, the contact problem of soft-rigid interfaces was investigated by performing micromechanical-based experiments on sand (rigid) grains in contact with granulated rubber (soft) particles subjected to different loading paths. The applicability of some contact mechanics models was examined, and necessary modifications were applied by considering the self-deformation of rubber. Finally, the stress-strain curves obtained from the element tests and the contact parameters quantified by the micromechanical tests were adopted in DEM simulations as the benchmark for double-scale calibrations. The DEM results provided some supplementary information that is not easily obtained from laboratory experiments.

The results showed that the inclusion of rubber amplified the intensity of soil creep even at very low rubber fractions. The time-dependent deformation and Gmax were strongly affected by the mechanical properties of the host sand. More specifically, the inclusion of rubber was more beneficial when soft components were mixed with crushable sand. The micromechanical tests showed that the constitutive behaviour at the sand-rubber composite interface had a similar soft-to-ductile response for different types of sand grains regardless of the very distinct behaviour at pure sand contacts. Commonly applied contact models in the normal and tangential directions in DEM were examined, highlighting the significance of considering the viscosity and self-deformation of the rubber grains into the constitutive behaviours at contact.

The Discrete Element Method (DEM) simulations for sand-rubber mixtures were calibrated using the experimental data from element-scale and grain-scale tests. The DEM simulations helped to calculate the absolute value of the contact force and contact stiffness at different types of contacts. The DEM results suggested that the coordination number of sand-rubber (S-R) and rubber-rubber (R-R) contacts increased as rubber content increased; the average contact force carried by S-R and R-R contacts also increased. The mean value of the contact stiffness was reduced with higher rubber content, which was closely linked to stiffness reduction. The matched data from the simulations justified that the calibration method used in the study provided considerable accuracy. The analysis also gave a more comprehensive image of the changing mechanical behaviour, which is gradually transformed from "sand-dominant" into "rubber-dominant" with increasing rubber content.