Micro and macro scale mechanical behaviour of polymer-coated sands

Abstract

The water repellent behaviour of polymers such as polydimethylsiloxane enables them to be utilized as coatings in grains to control wetting, with potential applications in barriers and fills. However, little exists on the mechanics of the polymer-coated soils. Here, the mechanical behaviour of a polymer-coated sand subjected to two mass ratios of dimethyldichlorosilane (0.05% and 3%) was investigated at the micro scale via inter-particle loading tests and macro scale via the triaxial tests. Dimethyldichlorosilane is used to treat the particles surfaces and produce the water repellent polydimethylsiloxane coatings. At the micro scale, the results from the inter-particle loading tests show that when the mass ratio of dimethyldichlorosilane is high (i.e. 3%), the soft polymer-coatings significantly decrease the normal stiffness and the tangential stiffness of the polymer-coated sand but increase its inter-particle friction coefficient which may be caused by the significant increase of adhesive friction force. This effect diminishes under a high normal force which could be attributed to the damage of the polymer-coatings. At the macro scale, the results from the triaxial tests show that when the mass ratio of dimethyldichlorosilane is low (i.e. 0.05%), the critical state locus in the e-lnp′ and q-p′ plane is similar to those of natural sand. Therefore, the effect of the thin polymer-coatings on the macro scale mechanical behaviour is negligible. While when the mass ratio of dimethyldichlorosilane is high (3%), the reduction of shear strength is evident, with the critical state locus in the e-lnp′ and q-p′ plane showing a significant difference compared to those of natural sand. The polymer-coatings significantly decrease the M-values (i.e. critical friction angle) and this effect also diminishes under high stress levels which could be ascribed to the damage of the polymer-coatings. The effect of the polymer-coatings on the particle shape and surface was also investigated. The thick polymer-coatings (i.e. 3%) by covering the grains significantly alter the particle surface by reducing its roughness. However, the polymer-coatings do not change the particle shape of the polymer-coated sand (3%) when compared to that of natural sand. Furthermore, quantification of the particle surface topologies prior to and after the triaxial tests as well as the inter-particle loading tests, clearly reflects the damage and crush of the polymer-coatings when the polymer-coated particles are subjected to relatively high external forces. A discussion on the relation between macro scale and micro scale mechanical behaviour of the polymer-coated sand is presented.