Effect of polymeric microcapsules on the mechanical behavior of sands

Abstract

Microcapsules have demonstrated to self-heal the micro cracks induced by external stimuli in cementitious materials, whereby repairing some properties such as permeability and strength. It is then of interest to apply self-healing microcapsules in soil to achieve long-term stabilization, exhibiting both strength enhancement and deformation mitigation, to develop more sustainable and adaptable infrastructures. This thesis aims to facilitate the potential application of microcapsules in soil by providing an insight into the effect of polymeric capsules on mechanical properties of sands. The specific objectives are to (1) investigate the microcapsules retention behavior in granular materials; (2) assess the effect of Tung oil on shear strength enhancement of silica sand; (3) encapsulate Tung oil, explore its controlled release mechanism, and investigate its applicability on shear strength enhancement of silica sand; (4) evaluate the effect of capsules on one-dimensional compression creep behavior of carbonate sand; (5) investigate the effect of capsules on drained triaxial creep behavior of carbonate sand with both global and local strain measurements. By adopting a constriction size-based model, followed by conducting washout tests on microcapsule-gravel/sand mixture to monitor the loss of microcapsules under flushing, a criterion for successful retention of microcapsules in granular materials was established, which considered the size change of microcapsules during wetting/drying and their adhesion nature. The effect of Tung oil on mechanical behavior of saturated silica sand was studied by triaxial compression tests, concentrating on isotropic compression, critical state, stress dilatancy and peak strength enhancement. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and thermogravimetric analyses (TGA) were utilized to prove bonding breakage and coating abrasion. Results highlighted the peak strength enhancement at p_0^'=200-400 kPa. Tung oil was enclosed in microcapsules for their use in sand. Some basic physical properties of sand-capsule composite such as specific gravity, global void ratio and Tung oil release were initially determined. The mechanical behavior and stabilization efficiency of sand-capsule composite under varying void ratios, capsule dosages and confining stress are investigated. A predictive model was established, suggesting the optimum capsule dosage of 3.85% and the peak strength enhancement at p_0^'=100-200 kPa. The deformation characteristics incorporating immediate and creep deformation, along with elastic and plastic deformation of both carbonate sand and its capsule-based composite in one-dimensional compression creep tests were investigated. The effects of relative densities, particle size and vertical stress on deformations are discussed. The results demonstrated the effectiveness of capsules on fine sand at relative density of 20-80% and stress of 100-1600 kPa. The last objective was met by exploring the drained triaxial creep behavior of fine carbonate sand and its capsule-based composite with local strain measurements. The global/local axial and volumetric creep strains of both materials are analyzed, showing decreased axial creep strain before failure and more dilative volumetric creep strain for sand-capsule composite. The stress-strain behavior and volume change from creep test are compared with those from monotonic loading, revealing the structuration of carbonate sand. Capsule compression and bonding breakage of sand-capsule composite appears to be the mechanism behind its creep behavior.