Characteristics of Glass-Polyurethane Beads Mixtures as Function of Weight Ratios: Experimental Approaches

In this study, a series of laboratory tests is conducted to examine the characteristics of mixtures composed of glass beads (GB) and polyurethane beads (PB) in different size ratios (s_r = D_50-GB/D_50-PB). Specimens are prepared in the same relative density but in various weight ratios between GB and PB called polyurethane content, PC (= W_PB/W_Mixture = 0, 0.5, 1.0, 2.0, and 5.0%). Small-strain shear modulus G_max is estimated by the shear wave velocity measured by bender elements installed in an oedometer cell with various confinements under K₀-loading condition. The normalized shear modulus (G/G_max) is also measured by a resonant column test. It is found that the behavior of mixtures could be divided into two regions as the contact control and the stress control by a value of “stress threshold” that decreases with an increase in PC due to the large deformation of polyurethane particles. Additionally, with the participation of PB, the mixtures experience a sudden rise in Λ factor incorporated with a small value of ζ exponent in the G_max = Λ(σ_v/1 kPa)^ζ at polyurethane content, PC ≈ 0.5–2%, indicating the optimum packing and minimum sensitivity to confining pressure of the mixture. Although PB works as a low stiffness material, they play an important role to enhance inter-particle contact behaviors, resulting in an increase of stiffness degradation at low stress range. However, the deformation of PB at high confinement causes a reduction in G/G_max curves.