Effect of Polyurethane Content on the Performance of Ballasts under Static and Cyclic Loading

The effect of polyurethane content on the behavior of ballast was assessed based on large-scale direct shear and cyclic loading tests. The large-scale direct shear tests were performed at normal stresses (${\sigma }_n$) of 40 to 120 kPa at a shearing rate ($S_r$) of $9\mathrm{mm}/\min$, while cyclic tests were performed at loading frequencies ($f$) of 16 to 32 Hz. The direct shear test results indicated that the increase in polyurethane content from 1.5% to 3.75% by weight of ballast has significantly improved the shear strength of ballast. However, the increment in shear strength is found to be insignificant for polyurethane contents greater than 2.25%, thus indicating 2.25% to be optimum for the stabilization of ballast. Further, the cyclic load tests were performed on unstabilized ballast and ballast stabilized with 2.25% polyurethane (${\mathrm{PSB}}_{2.25\%}$). The test results indicated that ${\mathrm{PSB}}_{2.25\%}$ has exhibited significantly reduced extent of vertical and lateral deformations, and had shown enhanced resilient modulus and damping ratio in comparison to unstabilized ballast (USB). Further, the vertical settlement of ${\mathrm{PSB}}_{2.25\%}$ at 2 million load cycles was observed to be less than the settlement of USB at 250,000 load cycles, which implies that the stabilized ballast continues to perform effectively even after higher load cycles.