Geotechnical Evaluation of Recycled Asphalt Shingles as Structural Fill

In this research, geotechnical properties of recycled asphalt shingles (RAS) were evaluated at constant and varying temperatures for use in high volume structural fill applications. Since compressibility of RAS is significantly higher than that of natural soils, addition of less compressible granular materials such as bottom ash (BA) or foundry slag (FS) to RAS, or stabilization of RAS using self cementing fly ash (FA) were considered to reduce the compressibility. To evaluate the effect of seasonal temperature on engineering properties of RAS mixtures, a thermo-mechanical system and the related testing procedures were developed. Systematic tests including hydraulic conductivity, one-dimensional compression, triaxial compression and deviatoric creep tests were conducted at constant and varying temperatures (between 5 °C and 35 °C). Results show that at room temperature, RAS mixtures have sufficient shear strength and drainage capacity for use in structural fills. Up to 50% RAS in granular materials and between 10 and 20% FA in the stabilized RAS reduced the compressibility to meet the settlement criteria for roadway design. The secondary compression index increased as a power function with stress level. As the temperature increases the shear strength decreased due to reduction in viscosity of the asphalt binder in RAS particles. However the shear strength of the mixture with RAS content up to 50% remained higher than 30°. The hydraulic conductivity increased with increasing temperature due to reduction of viscosity of permeating water. The compressibility of the compacted RAS mixtures exponentially increased with temperature. If the embankment containing RAS mixture is constructed during warm season of the year, the majority of the compression occurs during construction and the RAS embankment settlement during the rest of the year will be negligible. RAS mixtures were also susceptible to creep rupture under the applied deviatoric stress. When designing side slopes of the embankments containing RAS, the applied stress should be reduced to 80% of the maximum deviatoric stress to ensure no creep rupture will occur. Design graphs and analytical models were developed to predict shear strength and compressibility of RAS mixtures at constant and varying temperatures under temperature and stress levels typical to highway embankments.