Use of Shredded Rubber in Unbound Granular Flexible Pavement Layers

Abstract

The major objective of the research was to conduct a study into the feasibility of using shredded rubber as a partial replacement for aggregate within conventional base and subbase materials in a flexible pavement system. A graded aggregate base and sand subbase meeting specifications for the Maryland State Highway Administration were used. The rubber used in the study consisted of a shredded product with 60 to 70 percent retained on a 9.5-mm (⅜-in.) sieve. This size was selected because of the relatively inexpensive cost to produce it and because of its adaptability to an aggregate blend. Modified and standard Proctor, California bearing ratio (CBR), and resilient modulus tests were conducted on the base/subbase-rubber blends with up to 15 percent rubber content by weight. The aggregate base blend resulted in significant decreases in both CBR and nonlinear resilient modulus at 15 percent rubber. These significant reductions led the authors to conclude that the use of shredded rubber in a dense-graded aggregate base course is not feasible. In contrast, the sand-subbase blends resulted in insignificant changes to the CBR, friction angle, permeability, and resilient modulus at higher rubber percentages. It was concluded that the sand-rubber sub-base exhibits little change compared with the virgin sand-subbase material. As a result the use of shredded rubber may be a technically feasible alternative in the construction process. Finally, two constitutive models were used in the resilient modulus analysis: the conventional K1, K2 model and a universal model incorporating an octahedral stress term (k1, k2, k3 model). Direct comparisons revealed greatly improved predictability and accuracy with the universal model for assessing the nonlinear behaviors of both aggregate types evaluated.