EFFECTS OF SUPERPAVE RESTRICTED ZONE ON PERMANENT DEFORMATION

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The purpose of this study is to evaluate the restricted zone effect using four different aggregates: crushed granite, crushed limestone, crushed river gravel, and a mixture of crushed river gravel as coarse aggregate with natural fines. As the restricted zone is a component of Superpave, the blends prepared met most of the Superpave criteria, except the restricted zone in selected mixtures and fine aggregate angularity in three mixtures. Each type of aggregate was used for mixture design of three gradations: above, through, and below the restricted zone. The twelve mixtures designed were tested in the laboratory to evaluate their relative resistance to permanent deformation. Four types of tests were performed using Superpave equipment: simple shear at constant height, frequency sweep at constant height, repeated shear at constant stress ratio, and repeated shear at constant height. Rutting resistance of the mixtures was measured using the Asphalt Pavement Analyzer. Researchers found that there is no relationship between the restricted zone and permanent deformation when crushed aggregates are used in the mixture design. Superpave mixtures with gradations below the restricted zone were generally most susceptible to permanent deformation while mixtures above the restricted zone were least susceptible to permanent deformation. Recommendations include elimination of the restricted zone from hot mix asphalt design specifications.