Rheological Behavior of Asphalt–Rubber Binders Modified With Shale-Oil Residue and Polyphosphoric Acid

Abstract

Abstract The objective of this paper is to assess the effects of adding two modifiers of different chemical nature on the rheological behavior of asphalt–rubber binders: the shale-oil residue and the polyphosphoric acid. Four modified asphalt–rubbers were prepared: asphalt–rubber–PPA (18 % crumb rubber + 1 % PPA), asphalt–rubber–oil (18 % rubber + 10 % shale-oil residue), asphalt–rubber–oil–PPA–1 (18 % rubber + 10 % oil + 1 % PPA) and asphalt–rubber–oil–PPA–2 (9 % rubber + 5 % oil + 0.5 % PPA). A reference asphalt–rubber was prepared using 18 % crumb rubber. The samples were submitted to oscillatory-shear tests to obtain data (complex modulus, G*, and phase angle, δ) to construct master curves. Compared to the base asphalt binder, the pure asphalt–rubber improves the rutting and strain-controlled fatigue resistances. The addition of 1 % PPA enhances even more the rutting resistance and improves slightly the fatigue resistance below 20°C. The most substantial increase in fatigue resistance is obtained when 10 % oil is added to the pure asphalt–rubber, but its rutting resistance is drastically reduced. Mixtures 1 (18 % rubber + 10 % oil + 1 % PPA) and 2 (18 % rubber + 5 % oil + 0.5 % PPA) have lower rutting resistance compared to the base asphalt–rubber, but only mixture 1 has higher fatigue resistance. Compared to the base asphalt binder, all modified asphalt–rubber binders present higher G*/sinδ values (at all rutting temperatures) and lower G*·sinδ values (only below 20°C) when traffic speed reduces. Taking this evidence into account, the asphalt–rubber and the asphalt–rubber–PPA are the most appropriate. Although shale-oil residue has shown opposite results at intermediate and high pavement temperatures (enhancing the fatigue resistance and reducing the rutting resistance), it is able to reduce the viscosity of the asphalt–rubber, enhancing the workability of the HMA mixes during construction.