Sustainable solutions for road pavements: A multi-scale characterization of warm mix asphalts containing steel slags

Research and application concerning the use of environmentally friendly materials and technologies in road pavements have reached high relevance mainly due to the increasing public consciousness addressed to environmental protection and preservation. Warm Mix Asphalt (WMA) is a valid option in this regard. In fact, WMA is a cleaner asphalt concrete that can be prepared and compacted at lower temperatures than the traditional Hot Mix Asphalt (HMA). Moreover, the inclusion of recycled/waste materials in WMA can further enhance its environmental sustainability. Given this background, the present paper illustrates the overall results of a wide research study aimed at verifying the utilization feasibility of steel slags in warm-modified asphalt concretes. This was accomplished by investigating in the laboratory the midrange and high-service temperature properties of warm bituminous binders, as well as mastics and mixtures containing steel slag aggregates. The warm modification was performed using a chemical tensioactive additive; steel slags were produced in a metallurgical plant by electric arc furnace (EAF) treatment. To evaluate the combined effect of manufactured EAF steel slags and warm chemical additive, a comparative analysis was carried out taking into account unmodified binders as well as mastics and mixtures prepared with only natural aggregates. Binders, mastics and mixtures were studied in terms of stiffness, fatigue and permanent deformation resistance by carrying out dynamic tests on unaged and long-term aged samples. The results showed that cleaner materials prepared combining chemical warm technology and EAF steel slag aggregates seem to assure equal or even enhanced performance than the corresponding traditional hot mixed materials, demonstrating promising field applicability. In particular, the influence of lower mixing and compaction temperatures of the warm mixtures on stiffness, fatigue and rutting behaviour seemed to hide the contribution (positive or negative) due to the presence of EAF steel slag aggregates. In fact, the potential higher stiffness and rutting resistance of steel slag mixtures were limited whereas the possible lower fatigue resistance was positively counterbalanced.