Moisture Damage Evaluation of Asphalt Mixes that Contain Mining Byproducts

The availability of mineral aggregates for pavement construction is continuously depleting. The aggregate manufacturing process requires significant amounts of energy, which ranges from 10 to 30 MJ/ton. The process also produces 5 kg/ton of carbon dioxide (CO₂), which causes significant amounts of greenhouse gas emissions. With the annual consumption of approximately 1.2 billion tons of aggregates in the United States, the environmental impact is significant. More than 125 million tons of fine-grained, crushed siliceous material is generated annually through iron ore mining in northern Minnesota. Typically, this material is referred to as “taconite tailings” and usually ends up as landfill near mining operations. This paper describes a moisture damage evaluation of asphalt mixes that contained significant fractions of aggregate as taconite tailings. The evaluation was conducted with the use of the conventional AASHTO T 283 test procedure as well as an approach with a fracture energy basis. The paper presents comparative results for two mixes: one made with taconite tailings and the other with conventional granite aggregates. The results indicated that a mix that contained taconite had acceptable moisture-damage resistance. The results also pointed out the limitations of the AASHTO T 283 procedure, especially the process of moisture conditioning. The fracture energy results indicated that, although mixes underwent reduced tensile strength, the overall capability of mix to strain without cracking significantly increased after the AASHTO-recommended moisture conditioning process. The study also included a set of samples that were field-conditioned over the winter and spring months. The mechanical behavior of field-conditioned samples was quite different from the behavior of samples conditioned in the laboratory with the AASHTO procedure.