Aggregates comprise about 95% of hot mix asphalt (HMA), by weight. Some aggregates are more susceptible to microwave heating than others, based on their component minerals. For example, Mesabi aggregate rock generated by Minnesota's taconite industry contains magnetite, which is an excellent microwave absorber. Conversely, other aggregate rock types like granite do not absorb microwave energy as well because they contain minerals like quartz and feldspar, which are poor microwave absorbers.
This project would make use of a truck-mounted microwave unit to conduct a series of heating tests on selected HMA test cells at MnROAD. For example, the HMA pavement in Low Volume Road (LVR) Cell 31 was designed using taconite aggregates. The objective would be to create an HMA heating susceptibility document that provides an estimate of the energy required to heat HMA of varying aggregate composition. Ancillary information generated by this project could include testing of cores collected before and after microwave heating, to assess the effect of microwave treatment on asphalt microfracturing.
Infrared (IR) is typically used for pavement heating/repair, but IR heating is a top-down process. Microwaves can uniformly heat the pavement mass at depth, especially if the HMA aggregate is more microwave-susceptible. This susceptibility has longer term implications for new mix designs, road repair, longitudinal joint repair, and more efficient hot in-place recycling of asphalt pavements.