Effects of laboratory preheating procedures on recycled binder contribution in asphalt mixtures

Abstract

Recycled asphalt material (RAM) preheating procedures for fabricating laboratory-mixed asphalt mixture samples vary considerably, which may impact the recycled binder contribution. That is, the percentage of the recycled binder that is mobilized and incorporated into the virgin binder within the asphalt mixture. Three different preheating procedures were used to fabricate samples of four asphalt mixtures from different sources in the laboratory. In one procedure, both the virgin aggregate and RAM were conditioned to slightly above the mixing temperature. In another, the RAM was heated to 110 °C and combined with RAM preheated slightly above the mixing temperature. In the third method, the virgin aggregate was superheated and mixed with recycled material at room temperature. Three of the study mixtures contained reclaimed asphalt pavement (RAP) with contents spanning 21 to 40 percent and the fourth contained 25 percent RAP and 4 percent recycled asphalt shingles (RAS). Recycled binder contribution in the fabricated mixtures was quantified using tracer-based microscopy. Complete recycled binder contribution was not achieved in any of the laboratory-mixed samples. The two preheating methods that did not utilize superheating yielded statistically equivalent recycled binder contribution results for the three RAP mixtures. The superheating procedure yielded significantly higher recycled binder contribution compared to the other methods in some cases. The mixture with RAS demonstrated greater sensitivity in recycled binder contribution to the preheating procedure compared to the RAP only mixtures. The results of this study suggest preheating procedures should be specified to minimize variability in laboratory-fabricated asphalt mixture samples.