Laboratory Evaluation of Heat Exchange Between Superheated Virgin Aggregate and Reclaimed Asphalt Pavement Material

Abstract

The most common method of Reclaimed Asphalt Pavement (RAP) material processing in the production of Hot Mix Asphalt (HMA) in the United States is by using cold feed recycling. Superheating the virgin aggregates by hot gas to elevated temperatures above discharge temperature depends on the RAP content, RAP moisture content, and target discharge temperature. The superheated virgin aggregate is mixed with cold-wet RAP material in the dry mixing zone during HMA production. Drying and heating of the RAP material occur through heat transfer between virgin aggregate and RAP material before adding hot virgin binder. There are existing recommendations for superheated temperatures required for virgin aggregate in Asphalt Institute MS-20, National Asphalt Pavement Association IS-71, and ASTEC T-127. However, no research studies were found that validated these recommendations either in the field or in a controlled laboratory environment. Several research studies emphasize the importance of the thermal process during the dry blending stage. However, limited research has been conducted to understand the heat transfer process between superheated virgin aggregate and RAP. A better understanding of the heat transfer between virgin aggregate and RAP would be helpful in producing high-quality asphalt mixtures. The primary objective of this study was to characterize and investigate the heat transfer between virgin aggregate and RAP material as it relates to variations in RAP content, RAP moisture content, virgin aggregate gradation, RAP gradation, and virgin aggregate thermal properties. Another objective of the study was to validate current recommendations for superheated virgin aggregate temperatures needed. A heat transfer testing protocol was developed in this study for the laboratory evaluation of heat transfer. A laboratory blending chamber with minimal heat losses was fabricated to have a uniform and continuous mixing of virgin aggregate and RAP particles inside the chamber. An infrared camera was selected that is capable of measuring a representative sample size of the mixture in the chamber. Analysis methodologies to determine the time required for heat transfer completion and heat conductance between virgin aggregate and RAP were developed as part of the study. Discrete Element Model (DEM) simulation results were used to validate the assumptions made as part of the new test method. On all combinations of the experimental design, heat transfer between virgin aggregate and RAP was measured. For each of these combinations, heat transport parameters (Tequi and HC) developed as part of the study were determined. The developed test method was able to produce repeatable results with three replicates. Discharge temperatures obtained from experimental results were utilized to verify the existing superheated temperature recommendations. The existing equation was overestimating the superheated temperatures required; a modified equation is recommended. The time required for thermal equilibrium is affected by RAP size; Tequi was determined to be 45 sec for 10% dry, fine RAP compared to 116 sec for 10% dry, coarse RAP. The time required for thermal equilibrium was observed to take up to 5.0 times for mixtures with high RAP and moisture content (40% RAP and 5% moisture) compared to 10% dry RAP mixtures. Analysis of variance (ANOVA) test results showed that RAP content, RAP size, RAP moisture, virgin aggregate size, and virgin aggregate source affect heat transfer between virgin aggregate and RAP. A hypothesis was tested to check whether preheating RAP before adding to virgin aggregate would reduce the time required for thermal equilibrium or not. Based on the results, preheating RAP would help in asphalt mixtures to attain thermal equilibrium quickly compared to the cold feed recycling method. Preheat RAP temperatures for different RAP contents, and RAP moisture contents are recommended. Heat energy consumption analysis shows that both cold and warm feed methods of recycling would consume nearly the same amount of heat energy to process RAP during production. The outcomes of the study, including a new test protocol for heat transfer characterization, analysis frameworks to calculate heat transfer parameters, the modified equation for superheated temperature determination, and recommended RAP preheat temperatures, are expected to be useful tools for HMA production and future research on this topic.