United States Department of Transportation
i
Thermodynamics between RAP/RAS and virgin aggregates during asphalt concrete production : a literature review.
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2015-09-01
[PDF-3.64 MB]
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Abstract:In hot-mix asphalt (HMA) plants, virgin aggregates are heated and dried separately before being mixed with
RAP/RAS and virgin asphalt binder. RAP/RAS materials are not heated or dried directly by a burner to avoid
burning of aged binder coating on the materials so they are heated and dried indirectly by the hot virgin
aggregates. A literature review shows that virgin aggregate temperature has been predicted for drying and
heating RAP at batch plants only. In this study, thermodynamics and heat transfer principles are used to predict
virgin aggregate temperature for drying and heating RAP/RAS at a drum plant. Among many results, it was
shown that virgin aggregates become superheated (more than 1000°F) when both virgin aggregate and RAP
moisture content were in the range of 3% to 5% and the material proportions were in the range of 30% to 50%.
The size of virgin aggregates and RAP/RAS, the moisture content of virgin aggregates and RAP/RAS, and the
mix proportion of virgin aggregates and RAP/RAS were the major contributing parameters in predicting virgin
aggregate temperature in the drum plant. The plant’s moisture content data indicate that virgin coarse
aggregates hold a lower amount of moisture compared with virgin fine aggregates. However, in comparing same-size virgin aggregates and RAP, RAP contained a higher amount of moisture. The reason might be that the aged
binder coating of RAP holds moisture better than virgin aggregates do. Also, RAS contains a higher amount of
moisture compared with RAP of the same size.
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