Reversible aging in asphalt binders

X-ray diffraction, optical microscopy, and mass spectrometry techniques were used in an attempt to clarify the morphological and chemical features that are responsible for reversible aging processes that occur in asphalt binders during conditioning at low temperatures. The reversible aging term is used in this paper to capture all reversible processes (i.e., wax crystallization, free volume collapse, asphaltene aggregation, etc.) that lead to a reduction in low-temperature rheological and fracture performance. Crystalline content and asphaltene aggregate size at ambient temperatures, as measured by X-ray diffraction on thin asphalt films, are identified as two factors that correlate reasonably well with the reversible aging tendency at low temperatures. A coarse and unstable colloidal state for the asphaltene fraction is also identified as an important contributor to reversible aging. It was found that the saturates fraction has a particularly significant role in the aging process, with those asphalts containing higher amounts of linear paraffin losing more in terms of rheological performance. This important phenomenon is responsible for significant fracture distress in asphalt pavements in northern climates and therefore deserves further investigation. Some of the air-blown asphalts investigated in this study were found to show a high crystalline content and a coarse phase morphology and concurrent tendency for reversible aging during cold conditioning. This may be due to the crude source(s) employed, the chemistry of the air-blowing process, or resulting phase changes. Other air-blown binders did not show these features while they were still susceptible to reversible aging. Hence, the reason for this behavior appears to be due to multiple processes which are at present only poorly understood.