Ten asphalts, including seven Strategic Highway Research Program (SHRP) asphalts and three Texas asphalts, plus naphthene and polar aromatic Corbett fractions for five of the seven SHRP asphalts were aged at laboratory conditions. The oxygen content by direct measurement and the FTIR carbonyl content were obtained for samples which were aged to varying degrees. It was found that, for each material, the oxygen content correlates linearly with the carbonyl content. The correlation is independent of aging temperature and aging pressure over the ranges studied. Furthermore, each material has a unique correlation coefficient, and the correlation slope for a whole asphalt is smaller than those of its fractions, except for asphalt AAM-1. Since the major oxidation products, including ketones, carboxylic acids, and anhydrides, have different oxygen content−carbonyl content ratios, the correlation coefficients provide qualitative information about the distribution of oxidation products. Relations between oxygen, carbonyl, and asphaltene production are deduced from these and earlier results.
The naphthene aromatic (NA) fractions and polar aromatic (PA) fractions of five Strategic Highway Research Program asphalts (AAA-1, AAD-1, AAF-1, AAG-1, and AAM-1) along with six supercritical fractions of AAF-1 were subjected to oxidative aging. Asphaltenes and residual polar aromatics were recovered from the aged PA material. Produced asphaltenes and PAs and residual NAs were recovered from the aged NA samples. It was determined that NAs convert to PAs and subsequently become asphaltenes during oxidative aging of asphalt binders. However, within the extent of aging encountered in roadways, it is concluded that negligible asphaltenes are produced from NAs. Depending on the relative rates of oxidation of PAs and NAs, the content of PAs may either increase or decrease. Newly produced asphaltenes and PAs are smaller in molecular size than those present in the original whole asphalts. Also, the residual PAs in the aged PA fractions and the residual NAs in the aged NA fractions have smaller molecular size than those of the parent asphalts. The smaller molecular-sized material, whether PA or NA, is more slowly converted to asphaltenes or PAs, respectively. These data indicate that materials high in NAs and perhaps smaller PAs will harden very slowly in service.
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