ASPHALT COMPOSITION TESTS: THEIR APPLICATION AND RELATION TO FIELD PERFORMANCE

The application of chemical analysis to specific questions about asphalt and other tests to determine asphalt quality are discussed. Asphalt chemistry is complex; even with the analytical tools available, it would be almost impossible to identify and quantify all the components of even a single asphalt. Asphalt has commonly been analyzed by separating it into fractions on the basis of solubility, absorption, or molecular size. The fractions obtained are operationally or procedurally defined. The chemistry of the fractions has been only broadly defined. These fractional separation tests may be useful in fingerprinting an asphalt or in following changes that may occur during the manufacture, hot-mix processing, or in-use life cycle of a single asphalt. They do not, however, unravel the chemical composition of asphalt. Compositional tests based on fractional separation have not correlated reliably with field performance, nor have ratios based on the fractions. Physical and rheological tests have been shown to correlate with road performance on numerous test roads. These performance-related tests remain the most reliable guide to asphalt quality. Construction practices play a significant role in asphalt durability. High air void content has been shown to override any differences between asphalts. Asphalts from many sources perform well in roads. With this in mind, it appears unlikely that functional specifications based on composition could be devised. The performance-related thin film ovens and viscometers, and possibly new performance-related physical tests, will continue to provide a reasonable way of describing asphalt quality without directly confronting the almost impossible task of describing a most complex chemical material.