What are asphaltenes? While often discussions about the composition of asphalt will define asphaltenes in chemical terms, the basic definition is that asphaltenes are material in asphalt that is insoluble in certain solvents. For some, asphaltenes appear to be something that is in the way that must be tolerated. That is far from the truth. First, as mentioned above, the definition of asphaltenes is simply material insoluble in either pentane, hexane, heptan or octane, depending on the method used. Often the compositions are described as being saturates, aromatics, polar materials and asphaltenes. In fact asphaltenes might be considered to be simply the part of polar materials that are insoluble in some arbitrary solvent. If an asphaltenes-free asphalt is exposed to light, new asphaltenes will be formed. In fact pictures have been taken using asphaltenes-free asphalt as the “film”. Upon exposure to light a picture is formed.
What do they do? Asphaltenes have three important functions: 1) a bodying agent; 2) forming a complex structure that aids in performance in conjunction with the other polar materials; 3) and helps to reduce hardening with time.
Bodying agent. There was an asphalt a number of years ago that was essentially asphaltenes free. It acted almost like lubricating oil and was such a problem that the agencies insisted that it be blended with different more suitable asphalt. Normally once a hot mix is made, and cools, a gel like structure is formed that aids in the setting of a hot mix. With some asphalts there is a setting problem because the formation of that structure is slow or weak and mixes made with them are tender. Other asphalts result in hot mixes that set very well and are not considered to be tender. At one time California DOT had a test using what they called the cohesiograph that measured tenderness.
Performance. Tender mixes tend to rut easier and to be easily marred from power steering. If the aggregate gradation in the hot mix has too much sand, the mix will be very hard to compact. If the asphaltenes bodies up the mix well, compaction will go well even with oversanded mixes.
Aging. When the relationship between the polar-asphaltenes is optimum, oxidation will be controlled. In the asphalt “micelles” (a term coined by Dr. Claine Petersen, one of the foremost asphalt scientist in the world in my opinion) are formed such that oxidation is restricted for the material in the micelle so that the rate of oxidation decreases with time. With asphalts in which the asphaltenes are too well dissolved and micelles do not form, the rate of oxidation continues resulting in very hard asphalts in the pavement in a relatively short time. We have run across some cracked pavements that were not very old but the asphalt was very badly oxidized. The asphalt used was from a crude oil in which the micelles do not occur.