Relation of Asphalt Chemistry to Physical Properties and Specifications

About the report:

This report constitutes a synthesis of published information concerning the chemical composition and characteristics of asphalt cements used in highway construction. The general relations between chemical composition and physical properties and specifications are discussed from an engineering viewpoint. It is shown that asphalts are composed of so many complex molecules that no serious attempt has been made to identify specific compounds. Analytical methods are generally devised to define "generic" types but each generic type is of itself a combination of many compounds and the composition varies significantly with the source of the crude petroleum from which the asphalt is attained. The physical and engineering properties of asphalt depend not only on the chemical characteristics of the generic groups but also on the quantitative relations among the groups present. Functional groups and their relative position in the asphalt molecule also greatly affect physical behavior. It is evident from this synthesis that asphalts differing greatly in composition perform satisfactorily as a highway binder. Thus, control on asphalt composition does not appear feasible as a specification requirement. Specification changes, however, should be made to eliminate from the market materials having appreciable amounts of alkalies (sodium, potassium, or both). This can be accomplished by the addition of a maximum requirement of 0.40 percent ash to the specifications for asphalt cement.

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