Chemical Composition of Asphalt As Related to Asphalt Durability: State of the Art

Transportation Research Record 999 13 Chemical Composition of Asphalt as Related to Asphalt Durability: State of the Art J. CLAINE PETERSEN ABSTRACT For the purposes of this review, a durable as phalt is defined as one that (a) possesses the phys ical properties necessary to produce the desired The literature on asphalt chemical composi initial product performance properties and (b) is tion and asphalt durability has been re resistant to change in physical properties during viewed and interpreted relative to the cur long-term in-service environmental aging. Although rent state of the art. Two major chemical design and construction variables are major factors factors affecting asphalt durability are the in pavement durability, more durable asphalts will compatibility of the interacting components produce more durable pavements. of asphalt and the resistance of the asphalt The importance of chemical composition to asphalt to change from oxidative aging. Histori durability, although not well understood, cannot be cally, studies of the chemical components of disputed. Durability is determined by the physical asphalt have been facilitated by separation properties of the asphalt, which in turn are deter of asphalt into component fractions, some mined directly by chemical composition. An under times called generic fractions~ however, standing of the chemical factors affecting physical these fractions are still complex mixtures properties is thus fundamental to an understanding the composition of which can vary signifi of the factors that control asphalt durability. cantly among asphalts of different sources. The purpose of this paper is to examine the The reaction of asphalt with atmospheric literature dealing with the chemical composition of oxygen is a major factor leading to the asphalt and changes in composition during environ hardening and embrittlement of asphalt. The mental aging that affect durability-related prop hardening phenomenon is primarily a result erties. Both past and recent research important to of the formation in asphalt of polar oxygen the state of the art will be considered. Because of containing functional groups that increase the extreme breadth of the subject and the volumi asphalt consistency through strong molecular nous literature related to durability, a complete interaction forces. The identification and bibliography will not be attempted, but sufficient characterization of the chemical functional references will be cited to allow the serious re types normally present in asphalt or formed searcher to find additional literature. The author's on oxidative aging that influence molecular approach to the fundamental chemical factors affect interactions afford a fundamental approach ing asphalt properties and durability will also be to relating asphalt composition with asphalt presented. properties and thus the performance of both asphalts and asphalt-aggregate mixtures. In addition to the polar chemical functional PRELIMINARY CONSIDERATION OF FACTORS AFFECTING groups formed on oxidation, asphalt prop ASPHALT DURABILITY erties can also be significantly altered by molecular structuring, sometimes called To provide a background for the documented discus steric hardening. This potentially revers sions that follow, the major composition-related ible phenomenon, although highly elusive and factors affecting durability are briefly outlined. difficult to quantify in asphalt pavement The most important aspect of a durable asphalt, as mixtures, may also be a major factor con suming that it meets initial performance require tributing to pavement embrittlement. ments, is its resistance to change while in service. The dominant physical change leading to reduced as phalt durability is a change in flow properties re lated to excessive stiffening or hardening of the asphalt. Three fundamental composition-related fac Differences in the quality of asphalts from differ tors govern the changes that could cause hardening ent sources (different composition) and relation of asphalts in pavements: ships between composition and performance properties have long been recognized, as shown by the many pub lications on the subject, a few of which are cited 1. Loss of the oily components of asphalt by (l-13). Asphalts meeting the same specifications volatility or absorption by porous aggregates, often produce pavements with widely differing per 2. Changes in the chemical composition of as formance and serviceability. Admittedly such factors phalt molecules from reaction with atmospheric oxy as aggregate characteristics, design, construction gen, and variables, and environment play major roles in de 3. Molecular structuring that produces thixo termining pavement performance and often overshadow tropic effects (steric hardening). the contribution to performance made by var iabil ities in asphalt cement quality. However, such stud With current specifications and construction prac ies as the well-known Zaca-Wigmore Experimental Road tices, volatility loss is probably not a significant Test (4,9), in which construction variables were contributor to pavement hardening. Reaction with carefully- controlled and asphalt source was inten atmospheric oxygen is probably the major and best tionally varied, clearly demonstrate the importance understood cause. Molecular structuring, although of asphalt chemical composition in pavement dura elusive and difficult to quantify, may also be a bility. significant contributor. 14 Transportation Research Record 999 Irreversible adsorption of polar asphalt compo omatic ring systems has been estimated by correla nents by mineral aggregate surfaces, although not a tions based on carbon-hydrogen analyses and den factor that might be expected to harden asphalt, sities (~,17) and directly by nuclear magnetic will produce compositional changes in the asphalt resonance (NMR) (18) • Concentrations of aromatic that may also significantly affect asphalt prop carbon determined by NMR typically run from 25 to 35 erties and aging characteristics. Finally, it is percent for petroleum asphalts. The aromatic carbon recognized that environmental factors affecting the is incorporated in condensed aromatic ring systems properties of the asphalt-aggregate bond, partic containing from 1 to possibly 10 rings per aromatic ularly water, can seriously affect the performance moiety (18). These ring systems may be associated and durability of asphalt pavements i however, even with saturated naphthenic (cycloalkyl) ring systems, though moisture-induced damage may be related to as and both the aromatic and naphthenic ring systems phalt composition and adsorption of asphalt compo may have attachments composed of a variety of types nents on aggregate surfaces, it is primarily an in of normal or branched hydrocarbon side chains. By terfacial phenomenon and beyond the scope of this NMR, carbon associated with naphthenic ring systems paper. typically ranges from 15 to 30 percent (18). Normal and branched chain hydrocarbons are present either as individual molecules or as the previously men CHEMICAL COMPOSITION OF ASPHALT tioned moieties associated with naphthenic or aro matic rings. The nonaromatic and nonnaphthenic car Elemental and Molecular Composition bon content of asphalt would typically range from 35 to 60 percent. It should be emphasized that examples Before an attempt is made to discuss relationships outside these ranges may be found and the variety of between chemical composition and asphalt properties possible combinations of molecular structures in an affecting durability, the chemical composition of asphalt is astronomically large and may vary widely asphalt will be reviewed. Asphalt is not composed of from one crude source to another. The hydrocarbon a single chemical species but is rather a complex molecular structures are further complicated by the mixture of organic molecules that vary widely in heteroatoms sulfur, nitrogen, and oxygen, which are composition from nonpolar saturated hydrocarbons to often present in sufficient combined amounts so highly polar, highly condensed aromatic ring sys that, on the average, one or more heteroatom(s) per tems. Elemental analyses of several representative molecule may be present. These may be incorporated petroleum asphalts are presented in Table 1. Al within the ring or nonring components or in more though asphalt molecules are composed predominantly discrete chemical functional groups attached to of carbon and hydrogen, most molecules contain one these components. or more of the so-called heteroatoms nitrogen, sul The heteroatoms, particularly nitrogen and oxy fur, and oxygen together with trace amounts of gen, and the aromatic ring systems contribute con metals, principally vanadium and nickel. As seen in siderable polarity or polarizability to the mole- Table 1, the heteroatoms, although a minor component cules that roduce the fl!!jor association for~e~s'------- --------ompareu~ro-t.he-h ydrocarbon-mo±-e:ty , can vary---m---con affecting physical properties. This will be dis- centration over a wide range depending on the source cussed in more detail in a later section. of the asphalt. Because the heteroatoms often impart Because the number of molecules in asphalt with functionality and polarity to the molecules, their presence may make a disproportionately large contri different chemical structures and reactivities is bution to the differences in physical properties extremely large, chemists have not seriously con sidered

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