ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 329

The molecular structure of asphalt affects the physical and aging prop-
erties of asphalt, as well as how the asphalt molecules interact with each
other and with aggregate. Asphalt molecules have three arrangements, de-
pending on the carbon atom links: (1) aliphatic or parraffinic, which form
straight or branched chains, (2) saturated rings, which have the highest hy-
drogen to carbon ratio, and (3) unsaturated rings or aromatic. Heteroatoms at-
tached to carbon alter the molecular configuration. Since the number of
molecular structures of asphalt is extremely large, research on asphalt
chemistry has focused on separating asphalt into major fractions that are
less complex or more homogeneous. Each of these fractions is a complex
chemical structure.
Asphalt cement consists of asphaltenes and maltenes (petrolenes). The
maltenes consist of resins and oils. The asphaltenes are dark brown friable
solids that are chemically complex, with the highest polarity among the
components. The asphaltenes are responsible for the viscosity and the ad-
hesive property of the asphalt. If the asphaltene content is less than 10%, the
asphalt concrete will be difficult to compact to the proper construction den-
sity. Resins are dark and semisolid or solid, with a viscosity that is largely
affected by temperature. The resins act as agents to disperse asphaltenes in
the oils; the oils are clear or white liquids. When the resins are oxidized,
they yield asphaltene-type molecules. Various components of asphalt inter-
act with each other to form a balanced or compatible system. This balance
of components makes the asphalt suitable as a binder.
Three fractionation schemes are used to separate asphalt components,
as illustrated in Figure 9.11. The first scheme [Figure 9.11(a)] is partition-
ing with partial solvents in which n- butranol is added to separate (precipi-
tate) the asphaltics. The butranol is then evaporated, and the remaining
component is dissolved in acetone and chilled to to precipitate the
paraffinics and leave the cyclics in solution. The second scheme [Figure
9.11(b)] is selective adsorption-desorption, in which n- heptane is added to
separate asphaltene. The remaining maltine fraction is introduced to a chro-
matographic column and desorbed using solvents with increasing polarity
to separate other fractions. The third scheme [Figure 9.11(c)] is chemical
precipitation in which n- pentane is added to separate the asphaltenes. A
sulfuric acid

- 23°C

1 H 2 SO 4 2 is added in increasing strengths to precipitate other

fractions.
In addition, asphalt can be separated based on the molecular size with the
use of high-pressure liquid chromatography (gel-permeation chromatography).

9.5 Superpave and Performance

Grade Binders

In 1987, the Strategic Highway Research Program (SHRP) began developing
a new system for specifying asphalt materials and designing asphalt mixes.