Civil Engineering Reference
In-Depth Information
This chapter reviews the types, uses, and chemical and physical prop-
erties of asphalt. The asphalt concrete used in road and airport pavements,
which is a mixture of asphalt and aggregates, is also presented. The chapter
discusses the recently developed Performance Grade asphalt binder specifi-
cations and Superpave mix design. Recycling of pavement materials and ad-
ditives used to modify the asphalt properties are also included.
9.1
Types of Asphalt Products
Asphalt used in pavements is produced in three forms:
asphalt cement,
asphalt cutback,
and
asphalt emulsion.
Asphalt cement is a blend of hy-
drocarbons of different molecular weights. The characteristics of the asphalt
depend on the chemical composition and the distribution of the molecular
weight hydrocarbons. As the distribution shifts toward heavier molecular
weights, the asphalt becomes harder and more viscous. At room temperatures,
asphalt cement is a semisolid material that cannot be applied readily as a
binder without being heated. Liquid asphalt products, cutbacks and emul-
sions, have been developed and can be used without heating (The Asphalt
Institute 1989).
Although the liquid asphalts are convenient, they cannot produce a
quality of asphalt concrete comparable to what can be produced by heating
neat asphalt cement and mixing it with carefully selected aggregates. As-
phalt cement has excellent adhesive characteristics, which make it a supe-
rior binder for pavement applications. In fact, it is the most common binder
material used in pavements.
A cutback is produced by dissolving asphalt cement in a lighter molecu-
lar weight hydrocarbon solvent. When the cutback is sprayed on a pavement
or mixed with aggregates, the solvent evaporates, leaving the asphalt residue
as the binder. In the past, cutbacks were widely used for highway construc-
tion. They were effective and could be applied easily in the field. However,
three disadvantages have severely limited the use of cutbacks. First, as pe-
troleum costs have escalated, the use of these expensive solvents as a carry-
ing agent for the asphalt cement is no longer cost effective. Second, cutbacks
are hazardous materials due to the volatility of the solvents. Finally, applica-
tion of the cutback releases environmentally unacceptable hydrocarbons into
the atmosphere. In fact, many regions with air pollution problems have out-
lawed the use of any cutback material.
An alternative to dissolving the asphalt in a solvent is dispersing the as-
phalt in water as emulsion. In this process the asphalt cement is physically
broken down into micron-sized globules that are mixed into water contain-
ing an emulsifying agent. Emulsified asphalts typically consist of about 60%
to 70% asphalt residue, 30% to 40% water, and a fraction of a percent of
Search WWH ::
Custom Search