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OCR for page 15
CHAPTER 3
Asphalt Binders
Asphalt binders, sometimes referred to as asphalt cement binders or simply asphalt cement,
are an essential component of asphalt concrete--they are the cement that holds the aggregate
together. Asphalt binders are a co-product of refining crude petroleum to produce gasoline,
diesel fuel, lubricating oils, and many other petroleum products. Asphalt binder is produced
from the thick, heavy residue that remains after fuels and lubricants are removed from crude oil.
This heavy residue can be further processed in various ways, such as steam reduction and oxidation,
until it meets the desired set of specifications for asphalt binders. For demanding, high-performance
applications, small amounts of polymers are sometimes blended into the asphalt binder, pro-
ducing a polymer-modified binder.
Asphalt binders have been mixed with crushed aggregate to form paving materials for over
100 years. They are a very useful and valuable material for constructing flexible pavement world-
wide. However, asphalt binders have very unusual engineering properties that must be carefully
controlled in order to ensure good performance. One of the most important characteristics of
asphalt binders that must be addressed in test methods and specifications is that their precise
properties almost always depend on their temperature. Asphalt binders tend to be very stiff and
brittle at low temperatures, thick fluids at high temperatures, and leathery/rubbery semi-solids at
intermediate temperatures. Such extreme changes in properties can cause performance problems
in pavements. At high temperatures, a pavement with a binder that is too soft will be prone to
rutting and shoving. On the other hand, a pavement that contains a binder that is too stiff at low
temperatures will be prone to low-temperature cracking. Figure 3-1 illustrates the extreme change
in modulus that occurs in asphalt binders over the range of temperatures likely to occur in
pavements; at -30°C the modulus of this particular asphalt binder was about 37,000 times greater
than its modulus at 50°C. Specifications for asphalt binders must control properties at high, low,
and intermediate temperatures. Furthermore, test methods used to specify asphalt binders
usually must be conducted with very careful temperature control; otherwise, the results will not
be reliable. Asphalt binders are also very sensitive to the time or rate of loading. When tested at
a fast loading rate, an asphalt binder will be much stiffer than when tested at a slow loading rate.
Therefore, time or rate of loading must also be specified and carefully controlled when testing
asphalt binders.
Another characteristic of asphalt binders that complicates specification and testing of these
materials is that, for various reasons, such binders tend to harden with time. For example, when
asphalt binders are heated to high temperatures, as happens when mixing and transporting HMA,
some of the lighter volatile oil fractions of the asphalt vaporize, which can harden the remaining
asphalt binder. At the same time, some of the chemical compounds making up asphalt binders
can oxidize, which can also result in an increase in stiffness. Some oxidation occurs during mixing,
transport, and placement of the HMA. However, slow, long-term oxidation will continue to
occur in the asphalt binder in a pavement for many years, resulting in a slow but sometimes very
15
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16 A Manual for Design of Hot Mix Asphalt with Commentary
1.E+09
Modulus at -30 oC is 37,000
1.E+08
times the modulus
Modulus, Pa
1.E+07 at 50 oC
1.E+06
1.E+05
1.E+04
-40 -20 0 20 40 60 80
Temperature, oC
Figure 3-1. Change in dynamic shear modulus
with temperature for typical asphalt binder
(frequency = 10 rad/s).
significant increase in stiffness. Sometimes asphalt binder age hardening can be so severe that it
can lead to serious premature surface cracking of the pavement surface. Several other types of
hardening occur in asphalt binders without any loss of volatiles or oxidation; these include steric
hardening and physical hardening. These phenomena are not yet well understood, but appear
to be caused by a slow rearrangement of the molecules in the asphalt binder over time, resulting
in a gradual increase in stiffness. Unlike other types of hardening, steric hardening and physical
hardening are reversible--if the asphalt is heated until fluid and then cooled, all or most of the
hardening will be removed. This is one of the reasons it is important to thoroughly heat and stir
asphalt samples prior to performing any laboratory tests.
Asphalt binders are complex materials that are difficult to specify and test. Pavement engineers
and technicians have struggled for over 100 years to develop simple tests and effective specifica-
tions for asphalt binders. One of the earliest tests for asphalt binders was the penetration test, in
which a small lightly weighted needle was allowed to penetrate the asphalt for a set period of time
(typically 5 or 60 seconds). The distance the needle penetrated into the asphalt was measured
and was used as an indication of its stiffness. Other such empirical tests were the ring and ball
softening point temperature, and the ductility test. These tests were useful (many are still used
in specifications in Europe and other parts of the world), but had shortcomings. They did not
measure any fundamental property of the asphalt binder, like modulus or strength. The results
were also sometimes highly variable and were not always in close agreement from laboratory to
laboratory. In the 1960s, specifications based on viscosity measurements began to be adopted by
many highway agencies. Viscosity tests are superior to the earlier empirical tests--they provide
information on a fundamental characteristic of the asphalt binder and provide reasonably
repeatable results among laboratories. However, there are drawbacks to viscosity testing. First,
it is best used at high temperatures, where the behavior of the asphalt binder approaches that of
an ideal fluid. At low and intermediate temperatures, viscosity tests become difficult to perform
and even more difficult to interpret. Second, viscosity tests only provide a limited amount of infor-
mation on the flow properties of a material. Two different asphalt binders can have identical vis-
cosity values at a given temperature but might behave very differently because of differences
in the degree of elasticity exhibited in their behavior. When loaded, the asphalt binders might
deform the same amount, but when the load is removed, one might spring back, or recover, to
nearly its initial shape. The other might hardly recover at all, staying in its deformed shape. The
asphalt binder that showed more recovery--that behaved in a more elastic fashion--would tend
to provide better rut resistance in paving applications compared to the other binder with poor
recovery. However, viscosity tests provide no information about recovery or about the degree of
elasticity exhibited by a material under loading. The shortcomings in both older empirical tests
and in the newer viscosity tests eventually led to the development of a more effective system
