"Asphalt" is a dark brown to black, highly viscous, hydrocarbon produced from
petroleum distillation residue. This distillation can occur naturally,
resulting in asphalt lakes, or occur in a petroleum refinery. In 2001, the
U.S. produced almost 35 million tons of asphalt at a rough value of around $6
billion. Roads and highways constitute the largest single use of
asphalt at 85 percent of the
total (Asphalt Institute, 2002). In HMA, asphalt functions as a waterproof, thermoplastic, viscoelastic
adhesive. By weight, asphalt generally accounts for between 4 and 8 percent
of HMA and makes up about 25 - 30
percent
of the cost of an HMA pavement structure depending upon the type and quantity. The paving industry
also uses asphalt emulsions, asphalt cutbacks and
foamed asphalt.
Figure 1: Trinidad Lake Asphalt
Figure 2: Petroleum Refinery
"Asphalt cement" refers to asphalt that has been prepared for use in HMA and
other paving applications. This section uses the generic term, "asphalt binder", to represent the principal binding agent in HMA
because "asphalt binder" includes asphalt cement as well as any material
added to modify the original asphalt cement properties.
Asphalt can be classified by its chemical composition and physical
properties. The pavement industry typically relies on physical properties
for performance characterization. An aggregate's physical properties are a
direct result of its chemical composition. Typically, the most important
physical properties are:
Durability. Durability is a measure of how asphalt binder
physical properties change with age (sometimes called age hardening). In
general, as an asphalt binder ages, its viscosity increases and it becomes
more stiff and brittle.
Rheology. Rheology is the study of deformation and flow of matter. Deformation and flow of the asphalt binder in HMA is important in HMA pavement performance. HMA pavements that deform and flow too much
may be susceptible to
rutting and
bleeding, while those that are too stiff may be susceptible to
fatigue cracking.
Safety. Asphalt cement like most other materials, volatilizes (gives off vapor) when heated. At extremely high temperatures (well above those experienced in the manufacture and construction of HMA) asphalt cement can release enough vapor to increase the
volatile
concentration immediately above the asphalt cement to a point where it will
ignite (flash) when exposed to a spark or open flame. This is called the
flash point. For safety reasons, the flash point of asphalt cement is tested
and controlled.
Purity. Asphalt cement, as used in HMA paving, should consist
of almost pure bitumen. Impurities are not active cementing constituents
and may be detrimental to asphalt performance.
Slideshow 1: Asphalt Binder Testing Equipment Used with Superpave
Asphalt binders are typically categorized by one or more shorthand grading
systems according to their physical characteristics. These systems range
from simple to complex and represent an evolution in the ability to characterize
asphalt binder. Today, most States use the Superpave performance grading
(PG) system, although brief mention of the older systems is still worthwhile.
Older Grading Systems
The two major historical grading systems used in the U.S. are:
Penetration grading. Based on the depth a standard needle
will penetrate an asphalt binder sample when placed under a 100 g load for 5
seconds. The test is simple and easy to perform but it does not measure
any fundamental parameter and can only characterize asphalt binder at one
temperature (77°F). Penetration grades are
listed as a range of penetration units (one penetration unit = 0.1 mm).
Typical asphalt binders used in the U.S. are 65-70 pen and 85-100 pen.
This system is not used in Washington State.
Viscosity grading. Measures penetration
(as in penetration grading) but also measures an asphalt binder's viscosity at
140°F and 275°F. Testing can be done on virgin (AC) or aged (AR) asphalt
binder. Grades are listed in poises (cm-g-s = dyne-second/cm2)
or poises divided by 10. Typical asphalt binders used in the U.S. are
AC-10, AC-20, AC-30, AR-4000 and AR 8000. Viscosity grading is a
better grading system but it does not test low temperature asphalt binder
rheology.
Superpave Performance Grading (PG) System
The Superpave PG system was developed as part of the
Superpave research effort
to more accurately and fully characterize asphalt binders for use in HMA
pavements. The PG system is based on the idea
that an HMA asphalt binder’s properties should be related to the conditions
under which it is used. For asphalt binders, this involves expected
climatic conditions as well as aging considerations. Therefore, the PG
system uses a common battery of tests (as the older penetration and viscosity
grading systems do) but specifies that a particular asphalt binder must pass
these tests at specific temperatures that are dependant upon the specific
climatic conditions in the area of intended use. Therefore, a binder used in
Northeastern Washington would be different than one used in Western Washington.
Superpave performance grading is reported using two numbers – the first being
the average seven-day maximum pavement temperature (in °C) and the second being the minimum pavement design temperature likely to be experienced (in °C). Thus, a PG 58-22 is intended for use where the average seven-day maximum pavement temperature is 58°C and the expected minimum pavement temperature is -22°C. Notice that these numbers are pavement
temperatures and not air temperatures. WSDOT has analyzed pavement
temperatures across the State and come up with two baseline asphalt binder
grades (see Figure 3). If no other information is available, these grades
should be the default choices for use in HMA.
Some asphalt cements require modification in order to meet specifications.
Asphalt cement modification has been practiced for over 50 years but has
received added attention in the past decade or so. There are numerous binder additives available on the market today. The benefits of modified asphalt cement can only be realized by a judicious selection of the modifier(s); not all modifiers are appropriate for all applications. In general, asphalt cement should be modified to achieve the following types of improvements (Roberts et al., 1996):
Lower stiffness (or viscosity) at the high temperatures associated with construction
. This facilitates pumping of the liquid asphalt binder as well as mixing and compaction of HMA.
Higher stiffness at high service temperatures
. This will reduce rutting and shoving.
Lower stiffness and faster relaxation properties at low service temperatures
. This will reduce the likelihood of
stripping.
Figure 4 shows two aggregate samples from the same source after they have
been coated with asphalt binder. The asphalt binder used with the sample
on the left contain no anti-stripping modifier, which resulted in almost no
aggregate-asphalt binder adhesion. The asphalt binder used with the sample
on the right contains 0.5% (by weight of asphalt binder) of an anti-stripping modifier, which
results in good aggregate-asphalt binder adhesion.
Besides asphalt cement, three other forms of asphalt are used prominently in the paving industry:
Emulsified asphalt. Emulsified asphalt is a suspension of
small asphalt cement globules in water, which is assisted by an emulsifying
agent (such as soap). Emulsions have lower viscosities than neat
asphalt and can thus be used in low temperature applications. After an
emulsion is applied the water evaporates away and only the asphalt cement is
left. Emulsions are often used as
prime coats
and tack
coats.
Cutback asphalt. A cutback asphalt is a combination of
asphalt cement and petroleum solvent. Like emulsions, cutbacks are used
because their viscosity is lower than that of neat asphalt and can thus be
used in low temperature applications. After a cutback is applied the
solvent evaporates away and only the asphalt cement is left. Cutbacks
are much less common today because the petroleum solvent is more expensive
than water and can be an environmental concern. Cutbacks are typically
used as
prime coats and
tack coats.
Foamed asphalt. Foamed asphalt is formed by combining hot asphalt binder with small amounts
of cold water. When the cold water comes in contact with the hot asphalt
binder it turns to steam, which becomes trapped in tiny asphalt binder bubbles
(World Highways, 2001). The result is a thin-film, high volume asphalt
foam. This high volume foam
state only lasts for a few minutes, after which the asphalt binder resumes its
original properties. Foamed asphalt can be used as a binder in soil or
base course stabilization, and is often used as the stabilizing agent in
CIPR.
Asphalt binders are typically categorized by one or more shorthand grading
systems according to their physical characteristics. These systems range
from simple to complex and represent an evolution in the ability to characterize
asphalt binder. Today, most States use the Superpave performance grading
(PG) system, although brief mention of the older systems is still worthwhile. 