Page 51 - MODELING OF ASPHALT CONCRETE

Modeling of Asphalt Binder Rheology and Its Application to Modified Binders 29

than one form. A significant part could be dissipated in damping as opposed to damage
and thus testing for accumulated damage is necessary to differentiate between the two
mechanisms. This issue was found to be particularly important for modified asphalts
since most are manufactured to enhance elasticity thus causing an increase in delayed
elasticity (cause for damping). New testing protocols were, therefore, introduced to
capture the rate of damage accumulation in the linear and nonlinear viscoelastic range.
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Depending on G sind and G /sind parameters has proven to be insufficient because
these parameters are derived based on the concept that all energy dissipated is lost in
damage. The subject of damage resistance is covered in a subsequent section.
The Need for Asphalt Modification
Modification of asphalts is prompted mainly by the limitation of the conventional
refining practices used today in producing asphalts from crude petroleum that can
resist distresses. The chemical composition of asphalt and, in consequence its properties,
are largely dependent on the crude source and the refining process. Asphalt production
in most refineries is a secondary process that cannot compete with fuel and other
products in revenue generation. Therefore, production of better-performing asphalts is
not one of the common strategies in petroleum refining. When the produced asphalt
does not meet climate, traffic, and pavement structure requirements, modification has
been used as one of the attractive alternatives to improve asphalt properties.
In effect, what is called conventional asphalts or straight run asphalts have a range
of rheological and durability properties that are not sufficient for resistance on distresses
caused by the increase in traffic and total loading on current highways. Modification by
specialized refining practices, chemical reaction, and/or additives has been found to
improve contribution of asphalt binders to resistance of asphalt mixtures to various
modes or pavement distress. This improvement is recognized to result in life-cycle cost
savings and thus use of modified asphalts has been steadily increasing for the last 20
years or so.
In a recent survey of the state highway agencies in the United States, 35 out of 47
agencies that responded indicated that they plan on increasing the use of modified
binders in road construction, 12 were expecting to use the same amount of modified
asphalt, and none indicated that they plan on reducing amount of modified asphalts.
The majority of the agencies have cited premature distress such as rutting and fatigue
cracking as the main reasons for justifying the use of modified binders, which on
average increase the initial cost of construction (Bahia et al. 1998).
Asphalt modification using additives dates back to the last century (King et al. 1999).
Patents for using polymers to modify asphalts date back to 1823 (Isacsson and Lu 1995).
Test projects were placed in Europe in the 1930s and in North America in the 1950s. In the
early and mid-1980s the introduction of newer polymers and European technologies
resulted in proliferation of asphalt modification in the United States. By 1982 more than
1000 technical articles had been published on polymer modified asphalts or mixtures, and
more recently there is a continuing emphasis on this subject (Bahia et al. 2001).

Asphalt Modification Strategies
A modifier can be selected to improve one or more of the main performance related
properties of asphalts. Also, different modifiers that affect different properties can be
combined to improve several properties. There have been numerous tests used to
quantify each of the properties and measure the effectiveness of certain additives to