Page 57 - Mechanics of Asphalt Microstructure and Micromechanics

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50 Ch a p t e r w o

There are various image-based direct quantification methods for shape, angularity,
and texture. These quantities are often used in modeling the contacts between aggre-
gates, and the interfaces between aggregates and binder. As there are no consistent
models that directly incorporate these quantities into modeling, no details are present-
ed on this topic.

2.3 Mastics

Mastics are the mixtures of binder and fillers, and their properties are often character-
ized using similar methods as for asphalt binder. Many theories applicable to binder
and mixtures are often used to characterize the mechanical properties of mastics. There
are also theories that use micromechanics to evaluate mastic properties from binder
properties and the filler properties. The testing methods for binder are often applicable
to those of mastics as well.

2.4 Characterization of Mixture Properties

Due to the variety of asphalt binder, fillers, and aggregates, the mixture properties vary
widely. It is difficult to determine the typical values of these properties. One of the im-
portant branches of pavement mechanics is to predict the mixture properties from the
properties of their constituents. As a result, only the characterization methods are pre-
sented in this section.

2.4.1 Marshall Stability Test
The Marshall Stability Test (ASTM D 1559) was developed by the Corps of Engineers in
the 1940s and is used to measure the strength of an asphalt mixture sample compacted
to a standard laboratory compactive effort. The stability is the maximum load which the
sample can withstand or the load that must be applied to cause the sample to fail. In this
test procedure, a compressive load is applied to a 101.6 mm diameter and 63.5 mm thick
cylindrical specimen through semicircular testing heads. The displacement is applied at
a rate of 5 mm/min after the specimen is brought to a temperature of 60°C. This tem-
perature represents the assumed maximum pavement temperature in the summer in
the United States, thus being the weakest condition for the HMA mixture.

2.4.2 Hveem Stabilometer Test
The Hveem or stabilometer test is an empirical measure of the internal friction within
an asphalt mixture commonly used in the United States. In this test, a vertical axial load
is applied to an AC specimen that is 101.6 mm in diameter and 63.5 mm high and the
displacement is measured. The specimen temperature has to be 60°C at the time of the
test to simulate the most critical field condition of the mixture. Values of stability can
vary between 0 (for a liquid) and 100 (incompressible solid). For asphalt mixes, values
could range from 30 to 40.