Page 66 - Mechanics of Asphalt Microstructure and Micromechanics

CHAPTER3

Microstructure Characterization

hen asphalt concrete (AC) is treated as a continuum composed of three mi-
cro-continuums of aggregates, asphalt binder, and voids, microstructure of
WAC refers to the distributions and configurations of the three micro-contin-
uums. The implications of microstructure may be interpreted in three different perspec-
tives—the aggregate skeleton view, the inclusion view, and the distributed solid view.
In the aggregate skeleton view, aggregate particle shape, angularity and texture, and
the film thickness between two particles in contact are the quantities of interest. Corre-
spondingly, the modeling techniques in the skeleton category include the Discrete Ele-
ment Method (DEM) and the methods used in granular mechanics. The inclusion view
takes aggregates as individual inclusions and air voids as cavities and uses Ashelby
mechanics. In this view, the aggregate/air void shape and size, the spacial locations,
and orientations are important. The distances between two air voids/cavities are also
important. The third view is the distributed solid view. This view is consistent with the
mixture theories where the local solid (aggregates and binder) and air void volume
fractions and their gradients are the parameters.
There are several destructive approaches available including the thin sectioning
and serial sectioning approaches. These approaches are hardly useful to AC where heat
produced during the removal of the materials softens the asphalt binder and smears the
surfaces. Nevertheless, these sectioning approaches have been used to expose a few
sections for observation of the microstructure and their changes during testing, using
either a microscope, a camera, or a scanner. This technique has been replaced by more
advanced n ondestructive approaches, such as the X-ray computed tomography (XCT)
approach. This chapter will discuss the fundamentals of XCT, image analysis, and ste-
reology-based interpretation techniques. There are four helpful reference books on XCT
and stereology (Hsieh, 2003; DeHoff and Rhines, 1968; Underwood, 1970; Russ and
Dehoff, 2000).

3.1 X-ray Tomography Imaging

3.1.1 Fundamentals of X-ray Computerized Tomography (XCT)
X-ray was discovered by the German physicist Wilhelm Conrad Roentgen in 1895 dur-
ing his study of electrical discharge. In performing the discharge tests, Roentgen no-
ticed some invisible and unknown rays that could penetrate into metals, cause fluores-
cence, and affect photographic films. He named it X-ray, an unknown ray. The first
X-ray picture (radiogram), according to story, was the hand of Mrs. Roentgen. Due to
this important discovery, Roentgen received his Nobel Prize in 1901. The first picture
(a human hand) also inspired various applications of X-ray radiography in medical

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