Page 165 - Mechanics of Asphalt Microstructure and Micromechanics

Mixture T heor y and Micromechanics Applications 157

5.6 Micromechanics Applications for Pavement Analysis
5.6.1 Top-down Cracking (Wang et al, 2003)
Top-down cracking has been recognized as a severe cracking type that significantly
reduces the quality service life of pavements (Gerritsen et al., 1987; Dauzats and Ram-
pal, 1987). However, the causes of top-down cracking are very complicated. As a re-
sult, different causes, including tire-pavement contacting properties, thermal gradient,
type of mix, and construction factors have been identified (Svasdisant et al., 2002; Mat-
suno and Nishizawa, 1992; Uhlmeyer et al., 2000). So far, research in this field has
mainly focused on stress analysis using the FEM to identify causes that induce tensile
stress at the surface (Myers et al., 1998, 1999; Roque et al., 2000; Myers et al., 2001,
2002). Material inhomogeneity was qualitatively recognized as a cause; however, no
analytical and experimental methods have been developed to investigate how mate-
rial inhomogeneity would cause tensile stress and how significant the tensile stress
would be. In addition, this type of top-down cracking is currently assumed to be Type
I cracking. Whether there exists Type II or III cracking has not been verified. Due to the
lack of sufficient experimental investigations into the causes of top-down cracking,
and especially from the point of view of materials structure, no effective preventive
measures can be developed.
Investigation of top-down cracking causes in the view of materials properties is
important. For example, why are fewer top-down cracking cases reported in cement
concrete even when loading and tire configuration are the same? A most intuitive aca-
demic guess would be straightforward: the binding strength of cement paste is much
stronger than that of asphalt binder. Nevertheless, this straightforward answer involves
the micromechanics and microstructures of the material. From a materials standpoint,
AC is different from cement concrete in its binder. The weak and thermal sensitive
binder in asphalt concrete causes most of the distresses such as rutting, fatigue crack-
ing, low-temperature cracking, and moisture damage. Most of the current research uses
a continuum mechanics approach. This approach cannot account for the material struc-
ture and is not convenient for use in identifying the internal causes that lead to top-
down cracking.
Figure 5.8a presents a simple illustration for understanding the top-down cracking
from a materials standpoint. The spheres represent aggregate particles. It is apparent
that under vertical loading, the binder between the two bottom particles may be sub-
jected to tensile stress. The maximum tensile force between the two particles can be
calculated as large as:
T 3P/ 6 (5-175)

However, due to friction and restraint from the surrounding medium, this maxi-
mum force may not be reached.
Figure 5.8b presents another algorithm that will introduce tensile strains (tensile
stress) by shearing the materials, or the so-called dilatancy mechanism. In Figure 5.8b,
a densely packed granular system is subjected to a tensile force. When Particle C moves
away from Particle A, the mastic between the two particles will be in tension, and pro-
duces volume increase or dilatancy.
These two simple cases have important implications in better understanding the
causes of the top-down cracking. The aggregate particle skeleton structure and strength