Page 9 - Mechanics of Asphalt Microstructure and Micromechanics

2 Ch a p t e r O n e

In addition, due to the significant differences in chemical composition between
binder and aggregates, the bonding between asphalt binder and aggregates is weak and
has not been well understood. So far, there are not enough experimental and theoretical
backgrounds pinpointing the behavior of interfaces between aggregates and binder,
nor between fillers and binder.

1.1.2 Construction
Unlike other materials such as polymer and metals, the production of AC for real roads
presents difficulties in controlling its quality in the areas of: 1) aggregate moisture; 2) the
fines stuck at the aggregate surfaces; 3) temperature; 4) wind and moisture conditions at
compaction; and 5) roadbed conditions. The variations in these conditions affect the
bonding between aggregates and binder, binder content, void distribution, and even
segregation. More importantly, a typical lift thickness is around three times the nominal
maximum size of the aggregates, smaller than the required size for a representative vol-
ume, making compaction difficult and uniformity hard to achieve. These factors signifi-
cantly limit understanding or predicting the behavior of asphalt pavement.

1.1.3 Environment Exposure
Roads must be in a natural environment without protection or isolation. Variations in
temperature, moisture conditions, sunshine exposure, and the loading spectrum (load-
ing magnitude, speed, and interval) are difficult to characterize, making predicting the
performance of AC much more challenging than most other engineering materials. In
addition, the “strength” of AC is usually degrading (due to aging, sometimes also in-
creasing due to healing) with time, and since the degrading mechanisms are very com-
plicated to explore, they are not well understood.

1.1.4 Failure Modes
Unlike many other engineering structures, roads are not designed against failures such
as fracture or limited elastic deformation that can be accurately determined. They are
designed against several distresses such as rutting (permanent deformation), fatigue
cracking, thermal cracking, and roughness. These distresses accumulate with the repe-
titions of loads and thermal cycles. Due to the complexities described in previous sec-
tions, the performance characterized in the laboratory can hardly be used to predict
road performance. This makes empirical methods, such as the mechanistic-empirical
(M-E) method, necessary.

1.1.5 Moisture Damage
A special distress of AC is moisture damage. Moisture entrapped in AC produces large
excess pore water pressure. When asphalt film is soaked in water, its light components
may be dissolved and become weak. Moisture infused between aggregates and binder
interface will also weaken this interface. Moisture damage causes the removal of fine
materials, stripping the binder from the aggregate surface, and thus changing the mate-
rial structure. This phenomenon is not well understood.

1.1.6 Complex Coupling
Experiments have also discovered that rutting induces fatigue cracking and vice versa.
Moisture damage, weakening the mastics and binder-aggregate interface, leads to ac-
celeration of fatigue and rutting.