Page 44 - MODELING OF ASPHALT CONCRETE

22 Cha pte r T w o

Asphalt Viscoelastic Properties and Pavement Performance
Figure 2-2 is an isochronal plot that depicts rheological properties of asphalt in its
unaged condition and after aging in the field under a moderate climate for approximately
16 years. To relate asphalt properties to pavement performance, reference can be made
to three temperature zones. At temperatures in the range of 45 to 85°C, typical of highest
pavement in-service temperatures, the main distress mechanism is rutting (Chaps. 10
∗
and 11), and, therefore, the G and d need to be measured. A measure of viscosity alone
cannot be sufficient, since viscosity measurements are done on the assumption that
∗
asphalt response has only a viscous component. For rutting resistance, a high G value
is favorable because it represents a higher total resistance to deformation. A lower d is
favorable because it reflects a more elastic (recoverable) component of the total
deformation.
Within the intermediate temperature zone, asphalts are generally harder and more
elastic than at higher temperatures. The prevailing failure mode at these temperatures
is fatigue damage (Chaps. 12 and 13). For viscoelastic materials, like asphalt binders,
∗
both G and d play a role in damage caused by fatigue. They are both important because
during every cycle of loading the damage is dependent on how much strain or stress
is developed by the cyclic load and how much of that deformation can be recovered or
dissipated.
Under strain-controlled conditions, a softer material and a more elastic material
will be more favorable to resist fatigue damage because the stress developed for a given
deformation is lower and the asphalt will be more capable of recovering to its preloading
condition. Similar to rutting, a single measure of hardness or viscosity cannot be
sufficient to select better-performing asphalts with respect to fatigue resistance. Rutting
and fatigue damage are both functions of frequency of loading, and, therefore, the rate

∗
Unaged (G ) Unaged (delta)
∗
Field aged (G ) Field aged (delta)
10 100
9 90
8 7 80
70
G∗ at 10 rad/s, log Pa 6 5 4 3 60 Delta at 10 rad/s, °
50
40
30

1 2 20
10
0 0
Thermal cracking Fatigue cracking Rutting
−1 −10
−50 −30 −10 10 30 50 70 90
Temperature, °C
FIGURE 2-2 Typical rheological behavior of asphalt binders before and after aging in the fi eld in
relation to pavement main distress modes.