Page 417 - Mechanics of Asphalt Microstructure and Micromechanics

Characterization and Modeling Anisotropic Proper ties of Asphalt Concrete 409

Cyclic CTE Test

0:05:46 0:06:29 0:07:12 0:07:55 0:08:38 0:09:22
0
-0.005
Strains -0.015
-0.01

-0.02
-0.025
Time (seconds) Y Z X
FIGURE 12.13 Strain plot of the cyclic CTE test.

local slopes of the loading-unloading cycles may present a resilient modulus measure-
ment. Properties on compressibility, phase angles, stress-path dependency of elastic
moduli, creeping, accumulative plastic deformation, and dynamic modulus can be
evaluated.

12.7 Anisotropy in Compression and Tension
About 70 to 80% (by volume) of AC is aggregates, which form a skeleton with aggre-
gate-aggregate (aggregates coated with thin asphalt film) contact. By contact mechanics
(Johnson, 1985), the mixture should have different responses (stiffness) against com-
pressive and tensile loading. To further complicate this issue, the material properties of
asphalt mixture are loading-rate and temperature dependent. This is mainly due to the
viscoelastic asphalt binder in the mixture. In other words, the difference in the me-
chanical response of asphalt materials in compression and tension could vary signifi-
cantly as the properties of asphalt binder vary significantly with temperature and load-
ing rate.
In the current pavement design methods, and even the anisotropy considered in the
previous sections, the modulus in compression and tension are assumed to be the same
to simplify the analysis of pavement structure. However, two of three major distresses
considered in the pavement design, low-temperature cracking and fatigue cracking, are
mainly referred to as Mode I fracture, or the so-called tension fracture, and thus are close-
ly related to the mechanical response of asphalt materials in tension. Investigating the
difference of moduli in compression and tension serves as the first step to understanding
the different behavior of asphalt material in tension and compression and how signifi-
cantly this feature of asphalt mixtures could affect the current design system.
Bi-modularity (of asphalt mixtures), a material behavior that shows two different
moduli when subjected to tension and compression, has received attention from research-
ers in pavement engineering for a long time. Secor and Monismith (1965) noticed the
difference in the strain of AC in tension and compression using a creep testing procedure.
However, they did not estimate the modulus in tension and compression separately; in-
stead they calculated a combined modulus to describe the mechanical response of
asphalt material. Kallas (1970) conducted dynamic modulus tests on asphalt mixture