Page 96 - MODELING OF ASPHALT CONCRETE

74 Cha pte r T h ree

100

Percent passing Maximum density line departure
Large

Moderate
departure

0
Aggregate size
FIGURE 3-6 Effect of particle size distribution on “effective” poisson’s ratio.

Aggregate Particles
The principal reason for the emergence of the cross-anisotropic formulation of the
stiffness of asphalt concrete is because of the shape of the aggregate particles. When
they are compacted, the oblong particles tend to lie flat resulting in a modulus that is
greater in the vertical direction than in the horizontal direction. In addition to the shape,
the size and size distribution of the particles and the texture of the particles also have
an effect upon the directional stiffness and the effective Poisson’s ratio of asphalt
concrete. The graph in Fig. 3-6 shows qualitatively the effect of particle size distribution
on the “effective” Poisson’s ratio of the mixture.
The closer that the particle size distribution approaches the maximum density
distribution line, the higher will be the “effective” Poisson’s ratio.

Asphalt Binder Properties
The material properties of the asphalt binder which affect the stiffness of asphalt
concrete are the compliance, the mastic film thickness, the aging, and the wetting and
dewetting components of the surface energy. The compliance is a material response to
a constant applied stress. When a stress is applied and held constant in a uniaxial test,
the strain increases with the time after the load was applied. This time-dependent strain
divided by the constant stress is the creep compliance. A commonly used relation of the
creep compliance to the loading time is given in the modified power law, which is given
in Eq. (3-5) (Daniel and Kim 1998):

D − D
Dt() = D + ∞ 0 (3-5)
0 n
⎛ τ 0 ⎞
⎝ 1 + t ⎠

where D , D are minimum and maximum compliance and t , n are modified power law
0 ∞ 0
coefficient and exponent.
A log-log graph of the modified power law is shown in Fig. 3-7. The curve begins
with a nearly horizontal slope at short loading times and rises to approach a straight
line in its midrange and finally can return to horizontal at long loading times as it