Page 108 - MODELING OF ASPHALT CONCRETE

86 Cha pte r T h ree

The coefficient of this equation r is a scale factor and the exponent b is a shape
factor which has a considerable physical significance. It measures the logarithmic
p
rate of decrease of the plastic compliance of the asphalt concrete. The coefficient ε
0
is the asymptote that is approached by the accumulating plastic strain. If this
asymptote plastic strain is divided by the repeated stress that causes it, it is the
maximum plastic compliance of that material. The maximum plastic compliance is
given in Eq. (3-21):
ε P
D = Δσ (3-21)
0
P
∞
The logarithmic rate of change of the plastic compliance D of the material is a constant
b, as shown in Fig. 3-15.
The figure also shows that as the plastic compliance gets smaller, it becomes stiff
enough to promote the formation of microcracks. The curve departs from the straight
line curving upward as microcracks grow and multiply, softening the material and
allowing plastic deformation to accelerate. It can be shown mathematically that the
exponent b is equal to the creep compliance exponent n if the rate of change of the
P
plastic compliance D (N) with respect to log(N) is a constant. Even if this rate of
change is not constant, the slope b is closely approximated by the creep compliance
exponent n. This exponent plays an important role in the fracture and healing of
asphalt concrete, and it is also a close approximation of the logarithmic rate of
accumulation of plastic strain in the same material. It also governs the rate at which
the microcracks soften the asphalt concrete and accelerate the permanent deformation
of the material in a process that has been called tertiary creep. This means that these
two exponents are very important measures of the rate at which a pavement will
deteriorate. It will be wise to devise ways of measuring these properties in the field in
in-service pavements.
Microcracking and plasticity are closely related phenomena. The first reduces the
stiffness of asphalt concrete while the second increases it. Other types of damage,
which reduce this stiffness, include moisture damage and aging, the latter of which
makes the asphalt concrete more brittle and more susceptible to microcracking. The
causes, measurement, and prediction of moisture damage are discussed in detail in
Chap. 12.

Plastic deformation

1
p
∂ (log D ) b Microcracking
log
∂ (log N )

Iog N

FIGURE 3-15 Microcracking and plastic deformation.