Page 92 - MODELING OF ASPHALT CONCRETE

70 Cha pte r T h ree

Factors on Which Asphalt Concrete Stiffness Depends
Asphalt concrete depends upon the strain rate of loading, the temperature, the moisture
content of the bitumen, the stress state, the aggregate particles, the bitumen itself, the
fines in the mastic, the water in liquid and vapor form and its location with the mixture,
the air in the mixture, its age, and its reactivity with oxygen, and any additives or
modifiers that have been added to the mixture. Each of these is discussed subsequently.

Temperature and Rate of Loading
The stiffness of asphalt concrete depends on the temperature and strain rate of loading.
At any given temperature, asphalt concrete will deform slowly and permanently if it is
loaded slowly, while if it is loaded at a higher rate, it will be much stiffer and will be
subject to fracture. At any given strain rate of loading, there is a temperature above
which the material will relax quickly enough that no stress will accumulate in the test
sample. These two known facts about materials in general can be illustrated in a
qualitative graph of strain rate versus temperature that shows the stress-free temperature
and strain rate above which the material will experience microcracking and healing and
below which the material will undergo plastic flow and the properties of the aggregates
in the mixture will be important in limiting the size and shape of the flow patterns. This
conceptual graph is shown in Fig. 3-1.
Moisture has a similar effect on the stiffness and type of damage that occurs in
asphalt concrete. Although the mechanisms are distinctively different, high moisture
and high temperature both result in plastic flow. At any given strain rate, there is a
moisture content in the asphalt, above which the material relaxes faster than the stress
can build up in the material. On a graph of strain rate versus moisture content there will
be a similar stress-free boundary between the brittle, fracture-prone behavior and the
soft, plastic flow-prone behavior.

Stress State
The state of stress of asphalt concrete modifies its stiffness. In an isotropic mixture, the
stiffness depends upon the level of the first and the second deviatoric stress invariants
I and J ′. In an anisotropic mixture, which is the usual case, the stiffness is directional
1 2
and depends upon both the two stress state invariants just mentioned and the

Asphalt Concrete Damage

Fracture
(Binder in tension)
Stress free temperature
Strain rate Microcracking and strain rate Plasticity
healing

flow
(Aggregate properties
important)
Temperature (Moisture)

FIGURE 3-1 Asphalt concrete damage dependence on temperature and loading rate.