Page 107 - MODELING OF ASPHALT CONCRETE

Overview of the Stif fness Characterization of Asphalt Concr ete 85

Propagating
Microcracks
Shear cracks

FIGURE 3-13 Microcracks and propagating cracks under traffi c loading.

Microcracking
e R “Tertiary Creep”
0

Plastic pseudostrain e R

N
No. of load cycles
FIGURE 3-14 Plastic pseudostrain accumulation with repeated loading.

Another concern in the performance of asphalt concrete pavements is its plastic
stiffness. A material which ruts substantially or at a high rate is undesirable. A typical
graph of the plastic strain that an asphalt concrete undergoes under repeated loading is
shown in Fig. 3-14. The rate of rutting is rapid initially and slows down as the asphalt
work hardens and acquires lower plastic compliance, eventually approaching a
P
horizontal asymptote at a plastic strain level of ε .
0
Occasionally, the asphalt concrete becomes too stiff and brittle and becomes
susceptible to microcracking. At that point, the plastic strain curve begins to climb
steeply as the asphalt concrete softens in accordance with the microcracking process
described previously. The equation that describes this process of work—hardening and
slowing the rate of increase of the plastic compliance of the material is in Eq. (3-20):

⎛ ε P ⎞ − ⎜ ⎛ ρ ⎞ ⎟
DN() = ⎜ ⎝ σ ⎟ ⎠ e ⎝ N ⎠ (3-20)
0
P
Δ
where D (N) = plastic compliance of the asphalt concrete which increases at a decreasing
P
rate with repeated loading
P
ε = maximum or asymptote value of the plastic strain
0

r, b = scale and shape factor of the plastic compliance curve
Δs = repeated stress