Page 245 - Mechanics of Asphalt Microstructure and Micromechanics

Models for Asphalt Concrete 237

The initiation and propagation of the crack in the cylindrical specimen under conven-

tional indirect tensiletest (IDT) are modeled. This study is unique in its consideration of
material heterogeneity. It allows researchers to link the micro-scale damage with the

real pavement failure on the global scale.
7.11.3 Cohesive Zone Model Used in Modeling Fatigue of AC
Few studies (Song et al., 2005, 2006, 2008; Kim et al, 2006) have been conducted to inves-
tigate the fatigue of AC. Kim et al. (2006), Kim and Lutif (2008) started to incorporate
the cohesive zone model into a damage model to explain the mechanism of the AC fa-
tigue. Kim et al. (2006) developed a computational damage model which is able to ac-
count for material viscoelasticity, heterogeneity, and path-and rate-dependent energy
dissipation as cracks initiate and propagate. The model incorporates elastic behavior of
the aggregate particles, viscoelastic behavior of the asphalt matrix, and time-dependent
fracture both within the asphalt matrix and along boundaries between matrix and ag-
gregate particles. Rate-dependent progressive cracking up to failure was implemented
by incorporation of a cohesive zone fracture model. In the model, the tractions are de-
scribed in terms of the displacement differences across the cohesive zone. In Kim’s
study, a nonlinear cohesive zone model developed by Allen and Searcy (2001) was used
because that model can reflect nonlinear viscoelastic damage growth in the asphalt
mixtures and predict damage evolution, microcracking, post peak material softening,
and eventual fracture failure. The equation is shown below:
ut() t ∂λ t ()
Tt() = i [ −α t ( )][ ∫ C ( − π ) τ d ] (7-109)
CZ
1
t
τ
i λδ ∂τ
t ()
i 0
where i = n (normal direction), t (tangential direction), or r (radial direction),
T i (t) = cohesive zone traction,
u i (t) = cohesive zone displacement,
l(t) = cohesive zone strain,
d i = cohesive zone material length parameter,
a(t) = cohesive zone damage evolution function, and
C (t) = linear viscoelastic stress relaxation modulus of cohesive zone
CZ
The damage evolution function a(t) can be experimentally determined by perform-
ing small-scale fracture tests. The cohesive zones, which are subjected to damage and
fracture, are modeled by employing interface elements along the aggregate surface. The
elements were randomly specified in the sample to maximize the power of the model to
predict random crack initiation and propagation. The laboratory dynamic frequency
sweep test and dynamic shear rheometer test are used to determine the viscoelastic
property parameters in the model. The damage and fracture properties of the cohesive
zone interfaces were determined by utilizing a tensile fracture test. Damage growth was
captured by using an optical microscope and analyzed by video.

7.12 Other Fatigue Studies
Al-Qadi et al. (2005) discussed thermal fatigue cracking in flexible pavements based on
experimental results from the Virginia Smart Road. Thermal fatigue is caused by re-
peated thermal cycles; a finite element simulation model was developed to simulate the