Page 236 - Mechanics of Asphalt Microstructure and Micromechanics

228 Ch a p t e r Sev e n

There are many models in this category. Following are two examples.
The current Mechanistic-Empirical Pavement Design Guide (MEPDG) uses a model
shown below:
1 3 291β
1 0 854β
×
N = 0 00432 β × C × ( ) . f 2 ( ) . f 3 (7-74)
.
f f 1 ε E
t
a +
.
.
)
C = 10 484*[ V b /( V V b −069] (7-75)
b f1 , b f2 , b f3 = calibration factors
C = laboratory to field adjustment factor
e t = critical tensile strain
E = stiffness of the AC surface layer
V a = air voids (%)
V b = effective binder content (%)
The model is similar to that by El-Basyouny et al. (2005).
1 3 291.
1 0 854.
.
N = 0 00432 C( ) ( )
f ε E
t
C = 10 M
V
.
M = 484.( b − 069)
V + V
a b
where N f = number of repetitions to fatigue cracking,
t = tensile strain at the critical location,
C = correction factor,
E = stiffness of the material (psi),
M = power factor,
V b = effective binder content (%), and
V a = air voids (%)
Sousa et al. (1998) evaluated a series of phenomenological fatigue models under an
SHRP contract. These models were calibrated by applying shift factors based on field
observations to provide reasonable estimates of the in-service life of a pavement. The
shift factors, of the order 10 and more, are necessary to correct deficiencies in the ap-
proach. One such deficiency is the neglect of the crack propagation phase, which is
usually not properly represented or simulated in the conventional laboratory fatigue
tests, and thus the phenomenological models. Several other models in this category are
listed as follows.
The Monismith Model
Monismith et al. (1969) proposed a model as the following format:
a b
⎛ 1 ⎞ ⎛ 1 ⎞
N = K ⎜ ⎟ ⎜ ⎟ (7-76)
⎝ ε t ⎠ ⎝ S mix ⎠
Where S mix is the flexural stiffness; K is a factor that recognizes the influence of as-
phalt content and degree of compaction; e t = tensile strain applied; and a, b are the ex-
perimentally determined coefficients.