| U.S. Army and Air Force Design
The U.S. Department of Defense has developed a Pavement Design Manual for Roads,
Streets, and Open Storage Areas that includes a section for flexible pavements with stabilized
bases (UFC, 2004). Such structures would constitute a semi-rigid pavement when a CTB is used
underneath the HMA layer.
The pavement design software PCASE developed by the U.S. Army and Air Force for
airport pavements uses the procedure described in their guide that is based on an asphalt strain
criteria. Equation 7a is used to determine the limiting tensile strain at the bottom of the asphalt
layer (UFC, 2004):
A
AC
Allowable Strain
10
−
= ε =
(7a)
(
)
E
N 2.665 Log
0.392
14.22
A
5
+
×
+
=
(7b)
Where
N = log(coverage)
E = elastic modulus of asphalt concrete (psi).
Once the allowable strain is calculated, the allowable coverage of load repetitions is
approximated using the following equations:
X
Allowable Coverage 10
=
(7c)
( )
(
)
AC
E
X 5 Log
2.665 Log
0.392
14.22
= ×
ε
+
×
+
(7d)
where
AC
ε = allowable (tensile) strain.
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