It was shown in Figures 4.14 to 4.19 that D0 D4 in the modified
Dynaflect sensor array system was dependent only on the surface and base
course layers. Thus,
Di D4 = fn (El, t1, E, t2) = Eeq or E12 Eqn. 6.7
where Di D4 is the difference in modified Dynaflect sensor deflections
(see Figure 4.2),
El is modulus of the asphalt concrete layer,
E2 is modulus of the base course layer,
ti is the thickness of the asphalt concrete layer,
t2 is the thickness of the base course layer, and
E12 (or Eeq) is a composite (or an equivalent) asphalt concrete
and base course modulus.
Two equations were employed to combine the surface and base course
layer stiffnesses into a composite E12 value. The first formula is
essentially a weighted average formula, and is of the form
Et +Et
E = 1 1 2 2 Eqn. 6.8
12 t +t
1 2
Equation 6.8, which is a commonly-used weighting formula, has pre-
viously been utilized by Vaswani (124) to combine pavement layers over
the subgrade. The second method used to combine El and E2 follows the
approximation suggested by Thenn de Barros (115). The equation is of
the form