236
The resultant layer moduli are listed in Table 6.15. The modulus of the
asphalt concrete was generally found to be the same and close to the
value determined from rheology data. The base course modulus, E2,
decreased with load in the case of SR 15A and SR 15B test sections.
This again confirms that the pavement materials were stress-softening
rather than stress-stiffening. The subbase and subgrade layers had
modulus values showing no particular trends, except that higher values
were obtained at the lowest load. At higher loads the moduli for the
subbase and subgrade layers were very consistent.
Tables 6.14 and 6.15 suggest that FWD deflections should be mea
sured at higher loads, preferably 9 kips to minimize the effects of
nonlinearity. This also implies that equivalent moduli can be deter
mined using the theory of elasticity. The equivalent moduli would,
hopefully, produce a reasonable estimate of the deformations and strains
in the field.
6.6 Comparison of NDT Devices
The previous discussion indicated that different responses were
observed with the Dynaflect and FWD deflection basins. These differ
ences can be attributed to the magnitude of loading applied by each
device as well as differences between both devices. In this section a
comparison is made between the moduli predictions using Dynaflect and
FWD testing system. The layer moduli obtained from the plate loading
tests on some of the test sections will also be compared with the dyna
mic NDT devices.