to use the equations with field measured FWD deflections may result in
substantial prediction errors. This suggests that the measured FWD
deflections should be adjusted to compensate for the possible effects of
the rigid plate and other variables prior to the application of the
developed equations which were based on multilayered elastic theory.
6.5.4 Nonuniqueness of NDT Backcalculation of Layer Moduli
One of the major problems associated with backcalculation of layer
moduli from NDT deflection basins is the nonuniqueness of moduli. Theo-
retically, an infinite number of moduli combinations can produce the
same deflection basin. There are no closed-form solutions at present to
compute layer moduli if the NDT deflections are known. Therefore, a
completely erroneous set of moduli could be determined for a pavement
using the trial-and-error approach of matching measured deflection
basins. This is demonstrated in Table 6.13 using field measured Dyna-
flect basins from some of the test sections.
Table 6.13 shows that for the US 301 test section, the use of two
different combinations of E and E and same E and E values produced
practically the same deflection basin. The El value of 250.0 ksi is
that obtained from the modulus-viscosity-temperature relationships.
However, an El value which is five times as high as the above value also
produced similar deflections, with a slight reduction in E2.
In the case of US 441, when the El value was reduced from 290.0 ksi
to 100.0 ksi, predicted deflections were close to measured. It was
initially believed that the extensive block cracking on this site (Sec-
tion 5.2) had caused the reduction in the asphalt concrete modulus as
predicted from rheology data. However, because a third set of moduli
combination produced similar deflections indicate that the problem could