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The asphalt concrete modulus of 150 ksi used in the above para
metric study generally corresponds to pavements under warm temperature
conditions. To demonstrate the effect of low temperature or hard
asphalts on NDT deflections, the E1 value was increased to 600 ksi while
keeping the other parameters constant. Table 4.4 lists typical percent
changes in deflection for a tx value of 3.0 in. The trend is similar to
that of the E1 value of 150 ksi. Therefore, E4 has the greatest effect
on deflections whether the asphalt concrete modulus is high or low.
The effect of a bedrock or rigid layer at some finite depth beneath
the subgrade was briefly assessed by varying the subgrade thickness from
zero to infinity. The zero case corresponded to the case of the rock
layer being the subgrade, while the infinity case represented the
situation of no bedrock. The latter case--semi-infinite subgrade
thickness--is the classical representation with the use of layered
elastic theory. Figure 4.11 illustrates the variation of theoretical
FWD deflection basins with subgrade thickness. The figure shows that
the rock layer has considerable effect on NDT deflections unless the
depth of bedrock is 30 ft. or more.
4.2.2 Summary of Sensitivity Analysis
From the parametric study presented above, the following basic
conclusions can be made concerning the use of NDT and layered elastic
theory to determine pavement layer moduli.
1. E4 contributes significantly to NDT deflections, as compared with
the moduli of the upper layers. Small changes in E4 would have a
significant effect on deflections and vice versa.
2. The percent change in deflections from E4 increases from the first
NDT sensor to the last one. This suggests that the pavement