123
4.4.1.2 Base Course Modulus, E2, for Thin Pavements. For pave
ments with very thin asphalt concrete layers, tx < 2.5 in., Equations
4.8 through 4.10 can be used to predict E2 with an estimate of E .
Analysis of predicted E2 values indicated that errors were generally
within the order of 10 percent for tx values of 1.0, 2.0, and 2.5 in. A
few cases (8 out of 66) had errors up to +20 percent. These are listed
in Table 4.6.
Correlation between predicted and actual E2 values yielded the
following equation:
E2 (Predicted) = 2.3745 + 0.962E2 (Actual) Eqn. 4.34
(N = 66, R2 = 0.982)
An attempt was made to extrapolate the equations to predict pavements
with 3 inches or more of asphalt concrete layer. Errors as high as 40
percent were obtained, thus emphasizing the need to apply the equations
to the stipulated range.
4.4.1.3 Stabilized Subgrade Modulus, Eg. The stabilized subgrade
layer was initially found to be the most difficult layer for developing
a rational prediction equation. Equations 4.11 through 4.13 which were
developed for Case 1 with fixed tx and E1 values (3.0 in. and 100.0 ksi,
respectively) had 24 out of 88 cases with predictive errors exceeding
15 percent. When the equations were used to predict pavements with Ex
value of 1000.0 ksi, errors as high as 75 percent were obtained.
Equation 4.14 (for Case 2), which was developed from a multilinear
regression analysis, applies to pavements with AC thicknesses ranging