Project Report on the Continued Development and Analysis of the Flexible Pavements Database Page: 30
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with virgin materials. However, the results given in Table 6.2 suggest that the product of the two
bias correction factors for section 48-A502, in spite its 35% RAP mix, is much higher than all
the other sections that had a RAP percentage. This particular observation can be attributed to the
fact that it had an overlay thickness of merely 2.2 inches whereas all the other RAP sections had
an overlay thickness in excess of 4 inches.
It has already been stated that 11 and 13 are correlated and therefore estimating either of
the bias correction factors separately is not the most appropriate course. Situations that require
more than one response variable make use of simultaneous equations where both the equations
and their parameters are estimated together. One of the major benefits of using a joint estimation
approach is to improve the efficiency of the estimated coefficients. The analysis was conducted
using SAS (originally Statistical Analysis System) and the results are given in Table 6.6.
Table 6.6: Parameter Estimates for l1 and P3
Parameter Estimates for P1
Coefficient t-stat p-value
Intercept 102 6.6 0.007
13 -137 -5.4 0.012
Milling -4.94 -0.6 0.568
Overlay Thickness 2.65 1.3 0.289
Recycle Asphalt Mix -11.2 -1.3 0.288
Parameter Estimates for [3
Coefficient t-stat p-value
Intercept 0.719 9.5 0.003
P11 -0.00700 -5.4 0.012
Milling -0.0260 -0.5 0.666
Overlay Thickness 0.0170 1.1 0.346
Recycle Asphalt Mix -0.100 -1.9 0.159
The results given indicate that none of the experimental variables have a statistically
significant influence on 131, while the use of recycled asphalt may have a significant effect on the
value of 133, provided the null hypothesis is tested at an 85% level of significance. It should be
noted here that a level of significance higher than 85% will render all of the explanatory
variables insignificant. Thus no individual variable will have a significant effect on the values for
131. Nonetheless, a test of hypothesis with all the independent variables showed that they together
have a significant effect (p-value < 0.05) on the 131 and 13. Further investigation reinforced this
observation and the results so obtained are given in Table 6.7.30
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Banerjee, Ambarish; Aguiar-Moya, José Pablo; Sivaram, Prasad; Smit, Andre de F. & Prozzi, Jorge Alberto. Project Report on the Continued Development and Analysis of the Flexible Pavements Database, report, February 2011; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth281719/m1/44/?rotate=270: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.