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69
4.4 2000 NCAT TEST TRACK 4.4.1 Effect of Gradation
Gradation, aggregate type, and NMAS are experimental When the Superpave method was first implemented, the
factors at the NCAT test track (178). Eight aggregate types-- restricted zone excluded many aggregate blends close to the
quartzite, granite, a limestone-slag blend, gravel, a limestone maximum density line that had been used previously. Initially,
recycled asphalt pavement (RAP) blend, a limestone-gravel- it was felt that coarse-graded mixes would be more rut resis-
RAP blend, sandstone, and quartz gravel--are represented, tant than fine-graded mixes. However, the rapid failure of the
providing a range of aggregate consensus and source proper- coarse-graded mixtures in the WesTrack experiment created
ties. Three NMASs were used: 9.5, 12.5, and 19.0 mm. Four concern about coarse-graded Superpave mixes. Gradations
major gradation shapes are included: above the restricted passing above the restricted zone, through the restricted zone,
zone (fine), through the restricted zone, below the restricted and below the restricted zone were placed at the 2000 NCAT
zone, and SMA gap grading. Structural capacity was not a Test Track. Although each of the sponsoring agencies deter-
variable in the 2000 NCAT Test Track. The pavement sec- mined the mixes to be placed on their sections, there are a
tion consisted of two 2-in. experimental lifts on 15 in. of number of cases in which the effect of gradation can be com-
HMA base. The 19 in. of HMA were placed on top of 5 in. pared with the same aggregate source and binder grade.
of asphalt treated drainage layer, 6 in. of crushed stone, and Figure 31 shows comparisons between coarse- and fine-
12 in. of A-2 improved subgrade. graded mixes produced with PG 67-22 asphalt binder for
The maximum wire-line rut depth after 10 million ESALs three aggregate types. An analysis of variance was performed
was 7.27 mm (179). This level of rutting and the next highest with rut depth as the response variable and gradation and
rut depth occurred in two sections with unmodified binder that aggregate type as factors. Wire-line rut depths taken at three
were placed at optimum +0.5% binder content. This illustrates random locations within each section were used as factors.
that there were no true rutting "failures" at the 2000 NCAT As shown in Table 16, gradation is not a significant factor
Test Track; however, some observations can be drawn from affecting rut depth. However, aggregate type and the inter-
the track performance in relation to aggregate properties. action between aggregate type and gradation are significant.
Figure 31. Effect of gradation type on rut depth.
TABLE 16 Analysis of variance on effect of gradation on rut depth
Source Degrees of F-statistic p-value Significant
Freedom at 5%
Aggregate Type 2 6.68 0.011 Yes
Gradation 1 4.30 0.060 No
Aggregate Type*Gradation 2 8.15 0.006 Yes
Error 12
Total 17
