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24.0
22.0
20.0
18.0
Air Voids (AASHTO T166), %
16.0
14.0 0.842
y = 1.2486 x
12.0 2
R = 0.867 6
10.0
8.0
6.0
4.0
2.0
0.0
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 20.0 22.0 24.0 26.0
Air Voids (Vacuum-Sealing), %
Figure 41. Comparisons between AASHTO T166 and vacuum-sealing methods, field projects.
than about 5 percent, the two methods provided approximately tigated for 12.5 and 19.0 mm NMASs. The effect of lift thick-
similar results. Above 5 percent air voids, the vacuum-sealing ness was evaluated within the 9.5, 12.5, and 19.0 mm NMASs.
method resulted in higher air void contents. As air voids To determine if a general trend occurred between in-place air
increased, the two methods diverged and it is believed that the voids and t/NMAS, a regression was performed on the com-
reason for this divergence is the loss of water during the SSD bined data. Figure 42 illustrates this general relationship. From
method. Hence, at low air voids, both methods should be close this regression, a low R2 of 0.09 was found. The trendline sug-
to correct; however, at higher air voids the vacuum-sealing gested that as the ratio of lift thickness to NMAS increased,
method should be more correct. in-place air voids decreased.
To determine if the relationship between in-place air
voids and the t/NMAS ratio was significant, an ANOVA
4.11 FIELD VALIDATION OF RELATIONSHIPS
BETWEEN PERMEABILITY, LIFT was conducted on the regression. For the combined data,
THICKNESS, AND IN-PLACE DENSITY the p-value was 0.014, which indicated that the overall rela-
tionship was significant. Then the data were separated into
The main objective of the field portion of NCHRP 9-27 the three mix types. When an ANOVA was conducted on the
(Task 5) was to provide a field validation of the relationships regressions for the mix types, it was found that the relation-
between permeability, lift thickness, and in-place density so ship was not significant for any of the mix types (p-values
the overall objectives of the study could be accomplished. In of 0.956, 0.994, and 0.107 for fine-graded, coarse-graded,
order to field verify the relationships between air voids, lift and SMA, respectively). There is a lot of scatter in the data, but,
thickness, and permeability, 20 HMA construction projects as can be seen in Figure 42, every increase of 1 in the t/NMAS
were visited. Testing at these projects included tests on plant- results in a decrease in voids of approximately 0.6 per-
produced mix and on the compacted pavement. Testing of cent. This finding involves average numbers, and it must
the plant produced mix included compacting samples to both be realized that many other factors affect the density of these
the design compactive effort and to a specified height. Test- field projects.
ing on the compacted pavement included performing field Another factor to consider for these projects is the specifi-
permeability tests with the NCAT Field Permeameter. Selec- cation requirements were approximately the same for all of
tion of the 20 projects was based upon the following factors: these mixes. Hence, the contractor was trying to compact all
NMAS, gradation type (fine-graded, coarse-graded, and mixes to a low void content. Even with the same target density
SMA), and the lift thickness to NMAS ratio (t/NMAS). Table the t/NMAS affected the results.
15 presents the 20 projects evaluated. For Figure 43, a best-fit line was produced on the com-
Table 15 shows that both fine- and coarse-graded Superpave bined data for the 12.5-mm NMAS mixes. A low correlation
designed mixes were investigated for each of four NMAS, was also found for this regression (0.19), but the general
ranging from 9.5 to 25.0 mm NMAS. SMA mixes were inves- trend suggested that in-place air voids decreased as the lift
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TABLE 15 Field project summary information
Fine or Average Lift Actual Lift AC
Project Coarse Thickness Thickness/ Performance
ID NMAS Gradation (mm) NMAS Ratio Grade Ndesign
1 9.5 Fine 48.7 5.1:1 70-22 65
2 19.0 Coarse 65.7 3.5:1 64-22 65
3 9.5 Coarse 32.3 3.4:1 64-22 65
4 12.5 Fine 68.6 5.5:1 * 75
5 9.5 Fine 41.0 4.3:1 70-22 100
6 12.5 Coarse 50.3 4.0:1 58-28 75
7 9.5 Fine 40.6 4.3:1 64-28 75
8 19.0 Coarse 58.9 3.1:1 64-22 100
9 19.0 Coarse 96.4 5.1:1 64-22 100
10 19.0 Coarse 70.9 3.7:1 64-34 100
11 19.0 Coarse 38.0 2.0:1 64-34 125
12 25.0 SMA 42.6 1.7:1 76-22 50
13 25.0 Fine 70.0 2.8:1 67-22 100
14 9.5 SMA 26.8 2.8:1 76-22 75
15 19.0 Coarse 50.4 2.7:1 76-22 100
16 12.5 Coarse 43.8 3.5:1 67-22 86
17 12.5 Fine 43.3 3.5:1 64-22 75
18 12.5 Coarse 44.5 3.6:1 67-22 75
19 9.5 Fine 41.5 4.4:1 67-22 75
20 12.5 Fine 34.5 2.8:1 67-22 80
* Designated RA295 by the agency
thickness increased. An ANOVA conducted for the com- NMAS mixes, as well as for the individual mix types. For the
bined regression indicated that the relationship was signifi- combined data, the regression produced a low R2 value (0.09).
cant (p-value = 0.001). The data were then separated into the An ANOVA performed on the regression determined that the
different mix types to see if the relationship was significant relationship between t/NMAS and in-place air voids for the
for each mix type. For the fine-graded mixes, the relationship 19.0-mm NMAS mixes was significant (p-value of 0.000).
was significant (p-value = 0.000). The coarse-graded mixes The data indicate that an increase of 1 for the t/NMAS results
did not have a significant relationship between in-place air in an average decrease of 1.0 in the air voids.
voids and t/NMAS (p-value = 0.932). These data indicate In summary, even though there is a large amount of scatter
that an increase of 1 for the t/NMAS resulted in an average in the data for the three NMAS mixes, the results suggest that
decrease in air voids of 0.5 percent. the air voids dropped 0.5 to 1.0 percent for each increase of 1
Figure 44 shows the relationship between lift thickness and in the t/NMAS. This shows the importance of making sure
in-place air voids for the combined data set for the 19.0-mm that the t/NMAS is sufficiently high.
16.0
All Data y = -0.5809x + 10.792
14.0
Fine y = -0.0017x + 7.9818
Coarse y = -0.0488x + 9.1967
12.0
SMA y = -3.0894x + 16.837
In-place Air Voids, %
SMA Coarse
10.0
Fine
8.0
6.0
All Data
All Data: R2 = 0.09, p-value = 0.014
4.0 Fine: R2 = 0.00, p-value = 0.956
Coarse: R2 = 0.00, p-value = 0.994
2.0 SMA: R2 = 0.17, p-value = 0.107
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
Thickness-to-NMAS Ratio
Figure 42. Relationship between t/NMAS and in-place air voids--9.5 mm, all data.
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16.0
All Data y = -0.5205x + 10.079
14.0 Fine y = -0.6277x + 11.1
Coarse y = 0.0412x + 7.2391
12.0
In-place Air Voids, %
10.0
Fine
8.0
6.0 Coarse
All Data
2
4.0 All Data: R = 0.19, p-value = 0.000
Fine: R2 = 0.41, p-value = 0.000
2.0 Coarse: R2 = 0.00, p-value = 0.932
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
Thickness-to-NMAS Ratio
Figure 43. Relationship between t/NMAS and in-place Air Voids--12.5 mm NMAS.
16.0
14.0
12.0
In-place Air Voids, %
10.0
8.0
6.0 All Data (All Coarse)
y = -1.0455x + 11.09
4.0 R2 = 0.0913
p-value = 0.020
2.0
0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Thickness-to-NMAS-Ratio
Figure 44. Relationship between t/NMAS and in-place air voids--19.0 mm NMAS.
