Civil Engineering Reference
In-Depth Information
mixing of the asphalt and aggregates at a temperature where the kinemat-
ic viscosity of the asphalt cement is cSt and compacting tem-
perature corresponds to a viscosity of cSt.
The Asphalt Institute permits three different levels of energy to be
used for the preparation of the specimens: 35, 50, and 75 blows on each
side of the sample. Most mix designs for heavy-duty pavements use 75
blows, since this better simulates the required density for pavement
construction.
170
;
20
280
;
30
4.
Marshall Stability and Flow Measurement
The Marshall stability of
the asphalt concrete is the maximum load the material can carry when
tested in the Marshall apparatus, Figure 9.32. The test is performed at a
deformation rate of 51 mm/min. (2 in./min.) and a temperature of 60°C
(140° F). The Marshall flow is the deformation of the specimen when
the load starts to decrease. Stability is reported in newtons (pounds)
and flow is reported in units of 0.25 mm (0.01 in.) of deformation. The
stability of specimens that are not 63.5 mm thick is adjusted by multiply-
ing by the factors shown in Table 9.11. All specimens are tested and the
average stability and flow are determined for each asphalt content.
5.
Density and Voids Analysis
The values of VTM, VMA, and VFA are
determined as using Equations 9.8, 9.9, and 9.10, respectively.
6.
Design Asphalt Content Determination
Traditionally, test results and
calculations are tabulated and graphed to help determine the factors that
must be used in choosing the optimum asphalt content. Table 9.12 pre-
sents examples of mix design measurements and calculations. Figure 9.33
shows plots of results obtained from Table 9.12, which include asphalt
content versus air voids, VMA, VFA, unit weight, Marshall stability, and
Marshall flow.
T ABLE 9.11
Marshall Stability Adjustment Factors
Approximate Thickness
Adjustment
Approximate Thickness
Adjustment
of Specimen, mm (in.)
Factor
of Specimen, mm (in.)
Factor
50.8 (2)
1.47
65.1 (2 9/16)
0.96
52.4 (2 1/16)
1.39
66.7 (2 5/8)
0.93
54.0 (2 1/8)
1.32
68.3 (2 11/16)
0.89
55.6 (2 3/16)
1.25
69.8 (2 3/4)
0.86
57.2 (2 1/4)
1.19
71.4 (2 13/16)
0.83
58.7 (2 5/16)
1.14
73.0 (2 7/8)
0.81
60.3 (2 3/8)
1.09
74.6 (2 15/16)
0.78
61.9 (2 7/16)
1.04
76.2 (3)
0.76
63.5 (2 1/2)
1.00
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