Page 395 - Mechanics of Asphalt Microstructure and Micromechanics

Simulation of Asphalt Compaction 387

compactibility of asphalt concrete and developed the R f factor as defined in the fol-
lowing equation.
P − R τ 04 .
R = LD tcb ⎛ h⎞ [N.mm.sec] (11-19)
n
v⎠
⎝
f η ⎜ ⎟
m
where P = weight of the roller (N)
L = width of the roller drum (mm)
D = diameter of the roller drum (mm)
R t = roller type factor (dimensionless)
2
t cb = shear cohesion of bituminous mixtures (N/mm )
h m = viscosity of the compacted mixture (poise)
n = number of roller passes
h = thickness of asphalt layer (mm)
v = speed of the compactor (mm/s)

Vizi (1981) also used this approach to evaluate compactibility.

11.5 Microscopic Study of Lab Compaction Process

11.5.1 Void Distribution and Variation Using XCT
A more recent systematic study at the microscopic level on lab compaction methods
including Marshall, Gyratory, and LCPC rolling-wheel-compactor (RWC) has been
achieved by Partl et al. (2007a, 2007b). In this study, specimens of the same mix were
compacted using the above compaction methods at the initial, medium, and final com-
paction. The void distribution in these mixes was evaluated using computerized to-
mography. Its discoveries include: 1) the void reduction in the Marshall compaction is
almost linear; LCPC RWC is non-linear and the gyratory compaction is somewhere
between; 2) the void distribution within the specimen is non-uniform; and 3) there is
significant lateral movement of the mixtures during the RWC compaction. The struc-
turally different specimens produced in each of the compaction methods indicate that
these methods may not be able to produce specimens whose structure is similar to
those produced in the field.

11.5.2 Other Experimental Studies
Masad et al. (2002) studied the gyratory compaction process using XCT. They analyzed
the void distributions of the gyratory specimens and discovered that void contents at
both the bottom and top of the specimens are much higher than the void content at the
middle. In that investigation, compaction energy and the dissipation process were also
investigated.

11.6 Perspective on Gradation Effect Representation
In applied research, empirical representation of compatibility is highly practical to ma-
terial and paving engineers. Development of these formulations will help practicing
engineers to assess the compactibility of a mix conveniently. While Section 11.4 presents