petrologists, the shape of a coarse aggregate can
based image analysis has been developed for anal-
be described by its length, width, thickness, sphe-
yzing aggregate shape, angularity, and rough-
ricity, roundness, and angularity. Aggregate clas-
ness. A feasibility study conducted for the Federal
sification charts to evaluate shape, roundness,
Highway Administration (FHWA) on this tech-
and angularity of aggregates are shown in Figures
nology (Wilson et al. 1995) showed that image
2, 3, and 4. Surface texture is more difficult to deter-
analysis is a viable tool for distinguishing the
mine. Traces of magnified surfaces have been
shape and angularity of fine sands (manufac-
used to describe surface texture (Wright 1955).
tured vs. natural). The method involves first cap-
Others, such as Barksdale and Itani (1994), devel-
turing magnified images of the aggregates using
oped roughness scales to describe the texture of
a high-resolution video camera, and then using
an image analysis program to identify and separate
aggregates.
the objects and trace the edges of the aggregates.
For fine aggregates, shape and angularity can
Based on the traces, algorithms in the programs
be described with image analysis. Computer-
are used to determine the different characteristics
of the aggregates. The Quebec Ministry of Trans-
1.0
DISC
CUBIC
portation (QMOT) routinely uses image analysis
=
0.
95
of the HMA fine aggregates to distinguish round-
ness and angularity of the fine aggregates. Addi-
0.8
ψ
tional descriptions of this method can be found in
=
0.
Janoo (1998).
9
An alternate approach taken by engineers is to
0.6
infer these characteristics from the mass properties
of the aggregates. Several indices such as angu-
ψ=
0.8
larity number, time index, particle index, and
0.4
ψ=
rugosity have been identified in the literature.
0.7
For coarse aggregates, the angularity number
ψ= 0
(AN) developed by Shergold (1953) is recom-
5
0.2
.6
1.
ψ = 0.5
F=
mended by the British Standards Institution
.0
ψ = 0.4
F=3
(1989) for indexing the angularity of natural and
BLADE
ROD
crushed aggregates used in concrete. This tech-
0.0
0.0
0.2
0.4
0.6
0.8
1.0
nology can be easily transferred to HMA. Sher-
Short/Intermediate = p
gold found that when the aggregates were com-
pacted in a prescribed fashion, the percentage of
Figure 2. Aggregate classification chart.
Roundness = 0.4
Roundness = 0.5
Roundness = 0.1
Roundness = 0.2
Roundness = 0.3
0.3 0.4
0.4
0.5
0.5
0.4
Roundness = 0.9
Broken Pebbles
Roundness = 0.6
Roundness = 0.7
Roundness = 0.8
Figure 3. Roundness chart for 16- to 32-mm aggregates.
3
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