1.0
σ1/σ3 = 6
Granite
Gravel
0.8
Shale
0.6
σ1/σ3 = 4
σ1/σ3 = 6
σ1/σ3 = 6
0.4
σ1/σ3 = 4
σ1/σ3 = 4
0.2
σ3 = 6 psi
0.0
2
4
0
6
8
10
12
Percent of Fines
Figure 23. Influence of material type, stress level, and percent fines on plas-
tic strain. (After Barksdale and Itani 1994.)
decreased by 50% (compared to the optimum
flow through a known orifice. This method is
moisture content) for river gravel at a bulk stress
fairly simple and has been used for fine aggre-
of 103 kPa. For undrained conditions, this reduc-
gates; it can be modified for coarse aggregates up
tion will be much higher. He also reported that
to 50 mm in diameter.
the amount of fines in the base course materials
Several research studies have shown that the
had a significant effect on the amount of plastic
shape, angularity, and roughness have a signifi-
strain (Fig. 23).
cant effect on base performance. Some of these
studies have shown that there can be up to 50%
change in resilient modulus of base materials due
SUMMARY AND CONCLUSIONS
to geometric irregularities. It is not conclusive
There are several methods for quantifying the
whether the resilient modulus is a good indica-
shape, angularity, and roughness of base course
tion of the effects due to changes in shape, angu-
aggregates. Methods developed by geologists can
larity, and roughness of the aggregates. However,
be classified as direct methods and involve actual
what is clear in the literature is that moisture and
measurements on the aggregates. Basic measure-
amount of fines have a significant effect on base
ments of lengths are used to quantify the shape of
course performance. Increasing moisture content,
aggregates. The angularity and roughness can
as during the spring thaw, can increase the failure
also be determined with complex measurements,
rate of the base course. Further study is needed to
but several charts have been developed for sim-
consider the combined effect of aggregate irregu-
plifying the process. These methods are more in-
larities, moisture, and fines content on base
volved and time consuming. However, they can
course performance.
be considered as standards.
Indirect methods for quantifying the shape,
RECOMMENDATIONS
angularity, and roughness have been developed.
The shape, angularity, and roughness are usually
The results of this review indicate that for base
combined together as it is fairly difficult to sepa-
course materials with maximum sizes ranging
rate the effects of the individual components.
between 25 mm and 50 mm, the particle index
Most indirect measurements are based on the
test is best suited for indexing the angularity of
change in the voids of a single-sized bulk mass of
the material. The term "angularity" as used in
aggregates. The tests are similar to one another
this recommendation includes the shape and sur-
and involve compaction of the aggregate mass in
face characteristics of the aggregate. One of the
a given level. Some of the indices developed are
drawbacks of using this test is that it is time con-
the angularity number, particle index, and rugos-
suming.
ity. Another index test involves the amount of
VAOT requirements state that any test method
time required for a given aggregate gradation to
used for characterizing the angularity of base
20
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