Table 16. AIF values
either translated laterally or truncated. This in
of test aggregates.
turn produces resilient and plastic responses that
may not be representative of the actual material.
Base material
AIF
If there is a sufficient amount of large aggregates
Granite gneiss
500
(greater than 10%), large-scale triaxial testing is
50
recommended. Besides aggregate geometry,
Shale
1675
Barksdale and Itani (1994) looked at the influence
Quartzite
150
of the fines, the plasticity of the fines, gradation,
River gravel
3700
and moisture content on base course perfor-
mance.
This AIF value is specific to the aggregates tested
To quantify the effect of the aggregate geome-
by Barksdale and Itani (1994) and is not recom-
try and surface roughness, Barksdale and Itani
mended for other aggregates. If this approach is
(1994) developed the aggregate influence factor
taken to characterize base course performance,
(AIF), which was based on a multiple regression
new AIF values will have to be developed for oth-
between observed laboratory performance and
er aggregates. The AIF values for the test aggre-
various indices that described the aggregate
gates are presented in Table 16. The AIF values
shape and texture. For the aggregates tested, they
decrease with increasing angularity and rough-
found that the AIF was a function of the spherici-
ness.
ty, roundness, surface roughness, and angularity
The resilient modulus test was conducted using
of the aggregate. They employed methods used
the procedure prescribed in AASHTO T-274-82.
by geologists to quantify sphericity and round-
For permanent deformation studies, the speci-
ness as described earlier in this report. For surface
mens were subjected to 41 kPa confining pressure
roughness, Barksdale and Itani (1994) used a
and subjected to 70,000 load repetitions at a prin-
scale from 0 to 1000 to quantify glassy to very
cipal stress ratio of four or six. Barksdale found
rough particles, respectively. They then rated the
the resilient modulus of rough angular materials
roughness of the aggregates based on visual ex-
to be higher than the rounded material by about
amination. For example, for the aggregates in the
50% at low bulk stress values and about 25% at
study, the AIF was as follows:
high bulk stress values (Fig. 22). This is much
higher than the values reported by Thompson
AIF = 2500* (ψ + R) (A + SR)
and Smith (1990). There appears to be some in-
where AIF = aggregate angularity factor
consistency in the results found in the literature.
ψ = average sphericity value
The effect of moisture was also pronounced.
R = average roundness value
Barksdale and Itani (1994) reported that under
drained conditions, the resilient modulus
SR = surface roughness coefficient.
100,000
Granite Gneiss (γd=141 pcf, w=6.2%)
Gravel (γd=126 pcf, w=3%)
Shale (γd=130 pcf, w=4%)
10,000
5000
10
100
4
Sum of Principal Stresses (psi)
Figure 22. Influence of material type and state of stress on resilient modulus.
(After Barksdale and Itani 1994.)
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