The Vermont Agency of Transportation (VAOT) attempted to quantify the resilient modulus
and strength characteristics of its subbase material. Currently, VAOT defines the angularity of its
base/subbase material by visual identification of the number of fractured faces, a method used by
most state departments of transportation.
The study was conducted in two phases. In Phase 1, a literature review was done to determine
the various methods available for quantifying or indexing the shape, texture, and angularity of coarse
aggregates. (For the sake of brevity, "angularity" will include the particle shape, surface texture, and
angularity of the aggregate, unless otherwise noted.) At the end of Phase 1, the particle index test was
identified as VAOT's choice for quantifying the angularity of its base courses.
In the second phase, described in this report, a study was conducted to relate the particle index to
the mechanical resilient and shear properties of base course materials. Resilient modulus and shear
tests were conducted on base course aggregate gradation meeting VAOT base course specifications.
It is well documented that the scalping of the larger stones and replacing with equivalent smaller
aggregates changes the structure of the base course and in turn affects the resilient and shear proper-
ties. Tests were conducted on large-scale, 300-mm-diameter, 750-mm-height and standard 150-mm,
300-mm-height samples at ERDC/GSL in Vicksburg, Mississippi. In addition to the mechanical tests,
a comparative study was conducted at the Quebec Ministry of Transportation (QMOT) on the effect
of specimen size on the moisture density relationship. The tests were conducted in a 300-mm-diameter,
450-mm-height mold. The energy applied in the compaction process was similar to that applied on
AASHTO-T99 Standard Proctor test samples. On the average, density was found to be about 12%
higher from the large-scale QMOT tests than from the AASHTO T99 tests. The optimum moisture
contents for both tests were approximately the same.
Results from the 300-mm-diameter resilient modulus tests indicated that resilient modulus is a
function of the percentage of crushed aggregates and bulk stress. It was also found that, at lower bulk
stress levels, the resilient modulus of the natural aggregate mixture was higher than the 100% crushed
aggregate. The trend reversed when the bulk stress was greater than 300 kPa. This suggests that, at
lower depths in a thick (≥ 60 cm) base course layer, the lower half of the base course can be con-
structed with natural material. Results also indicated that the void ratio affected the resilient modulus
of aggregates containing 50% or less of crushed aggregates.
The resilient modulus of the 100% natural material was higher than the 100% crushed material
for the standard 150-mm-diameter samples. We believe that the effect of the larger stones (+19 mm)
significantly affected the resilient modulus, which was about 35 to 50% higher than that obtained
from the large-scale tests.
Angle of internal frictions ranged between 31 and 51 for the large-scale shear tests. The effect
of percent crushed material on the angle of internal friction was minimal at 50% and higher. How-
ever, there was a significant difference when the aggregate was 100% natural. The difference in the
The particle index as modified by the Michigan Department of Transportation used the complete
gradation and was a good indicator of the crushed (angular) content of a given base course gradation.
The particle index test appears to be a fair indicator of the resilient modulus. However, it may be used
to indicate the shear properties of the base course aggregate gradation.