effect of moisture on the fines in the base course mix.
Mr
The higher the fine content, the more pronounced is
[MPa]
the effect. Janoo (1997) reported that base course ma-
terials containing more than 3% fines were prone to
500
thaw weakening (strength loss). Haynes and Yoder
(1963) found that granular materials with a degree of
saturation higher than 80% became unstable under re-
peated loading. Therefore, the base course design in
cold regions is a compromise between density (fines
200
content) and its permeability, since its strength is a
function of its internal friction, which in turn is a func-
tion of its density, gradation, and particle shape (Yod-
100
er and Witczak 1975). Increase in density is usually
Masonary Sand
Masonry Sand
obtained by increasing the fine content of the mix,
150 mm φφ
150 mm
which in turn may make it frost-susceptible.
400 mm φφ
400 mm
Finally, data on the effect of aggregate shape, tex-
50
ture, and angularity on base course performance are
limited. Shape, texture, and angularity can be quanti-
fied as specific measurements using petrological tech-
θ [kPa]
50
100
200
500
niques or indexed as a lumped parameter, such as the
angularity number, particle index (PI), etc. Details of
Figure 2. Resilient modulus as measured in 400-mm-
both techniques can be found in Janoo (1998). Janoo
and 150-mm-diameter specimens of coarse crushed
masonry. (After Sweere 1990.)
cited two studies; one was on the effect of crushed
base course material on creep strain and on the angle
of internal friction (Holubec and Wilson 1970). The
Most resilient modulus tests conducted on granular
other was on base course material type (granite, gravel,
materials have been with aggregates not larger than 19
and shale) on the resilient modulus (Barksdale and Itani
mm. Sweere (1990) concluded from his study on
1994). The results were used to infer the effect of aggre-
unbound granular bases that specimen size does influ-
gate shape, texture, and angularity and are presented
ence the measured resilient properties. He reported that
in Figure 3.
the resilient modulus from large-scale (400-mm diam-
The Vermont Agency of Transportation (VAOT) is
eter) testing, can be about 70% of the standard 150-
interested in determining the resilient modulus and
mm-diameter samples (Fig. 2). Specimen size is defined
strength characteristics of its subbase material. The
by maximum aggregate size. However, it has generally
focus is on the effect of the aggregate angularity on
been recognized that with base course materials hav-
the resilient modulus. VAOT defines the angularity of
ing a significant amount of large aggregates (> 25 mm),
its base/subbase material by visual identification of
scalping the aggregates to 19-mm maximum size
the number of fractured faces, a method commonly
changes the gradation and thus the material properties.
used by most state departments of transportation. The
For base course materials, it is recommended that resil-
study was conducted in two phases. In Phase 1, a liter-
ient modulus tests be conducted with specimens larger
ature review was conducted on the various methods
than 150 mm.
available for quantifying or indexing the shape, tex-
The effect of density on resilient modulus seems to
ture, and angularity of coarse aggregates. (For the sake
be dependent on material type. Kolisoja (1997)
of brevity, "angularity" will include particle shape,
reviewed the literature and reported that some studies
surface texture, and angularity of the aggregate, unless
concluded that the effect of density on resilient modu-
otherwise noted.) Also, any available laboratory or
lus was insignificant (Thom 1988). Others, such as
field test results were documented.
Hicks and Monismith (1971), Allen and Thompson
From Phase I, VAOT decided to use the PI as an
(1974), and Rada and Witczak (1981) reported that in
indicator of the angularity of the base material. The PI
many cases the resilient modulus increased with den-
(Ia) is calculated using the formula
sity. The effect of moisture content on the resilient mod-
ulus of base course materials is through an interaction
Ia = 1.25V10 0.25V50 32
(2)
of the moisture content with the gradation and fines con-
where V10 = % voids in aggregates at 10 strokes per
tent. Initially, an increase in moisture content will
layer and V50 = % voids in aggregates at 50 strokes
increase the resilient modulus. As moisture content is
per layer.
further increased (increase in degree of saturation), the
The PI test initially was developed for three differ-
modulus will decrease. This is due to the lubrication
2
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