(Thompson and Smith 1990). They reported the
LABORATORY RESPONSE OF
range to be about 200 to 240 MPa under a bulk
ANGULAR MATERIAL
stress of 138 kPa. Ishai and Gelber (1982) also
From static triaxial tests, it has been shown that
found similar trends for bituminous mixes. They
rounded aggregate materials produce significant-
suggested that this was to be expected because in
ly higher permanent deformation than angular
the resilient modulus test, the applied stress levels
aggregate soils. This is reflected in the angle of in-
are very low and only with near-failure conditions
ternal friction (φ). During compression, it is com-
will the effect of the geometric irregularities be ev-
monly found that rounded particles are able to
ident; for example, in terms of particle interlock-
slip easily, whereas angular materials have to
ing. However, the deformation under the same
overcome the higher frictional forces at the contact
applied stress level, when the base course is satu-
interfaces. Generally, φ increases with increasing
rated, will be high, and the angularity and rough-
angularity (Holtz and Kovacs 1981). Holtz and
ness of the aggregates will have a greater effect on
Kovacs (1981) also found that as the surface
pavement performance.
roughness increased, so did φ. The effects on φ by
There is very little information in the literature
angularity, surface roughness, and other factors
on the performance of base course layers as a func-
are shown in Table 14.
tion of the shape, angularity, and roughness of the
aggregates. The results of only two studies on the
Table 14. Summary of factors affecting φ. (After
effect of aggregate geometry and surface rough-
Holtz and Kovacs 1981.)
ness were found in the literature. One was by Hol-
ubec and Wilson (1970) and the other by Barksdale
Factors
Effect
and Itani (1994). Holubec and Wilson (1970) used
e↑φ↓
Void ratio (e)
crushed gravel and crushed stone in their study.
A↑φ↑
Angularity (A)
They looked at the effects of angularity and the
Cu↑φ↑
Grain size distribution
proportion of crushed material on base course per-
R↑φ↑
Surface roughness (R)
formance. The samples were compacted at an
w↑φ↓ slightly
Moisture content (w)
optimum moisture content of 4.5% using a 4.5-kg
Particle size (S)
No effect (with constant e)
hammer dropped 457 mm and applying 25 blows
φps ≥ φtx
Intermediate principal stress
per layer. The test samples were 102 mm in diame-
Overconsolidation or prestress
Little effect
ter and 203 mm in height. The maximum aggre-
gate size was 9.5 mm.
Holubec and Wilson (1970) report in terms of
The information in Table 14 relates to static
the cyclic creep strain. The definition of cyclic
loading conditions. However, the loading condi-
creep strain is unclear in the report and is taken by
tion on a pavement structure is not static, but cy-
this author to refer to the total strain minus the
clic. Also, the load levels applied are usually not
elastic strain. They found that the cyclic creep
near the shear strength of the material except
strain after 5000 load repetitions decreased as the
when the material is saturated, as is the case dur-
percent of crushed particles increased (Fig. 18).
ing spring thaw. The data in Table 14 may or may
The samples were made by blending different
not apply to pavement structures. Very little data
amounts of crushed aggregate with the parent
are published on the performance of base course
rounded gravel aggregates. With respect to sphe-
materials under cyclic loading. This is probably
ricity and angularity, they reported that, as the ag-
due to the erroneous assumption that failure of
gregates became angular, the cyclic creep strain,
base course materials does not occur.
after 5000 load repetitions, increased (Fig. 19). The
With the advent of mechanistic pavement de-
results appeared to be contradictory, as one would
sign procedures, several studies have been pre-
expect the creep strains to decrease with increas-
sented on the resilient modulus of base materials.
ing angularity. The results may be explained by
Base materials have been identified as stress de-
the fact that there is a larger contact area with
pendent and several models exist for predicting
rounded material than with angular material.
the resilient modulus of base materials, such as
Thus it will require a larger shear force at the
the K1K6 model in SUPERPAVE (Lytton et. al
points of contact to move the aggregates. This
1993) and the "universal" K1K3 model (Witczak
could also explain why the resilient modulus re-
and Uzan 1988). However, the resilient moduli re-
ported in the literature for rounded and angular
ported in the literature for natural gravel and
materials are similar. This is only speculation and
crushed rock were quite similar to one another
16
Previous Page
