Macro-Surface Voids
Specific rugosity index
Tons and Goetz (1968) developed the
packing volume concept to characterize the
shape, angularity, and roughness of the ag-
gregates used in bituminous mixtures. The
test was developed for both the coarse (12.7-
mm max) and fine fractions. They decided
Micro-
that the shape of the particle could possibly
Surface
Packing
Voids
be quantified as a separate value; however,
Porosity
Voids
it was difficult to separate the interaction of
angularity and roughness on aggregate per-
formance. They proposed that the effect of
Solids
both angularity and roughness be combined
and considered in one term, "rugosity."
The assumption is that the volume of an
Packing Volume Membrane
individual particle in an aggregate mass de-
termines the density and voids in the bulk. Figure 13. Packing volume, packing porosity, and geometric
This volume, called the "packing volume," irregularity of aggregate particles. (After Ishai and Tons 1977.)
had to account for the volume of the solid
particle, the volume of internal voids, and the vol-
ume of "outside voids." The "outside voids" vol-
The apparent specific gravity of the aggregates
ume is made up of the volume of the dips and
can be calculated using ASTM Test for Specific
valleys on the aggregate surface (Fig. 13). The
Gravity and Absorption of Coarse Aggregate (C
packing volume can be imagined as a membrane
127). Gpx is determined from the pouring test
around the aggregate.
developed by Ishai and Tons (1977). The test in-
The packing volume membrane divides the
volves taking two one-sized particles and pour-
voids into two components, the interparticle voids
ing them into a standard container using a stan-
and particle surface voids (Ishai and Tons 1977)
dard procedure. One of the particles is used as a
(Fig. 13). For one-sized particles, it is assumed that
standard (smooth, spherical glass beads) with a
the interparticle voids are constant and this poros-
known packing specific gravity, Gps. The other is
ity is the same as that obtained from same one-
the test particle for which Gpx is sought. Gpx is a
sized smooth spherical particles. Tons and Goetz
function of the ratio of the weight of the test parti-
(1968) assumed that aggregate shape can be math-
cle to the standard particle:
ematically defined as ellipsoids. They found that
the porosity calculated from ellipsoids or spheres
ΣWx
Gpx =
Gps
of the same size had the same amount of voids.
ΣWs
Any difference, then, between the porosity of the
smooth spherical particles and the aggregates is
where Ws = weight of the standard
due to the irregularities of the aggregates.
Wx = weight of the test material
Ishai and Tons (1971) developed a specific rug-
Gps = packing specific gravity of standard
osity (Srv) index to express the total geometric ir-
material
regularity of the particle. Srv will be approximately
Gpx = packing specific gravity of test mate-
equal to zero for smooth, spherical particles:
rial.
Gpx
V
A schematic of the pouring test is given in Fig-
sr = 1001 -
Srv = 100
Gap
Vp
ure 14. The specifications for the pouring test ap-
paratus for the coarse aggregate fraction are giv-
en in Table 12.
where Srv = specific rugosity (%)
Some typical results as presented by Ishai and
Vsr = volume between the packing volume
Tons (1977) are shown in Figure 15. The specific
membrane and the volume of macro
rugosity of two sizes, 12.7 to 15.9 mm and #3 to
and micro surface voids
#4 sizes for natural gravel, crushed gravel, lime-
Vp = packing volume of the particle
stone, and beach pebbles are plotted. The specific
Gpx = packing specific gravity
rugosity increases as the material becomes more
Gap = apparent specific gravity.
13
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