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chemical contribution was captured by KE. These parameters 2.10 EFFECT OF CRUSHING OPERATIONS
had a physical basis and were a function of many factors that ON AGGREGATE PROPERTIES
affected the stiffening potential of fillers.
It was found that the volume-filling contribution to stiff- Barksdale (25) states, "Rock is broken or crushed when a
ening (m) was able to better distinguish between fillers with force is applied with sufficient energy to disrupt internal
"good" and "bad" performance in SMA in the cases studied. bonds or planes of weakness that exist within the rock." For
It was able to predict the performance accurately even in quarried aggregates, the crushing process begins with the
cases in which prediction by Rigden voids had failed. The blast that turns solid rock into particles of a size range that
authors concluded that the stiffness of asphalt by a filler can can be accepted by the primary crusher. The resulting parti-
be fully characterized by measuring the maximum packing cles from the initial blast are called "shot rock." Additional
fraction, m, and the generalized Einstein coefficient, KE, of crushing of shot rock or gravel is performed (1) to reduce the
the system. The data obtained show that m is a better pre- aggregate to product size; (2) to improve the aggregate
dictor of the performance of filler in SMA than Rigden voids. shape; and (3), in the case of gravel for HMA, to create frac-
The m denotes the maximum filler one can put into the tured faces. Aggregate is produced in a variety of sizes and
system and the volume-filling contribution to stiffening. for a variety of purposes besides HMA. In some cases, the
Mathematically, it is an asymptote at which the stiffening is properties desired for HMA may conflict with those desired
infinite. m is analogous to bulk density in many ways. KE is for another product.
the physicochemical contribution to stiffening. It is a mea- The ratio between the sieve size representing 80% passing
sure of the rate of increase in stiffness ratio with the addition of the crusher feed stock and the sieve size representing 80%
of fillers: the higher the KE, the higher the slope of the curves. passing for the product of the crusher is termed the "reduc-
tion ratio" (25). When processing aggregate, a 21 reduction
ratio will result in at least one fractured face (154). Therefore,
2.9.2 Summary of Research Related
to Fines and Fillers when determining the possible number of fractured faces for
gravel sources, the feed size and the resulting product size
It is widely believed that depending on the particle size, must be considered. It is impossible to create a 12.5-mm
fines can act as a filler or as an extender of asphalt cement crushed gravel having 100% of the particles with one frac-
binder. Some fines have a considerable effect on the asphalt tured face if the feed stock is only 19.0 mm.
cement, making it act as a much stiffer grade of asphalt Crushers reduce the size of aggregate particles through three
cement compared with the neat asphalt cement. Early work mechanisms: abrasion, cleavage, and impact (Figure 17) (25).
indicated that both the size of the filler and the asphalt binder Abrasion occurs in localized areas when insufficient energy
composition had an impact on the stiffening effect. As much is applied to the particle to cause significant fracture. Abra-
as a 1,000-fold increase in viscosity of the neat asphalt cement sion results in limited size reduction and the production of
was measured when certain fillers were added to asphalt fines. Cleavage results when the compressive forces applied to
cement. Some fines may also make HMA mixtures more sus-
ceptible to moisture-induced damage.
Numerous studies have evaluated the effects of fines,
filler, and mortar on HMA performance in the laboratory and
in the field. Efforts to characterize fillers have generally fol-
lowed three paths:
1. Characterization of particle size or packing. Several
research studies have been conducted to develop suitable
test parameters related to particle size or packing to eval-
uate the fines and fillers. D60 (the particle size of P200
at 60% passing) and methylene blue values were found
to be related to rutting, and D10 and methylene blue val-
ues to stripping. The modified Rigden voids test has
been used to characterize the stiffening potential of
baghouse fines.
2. Binder tests performed on a mortar. Superpave
binder tests, BBR, DSR, flexural creep, and direct ten-
sion tests have been used by several researchers to char-
acterize the fine mortar or voidless mastics properties.
3. Modeling of the overall interaction between the
filler and binder. Recent efforts involve modeling the Figure 17. Mechanisms of rock fracture (Figure from
physical-chemical interaction between fillers and binder. Kelley [155] published in Barksdale [25]).
