Aggregate Degradation In Bituminous Mixtures: Technical Paper

AGGREGATE DEGRADATION
IN
BITUMINOUS MIXTURES

* tiii

Technical Paper
AGGREGATE DEGRADATION IN BXTUMBSDUB MIXTURES

TOi K. B. Woods, Director January 30, 1963
Joist High-way Research Project

FROM; E. L. Michael, Associate Director Filet 2-8-3

Joint Highway Research Project Project: C-36-21C

Attached is a paper titled "Aggregate Degradation in Bituminous
Mixtures" Which has been authored by F. Moavenzadeh, formerly of our
staff, and W. H. Goetz. The paper was presented at the 1963 Annual
Meeting of the Highway Research Board in Washington, B.C., on January 30 •

The paper is a summary of the research performed by Mr. Moavenzadeh
under the direction of Professor Goetz -which "Has presented to the Board
several months ago. It is proposed that the paper be offered to the
Highway Research Board for publication.

The paper is presented to the Board for the record and for
approval of the proposed possible publication.

Respectfully submitted,

Harold L. Michael, Secretary

HIM/llsc

Attachments

Copy: F. L. Ashbaucher F. S. Hill R. E. Mills

J. R. Cooper G. A. Leonards M. B. Scott

W. L. Bolch J. F. Mclaughlin J. 7. Smythe

V. Bo Goetz R. D. Miles J. D. Haling
F. F. Havey E. J. Ybder

Technical Paper
AGCREG&3E lECSUU&aiCH US BOTfflG&XJB JUXKE^

by

F. Kosveaaadeh

and

W. 2. Goets

Joint Higltfiay Besearch Project
Files 2-8-3
Project: C-36-21C

Purdue University
Lafayette, Indiana

January 30, I963

Digitized by the Internet Archive

in 2011 with funding from

LYRASIS members and Sloan Foundation; Indiana Department of Transportation

http://www.archive.org/details/aggregatedegradaOOmoav

INTRODUCTION

A bituminous mixture is essentially a three-phase system consisting of
bitumen, aggregate and air. In order for such a mixture to serve its purpose,
it is compacted to a certain degree during construction. During its life,
the mixture is subjected to further compaction due to the action of traffic.
This further densification of a bituminous mixture under traffic may
produce progressive deterioration of the pavement, either by reduction of
voids to the point where a plastic mixture results, or by producing ravelling.
In either case, degradation of the aggregate may play an important role.

Compaction is an energy-consuming process, which results from the
application of forces to the mixture. The mixture withstands these forces
in many ways, such as by interlock, by frictional resistance, and by viscous
or flow resistance. T/uhen the applied forces have a component in any direction
greater than the resistance of the mat, the material will move and shift
around until a more stable position is attained. This rearrangement of
the material, especially the aggregate phase, causes a closer packing of
particles, a new internal arrangement or structure, and a higher unit weight.

The energy required for the relocation or rearrangement of particles
is provided by contact pressure, and the particles while adjusting to their
new locations are subjected to forces which cause breakage and wear at the
points of contact. This phenomenon, called degradation, reduces the size
of particles and changes the gradation of aggregate which in turn causes
a reduction in void volume and an increase in density. Any change in the
gradation of the aggregate in a mix causes an associated change in basic
properties of the bituminous mixture, namely, stability and durability.
In some mixtures the change of gradation due to degradation of aggregate
causes the asphalt present in the voids to be pushed out and an unstable

It was the purpose of this investigation, then, to evaluate the
degradation characteristics of aggregates in bituminous mixtures and to
analyze the factors which are effective in c ausing this degradation.
In so doing, the following factors were investigated: (l) type of aggregate,
(2) gradation of aggregate, (3) aggregate shape, (4) aggregate size,
(5) asphalt content, and (6) compactive effort.

MATERIALS AiMD PROCEDURE

Three kinds of aggregates were used in this study, dolomite, limestone
and quartzite. Their selection was based on a relatively wide range of
Los Angeles values and on petrographic structure. Table 1 includes data
on origin, specific gravity, Los Angeles value, and compressive strength,
while Table 2 shows a summary of petrographic analysis results for the
materials used.

An 85-100 penetration grade asphalt cement was used in this study.
The results of tests *n the asphalt are presented in Table 3.

The three gradations selected for this investigation are shown in
Table 4. They ranged from an open grading, consisting only of the top four
sizes, to a Fuller gradation for well-graded material. The maximum size
of all three gradations was 5 in. Figure 1 shows these three aggregate
gradations graphically.

The aggregates used for each specimen were batched by component
fractions according to the blend formula. A batch consisted of 1000 grams.
The blended aggregates for specimens containing asphalt were heated to
275° * 10°F. The asphalt was heated separately to 290° - 300°F. The mixing
was accomplished using a Hobart electric mixer modified with a special aslxLng
paddle and a scraper. The mixing continued for two minutes. For those cases
in which the aggregate was tested without asphalt, the aggregate was not heated
or subjected to the mixing operation with the Hobart mixer.

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Due to the fact that this study was solely a laboratory investigation,
a fundamental part of it was the selection of testing equipment which
would produce specimens similar to the pavement with respect to density and
structure. Many methods of compaction have been devised and used to
simulate field compaction in the laboratory. Most of these methods are
based principally upon the concept of equal density. Equal density without
regard to orientation and degradation of particles cannot produce representative
specimens and unfortunately there is no way to measure the structure of
specimens quantitatively. The only way in which it seems possible to compare
the structure of the compacted materials is to compare the forces involved
in producing the laboratory specimen and the field mat. The methods that
incorporate horizontal forces and apply shear to the specimen throughout its
depth would seem to be the most suitable ones. Therefore, of all available
methods, gyratory compaction appeared to be the most promising one to produce
specimens similar to the field mat from the density and structure standpoint.

A gyratory testing machine of the design shown in Figure 2 was used
in this study. With this equipment it was possible to change the compactive
effort in two different ways, (1) change in magnitude of load, and (2) change
in repetition of load. The magnitude of load, controlled by vertical pressure,
was varied from 50 to 250 psi, and the repetition of load, controlled by
the number of gyrations, ranged from 30 to 250, for the most part, but in
some cases up to one thousand gyrations were used.

The mixtures were brought from the mixing temperature to 230°F and
were placed in the gyratory machine for compaction. Electric heating elements
around the mold were used to provide an elevated temperature throughout the
test. After each mix had been subjected to the gyrating action, an extraction
test was made on the whole specimen and the gradation of the extracted
aggregate was determined for comparison with the gradation before mixing and
compaction.

FIG. 2 GYRATORY TESTING MACHINE

In order to study the effect of shape of particles on degradation, it
was desirable that the rounded pieces not differ from the crushed ones in
their composition. Therefore, artifically rounded pieces were produced
by subjecting angular pieces to a few thousand revolutions in a Los
Angeles machine. See Figure 3.

To investigate how various sizes of aggregate degrade in an aggregation
of pieces of different sizes, the three top sizes were dyed different colors
so that after compaction and extraction of asphalt the newly-produced pieces
could be associated with the original piece by colored faces. For this
purpose the dyes had to be soluble in water, stay on the surface of the piece,
and not be soluble in asphalt or the trichloroethylene used in extraction.
The following dyes were found to have such characteristics: (l) Orseillin
BB Red, (2) Crystal Violet, (3) Malachite Green Oxalate.

RESULTS

Of the several methods available to represent the degradation characteris-
tics of aggregate, two were chosen for this study; one was a simple gradation
curve of percent smaller than certain sizes, and the other was based on surface-
area concepts. Using the surface area concept, measurements of the degradation
were made on the basis of surface-area increase as determined by sieve analysis.
The factors used for computing surface areas are given in Table 5 for an assumed
specific gravity of 2.65. These values were calculated on the assumption that
all material passing the No. 4 sieve was spherical and that retained was one-
third cubes and two-thirds parallelepipeds with sides of 1:2:4 proportions.

It was decided that numerical increase in surface-area, which is merely
the difference between the final surface area and the original surface area,
is not a satisfactory measure of aggregate degradation. For example, when a
mixture with an original surface area of 2.2 cm /gr has increased 2.2 cwr/gv

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in surface area after compaction, and another mixture with 67.3 cm2/gr has
increased the same amount, we cannot consider that the two mixtures have under-
gone equal degradation. The first mixture has gained 100 percent in surface
area or, in other words, its final surface area is twice the original, while
the second mixture has increased only 3 percent in surface area. Therefore,
it was decided to express the data in percent increase in surface area rather
than increase in surface area. Another advantage of the percentage method
is the elimination of the necessity for correction of surface area values for
specific gravity.

The term degradation is used in this study to include all of the aggregate
breakdown due to mechanical action regardless of the type of mechanical action
causing it. Degradation can result from aggregate fracture or breakage through
the piece, from chipping or corner breakage, and f rom t he rubbing action of
one piece or particle against another. In parts of this study, attempts were
made to separate degradation into two parts, one due to fracture through the
piece and designated as breakage, and the other due to corner breakdown and
attrition which collectively has been designated as wear.

Degradation of One-sized Aggregate
Size of particles and maximum size of particles are cited in the literature
among the factors controlling degradation. In order to determine whether or
not change of size will change the degradation characteristics of an aggregate,
and in order to investigate the effect of combinations of pieces of different
sizes on degradation, specimens of one-sized aggregate were tested. The results
are presented in Table 6. This table includes the results of sieve analysis
together with percent increase in surface area for 12 specimens. Specimens
containing one thousand grams of one-sized aggregate of g" - 3/8", 3/8" - #3*

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#3 - #4 * and #4 - #6 of each of the three aggregates, dolomite, limestone and
quart zite, were compacted in the gyratory compactor under 200 psi ram pressure
and 100 revolutions.

Figure 4 shows the results of sieve analysis on specimens made of lime-
stone aggregate. These results show that regardless of size of aggregate, all
the curves appear to be approaching a parabolic shape. A plot of the data
in Table 6 for the other two aggregates would show that this statement can be
made with respect to type of aggregate as well. The results also indicate
that as original size of particles decreases there is a corresponding increase
in fine material, which might suggest that degradation increases as size of
the particle decreases. Figure 5 presents the percent increase in surface area
versus average size of original particles for the three kinds of aggregate.
This figure shows that as the size of one-sized aggregate increases, the
degradation under equal compactive effort (200 psi and 100 revolutions)
increases.

Therefore, at first glance it appears that the results of the two methods,
sieve analysis and percent increase in surface area, are in conflict. Clarifi-
cation lies in the fact that sieve analysis representation only indicates what
percent of material is of which size, without considering through what changes
this material has gone and what was its original condition. A piece of larger
size has to undergo more breakdown than a smaller particle to be reduced to a
certain size. Therefore, it can be seen that sieve analysis representation,
although it is an excellent means for studying the pattern of degradation,
by no means can be used as a measure of degradation and the concept of percent
increase in s urface area, obtained by relating the produced area to the
original area, is a much better means of measuring degradation.

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Figure 5 also shows that degradation increases from quartzite to lime-
stone to dolomite, which follows the same pattern as indicated by the Los
Angeles rattler test. In other words, degradation of one-sized material
increases as the material becomes weaker and softer (higher Los Angeles
value ) .

Figure 6 shows the percent increase in surface area for different
original one-sized fractions versus Los Angeles values of the three kinds
of aggregate. This figure indicates that there is a linear relationship
between the Los Angeles values of the three kinds of aggregate used in this
study and the degradation of the one-sized aggregate when tested in the
gyratory compactor and measured in percent increase in surface area.

The effect of change of compactive effort on the degradation of one-
sized aggregate was studied by changing the number of revolutions of gyra-
tory compaction. Five specimens of each kind of aggregate having an original
size of 3/8" - No. 3 were compacted under 100 psi ram pressure and five
different numbers of revolutions in the gyratory machine. Table 7 gives
the results of sieve analysis and percent increase in surface area for each
specimen. Figure 7 shows the results of sieve analysis of dolomite aggregate
after compaction* These results also indicate that the general shape of
the gradation curve is not changed by a change in compactive effort; as
compactive effort increases the curve shifts upward. Figure 8 shows the
degradation versus number of revolutions. It can be seen that as compactive
effort increases the degradation also increases, but generally a significant
portion of the degradation occurs under the first few hundred revolutions
and then the curves start leveling off. The figure also indicates that as
the material becomes softer or weaker, the slope of the latter part of the
curves increases, which indicates that the degradation of such materials is. more
susceptible to change in compactive effort.

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FIG. 6 DEGRADATION VS LOS ANGELES VALUE- GYRATORY
COMPACTION, ONE-SIZED AGGREGATES

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Degradation of Individual Sizes in an Aggregation of Sizes

From the previous section it was found that degradation of one-sized
aggregates when illustrated by sieve analysis curves has a constant pattern
of a smooth curve approaching a parabolic one. It also was found that size
of aggregate, kind of aggregate, and degree of compaction have no influence
on the shape of the sieve analysis curve, while the magnitude of degradation
is a function of these variables. In addition it was found that; the larger
the size of particles, the greater the degradation; increase in compactive
effort increases degradation; and aggregates with high Los Angeles values
degrade more than those with low Los Angeles values.

Before making a detailed analysis of the effect of variables on degrada-
tion of different mixtures, it was necessary to investigate the changes which
might occur in degradation characteristics of each size of particle due to the
presence of other sizes in the specimen. For this purpose, a dyeing process
was utilized to determine the size fraction from which each particle was
produced when degradation occurred. Because it was found from studies on single-
sized aggregates that kind of aggregate only changes the magnitude of degradation
and has no effect on its pattern, it was decided to use only one kind of
aggregate for this part of the study. The limestone which had the intermediate
Los Angeles value and which could be satisfactorily dyed was used. Due to the
time-consuming process of separating the fractions of different colcrs by hand,
it was decided to dye only the top three sizes; namely l/2" - 3/8", 3/8"-#3,
and #3 - #4. If a difference in pattern of degradation due to the size was
noticed, then other sizes would have been dyed also. The materials were
separated only down to the #30 sieve. The factors which were considered as
variables in this part of the study were gradation of aggregate, compactive
effort, and presence or absence of asphalt.

18

The three gradations which are given in Table 3, gradings 0, B, and F,
were used in this part of the study. Twenty-four samples were used which
were of three gradations, without asphalt and with k percent asphalt, and were
tested under four different compactive efforts in the gyratory machine. The
results of sieve analysis of each fraction (colored for identification), along
with sieve analysis of the total specimen are presented in tabular form in
Tables 8, 9, 10, 11, 12 and 13.

Figure 9 shows the sieve analysis of each fraction of a specimen without
asphalt having an original open gradation and being subjected to 200 psi ram
pressure and 100 revolutions in the gyratory compactor. From left to right
the curves show the degradation of particles of original sizes of l/2"-3/8",
3/8" - #3> ff3 — #4, and ftU - ti&° These curves indicate that the degradation of
each fraction has a constant pattern of a smooth curve approaching a parabolic
one. Figures 10, 11, and 12 which show the sieve analysis of each fraction
for specimens with four percent asphalt and original gradings 0, B, and F, also
indicate that the pattern of degradation of each fraction is a constant.

From the results obtained with the aid of colored aggregate it can be seen
that, when particles of different sizes are mixed together and subjected to a
certain compactive effort, each size will break down into smaller particles whose
new gradation has a characteristic size distribution. The produced size distri-
bution follows a curve which is smooth and approaches a parabolic one similar
to the curves obtained for specimens made of one-sized aggregates tested separ-
ately. Therefore, this portion of the study indicated that degradation of one-
sized particles follows a definite pattern regardless of its size or the gradation
with which it is associated, magnitude of compactive effort, or presence of
asphalt. Also, from the first part of the study it was found that the degradation
pattern is independent of kind of aggregate. Hence ; it can be concluded that when the

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pattern of degradation of each fraction is constant, then the combination
of particles of different sizes will have a pattern which depends only on
the blending ratios of these sizes rather than on type of aggregate or magni-
tude of compactive effort.

Thus, it can be stated that if pattern of degradation is a matter of
concern, which is the case in ore treatment and in mining and metallurgical
engineering, then this pattern can be predicted beforehand by knowing the
gradation of feed material. But if magnitude of degradation is a matter of
concern, additional variables have to be investigated thoroughly before any
prediction can be made concerning this factor. In other words, in addition
to gradation, the magnitude of degradation in a degradation process is dependent
upon compactive effort, shape of particles, and type of rock even though these
factors do not affect its pattern. For example, a change of gradation will
not eliminate production of a certain size of particles when particles of
larger size than this size are produced. The change in gradation will reduce
or increase each size in such a proportion that the final gradation of each
fraction will follow a smooth curve approaching a parabolic one. However, this
change of gradation will change the magnitude of degradation, because the
magnitude of degradation depends on energy consumed for breakage. So any factor
affecting the breakage energy will affect the magnitude of degradation. For
example, higher compactive effort corresponds to higher breakage energy and
thus has to result in higher degradation. But the pattern of degradation is
not energy dependent and can be considered as a constant c

Since, for any original gradation, the pattern of degradation is constant,
and it is only the magnitude of degradation which varies with other factors,
we can deduce that the effects of degradation on the properties of a given

24

bituminous mixture have to be due to the magnitude of degradation. Therefore
in the detailed study which follows only the magnitude of degradation has been
considered, and attempts are made to find which factors are more effective in
reducing the magnitude of degradation and what protective measures can be
taken against degradation of aggregate in bituminous mixtures.

Effect of Mixture and Compaction Variables
In this portion of the investigation, the magnitude of degradation,
measured by percent increase in surface area, was determined for the three
types of aggregate, dolomite, limestone, and quart zite. Three gradations,
grading 0, grading B, and grading F, were used. Compactive effort applied
by the gyratory compactor was changed both in ram pressure and number of
revolutions. For this purpose 450 specimens were formed and tested, the
asphalt was extracted, and a sieve analysis made on the dry aggregate from
which the percent increase in surface area for each specimen was calculated.
Tables 14, 15 and 16 present data for the percent increase in surface
area for each of the three kinds of aggregate. Each value is for a specimen
whose original gradation, percent asphalt, and effort used in testing it can be
read from the table. Similar data for specimens made of rounded quartzite are
given in Table 17.

Ram Pressure and Number of Revolutions
Figure 13 illustrates the percent increase in surface area versus number
of revolutions for specimens made of limestone with zero and 4 percent asphalt.
All specimens were made of grading 0. The ram pressures are indicated on each
curve. This figure shows that degradation increases very rapidly in the first
part of the test and then continues to increase at a decreasing rate until

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about 250 revolutions after which the rate of increase remains constant in each

case. It can also be noticed that as ram pressure increases the degradation

in the first few revolutions increases drastically. For a ram pressure of

250 psi, almost 70 percent of the degradation that occurred at 1000 revolutions

had occurred in the first hundred revolutions, while at 50 psi ram pressure

only 50 percent of the degradation had occurred in the first hundred revolutions.

Figures 14 and 15 show degradation versus ram pressure for specimens
made of limestone with zero and 4 percent asphalt, In this case the results
for all three gradings are shown „ Degradation on the ordinate is plotted on
a log scale, while ram pressure on the abscissa is plotted to an arithmetic
scale. Gradation designations of original mixtures are shown at the left side
of the curves. These figures indicate that degradation increases both with
increase in ram pressure and increase in number of revolutions. This means that
degradation increases with increase in compactive effort.

In Figures 16 and 17 degradation is plotted versus number of revolutions,.
Each curve is for a single ram pressure as indicated on the curve. In these
figures degradation for each gradation is plotted on different scales, and
from left to right the results are for gradings 0, B, and F, respectively.
These figures also indicate that as compactive effort increases degradation also
increases.

It can be seen that when ram pressure was kept constant and compactive
effort was increased only by the number of revolutions, the increase in degradation
depended on type of aggregate and gradation of aggregate. The softer and weaker
the aggregate (higher Los Angeles value) the greater was the increase in degra-
dation caused by increase in number of revolutions, while the harder (lower
Los Angles value) the aggregate the less was the increase in degradation from

27

100 200 250

Ram Pressure in psi
FIG. 1 4 DEGRADATION VS RAM PRESSURE FOR
LIMESTONE- 0% ASPHALT

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200

100 200 250

Ram Pressure in psi

FIG. 15 DEGRADATION VS RAM PRESSURE FOR
LIMESTONE-4% ASPHALT

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this cause. These figures also show that increase in degradation caused by-
increase in number of revolutions depends upon gradation. The slopes of curves
for open-graded mixtures are much steeper than those for dense-graded ones.

Type and Gradation of Aggregate
Even more pronounced than the effect of compactive effort is the effect
of the original gradation of the mixture on the degradation of aggregate. It
can be noted from Figures 14 and 15 that as gradation becomes more dense,
degradation decreases,, Open-graded mixtures which contain only the four top
sizes of aggregate produced the highest degradation for all three kinds of
aggregate, at all compactive levels, and for all asphalt contents. At the same
time, grading F which corresponds to Fuller's gradation for maximum density
gave the lowest values of degradation under the same conditions. Although it
isn't at once apparent because a log scale has been used to plot degradation,
it should be noted that open-graded mixtures experienced some twenty times
more degradation than dense-graded mixtures under the same conditions.

Figures 16 and 17 indicate that the amount of degradation also depends
on kind of aggregate. The softer and weaker (higher Los Angeles value) the
aggregate the more the degradation. The curves for dolomite always lie above
the curves for the other two kinds of aggregate. However, the effect of aggre-
gate softness and strength on degradation also depends on gradation of the
mixtures e For example, in Figure 16, the change in degradation due to kind
of aggregate is a matter of a few hundred percent for the case of the open-
graded mixtures, while for the dense-graded mixtures this change is around
50 percent at most.

Cognizance of the scale of degradation for each gradation in Figures
16 and 17 makes one aware that original gradation of aggregate has a very

32

pronounced effect on magnitude of degradation. Degradation for open-graded
mixtures (grading 0) ranges from 100 percent to 1400 percent depending on
the type of aggregate and compactive effort, while for dense-graded mixtures
(grading F) this range is between 5 and 40 percent, or only about l/20 to
l/35 pf the values obtained for open-graded mixtures. This indicates that
the original aggregate gradation is the most important factor in degradation,
because the results indicate that changes in compactive effort, changes
in kind of aggregate, or changes in aggregate shape (as discussed later),
did not produce as much change in degradation as changes in original gradation.
This point can easily be related to the previous finding with regard to
mechanism of degradation. In a previous section it was said that magnitude
of degradation cepends on distribution and magnitude of forces applied to the
specimen. When a dense mixture is used the number of contact points is
numerous and any applied force will be distributed to many more points in much
less intensity than for more open mixtures, which in turn produces much less
breakage. In open mixtures the number of contact points are few, and particles
are subjected to much higher contact pressures, which in turn causes much
more breakage than in dense-graded mixtures.

Asphalt Content
Figure 18 illustrates the effect of change in asphalt content on degra-
dation for the three gradings of limestone aggregate. This figure, as well as
the results for the other two kinds of aggregate, indicates that depending on
compactive effort, kind of aggregate, and gradation of aggregate there is in
general an asphalt content for which the degradation is minimum. The results
also indicate that asphalt content is not an independent variable with respect
to degradation as was shown to be the case for kind of aggregate and aggregate

33

<£>

CVI

D9^y aoDpns u| asoajoui iiiaojad

34

gradation. For an independent factor, such as kind of aggregate, it could be
said that when aggregates become softer and weaker the degradation increases
regardless of other variables, but for the asphalt content variable there is
no such trend.

This result may be viewed with respect to the role of asphalt in the
mechanism of degradation. It was found that magnitude of degradation depends
on distribution of load and intensity of contact pressure. Considering asphalt
as a viscous material which covers the particles, its effect on degradation
may be influenced by the effect of its viscosity on magnitude of contact pressure,
Also, for a particular arrangement of particles and a particular condition
of load the asphalt may help the particles to rotate and slip over each other.
Rotation and slippage of particles will increase the probability of wear of
corners of particles and will also increase the probability of obtaining a
denser mixture. If these effects result in an increase in contact pressure,
degradation will increase, but if the effect is to reduce contact pressure,
degradation will be decreased. Since these effects of asphalt change as the
specimen undergoes densification, the net result is a complex one in which no
definite pattern for effect of asphalt on degradation is apparent.

Aggregate Shape
In order to investigate the effect of aggregate shape on degradation, a
limited number of tests were performed on specimens made of rounded pieces
of quartzite. Table 17 contains the percent increase in surface area for
such specimens. The same gradings (0, B, and F) as used before were used in
this part of the study„ The levels of compactive effort used were 100, 200,
and 250 psi ram pressure, and 30, 100, and 250 revolutions. Eighteen speci-
mens of each grading were tested, half of them without asphalt and the other

iey 0

35

half with 4 percent asphalt. Therefore, a total of 54 specimens were used.
Figure 19 presents the results obtained from specimens with 4 percent
asphalt. The degradation of rounded and angular quart zite are compared.

This figure shows that curves for rounded aggregate lie below those
for the angular material. Also, both the flatness and spacing of the curves
for rounded pieces are less than those for angular ones, indicating that
increase in compactive effort produces less degradation in the case of rounded
aggregate regardless of whether the increase is due to pressure or number of
revolutions. The cause of this phenomena can be attributed to the reduc-
tion, in the case of rounded aggregate, of that part of degradation which
is due to wear rather than breakage. V.ear phenomenon occurs due to the rounding
off of corners of particles when they rotate or slip over each other.
Breakage occurs when the contact pressure between two particles exceeds their
strength, resulting in fracture or splitting. Theoretically, by using rounded
particles we should be able to eliminate that portion of degradation due
to wear. Practically, however, we can only reduce this portion rather than
eliminate it, because when particles start to break, the newly produced
pieces are no longer rounded and wear starts to occur.

This reasoning leads to the conclusion that the major part of the
difference between degradation of rounded and angular particles can be considered
as reduction of wear. Figure 19 shows that the rounded aggregate experienced
almost 50 percent less degradation than the angular one, which then can be
considered as almost 50 percent less wear. This reduction of degradation due
to the shape of particles should decrease as softer material is used, because
in soft aggregates probability of breakage is high and, thus, after few
applications of load, the amount of angular pieces should increase and wear
start o This was one reason that in this portion of the study the quart zite
which had the lowest Los imgeles value was used.

:■ ■ ■!':-.;■*•[■ i

36

D9J\/ 9DD^jns ui asDajoui juaaod

37

Degradation Versus Los Angeles Value
In order to see whether there is any relationship between the Los
Angeles value and degradation of aggregate, degradation values were plotted
versus the Los Angeles values for the three kinds of aggregate used in this
investigation. Among the three gradings used for the Los Angeles test
(Table l), grading C was used to determine the correlation between Los Angeles
value and degradation merely because the maximum size of grading C is the
closest to the maximum size used in this investigation.

Figures 20, 21, and 22 show the results obtained from testing gradings
0, B, and F respectively. Each curve is for a certain number of revolutions
which can be read on the curve. The three points on each curve are the results
obtained from specimens made of the three kinds of aggregate tested under
equal efforts.

Figure 20 shows that as the Los Angeles value increases the degradation
value also increases, but the rate of increase is not constant, and the
relationships are not linear until the compactive effort is about 200 psi ram
pressure and 250 revolutions. Below this level of compactive effort the
Los Angeles machine produces more degradation for soft or weak aggregate than
the gyratory machine, while above 250 revolutionr oiore degradation is experienced
by the less resistant material in the gyratory compactor than in Los Angeles
machine because the curve for 500 revolutions is concave rather than convex.
Figure 21 shows that for grading B this linearity occurs somewhere between
200 psi ram pressure and 250 revolutions, and 200 psi ram pressure and 500
revolutions, while Figure 22 shows that such linearity was not reached for
specimens with grading F under compactive efforts used in this study.

38

1200

1000

800

0)

o
o

H-

</>
O

2?

o

c

c

§ 600

Q_

400

200

Grading 0

0% Asphalt

200 psi

500 Rev.

250

olOO

60

30

22

34

26 30

Los Angeles Value for Grading C

FIG. 20 DEGRADATION VS LOS ANGELES VALUE,
GRADING 0, 200 PSI

39

140

120

S 100
<

o

o

M—

L.

C
0)

80

60

40

Grading B

0% Asphalt

200 psi

500 Rev.

p250

o 100

o 60

30

34

22 26 30

Los Angeles Value for Grading G
FIG 21 DEGRADATION VS LOS ANGELES VALUE,
GRADING B, 200 PSI

40

Grading F

o500 Rev.

0% Asphalt

40

200 psi

/ o250

o y^

ol00

o

0)

/^

< 30

/

y^ 0 60

/

(D
O
O

/

in

/ / o /^

' o 30

Percent increase
o

0 /
1

o

o /

/ / °

o / /

10

o /

l
i

1

i

o

i

1

1

1
__1

22 26 30 34

Los Angeles Value for Grading G
FIG. 22 DEGRADATION VS LOS ANGELES VALUE
GRADING F, 200 PSI

41

The foregoing discussion indicates that, depending on gradation of the
aggregate, there is a certain level of compaction for which the plot of
degradation versus Los Angeles value of the aggregate is a straight line.
For compactive efforts higher than that, soft and weak aggregates experienced
more degradation in the gyratory machine than in the Los Angeles machine,
and for compactive efforts below that soft and weak materials experienced
more degradation in the Los Angeles machine. Therefore, as far as degradation
is concerned, depending on the gradation of the material, the Los Angeles
test corresponds only to a certain level of compaction. This level of
compaction, as can be seen in Figures 20, 21, and 22 increases as gradation
of material becomes more dense. Noting that these levels of compaction,
especially in dense-graded materials, are much higher than those the material
is normally subjected to in the field, imposes some doubts on the validity
of the Los Angeles test as a measure of quality of aggregate with respect to
degradation. This becomes especially apparent when it is noted that the
dolomite aggregate with a high Los Angeles value (Figures 16 and 17) when
tested in a Fuller gradation produced less than one-tenth of the degradation
under equal compactive effort of that produced by the low Los Angeles value
quartzite when tested in the open gradation.

It was mentioned before that degradation occurs due to two phenomena,
wear and breakage. Wear was considered responsible for that portion of
degradation which is caused by rotation and slippage of particles over each
other, while breakage was considered to occur when the contact pressure
exceeds the strength of the particle in a certain direction. Thus under
traffic compaction the particles either break or rotation wears off their
corners,, In either case the result is production of particles of smaller

42

sizes. These two actions, rotation and breakage will result in a denser packing,
thus producing a mat whose particles have more contact points and less chance
for rotation. This reduces the rate of degradation under further compaction.
But in the Los Angeles rattler test the particles do not experience this dense
packing or cushioning effect which occurs in a road mat and consequently the
material is subjected to a more severe degradation condition than actually
exists in the field.

Petrographic analysis
A comparison of petrographic analysis (Table 2) with degradation and
Los Angeles values of the materials reveals that nature of grain boundaries,
cementation, and percent of voids influence the resistance of aggregates to
degradation. Good interlocking between the grains present in limestone,
results in a low Los Angeles value and low degradation. Loose interlocking,
present in dolomite, results in a high Los Angeles value and high degradation.
In quartzite strength is due to silica cementation^ which results in a compara-
tively strong and resistant rock. If the material had not been highly
stressed, this strong cementation would have resulted in a very low Los Angeles
value. But the directional weakness due to cracking and fracturing makes the
material susceptible to impact breakage, which may be the reason for its high
Los Angeles value as compared to the nature of its cementation. The results
also show that degradation increases as percent voids of the material increases.

.[.:' ,■■ ■JJ.flr

UrL iiG-I

43

CONCLUSIONS
The results obtained from this study appear to justify the following
conclusions. It should be realized that they are specifically applicable
only to the particular kinds of aggregate used in this study. Furthermore,
it should be noted that all the tests were performed in the laboratory,
and there exists no field correlation study to specifically evaluate the
field behavior of the materials. Also, it has to be noted that all con-
clusions and recommendations deal with degradation characteristics of
mineral aggregate. Protective measures suggested in this study are made
only with respect to the reduction of aggregate degradation without con-
sidering their effects on other properties of mixtures.

1. Vnithin the range of the materials and procedures used in this
study, there appears to be a unique pattern for degradation of
each aggregate fraction of a bituminous mixture. This pattern
does not vary with kind of aggregate, compactive effort,
presence of asphalt, or original gradation of the mixture.

2. The magnitude of degradation of a bituminous mixture, as measured
by percent increase in aggregate surface area, depends on the fol-
lowing factors; kind of aggregate, gradation of the aggregate,
compactive effort, and shape of particles. The effect of asphalt
on the magnitude of degradation is dependent on other factors

and cannot be considered as an independent variable.

3. Physical characteristics of the aggregate, as reflected by its
Los Angeles value or by petrographic analysis, has a dominant
effect on degradation. Mineral aggregates with low Los Angeles
values will produce less degradation than those with high Los

44
Angeles values. Rocks with good interlocking or cementation
between grains are more resistant to degradation than others.

4. From the results of tests on mixtures ranging in gradation from
open to dense, tested with compactive efforts ranging from low
to high, it can be concluded that some aggregates having a

Los Angeles loss greater than the minimum commonly specified
may, from the standpoint of degradation, be satisfactory
materials especially if used in dense gradings subjected to
low compactive effort.

5. Gradation of the mixture is the most important factor controlling
degradation, as the gradation becomes more dense, degradation
decreases. The magnitude of this decrease is much greater than
that brought about by changes in other variables. Soft or weak
materials with high Los Angeles values can produce much less
degradation than hard and strong materials if the former are
used in dense-graded mixtures and the latter in open mixtures.
Therefore, from a degradation point of view, dense-graded
mixtures offer the best use of local aggregates with high

Los Angeles values.

6. Increase in compactive effort results in increase in degradation
of the mixture regardless of the form of this increase in effort,
but degradation is more susceptible to change in magnitude of
load than to change in repetition of load. The rate of change
in degradation is high during the initial part of the appli-
cation of compactive effort, and thereafter becomes less as

the compactive effort is increased.

k5

7. When the degradation of rounded particles is compared with that
of angular particles of the same kind of aggregate, the rounded
aggregate can be expected to produce less degradation because
of a reduction of that portion of degradation which is due to
wear. Use of rounded material will be helpful in reduction of
degradation providing its use does not impair other properties
of the mixtures.

46

LIST OF REFERENCES

1. Aughenbaugh, N. B., Johnson, R. B,, and Yoder, E. J., "Available Information
on Aggregate Degradation (a Literature Review)," Purdue University, April
1961, (unpublished).

2. Bond, F. C, "The Third Theory of Comminution," Transactions , American
Institute of Mining Engineers, Vol. 193> 1952.

3. Charles, R. J., "Energy-Size Reduction Relationships in Comminution,"
Transactions , American Institute of Mining Engineers, Vol. 208, 1957.

4. Collet, F. R., Warnick, C C, and Hoffman, D. S., "Prevention of
Degradation of Basalt Aggregates Used in Highway-Base Construction,"
Proceedings , Highway Research Board, Vol. 41* 1962,

5. Cook, F. C„, "Report of the Road Research Board," Department of
Scientific and Industrial Research, London, England, 1935.

6. Croeser, H. M. W„, "Bituminous Mixtures," Unpublished M.S. Thesis,
University of Witwatersrand, Johannesburg, South Africa, 1944.

7. Day, H. L., "A Progress Report on Studies of Degrading Basalt Aggre-
gate Bases," Proceedings;, Highway Research Board, Vol. 41, 1962.

8. Ekse, M. and Morris, H. C, "A Test for Production of Plastic Fines

in the Process of Degradation of Mineral Aggregates," Special Technical
Publication No„ 277, American Society for Testing Materials, 1959.

9. Endersby, V. A. and Vallerga, B. A., "Laboratory Compaction Methods and
Their Effects on Mechanical Stability Tests for Asphaltic Pavements,"
Proceedings , The Association of Asphalt Paving Technologists, Vol. 21,
1952.

10. Erickson, L. F. , "Degradation of Aggregate Used in Base Courses and
Bituminous Surfacings," Circular 416, Highway Research Board, March
I960.

11. Erickson, L. F., "Degradation of Idaho Aggregates," Pacific Northwest
Soils Conference, Moscow, Idaho, February 1958„

12. Faust, A. S., Wengel, L. A,, Clump,, C, W„, Maus, L„, and Anderson,

L. B., "Principles of Unit Operations," John Wiley & Sons, Inc., I960.

13. Goetz, W. H., "Flexible Pavement Test Sections for Studying Pavement
Design," Proceedings , Thirty-Seventh Annual Purdue Road School, 1952.

47

14. Goldbeck, A. T., "Discussion on the Los Angeles Abrasion Machine,"
Proceedings , American Society for Testing fete rials, Vol. 35, Part II,
1935.

15. Goldbeck, A. T., Gray, J. E., and Ludlow, L. L., Jr., "A Laboratory
Service Test for Pavement Materials," Proceedings , American Society
for Testing Materials, Vol. 34, Part II, 1934.

16. Gross, J., "Crushing and Grinding," Bulletin No. 402, U. S. Bureau
of Mines, 1938.

17. Gross, J. and Zimmerlgy, S. R. , "Crushing and Grinding," Transactions ,
American Institute of Mining Engineers, Vol. 87, 1930.

18. Havers, J. A. and Yoder, E. J., "A Study of Interactions of Selected
Combinations of Subgrade and Base Course Subjected to Repeated Load-
ing," Proceedings. Highway Research Board, Vol. 36, 1957.

19. Herrin, M. and Goetz, W. H., "Effect of Aggregate Shape on Stability
of Bituminous Mixes," Proceedings , Highway Research Board, Vol. 33,
1954.

20. Holmes, J. A., "A Contribution to the Study of Comminution - A Modified
Form of Kick's Law," Transactions, Institute of Chemical Engineers,
Vol. 35, 1957.

21. Idaho Department of Highways, "Standard Method of Test for Degradation
of Aggregates," T-15-58, State of Idaho, Boise, Idaho, 1958.

22. Laburn, R. J., "The Road Making Properties of Certain South African
Stones," Unpublished M.S. Thesis, Part II, University of Witwatersrand,
Johannesburg, South Africa, 1942.

23. Macnaughton, M, F. , "Physical Changes in Aggregates in Bituminous
Mixtures Under Compaction," Proceedings , The Association of Asphalt
Paving Technologists, Vol. 8, January 1937.

24. Mather, B., "Shape, Surface Texture, and Coatings," Special Technical
Publication No, 169, American Society for Testing Materials, 1955.

25. McLaughlin, J. F., "Recent Developments in Aggregate Research," A paper
presented at the IV World Meeting of the International Road Federation,
Madrid, 1962.

26. McRae, J. L and Foster, C. R. , "Theory and Application of a Gyratory
Testing Machine for Hot-Mix Bituminous Pavement," Special Technical
Publication No, 252, American Society for Testing Materials, 1959,

27. Minor, C. E., "Degradation of Mineral Aggregates," Special Technical
Publication No. 277, American Society for Testing Materials, 1959.

28. Nevitt, H. G., "Compaction Fundamentals," Proceedings , The Association
of Asphalt Paving Technologists, Vol. 26, 1957,

48

29. Pauls, J. T. and Carpenter, C. A., "Mineral Aggregates for Bituminous
Construction," Special Technical Publication No. 83, American Society
for Testing Materials, 1948.

30c Piret, E. L., Kwong, J. M. , Adams, J. T., and Johnson, J. F., "Energy-
New Surface Relationship in the Crushing of Solids," Chemical Engineer-
ing Progress, Vol. 45, 1949=

31. Rhodes, R„ and Mielenz, R, C, , "Petrographic and Mineralogic Charac-
teristics of Aggregates," Special Technical Publication No. 83,
American Society for Testing Materials, 1948.

32-. Scott, L, E., "Secondary Minerals in Rock as a Cause of Pavement and
Base Failure," Proceedings, Highway Research Board, Vol. 34> 1955.

33. Shelburne, T. E«, "Crushing Resistance of Surface-Treatment Aggregates,"
Engineering Bulletin 8 Purdue University, Vole 24, No. 5, September 1940.

34. Shelburne, T. E„, "Surface Treatment Studies," Proceedings, The Asso-
ciation of Asphalt Paving Technologists , Vol,, 11, 1940.

35. Shergold, F„ A0J "A Study of the Crushing and Wear of Surface-Dressing >
Chippings Under Rolling and Light Traffic," Research Note No. RN/2298/FAS,
B. P. 397, Road Research Laboratory, London, 1954»

36. Turner, R. S„, and Wilson, J = Ds, "Degradation Study of Some Washington
Aggregates," Bulletin No, 232, Washington State, Institute of Technology,
1956.

37. U„ S. Army, Corps of Engineers, Waterways Experiment Station, Vicks-
burg, Mississippi, "Development of the Gyratory Testing Machine and
Procedures for Testing Bituminous Paving Mixtures," Technical Report
No. 3-595 . February 1962,

38. Woods, K. B. , "Highway Engineering Handbook," Section 16, "Distribu-
tion, Production, and Engineering Characteristics of Aggregates," by
McLaughlin, J„ FQ, Woods, K0 B„, Mielenz, R. C., and Rockwood, N. C,
McGraw-Hill, i960.

39. Woolf, D. 0., "Results of Physical Tests of Road Building Aggregates,"
Bulletin, Bureau of Public Roads, 1953.

40. Moavenzadeh, F„, "A Laboratory Study of the Degradation of Aggregates in
Bituminous Mixes," Thesis submitted in partial fulfillment of the
requirements for the Ph.,D„ degree, Purdue University, July, 1962
(unpublished).

V J. ,

' : c

:-tr-:

#

TABLE 1

RESULTS OF LOS ANGELES ABRASION
AND COMPRESSIVE STRENGTH TESTS*

Los Angeles Abrasion

Grading *">:~

Type of Aggregate A B.

Dolomite 40.0 41.0 33.0

Limestone 26.7 25.0 27.5

Quartzite 22.0 23.7 24.9

Compressive Strength PST**-"-

Size

of Spe<
Inches

:imen

Type of Aggregate

1,

,0

x 1.0 x

1.0

1.

,0

x 1.0 x 2.0

Dolomite

10,100

8,500

Limestone

15,000

14,300

Quartzite

25,200

29,600

* Each value is the average of three tests
** According to aSTH Method C 131
*** Rate of loading .025 in/min

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51

TABLE 3
ORIGINAL GRADATIONS
Percent Passing

Sieve

Grading

1/2"

100.0

3/8"

75.0

£3(1/4")

50.0

#4

25.0

#6

0.0

#8

#12

#16

#30

#50

#100

#200

ding B

Grading F

.00.0

100.0

86.0

86.6

62.0

70.7

50.0

61.2

45.0

51.4

36.0

43.3

25.0

36.3

16.0

30.0

11.0

22.0

6.0

15.0

4.0

10.9

3.0

7.7

52

table 4

RESULTS OF TESTS ON ASPHaLT CEMEWT

Specific Gravity, 77/77°F 1.032

Softening Point, Ring and Ball, °F 114.0

Ductility, 77°F, cm. 200 +

Penetration, 100 grams, 5 sec, 77°F 90

Penetration, 100 grams, 5 sec, 32°F 20

Flash Point, Cleveland Open Cup, °F 600

Solubility in CC1, , percent 99.8

53

TABLE 5
SURFACE -AREA FACTORS

Fraction of Material Factor

Passing Retained Sq. cm. per gram

2.2

3.2

4.5

5.7

7.9

12.7

30.0

100 o0

205.0

615.0

Note: Assumed sp,, gr„ = 2,65. For values other
than 2„65j multiply the above factors by
2.65
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65

T..BLE 17

PERCENT INCREASE IN SURFACE AREA
Rounded Quartzite

Original
Grading

G
Grad

ing 0

Grading B

Grading F

Rev.

% AS

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% Asphalt

% asphalt

PSI

0

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116.0
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0.7
3.2
6.0

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178.0
212.0

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173.4
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23.5
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2.6
4.8
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2.5
5.5

8.0

250

30
100
250

128.0
185.0
231.0

175.0
215.0
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13.3
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29.0

23.3
27.5
32.0

2.9
5.7
8.6

4.5
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