MIX DESIGN AND AGGREGATE REQUIREMENTS
FOR STONE MATRIX ASPHALT MIXTURES
RANDY CLARK WEST
DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
To my mother and father, who nurtured me, inspired me, supported me, and gave
me the confidence to venture toward my own goals.
To my girls: Ronda, Anna Grayce and Clara, who have lovingly and patiently
helped me reach this goal with my identity intact.
The author thanks all of those individuals who provided assistance during the
progress of this research.
The author is deeply appreciative of the many forms of support
provided by the Florida Department of Transportation including the funding for this study,
the educational leave granted to pursue this degree, and the courtesy of permitting much
of the work to be performed by fellow students at the Bituminous Laboratories of the
State Materials Office.
Those individuals from the State Materials Office which provided significant
contributions to this work were Gale Page, Edd Leitner, Herb Cone, Toby Dillow,
George Lopp, Aaron Turner, and Randy Burnett.
Much of the testing was performed by students from the University of Florida
including Ivo Moroni, Bouzid Choubane,
William Freeman, Raja Subramanian,
Mark Davies, John Veilleux, Howard Eshram, John Bertram and Shin-Che Haung.
TABLE OF CONTENTS
ACKNOW LEDGEM ENTS ................... ................... ................... ................... .* 4
ABSTRACT .................................... ............................... .... .. .. -.- .
Significance of Pavement M materials Research .............................................
Description of SM A M ixtures .......................... . .. ............. .... ...............
Issues to be Resolved .................................................................................
Objectives and Approach to Research ........................................................
Organization ................... ................... .................. ....... .................. ... .... ..
HISTORIC OVERVIEW OF SMA TECHNOLOGY
Design of SM A M ixtures .................................
The European Asphalt Study Tour ...................
Early SMA Projects in the United States ..........
Development of United States SMA Guidelines
S*. . .. .. . . .. *** ** * *
. . . 111 .. .. .( .m .
. . . . . .
REVIEW OF TESTING AND MIX DESIGN FOR SMA MIXTURES
Purpose of M ix D design ...................................
Current Methods of SMA Mix Design ............
Review of Mix Tests for SMA
TESTING PROGRAM ..........
Selection of Tests ...................................... ................... .............................
. . .. .. .1 i wo .*0 e .. *. ** .* *** *
. . 1 .. ... .. ... .. ... .. ... .. ... .. ... .. ---
. .. .. .. .. .. .. ... .. .. .. .. .. .. ... .. .. .. -- -- .-
Origin of Stone M atrix Asphalt ..................................................................
Comparison of SM A and DGAC M ixtures .................................................
Selection of M materials ................................................................................
EVALUATION OF M IXTURE TEST DATA ...........................................
GTM Compaction ......................................................................................
Design of Experimental M fixtures ...............................................................
GTM Densification/Shear Tests .................................................................
Georgia Loaded Wheel
Triaxial Repeated Load Tests .....................................................................
Correlation of Rutting Tests .......................................................................
Resilient M odulus Tests .............................................................................
Indirect Tensile Strength Tests ...................................................................
Diametral Creep Tests ...............................................................................
Tests ... . .... ..... . . . . . .. .. .......... . .
SMA AGGREGATE CHARACTERISTICS AND TEST METHODS ......
* **. **
. . *T k. f
*. ** .* .* *
The Aggregate Issues .................................................................
Review of Desired Aggregate Characteristics ..............................
Aggregate Characteristics Related to Shear Strength ...................
Aggregate Resistance to Degradation .................................... .....
Tests for Aggregates ...................................................................
Aggregate Specifications ............................................................
Test Procedures ..........................................................................
EVALUATION OF SMA AGGREGATE PROPERTIES ..........
Traditional Aggregate Properties ................................................
Particle Index .............................................................................
Crushing Strength .......................................................................
GTM Aggregate Strength and Degradation .................................
Correlation of Aggregate Characteristics to Mixture Test Results
ASSESSMENT OF TESTS AND MATERIAL
* .. ... .. **** *
. .* * . *
Assessment of Mixture Tests .......
Assessment of Study Mixtures .....
Assessment of Aggregate Tests ................... ........................................... ... . .
Assessment of Florida Aggregates ................... ................... ................... .....
IMPACT OF SMA MIXTURES ON FLORIDA .................... .. ................
Projecting SM A Usage ..............................................................................
Effects on M material Supply .........................................................................
CONCLUSIONS AND RECOMMENDATIONS ......................................
T ests ................................ ......... ... . ...... ..... ... .
. .......... ., .. *. ., > .* -* ** ** . ** ** . -
- . ** ** .* .* .* ** ** . ** .* .(* .
Cost Analysis ...................................
MIX DESIGN INFORMATION FOR FIELD MIXTURES .......................
BATCH SHEETS FOR EXPERIMENTAL FLORIDA MIXTURES .........
GYRATORY COMPACTION AND DENSIFICATION DATA ................
COMPARISON OF STABILIZING ADDITIVES FOR SMA MIXTURES
SM A Production ....................................................................................
SM A Placement ......................................................................................
BIOGRAPHICAL SKETCH .....................................................................
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
MIX DESIGN AND AGGREGATE REQUIREMENTS
FOR STONE MATRIX ASPHALT MIXTURES
RANDY CLARK WEST
Chairman: Byron E. Ruth
Major Department: Civil Engineering
Stone Matrix Asphalt (SMA) mixtures were developed in Europe where they have
proven to be very resistant to rutting, cracking, and polishing.
SMA mixtures are
characterized by a stone-on-stone aggregate structure filled with a rich binder-filler mastic.
Implementation of SMA in the United States began in 1991.
The traditional method of
design for SMA mixtures has utilized the Marshall procedure, but recipe formulations of
component percentages and restrictive material guidelines persist.
A better design method
was needed that could provide an engineering basis for the selection and optimization of
In this research study, SMA mixtures from several field projects and experimental
v S I *.1 44
compaction and shear strength testing with the Gyratory Testing Machine; a rutting test
using the Georgia Loaded Wheel Tester; a rutting test using a confined repeated-load
procedure; resilient modulus tests at low, moderate, and high temperatures; the indirect
tensile strength and strain test; wet/dry moisture damage tests, and a low-temperature
diametral-creep compliance test.
Each of the rutting tests do indicate that the SMA mixtures are generally very
resistant to permanent deformation.
However, the procedure used to evaluate fatigue
cracking indicated that many SMA mixtures were less resistant to fatigue than standard
The validity of this assessment is uncertain.
Tests on the mixtures
and the aggregates show that some of the softer Florida limestones tend to be more prone
to degradation which may lead to moisture damage and increase the potential for ravelling
or loss of skid resistance. A preliminary assessment of the impact that SMA may have on
the paving and aggregate supply industries indicates that due to limited application of this
technology to heavy-traffic-type roadways the effect will be very minor.
1.1 Significance of Pavement Materials Research
The quest to extend pavement life has great economic importance.
In the United
States, an estimated $10.5 billion of hot mix asphalt is produced each year for highway
construction and resurfacing pavements.
In Florida, approximately $150 million is spent
each year on hot mix asphalt placed on the state's highway system.
Most of that is used
for resurfacing of existing pavements that have reached their useful life .
Highway engineers must continually search for ways to improve the performance
The amount of traffic and the loads carried by our highway pavements has
steadily increased through the years.
It has become apparent over the past decade that
many of the materials and procedures that have been conventionally used to design
pavements are not suitable for the increase in traffic and heavy loads imposed upon them.
One technology that may have potential for improving the performance of heavily
loaded pavements is the European concept of stone matrix asphalt (SMA).
SMA mixtures have been heralded to have better highway performance than conventional
dense-graded asphalt concrete (DGAC) mixtures used in the United States.
SMA mixtures have been said to have three primary advantages: high deformation
These attributes are a result of several factors which distinguish SMA mixtures from
Description of SMA Mixtures
The most unique characteristic of SMA mixtures is the aggregate gradation.
mixtures are said to be gap-graded with a very high proportion of coarse aggregate and a
high mineral filler content.
The gap in the aggregate gradation of SMA mixtures occurs in
the size range between 4.75mm and 75 im, where little material is retained
the gap-gradation is evident in the core sample shown in Figure 1
The effect of
. The contact between
coarse aggregate particles has been described as a stone-on-stone skeleton which provides
the strength (i.e. rutting resistance) of the SMA mixture.
rca....-nr&'.- S V VS.-Eb
The other unique feature of SMA mixtures is the binder mastic.
In SMA mixtures,
the asphalt binder combines with the mineral filler and added stabilizers to form a stiffened
binder mastic to bond the aggregate particles together and provide durability to the
Several types of stabilizing additives have been used in SMA mixtures including
cellulose fibers, mineral fibers, and polymers.
Without a stabilizing additive the binder will
tend to drain to the bottom of the mixture during storage or transport between the hot-mix
production facility and the paving operation at the roadway.
The filler and stabilizing
additives stiffen the binder at high temperatures without sacrificing flexibility of the binder
to resist cracking.
Issues to be Resolved
There are, however, several issues regarding SMA mixtures that must be resolved
before the technology can be effectively applied in Florida and many other states.
most important issue is cost effectiveness.
European experience has been that SMA
mixtures cost ten to twenty percent more than the conventional DGAC mixtures [2,3].
The increased costs are attributed to several factors such as the use of premium quality
aggregates, the cost of additives, and slower production rates.
Compared with the current
DGAC mixtures, can SMA mixtures provide longer pavement service lives to offset the
higher initial costs?
In Florida, the economics of SMA mixtures will depend primarily upon the
European recommendations adopted by the Federal Highway Administration (FHWA) 
have been to limit SMA aggregates to very high quality materials.
The SMA aggregate
specifications suggested by FHWA are so restrictive that no native Florida aggregates
meet the requirements.
If it is determined that Florida's aggregates are not suitable for use
in SMA mixtures, it is expected that importing acceptable aggregates from out of state
would make SMA mixtures cost prohibitive in most of the state.
Another critical area of concern is the current practice of SMA mixture design.
Selection of materials and component proportions for SMA mixtures has been traditionally
based upon recipes and rules-of-thumb developed from experience in Europe.
optimum asphalt content for most SMA mixtures has been determined using only the
volumetric elements of the Marshall mix design method.
Considering the widely
recognized deficiencies of the Marshall procedure, it is not surprising that many SMA
designs are adjusted in the field after the start-up of production.
Possibly the most significant problem with the current practice of SMA mixture
design is the total absence of performance-related, mixture strength or deformation tests
which may indicate how well the mixture resists loading and environmental conditions in
Without such tests, it is impossible to compare the relative benefits of using
alternative materials or to evaluate expected performance of SMA mixtures versus dense-
graded mixtures. The primary reason for the lack of such testing is because SMA mixtures
L -_- ... -l. -- ,,* ^ k .* ** .. nn. 1 nrh a n f. u a A a a' ,n I.. n r4 a a^ nr^ rn na n a
are inadequate rather than the SMA mixtures.
Good mixture design tests should be able
to predict, or at least indicate, the mixture's performance.
1.4 Objectives and Approach to Research
It is important to research the viability of the SMA mixtures containing currently
Likewise it is also important to evaluate these SMA mixtures relative to
the standard dense-graded mixtures.
If it can be demonstrated that the current Florida
aggregates are suitable in SMA mixtures, and that SMA mixtures will perform better than
typical DGAC mixtures, the savings earned from longer lasting pavements may amount to
several million dollars each year.
One approach to this research could be to simply construct test pavements with
SMA mixtures and evaluate field performance.
However, this approach is very costly,
limited to the evaluation of only a few of the variables, and would probably take ten to
twenty years to fully evaluate all aspects of performance.
A more sensible approach is to
conduct the research in the laboratory using tests which are representative of field
conditions and clearly related to field performance.
Therefore, the objectives of this
project were developed to address two issues regarding SMA mixtures:
and aggregate requirements.
To evaluate improved mix design for SMA mixtures, a combination of mixture
tests were selected to assess the resistance of SMA mixtures to the four major types of
of Engineers Gyratory Testing Machine, rutting tests with the Georgia Loaded Wheel
Tester, and deformation resistance measured in triaxial repeated-load tests.
stripping resistance of SMA mixtures were evaluated using the indirect tensile strengths
and resilient moduli of conditioned and unconditioned samples.
potential was evaluated using indirect tensile creep and resilient moduli tests.
Current aggregate requirements for SMA mixtures were thoroughly reviewed.
Arguments of why certain aggregate requirements may or may not be necessary to ensure
good mixture performance are presented.
SMA aggregates from several field projects
were evaluated in terms of traditional aggregate tests and a couple of new tests felt to be
more indicative of performance.
In particular, a new test was developed to assess
aggregate degradation potential by comparing sample gradations before and after
compaction tests of dry aggregates and SMA mixtures in the Corps of Engineers Gyratory
A cost-effectiveness comparison of SMA mixtures and conventional asphalt
mixtures can not be accurately assessed until typical production costs and life cycles are
A brief discussion is presented to focus on the implementation of SMA
mixtures and its potential impact on the paving and materials industries in Florida.
the specific research objectives of this research are as follows:
Identify mixture tests which can be used to indicate the performance of SMA
Recommend an appropriate testing protocol for the design of SMA mixtures that
will enable the designer to choose the materials and proportions that will provide
the greatest resistance to the major forms of pavement distress.
Establish more rational limits for aggregate quality parameters which are necessary
to assure success in SMA mixtures.
Identify the aspects of the paving industry that will be most affected by the use of
SMA mixtures in Florida.
The scope of the research was established so that test results could be related to
This was accomplished by obtaining eleven mixtures (or materials) from
SMA projects constructed in six states between 1991 and 1993.
The laboratory test
results of these field proven materials and mixtures were used to establish a baseline of
results for use in the evaluation of Florida materials and experimental mixtures.
aggregate sources, typically used in asphalt mixtures and representative of the geologic
and geographic range of aggregates used in Florida, were selected for the research.
This dissertation has been organized to present the literature review, research plan,
testing, analysis, and findings in as clear and logical sequence as possible.
introductory chapter is a chapter on the historical development of SMA mixtures and their
A rxis7nz 7 nf -^MA mey
Inrr^rrln*/^l /^/^rao r ltfl r mll rta-F^^a^a c^rl a nm; l-i+r iri1/rn i -T-Ql / iciTA
of the materials and the mixture test procedures.
The results of the SMA mixture tests are
presented and analyzed in Chapter 6.
A review of
aggregate characteristics and aggregate
tests is presented in Chapter
. Chapter 8 consists of evaluations of the aggregate
properties and characteristics for the materials used in this study. Chapter 9 presents an
assessment of the mixture and aggregate tests covered in this study. Chapter 10 projects
the impact of SMA mixtures on Florida highway construction, the probable role and
extent of usage, and the changes that will be necessary in the hot-mix asphalt (HMA)
industry and in the aggregate industry.
Conclusions and recommendations are presented
in Chapter 11.
HISTORIC OVERVIEW OF SMA TECHNOLOGY
Origin of Stone Matrix Asphalt
SMA mixtures were developed over the past three decades in several of the
northern European countries.
Germany, where the SMA technology originated, has the
most experience, with over 14,000 lane miles of SMA pavement.
Germany placed nearly a million tons of SMA .
In 1989 and 1990,
Sweden, Denmark, Norway, the
Netherlands, Finland, Austria, France, and Switzerland all utilize the SMA concept to
Each country has different requirements and different terminology.
Splittmastixasphalt, grit mastic asphalt, stabinor, and HABS (hot asphalt with stone) are
all names synonymous with stone matrix asphalt as it has been titled in the United States.
The SMA concept was initially developed in the 1960s as a pavement surface
course to provide improved abrasion resistance under studded tires .
tires were banned from most European countries in the early 1970s, the use of the SMA
However, from the performance of the early SMA pavements, it was
recognized that the SMA mixtures were also very resistant to rutting and shoving under
Since most countries now prohibit studded tires, SMA mixtures have
become used primarily to provide a highly stable pavement surface.
In 1984, standard
European countries adopted slightly variant specifications for SMA mixtures in the late
In addition to the wear resistance and rutting resistance of SMA mixtures, several
other attributes have also been cited: improved durability [2,3,4], improved cracking
resistance [2,6], very good skid resistance , reduced water spray , and reduced
Comparison of SMA and DGAC Mixtures
Like conventional asphalt concrete, SMA mixtures are hot-laid, densely compacted
mixtures composed of about 94 to 95 percent aggregate and about 6 to 7 percent asphalt
by weight of total mix.
However, the primary difference between conventional DGAC
mixtures and SMA mixtures is the aggregate gradation.
As shown in Figures
and 3, a
typical aggregate gradation for DGAC consists of a uniform distribution of particle sizes,
whereas SMA mixtures utilize a gradation which is dominated by a single particle size,
typically between 9.5 mm and 4.75 mm, and contains few aggregate particles between
4.75 mm and 75 jpm.
Figure 4 illustrates the effect of gradation for an SMA mixture and a typical
DGAC mixture containing the same maximum aggregate size.
The obvious and important
detail evident from this figure is that the SMA gradation creates more contact between
coarse aggregate particles.
In the DGAC gradation, the large aggregate particles float in a
75un 300m em0pm 2.36mm 4.75mnm .Smn 12.Smm o.anmm
SIEVE SIZE (.45 POWER)
Plot of Typical DGAC and SMA Gradations
Typical Percentages of Aggregate Particle Sizes in SMA and DGAC Mixtures
The high percentage of essentially one-size coarse aggregate creates excellent particle
The durability of SMA mixtures is derived from the rich voidless mastic of asphalt,
filler, and stabilizer. SMA mixtures in Europe generally have higher binder contents than
dense-graded mixtures. The coarse aggregate gradation and the higher binder content
together yield a thicker binder coating on the aggregate particles which holds the particles
tightly together and provides greater resistance to binder aging.
Selection of Materials
In Europe, selection of materials for SMA mixtures is based primarily on
This applies to the selection of aggregates, fillers, stabilizers, and grades of
Most references state that premium-quality, crushed aggregates must be used.
Bellin  states that Germany requires "double crushed, tough, premium aggregates of
definite size, soundness, shape, etc." and that the aggregates must have a high Polished
Stone Value (PSV) for good skid resistance.
The term "double-crushed" is unclear, and
quantitative criteria corresponding to the descriptive requirements of "toughness" or
"soundness" were not cited. A requirement that aggregates contain less than 20 percent
flat and elongated particles was given. Aggregates commonly used in German SMA
mixtures are gabbro, diabase, basalt, and granite. Limestones, sandstones, and "soft"
index, and abrasion resistance .
Swedish SMA mixtures. Danish s
Quartzite is cited as a commonly used aggregate in
specificationss also require 100 percent crushed rock with
a minimum flakiness value and a minimum brittleness value .
In references from other
countries, specific aggregate requirements were not given although "crushed rock" was
often used to describe the coarse aggregate.
Requirements for the fine aggregates are also unclear.
Several references stated
that fine aggregate shall be manufactured sand [11,12], whereas others appeared to allow
manufactured sands, natural sands, or a combination of the two [10,13,14,15].
requirements for fine aggregates were given in the literature.
The most common type of filler used in European SMA mixtures appears to be
limestone dust [2,12].
No specific requirements are provided in the literature.
 suggests that the filler should be as fine as possible so that it embeds in the asphalt
According to Bellin , baghouse fines are not used much as filler in SMA mixtures.
A variety of stabilizers have been used in European SMA mixtures.
fibers were used in early SMA mixtures before studies indicated potential hazards with this
Cellulose and mineral fibers appear to now dominate the stabilizer market.
Cellulose fibers are used exclusively in Denmark and the Netherlands [10,13].
reportedly utilizes cellulose and mineral fibers on a equal basis .
uses cellulose fibers, but also permits mineral fibers, as well as siliceous acid and polymers
ra- L tl I 11 I. .. .-^ __ 1- t-- ....
European asphalt cements are graded by the penetration test.
utilize a 60 to 70 penetration asphalt for SMA mixtures [6,9,10,12].
In some cases a
lower penetration asphalt is used for heavy trafficked pavements [13,14], or higher
penetration asphalts are used for low volume roads or cold climates [9,10,12,13].
Design of SMA Mixtures
The European approach to design of SMA mixtures has been largely based upon
rules-of-thumb and recipes for selecting and proportioning components.
indicates that proportioning of aggregates has been controlled to a large degree by narrow
gradation bands developed from years of experience in each country.
Most of the
countries have several SMA gradations which are identified by the maximum particle size.
Germany, for example, has gradation specifications for SMA mixtures with
mm, 8 mm,
and 11 mm top size.
Sweden utilizes larger top sizes of 12 mm and 16 mm.
10 rule (30% passing the 4.75 mm sieve, 20% passing the 2.36 mm sieve, and 10%
passing the 75 Cjm sieve) also appears to be used to determine aggregate proportioning in
The practice of selecting the design binder content varies among the European
countries, although all appear to require at least 6.0 percent binder in SMA mixtures.
some countries, such as the Netherlands , a specific binder content for each gradation
is also specified as part of the recipe.
In Sweden  and Denmark , minimum binder
practice is apparently to fix the binder content and to vary the gradation to achieve
percent voids using the Marshall procedure.
2 to 4
Although none of the available European
references stated how many blows are used in the Marshall compaction, it is assumed by
nearly all U.S. references that 50 blows-per-face is used.
Other than air voids, the
European specifications did not identify other volumetric requirements, except for
Denmark which requires a minimum VMA of 16 and voids filled between 78 and 93
Stability and flow requirements are apparently not utilized by any of the
Several references [2,19],
in fact, cautioned that misleading conclusions could
be made by comparing low stability and high flow values obtained for SMA mixtures to
properties of dense-graded mixtures.
2.5 The European Asphalt Study Tour
In September 1990, a group of U
pavement specialists toured six European
countries to observe and learn about the asphalt pavement technologies, practices, and
experiences that could be brought back to the U.S.
The group consisted of 21 participants
from FHWA, the paving industry, state highway agencies, and research organizations.
The group was very impressed with the European pavements and shamelessly admitted
that U.S. pavements were inferior .
One of the reasons that many of the European
pavements performed so well was attributed to the durable SMA surface mixtures.
the return of the study tour, the heralded SMA technology has been aggressively
Early SMA Proiects in the United States
During 1991, four states (Wisconsin, Georgia, Michigan, and Missouri)
constructed SMA test sections. Since then the number of states and test sections has
Table 1 summarizes the SMA test projects constructed in the United
States. through 1993.
In 1994, the total tonnage of SMA mix
projected to be placed in
the United States was over 240,000 tons.
Development of United States SMA Guidelines
In the United States, guidelines for the selection of materials and the design and
construction of SMA mixtures have been developed by a Technical Working Group
(TWG) sponsored by the Federal Highway Administration and the National Asphalt
The SMA TWG is a panel of knowledgeable pavement engineers
that was organized to provide guidelines for the transfer of the European SMA technology
to the United States.
The SMA TWG has developed model specifications for the materials, design
requirements, and construction of SMA pavements.
Early versions of the model
specifications attempted to duplicate, as closely as possible, the specifications used in
However, this was not a simple task due to the fact that each of the European
countries appeared to have unique requirements and often used different test methods.
Since Germany and Sweden have the most SMA experience, their specifications were
. In the United States, it is not appropriate to specify particular
The development of the model specifications has therefore been an evolutionary
process in which experience with the technology and materials in the United States has
provided a basis for adjusting the requirements.
The current version of the model
specifications is divided into three areas: mix design, aggregate requirements, and
The specifications for each of these areas are presented in the
Table 1. U.S. SMA Test Sections and Projects Through 1993
YEAR STATE PROJECT LOCATIONS) TONNAGE
1991 WISCONSIN 1-94 1,000
GEORGIA I-85 3,000
MICHIGAN M-52 3,000
MISSOURI I 75, St. Louis 1,000
1992 INDIANA 1-70 1,000
CALIFORNIA 1-40 1,000
ALASKA STEWARDS HWY 4,000
MARYLAND US-15 10,000
I 1-70 25,000
GEORGIA I1-75 2,000
NEW JERSEY US- 4,000
NEW YORK Rt. 281 1,000
MICHIGAN 1-94 12,000
MISSOURI I 70, Jefferson City 4,000
OHIO US-33 16,000
TEXAS I-35/SH 171 8,000
VIRGINIA US-29 2,000
WISCONSIN 1-43, 3,000
1993 ARKANSAS 1-40 11,000
CALIFORNIA Rt. 152, Santa Clara 1,000
GEORGIA 1 95, Savannah 62,000
ILLINOIS 1- 80 12,000
US 24 8,000
Rt. 121 3,000
Rt. 1 5,000
Lamont Rd., 11,000
KANSAS US-54 1,000
MARYLAND 1 95, Toll Rd. 55,000
S- 83 14.000
Table 1. continued
YEAR STATE PROJECT LOCATIONS) TONNAGE
1993 MICHIGAN 1-96 9,000
1 94 9,000
US- 131 22,000
MISSOURI I 70, St. Charles 9,000
I 70, Callaway 7,000
I 70, Saline 16,000
NEBRASKA Hwy 75 27,000
NORTH CAROLINA US 264 2,000
TEXAS US-79 5,000
US 323 7,000
US 60/83 1,000
VIRGINIA 1-66 10,000
OHIO US 23, Sandusky 60,000
I 75, Findlay 20,000
WISCONSIN US- 51 5,000
US 63 5,000
US 45 5,000
REVIEW OF TESTING AND MIX DESIGN FOR SMA MIXTURES
3.1 Purpose of Mix Design
The design of any asphalt-aggregate mixture is basically the selection of material
proportions to produce a mixture having certain desired properties or characteristics.
Ideally, a mix design procedure should optimize component materials and proportions to
yield a mixture that is resistant to all forms of distress which may reduce its performance
in the pavement structure.
SMA mixtures are intended to be surface mixtures.
obviously have good skid resistance.
Surface mixtures must
Rutting resistance of surface mixtures is also critical
since the highest shear and compressive stresses occur at or near the pavement surface.
Environmental elements (e.g., temperature changes, rain, sunlight, etc.) are also more
active on the pavement surface, therefore, surface mixtures must be able to withstand
In other words, SMA mixtures must be resistant to polishing, rutting,
thermal cracking, ravelling, moisture damage, and aging.
3.2 Current Methods of SMA Mix Design
In following with the European practice, the Marshall procedure has been used to
I 1 1 1 1 r1 .fi- A
1 /^I ms A ,1 TT ". 1 ttI -
AASHTO T 245,
Marshall Method (50 blows to each side of specimen)
Voids in Total Mix,
Voids in Mineral Aggregate,
Flow. 0.01 inch
Asphalt Content, percent:
The Marshall method is also used by most states for the design of conventional
dense-graded asphalt concrete mixtures.
Therefore, it is a well known procedure and
most agencies and contractors are equipped to perform the tests.
Many states have
successfully placed SMA mixtures that were designed by the Marshall procedure, although
field adjustments in asnhalt content have been common
Each laboratory used 50 blows per face with the standard Marshall hammer
except for Payne & Dolan Inc. who used a rotating base and bevelled foot hammer.
Optimum asphalt contents were selected at three percent air voids.
robin studies are shown in Table
Results of the round-
Notes from the TWG meeting  state that the
variability of optimum asphalt content for SMA mixtures is significantly greater than for
Results of the SMA TWG Sponsored Marshall Procedure Round Robin
First Set Second Set
Agency Content VMA Stability Flow Content VMA
(%) (%) (lbs) (.01 in) (%) (%)
Asphalt Institute 5.2 14.8 1800 10 7.0 20.6
Turner Fairbank 5.2 14.7 2350 13 6.4 19.4
FHWA OTA 5.2 15.0 2300 12 6.6 19.7
GA DOT 5.9 16.0 1650 10 6.9 20.2
MDDOT 5.2 15.0 1880 11 6.1 18.4
MI DOT 4.7 14.0 2300 9 5.9 18.3
MO DOT 4.5 13.5 1900 10 7.3 21.2
NCAT 5.9 16.1 1615 12 7.3 21.0
Payne & Dolan 4.9 13.9 2500 13 5.5 17.7
VADOT 5.8 16.4 1610 11
KY DOT 6_.4 19.5
primarily to the impact compaction of the Marshall hammer which fails to orient the
aggregate particles as does field compaction and create the aggregate structure which is
important to SMA mixture strength.
Therefore, the procedure is merely a volumetric
exercise that is based on an inappropriate compaction method.
3.3 Review of Mix Tests for SMA
3.3.1 Drain-down Tests
A drain-down test is necessary to determine if the stabilizing additive is effective in
mitigating run-off of the binder between production and placement of SMA mixtures.
Schnellenberg drain-down test, borrowed fiom Germany, was recommended in early
versions of the model specifications .
The Schnellenberg drain-down test consists of
pouring about 1000 grams of SMA mixture immediately after mixing at 1500C into an 800
ml glass beaker, then placing the beaker and mixture in an oven at 1700C for one hour.
At one hour, the mixture is dumped out of the beaker without shaking or vibrating, and
the weight loss of the mixture is determined.
In 1993, the National Center for Asphalt
Technology (NCAT) began development on an alternate procedure .
drain-down test utilizes a cylindrical wire basket rather than a beaker to contain the
mixture while in the oven.
The advantage of this procedure is that the drained binder drips
out of the container which makes determination of the weight loss more straight forward.
"*> r) / T-1-: 1/",,-T,.
unconfined uniaxial creep test to evaluate field cores from Georgia's first SMA project.
Tests were performed at 40 C with axial stress levels of 20, 70, and 100 psi for one hour.
Using criteria developed in the Asphalt-Aggregate Mixture Analysis System (AAMAS)
study , Little concluded that the SMA mixtures had excellent resistance to
He added that the test was very demanding for an SMA mixture, and that
confinement would substantially improve the creep response.
In a separate study ,
Little used the creep test to evaluate laboratory prepared
SMA mixtures. Four inch diameter by eight inch high specimens were prepared using a
kneading compactor. The compactive effort was selected to produce specimens with
approximately four to five percent air voids.
and with 30 psi confining pressure. In the u
applied; for the confined tests the creep load was 50 psi.
In this study, samples were tested unconfined
confined state, an axial stress of 20 psi was
All tests were performed at
In both cases, samples containing a low density polyethylene (LDPE) modified
asphalt and polyester fibers performed well, whereas samples with only fibers failed.
Brown  and Brown and Manglorkar  also used confined creep tests, but
using different conditions.
Using samples compacted in the Gyratory Testing Machine,
creep tests were performed at 60C with 20 psi confining pressure and 100 psi axial
Several SMA mixtures were evaluated with variations in asphalt content,
fiber type and content, filler content, and percent passing the No.4 sieve.
;nrl;ratP t t htlip mnvtilrpv nprpt n-nrr rscctrnt trn JfiMnrn itnn at Inrpr aenhalt r nntrntec
around 30 percent.
Optimum filler contents were dependent on each mixture.
particular fiber type appeared to be superior based on the creep test results.
3.3.3 Repetitive Uniaxial Tests
Several research projects have also used repeated-load uniaxial tests.
Manglorkar  used a repeated-load sequence as a modification to the confined,
uniaxial, creep test.
Test conditions were the same as used in their creep tests (20 psi
confining stress, 100 psi deviator stress, and 600 C) except that repeated loads were
applied using a haversine waveform over 0.1 second followed by 0.9 seconds unload.
Repeated-loads were applied for 3600 cycles, then the sample was allowed to recover for
The repeated load condition was much more severe than the static-load
Accumulated permanent deformations were generally
to 3 times greater than
for the static creep tests.
The repetitive load tests showed more sensitivity to asphalt
content, but less sensitivity to filler content, fiber content, and fines content.
and Veizer  performed repeated uniaxial tests on SMA mixtures
containing various percentages of ground tire rubber.
They used Marshall specimens in an
Loading was applied with a square wave with maximum stress of 100
psi for 0.2 seconds followed by 1.8 seconds rest.
Tests were performed at
Based on the accumulated permanent strains and the slopes of the permanent deformation
versus cycles plots, they concluded that five to ten percent rubber by weight of asphalt
3.3.4 Loaded Wheel Tests
The Georgia Loaded Wheel Tester (GLWT) was used by FHWA's Turner-
Fairbank Highway Research Center [28,29] and the Georgia Department of
Transportation [30,31] to evaluate the rutting resistance of SMA mixtures.
studies included tests with the GLWT and the French Pavement Rutting Tester to evaluate
the effects of different stabilizers  and to compare SMA mixtures to dense-graded
The study to evaluate the stabilizers concluded that the laboratory
prepared SMA mixtures had good rutting resistance in both wheel rutting devices and that
the results were not significantly different for the different stabilizers.
Likewise, in the
study comparing DGAC mixtures to SMA mixtures, the results with both devices
indicated that all mixtures were rut resistant and that no statistical difference was evident
between the DGAC mixtures and the SMA mixtures.
The Georgia DOT also used the GLWT to test plant-produced mixtures and cores
from their first two SMA projects.
conventional mixtures were tested.
On the first project, three SMA mixtures and two
Although the GLWT results indicated that all five
mixtures were very resistant to rutting, the SMA mixtures were clearly superior .
the second SMA project, plant mix and cores of both conventional DGAC and SMA
mixtures were again tested.
Both mixture types again were very resistant to rutting, but
for this project, the GLWT rut depths for the SMA mixture were slightly greater than for
+1. nc m r / t raI *a 1 .. r n [I 1 1
3.3.5 Gyratory Tests
Compaction characteristics of SMA mixtures using gyratory-type machines have
been evaluated by several researchers.
Brown and Manglorkar at NCAT  used several
tests including the Corps of Engineers Gyratory Testing Machine (GTM) to study the
effects of variations in component proportions in SMA mixtures. This study utilized a
GTM equipped with an oil-filled roller, and samples were compacted to 75 revolutions
with a vertical pressure of 120 psi and a one-degree angle of gyration. Three GTM
parameters were evaluated for each sample: Gyratory Shear Index (GSI), Gyratory Elasto-
Plastic Index (GEPI), and the shear stress produced at a one degree angle.
indicated that all mixtures were very stable regardless of variations in asphalt content, filler
content, fine aggregate content, and fiber type.
evident between GEPI and mixture proportions.
one-degree angle was found to be more sensitive
Likewise, no relationships or trends were
Gyratory shear strength produced at a
e. Results indicated that increasing the
asphalt content reduced shear strength slightly, but the small strength loss was evidence
that the SMA mixtures were highly tolerant to changes in asphalt content.
The effect of
changes in fiber type, fiber content, and mineral filler content on shear strength was found
to be dependent on the type of aggregate used in the mixtures.
The two FHWA studies by Stuart et al. [28,29] also used the Corps of Engineers
GTM to compare the effects of stabilizers on SMA properties and to compare DGAC
t-h^ 'roi isn+-i, CAL A A wniv+inra
TI,0 cltn 1, cflltn't [4)01 IIA 1002 n naA r rr^ 1 'rartt^v +
the point where the change in density decreases to less than 16 kg/m3 per 100 revolutions.
This typically required between 260 and 300 revolutions.
Shear strength, GSI, and GEPI
All three gyratory parameters for each mixture containing different
stabilizers were found to be statistically equal. It was also found that the three parameters
were not significantly affected by change in binder content. This was interpreted as an
indication that binder contents higher than designed by the Marshall procedure could be
used without detrimentally affecting rutting resistance.
The study comparing DGAC mixtures with SMA mixtures  used a slightly
The report states that the mixtures were tested at 600C in
accordance with the National Cooperative Highway Research Program (NCHRP)
Asphalt-Aggregate Mixture Analysis System (AAMAS) procedure .
tested in the GTM with an oil roller using 120 psi vertical pressure and a one-degree
Samples were compacted to 300 revolutions.
Gyratory Stability Index (GSI) and
refusal density were reported for each mixture.
The study concludes that GSI values for
the DGAC mixtures and the SMA mixtures were statistically equal and that all of the
mixtures were resistant to rutting.
with the DGAC mixtures. Extrac
Refusal air voids were lower in the SMA mixtures than
tions were also performed on the tested samples to
compare aggregate breakdown in the GTM to the Marshall hammer.
These data indicated
that the breakdown caused by the above GTM procedure was nearly identical to the
lrnniA,1, a, ,r nfl. oat 1, *1 A ,1 1a1n,,i p n ,rcninll nn'mnfn49 ra nT+^
kneading compaction and 50-blow Marshall compaction.
Results showed that at equal
asphalt contents, the Texas gyratory produced the lowest air voids, followed by kneading
compaction and then Marshall compaction.
However, the low air void content gyratory
samples produced the highest Hveem stabilities, which according to Little indicated that
when the high binder content SMA mixtures were compacted to ultimate densification
(such as with the Texas gyratory) the mixtures did not suffer a loss of strength.
that the gyratory compactor apparently produces a better interparticle matrix of the coarse
aggregate for load distribution than the kneading compactor.
3.3.6 Resilient Modulus Tests
Diametral resilient modulus tests have been performed on SMA mixtures by
Generally, most researchers have followed the resilient modulus
ASTM D 4123.
Little  used the resilient modulus test in the evaluation of
the SMA mixtures used in Georgia's first SMA project.
Using cores taken from the
Georgia SMA test sections, modulus values were determined at three temperatures: 0,
These results indicated that the stiffness of the SMA cores tended toward the
low end of the modulus range typical of DGAC mixtures.
At 400C, the resilient moduli of
the SMA cores were actually slightly below the range for dense graded mixtures.
prompted Little to postulate that diametral testing of SMA mixtures may not be
appropriate at higher temperatures.
Little also evaluated other tensile properties such as
AAMAS procedure .
This evaluation led him to conclude that the SMA mixtures had
a favorable fatigue response which was consistent with the concept of using a rich binder
In another report ,
Little used the diametral resilient modulus to demonstrate
the effects of a low density polyethylene (LDPE) additive on tensile stiffness of an SMA
His data showed a significant increase in mixture stiffness at
and 400C when
4.5% LDPE was added to the binder.
Another interesting note about his results was that
the binder content did not appear to effect the moduli of mixtures containing a 0.15%
polyester fiber alone, but when the fiber and LDPE were added together, the stiffness
peaked at 5.5% binder content.
Brown and Manglorkar  also included diametral resilient modulus testing in
their research to evaluate variations in mix proportions.
They found no consistent
correlations between moduli and fiber content, dust content, fine aggregate content, or
The lack of any significant trends was attributed to high variability of the
SMA mixtures prepared with a granite aggregate typically had moduli approximately
equal to that of dense-graded mixtures; whereas those containing a gravel aggregate had
moduli lower than typical dense-graded mixtures.
Stuart and Mogawer's comparison of DGAC and SMA mixtures  included the
resilient modulus test to evaluating several mixture characteristics.
ratfl~an rntnnun,. +nn+ nn '112. A/"^ A* A OX A A eb.terd..a-a n* i/"\ A- ri.4-3-a...--S -
moduli were statistically equal for SMA and DGAC mixtures.
Resilient moduli of
mixtures before and after moisture conditioning were also evaluated to
This analysis indicated that the SMA mixtures were less susceptible to
moisture damage than the DGAC mixtures.
Resilient modulus ratios for the SMA
mixtures were greater than 70 percent; whereas resilient modulus ratios for the DGAC
mixtures were below
These conclusions were also confirmed by tensile
This same type of analysis was also performed to evaluate changes
occurring upon aging in accordance with the procedures recommended from the Strategic
Highway Research Program.
The conclusions from this part of the study was that tensile
strength and stiffness of SMA mixtures increased significantly less than DGAC mixtures
due to aging, which indicated that SMA mixtures would be more resistant to cracking
3.3.7 Tensile Strength Tests
Indirect tensile strength has also been used by many researchers to evaluate SMA
Most researchers have measured indirect tensile strength at 25 C using a
vertical deformation rate of 2 inches per minute.
Brown and Manglorkar  found that tensile strengths of SMA mixtures were
lower than DGAC mixtures.
Increasing the asphalt content and filler content of SMA
mixtures did result in a gradual increase in tensile strengths.
Likewise, shifting the
sieve) also increased the tensile strength.
Fiber content and fiber type, however, did not
appear to have a significant effect on tensile strength.
Little  used the indirect tensile strength and the strain (horizontal) at failure to
evaluate several mixtures.
He found that LDPE and fibers added to an SMA mixture
substantially increased tensile strength but no statistical difference was evident in strain at
He also measured tensile strengths and strains at failure at 25 C using a
deformation rate of 0.5 inches per minute and found the same trends.
Compared to typical
DGAC mixtures, Little concluded that SMA mixtures had about the same tensile strength,
but higher strains at failure.
However, he also made several precautionary comments on
the use of the indirect tensile testing mode for SMA mixtures, stating that "this type of
testing may not be indicative of the relative response of SMA mixtures to traditional
densely graded mixtures in the field...Probably the most effective use of the IDT (indirect
tensile) test is to simply compare candidate SMA mixtures and not to compare the results
of SMA and densely graded mixtures."
The indirect tensile test has also been used to evaluate moisture damage potential
of SMA mixtures by several researchers.
This type of evaluation typically consists of
comparing the tensile strengths of two sets of samples: one set conditioned by water
saturation, soaking in a 600C temperature bath and then, in some cases, one or more
freeze-thaw cycles, the other set unconditioned.
A A CTT-rr'." P _-a A P fT'l T' AOCT 'Tl^ /"l
Standardized procedures for this test are
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3.3.8 Diametral Creep
Diametral creep, also known as indirect tensile creep, was measured by Little
[22,24] on cores from the first Georgia SMA project and on laboratory prepared samples.
Results on the cores, apparently at low test temperatures, showed that the SMA mixtures
deformed more than typical dense-graded mixtures which indicated a greater ability of the
SMA mixtures to relax and shed tensile stresses during a decrease of pavement
The laboratory prepared samples were tested at warmer temperatures and
used to evaluate relative differences in fatigue resistance.
Little concluded that the SMA
mixtures are more resistant to fracture and crack propagation than typical dense-graded
He also stated that polymer modification was necessary to keep tensile strains at
a reasonable level to reduce accumulated damage due to distortion .
There are several reasons to develop an improved procedure for SMA mix design.
The Marshall procedure, although it is familiar, has some deficiencies including hammer
compaction, high variability, and probably most significant, it lacks a measure of
The recipe based formulation of SMA mixtures may provide some
assurance based on historical performance, but this can also be restrictive to alternative
component blends which may perform better or be more economical.
Mix design should
include tests that indicate which materials and combinations are superior in terms of
deformation resistance, cracking resistance, and moisture damage resistance.
program for this study was set up to identify a set of test methods which better represent
the important mixture placement and distress conditions which exist in the field.
Selection of Tests
Selection of the tests to be evaluated for potential use in SMA mix design was
based on a variety of factors. It was desired to have at least one test to evaluate each of
the major forms of pavement distress. It was important that the test simulate field
-- -: -- .a t. -- t.--: -- -i-:- --*--- ci ,-_ _- -^-- i-- t.. 1 -
From a general review of recent literature, some of the most promising
performance related test procedures were identified.
Many of the tests were identified
from the SMA literature review presented in the previous section.
Other promising tests
had been used for evaluation of dense-graded asphalt mixtures, but had not yet been used
for testing of SMA mixtures.
The final factor, or constraint, used to select tests for this project was equipment
Funding for this research project did not include any funds for the purchase of
equipment or for subcontracting tests to other laboratories.
Therefore, it had to be
possible to perform the selected tests with equipment available at the University of Florida
Civil Engineering laboratories or the Florida Department of Transportation's Central
Bituminous Laboratory or Bituminous Research Laboratory.
The selected tests, categorized by distress mode, are shown in Table 3.
were selected to characterize rutting potential: the GTM densification/shear test, the
Georgia loaded wheel test, and the triaxial repeated-load test.
This redundancy was felt to
be beneficial considering the possibility that one of the tests may not be representative of
actual performance, or if all three tests were satisfactory, it would allow other agencies to
determine rutting resistance of SMA mixtures with only one test device.
To provide an
indication of fatigue resistance, two tests were selected: the indirect tensile strength test
and the resilient modulus test.
For evaluation of low temperature or thermal cracking, the
I. 1 1 1 I -
Table 3. Selected Mixture Tests
Distress Test References
Rutting GTM Densification/Shear Ruth et al 
Georgia Loaded Wheel Tester Lai , West et al. 
Triaxial Repeated Load Brown , Gabrielson 
Fatigue Cracking Indirect Tensile Strength & Strain Von Quintus et al. 
Thermal Cracking Diametral Creep Von Quintus et al. 
Low Temperature Resilient Modulus
Moisture Damage Wet/Dry Resilient Modulus Von Quintus et al. 
Wet/Dry Indirect Tensile Strength AASHTO T 283 ,
ASTM D 4123 
Compaction of all test specimens, except for the Georgia Loaded Wheel test
beams, was accomplished with the Corps of Engineers Gyratory Testing Machine (GTM).
The GTM compactive effort was adjusted by using different numbers of gyrations so that
the air void content of the compacted samples were as close as possible to the average in-
place air void content for each particular mixture.
For experimental mixtures that did not
have any field data, a standard compactive effort was used that had been selected from the
data generated from the mixtures with field data.
A flow chart showing the general order of testing and number of specimens per
test is shown in Figure
As indicated, test specimens were often used for more than one
This was possible due to the fact that some tests were non-destructive to the
This permitted a significant reduction in the material and sample preparation
T rT ":1L.2I zL ^A--tI.1- --. '-2I ..... 1...... ....... f __
Flow Chart of Testing
compactive effort, the step where the compactive effort is determined (shown as the
shaded box) was eliminated.
Compact Samples to
Initial, In-place Density
/ Shear Test
Strength @ 25C
@ 525 & 40oC
@ 5,25 & 40C
Strength @ 250C
MATERIALS AND TEST PROCEDURES
5.1.1 SMA Mixtures from Field Projects
Material components for some of the early SMA mixtures placed in the U.S. were
obtained in cooperation with several highway agencies and paving contractors.
mixtures from eleven field test sections were duplicated in this study.
For each mixture, the average of the field construction data (gradation, binder
content, and in-place density) was used as the target for preparing the laboratory test
Table 4 includes descriptions of the components and the field data used to
prepare the laboratory mixtures.
Complete descriptions of each mixture are included in
For the purpose of identification, each mixture was given a "Mix I.D." which
includes the state abbreviation for easy recognition.
For states that have had several SMA
mixtures placed, the number following the state abbreviation should correspond to the
chronologic order in which each mix was placed.
This is a coarse SMA mixture that was used as a binder layer on Georgia's
first SMA project which was placed in late July 1991 on Interstate 85 northeast of Atlanta
N r^ oo
\ A^ IT e, da I I
0 0 9Rws^^zRs
1-1 ** yis y w -
consisting of a marble dust mineral, filler mineral fibers, and an AC-20 modified with 5%
low density polyethylene (LDPE).
This mixture is unique among the other mixtures
included in this study due to its gradation and its use as a binder layer.
The top size
aggregate used in this mixture was
Although specific performance data is not
available on this mixture due to its location in the pavement structure, the pavement as a
whole is performing very well.
This mixture contains the same component materials as GA1,
but has a
much finer aggregate gradation .
The top size aggregate used in this mixture was 3/%
This mixture, placed on Georgia's first SMA project in September 1991, was used
as a surface layer.
The available performance data (Table 5) shows that the section is
performing very well with very low rut depths and excellent skid test results.
Performance Data for GA2
This is the third SMA mixture placed on Georgia's first SMA project .
This mixture contains a different granite aggregate (Ruby mine) which has a lower LA
abrasion value than the Buford granite.
Compared to GA2, the gradation of GA3 is
Date (Age) April, 1993 (1.6 years) June, 1994 (2.75 years)
Average Rut Depth (inches) 0.01 0.09
Average Friction Number 52 not available
Average Air Void Content (%) 7.3 5.8
section is also performing very well.
Rut depths are practically negligible and skid test
results are very good.
Table 6. Performance Data for GA3
Date (Age) April, 1993 (1.6 years) June, 1994 (2.75 years)
Average Rut Depth (inches) 0.05 0.09
Average Friction Number 48 not available
Average Air Void Content (%) 7.6 8.0
This SMA mixture was placed on Georgia's second SMA project which
was constructed on Interstate
mines in Tyrone and Mt. Viev
values for the Tyrone and Mt.
south of Atlanta in 1992 [31 ].
v, Georgia were used in this mi
View granites were 41 and 37
Granite aggregates from
e. The LA abrasion
, respectively, which are well
above the recommended maximum limit of 30, and are believed to be the highest used in
an SMA project to date.
used in GA3.
The gradation for the GA4 mixture was nearly identical to that
The mixture also contains marble dust mineral filler, domestic cellulose
fibers, and an SBS modified AC-20 special.
A thin overlay was placed over this project in
late 1992 due to motorists overreacting to the change in surface texture of an adjacent test
section. There were no signs of distress in the SMA or other sections at the time of the
Consequently, no performance data was available for this section during
preparation of this dissertation.
filler, and an 85-100 penetration asphalt.
A pelletized polyolefin was added as a stabilizing
The gradation of this SMA mixture is one of the few included in this study which
meets the TWG's recommended gradation band.
occurred in an area where a vibratory roller was t
Brown  reported that flushing had
ised. Early friction test results showed
that the SMA had lower skid numbers than the adjacent conventional mixture.
discussions with Wisconsin DOT personnel indicate that the test section is performing
No other performance data was available.
The second SMA mixture from Wisconsin was placed on Interstate 43 in
Waukesha County during August, 1992.
This mixture contains specially screened
limestone aggregates from Crowbar quarry, a limestone dust mineral filler, and an 85-100
Several test sections were constructed with this base mixture.
contains a different stabilizing additive. The mi
polyolefin additive was duplicated in this study.
xture from the section containing 7%
A unique feature of this mixture is that
the gradation has a 3/% inch top size rather than a
inch top size.
indicate that this mixture had high in-place pavement air voids.
The average in-place air
void content for this SMA pavement was 10.5 percent, whereas most other SMA
pavements in the U.S. have had initial air void contents in the range of 5.5 to 8.0 percent.
Post construction performance of this pavement has not been reported.
c ',l,-_,1- 1 r
This SMA mixture was placed on Ryan Road in the Milwaukee area during
I C^. a11, ni,..n-n.n.naA., 1n-tr,-,,+nn*-,A n-ran.,fa, o CRrtnrn raynlrlin nntornr lirMaectnn
mixture was very near the same as used in WI2.
No performance information has been
reported for this SMA section.
This SMA mixture, which was also placed in September 1993, was used on
a project in Three Lakes,
Wisconsin on US 45.
This mixture contains a crushed gravel
aggregate from Consolidated mine. A limit
the binder was a 85-100 pen asphalt. Six t
additives, were constructed on the project.
stone dust was used for the mineral filler, and
:est sections, each with different stabilizing
Only the mixture using inorganic fibers was
duplicated in this study since the construction data showed that this mixture had the least
variation during production.
This SMA mixture was placed on a project on US 29 in Lynchburg,
Virginia in the fall of 1992 . This mixture contains specially screened aggregates
quarried from an anorthosite traprock. Agricultural lime from an argillaceous marble was
used as the fine aggregate and mineral filler.
AC-20 was used as the binder.
sections were placed on the project: one containing polyolefin and the other containing
The mixture containing cellulose fibers was duplicated in this study.
August 1993, flushing of the binder to the pavement surface was observed in isolated
areas of both sections.
A brief investigation  indicated that the mixtures were close to
the job-mix-formulas and no definitive reason could be given for the flushing.
flushing, no rutting had occurred in the SMA sections.
'k1 rim. o' EA _- .1 --TT it m. rE_- -i. 1. ~I.._i-2 2
Test sections were constructed with cellulose fiber and polyolefin stabilizing
The mixture containing the polyolefin additive was evaluated in this study.
Shortly after the sections were constructed, the surface of the cellulose section appeared
"tight" or seemed to have less texture.
However, no rutting was reported.
discussions with Maryland DOT personnel indicates that the SMA sections are performing
No performance measurements have been reported.
This SMA mixture was placed in August 1992 on the business loop of
in Alvarado, Texas .
The mixture was also placed on Texas State
Highway 171 near Cleburne.
Components of the mixture were limestone coarse
aggregate, limestone dust mineral filler, AC-20, and pelletized cellulose fibers.
gradation of this mixture was within the range recommended by the SMA TWG.
Discussions with Texas DOT personnel indicate that the SMA is performing well.
measured performance data is available.
Field performance. In general, all of the SMA mixtures obtained from other
states have performed well in the field. Some states have reported isolated areas of
flushing of the mastic to the pavement surface, but in each case the flushing was not
associated with any rutting. At the time this dissertation was prepared, very little
performance data was available. Several of the mixtures included in this study have been
in service for over three years.
5.1.2 Experimental SMA Mixtures with Florida Aagregates
It was necessary to include in the study aggregates that are available within the
state of Florida to evaluate their potential for use in SMA mixtures.
Six sources of coarse
aggregate were selected representing the range of properties for materials currently used
in asphalt mixtures in Florida. An attempt was made to include aggregates covering the
geologic and geographic areas of the state. Brief descriptions of the selected coarse
aggregates are given below.
Each description includes a Mix I.D.
The order of the I.D.
numbers is not significant; the mixtures were simply
numbered in the order that they were tested.
Limestone aggregates from Dade County (FDOT mine no.
used in this mixture.
This and other similar aggregates mined from the Fort Thompson
and Miami Limestone Formations in Dade County
are widely used as construction
aggregates in southeast Florida and along the east coast of the state.
possesses unique wear characteristics due to the significant silica content of this limestone
Therefore, it is used extensively for friction course mixtures throughout the
The aggregate tends to be chalky and typically has an LA abrasion value of 31,
and water absorption of about 3 percent.
This mixture contains limestone aggregates mined from the Ocala and
Suwannee Limestones in Hernando County (FDOT mine no.
nancr4nra t ,ryCt1 rolotrt nli, nnro mnno 1tnna r iCrn th a noa?_antral naaf+ n0^fthia
abrasion for this aggregate is typically about 36, and water absorption is about 2.3
This aggregate also tends to be chalky.
This mixture contains a dolomitic limestone coarse aggregate mined from
the Ocala and Suwannee formations in Taylor County (FDOT mine no. 38-268).
aggregate source is one of the few limestone mines in the northern part of the state.
abrasion for this aggregate is usually around 40, and water absorption about 2.7 percent.
The aggregate contains more porous particles than the calcium carbonates from mid and
It is presently considered as marginal in quality relative to the other
aggregates mined in the state.
This mixture contains aggregate derived from a crushed concrete pavement
taken from Interstate 75 in Hillsborough County.
FDOT mine no.13-395 in 1991.
This crushing operation was given
LA abrasion for this crushed concrete aggregate has
averaged around 48, and water absorption is about 5.1 percent.
results this aggregate would be considered poor quality. Howe
Based on these test
ver, it is apparently being
used successfully in conventional asphalt mixtures.
The aggregate used in this mixture is from a dolomitic limestone quarry in
the Newala Formation in Shelby County, Alabama (FDOT mine no. AL-149).
aggregate is included in the study because it is commonly used in asphalt mixtures in the
panhandle of the state.
.. ... 1 A- n a..-^ 4- 1
It typically has an LA abrasion value of 23, and water absorption
i .:.l r,. .- .-.: -- i- _. 1_ -_-i -- 1 at,- -- .i
This mixture contains crushed river gravel aggregates obtained from
deposits along the Chattahoochee River in Gadsden County (FDOT mine no. 50-120).
This mostly quartz gravel is mined from the Citronelle formation. Th
approved, non-limestone source of coarse aggregate within the state.
used for asphalt mixtures in the central part of the panhandle. Due to
is is the only FDOT
It has been widely
)its polish resistance,
it was also used as a coarse aggregate in open-graded friction course mixtures for over ten
It was disallowed in this application in 1993 due to problems with ravelling.
aggregate typically has an LA abrasion value of 41, and water absorption of about 0.4
Other components of experimental mixtures.
In order to evaluate the differences
in mixture characteristics which were due only to the aggregates, the gradation and the
other mix components were kept constant.
The gradation used for all experimental mixtures, shown in Table 7
was near the
middle of the gradation range of the early versions of the FHWA's model SMA
This gradation is similar to that used in mixture WI1.
In July 1993, the
range for passing the No. 4 sieve was adjusted to 20
- 28 percent by the TWG .
Therefore, the gradation of the experimental mixtures is slightly above the current
recommended range at the No.
Generally, to meet this gradation with the different aggregates, it was necessary to
C.~, c ct..2 a n,,1 a a. ay a in is. a pE. .t .a a Aa n .L to a a a aaSr a
Interpolated from Standard
. Gradation Used For Experimental SMA Mixtures
The mineral filler used in the experimental mixtures is a marble dust, known as
R04, obtained from Georgia Marble Co.
in Marble Hill
This is the same filler that
was used in all of the Georgia SMA mixtures tested in this study.
Characteristics of this
mineral filler are given in Table 8.
The asphalt cement selected for the experimental mixtures was an AC-30 from
Coastal Fuels' terminal in Jacksonville.
Forty gallons of this
asphalt were obtained on
September 14, 1992. This asphalt was tested according to the SHRP binder specifications
and graded as a PG 64 -22.
Cellulose fibers and FDOT Type A ground tire rubber were used as stabilizing
additives in the experimental mixtures.
Cellulose fibers, CF31005,
from Custom Fibers
Sieve Gradation (Percent Passing) TWG Spec. Range
3/4 (19.0 mm) 100 100
1/2 (12.5 mm) 95 85-95
3/8 (9.5 mm) 70 60 -75
No.4 (4.75 mm) 30 20 -28
No.10 (2.0 mm) 20 15 -23
No.40 (425 pm) 14 12- 15.5'
No.80 (180 pm) 11 10-13'
No.200 (75 pm) 9 8- 10
Mineral Filler Properties
Half gallon quantities of GTR and asphalt were preblended at about
F with a high speed paddle mixer for about five minutes to achieve uniform
dispersion and reaction of the rubber and asphalt.
.2.1 GTM Compaction
Gyratory compaction was selected based on several studies which have shown that
this mode of compaction best simulates field compaction by rollers in terms of the
resulting density, particle orientation, and engineering properties of the compacted
Although these studies were performed using traditional dense-graded
mixtures, it is Presumed that the conclusions apply to SMA mixtures as well.
Due to the
Composition Mg,CaCO3 95% min. Hydrometer Analysis
Acid Insolubles 3% max. Grain Diameter (mm) Percent Finer
pH 9.0-9.5 .075 99
Specific Gravity 2.710 .034 47
Sieve (Opening) Percent Passing .010 13
No.40 (.425mm) 100 .007 10
No.80 (.180mm) 95 .003 5
No. 200 (.075mm) 71 .001 2
For this research, a gyratory compaction procedure initially developed by Ruth and
Schuab  was used. This procedure, which utilizes a GTM equipped with an air roller
is intended to compact the samples to the initial in-place density following
construction, typically between 92 and 94 percent of the theoretical maximum density.
The settings used to achieve this level of compaction with the GTM Model 6B/4C are 100
psi ram pressure, 3 degree initial angle of gyration, and 9 psi initial air roller pressure.
Work by Ruth et al.  determined that 18 gyrations generally produced the desired
density for many dense-graded mixtures.
To establish the compactive effort necessary for
the SMA mixtures in this study, compaction curves were developed for each mixture.
Three samples for each mixture were prepared and compacted in the GTM to
determine the number of gyrations necessary to achieve the average initial in-place density.
Each sample was compacted to a different number of gyrations.
Sample heights, recorded
with each gyration, and the bulk specific gravity of the sample measured following
compaction were used to backcalculate densities at intermediate numbers of gyrations
using Equation 1.
Gmb, = bulk specific gravity of compacted mixture at x gyrations
,,,.., L .. 1. 1.. 1SA a .. : :,n aC1a aAaa. aan.
(OIL- FILLED CHAMBER)
AIR- FILLED CHAMBER
Schematic of the Gyratory Testing Machine
An example of the compaction results is shown in Figure 7
. This graph of %Gmm
versus gyrations shows how the number of gyrations to achieve the average, initial, in-
place field density was selected.
GTM Densification/Shear Test
The GTM densification/shear test, which was also initially developed by Ruth and
Schuab , is intended to simulate traffic densification.
This procedure was
recommended as a part of the AAMAS study  for the evaluation of rutting potential
for dense-graded mixtures.
The test was performed at 600C to be representative of the
pavement during the hottest part of the year.
The densification/shear test was performed
on previously compacted samples that were left in the molds and using the same GTM
settings as used in compaction: 100 psi ram pressure, 3 degree initial angle of gyration,
and 9 psi initial air roller pressure.
Generally, the samples were densified to 300 gyrations
or until a significant loss of shear strength occurred.
2 was used to calculate the
gyratory shear strength of the mixture during the test.
= gyratory shear strength at gyration
ARP-= Air Roller Pressure at gyration
- 1. atF1 nnIUY~ 1 aC n*~C rn
* C.. ...
(N Ir;I .
| 1 | |co 3
w "4 U*
;~ i I :r i ;
O 'F I I O
C\ ) 0 a
l I : k-
\ i : 'a~ ,Q
The change in the percent of air voids (i.e. Percent Densification) during the test
was calculated using equation 3.
= %Grmm -
percent densification at gyration
= percent of maximum specific gravity at start of the densification test
nx = percent of maximum specific gravity at gyration
= percent air voids at start of the densification test
= percent air voids at gyration
Typical results from the GTM densification/shear test are shown in Figure 8.
Georgia Loaded Wheel Tester
The Georgia Loaded Wheel Tester (GLWT), shown in Figure 9, was developed by
Lai [33,42] for the Georgia Department of Transportation to evaluate the rutting potential
of asphalt mixtures.
Several studies have shown that results of the GLWT can accurately
discriminate the rutting potential of dense-graded mixtures relative to their actual known
field performance [33,34].
The Georgia DOT has reported very good results from GLWT
tests on their SMA mixtures [30,31].
The GLWT test specimens in this study were 1.5"x3"x15" asphalt concrete beams.
The prepared materials were heated to 300F, mixed, then placed in molds and returned to
the oven prior to compaction.
Compaction was accomplished with a hydraulic
^rnnr~f^"nn ractsr n mot'h*n 1nrrr .~nnIvna I r *k r nina1 fl1n4n^ nn* vn.. 4nn. a^-frt*^ L.La t
2o gn 40
(%) SGOlOA IV
(isd) IV3HS AO.VTAO
Illustration of the Georgia Loaded Wheel Tester
0 1 2 3 4 5 6 7 8 9
fifth application, the load was held at 60,000 lbs. for five minutes.
The compacted beams
were removed from the molds the next day and allowed to cure at room temperature for
one day. During the curing period, bulk density and air void content for each beam was
determined. Prior to testing, each beam was placed in the preheating chamber at 105F
for 24 hours.
The beams were rigidly confined on the sides and ends with angle brackets during
A stiff hose pressurized to 100 psi was mounted along the top of the beam acts
as a tire to transfer the load from the wheel of the moving chassis to the beam.
chassis, loaded with 122 pounds, was driven by the motor / gear-box assembly through the
One loading cycle consisted of a forward and return pass of the loaded
The entire GLWT assembly was housed in a heated and insulated temperature
chamber which maintained the test temperature at
within the chamber to circulate the air. Rut depth
105 +3 F. Small fans were placed
measurements were made at three
locations near the center of the beams with a dial gauge and reference template at 0, 1000,
4000, and 8000 cycles.
Figure 10 shows how typical results for the Georgia Loaded Wheel Tester are
graphed to illustrate rut depths versus cycles.
A criteria for maximum acceptable rut
depth for dense-graded mixtures was tentatively set at 0.15 inches at 8000 cycles by
West et al. .
5.2.4 Triaxial Repeated-Load Test
Triaxial repeated-load tests have been used by a number of researchers to evaluate
the permanent deformation (rutting) potential of asphalt mixtures.
evaluated several rutting tests using dense-graded mixtures with known performance and
concluded that the triaxial repeated-load test best models in-situ permanent deformation.
A good correlation was shown to exist between laboratory strain and rutting rate (rut
depth/square root of ESAL's in millions).
The same test configuration was used by Brown
and Manglorkar  to evaluate SMA mixtures with variations in mixture components.
The triaxial repeated-load test consisted of applying a constant confining pressure
to a standard cylindrical specimen and a repeated axial stress for 3600 cycles to simulate
The test conditions were set to realistically simulate the critical field
conditions which lead to rutting as close as possible.
The test temperature was 600C to
simulate high pavement temperatures. The axial deviator stress was applied using a 0.1
second haversine load period followed by a 0.9 second rest period to simulate 60 mph
Although a single confining pressure cannot replicate the horizontal stress
distribution existing in a pavement, Gabrielson found that 20 psi confining pressure gave
very low variation in test results.
The deviator stress level was set at 100 psi to give a
total axial stress of 120 psi which is typical of the high contact pressures existing beneath
the pavement and tires of heavy trucks.
Figure 11 is an illustration of the triaxial repeated-
1 1 1 1
Figure 11. Illustration of the Triaxial Load Test Apparatus
Specimens for the triaxial repeated-load test were standard 4 inch diameter by 2.5 inch
thick specimens compacted in the Gyratory Testing Machine.
smoothed with sandpaper.
Specimen ends were
Prior to testing, specimens were preheated to 600C for 4 to 6
Thin Teflon sheets were also placed between the specimen and platens to eliminate
shear stresses on the specimen faces.
A rubber membrane was placed around the
specimen and platens and held by o-rings at the top and bottom platen.
Once the rubber
membrane was in place, the triaxial cell was placed over the specimen and the confining
pressure was applied.
The entire triaxial assembly was then placed in the test
environmental chamber and allowed to stabilize at 60C for 20 to 30 minutes. Following
the temperature equilibration time, 100 cycles of conditioning load was applied. The
conditioning load was set to produce 20 psi of axial deviator stress.
The cyclic deviator stress was applied with a MTS servo-hydraulic test station.
Axial deformations were measured by two independent LVDT's mounted inside the
triaxial chamber on opposite sides of the specimen.
Test control and data acquisition was
accomplished through a MTS Model 458.20 microconsole using modified MTS software
on a 386 P(
An example plot of results from the triaxial repeated-load test is shown in
The slope of the permanent strain versus cycles (i.e. permanent strain rate) was
used to characterize rutting potential of the study mixtures.
Permanent strain rates were
calculated using Equation 4.
Sp = Permanent Strain Rate, in/in/cycle
hx = height of sample at cycle x, inches
h, = height of sample at end of conditioning, inches
cx = cycle number
Permanent Strain (in/in)
800 1200 1600 2000 2400 2800 3200
5.2.5 Indirect Tensile Strength Test
Tensile properties of the SMA mixtures were determined using the indirect tensile
(diametral) test for evaluating cracking potential and moisture damage resistance.
indirect tensile strength tests were performed at 77F
(25C) following the loading
guidelines of ASTM D 4123 with a ram rate of
2 inches/min (50 mm/min).
performed on GTM compacted samples using an MTS servo-hydraulic testing machine
with a model 458.20 controller. Horizo,
model 632.94E-20 extensometer fixture.
illustrated in Figure 13.
ntal deformations were measured using an MTS
The test set-up for the indirect tensile test is
Test control, data acquisition and calculations were performed by
software originally developed by MTS.
The program for indirect tensile strength was
modified to capture horizontal strain data and to calculate fracture energy.
strengths and strains at failure were calculated using Equations
and 6, respectively.
Fracture energy for each test was determined as the area beneath the stress-strain curve to
the point of failure.
r = tensile strength, psi
I = ultimate applied load required to fail sample, lbs
'= diameter of sample, inches
C C fl S 4
f= strain at failure, %
6h = horizontal deformation corresponding to Pu,,, inches
Figure 13. Illustration of Diametral Testing Configuration
Total resilient moduli of the GTM compacted mixtures were determined in
accordance with ASTM D 4123. Mod
77F and 1040F (5C, 25C, and 400C).
uli were determined at three temperatures: 41F,
Assumed Poisson's ratios were input for each
temperature to avoid errors resulting from inaccurate vertical deformations caused by
.A1 I 1I I I 1*
1 -_~ 1.. 1 _- -_ _I 1iir
MTr = total resilient modulus, psi
P = repeated load, lbs
v = Poisson's ratio, assumed v=0.2 at 50C, v=0.35 at 25C, and v=0.5 at
= total recoverable horizontal deformation, inches
5.2.7 Creep Compliance
Static diametral creep tests were performed using guidelines taken from several
sources including the AAMAS procedure  and the provisional AASHTO standard
TP9  developed in the Strategic Highway Research Program.
A program was
developed to provide test control, data acquisition, and perform calculations for the
creep compliance test.
All tests were performed at 14F (-10C) by applying a static
load to the specimen's vertical diameter through 0.5 inch loading strips as used in the
indirect tensile and resilient modulus tests.
The magnitude of the load was set to
produce between 50 and 200 horizontal microstrains at 60 seconds.
The load was
maintained for 1000 seconds, then removed and recovery of the horizontal strain was
recorded for an additional 1000 seconds.
Creep compliance was calculated using
= Creep Compliance at time t, square inches per pound
, = horizontal deformation at time t, inches
= percent recovery
= horizontal deformation at end of unload period
= horizontal deformation at end of load period
5.2.8 Moisture Conditioning
To evaluate the potential for moisture damage, a.k.a. stripping, of asphalt
mixtures, wet/dry tensile strength tests or resilient modulus tests are commonly performed.
Standardized procedures for the wet/dry indirect tensile strength tests include
ASTM D 4867  and AASHTO T 283 .
In this study, comparisons were made between wet (moisture conditioned) and dry
(unconditioned) resilient moduli and indirect tensile strengths.
showed how the
A --A-_. -----------t-- -t. .- a i -- -- n nna. n4 pa
Moisture conditioning consisted of vacuum saturation in tap water until specimens
reached 60 to 100 percent saturation.
vacuum of 100 mbars.
Generally, this required one to two minutes under a
For specimens with lower air voids, the vacuum time was extended
to meet the saturation requirement. Following vacuum saturation, the specimens were
placed in a 600C water bath for 24 0.5 hours. Testing at 25 C was initiated within 24
hours after removal from the water bath.
EVALUATION OF MIXTURE TEST DATA
6.1 GTM Comoaction
Compaction results of the eleven duplicated field mixtures are shown in Table 9.
The number of gyrations to achieve the field density (N1.) was estimated from the
compaction curves generated from three samples typically compacted to 12, 15 and 18
Estimated values of N, ranged from
to 20, with an average of 12.1 and a
standard deviation of 4.5.
Excluding the data from WI2, which was apparently
undercompacted in the field, does not significantly alter these statistics. The average
initial in-place air void content for the field SMA mixtures was 6.9 percent. Excluding
WI2 does drop the average to 6.5 percent.
The air void content and VMA data for each mixture, shown in Table 9, are the
results of six samples compacted to N e. Several mixtures had very small variations in
air void contents, for others the repeatability was poor. In most cases, the estimated value
of Nf~ did yield sample air void contents that were reasonably close to the target field air
void content for the six compacted samples.
Using the data generated from the six
samples from each mixture, the adjusted estimate of Nh was made, but these values
were only slightly different.
Table 9. Compaction Results for the Duplicated Field Mixtures
Average Compacted Compacted
Mix I.D. Field Air Estimated Air Voids VMA Adjusted
Voids (%) N (%) (%) Ni
GA1 5.0 12 4.3 4.3 16.5 16.5 10
4.1 4.2 16.4 16.5
3.7 4.0 16.0 16.3
GA2 7.0 5 7.0 5.2 18.2 16.7 4
6.8 6.6 18.1 17.9
6.3 6.3 17.7 17.6
GA3 6.3 16 6.5 6.3 19.6 19.3 15
5.7 6.0 18.8 19.1
5.8 6.5 18.9 19.5
GA4 5.7 15 6.2 5.7 18.5 18.0 16
6.0 5.9 18.2 18.2
6.3 6.1 18.6 18.4
WI1 6.8 20 6.8 7.5 17.4 18.0 22
7.2 6.7 17.7 17.3
7.5 7.9 18.0 18.4
WI2 10.5 5 10.1 9.9 20.6 20.4 4
10.2 9.9 20.7 20.4
9.8 9.2 20.4 19.8
WI3 7.6 12 6.9 4.7 18.5 16.6 9
6.4 5.5 18.1 17.3
5.7 7.0 17.4 18.6
WI4 6.5 11 6.5 4.7 18.1 16.5 11
6.9 6.0 18.4 17.7
6.4 5.4 18.0 17.2
MD1 5.9 12 5.9 6.5 20.9 21.5 13
6.5 5.8 21.5 20.9
7.1 6.1 22.0 21.1
TX1 7.0 10 7.3 6.8 20.3 19.9 10
7.0 8.3 20.1 21.3
6.9 6.3 20.0 19.5
VAl 7.3 15 6.2 6.4 19.8 20.2 9
4.9 5.6 18.7 19.3
5.9 4.6 19.5 18.4__
One purpose of the compaction tests on the duplicated field mixtures was to
develop a standard compactive effort to be used for the design of experimental mixtures.
When the first few mixtures of this study had ben tested, a quick, rough analysis indicated
that about 15 gyrations would generally provide the appropriate level of compaction.
was used as the standard compaction effort for the experimental SMA mixtures using
Based on more complete data, it now appears that about 12 gyrations
would be closer to the average N1t for the mixtures evaluated.
6.2 Design of Experimental Mixtures
Since the experimental mixtures had fixed gradations and components, the only
variable in the mix design was selection of the design asphalt content.
Samples for each
experimental mixture were prepared with a range of asphalt contents and compacted in the
GTM using 100 psi vertical pressure, 3 degree initial angle of gyration, 9 psi initial air
roller pressure and 15 gyrations. Sets of samples were prepared with cellulose fibers and
with ground tire rubber as stabilizing additives. The design asphalt content for each
mixture was selected at 6.5 percent air voids. This was the average initial in-place air void
content of the field mixtures included in this study. Tensile strengths and failure strains
were also measured on each of the samples prepared for the asphalt content determination
Results are shown in Table 10.
Conclusions drawn from this data are:
A half percent increase in asphalt content generally produces about a one percent
For most mixtures, the highest tensile strengths occur at asphalt contents that yield
between 6.0 and 7.0 percent air voids.
Tensile strains at failure generally tend to
increase with increasing asphalt contents and decreasing air void contents.
Results of Experimental Mix Design Tests at
Mix I.D. Asphalt Air Tensile Failure Air Tensile Failure
Content Voids Strength Strain Voids Strength Strain
(%) (%) (psi) (%) (%) (psi) (%)
FLI 5.0 7.7 83.7 0.75
6.0 6.7 83.7 1.41 5.4 105.3 1.14
5.0 8.9 82.4 0.79 7.5 106.4 0.62
FL5.5 7.9 85.3 0.85 6.3 108.2 0.64
6.0 6.6 95.2 0.85 5.8 105.6 0.73
5.0 11.2 65.9 0.79 9.0 86.3 0.62
5.5 9.1 71.0 0.96 8.0 89.1 0.79
6.0 8.0 78.0 0.83 7.2 90.6 0.77
6.5 7.7 73.8 0.92 5.6 105.2 0.75
5.5 8.1 73.5 0.92 7.0 89.4 0.75
6.0 6.8 74.8 0.85 6.2 100.5 0.77
6.5 5.2 87.9 0.89 4.8 105.5 0.71
7.0 4.4 84.0 1.08 3.9 107.9 0.89
4.5 9.1 86.5 0.79 7.9 109.8 0.75
5.0 8.0 97.0 0.92 5.8 151.1 0.71
5.5 7.2 105.3 0.79 5.8 126.9 0.71
6.0 5.1 113.1 0.81 4.2 130.0 0.96
4.0 10.8 62.1 0.79 7.0 110.2 0.71
4.5 9.5 62.8 0.73 5.8 107.6 0.73
5.0 7.3 72.5 0.87 4.9 113.2 0.85
5.5 6.7 71.8 0.98 4.2 110.7 0.87
Cellulose fibers tend to bulk mixtures more than ground tire rubber (and most
other additives as well) and therefore, will allow for more asphalt in mixtures at
equal air void contents.
Increased asphalt contents will likely result in mixtures
that are more resistant to thermal and fatigue cracking.
At equal asphalt contents, mixtures with ground tire rubber have higher tensile
strengths but lower failure strains than cellulose mixtures.
Also, at approximately
equal air void contents (equal densities), mixtures containing ground tire rubber
have higher tensile strengths and lower failure strains.
These effects are related to
After selecting the design binder content, additional samples were compacted for
the other tests.
Average weight-volume data from these compacted samples are shown in
Weight-Volume Data for Experimental Mixtures Compacted 15 Gyrations
Stabilizer Cellulose Ground Tire Rubber
Mix I.D. A.C. Content Air Voids VMA A.C. Content Air Voids VMA
(%) (%) (%) (%) (%) (%)
FLI 7.0 6.4 17.4 6.0 7.1 16.0
FL2 5.9 6.8 16.1 5.3 7.3 15.4
FL3 6.7 6.2 17.9 6.2 6.7 17.2
FL4 6.1 6.2 17.0 5.8 6.4 16.6
6.3 GTM Densification/Shear Tests
Results of the GTM densification/shear tests for all mixtures are summarized in
The shear strength trends (in the first row) indicate the general pattern of the
gyratory shear strength versus gyrations plots between 200 and 300 gyrations.
reported as rising (increasing shear strength), stable (little or no change in shear strength)
or declining (shear strength loss and tending toward failure).
Trends for most of the SMA
mixtures are either stable or rising which indicates that the mixtures would provide good
shear resistance under traffic.
Three mixtures (GA2, GA4, and WI1) had declining shear
Mixtures GA2 and WI1 were retested under the same conditions but with a
Results of the retests are shown in parentheses.
was available to retest GA4.
The retest results for GA2 and WI1 show that these mixtures
performed much better than indicated by the original test results.
All data for these
mixtures were carefully reviewed, but no reason was evident for the difference between
the original and retest results.
Based on the available field data it is expected that all of
the duplicated field mixtures would have stable shear strengths.
The value of the shear strength at 200 gyrations is shown in the second row.
Shear strengths for the SMA mixtures vary considerably, ranging from 50.4 psi for one
WI3 sample to 67.0 psi for one of the FL1 samples.
replicate samples of the same mixture.
There is also some variation within
An analysis of variance was performed to
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Since unbalanced data exist in some cases, SAS procedure PROC GLM was selected and
used throughout this study. A Duncan's multiple range test with a significance level (a) of
0.05 was applied to compare the means for the different mixtures.
Results of this statistical analysis are illustrated in Figure 14.
This bar graph shows
the average gyratory shear strength for each mixture, with the mixtures ordered from
highest to lowest shear strength.
Letters on the bars indicate mixtures which have mean
shear strengths that are not significantly different.
For example, the shear strength,
for FL6 is not significantly different than that of GAl (shown with letter "A") and also not
significantly different than FL3 (shown with letter "B").
However, FL6 is significantly
different than FL1 and all other mixtures with lower shear strengths.
Based on criteria established by Ruth et al.  using dense-graded mixtures, the
shear strength at 200 gyrations should be above 54 psi to assure good rutting
Application of that criteria to these SMA mixtures indicates that most of the
mixtures are very rut resistant.
The exceptions are GA4, TX1, and FL4.
results of GA2 and WII were also below 54 psi, but the retest results were well above the
For the mixtures which failed the criteria, only GA4 displayed a declining shear
strength, yet this trend was gradual and not indicative of a catastrophic failure.
TX1 and FL4 were actually still gaining strength when the test was terminated.
The other parameter evaluated from the GTM densification test is the percent
Th;E rtnaromnar ic cimrnl^, tb rinno in air tmrArlc nfthA c2mflla at ani nnrfnt
General Linear Models Procedure
Dependent Variable: Gyratory shear strength at 200 gyrations
MIX 15 968.53308 64.5688725 2.231 12.97 0.0001
a = 0.05
Means with the same letter are not significantly
A A A A
i B B B B
C C C C
B E B
FL1 GA1 FL2 FLS FL6 GA3
FL3 3I3 WI4 FL4 WI2 MDI GA4 TX
The general criteria used for dense-graded mixtures has been:
3.5% is acceptable,
4.0% is undesirable.
Results for many of the SMA mixtures (Tablel2, Row 4) indicate a potential for
excessive densification. This data was analyzed with the same statistical procedures used
to evaluate shear strengths. The results shown in Figure 15 show a more visually dramatic
spread in the percent densification results.
Yet due to the variation among replicates,
mixtures having as much as 1.0 percent difference in means are not significantly different.
Initial air void contents and VMA's of the test samples are shown in the fifth and
sixth rows, respectively.
These parameters were believed to possibly have some affect on
shear strength and percent densification results.
Several plots were made to assist in
identifying relationships between results and with other mix parameters.
shown in Figures 16 through 20.
These results are
It can be seen that the only apparent correlation is
between percent densification and initial air voids. This intuitive relationship indicates that
mixture densification (consolidation rutting) will be lower when the as-compacted air void
content is low.
However, the gyratory shear strength of the SMA mixtures is not affected
by initial air void content or percent densification.
This supports findings by others [7,26]
that the deformation resistance of SMA mixtures is not sensitive to low air void contents.
This departure from the conventional wisdom used with dense-graded asphalt mixtures
:- ~ r + Lt 4+l- t n !* +.. nt Ci A A ean --.4. -.. ... 2 g 1- -- _- -_
General Linear Models Procedure
Dependent Variable: Percent densification
MIX 15 62.185188 4.14567923 0.679 9.00 0.0001
a = 0.05
Means with the same letter are not significantly different
WI2 FA F IX1 FL2 MDI GA2 WI3
FLS WI4 FLI WII GA4 GA3
Gyratory Shear Strength versus Percent Densification Results at 200 Gyrations
cP B O
0 0 D
A _. '
16 18 20 22 24
Voids in mineral aggregate, %
Gyratory Shear Strength at 200 Gyrations versus Initial VMA
. c atJ
O CD D
Voids in mineral aggregate, %
Percent Densification versus Initial VMA
The bottom row of Table 12, average weight loss, was included in the results as a
potential indicator of flushing due to excessive asphalt content.
densification/shear test, some samples would consolidate to a degree at which some mastic
would flush out of the sample and squeeze out of the mold during the test. F
mastic often occurred well before the calculated sample air voids were below
lushing of the
The amount of extruded binder was determined indirectly from the difference in average
I;rrhc* 4nf ctnnlc rA r+ntrlntA r4r A +an 0x t,, ntrornn n10;r^+0o af T+rit A ; 0C.t/A
0 [0 a
o cc 0
losses, but their shear strengths were high and very stable.
As mentioned earlier, some
flushing has occurred on several field SMA projects, yet these SMA mixtures have also
remained very stable.
6.4 Georgia Loaded Wheel Tests
Results of the Georgia Loaded Wheel Tests (GLWT) are shown in Table 13
to shortages of some materials, triplicate samples were not possible for all mixtures.
Nearly all of the SMA mixtures performed very well in the GLWT
inspections of the samples after the test indicated that the low magnitude ruts were due
primarily to wearing away of the mastic, reorientation or seating of surface aggregate
particles and minor consolidation.
No lateral displacement was evident for any mixture
except for TX1
which was very minor.
Little rutting was apparent at the ends of the
beams, which are often greatly distorted in DGAC samples.
TX1 easily met the maximum rut depth criteria of 0.1
All of the mixtures except for
Results of the Georgia Loaded Wheel Tests at 8000 Cycles
Mix GA1 GA2 GA3 WI2 WI3 WI4 MD1 TX1 VAI FL1 FL2 FL3 FL4 FL5 FL6
Initial 3.9 7.4 6.9 6.0 5.1 7.2 5.3 5.1 6.1 8.9 6.0 5.7 5.8 5.6 7.3
Air 4.0 7.3 7.4 6.5 3.8 5.5 4.7 5.4 8.5 6.4 4.7 5.0 6.1 7.4
Voids 4.0 7.4 6.0 4.3 5.2 8.9 5.0 4.4 5.6 6.1 6.0
Rut .079 .077 .102 .071 .095 .111 .124 .150 .104 .073 .092 .143 .108 .110 .124
Depth .038 .061 .098 .100 .094 .143 .115 .150 .104 .079 .142 .097 .143 .130
The air void data shown in Table 13 indicates that a reasonable level of
compaction was achieved with the crude static compaction method.
Nearly all sample air
void contents were within the range expected to exist in the field after construction.
As can be seen in Figure 21, some mixtures were clearly more rut resistant than
Statistical analysis of the rut depth data revealed a large number of statistical
groupings for the fifteen mixtures. In general, the Florida experimental SMA mixtures
compare well with the duplicated field mixtures. FL3 and FL5 were among the worst
mixtures in terms of GLWT mean rut depths, yet they were statistically the same as GA3
which is known to be a very rut resistant mixture.
grouped with this baseline mixture. However, as
The other FL mixtures can also be
indicated by the coefficient of variation,
repeatability of this test is poor.
A plot of air void contents versus rut depths, shown in Figure
apparent correlation. This provides additional support to the conclusion that rutting
resistance of SMA mixtures is not affected by air void contents.
6.5 Triaxial Repeated Load Tests
Results of the Triaxial Repeated Load Tests are shown in Table 14.
this data, most of the mixtures appear to have very good deformation resistance.
developed by Gabrielson  based on the field performance of 16 dense-graded mixtures
was a maximum strain of 0.13 in/in for a test of 3600 cycles, or 3.61 E-5 strain/cycle.
General Linear Models Procedure
Dependent Variable: Rut depth
MIX 14 0.02939337 0.00209953 0.015 9.65 0.0001
Means with the same letter are not significantly different
TX1 FI FLS
WI4 FL6 MD VAl
WI2 FL2 GA2 GA1
B B B B B B B
ccc c c c
D DD D D D
E E E E E E E
GLWT Rut Depth (inches)
Plot of GLWT Rut Depths Versus Air Voids
Results of the Triaxial Repeated Load Tests
below the criteria, these mixtures were considered as having marginal deformation
Mix GAl GA2 GA3 GA4 Wll WI2 W13 WI4 MD1 TX1 VAl FLI FL2 FL3 FL4 FL5 FL6
Initial 4.2 6.6 6.0 5.9 6.8 10.1 6.5 4.7 5.9 6.8 5.1 8.8 6.9 6.3 6.4 7.0 5.5
Voids 4.0 6.3 6.0 6.1 6.7 10.2 6.4 6.0 5.8 8.3 6.2 8.7 6.7 6.6 6.7 7.0 6.3
c rate 1.25 1.31 1.31 2.50 2.14 1.82 1.65 3.69 2.93 6.85 2.50 1.52 2.24 1.86 1.93 4.22 2.31
(E-5/ 1.27 1.29 1.30 2.51 2.65 1.94 2.12 2.58 3.70 6.47 3.49 1.63 1.63 1.77 1.49 3.76 3.95
The statistical comparison of the TRL results for the study mixtures is shown in
Mixture TXI is obviously significantly different from the other mixtures.
Otherwise, there are broad groupings of results.
Variability of the test is high as indicated
by the high coefficient of variation.
6.6 Correlation of Rutting Tests
Comparisons were made between the results from the three tests which were used
to indicate rutting potential to infer the adequacy of the tests and the data.
First, if one or
more of the tests results agree that a particular mix is good or bad then the confidence that
the results are reasonable is increased.
Conversely, if the results of one test are clearly out
of the trend, it may be an indication that the test results are in error.
one of the tests fails to consistently
Also, if results from
correlate with the results of the others, then it may
indicate that the test is not representing the conditions critical for rutting or that the test
affected by other variables.
To aid in the comparisons, results were plotted and least-square linear regressions
First, shown in Figure 24, is the plot of GTM shear strength (rG200)
versus GLWT rut depth at 8000 cycles.
The expected trend of increasing rut depth for
lower shear strength is apparent, however, as indicated by the R2 value of 0.34 the
correlation is weak.
The regressions were run again without the data from the FL
mixtures and a much improved coefficient of determination was given (R2
General Linear Model
Dependent Variable: Permanent strain rate
Tvoe I SS
MIX 16 0.00000001 0.000004 19.71 0.0001
a = 0.05
Means with the same letter are not significantly different.
B BB BB -
C C C C C C
D DI D D D
EE E E EE E EE
TX1 FL5 MD1 VAl WI4 FL6 GA4 WI1 FL2 WI3 WI2 FL3 FL4 FLI GA3 GA2 GAl
GLWT Rut Depth (inches)
Plot of Gyratory Shear Strength at 200 Gyrations versus GLWT Rut Depth
I I I I
0.08 0.1 0.12 0.14 0.16
GLWT Rut Depth (inches)
Figure 26. Plot of GLWT Rut Depth versus TRL Permanent Strain Rate
regression excluding the FL mixtures had an R2 of 0.53.
These two tests were expected to
have a better correlation since they both utilize samples compacted in the GTM and are
performed at the same temperature.
The plot of GLWT rut depth versus TRL permanent
strain rate, Figure 26, shows that higher rut depths correspond to higher strain rates.
second order regression indicated a good correlation with an R2 of 0.92.
regression excluding the FL mixtures produced an R2 of 0.78.
Table 15. Output of Rut Test Regressions
All Mixtures Excluding FL Mixtures
X Y Enuatinn R2 Enmuation R2
It is apparent from the regressions that the FL mixtures diminish the correlations
involving GTM results.
It is observed that FL5 and FL6 have higher shear strengths than
may be expected from the results of the other tests. It is probable that the shear strengths
for these two mixtures and possibly also FL1 are in error. The cause of such an error may
due to improper alignment of the mold and sample in the GTM or incorrect
readings of the air roller pressure gauge.
Exclusion of this maverick data does indicate a fairly good correlation between the
different rutting tests.
Also, comparison of results with previously reported criteria tend
to indicate that most of the SMA mixtures have good rutting resistance.
the rutting tests indicate that GA1, GA2, and GA3 are very rut resistant.
Results for all of
The very stiff
low-density-polyethylene modified asphalt used in these mixtures probably provided a
substantial increase in their deformation resistance<
these mixtures also corroborates the test results.
mixture TX1 has marginal to poor rutting resistar
The excellent field performance for
Each of the tests also indicate that
ice. However, no field performance data
is available for this mixture, even though it reportedly "appears" to be doing well.
6.7 Resilient Modulus Tests
Resilient Modulus results at
25 and 40C are shown in Table 16.
arP rp nnrted fnr patrh mixrhtre
The tePt wan nerfnrmed on dunlicate samnles and on two
graphically illustrated in Figures
Reasonably good coefficients of variation
were obtained at each of the temperature conditions.
At low temperatures, a low elastic modulus is beneficial to prevent a rapid
accumulation of tensile stresses and strains during temperature drops.
viscous element of the mixture's stiffness is more important to low temperature cracking.
From Figure 27
have the lowest moduli.
it can be seen that the Florida experimental mixtures, except for FL5,
In diametral tests, the properties of the binder control mixture
The low moduli of the experimental mixtures is probably due to the properties
of the asphalt-rubber binder which permits greater elastic deformations.
Although rutting resistance is primarily dependent on the aggregate structure, the
stiffness of the binder also plays a role in the mixture's deformation resistance.
at high temperatures, a higher stiffness (i.e. higher resilient modulus) is beneficial to
minimize rutting. From Figure 29, it is evident that a wide range of moduli exist among
the SMA mixtures. The three Georgia mixtures which have the highest resilient moduli all
contain a very stiff modified asphalt.
Fatigue resistance of the SMA mixtures was evaluated with resilient moduli and
tensile strain at failure using the technique presented in the AAMAS study .
analysis is presented with the strain at failure data in section 6.8.
Plots of resilient moduli versus temperature are shown in Figures 30, 31 and 32.
The reference lineP which pYtpnd the lpnoth nf thp aranhlc ar, rearnmmt ndAonA nnar oan,
Results of Resilient Modulus Tests
Mix I.D. MR at 50C MR at 25oC MR at 400C
(ksi) (ksi) (ksi)
GA1 982 981 424 449 188 183
1051 1110 458 503 198 207
GA2 964 939 452 451 174 173
957 992 444 436 184 176
GA3 1128 1066 514 502 195 201
1178 1110 508 497 206 208
GA4 913 912 267 251 72 68
953 864 260 244 78 82
WI1 1293 1279 525 507 139 146
1227 1177 449 445 120 130
WI2 1080 1050 394 369 103 100
1017 997 357 345 109 118
WI3 1038 973 381 347 81 76
962 948 383 365 90 90
WI4 953 904 250 264 43 50
961 953 299 297 57 70
MDI 906 987 210 217 69 64
885 943 218 211 73 66
TX1 1038 1072 332 336 67 85
976 951 323 334 60 76
VA1 989 935 273 256 88 81
1022 992 270 261 81 92
FL1 809 905 375 370 115 117
752 848 373 370 105 101
FL2 952 858 362 363 85 90
916 912 387 353 94 98
FL3 941 875 342 330 60 74
845 812 308 292 66 72
FL4 774 766 308 307 100 92
787 775 328 319 92 89