DEVELOPMENT OF A NON-SOLVENT BASED TEST METHOD FOR
EVALUATING RECLAIMED ASPHALT PAVEMENT MIXES
Except where reference is made to the work of others, the work described in this
dissertation is my own or was done in collaboration with my advisory committee. This
dissertation does not include proprietary or classified information
Alan Carter
Certificate of Approval:
Dr. Dan Brown Dr. Mary Stroup-Gardiner, Chair
Associate Professor Associate Professor
Civil Engineering Civil Engineering
Dr. David Timm Dr. Stephen L. McFarland
Assistant Professor Acting Dean
Civil Engineering Graduate School
DEVELOPMENT OF A NON-SOLVENT BASED TEST METHOD FOR
EVALUATING RECLAIMED ASPHALT PAVEMENT MIXES
Alan Carter
A Dissertation
Submitted to
The Graduate Faculty of
Auburn University
In Partial Fulfillment of the
Requirements for the Degree of
Doctor of Philosophy
Auburn, Alabama
August 8, 2004
iii
DISSERTATION ABSTRACT
DEVELOPMENT OF A NON-SOLVENT BASED TEST METHOD FOR
EVALUATING RECLAIMED ASPHALT PAVEMENT MIXES
Alan Carter
Doctor of Philosophy, August 8, 2005
(M. Eng., Ecole de technologie sup?rieure, 2002)
(B.Eng., Ecole de technologie sup?rieure, 2000)
118 Typed Pages
Directed by Mary Stroup-Gardiner
The objectives of this research are twofold. The first is to develop and validate an
indirect tension stress relaxation test methodology for assessing asphalt binder properties
using compacted hot mix asphalt (HMA) samples. The second is to evaluate the
influence of adding various percentages of reclaimed asphalt material (RAP) to HMA
mixtures using this stress relaxation test. More than 32 different HMA mixtures (two
binders, two aggregate sources, two sources of reclaimed asphalt pavement (RAP) used at
five different percentages, three replicates) were compacted and tested using indirect
tension stress relaxation test at two temperatures (5 and 22
o
C).
iv
Two relaxation characteristics were used to evaluate binder properties in the compacted
HMA samples: 1) the initial stress relaxation modulus (at time=1 second), and 2) the
curvature coefficient (exponent) from a power law model fit to the data for each mix.
The stress relaxation test showed that the addition of RAP increases the modulus
linearly from 0 to 50% of RAP but that there is no statistically significant change in
modulus when increasing the percent RAP above 50%. The curvature coefficient
decreases linearly with the increase of the percent RAP from 0 to 50%, but like for the
modulus, there is no statistically significant change in curvature coefficient when
increasing the percent RAP above 50%. Similar trends are seen at both test temperatures.
Statistical analyses also showed that there are no statistical differences in the
power model constants due to changes in the gradation, aggregate source, or RAP source.
Since the initial hypothesis was that this test should be primarily a function of binder
properties, the lack of significant influence by the aggregate gradation and aggregate type
was expected. It was originally thought that RAP sources from different regions of the
country (Georgia and Minnesota) would produce measurable differences in the effective
mix binder properties since different performance grades (PG) are used in the different
regions. However, testing of the extracted RAP binders showed that there was little
difference in the recovered, aged binder. Therefore, little change in the effective HMA
binder is expected.
One of the potential uses for this test method is as an HMA contractor quality
control (QC) test. QC samples, compacted for volumetric testing by a local HMA
contractor, were used to evaluate this premise. Results show that both the stress
relaxation modulus and the curvature coefficient for the unmodified asphalt binder used
v
by the contractor matched the expected values obtained during the initial laboratory
study. This testing also showed the coefficient of variability in the stress relaxation
modulus can be reduced from 30 to 18%, and from 15 to 7% for the curvature coefficient
by testing each sample three times instead of only once.
vi
ACKNOWLEDGEMENTS
I would like to thank Dr. Mary Gardiner for her guidance and assistance throughout this
project. It is not often that one finds an advisor who always finds time for listening and
help to solve all the little problems that are an unavoidable constant in any research work.
I wish to acknowledge the help from the members of my committee, Dr. Dave Timm and
Dr. Dan Brown for their help in correcting the dissertation. I would also like to
acknowledge the assistance provided by the NCAT for the binder testing and TA
Instrument for the loan of a new DSR.
Finally, I want to thank my wife, Annie, and my two sons, Sean and Kyle, for their
patience, support, encouragement and love, which have kept me going during this project.
vii
Style manual or journal used: The Chicago Manual of Style
Computer software used: Microsoft Word, Excel and PowerPoint
viii
TABLE OF CONTENT
Page
CHAPTER 1 INTRODUCTION
1.0 Introduction 1
1.1 Objectives 3
1.2 Scope 4
1.3 Organization 5
CHAPTER 2 LITERATURE REVIEW
2.0 Introduction 7
2.1 Reclaimed asphalt pavement (RAP) 8
2.1.1 Candidate Projects for Recycling 9
2.1.2 Asphalt Recycling Methods 9
2.1.2.1 Hot Recycling 10
2.1.2.2 Hot In-Place Recycling (HIR) 10
2.1.2.3 Cold Recycling (CR) 11
2.1.2.4 Full Depth Reclamation (FDR) 11
2.2 Current Practices for Estimating Binder Properties of HMA with RAP 11
2.2.1 Blending Charts 12
2.3 Problems with the current practice 15
2.4 Influence of the Addition of RAP in a new mix 16
2.4.1 HMA Modulus 18
2.5 RAP mix field applications 20
2.6 Summary 22
CHAPTER 3 THEORETICAL MODEL DEVELOPMENT
3.0 Introduction 24
ix
3.1 Dynamic Shear Rheometer (DSR) 24
3.1.1 DSR Geometry 25
3.1.1.1 Cone and Plate 26
3.1.1.2 Concentric Cylinder 26
3.1.1.3 Parallel Plate 27
3.1.2 Stress and Strain Calculation Using Parallel Plate 27
3.2 Viscoelastic Models 29
3.2.1 Basic Equations 29
3.2.2 Relaxation Modulus Modeling 30
3.2.2.1 Maxwell-Wiechert model 32
3.2.3 Dynamic Modulus 35
3.3 Construction of Master Curves 41
3.4 Summary 41
CHAPTER 4 MATERIALS AND METHODOLOGIES
4.0 Introduction 43
4.1 Materials 43
4.1.1 Binders 43
4.1.2 Aggregates 44
4.1.3 Reclaimed Asphalt Pavement (RAP) 46
4.1.4 HMA Mixes 47
4.2 Test Method Descriptions 48
4.2.1 Dynamic Modulus of Binders 48
4.2.2 Stress Relaxation of Binders 51
4.2.3 HMA IDT Stress Relaxation Test 54
4.2.3.1 Test Method Development 54
4.3 Summary 55
CHAPTER 5 RESULTS AND DISCUSSION
5.0 Introduction 57
5.1 Relation Between Dynamic Modulus and Relaxation Modulus (Binders only) 57
x
5.2 Comparison of binder and HMA stress relaxation test results 59
5.2.1 HMA mixtures with Model Aggregates 59
5.2.2 HMA Mixtures with Standard Aggregates 62
5.3 Statistical Analysis 63
5.3.1 Effect of Gradation, Aggregate Source and RAP Source 65
5.3.2 Effect of PG Grade 65
5.3.3 Effect of the Percentage of RAP 66
5.4 Evaluation of Current Blending Chart Practices for Estimating the Percent
of RAP or Grade of Virgin Binder 68
5.5 Practical Application of Findings 70
5.5.1When to Change Virgin Binder Grade 70
5.5.2 Determining the Percent RAP Actually Used in Construction 72
5.6 Test Method Refinements ? Field Study 74
5.7 Summary 77
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS
6.1 Summary 79
6.2 Conclusion 79
6.3 Recommendations 81
REFERENCE 82
APPENDIX A: Maximum RAP allowed in Pavement for each state 88
APPENDIX B: Tables of results for the HMA indirect stress relaxation test 90
APPENDIX C: Table of results for the field tests 93
APPENDIX D: Draft standard for Indirect Tension Stress Relaxation Test on HMA to
Evaluate the effect of the addition of RAP on the Binder Related
Properties in the ASTM format 95
xi
LIST OF TABLES
Page
Table 2.1 Guidelines for binder selection for RAP mixtures 12
Table 4.1 Binder properties 44
Table 4.2 Aggregate Properties 45
Table 4.3 Gradations of materials used in this study 46
Table 4.4 Average air voids content for the mixes 49
Table 4.5 Strain used in the dynamic modulus test for both binders (DSR AR 1000) 51
Table 4.6 Strain level as a function of the temperature for both binders tested
(Parallel plate configuration) 52
Table 5.1 Influence of the PG grade on the relaxation modulus and the curvature
coefficient (no RAP) 66
Table 5.2 Influence of the percentage of RAP on the relaxation modulus and the
curvature coefficient 67
Table 5.3 Alternative guidelines for when to consider changing PG grades 72
xii
LIST OF FIGURES
Page
Figure 2.1 Example of blending chart for high temperature (% of RAP unknown) 14
Figure 2.2 Example of blending chart for low temperature (% of RAP unknown) 14
Figure 3.1 DSR geometry 26
Figure 3.2 Parallel plate geometry 28
Figure 3.3 Spring and dashpot element and the Maxwell model 30
Figure 3.4 Comparison of experimental data with model
(Based on Macosko 1994) 34
Figure 3.5 Maxwell-Wiechert model 35
Figure 3.6 Dynamic experiment 36
Figure 3.7 Stress wave decomposition 36
Figure 3.8 Example of master curve built from response curves at different
temperatures 42
Figure 4.1 Binder G* master-curve at 22
o
C 44
Figure 4.2 Example of linearity limit (PG 64-22 at 30
o
C) 50
Figure 4.3 Example of time needed to reach constant strain for binder relaxation
test (PG 64-22 at 22
o
C) 53
Figure 4.4 Setup time with steel cylinder in HMA IDT stress relaxation equipment
(22
o
C) 56
Figure 5.1 Relation between measured and calculated shear dynamic modulus for
both binders at 22
o
C 58
Figure 5.2 Relation between measured and calculated shear dynamic modulus for
both binders at 5
o
C 59
Figure 5.3 Relation between binder relaxation modulus (PG 76-22) and HMA
xiii
indirect tension stress relaxation modulus (model aggregate) prepared
with the same PG 76-22 at 22
o
C 60
Figure 5.4 Relation between binder relaxation modulus and HMA relaxation
modulus 62
Figure 5.5 Typical relaxation curve and best fit curves 63
Figure 5.6 Linear relation between percent of RAP and modulus
(MN RAP, Gravel, PG 64-22, 22
o
C) 68
Figure 5.7 Relation between percent of RAP and modulus
(MN RAP, Gravel, PG 64-22, 22
o
C) 69
Figure 5.8 Linear relation between percent of RAP and the curvature coefficient
(MN RAP, Gravel, PG 64-22, 22
o
C) 70
Figure 5.9 Relation between percent of RAP and the curvature coefficient
(MN RAP, Gravel, PG 64-22, 22
o
C) 71
Figure 5.10 Modulus process control chart for field study 75
Figure 5.11 Curvature coefficient process control chart for field study 76
1
CHAPTER 1 INTRODUCTION
1.0 Introduction
Reclaimed asphalt pavement (RAP) is commonly used in combination with asphalt
binders, or mixed with various percentages of new aggregates and asphalt binders to
produce fresh hot mix asphalt (HMA) pavements. It can also be used in the lower
pavement layers (i.e., binder and base layers) to provide improved layer support for
traffic loads. However, limits on the amount of RAP are usually set because of the
nation-wide implementation of the new Superpave binder specifications (Asphalt
Institute, 1996). These specifications require the binder in the mixture to have
rheological properties which will optimize long term pavement performance for a given
set of environmental and traffic conditions. When RAP is added, the residual aged binder
in the RAP mixes to some degree with the virgin binder. This produces a composite
effective binder system with unknown material properties and hence unpredictable
pavement performance.
For the purposes of this research, effective binder refers to the combination of
neat asphalt binder and RAP binder that governs HMA properties that are primarily a
function of the HMA binder. These properties include, but are not limited to, HMA
critical low temperature (i.e., thermal cracking) and resistance to rutting. Changes in
2
these properties are due to the incorporation of the stiffer, aged RAP binder into the
effective binder of the mix.
The NCHRP 9-12 (Use of reclaimed asphalt pavement (RAP) in the Superpave
design method) was recently completed (McDaniel et al., 2000). The researchers found
that RAP should not be considered a ?black rock? because significant blending occurs
between the RAP and neat asphalt binders. Researchers recommended that blending
charts using neat and recovered RAP binder properties be used to account for the RAP
contribution to the total binder properties or conversely, to select a softer (lower grade)
neat binder when RAP is used in lower quantities. In blending charts, a linear relation is
assumed between the amount of recovered RAP binder in the combined binder and the
grade of the combined binder. Unfortunately, those blending charts are used for the
binder alone, not the complete mix (binder and aggregates).
To use blending charts, the RAP binder must be extracted, recovered and graded.
The extraction of the binder from a mix can be done in different manners, but to be able
to recover the binder and grade it afterwards, the extraction must be done with solvents.
Those solvents are dangerous and the extraction process can alter the properties of the
binder. In the recovering process, the binder is heated which results in making the binder
stiffer than it was in the mix. The gradation of the recovered binder is done with a
dynamic shear rheometer (DSR). The complete extraction, recovery grading process can
take up to a week.
The primary hypothesis for this research is that the tensile stiffness of mixtures
within the linear viscoelastic range is primarily an indication of binder properties.
Changes in mixture stiffness due to the inclusion of RAP in the mixture should therefore
3
reflect the contribution of the RAP binder to effective (combination of both neat and RAP
binder) binder content. Using an indirect tension stress relaxation test rather than either
tensile strength or a tensile creep test allows for the direct application of general linear
viscoelasticity theory for predicting the dynamic (oscillatory) response. Therefore, the
mixture test results can be theoretically converted into predictions of dynamic results as
used in Superpave binder specification.
1.1 Objectives
The main hypothesis for this research is that the tensile properties of compacted HMA
mixes will be governed by the properties of the asphalt binder, since this is the only
component in the compacted sample that can withstand a tensile load. This hypothesis
builds on the current low temperature indirect tensile strength creep test that is used to
predict the critical low temperature at which the HMA pavement will exhibit thermal
cracking. Thermal cracking has been shown to be primarily a function of asphalt binder
properties (Carter, 2002). If this hypothesis can be applied to the results from a
simplified indirect tensile stress relaxation test, then the properties of the asphalt binder
can be evaluated by testing the compacted HMA samples rather than needing to extract,
recover, and test the binder. Also, there should be a limited influence of gradation on the
stress relaxation characteristics if the stress relaxation characteristics are mostly a
function of only the asphalt binder properties.
The main objectives of this research were to:
? Develop and validate a simple, quick indirect tension stress relaxation
test for assessing the asphalt binder properties using compacted HMA
4
samples that do not contain reclaimed asphalt pavement.
? Evaluate the effect of adding RAP to HMA mixtures on the stress
relaxation characteristics of the mix.
1.2 Scope
Two experimental designs were used, one to accomplish each objective. The first
experiment was developed to compare the asphalt binder stress relaxation properties to
the HMA indirect tensile stress relaxation properties of the compacted HMA mixture. A
parallel plate rheometer was used to develop shear stress relaxation master curves for two
asphalt binders (PG 64-22, PG 76-22). These master curves were compared to indirect
tensile stress relaxation master curves developed for the HMA mixtures prepared using
the same two binders with either of two gradations. If the initial hypothesis is correct,
and the indirect tensile stress relaxation test represents primarily binder properties, then
there should be a good correlation between the two test results once shear modulus is
converted to tensile modulus. A range of eight test temperatures was used to construct
the master curves for the asphalt binder. Only two test temperatures were used to
construct the master curves for the HMA mixes because of a time constraint. All tests,
both asphalt binder and HMA, were done within the linear viscoelastic range.
The second experiment was designed to determine if the HMA indirect tensile
stress relaxation characteristics are sensitive to changes in HMA mixture composition,
such as those anticipated with various percentages of RAP. Once the HMA indirect
tension stress relaxation test was validated as being representative of neat binder
properties, a range of HMA mixes were tested and statistical analyses were used to assess
5
the influence of RAP on HMA mix properties. A Superpave gyratory compactor was
used to prepare samples at mix design air voids (i.e., 4 percent) for two binder, two
gradations, two aggregate types and two sources of RAP at one of five concentrations (0,
15, 25, 50, and 100 percent). The 100 percent RAP mixtures were used as a mixture
representation of the recovered binder properties used in the blending charts
recommended by McDaniel et al. (2000).
The potential for using this test as an additional QC test to monitor the
consistency and grade of asphalt binder was evaluated once the test method was
rigorously evaluated in the initial laboratory experiment. Compacted HMA samples
prepared by a local HMA contractor for quality control (QC) testing of mix volumetric
were collected and tested. The mixes, tested over one week of production, did not
contain RAP; indirect tensile stress relaxation was conducted at only at two temperatures,
5 and 22
o
C.
1.3 Organization
The organization of this document is as follows:
Chapter 1 ? Introduction of problem statement and initial hypotheses.
Chapter 2 ? Literature Review
Chapter 3 ? Theoretical Model Development
Chapter 4 ? Materials and Methodologies
Chapter 5 ? Results and Discussion
Chapter 6 ? Conclusions and Recommendations
6
Chapter 2 provides background information on RAP, the different methods used to
produce it, current practices for its design and use, and different problem associated with
determining how to estimate the influence of the RAP binder on the effective HMA mix
binder. Chapter 2 also includes a literature review that covers the influence of the
addition of RAP to HMA mixes as evaluated in both the laboratory and field projects.
Chapter 3 provides information regarding dynamic shear rheometer and viscoelastic
models used in defining binders? behavior. It also establishes the fundamental relations
between stress relaxation modulus and dynamic modulus. Chapter 4 documents all the
materials used in this research program, sample preparation, sample compaction, and
testing methodologies. Chapter 5 explains the relationship between dynamic shear
modulus and stress relaxation tests on binder alone. The relationship between the stress
relaxation for binder and for the mixes, with and without RAP, is also discussed in this
chapter. The last sections in Chapter 5 present the results of the field study that uses the
test method as a possible QC test during construction. Finally, Chapter 6 summarizes the
results of the research program and suggests future work.
7
CHAPTER 2 LITERATURE REVIEW
2.0 Introduction
It has been well established that binder properties have a large effect on the properties of
hot mix asphalt (HMA) mixes (Roberts et al., 1991). Binder viscosity needs to be
sufficiently low at high temperatures to allow the material to be moved through the HMA
plant. It also needs to be sufficiently stiff at the average maximum high in-service
temperature so that load-induced deformation (rutting) is minimized. At the same time,
the binder needs to be flexible (ductile) at cold temperatures so that thermal cracking is
minimized by the material?s ability to dissipate stresses through deformation.
The performance graded (PG) asphalt binder specification was a product of the
Strategic Highway Research Program (SHRP) program (circa 1993) and was developed
to evaluate fundamental properties of binders under the expected local environmental
conditions. This specification incorporates three rheometers for defining key asphalt
binder properties over a wide range of temperatures. A concentric cylinder rheometer at
135
o
C is used to set a maximum viscosity for workability (i.e., pumpability). A dynamic
shear rheometer (DSR) is used to define the complex shear modulus (i.e., viscoelastic
stiffness) at both high (summer) and intermediate (spring/fall) in-service temperatures. A
bending beam rheometer evaluates cold (winter) temperature stiffness and rate of
deformation.
8
Ideally, the PG binder grading specification should be used to evaluate the binder
properties of reclaimed asphalt pavement (RAP)-modified HMA as well as the blend of
virgin and RAP binder that will form the effective binder in the final RAP mix. However,
current methods for estimating the influence of RAP on binder properties requires that
the aged asphalt in the RAP be solvent extracted, hot-recovered from the solvent, and
tested. These results, mathematically combined with the results with virgin asphalt test
results, are used to estimate composite binder properties. The amount of RAP that is
actually incorporated into the effective HMA binder cannot currently be assessed.
This chapter is separated into four sections. First, a summary of RAP production
and how it is used in new mixes is presented. Second, a brief summary of the current
practice for estimating binder properties of HMA with RAP is shown as well as problem
with the current practice. The third section shows the influence of the addition of RAP in
a new mix and how that influence is measured.
2.1 Reclaimed Asphalt Pavement (RAP)
RAP is the acronym for Reclaimed Asphalt Pavement. ?Recycled? rather than
?Reclaimed? Asphalt Pavement is also an often-used term. RAP is produced when a
paved road is milled during rehabilitation; it is not a new process. The first documented
case of hot in-place recycling was done in the 1930?s (ARRA 2001). The interest in
asphalt recycling has increased in the 1970?s because of the petroleum crisis and because
a large scale milling machine was developed in 1975.
The Federal Highway Administration (FHWA) reports that around 91 million
metric tons of pavements are milled every year in the USA. About 80 percent of that is
9
reused in new roads, road bed, shoulders and embankments (APA 2004) saving taxpayers
almost $300 million annually (Eighmy and Magee 2001). Each state has different rules
as to how much RAP is allowed in the different layers of the pavement. The percent used
ranges from 0 to 70% in the base course, the binder course or the surface course
(Banasiak 1996). A complete table showing the percentage allowed for each state is
shown in Appendix A.
2.1.1 Candidate Projects for Recycling
Almost any HMA rehabilitation project that requires an overlay is a candidate for
recycling. However, there is some limitation to the process. For example, if the
distressed pavement layer is very thin, the milling machine may break it into chunks
which would need additional crushing before being reused. In this case, it may be more
economical to simply put an overlay on top of the existing surface (Roberts et al., 1991).
Also, if the aggregates in the existing pavement do not meet the materials
specifications for the new mix, the old pavement will be simply discarded or used in
other projects in the lower layers. This will be done if there is too much fine material, if
the aggregate tends to polish or if it breaks too easily.
Pavement recycling is used to removed rutted or oxidized pavements or to correct
the profile of the road. If the problem of the pavement is structural, recycling is not the
solution.
2.1.2 Asphalt Recycling Methods
There are four main asphalt recycling methods: Hot recycling, Hot In-Place recycling
10
(HIR), Cold Recycling (CR) and Full Depth Reclamation (FDR). The information in this
section comes from ARRA (2001) unless stated otherwise.
2.1.2.1 Hot Recycling
Hot recycling is the most popular asphalt recycling technique in the world. In the USA,
over 50 millions tons of RAP are generated annually by State Highway Agencies and
around 33 percent of that is used in hot recycling (APA 2004). Hot recycling is the
mixing in a plant of the RAP and new aggregates, binder and sometimes recycling agent.
The amount of binder contained in the RAP and the viscosity of the recycled binder is an
important factor in the choice of the virgin binder. Once the new mix is produced, it is
placed and compacted with conventional HMA equipment.
2.1.2.2 Hot In-Place Recycling (HIR)
In hot in-place recycling, the pavement is softened by heating then scarified. The
recycled HMA can be used by itself, but often new aggregates and binder are used to
correct the mix composition and volumetric. Oxidized binder can be rejuvenated during
HIR by adding a recycling agent in the mixing process.
Since HIR is done in one step, the traffic disruption is very limited. However, the
maximum treatment depth is around three inches. If the distress in the pavement layers is
deeper than this depth, another method of recycling will be necessary.
11
2.1.2.3 Cold Recycling (CR)
Cold recycling can be done in place or at the plant. In cold recycling, the asphalt
pavement is removed without the use of heat. Cold in-place recycling processes often
mix the RAP with an emulsion, then place and compact the mix. This new mix can be
used as a stabilized base, or, less frequently, as the wear course on low volume roads.
The in-plant cold recycling process is the same except that the RAP in stocked and mixed
only when needed. CR is usually done on the top two to four inches of pavement.
2.1.2.4 Full Depth Reclamation (FDR)
FDR is a cold in-place recycling technique that pulverizes the top four to twelve inches of
materials. The pulverized material usually includes the asphalt layer and a part of the
base layer. The reclaimed pavement is then mixed and more materials can be added
(aggregates, binder, etc.) before being placed. This new material is considered a treated
base; it is compacted and graded before being covered by a new asphalt pavement layer.
2.2 Current Practices for Estimating Binder Properties of HMA with RAP
Selection of the virgin asphalt PG grade depends on the quantity of RAP added.
According to McDaniel et al. (2000), if less than 15% of RAP is used, there is no need to
change the binder grade. If between 15% and 25% of RAP is used, the virgin binder
grade is commonly decreased by one grade (6
o
C) on both ends (e.g. a PG 64-22 is
changed to a PG 58?28). If more than 25% of RAP is used, then the RAP binder needs to
be extracted, recovered and graded using the performance-graded binder tests. This
information, along with the same test information for the new binder is then used to
construct blending charts.
12
Other guidelines based only the recovered RAP binder properties were presented
by McDaniel et al. (2001). These guidelines (Table 2.1) are a more precise since the
quantity of RAP allowed is based on the recovered binder properties which are different
with every RAP stockpile. This table indicates that a warmer lower temperature grading
of the RAP binder results in a decrease in the amount of RAP that can be used in the mix.
That is, a grading of -22
o
C allows the use of at most 20% RAP; when the grading of the
RAP is -10
o
C, only 10% at most RAP can be used.
Table 2.1: Guidelines for binder selection for RAP mixtures (McDaniel et al., 2001)
RAP Percentage
Recovered RAP Grade
Recommended Virgin Asphalt Binder
Grade
PG xx-22
or lower
PG xx-16
PG xx-10
or higher
No change in virgin binder selection < 20% < 15% < 10%
Select virgin binder one grade softer than
normal (ex.: PG 58-28 instead of PG 64-22)
20-30% 15-25% 10-15%
Follow recommendations from blending
charts
>30% >25% >15%
2.2.1 Blending Charts
The blending charts are used to determine the percentage of RAP needed to produce a
mix with a specific PG binder grade when the percent of RAP to be used is more than
25% (McDaniel et al., 2001). Although it can be used for any percentage of RAP, the
approach requires significant testing and time in order to obtain results so it is usually
limited to only the higher percentage of RAP mixes. There are two ways to use the
blending charts:
? A known percentage of RAP to be used and the virgin binder grade needs to be
selected to achieve a specific blended binder grade.
13
? A known virgin binder grade and the percentage of RAP that can be used to
achieve a specific blended binder grade needs to be selected.
The charts are built following the Superpave grading specification. To build a
blending chart, the recovered binder is tested with a dynamic shear rheometer (DSR) and
a bending beam rheometer (BBR) in order to estimate the PG grading of the RAP binder.
The same tests are done for the virgin binder. Two charts need to be constructed, one for
the high temperature properties (i.e, DSR results) and one for the low temperature
properties (i.e., BBR results). For example, if the grade of the recovered binder is found
to be PG 82-10 and the grade of the virgin binder is PG 58-28, the blending chart for high
temperature (Figure 2.1) shows that between 25% and 50% of RAP can be added to
achieve a blended grade of PG 64-22. The low temperature blending chart (Figure 2.2)
shows that a maximum of 33% of RAP can be added to achieve a PG 64-22. So
anywhere between 25% and 33% would be appropriate. Instead of using the graphical
solution, one can use the following equation:
virginRAP
virginblend
TT
TT
RAP
?
?
=%
Eq. 2.1
where:
%RAP is the percentage of RAP expressed in decimal
T
blend
is the critical temperature of the blended asphalt binder
T
virgin
is the critical temperature of the virgin asphalt binder
T
RAP
is the critical temperature of the recovered RAP binder
14
50
55
60
65
70
75
80
0% 20% 40% 60% 80% 100%
% of RAP
T
cr
i
t
i
c
a
l
(
o
C)
50%
25%
70
o
C
64
o
C
Figure 2.1: Example of blending chart for high temperature
(% of RAP unknown)
-30
-28
-26
-24
-22
-20
-18
-16
-14
-12
-10
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
% of RAP
T
c
r
it
ic
a
l
(
o
C)
33%
-22
o
C
Figure 2.2: Example of blending chart for low temperature
(% of RAP unknown)
15
Even though the Superpave method recommends using both original and rolling
thin film oven (RTFOT) high temperature blending chart, usually only the original binder
property blending chart is used to avoid running the time consuming RTFOT testing
(ARRA 2001).
2.3 Problems with Current Practice
Every method that can separate the binder from the aggregates in an HMA uses a solvent
such as 1,1,1 trichloroethane (TCA), trichloroethylene (TCE), methylene chloride, or
toluene. The solvent removes the binder from the aggregate, and by distillation, the
binder is separated from the solvent. The first problem with this practice is that the
solvent does not completely remove the binder from the aggregate (Cipione et al., 1991).
After a TCE extraction, there can be up to 2% of binder left on the aggregate (Peterson et
al., 1982). The binder that is not extracted represents a part of the binder that is strongly
adsorbed on the surface of the aggregate. Because this part of the binder is not present in
the extracted binder, it is possible that the properties determined for the extracted binder
do not represent the actual RAP binder properties (Peterson 1984).
The second problem with the use of solvent is that the binder typically becomes
stiffer after extraction (Burr et al., 1991). This hardening appears in all solvent extraction
methods, but to a lesser extent when cold extraction processes are used as compared to
hot extraction methods. Once again, the hardening of the binder will results in binder
properties different than the actual binder properties in the mix.
Another problem lies in the fact that solvent extraction is expensive. The price of
the solvent is not too high, but the cost associated with disposing of the waste is high
(Behrens et al., 1999). When TCE is used, the cleaned aggregates are considered
16
hazardous waste because the flash point is under 140
o
F (McGraw et al., 2001). The flash
point is the temperature at which the vapor of the heated material ignites in the presence
of a spark or an open flame (Roberts et al., 1996). There are also health problems
associated with the use of solvent. TCE for example, has been proven to causes cancer in
mice and rats and is suspected of causing cancer in humans (Fialka 2004).
Finally, there is a large variability in the properties of the recovered binders. In a
research by Stroup-Gardiner and Nelson (Stroup-Gardiner and Nelson 2000), it was
shown that the within-laboratory variation for binder testing is about 23 to 30% and about
38 to 45% between-laboratory. This variation can be explained by the problems noted
above. In addition, some solvent may remain in the binder after recovery, which may
also alter the properties (Collins-Garcia et al., 2000).
2.4 Influence of the Addition of RAP in a New Mix
Previous research has shown that tests to evaluate the effect of the addition of RAP in a
new mixture can be used to adjust the new HMA mix design. These studies concentrate
on evaluating changes in key mix properties that are related to pavement performance
such as fatigue (Huang et al. 2004) and mix stiffness. The fatigue tests done by Huang et
al. (2004) were done in indirect tension (IDT). Indirect tension tests are easy to perform,
but they are more complicated to analyze due to the complicated stress contours within
the sample.
The tensile strength (ITS) and the toughness index (TI) have also been used in
that research (Huang et al., 2004). TI is a parameter describing the toughening
characteristic in the post-peak stress region. A perfectly plastic material will have a TI of
1 and an ideal brittle material will have a TI of 0, therefore HMA with and without RAP
17
will have TI values between these two limits. This research showed that there is no
significant difference in ITS and TI between mixes containing 0 or 10% of RAP. There
is also no significant difference between mixes containing 20% to 30% of RAP.
However, between 0 and 20% of RAP, ITS increase significantly and TI decrease
significantly (Huang et al. 2004). If related to fatigue, this would mean that mixes
containing at least 20% of RAP would have the potential to absorb more strain energy
before they start to fail but after failure (cracks), the specimen would fail faster (lower
post-failure tenacity).
As for the effect of the addition of RAP to the complex modulus, it was found that
the complex modulus increases with the percentage of RAP (Daniel et al. 2004 and
Sondag et al. 2002). According to Sondag et al. (2002), the source of RAP also has an
influence on the complex modulus since different RAP sources have different binder
properties. The phase angle, measured during the complex modulus test, decreases with
the addition of RAP (Sondag et al. 2002). The rate of decrease is high between 0 and
15% of RAP and much smaller between 15 and 40%.
It was also been found that the resilient modulus tends to increase with the
addition of RAP (Sargious and Mushule 1990, Sondag et al. 2002 and Garg and
Thompson 1996). On the other hand, Kandhal et al. (1995) has shown that the difference
in resilient modulus between an HMA with virgin material and a mix containing RAP is
often not statistically different. This is probably due to the fact that binders with lower
viscosity were used when large amount of RAP was used. It could also be a function of
the variability in the test method.
18
2.4.1 HMA Modulus
There are currently two main methods to determine the modulus of asphalt concrete; a
dynamic or repeated load axial compression test and a dynamic or repeated load
diametral indirect tension test (IDT). The main difference between these two tests is the
state of stress in the specimen (Kim et al. 2004). In the axial compression test, there is a
uniaxial state of stress, while the diametral configuration produces a biaxial state of
stress. Therefore it is more complicated to calculate the modulus using the IDT test than
the axial compression test.
The results from IDT and compression tests are not always in good agreement
when testing HMA because this material can have different properties in tension as
compared to compression. In fact, the modulus calculated from both methods is only
similar at low temperature (Khanal et al., 1995). The difference in results from the two
test methods can be explained by the fact that at low temperature, the binder is stiff and
does not allow the movement of the aggregates as well as it does at higher temperatures.
Both tests are more representative of the binder properties than the aggregates properties
when done at low temperatures. It should be noted that at room temperature and higher,
compression test results represent more of the aggregates properties than the binder
properties. This is because in compression, the contact between the aggregates (i.e.,
aggregate interlock) that mainly control the strain. In a tension test, the aggregate
interlock does not have a big effect because the only thing keeping the aggregate particles
from separating is the binder. The modulus calculated from a compression test can be 35
to 45% higher than the modulus calculated from the indirect tension test (Tayebali et al.,
1995).
19
Even though IDT tests are simple to perform, one needs to know or to determine
Poisson?s ratio in order to calculate a precise modulus (Roque and Buttlar 1992).
Poisson?s ratio is usually assumed to be 0.35, but it changes depending on the mix, the
test temperature and the frequency of the test (Tayebali et al., 1995). It is generally
accepted in HMA analysis that Poisson?s ratio is assumed to be 0.2 at temperatures lower
than 10
o
C, 0.35 between 10 and 30
o
C, and 0.5 above 30
o
C. Assumptions are made about
Poisson?s ratio because this property is very difficult to measure during testing; problems
are related to the sensitivity of sensors at very low strain measurements, equipment
vibrations, gauge mount slippage, and the localized but high compressive zone directly
under the loading strips (Wallace and Monismith 1980).
In an IDT test, the horizontal and vertical strain can be measured externally or
internally. When the vertical displacement is measured by an LVDT on the ram, it is
called external. The horizontal strain is then calculated using an estimated Poisson?s
ratio. An internal measurement uses strain gages glued to the center of one or both flat
faces of the sample. The main disadvantage of using external measurement is that the
strain near the point of contact on the specimen will be taken into account in the
calculation even if it is not representative of the failure plane (Roque and Buttlar 1992).
External measurement will also measure any rocking of the samples. Internal
measurements will measure only what happens in the center part, but the strain gages are
so small that any segregation in that section of the sample will results in erroneous results
(Wallace and Monismith 1980).
In the IDT creep test used to estimate thermal cracking potential, the master creep
compliance curve is transformed in the master relaxation modulus curve by a Laplace
transformation, which is then used to compute the thermal stresses in the pavement
20
according to a constitutive equation (Lytton et al. 1993). The indirect creep test was
preferred to the indirect stress relaxation test because, according to Baumgaertel and
Winter (1989), it is easier to conduct and more reliable than the relaxation test. One of
the reasons that makes the relaxation test more complicated is the need to reach a precise
strain in a very short time (Menard 1999). Another reason given for the preference of
creep testing over relaxation testing is that creep tests can be more useful to engineers
and designers (Nielson and Landel 1994).
2.5 RAP Mix Field Applications
RAP has been extensively used in projects all over USA and Canada. In most cases, the
pavement constructed with RAP had mechanical behavior comparable to pavement made
with virgin mixes (McDaniel et al. 2002, Emery 1993, Kandhal et al. 1995, Garg and
Thompson 1996, Hossain et al. 1993, Tam et al. 1992, Paul 1996, Larsen 2003, TFHRC
2004 and Cosentino et al. 2003). RAP is now used in almost all 50 states in the base, the
binder course, and the surface course and as noted before, each state has their own
specifications regarding the amount of RAP that can be used and where it can be used
(Appendix a, Table A1) (Basaniak 1996).
Historically, the amount of RAP that could be added in a mix was limited by the
heating capacity of the equipment (Shoenberger et al. 1995). RAP was initially added
with the aggregate, which created problems when the old binder came into contact with
the open flame. As technology improved, drum plants were modified with a collar further
down the drum and away from the flames for the addition of the RAP.
21
When RAP is used in the base material, it was found that the California Bearing
Ratio (CBR) is lower compared to a base made of crushed aggregate (Enery 1993, Garg
and Thompson 1996, Cosentino et al. 2003). Taha et al. (1999) hypothesize that the CBR
is lower with the addition of RAP when compared to virgin aggregate because the RAP
binder creates a slip plane between the aggregate, which facilitates the movement of the
aggregates.
In any cases reported in these studies, there was no difference in rutting between
HMA overlays with or without RAP. Fatigue cracking was found to increase when RAP
was used in the wearing course (Paul 1996). There is some evidence that if 20% RAP or
more is used in the wearing course, the viscosity of the combined binder will likely
increased over a limit (12 000 poise at 60
o
C), above which pavements tend to crack.
However, Hossain et al. (1993) observed that mixes used as wearing course containing
50% RAP have a lower rate of fatigue crack increase than virgin mix. It should be noted
that the virgin mix contained an AR-8000 binder and that the recycled mix, which has
0.5% more asphalt binder, contained an AR-4000 binder.
According to Tam et al. (1992), if less than 50% of RAP is used, the amount of
thermal cracking should be limited. In this study, thermal cracking was directly related to
the penetration of the binder. If the penetration of the recovered combined binder (virgin
+ RAP) is under 20 dmm, over 20 000m/km of crack length can be expected (Tam et al.,
1992). This means that the pavement will be completely disintegrated due to thermal
cracking. In another case documented by Sargious and Mushule (1991), mixtures
containing 45% of RAP were found to crack at colder temperature than mix without
RAP. The softer virgin binders used in with RAP mixes (400/500 penetration grade in
22
the RAP mixes and 150/200 penetration grade in the virgin mixes) may be the reason for
the lower amount of cracking seen in these studies. Larsen (2003) reported that in a case
of a project in Connecticut, the only differences between pavement made with a wearing
course containing 20% RAP and pavement made with virgin mixes came from the layers
underneath. In that project, there were no significant differences in any types of cracking
or in rutting between virgin and RAP mixes after five years of monitoring.
2.6 Summary
The current practice to use RAP in new mixes include the extraction of the binder in the
RAP and its gradation. Unfortunately, the extraction process requires solvent that can be
a health hazard, that are expensive to get rid of, and that can change the binder properties.
The amount of RAP to be used is set depending on the PG grade of the RAP binder, the
PG grade of the virgin binder, and the PG grade needed from the combined binders.
Blending charts are currently used to evaluate the effect of the RAP binder on the PG
grade of the combined RAP binder. Blending charts assume that there is a linear relation
between the amount of RAP binder included in the combined binder and the change in
PG grade.
The addition of RAP to a mix makes the resulting HMA stiffer. An addition of 15
to 20% of RAP is enough to increase the complex modulus and to alter the fatigue
characteristics. Depending upon the PG grade of the virgin binder and the pavement
structure, the fatigue properties may improve or deteriorate. In some cases, it has been
shown that only 20% of RAP in the wear course is necessary to increase fatigue cracking.
This literature review emphasizes the need to be able to evaluate the actual HMA mix
23
properties in order to select the most appropriate percentage of allowable RAP, or to
select the PG grade of virgin binder to use with a preselected percentage of RAP.
24
CHAPTER 3. THEORETICAL MODEL DEVELOPMENT
3.0 Introduction
In polymer science, the relation between dynamic modulus, creep modulus and relaxation
modulus has been well established. Since asphalt binders are considered to behave like
short-chain polymers (i.e., oligomers), the same relations can applied. Lately, the
relaxation spectrum has been used to determine the zero shear viscosity of binders (Rowe
and Pellinen 2003).
In this chapter, the instrumentation used to analyze the binder rheology is
described, the mathematical model used to represent the binder behavior, and the relation
between the dynamic and the stress relaxation modulus are explained.
3.1 Dynamic Shear Rheometer (DSR)
The main advantage to using the DSR to determine binder stiffness is that the viscoelastic
(i.e., time-temperature) properties of the material can be considered. The DSR is a
parallel plate rheometer that is set up by sandwiching an asphalt sample between two
circular plates. The asphalt is sheared by the oscillatory movement of the upper plate at a
speed of 10 rad/s (frequency set by PG specification). The applied shear stress is a
sinusoidal signal oscillating around zero. The actual test temperatures are based on the
anticipated in-service temperature in which the binder will be used.
25
To correctly describe the behavior of a binder, the complex shear modulus, G*,
and the phase angle, ?, are needed. G* is the maximum shear stress divided by the
maximum shear strain of the oscillatory signal. The phase angle is the time lag between
the stress and strain signals. A perfectly elastic material has a phase angle of zero since
the strain follows the stress perfectly. With a purely viscous material, there is a 90
o
difference in the position of the stress and strain signals. Materials that exhibit a
combination of both elastic and viscous behavior are call viscoelastic and have a phase
angle somewhere between 0 and 90
o
(Roberts et al. 1996).
Two parameters measured with the DSR are used in the PG binder specification
to quantify binder properties for adequate pavement performance with regards to rutting
resistance and fatigue resistance (Roberts et al. 1996). For rutting, the binder must be
stiff (high G*) and elastic (low phase angle) to resist the shear stresses from traffic loads
and to have as little permanent deformation as possible. For fatigue resistance, the binder
should be soft (i.e., low G*) and elastic (i.e., low phase angle) to allow the HMA layer to
flex without cracking after a number of repetitive loading cycles.
3.1.1 DSR Geometry
The three most commons geometries used with a DSR are the parallel plate, cone and
plate and the concentric cylinder (Macosko 1994) (Figure 3.1). These three geometries all
have advantages and disadvantages.
26
3.1.1.1 Cone and Plate
The cone and plate geometry (Figure 3.1a) is used with sample with submicron size
particles. Samples with particle matter in them, such as binder mastics, should not be
tested with the cone and plate geometry since the solid particles will tend to migrate to
the apex of the cone and get jammed. This would results in erroneous results since one of
the underlying assumptions for this test is that the material is homogenous (TA 2003).
The main advantage of the cone and plate geometry is the fact that the shear strain and
shear rate are constant in the sample (Macosko 1994).
3.1.1.2 Concentric Cylinder
The concentric cylinder (Figure 3.1b) is for testing materials with a low viscosity that can
be poured into the bottom cylinder. In the HMA industry, concentric cylinder
rheometers are used to evaluate the high temperature viscosity of the binder. A
maximum viscosity is set in the PG specification to make sure that the binders can be
pumped during HMA production. High temperature viscosity values for at least two
temperatures can be used to set the laboratory mixing and compacting temperatures.
c) Parallel
plate
a) Cone and
plate
b) Concentric
cylinder
Figure 3.1: DSR Geometry
27
3.1.1.3 Parallel Plate
The parallel plate geometry (Figure 3.1c) produces a shear strain that is not
homogeneous; it depends on both the radial and vertical position within the sample. Since
the calculation of shear stress and strain use equations for a cylindrical geometry, care
needs to be taken during sample preparation to make sure that the sides of the sample are
perpendicular to the upper and lower plates. This configuration can be used to test
materials with discrete particles in the binder, however there is a general guideline that
indicates the gap between the plates needs to be no less than 10 times the diameter of the
largest discrete particle (TA 2003).
3.1.2 Stress and Strain Calculations Using Parallel Plate
The sample used with the parallel geometry is usually 1 or 2 mm thick and the plates
have 8 or 25mm in diameter when testing asphalt binders (Kennedy et al., 1994). The
diameter of the plate is chosen depending on the test temperature since the DSR has a
limited torque capacity. A smaller diameter of the cylindrical geometry is needed with a
stiffer material or a colder test temperature.
The shear stress is calculated by dividing the applied force, the torque in this case,
by the area on which the force is applied:
3
2
r
T
?
? = Eq. 3.1
where:
? = shear stress (Pa)
T = torque (N-m)
r = radius (m) (Figure 3.2)
28
Also, by definition, the shear strain is the deformation caused by forces that
produce an opposite but parallel sliding motion of the body?s plane. In the DSR test with
parallel plate, the shear strain is calculated by dividing the angular movement by the
thickness of the specimen:
h
r?
? = Eq. 3.2
where:
? = shear strain (mm/mm)
? = angular displacement (rad)
r = radius (mm)
h = thickness (distance between plates) (mm)
a) Side view b) Top view
r
P
Torque: T = P * r
?
Figure 3.2: Parallel plate geometry
Radius (r)
Height (h)
T
29
The shear strain commonly used is the shear strain measured at the edge of the
plate (Kennedy et al. 1994). The angular displacement is measured by relative
displacement of the upper and lower shaft. With the stress and the strain, it is possible to
calculate a modulus. If the stress is maintained constant, it is called the creep modulus.
If it is the strain that is held constant, it is called the relaxation modulus.
3.2 Viscoelastic Models
Theoretically, relaxation modulus can be used to predict dynamic modulus. The
following section shows how the equations from each part are derived so the results from
one test can be compared to the other.
3.2.1 Basic Equations
When a purely elastic body without any inertial effect is subjected to an instantaneous
stress, it responds with an instantaneous strain; it is the Hooke law (Aklonis et al., 1983).
For a shear displacement, the equation is:
?? G=
Eq. 3.3
Where:
? = stress (Pa)
? = strain (mm/mm)
G = shear modulus (Pa)
On the other hand, a fluid with no elastic behavior but simple linear viscous
behavior will obey Newton?s law (Menard 1999):
30
dt
d?
?? =
Eq. 3.4
Where:
? = viscosity (Pa.s)
d?/dt = shear strain rate (s
-1
)
G for elastic materials as well as ? for viscous materials are proportionality
constants. The biggest difference between those two expressions is the fact that there is a
time dependency for a fluid.
3.2.2 Relaxation Modulus Modeling
The Maxwell model provides a mathematical combination of elastic (spring, Figure 3.3a)
and viscous (dashpot, Figure 3.3b) material behavior. The individual components are in
series (Figure 3.3c). In a Maxwell model, the stress is the same in the two elements since
they are in series, but the strain differs.
G
G
?
?
a) Spring
Element
b) Dashpot
Element
c) Maxwell
Model
Figure 3.3: Spring and dashpot element and the Maxwell model
31
Since G and ? are two proportionality constants, the relationship between the two
is (Haddad 1995):
G?? =
Eq. 3.5
where:
? = relaxation time of the element (a constant)
To calculate the behavior of a material with the Maxwell model, the equation of motion
of the Maxwell model is used. In this equation, the derivative of the strain as a function
of time is the result of the immediate elastic behavior (first part on the right side) and the
viscous behavior (second part on the right side):
?
???
+=
dt
d
Gdt
d 1
Eq. 3.6
In a stress relaxation experiment, an instantaneous strain of ?
o
is imposed to the sample
and the change in stress with time is recorded (?(t)) (Rosen 1993). Since the strain is
theoretically an instantaneous strain, d?/dt = 0 after the load application, so equation 3.6
becomes:
dt
d
G
or
dt
d
G
?
?
?
?
?? 11
0 =?+= Eq. 3.7
If equation 3.5 is combined with equation 3.7, the result is:
dt
d
GG
?
?
? 1
=? Eq. 3.8
This equation can be simplified to:
??
? dtd
?= Eq. 3.9
32
The integration of that equation from a time 0 to a time t is:
() ( )
??
=
?=???=
tt
t
t
tdt
d
t
00
0lnln
1
?
??
??
?
?
?
Eq. 3.10
Equation 3.10 can be transformed in an exponential and then both sides of the equation
are divided by ?
0
.
()
()
()
()
() ()
() ()
00
0
0
00
ln
?
?
?
?
??
?
?
??
?
?
??
te
te
t
e
tt
t
tt
=?=?=??=
?
??
Eq. 3.11
The right side of equation 3.11 is the stress relaxation modulus and on the left side, the
shear modulus is multiplying the exponential. This can be simplified as:
()tGeG
t
=
?
?
0
Eq. 3.12
It should be noted that the larger ? is, the slower the stress will relax (Wineman
and Rajagopal 2000). This means that the relaxation time is a measure of how quickly
the stress relaxes.
3.2.2.1 Maxwell-Wiechert model
Unfortunately, for the Maxwell model to represent experimental test results, a series of
relaxation time and elastic modulus have to be used (figure 3.4). This series, called a
Prony series (Wineman and Rajagopal 2000). Equation 3.12 becomes:
()
k
t
nk
k
k
eGtG
?
?
=
=
?
=
1
Eq. 3.13
33
In Figure 3.4a, only one relaxation time and modulus is used in the model. In
3.4b, five different relaxation time and modulus are used to better fit the experimental
data.
According to Tsai et al. (2004), 12 couples of relaxation time and modulus should
be used to have a good model. The sum in equation 3.13 can be represented as a series of
Maxwell elements placed in parallel (Figure 3.5) (Barnes 2000). This model is called the
Maxwell-Wiechert model (Aklonis 1983).
To obtain the relaxation time and modulus for this model, a relaxation time that is
evenly spaced is chosen and the modulus is determined with a linear regression equation
that minimizes the least square. The relaxation time used in the Maxwell model for the
relaxation experiment can then be used to simulate the dynamic modulus. The relaxation
time is the constant that links the two types of experiment.
It should be noted that the Maxwell model is suitable for relaxation experiments
on polymer material, but not for the creep experiments. Also, the Maxwell model
predicts that the stress will relax to zero over a long period of time; this is not really the
case with polymer (Young et al., 1994).
All the equations shown above were written for shear experiments. The same
equations are valid for a linear relaxation experiment. In terms of axial loading stress,
strain, and modulus, Equation 3.13 becomes:
()
k
t
nk
k
k
eEtE
?
?
=
=
?
=
1
Eq. 3.14
where:
E = elastic modulus (Pa)
34
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02
Time (s)
G (
P
a)
Experimental data
Model
a) Curve fitting with only one exponential
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02
Time (s)
G (Pa)
Experimental data
Model
b) Curve fitting with a five constant exponential model
Figure 3.4: Comparison of experimental data with model
(Based on Macosko 1994)
35
Equations 3.13 and 3.14 should not be used when trying to model the behavior
over a wide time because these estimates will get less and less precise the longer the
experiment lasts (Nielsen and Landel 1994).
3.2.3 Dynamic Modulus
When a sinusoidal oscillation test in shear strain control is done, the strain and the stress
can be written as (Figure 3.6):
t??? sin
0
=
Eq. 3.15
()???? += t
o
sin
Eq. 3.16
where:
?
o,
?
o
= amplitude of shear strain and stress
? = phase angle
? = angular frequency (2? times the frequency in Hz)
The stress wave can be separated in two different waves:
Figure 3.5: Maxwell-Wiechert model
36
tt ??????? cos"sin'"'
00
+=+=
Eq. 3.17
The two stress waves have the same frequency but different amplitude and a 90
o
phase
angle in between (Figure 3.7) (Riande et al. 2000). The t? wave is in phase with the strain
wave.
-1.6
-1.1
-0.6
-0.1
0.4
0.9
1.4
Time
-1
-0.5
0
0.5
1
1.5
2
?
?
?
?
0
?
0
Figure 3.6: Dynamic experiment
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time
?
??
???
?
0
??
0
???
0
? = 90
0
Figure 3.7: Stress wave decomposition
?
37
Since phase angle between the two waves is 90
o
, the phase angle can be defined as:
From this, the elastic modulus can also be separated in two equations: the in phase or
elastic modulus (G?) and the out-of-phase, viscous or loss modulus (G??). These
equations are:
?
?
?
?
oo
GandG
"
"
'
' ==
Eq. 3.19
Equation 3.18 can now become:
'
"
tan
G
G
=?
Eq. 3.20
From geometry, it can be stated that:
()
22
'''* GGG +=
Eq. 3.21
where:
?G*? = magnitude of the complex modulus (Pa)
?
G?
G''
?G*?
?
o
?
?
o
''
o
o
'
"
tan
?
?
? =
Eq. 3.18
?
o
38
The oscillation can also be written in terms of sinusoidal strain rate. The strain rate is:
tt
dt
d
o
?????
?
? coscos
0
??
===
Eq. 3.22
In equation 3.22, a cosine is used since the strain rate is maximal when the strain is equal
to zero and the strain rate is minimal when the strain is maximal. From this observation,
it can be said that the strain rate wave is in phase with the out-of-phase stress wave (???).
From equation 3.22, it is clear that
o
?
= ???
0
. Now, the strain rate can be
substituted in equation 3.19 to obtain:
?
?
?
?
?
?
?
?
=?=
"
"
""
0
0
GG
Eq. 3.23
and
?
?
?
?
?
?
?
?
=?=
'
'
''
0
0
GG
Eq. 3.24
Now consider a small change in stress due to a small change in strain:
?? Gdd = Eq. 3.25
This equation can also be written as:
dtGdt
dt
d
Gd
?
== ?
?
?
Eq. 3.26
The integral of equation 3.26 is:
() ''
'
dttttGdtGd
tt
?
????
?
???
?=== ????
Eq. 3.27
Where:
39
t? = variable from -? to the present time t; so t-t? equals zero
The strain rate from equation 3.22 is substitute in equation 3.27 to obtain:
() () ( )dsstsGdttttG
o
t
?=?=
??
?
?
??
???? cos''cos
0
'
Eq. 3.28
where:
s = t-t?
By using the trigonometric relation that relates cosine and sin:
() () tdsssGtdsssG ??????? sinsincoscos
0
0
0
0
??
?
?
?
?
+=
Eq. 3.29
Equation 3.29 can be separated in two different waves like in equation 3.17 to obtain:
() dsssE ??? sin
0
00
?
?
?
=? Eq. 3.30
() dsssE ??? cos
0
00
?
?
?
=?? Eq. 3.31
With equations 3.23 and 3.24 substituted in equations 3.30 and 3.31:
==
?
?
?
?
'
'
0
G
() dsssG ?? sin
0
0
?
?
?
and
?
?
?
?
=
"
"
0
G
() dsssG ?? cos
0
0
?
?
?
=
() dsssGG ??? sin'
0
0
?
?
?
= Eq. 3.32
() dsssGG ??? cos"
0
0
?
?
?
= Eq. 3.33
Now, if G is transformed with equation 3.12, equations 3.32 and 3.33 become:
40
dsseGG
s
???
?
sin'
0
00
?
?
?
?
= Eq. 3.34
dsseEG
s
???
?
cos"
0
00
?
?
?
?
= Eq. 3.35
Finally, solving for the two integrals before using equation 3.13:
22
22
0
1
'
??
??
+
=
G
G Eq. 3.36
22
0
1
"
??
??
+
=
G
G Eq. 3.37
so
22
22
1
1
'
??
??
+
=
?
=
=
nk
k
k
GG Eq. 3.38
and
22
1
1
"
??
??
+
=
?
=
=
nk
k
k
GG Eq. 3.39
As with the relaxation portion, equations 3.38 and 3.39 can be transformed to use the
elastic modulus instead of the shear modulus, in which case Equations 3.38 and 3.39
become:
22
22
1
1
'
??
??
+
=
?
=
=
nk
k
k
EE Eq. 3.40
22
1
1
"
??
??
+
=
?
=
=
nk
k
k
EE Eq. 3.41
41
3.3 Construction of master curves
It is often practical to reduce all the data from the same test at different temperature to a
reference temperature. By building a master curve, one can see the behavior of the
material over a wide range of time or frequencies.
The Williams, Landell and Ferry (WLF) superposition principle (Williams et al.,
1955) is one of the most popular principles relating the shift factor to the temperature
(Findley et al., 1989). The WLF was successfully used to construct master curves for
rheological response of asphalt binders (Anderson et al., 1994). Once the reference
temperature is selected, the different frequency sweep curves collected at different
temperatures can be shifted horizontally by a shift factor:
()
()
o
o
t
TTC
TTC
aLog
?+
??
=
2
1
Eq. 3.42
where:
a
t
= the shift factor
C
1
and C
2
= material constants
T = temperature of curve to be shifted
T
o
= reference temperature
Figure 3.8 shows an example of master curve constructed from individual frequency tests
done at different temperatures.
3.4 Summary
Dynamic modulus can be mathematically converted to stress relaxation modulus by using
a modified Maxwell model. In stress relaxation, the relaxation time and the initial
42
modulus are the two variables that are needed in order to apply the models. Five sets of
relaxation time and initial modulus are enough to model the complete relaxation curves.
The relaxation time is the variable that links the dynamic modulus with the stress
relaxation modulus.
2
3
4
5
6
7
8
-5 -4 -3 -2 -1 0 1 2 3 4 5
Reduced Frequency (rad/sec)
L
og G
'
(
P
a)
10C
20C
30C
40C
50C
60C
70C
Master
Figure 3.8: Example of master curve built from response curves at different
temperatures.
43
CHAPTER 4 - MATERIALS AND METHODOLOGY
4.0 INTRODUCTION
This chapter describes the materials and test methods used in this research program. The
material portion is separated in three sections: binder, aggregates and HMA mixtures.
Binder and HMA tests are described in the methodology section.
4.1 Materials
Two different binders, three different aggregates types and two RAP sources were used in
this research. This section describes those materials.
4.1.1 Binders
In order to evaluate the effect of the addition of RAP to HMA mix binder properties, two
different PG virgin binders were chosen: 1) PG76-22, a polymer modified binder (SBS),
and 2) PG64-22, an unmodified binder. These two binders are commonly used binders in
Alabama. Table 4.1 shows the standard PG binder specification properties for both of
these asphalt binders.
Figure 4.1 shows master curves for both of these binders using 22
o
C as a
reference temperature. Note that there is little difference in the binder modulus at the
colder temperatures (i.e., higher frequencies). Only at the warmer temperatures is there
any appreciable difference in the modulus. Given these data, little difference is expected
in the mix modulus when tested at 22
o
C or colder.
44
TABLE 4.1
Binder properties
Properties PG 64-22 PG 76-22
Recovered
Minnesota
RAP
Binder
Recovered
Alabama
RAP
Binder
64
o
C 4.228 - - -
76
o
C - 3.558 - - G* / sin ?, kPa (RTFOT)
88
o
C - - 4.65 2.613
0
o
C - - 101 - Bending Beam Stiffness, S,
MPa -12
o
C 179 127 - 169
0
o
C - - 0.315 -
-6
o
C - - - 0.348 Bending Beam Slope, m
-12
o
C 0.323 0.363 - -
PG Grading PG 64-22 PG 76-22 PG 88-10 PG 88-16
RTFOT = rolling thin film oven test
4.1.2 Aggregates
Two sources of aggregates, a granite and a partially crushed river gravel, were selected to
provide a range of aggregate shape and water absorption characteristics. Since selective
absorption and adsorption of the asphalt binder or its components is possible, it was
Figure 4.1: Binder G* master-curves at a 22
o
C reference temperature.
y = -0.0223x
2
+ 0.5925x + 5.874
R
2
= 0.9899
y = -0.0545x
2
+ 0.7465x + 5.7752
R
2
= 0.9935
0
1
2
3
4
5
6
7
8
9
-6 -4 -2 0 2 4 6
Log Reduced Time (s)
L
og G
*
(P
a
)
PG 76-22 PG 64-22
45
considered desirable to also use a ?model? aggregate that could be expected to have very
low absorption and consistent, if any, adsorption at the asphalt-aggregate interface (Curtis
et al., 1993).
Initially, glass beads of various sizes were considered for the model aggregate,
assuming that the glass surface should not absorb any (or very little) of the binder. It was
also assumed that the glass beads, being siliceous in nature, would be a good consistent
representation of common aggregate mineralogy. However, the spherical shape of the
beads is not representative of aggregate shapes used in HMA construction. Therefore,
crushed and graded recycled glass was selected as the ?model? aggregate.
All of the aggregate properties are shown in Table 4.2. Each of these aggregate
stockpiles (natural and model) were sieved into individual fractions, and recombined to
produce one of two gradations (Table 4.3).
TABLE 4.2
Aggregate properties
Properties Granite Gravel
Model
Aggregate
(recycled
glass)
MN
RAP*
AL
RAP*
Bulk specific gravity 2.658 2.598 2.413 2.126 2.340
Bulk specific gravity,
SSD
2.676 2.618 2.423 2.161 2.428
Apparent specific
gravity
2.707 2.652 2.435 2.204 2.470
Water absorption, % 0.7 1.2 0.0 1.7 1.2
% Crushed Faces 100% 100% 100% 100% 100%
Flat and elongated, %
(5:1)
0% 0% 100% 0% 0%
% Asphalt Binder NA NA NA 5.6% 4.3%
NA = not applicable
* Values obtained on the RAP aggregate after solvent extraction.
46
TABLE 4.3
Gradations of materials used in this study
Gradation Cumulative Percent Passing, %
Sieve Size
Coarse
Gradation
Coarse
50% AL
RAP
Coarse
50%
MN
RAP
Fine
Gradation
MN
RAP
AL
RAP
19.0 mm 100 100 100 100 100 100
12.5 mm 95 92 98 95 97 84
9.5 mm 85 82 92 85 92 76
4.75 mm 50 48 60 69 79 50
2.36 mm 31 31 40 55 66 32
1.18 mm 20 20 28 40 51 25
0.60 mm 15 15 19 30 33 19
0.30 mm 11 11 10 20 15 14
0.150mm 9 8 6 9 7 9
0.075 mm 5 5 4 5 4 6
4.1.3 Reclaimed Asphalt Pavement (RAP)
Two different sources of RAP were used in order to evaluate the sensitivity of the
indirect tensile stress relaxation test method to a range of RAP properties. Alabama RAP
was selected to represent RAP obtained from a region of the country that typically uses a
PG 64-22. The Minnesota RAP was selected to represent region of the country that uses
a softer grade of binder (e.g., PG 58-22). Both RAP sources were used at each of three
concentrations of RAP: 15, 25 and 50%. Mixtures without RAP were used as the control
mixtures (i.e., 0% RAP).
Table 4.1 shows binder properties for RAP binders extracted by ASTM D2172
(centrifuge) and recovered with a Rotavapor distillation process. Table 4.2 presents the
aggregates properties of the RAP and Table 4.3 presents the after-extraction gradations
for both sources.
47
The Alabama RAP source was visually more variable in it?s content than the
Minnesota RAP source. In order to minimize the heterogeneity of the RAP, three bags of
RAP were mixed together before the amount of RAP used to prepare samples was
obtained.
4.1.4 HMA mixes
The Brookfield concentric cylinder viscometer was used to evaluate the viscosity of the
two binders over a range of temperatures so that the mixing and compacting temperature
could be determined. According to Roberts et al. (1996) the mixing temperature is the
temperature which gives a viscosity of 170 cPoise and the compacting temperature is the
one that gives the binder a viscosity of 280 cPoise. For the PG64-22 (unmodified), the
mixing and compacting temperature are respectively 150
o
C and 105
o
C.
This method of determining the mixing and compaction temperatures could not be
used for the PG76-22 because it is a polymer-modified asphalt binder. Modified binders
have higher shear rate-dependent viscosities, which results in artificially high temperature
estimates of mixing and compaction temperatures (Azari et al., 2003). At the higher
temperatures, some modified binders can start to degrade and change in composition
(Shenoy 2001). In these cases, mix and compaction temperatures are typically estimated
as 10
o
C higher than needed for an unmodified binder (Yildirim et al., 2000). Many
authors have proposed different methods for estimating mixing and compacting
temperature based on different shear rates; these are usually different than those used in
the laboratory preparation of compacted samples. (Shenoy 2001, Yildirim et al., 2000,
Azari 2003). Another author has proposed the mixing temperature can be increased
48
higher than 163
o
C as long as the binder does not start to emit smoke (Stuart 2002). In
that study, they also proposed the use of a viscosity level of 1100 cPoise to find the
compacting temperature for all binders with PG64-XX and above. For this research
project a mixing temperature of 163
o
C and a compacting temperature of 120
o
C were used
for the PG 76-22.
Conventional compacted HMA samples were prepares with a Superpave gyratory
compactor following the ASTM D4013 standard using 100 gyrations. All specimens had
a diameter of 150 mm and a height of around 115mm which results in air voids of around
4%. Pure (100 %) RAP samples were also prepared. This was done by heating the RAP
at 160
o
C for four hours, then compacted with 100 gyrations of the SGC compactor.
Table 4.4 shows all the conventional HMA samples with and without RAP that were
compacted. Three replicates were fabricated for each HMA mixture.
The HMA mixtures with the model aggregates were compacted with a manual
Marshall hammer to avoid breaking the glass particles during compaction; these samples
had air voids of around 3%.
4.2 Test Method Descriptions
A parallel plate DSR was used to determine both the dynamic and relaxation modulus of
both the virgin binders used in this study. The shear complex modulus and shear stress
relaxation modulus values were calculated from this testing.
4.2.1 Dynamic Modulus of Binders
A TA Instruments AR1000 constant stress parallel plate DSR was used for the dynamic
experiment. A frequency sweep covering a range of 0.6 to 250 rad/s on samples with
49
both 8 or 25mm in diameter was used to establish the minimum and the maximum
frequencies that could be used with this DSR. In order to be able to build master curves
for the dynamic modulus of the binders, testing was completed for a range of
temperatures (5, 10, 22.5, 30, 40, 50, 60, 70 and 80
o
C).
For the purpose of this research, the upper temperature was selected to encompass
the highest PG temperature (i.e., 76
o
C for the PG 76-22). The lowest temperature (5
o
C),
while -22
o
C was ideal, was the coldest practical temperature that could be used with this
equipment. Three samples of the same binder were tested at each temperature. The first
test was done at 5
o
C and once completed, the temperature was increased to the next
warmer test temperature in the sequence. To ensure that the temperature was
homogeneous throughout the sample, the sample was held at the test temperature for 10
Average Air Void (%)
Granite Gravel Asphalt Type of
RAP
% of
RAP Fine Grad. Coarse
Grad.
Fine Grad. Coarse
Grad.
None 4.5 3.8 5.0 4.6
15% 3.4 3.5 5.8 5.0
25% 3.7 4.2 5.4 3.8 AL RAP
50% 3.4 4.0 3.4 4.5
15% 3.1 2.5 5.6 5.5
25% 3.5 3.0 2.8 2.8
PG 64-22
MN RAP
50% 2.0 2.8 2.5 2.2
None 4.1 3.6 4.5 3.7
15% 2.9 2.7 5.2 4.7
25% 3.7 4.4 4.2 4.2 AL RAP
50% 2.4 3.2 5.6 3.2
15% 3.5 2.3 4.9 5.0
25% 3.3 3.0 4.1 4.2
PG 76-22
MN RAP
50% 1.6 3.3 3.8 2.7
AL RAP 100% 3.0
MN RAP 100% 2.9
Table 4.4 Average air voids content for the mixes
50
minutes after the test temperature in the environmental chamber was reached and
stabilized (ASTM P246). This procedure was followed for each incremental increase in
test temperature.
One of the assumptions in DSR testing is that the binder modulus is constant in
the linear viscoelastic region. This assumption was verified prior to any dynamic
modulus testing by conducting tests at different strain level and different frequencies,
then comparing the resulting modulus. When the modulus starts to change, the linearity
limit is reached. In order to fix the linearity limit, a threshold value needs to be set. In
this case, when the modulus changed of more than 10%, the limit was considered reached
(ISAP 2005) (Figure 4.2).
At the lower temperatures (5 and 10
o
C) the strain was controlled by the capacity
of the DSR and not the linearity limit. At low temperature and high frequencies (above
0.5
0.6
0.7
0.8
0.9
1
1.1
0 5 10 15 20 25
% strain
G*
/
G
*
0 1 Hz
Linearity
limit
Figure 4.2: Example of linearity limit (PG 64-22 at 30
o
C)
51
60 rad/s) the binder is so stiff that the DSR does not have enough torque to provide the
desired strain. For example, the strain limit at 5
o
C is 0.35% because that is the maximum
strain that the DSR can achieve at 250 rad/s. It should be noted that the sample geometry
was also chosen as a function of the temperature. For temperature below 40
o
C, the
sample had an 8 mm diameter and a thickness of 2 mm. At higher temperature, the
samples had a larger diameter of 25 mm in diameter and a thickness of 1 mm. The
different strain levels used in this testing program are shown in Table 4.5. The same
limits were used for both binders. Since higher strain will result in higher stress, the
maximum limits shown in this table were used. By using higher strains, there is less error
in the results since the amplitude of both the strain and stress are much bigger than the
sensor noise range in the apparatus.
Table 4.5: Strain used in the dynamic modulus test for both binders
(DSR AR 1000)
Temperature (
o
C) Strain (%)
5 0.35
10 0.5
20 1
30 5
40 10
50 10
60 10
70 15
80 15
4.2.2 Stress Relaxation of Binders
The binder stress relaxation test was conducted with a parallel plate DSR. This test was
used to evaluate stress relaxation characteristics of virgin asphalt binders. A TA
52
Instruments AR2000 was used to conduct 8 and 25mm diameter parallel plate stress
relaxation testing at five different temperatures (5, 22, 30, 40 and 50
o
C). It should be
noted that different DSR equipment were used for the dynamic and stress relaxation tests.
While the DSR for dynamic modulus was available through the NCAT laboratory,
arrangements had to be made for the temporary loan of a research-grade DSR unit
capable of stress relaxation testing with TA Instruments.
As with the dynamic modulus testing, all stress relaxation tests were performed
within the linear viscoelastic (LVE) range of the behavior of the binder. To find the limit
of the LVE range, relaxation tests were performed at different levels of constant strain.
The same threshold, a decrease of more than 10%, was used to set the LVE limit. Table
4.6 shows the strain levels used as a function of the temperature.
TABLE 4.6
Strain level as a function of the temperature for both binders tested
(parallel plate configuration)
Temperature (
o
C) Strain (%)
5 0.1
10 0.5
22 2.0
30 5.
40 5.0
50 6.
Another important point that needs to be considered in relaxation test is the time
needed to reach a constant strain level. As noted in Chapter 2 - Literature Review, one of
the main reasons for other researchers not using stress relaxation testing is the inability to
quickly achieve the desired, and stable, strain level. Ideally, the relaxation modulus
should only be calculated from the moment at which the constant strain is achieved (Chen
53
2000). With the DSR used in this experiment, it took between 0.1 and 0.35 seconds to
reach the desired strain; the length of time varies depending on the testing temperature
(Figure 4.3). Since consistency along the time axis was needed for comparing the results
from different temperatures, a time of 0.35 seconds was selected as the starting point for
the calculation of the stress relaxation modulus for the binder testing.
The strain was maintained for a maximum of two minutes because preliminary
testing indicated that the stress relaxation was substantially achieved within that time,
regardless of temperature. Measurable loads were not usually obtainable at test times
longer than 2 minutes. By default, the DSR acquires a thousand points of data per
second. This amount of data was found to be sufficient to define the material behavior.
Three samples of both binders (PG 76-22 and PG 64-22) were tested at each of
the same temperatures used for dynamic modulus testing. The average result for each
temperature was then used for analysis.
0.001
0.01
0.1
1
10
100
1000
0.001 0.01 0.1 1 10 100
Time (s)
R
e
l
axati
o
n
M
odul
us
(
M
P
a
)
0
0.5
1
1.5
2
2.5
% s
t
r
a
i
n
Modulus % S t r a i n
Figure 4.3: Example of time needed to reach constant strain
for binder relaxation test (PG 64-22 at 22
o
C)
54
4.2.3 HMA IDT Stress Relaxation Test
An Instron 8501 was used to conduct the IDT stress relaxation test on compacted HMA
samples. The strain was set using the ram control displacement sensor, and a 22KN load
cell was used to measure stress relaxation every 0.1 seconds over a time interval of up to
two minutes. Eventually, the test time was limited to 45 seconds because only minimally
measurable decreasing changes in stress, regardless of temperature, were obtained after
this time.
Originally, the jig for indirect tensile strength testing when evaluating the
moisture sensitivity of HMA mixtures (ASTM D4867) was used to hold the sample. The
posts appeared to generate some friction so the first modification to the test equipment
was to refit the load frame so that the upper and lower platens were vertically aligned but
not mounted on guide posts.
The test method development included:
? Defining of a range of strain levels within the linear viscoelastic range,
? Evaluation of the time needed to achieve a constant strain, evaluation of
the time needed for data collection, and
? Development of an analysis approach for estimating initial modulus and
the rate of stress relaxation.
Testing parameters were evaluated at two test temperatures: 5 and 22
o
C.
4.2.3.1 Test Method Development
Like for the binder experiment, it was necessary to establish the limit of the linear
viscoelastic range for the different HMA mixtures when tested at different test
55
temperatures. Tests were performed at different strain levels and the relaxation modulus
was calculated. The LVE limit was considered attained when the relaxation modulus has
decreased 10% compared to the modulus measured at the lowest strain level. At 22
o
C, the
average strain level was 0.0013 with a coefficient of variation of 20%. At 5
o
C, the strain
level was 0.0006 with a coefficient of variation of 30%.
The ramp speed was found to have a large influence on the precision of the level
of strain achieved. The faster the ram moves, the less control there is on the level of
strain. Since in a relaxation experiment it is desirable to reach a known constant strain as
fast as possible without over-shooting the target strain, a ram speed of 100 mm/min was
chosen.
The time to constant level of strain was also analyzed. The Instron software used
to gather the data was set to only start acquiring data when the strain was constant, so the
change in stress before this point could not be considered in the analysis. To ensure that
the stress relaxation analyzed comes from the sample tested and not the equipment setup,
an elastic (steel) cylinder was tested. Steel, under the low level of stress used in this
experiment, is not supposed to show any relaxation; so if there is relaxation, it can be
attributed to one or more equipment components. Figure 4.4 shows that it takes less than
3 seconds to reach a modulus that is constant for the steel cylinder; a difference of 1% or
less was considered to be acceptable.
4.3 Summary
For this research project, different materials were used: 1) a PG 64-22, an unmodified
asphalt binder, and 2) a PG 76-22 polymer modified asphalt binder. Both binders were
56
found to have similar master curves for temperatures at or below 22
o
C; above 22
o
C, the
difference in the stiffness and the relaxation capacities of the binders increases with
increasing temperature. Dynamic modulus and stress relaxation tests were performed on
both binders. For both tests, the controlled strain was kept in the limit of the linear
viscoelastic region of the behavior of the binders.
The dynamic modulus was determined for these binders using a frequency sweep
between 0.6 and 250 rad/sec at each of 9 different temperatures between 5 and 80
o
C.
Stress relaxation testing was conducted at only 6 different temperatures between 5 to
50
o
C were used due to equipment limitation.
HMA mixtures were prepared with one of three aggregate sources: 1) a model
aggregates simulated with crushed glass, 2) a 100% crushed granite, and 3) a
predominately crushed river gravel with a high absorption capacity.
-5%
0%
5%
10%
0 5 10 15 20 25 30 35 40 45
Time (s)
C
h
a
nge
i
n
M
odu
l
u
s
(
%
)
Modulus
Figure 4.4: Setup time with steel cylinder in HMA IDT stress
relaxation equipment (22
o
C)
57
CHAPTER 5 - RESULTS AND DISCUSSION
5.0 Introduction
This chapter presents the results and analyses, in the following order:
? Relation between the dynamic and the relaxation modulus for the binders.
? Comparison of the asphalt binder and HMA mixture stress relaxation results.
? Statistical analyses.
? Methodology for selecting either the percent of RAP or grade of virgin asphalt
binder based only on testing compacted HMA samples.
5.1 Relationship Between Dynamic Modulus and Relaxation Modulus
(Binders Only)
As described in Chapter 3 ? Original Analysis Hypothesis, it is mathematically possible
to use dynamic modulus results to obtain relaxation modulus. Figures 5.1 shows an
example of the relation between the measured asphalt binder shear dynamic modulus and
the stress relaxation predicted dynamic modulus. It shows that the relation between the
measured and predicted moduli is linear, but it is not perfectly on the equality line (i.e.,
slope of 1.00). Possible reasons for this are:
? Equipment differences in the DSR devices that needed to be used for dynamic
modulus testing (NCAT DSR) and the stress relaxation testing (TA DSR).
58
? The stress relaxation test results obtained with the TA AR2000 model may be slightly
off because of the time needed to reach constant strain. The very early time results
had to be discarded due to the strain levels not having reached a constant value
quickly (see Figure 4.3). It is anticipated that higher moduli values would be those
obtained from very short loading times.
? The time between the sample preparation and the test was longer for the relaxation
tests than for the dynamic modulus tests which results in stiffer binder due to internal
structuring of the molecules (Bell 1989).
Results are similar at both temperature. Modulus values range from about 0 to
close to 30 MPa when binders are tested at 5
o
C; they only range from about 0 to less than
2 MPa at 22
o
C. The good correlation indicated by R
2
values of at least 0.96 indicate that
PG 76-22
y = 0.94x
R
2
= 0.96
PG 64-22
y = 1.26x
R
2
= 0.99
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
G* from DSR (MPa)
G
*
f
r
o
m
R
e
laxat
io
n
(
M
P
a
)
Figure 5.1: Relation between measured and calculated shear dynamic
modulus for both binders at 22
o
C
59
changes in the stress relaxation modulus for a given binder is a good representation of
changes in the binder?s dynamic modulus. However, the slope of the relation changes
with each test (Figure 5.2). The fact that the slope change and that the coefficient of
variation of the slope is high make the use of those relation complicated.
5.2 Comparison of Binder and HMA Stress Relaxation Test Results.
5.2.1 HMA Mixtures with Model Aggregates
It was hypothesized that the aggregate in the mix should have little to no effect on the
indirect tension stress relaxation modulus of the HMA mixes since the asphalt binder is
the only component that can provide tensile strength. The theoretical basis of this
hypothesis was explored through the testing of HMA mixtures made with the model
Figure 5.2: Relation between measured and calculated shear dynamic
modulus
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
C
o
r
r
el
ati
o
n
C
o
ef
fi
ci
en
t
1357911131517192123252729313335
Test Number
AVG
60
aggregate. Figure 5.3 shows the relationship between the average stress relaxation
modulus at 22
o
C of the PG 76-22 asphalt binder and the average HMA indirect tension
stress relaxation modulus.
The strain for the model HMA stress relaxation modulus was calculated using an
estimated horizontal strain. The vertical displacement of the ram was divided by the
diameter of the sample to calculate the vertical strain. The horizontal strain was then
estimated using an assumed Poisson?s ratio, ?, which was selected base on the
temperature (0.35 for 22
o
C and 0.2 for 5
o
C):
?
h
= ?
v
? Eq. 5.1
Figure 5.3: Relation between binder relaxation modulus (PG 76-22) and HMA
indirect tension stress relaxation modulus (model aggregate) prepared
with the same PG 76-22 and tested at 22
o
C
y = 188.66x + 8.59
R
2
= 0.99
1
10
100
0.01 0.1 1
Binder stress relaxation modulus (MPa)
Gla
ss sp
ecim
e
n
st
r
ess
r
e
laxat
io
n
Mo
dul
u
s
(
M
P
a
)
61
Where:
?
h
= horizontal strain
?
v
= vertical strain
? = Poisson?s ratio
It should be noted that linear stress relaxation modulus of the HMA, E, was
converted to a shear relaxation modulus, G, by the following equation (Cook and Young
1999):
E = 3 G Eq. 5.2
Figure 5.3 shows that there is a good linear log-log relationship between both relaxation
modulus, however there is a pronounced effect on the modulus with the inclusion of the
solid particles in the binder film. That is, the model HMA stress relaxation modulus is
consistently higher (stiffer) than the stress relaxation modulus of the virgin asphalt
binder. This is expected since the inclusion of solid particles in any liquid increases the
viscosity of the liquid, therefore the higher stiffness for the mixes is assumed to be a
function of the percentage of fines that is incorporated into the combined binder-
aggregate film (Buttlar et al., 1999). The stress relaxation modulus of the model HMA
mix is about 10 MPa when the binder stress relaxation modulus is 0.01 MPa and 100
MPa when the binder stress relaxation modulus is 1 MPa. These results indicate that the
stress relaxation modulus of the binder will be significantly increased by the presence of
solid particles.
While the stress relaxation moduli magnitudes are different, the indirect tension
stress relaxation testing of an HMA mixture can be expected to represent corresponding
changes in the stress relaxation modulus of the asphalt binder.
62
5.2.2 HMA Mixtures with Standard Aggregate
The work with the model HMA mixtures showed that indirect tension stress relaxation
test on HMA mixture is a good indicator of the binder properties. Work continued with
the test method development using HMA mixtures with two gradations with each of two
aggregate sources. Like for the model aggregate, the correlation coefficient of the
relation between the binder relaxation modulus and the HMA relaxation modulus is very
good (r
2
> 0.9), but the slope of the relation varies a lot. Figure 5.4 presents the average
slope of the relation for the different mixes. It can be noted that the slope differs from
one type of mix to another and that the coefficient of variation is high in all cases
0
50
100
150
200
250
300
glass fine glass coarse PG64 fine PG64 coarse PG76 fine PG76 coarse
Mix type
A
ver
age s
l
ope of
rel
a
t
i
on
Figure 5.4: Average slope of the relation between binder relaxation modulus and
HMA relaxation modulus
63
5.3 Statistical Analysis
A best fit curve approach was used to model the changes in indirect tension stress
relaxation modulus with time (Figure 5.5). A power curve was, in every case, the best fit
curve with an R
2
of 0.9 or above, regardless of the test temperature. The influence of
when the data collection is started was investigated next.
On Figure 5.5, two curves for which the data collection was started at different
times are shown. The resulting correlation equations have a difference of less than 1% in
either the intercepts or exponents. These results indicate that while obtaining data at
times closer to zero would provide more accurate zero-time information, there is a
sufficient amount of stress relaxation information to estimate the mix characteristics
using less than ideal testing conditions.
There are two key parameters in the power model that can be used to evaluate
changes in the mix properties due to changes in the asphalt binder properties. These are
Starts at 5 seconds
y = 145.43x
-0.2816
R
2
= 0.99
Starts at 2 seconds
y = 145.03x
-0.2807
R
2
= 0.99
0
20
40
60
80
100
120
0 1020304050
Time (s)
R
e
l
a
x
a
t
i
on Mo
dul
u
s
(
M
Pa
)
Figure 5.5: Typical relaxation curve and best fit curves.
64
the intercept and the exponent. The intercept represents the maximum modulus as time
approaches zero. The exponent, which is function of the degree of curvature, represents
the ability of the sample to relax. The exponent with be referred to hereafter as the
curvature coefficient. The higher the value of the exponent, the more the HMA mixture
relaxes in a given period of time. Statistical analyses were completed using these two
parameters. The first statistical evaluation calculated the Pearson?s correlation matrix to
determine if there were any well-correlated single variable comparisons. This analysis
showed there were only two fair correlations found for each of the two test temperatures:
? Modulus increased with the addition of RAP (R=0.59 at 5
o
C and 0.65 at 22
o
C).
? Curvature coefficient decreased with the addition of RAP (R=0.76 at 5
o
C and 0.66 at
22
o
C).
These correlations are as expected. That is, modulus should increase as more
RAP binder is incorporated into the HMA effective binder. Also, as more RAP binder is
included in the overall HMA binder properties, the ability of the mix to deform with time
(i.e., relax) should be reduced. This is because asphalt binders lose their ductility as they
age.
The next statistical evaluation used an analysis of variance (ANOVA) followed
by the Duncan multiple range test (Cody and Smith 1997). With the Duncan analysis, it
is possible to separate the data into groups that have significant difference in their means.
The effect of PG grade, percentage of RAP, gradation, aggregate source, and RAP source
on the stress relaxation modulus and the curvature coefficient were analyzed. The
complete results used for the statistical analyses are shown in Appendix B.
65
5.3.1 Effect of Gradation, Aggregate Source and RAP Source
No statistical differences in either the modulus (intercept) or curvature coefficient
(exponent) were found as the result of changes in the gradation, aggregate source, or RAP
source. This is in agreement with the preliminary findings that showed minimal
influence of the gradation. As for the aggregates sources, it was also expected since the
IDT stress relaxation test is primarily an indicator of the binder properties. Finally, the
RAP source did not make any statistical difference because there is little difference
between the PG grades of both RAP binders. It should be noted that the high variability
in the Alabama RAP, noted previously as visibly variable material, may have produced
enough variability in the HMA mixture properties to hide any statistical difference that
could have been seen otherwise (e.g. due to the RAP gradation).
5.3.2 Effect of Virgin PG Grade
For the analysis on the effect of the PG grade on the modulus and the curvature
coefficient, only data from the HMA samples without RAP were used (Table 5.1). The
Duncan multiple means test shows that neither the average initial stress relaxation
modulus nor average curvature coefficient were significantly different at a given test
temperature. However, the trends in both mix properties are consistent with expectations.
The modulus increases with decreasing temperature, with little difference between the
binders at the cold temperatures and a slight, but not statistically different, change at the
warmer temperature. This agrees with the differences seen in the binder master curves
(Figure 4.1 in Chapter 4).
66
The curvature coefficient is lower at the colder temperature, and dependent upon
the binder grade at the warmer temperature. The curvature coefficient is less for the
polymer modified PG 76-22 than for the PG 64-22 at 22
o
C, indicating that the polymer
modified asphalt will take longer to relax at the warmer temperature.
Table 5.1
Influence of the PG grade on the relaxation
modulus and the curvature coefficient (No RAP)
Temp.
(
o
C)
Duncan
Grouping
Mean
COV
(%)
n
Binder
Grade
220.6 24 24 PG 64-22
5 A
206.6 34 24 PG 76-22
112.7 33 24 PG 64-22
Modulus
(MPa)
22 A
103.3 34 24 PG 76-22
0.123 18 24 PG 64-22
5 A
0.116 24 24 PG 76-22
A 0.371 20 24 PG 64-22
Curvature
Coefficient
22
B 0.267 51 24 PG 76-22
5.3.3 Effect of the Percentage of RAP
For this analysis, the results from mixes with and without RAP were used (Table 5.2). At
5
o
C, adding 15% of RAP at least doubles the initial stress relaxation modulus; no
significant increases are seen with further increases in RAP. The curvature coefficient is
more sensitive to the percentage of RAP as is seen by the continually decreasing ability
of the HMA to dissipate stress over time with an increasing percentage of RAP. Both
the control (no RAP) and 15% RAP mixtures have statistically similar curvature
coefficients; at least 25% RAP is needed to produce a statistically significant decrease.
The curvature coefficient is similar for both the 50 and 100% RAP mixtures, and both are
significantly different from the control (no RAP).
67
Table 5.2
Influence of the percentage of RAP on the
relaxation modulus and the curvature coefficient
Temp.
(
o
C)
Duncan
Grouping
%RAP Mean
COV
(%)
n
A 0 213.6 29 48
15 507.9 29 48
25 513.1 27 48
50 517.1 34 48
5
B
100 443.6 47 11
A 0 108.0 34 48
15 190.0 36 48
C
25 204.9 35 48
B
50 237.6 29 48
Modulus
(MPa)
22
D
100 258.8 18 11
0 0.318 21 48
A
15 0.281 32 48
B
25 0.242 29 48
50 0.148 46 48
5
C
100 0.153 101 11
A 0 0.119 38 48
15 0.097 21 48
B
25 0.082 16 48
50 0.051 46 48
Curvature
coefficient
22
C
100 0.057 41 11
At 22
o
C, the initial stress relaxation modulus doubled with the inclusion of any
percent RAP in the mixes. There is a statistically significant increase in the modulus
when using as little as 15% RAP at this temperature. There is no difference between
mixes with either 15 or 25% RAP. Mixes with either 50 or 100 % RAP have similar
modulus and both are significantly higher than mixes with lower percentages of RAP.
The same trends are seen for the curvature coefficient at this temperature.
68
5.4 Evaluation of Current Blending Chart Practices for Estimating the Percent of
RAP or Grade of Virgin Binder
The current practice for determining the percent of RAP to use in a new HMA mix uses
blending charts, which assumes that there is a linear relation between the amount of RAP
and the binder properties of the composite mix from 0 to 100% RAP. When mixtures
with a range of RAP from 0 to 50% are considered, the assumption of linearity between
modulus and the percent of RAP is valid. Figure 5.6 shows the relationship for mixes
containing Minnesota RAP and PG 64-22 binder tested at 22
o
C; the other mixes, while
not shown, have similar relationships.
y = 3.44x + 150.94
R
2
= 0.87
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25 30 35 40 45 50
% of RAP
I
n
i
t
i
a
l
Modul
u
s
(
P
a)
Coarse PG64 GV
Figure 5.6: Linear relation between percent of RAP and
modulus (MN RAP, Gravel, PG 64-22, 22
o
C)
69
To represent the 100% recovered binder used for constructing the binder-only
blending charts, 100% RAP samples were compacted and tested. As shown previously,
there is no significant difference between samples with 50 and 100% RAP. Because of
that, if the 100% RAP samples are added to the Figure 5.6, it is obvious that the relation
is no longer linear (Figure 5.7).
This figure shows that an assumption of a linear relationship between the limits of
no RAP and 100% RAP are not supported when the HMA mix is evaluated; the
relationship is in fact non-linear. These same trends were seen for all of the other binder-
RAP mixes (not shown).
If the guidelines for blending RAP in a new mix are applied to the indirect tensile
stress relaxation modulus results, an addition of 10, 15 and 25% of RAP would result in
an increase of modulus of 5, 7.5 and 12.5% respectively, according to Figure 5.7. But
Figure 5.7: Relation between percent of RAP and
modulus (MN RAP, Gravel, PG 64-22, 22
o
C)
y = 1.13x + 192.89
R
2
= 0.41
0
50
100
150
200
250
300
350
400
0 10203040506070809010
% of RAP
In
i
t
ia
l M
o
d
u
lu
s
(
P
a
)
Coarse PG64 GV
70
since the relation is really non-linear, the increase of modulus should more realistically be
estimated as 15, 22.5 and 37.5%, respectively. These blending charts were constructed
with the data presented in Tables 5.4a and 5.4b.
The curvature coefficient can also be used to create blending charts. Figures 5.8
and 5.9 show that, as for the stress relaxation modulus, there is a good linear relation
between the curvature coefficient up to 50% of RAP but the relation is not as linear if the
blending chart includes 100% RAP.
5.5 Practical Application of Findings
5.5.1 When to Change Virgin PG Binder Grade
Guidelines for when to reduce the PG grade of the virgin binder, shown in table 2.1, are
based on the grade of the recovered RAP binders (McDaniel et al. 2001). The AL RAP
Figure 5.8: Relation between percent of RAP and
curvature coefficient (MN RAP, Gravel, PG 64-22, 22
o
C)
y = -0.006x + 0.4436
R
2
= 0.9575
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15 20 25 30 35 40 45 50
% of RAP
C
o
ef
f
i
c
i
ent
of
Cur
vat
ur
e
Coarse PG64 GV
71
and MN RAP were graded as PG 88-16 and PG 88-10, respectively, so at the current
ALDOT maximum of 15% RAP, mixes with either RAP source should have their PG
binder grade reduced.
However, since ALDOT uses a midpoint PG grade (PG 67-22 instead of a PG 64-
22) for specifying unmodified asphalt binders, this would mean that the PG grade would
have to be reduced to a PG 58-28 to keep with the standard PG grading system. This
may be too much of a reduction in the upper temperature stiffness, and could result in a
substantial increase in rutting problems. RAP used in ALDOT mixes with a specified PG
76-22 would be increased to a PG 70-28.
An alternative approach can be developed based on the results of this study.
Table 5.3 shows how the indirect tension stress relaxation parameters can be used to
Figure 5.9: Relation between percent of RAP and
curvature coefficient (MN RAP, Gravel, PG 64-22, 22
o
C)
y = -0.0034x + 0.3959
R
2
= 0.84
0
0.1
0.2
0.3
0.4
0.5
0.6
0 10203040506070809010
% of RAP
Coef
f
i
ci
ent
of
Cu
rva
t
ure
Coarse PG64 GV
72
replace the recovered binder NCHRP guideline recommendations. The data in tables in
Tables 5.4a and 5.4b were used to estimate the percent change in the properties when
10% RAP, for the NCHRP guidelines, and 15% RAP, for the current ALDOT maximum
allowable RAP. Estimated changes in properties assumed that changes are linear
between 0 and 50% RAP.
Table 5.3 Alternative guidelines for when to consider changing PG grades
NCHRP Guidelines Current ALDOT Practice
Stress
Relaxation
Modulus
(MPa)
Curvature
Coefficient
Stress
Relaxation
Modulus
(MPa)
Curvature
Coefficient
Allowable change in mix
properties before grade of
virgin binder needs to be
changed
1,2
No more
than 50%
increase
No more
than 12%
decrease
No more
than 75%
increase
No more
than 19%
decrease
1
% Changes expressed as changes in the mix properties compared with the same mix
with no RAP
2
Conditions for both the stress relaxation modulus and curvature coefficient need to be
met
5.5.2 Determining the Percent RAP Actually Used in Construction
A practical application of this research is the determination of the percent RAP that can
be used in a given HMA mixture. Using the concept that there is a linear relationship
between the percent RAP from 0 to 50%, and both the initial stress relaxation modulus
and the curvature coefficient, the percent RAP in a given mix can be determined as
follows:
1. Mix and compact three samples for a control mix (no RAP) and three samples for
a mix with the same aggregates and binder but with 50% RAP. The control
gradation should be similar to that of the mix containing RAP. Use the same
73
virgin asphalt binder for both sets of samples.
2. Determine the initial stress relaxation modulus and curvature coefficient for the
mix with no RAP and for the mix with 50% RAP.
3. Determine, b, the change in modulus for a percent change in RAP:
b = (M
SR50
? M
SR0
) / 50 Eq. 5.3
where:
M
SR50
= initial stress relaxation modulus with 50% RAP
M
SR0
= initial stress relaxation modulus with no RAP
4. Determine, M
SR?
, the initial stress relaxation modulus for a mix with an unknown
percent of RAP
5. Estimate the percent RAP in the mix:
%RAP
M
= (M
SR?
? M
SR0
) / b Eq. 5.4
where:
%RAP
M
= estimated %RAP from initial relaxation modulus
The same process can be done with the curvature coefficient. Steps 1 and 2 would be the
same, but then:
3. Determine, C, the change in curvature coefficient for a percent change in RAP:
C = (C
50
? C
0
) / 50 Eq. 5.5
where:
C
50
= curvature coefficient with 50% RAP
C
0
= curvature coefficient with no RAP
4. Determine, C
?
, the curvature coefficient for a mix with an unknown percent of
RAP
74
5. Estimate the percent RAP in the mix:
%RAP = (C
?
? C
0
) / C Eq. 5.6
where:
%RAP
C
= estimated %RAP from curvature coefficient.
5.6 Test Method Refinements ? Field Study
An evaluation of the initial test results showed that the standard deviation of both the
initial stress relaxation modulus and curvature coefficient were dependent upon the mean
value. The coefficient of variation (COV) was about 30% for the modulus and about
15% for the curvature coefficient. It was felt that the variability of both parameters could
be reduced by conducting three tests per sample, then using the average of these three
tests per sample as the single test result reported for each sample. Three tests can easily
be obtained from the same sample by conducting one indirect tensile stress relaxation
test, then removing the load, rotating the sample 30 degrees, testing again, then repeating
the procedure for a third time. This way each test is conducted on a different area of the
sample with each loading. Because the test method was originally set up so that the
strains were within the LVE limits, then the assumption is that there is little or no
significant damage to the sample. Since the time from start of loading the sample to the
completion of one loading cycle takes less than 2 minutes, testing the same sample three
times should only increase the testing time by about 3 to 4 minutes.
This change in the test method was combined with a preliminary evaluation of the
consistency of binder properties during the production of a typical Alabama HMA
(polymer modified asphalt binder) plant mixes which was obtained from East Alabama
75
Paving (EAP), a local paving company. One bag of HMA, taken by the plant staff during
the day?s production, was picked up each day for about 1 week by Auburn University
researchers. This mix was reheated, split, and used to prepare five gyratory samples (100
gyrations). Samples for four days of HMA production were obtained, which means that a
total of 20 samples were compacted and tested (see Appendix C). The same mix (coarse
gradation, PG 76-22, no RAP) was produced on all four days of paving.
The indirect tension stress relaxation test was performed for this last portion of the
research with the following test parameters:
? ram speed of 100 mm/min
? strain level around 0.0013
? test duration of 45 seconds
Figure 5.10: Modulus process control chart for field study
50
75
100
125
150
0 5 10 15 20 25 30
QC Sample No.
S
t
r
ess
R
e
l
axat
i
o
n
M
odul
us
,
MP
a
Modulus
+ 2 std dev.
- 2 std dev.
Mean
Sample with
high variability
76
? data acquisition at every 0.1 second
? first three seconds of the test were not included in the analysis
The complete procedure, in ASTM format, can be seen in Appendix D.
Figure 5.10 shows the process control chart that is obtained for initial stress
relaxation modulus for four days of paving as well as the plus and minus two standard
deviation limits. The per bag variability, rather than per sample variability, is used to set
the limits. All of the modulus values are well within the upper and lower limits.
Figure 5.11 shows the process control chart for the daily results for the curvature
coefficient. While all of the values are within the limits, the curvature coefficient is more
variable for the fourth day (samples 16 through 20), and once the outlier is removed, day
4 data shows a tendency to be higher than the other days.
0.2
0.25
0.3
0.35
0.4
0102030
QC Sample No.
S
t
r
e
ss R
e
l
a
xati
o
n
C
u
r
vatu
r
e
C
o
ef
fi
ci
en
t
Curvature Coefficient
+ 2 std dev.
- 2 std dev.
Mean
Sample with
high variability
Figure 5.11: Curvature coefficient process control chart for field study
77
If the results from the EAP samples are compared with the results of the lab
mixtures without RAP with the binder PG 76-22, it is found that the results are not
statistically different for the modulus and for the curvature coefficient.
By doing three tests on each plant-produced sample without RAP, the COV was
reduced from 30% to 18% for the initial stress relaxation modulus and to 7% (from 15%
for the curvature coefficient.
5.7 Summary
When comparing the measured dynamic modulus and the calculated dynamic modulus
from the stress relaxation test on the binder alone, it is obvious that the relation is really
good, but not perfectly on the equality line. This is probably because two different DSR
were used.
The relationship between the asphalt binder stress and the HMA mixtures stress
relaxation moduli shows that the inclusion of any aggregate increases the initial stress
relaxation by 1.5 to 2 orders of magnitude. There was little statistical difference in the
initial stress relaxation modulus when one gradation was compared to another. These
results indicate that while there is a large difference in the magnitude of the modulus
when comparing binder-only to HMA samples, comparisons of one HMA mix result with
another is not strongly dependent upon the gradation. Therefore, any changes in the
modulus should represent a change in the effective HMA binder properties.
The use of a best fit power law curve is a good way to differentiate the stress
relaxation results from different mixes. Two parameters are obtained from this curve fit:
the initial stress relaxation modulus, and the curvature coefficient (exponent). The results
78
show that gradation, aggregate type and RAP source did not significantly influence either
the stress relaxation modulus or the curvature coefficient. The grade of the binder had a
small influence; reflecting the limited difference between the properties of the recovered
RAP binders used in this study. The percent of RAP included in the mix had the largest
effect on either parameter. The stress relaxation modulus increased with the addition of
RAP up to 50%; there was no statistical difference in the modulus between mix
containing 50 or 100% RAP. There was only a linear relationship between the percent of
RAP and either parameter between 0 and 50% RAP. If 100% RAP is included, the
relationship was non-linear for both the initial stress relaxation modulus and curvature
coefficient.
79
CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS
6.1 Summary
The main objectives of this research were to develop and validate an indirect tension
stress relaxation test using compacted HMA samples to evaluate binder properties and to
evaluate the effect of adding RAP to an HMA mixture. Tests were performed in order to
compare the stress relaxation results on binder alone with stress relaxation results on
HMA. Then, mixes containing 0, 15, 25, 50 or 100% RAP were compacted with a
Superpave gyratory compactor and tested in indirect tension stress relaxation at 5 and
22
o
C. The stress relaxation characteristics used in the analysis were the initial modulus
and the curvature coefficient. The results were verified and the test method was refined
in a field testing program.
6.2 Conclusion
The following conclusions are based on the observations and analysis presented in this
paper:
1. Indirect tension stress relaxation test is a good indicator of the binder properties of
the mix.
2. The gradation, aggregate source and RAP source have minimal influence on the
stress relaxation characteristics in an indirect relaxation test for the range of mix
variables used in this study.
80
3. There is a good correlation between the percentage of RAP in a mix and increases
in the modulus of the mix. This relationship is linear only between 0 and 50% of
RAP. This research suggests that the currently used assumption of a linear
relationship between a change in binder stiffness and the percent of RAP (i.e.,
blending charts) is not a valid assumption. The relationship between modulus and
the percent of RAP is non-linear, with the relationship between 50 and 100%
being about asymptotic.
4. There is no significant difference in the modulus and the curvature coefficient for
mix containing 50 or 100 percent RAP.
5. There is a good correlation between the percentage of RAP in a mix and the
decrease of the curvature coefficient when between 0 and 50% RAP is used. This
means that as the percentage of RAP increases, the ability of the mix to dissipate
stress through deformation decreases.
6. Testing at an intermediate temperature, such as 22
o
C can be used as a single test
temperature in order to evaluate the effect of RAP on HMA mixes.
7. The use of three tests per sample can decrease the coefficient of variation of the
results.
8. The test is quick (5 minutes), simple to conduct, requires basic equipment (i.e.
load cell, computer acquisition of data and time log, small load frame, and jack).
With minor modifications, old Marshall stability load frames (with electronic load
cells and RS232 port) can be used. Temperature control can be accomplished by
placing sample in zip-lock bag, and bringing it to test temperature in a bulk
specific gravity water tank.
81
9. Indirect tension stress relaxation testing has the possibility of being used as a
process control test for contractors to monitor the consistency of the mix being
produced.
6.3 Recommendations
These results suggest the following guidelines for the use of RAP:
? As little as 15% RAP will increase the mix modulus and decrease its ability to relax
over time at cooler temperatures. RAP mixes should be used in the lower lifts of
HMA pavements where an increase in stiffness could be beneficial and where daily
thermal changes will be minimized.
? Using 15% and 25% RAP results in mixes with similar modulus and curvature
coefficient at either test temperature. Therefore, it is preferable to use the higher
percentage of RAP in lower lifts.
? Use of RAP in the wearing course is not recommended as this is the lift that will
experience the highest temperature gradient. The highest curvature coefficient value
is needed in this location in order to provide the most resistance to thermal and/or
block cracking.
82
REFERENCES
1. Aklonis J.J., MacKnight W.J., Introduction to Polymer Viscoelasticity, Wiley-
Interscience Publication, Second edition, 1983
2. Anderson D.A., Christensen D.W., Bahia H.U., Dongre R., Sharma M.G., Antle
C.E., and Button, J., Binder Characterization and Evaluation, Volume 3:
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88
APPENDIX A
Maximum RAP allowed in Pavement for each state
89
Use of RAP in all states as of 1996
Max RAP for Batch Plants (%) Max RAP for Drum Plants (%)
State
Base Binder Surface Base Binder Surface
Alabama 40 40 15 50 50 15
Alaska - - - - - -
Arizona 30 30 30 30 30 30
Arkansas 70 70 70 70 70 70
California 50 50 50 50 50 50
Colorado 15 15 15 15 15 15
Connecticut 40 40 40 40 40 40
Delaware 35 35 25 50 50 30
Florida 60 50 None 60 50 None
Georgia 25 25 25 40 40 40
Hawaii 30 None None 40 None None
Idaho Open Open Open Open Open Open
Illinois 50 25 15 50 25 15
Indiana 50 50 20 50 50 20
Iowa Open Open Open Open Open Open
Kansas 50 50 50 50 50 50
Kentucky 30 30 30 30 30 30
Louisiana 30 30 None 30 30 None
Maine 40 40 None 40 40 None
Maryland Open Open Limit Open Open Limit
Massachusetts 20 20 10 40 40 10
Michigan 50 50 50 50 50 50
Minnesota 59 50 30 50 50 30
Mississippi 30 30 15 30 30 15
Missouri 50 50 50 50 50 50
Montana 50 50 10 50 50 10
Nebraska Not used Not used Not used Open Open Open
Nevada 50 50 15 50 50 15
New Hampshire 35 35 15 50 50 15
New Jersey 25 25 10 25 25 10
New Mexico Open Open Open Open Open Open
New-York 50 50 None 70 70 None
North Carolina 60 60 60 60 60 60
North Dakota 50 50 50 50 50 50
Ohio 50 35 20 50 35 20
Oklahoma 25 25 None 25 25 None
Oregon 30 20 20 30 20 20
Pennsylvania Open Open Open Open Open Open
Rhode Island 30 30 None 30 30 None
South Carolina 30 25 20 30 25 20
South Dakota Not used Not used Not used 50 50 50
Tennessee 15 Open None Open Open None
Texas 15 Open Open Open Open Open
Utah Not used Not used Not used 25 25 25
Vermont Specs Specs Specs Specs Specs Specs
Virginia 25 25 25 25 25 25
Washington Open Open Open Open Open Open
West Virginia Open Open Open Open Open Open
Wisconsin Open 35 20 Open 35 20
Wyoming 50 50 50 50 50 50
90
APPENDIX B
Tables of results for the HMA indirect stress relaxation test
RELAXATION CHARACTERISTICS (MODULUS / CURVATURE COEFFICIENT)
FOR HMA MIXES AT 5
O
C
Granite Gravel
Fine Grad. Coarse Grad. Fine Grad. Coarse Grad.
Temp.
o
C
Asphalt Source
of
RAP
% of
RAP
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
None None 255 0.135 211 0.111 235 0.117 237 0.144
15% 525 0.078 528 0.071 554 0.102 632 0.099
25% 564 0.068 487 0.087 527 0.094 604 0.066
50% 468 0.084 606 0.067 428 0.081 614 0.062
AL
RAP
100% 595 0.036 575 0.036 595 0.036 575 0.036
15% 516 0.126 521 0.100 557 0.108 499 0.138
25% 486 0.080 401 0.096 523 0.064 494 0.077
50% 475 0.042 375 0.060 521 0.047 504 0.036
PG 64-
22
MN
RAP
100% 625 0.018 625 0.018 625 0.018 625 0.018
None None 224 0.116 227 0.131 211 0.099 210 0.117
15% 413 0.071 494 0.096 635 0.054 617 0.094
25% 669 0.081 479 0.105 498 0.059 519 0.055
50% 574 0.063 629 0.064 565 0.055 701 0.036
AL
RAP
100% 595 0.036 575 0.036 595 0.036 575 0.036
15% 357 0.135 349 0.138 421 0.071 507 0.073
25% 444 0.103 602 0.096 472 0.078 441 0.102
50% 363 0.027 316 0.034 632 0.026 504 0.036
5
PG 76-
22
MN
RAP
100% 625 0.018 625 0.018 625 0.018 625 0.018
91
RELAXATION CHARACTERISTICS (MODULUS / CURVATURE COEFFICIENT)
FOR HMA MIXES AT 22
O
C
Granite Gravel
Fine Grad. Coarse Grad. Fine Grad. Coarse Grad.
Temp.
o
C
Asphalt Source
of
RAP
% of
RAP
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
Modulus
(MPa)
Curvature
coefficient
None
None 101 0.349 86 0.376 120 0.334 131 0.434
15% 322 0.268 189 0.267 139 0.318 166 0.401
25% 304 0.244 246 0.256 191 0.318 152 0.301
50% 231 0.253 245 0.226 203 0.255 163 0.318
AL
RAP
100% 217 0.209 274 0.201 217 0.209 274 0.201
15% 211 0.360 181 0.289 219 0.325 203 0.388
25% 210 0.210 175 0.249 307 0.255 276 0.265
50% 257 0.068 305 0.165 236 0.184 303 0.148
PG 64-
22
MN
RAP
100% 267 0.103 267 0.013 267 0.103 267 0.103
None None 94 0.220 79 0.268 120 0.207 103 0.290
15% 150 0.200 180 0.226 221 0.212 169 0.258
25% 173 0.199 140 0.225 233 0.210 208 0.235
50% 270 0.118 165 0.186 253 0.136 169 0.109
AL
RAP
100% 217 0.209 274 0.201 217 0.209 274 0.201
15% 127 0.273 116 0.233 260 0.020 187 0.257
25% 194 0.266 100 0.231 191 0.188 176 0.219
50% 280 0.091 266 0.091 237 0.050 192 0.067
22
PG 76-
22
MN
RAP
100% 267 0.103 267 0.103 267 0.103 267 0.103
92
APPENDIX C
Table of results for the field tests
93
RESULTS FROM FIELD TESTING
Modulus (MPa) Curvature
bag 1 2 3 average std deviation COV 1 2 3 average std deviation COV
1 112.0 86.3 95.2 97.8 13.1 13.4% 0.278 0.267 0.281 0.275 0.007 2.7%
1 103.9 79.8 50.9 78.2 26.5 33.9% 0.341 0.352 0.271 0.321 0.044 13.7%
1 87.1 112.7 85.8 95.2 15.1 15.9% 0.376 0.352 0.339 0.356 0.019 5.3%
1 146.5 74.0 110.8 110.4 36.3 32.9% 0.316 0.346 0.368 0.343 0.026 7.6%
1 100.4 87.3 65.4 84.4 17.7 20.9% 0.359 0.356 0.391 0.369 0.019 5.3%
2 106.5 93.2 74.8 91.5 15.9 17.4% 0.346 0.307 0.306 0.320 0.023 7.1%
2 120.3 109.6 115.9 115.3 5.4 4.7% 0.270 0.304 0.305 0.293 0.020 6.8%
2 96.7 103.4 88.2 96.1 7.6 7.9% 0.346 0.315 0.305 0.322 0.021 6.6%
2 97.1 51.4 85.7 78.1 23.8 30.5% 0.271 0.330 0.262 0.288 0.037 12.8%
2 117.9 97.4 71.6 95.6 23.2 24.2% 0.348 0.299 0.327 0.325 0.025 7.6%
3 103.1 125.0 104.6 110.9 12.2 11.0% 0.291 0.271 0.266 0.276 0.013 4.8%
3 122.2 90.9 72.9 95.3 24.9 26.2% 0.339 0.281 0.360 0.327 0.041 12.5%
3 94.9 118.9 113.9 109.2 12.7 11.6% 0.310 0.273 0.270 0.284 0.022 7.8%
3 86.0 93.6 99.1 92.9 6.6 7.1% 0.381 0.345 0.380 0.369 0.021 5.6%
3 120.5 92.7 106.1 106.4 13.9 13.0% 0.332 0.318 0.369 0.340 0.026 7.8%
4 122.2 114.6 87.6 108.1 18.2 16.8% 0.294 0.269 0.285 0.283 0.013 4.5%
4 88.8 80.3 111.6 93.5 16.2 17.3% 0.360 0.402 0.410 0.391 0.027 6.9%
4 182.8 92.6 99.6 125.0 50.2 40.1% 0.293 0.248 0.269 0.270 0.023 8.3%
4 94.2 93.3 83.4 90.3 6.0 6.6% 0.382 0.371 0.387 0.380 0.008 2.2%
4 68.1 103.4 73.9 81.8 18.9 23.1% 0.250 0.357 0.342 0.316 0.058 18.3%
94
95
APPENDIX D
Draft standard for Indirect Tension Stress Relaxation Test on HMA to Evaluate the effect
of the addition of RAP on the Binder Related Properties in the ASTM format
Designation: X XXXX-XX
96
Standard Test Method for
Indirect Tension Stress Relaxation to for Compacted Bituminous
Mixtures
1
This standard is issued under the fixed designation D XXXX; the number immediately following the
designation indicates the year of original adoption or, in the case of revision, the year of last revision. A
number in parentheses indicates the year of last reapproval. A superscript epsilon (?) indicates an editorial
change since the last revision or reapproval.
1. Scope
1.1 This test method covers the determination of the asphalt binder related properties
of compacted HMA specimens by means of indirect tension stress relaxation. This test
method can be used to evaluate the effect of the addition of RAP to a HMA mix.
1.2 The values stated in SI units are to be regarded as the standard. The value given
in parentheses are for information only
1.3 This standard does not purport to address all of the safety concerns, if any,
associated with its use. It is the responsibility of the user of this standard to establish
appropriate safety and health practices and to determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM D979 Standard Practice for Sampling Bituminous Paving Mixture
2.2
D6925 Standard Test Method for Preparation and Determination of the Relative
Density of Hot Mix Asphalt (HMA) Specimens by Means of the Superpave Gyratory
Compactor
2
1
This test method is under the jurisdiction of ASTM Committee and is the direct responsibility of
Subcommittee .
2
Annual Book of ASTM Standards, Vol 04.03
Current edition approved XXX. XX, XXXX. Published XX XXXX.
X XXXX
97
2.3 ASTM D3203 Standard Test Method for the Percent Air Voids in Compacted
Dense and Open-Graded Bituminous Paving Mixtures.
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 Curvature coefficient, n ? exponent of a power law curve equation fit through
indirect tension stress relaxation modulus over at least a 45 second time period from the
start of the load application. The curvature coefficient represents the ability of the
bituminous mixture to dissipate stress through deformation.
Initial stress relaxation modulus, n ? the intercept for a power law curve fit through
indirect tension stress relaxation modulus over at least a 45 second time period from the
start of the load application. The stress relaxation modulus represents the maximum
tensile stress the bituminous mixture exhibits immediately after the application of a given
strain.
4. Significance and Use
4.1 The curvature coefficient and the initial stress relaxation modulus can be used to
monitor changes in the bituminous mixture properties that are predominately related to
the asphalt binder properties.
4.2 These values can be used to evaluate the influence of the percent of reclaimed
asphalt pavement (RAP) on bituminous mixture properties. These values can also be used
to evaluate the uniformity of RAP stockpiles.
5. Apparatus
X XXXX
98
5.1 Caliper? digital or manual caliper with a precision of 0.1 mm.
5.2 Water Bath- capable of maintaining by any means, a constant temperature
between 20 and 30?C (70 to 85?F). The water bath must be suitable for immersion of the
sample to be tested.
5.3 Diametral testing jig - as shown in Figure 1.
5.4 Load cell
5.5 Load frame - a load frame with an adjustable platform. The movement of the
platform shall be sufficient to allow the application of a maximum of a 10 lb seating load
on the sample after it has been loaded in the diametral testing jig.
Note 1: A Marshall stability load frame is an example of an acceptable load frame.
6. Sampling
6.1.1 Obtain field samples in accordance with Practice D 979.
6.2 Alternatively, obtain 150 mm (6 inch) diameter cores from bituminous
pavements. Care shall be taken to avoid distortion, bending, or cracking of the specimens
during and after removal from pavements. Specimens shall be stored in a safe, cool place.
Specimens shall be free of foreign materials such as seal coat, tack coat, foundation
material, soil, paper, or foil.
X XXXX
99
7. Sample Preparation
7.1 Compact three specimens according to ASTM D6925 using either laboratory
prepared or field sampled bituminous mixtures.
7.2 All samples shall be compacted so that the air voids target 4% air voids +/- 1%
according to ASTM D3203.
7.3 If cores are used, specimens shall be separated from other pavement layers by
sawing or other satisfactory means. Each specimen shall represent, as closely as possible,
a single layer of homogeneous bituminous mixture with no height less than 25 mm (1
inch).
8. Procedure
8.1 Use a marker such as a construction crayon or paint pen to mark points around the
circumference of the specimen at 0, 30, and 60
o
. Mark three additional points at 180
o
from each of these positions.
8.2 Place the specimens in a waterproof bag, such as a zip-lock
TM
bag, and
submerged in the water bath at 25
o
C for a minimum of 1 hour. Care should be taken so
that the plastic does not allow water into the sealed bag,
8.3 After 1 hour, remove the specimen from the water, and then remove the specimen
from the bag.
8.4 Immediately place the specimen on the lower platen of the diametral jig.
8.5 Align the top and bottom loading strips with the first point on the circumference
of the specimen.
X XXXX
100
8.6 Move the loading frame platform up so that no more than 10 lb of seating load is
applied to the specimen.
8.7 Quickly apply the step strain.
8.8 Maintain the firm contact for 45 seconds and record the changing stress during
that period.
8.9 Release the strain and ensure that the specimen will not fall off the loading strips.
If the specimen falls from the setup, take note of it because it might be damaged and the
results can be misleading.
8.10 Rotate the specimen 30
o
, so it is aligned with the second diameter line, and start
the test over. Redo the same thing on the third diameter line.
8.11 Determine the diameter of the test specimen by averaging three measurement of
the diameter taken evenly spaced on the specimen.
8.12 Determine the height of the test specimen by averaging three measurement of the
height taken evenly spaced on the specimen.
9. Calculation or Interpretation of Results
9.1 Calculate the tensile stress in the specimen with:
dl
P
T
?
2
=
Where:
T = tensile stress (kPa)
P = applied load by the testing machine (kN)
l = length of the specimen (m)
X XXXX
101
d = diameter of the specimen (m)
9.2 Calculate the horizontal strain with:
???
vh
=
Where:
?
h
= horizontal strain
?
v
= vertical strain
? = Poisson?s ratio (0.2 at 5
o
C and 0.35 at 22
o
C)
9.3 Calculate the stress relaxation modulus (E) for every time increment available
with:
h
T
E
?
=
9.4 Draw the stress relaxation modulus versus the time and calculate a best fit power
curve starting at 3 seconds. If the load frame used can reach and keep a constant strain in
a shorter time, than plot the best fit curve starting earlier.
9.5 Use the initial modulus and the curvature coefficient of the equation of the best fit
power curve to do the analysis.
10. Report
10.1 Report the following information:
10.1.1 Proper identification of the samples,
10.1.2 Constant horizontal strain and the poisson?s ratio used to calculate it,
10.1.3 Graphical representation of the relaxation modulus versus time,
X XXXX
102
10.1.4 Initial modulus, curvature coefficient and the r
2
correlation factor of the power
curve,
10.1.5 Apparatus used to determine test values.
11. Precision and Bias
11.1 precision ? The single operator and multilaboratory precision of tests of
individual Superpave gyratory compacted HMA samples is given for samples made in a
laboratory environment and under normal field conditions.
A
These numbers represent, respectively, the (1s%) and the (d2s%) limits as described in Practice C 670
11.2 Bias ? Since the relaxation modulus and curvature coefficient can be defined
only in terms of a test method, no bias statement is being made.
Coefficient of
Variation
(percent of mean)
A
Acceptable Range of
Two Results
(percent of mean)
A
Single Operator Precision
Relaxation Modulus (22
o
C) 0.18 0.50
Curvature Coefficient (22
o
C) 0.07 0.20
Multilaboratory Precision
Relaxation Modulus (22
o
C) 0.21 0.59
Curvature Coefficient (22
o
C) 0.13 0.36
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12. Keywords
12.1 Reclaimed Asphalt Pavement, RAP, Bituminous Mixtures, Indirect Tension, Relaxation
Modulus, Curvature Coefficient
Figure 1: Indirect Tension Setup