ABSTRACT
MALLADI, HARITHA. Investigation of Warm Mix Asphalt Concrete Mixtures with
Recycled Asphalt Pavement Material. (Under the direction of Dr. Akhtarhusein A.
Tayebali).
Today’s world faces the challenge of sustainability in all facets of existence,
production and consumption. Since roads have a large areal as well as carbon footprint,
it is imperative that the pavement industry take serious steps to address their impact
on the environment. Warm Mix Technology (WMA) and Reclaimed Asphalt Pavement
(RAP) material can enable us to “Reduce, Reuse and Recycle” our way to sustainable
pavements.
WMA, with its lower production temperatures, can reduce fuel consumption and
lower harmful emissions during construction. However, there is a concern that the
lower production temperatures can lead to softer mixtures, affecting rutting and
moisture susceptibility, especially in those WMA technologies that employ water as a
mechanism to lower production temperatures.
RAP material has been used in the pavement industry since decades and is an
obvious move towards sustainability. RAP material is stiffer than freshly produced
asphalt concrete. This extra stiffness can be beneficial in terms of improving resistance
to permanent deformation. However, concerns related to workability and long-term
durability, hinder the usage high amounts of RAP in construction.
As the behavior of softer WMA mixtures is converse to that of the stiffer RAP
mixtures, it is believed that these technologies in conjunction can complement each
other’s perceived deficiencies. There is a potential for large environmental and
economic gain if this combination of sustainable technologies is successful in
addressing material quality concerns. Thus, there is a need to study the compatibility of
different types of WMA technologies with the usage of RAP. Material performance
characteristics like workability, moisture-susceptibility and stiffness need to be
analyzed for WMA–RAP mixtures.
This research study focuses on combining two WMA technologies—Evotherm®
3G and The PTI Foamer with two RAP percentages—20% and 40%, along with control
HMA and virgin (no RAP) mixtures. The resulting mixture combinations were evaluated
for workability and moisture susceptibility, and their pavement performance was
predicted based on dynamic moduli.
The evolution of %Gmm during mixture compaction was used to evaluate
workability. Even without a change in binder grade, WMA mixtures with 40% RAP
exhibited similar %Gmm trends the HMA mixtures with 40% RAP that incorporated a
softer binder grade.
Tensile Strength Ratio (TSR) was used to evaluate the moisture susceptibility.
TSR values decreased with increase in RAP content in HMA as well as the two WMA
mixtures. Dynamic modulus tests were conducted to obtain the E* master curves for all
mixtures. They were also used to compute the E* Stiffness Ratio, i.e. the ratio of
dynamic modulus values of moisture-conditioned specimens to that of unconditioned
specimens, analogous to the Tensile Strength Ratio. The ESR value of HMA mixture with
40% RAP was significantly lower than all the other mixtures. This may be because of
the softer binder grade used in this mixture while all other mixtures used standard
binder grade.
AASHTOWare Pavement ME software was used to analyze the rutting and
fatigue performance of the mixtures for a design life of 20 years. For a typical pavement
section, none of the mixtures exceeded the threshold failure criteria. The only
difference in production costs amongst the mixtures are the costs of additives and
technology installation for WMA mixtures, screening and processing costs of RAP and
energy savings from WMA. Despite these initial costs, using WMA and RAP prove to be
more economical than HMA.
© Copyright 2015 Haritha Malladi
All Rights Reserved
Investigation of Warm Mix Asphalt Concrete Mixtures with Recycled Asphalt Pavement
Material
by
Haritha Malladi
A dissertation submitted to the Graduate Faculty of
North Carolina State University
in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
Civil Engineering
Raleigh, North Carolina
2015
APPROVED BY:
_______________________________
Dr. N. Paul Khosla
_______________________________
Dr. Cassandra Castorena
_______________________________
Dr. Mohammad Pour-Ghaz
_______________________________
Dr. Justin Post
_______________________________
Dr. Akhtarhusein A. Tayebali
Committee Chair
DEDICATION
To Kerala, Carolina and Carl
Keeping the Verdant Specks Alive on the Pale Blue Dot
ii
BIOGRAPHY
Haritha Malladi was born in the capital city of Thiruvananthapuram, India, where she
spent all her childhood years until college. She received her bachelor’s degree in Civil
Engineering from National Institute of Technology, Warangal, India in 2010. During her
undergraduate studies, she became interested in experimental research on civil
engineering materials. She was awarded summer fellowships and internships at RWTH
Aachen University, Germany and Indira Gandhi Center for Atomic Research, India to
work on nondestructive testing of concrete. Her undergraduate senior project focused
on using recycled plastic and crumb rubber in asphalt concrete. This work enhanced
her interest in sustainable civil engineering material practices. In 2010, she started
working under Dr. Akhtar Tayebali at North Carolina State University on a project
funded by North Carolina Department of Transportation (NCDOT) to test different
types of Warm Mix Asphalt technologies; a part of this work formed her master’s thesis.
She received her Master of Science degree from North Carolina State University in 2012
and continued to work for her PhD in Civil Engineering with a minor in Statistics. Her
doctoral dissertation was based on a research project, also funded by the NCDOT, to test
combinations two sustainable pavement construction practices—Warm Mix Asphalt
and Recycled Asphalt Pavement. Alongside research, she is passionate about science
communication and outreach and has utilized multiple opportunities to engage the
public in science and technology. In her spare time, she likes to read, practice playing
the “Veena” (an Indian classical instrument) and spend time outdoors.
iii
ACKNOWLEDGMENTS
First and foremost, I would like to sincerely thank my advisor, Dr. Akhtar
Tayebali for his constant support and guidance throughout my graduate career. He
always awarded me the highest form of independence in decision-making and was
always ready to provide any form of support and encouragement whenever I needed it.
Working under him and learning from him during my doctoral studies has been my
honor and privilege.
I would like to thank Dr. Paul Khosla for his words of advice and supervision
throughout the past five years. His support and guidance has been an integral part of
my success in graduate school. I am also fortunate to have Dr. Cassie Castorena, Dr.
Mohammad Pour-Ghaz and Dr. Justin Post on my advisory committee. I thank them for
their time and valuable suggestions that contributed positively to the scientific rigor
and readability of my doctoral dissertation.
I thank the North Carolina Department of Transportation for funding this
research project. I especially thank Mr. James Budday for his help with procurement of
materials and technical guidance. I also thank the members of the technical committee
for this project and the staff of the Materials and Tests Unit for their feedback.
My sincere gratitude goes to my colleague and research partner, Abhilash
Kusam. He always lent a helping hand whenever I needed it. I also thank my present
and former colleagues Dr. Dinesh Ayyala, Srikanth Sree Ramoju and Dr. Haritha Musty
iv
for all their help and friendship during my graduate studies. A special thanks to Michael
Elwardany for his help with the dynamic modulus tests.
My parents have always encouraged me in all my aspirations. I thank them for
their omnipresent love and support that surpasses the thousands of miles that separate
us. Last but certainly not the least, I’m grateful for the strength and presence
throughout my grad school years from Phalgun Nelaturu. Thank you for the memories.
v
TABLE OF CONTENTS
LIST OF TABLES …………………………………………………………………………………………………xi
LIST OF FIGURES ……………………………………………………………………………………….………xiv
CHAPTER 1. INTRODUCTION ................................................................................................... 1
1.1.
Background....................................................................................................................... 1
1.1.1.
Recycled Asphalt Pavement (RAP) ................................................................... 2
1.1.2.
Warm Mix Asphalt (WMA) ................................................................................... 3
1.2.
RAP and WMA in Conjunction .................................................................................... 3
1.3.
Need for Study.................................................................................................................. 4
1.4.
Research Objective ......................................................................................................... 5
1.4.1.
Experimental Design ............................................................................................. 5
1.4.1.
Sequence of Tasks................................................................................................... 6
CHAPTER 2. LITERATURE REVIEW ........................................................................................ 7
2.1.
Recycled Asphalt Pavement ........................................................................................ 7
2.2.
Warm Mix Asphalt ........................................................................................................ 17
2.2.1.
Production of WMA .............................................................................................. 19
2.2.2.
Economic and Environmental Benefits of WMA ........................................ 27
2.2.1.
Performance of WMA Mixtures........................................................................ 29
2.3.
Studies on RAP-WMA ................................................................................................... 40
vi
CHAPTER 3. MATERIAL CHARACTERIZATION ................................................................. 44
3.1.
Aggregates ....................................................................................................................... 44
3.1.1.
Gradation ................................................................................................................. 44
3.1.2.
Bulk Specific Gravity ............................................................................................ 46
3.2.
Asphalt Binder ............................................................................................................... 46
3.3.
RAP Aggregate ............................................................................................................... 47
3.3.1.
Ignition Oven Test ................................................................................................ 47
3.3.1.
Bulk Specific Gravity of RAP ............................................................................. 48
3.3.1.
RAP Batching .......................................................................................................... 50
3.3.1.
RAP Heating Procedure ...................................................................................... 50
3.4.
Asphalt Additives .......................................................................................................... 51
3.4.1.
Liquid Anti-Strip ................................................................................................... 51
3.4.2.
Warm Mix Additive .............................................................................................. 51
3.4.1.
Additive Weight Calculation ............................................................................. 52
CHAPTER 4. SUPERPAVE MIX DESIGN ................................................................................ 53
4.1.
Aggregate Gradation .................................................................................................... 53
4.2.
Mixing and Compaction Temperatures ................................................................ 54
4.3.
Optimum Asphalt Content ......................................................................................... 55
vii
4.3.1.
Volumetric Data of Virgin (0% RAP) Mixtures ........................................... 57
4.3.1.
Volumetric Data of 20% RAP Mixtures ......................................................... 58
4.3.2.
Volumetric Data of 40% RAP Mixtures ......................................................... 59
CHAPTER 5. EVALUATING WORKABILITY USING %Gmm.............................................. 62
5.1.
Procedure ........................................................................................................................ 62
5.2.
Effect of RAP in Each Mixture Type ........................................................................ 63
5.2.1.
HMA Mixtures......................................................................................................... 63
5.2.2.
Evotherm® Mixtures ........................................................................................... 64
5.2.3.
Foamer Mixtures ................................................................................................... 65
5.3.
Effect of Mixture Technology in Each RAP Content .......................................... 67
5.3.1.
0% RAP Mixtures .................................................................................................. 67
5.3.2.
20% RAP Mixtures ............................................................................................... 68
5.3.3.
40% RAP Mixtures ............................................................................................... 69
5.4.
Conclusions ..................................................................................................................... 70
CHAPTER 6. TENSILE STRENGTH RATIO ........................................................................... 71
6.1.
Specimen Preparation ................................................................................................ 71
6.2.
Test Procedure .............................................................................................................. 72
6.3.
Test Results..................................................................................................................... 73
viii
6.3.1.
Virgin Mixtures ...................................................................................................... 73
6.3.2.
20% RAP Mixtures ............................................................................................... 73
6.3.3.
40% RAP Mixtures ............................................................................................... 77
6.4.
Analysis of Test Results .............................................................................................. 77
6.5.
Inferences ........................................................................................................................ 86
6.5.1.
Virgin Mixtures (0% RAP) ................................................................................. 86
6.5.2.
20% RAP Mixtures ............................................................................................... 87
6.5.3.
40% RAP Mixtures ............................................................................................... 87
CHAPTER 7. E* STIFFNESS RATIO ........................................................................................ 89
7.1.
Background on Dynamic Modulus .......................................................................... 89
7.2.
ESR Test Description ................................................................................................... 91
7.3.
Specimen Preparation and Conditioning ............................................................. 92
7.4.
ESR Test Results ............................................................................................................ 93
CHAPTER 8. DYNAMIC MODULUS...................................................................................... 101
8.1.
Specimen Preparation and Test Description ................................................... 101
8.2.
Results and Master Curves ..................................................................................... 101
CHAPTER 9. PERFORMANCE PREDICTION AND ECONOMICS .................................. 116
9.1.
Pavement Performance Prediction ..................................................................... 116
ix
9.2.
Economic Analysis ..................................................................................................... 125
CHAPTER 10.
SUMMARY, CONCLUSION and RECOMMENDATIONS ...................... 131
10.1. Summary ....................................................................................................................... 131
10.2. Conclusions .................................................................................................................. 132
10.3. Recommendations for Further Studies .............................................................. 134
REFERENCES ……………………………………………………………………………………………………135
APPENDIX ……………………………………………………………………………………………………..…141
APPENDIX A ……………………………………………………………………………………….……………142
x
LIST OF TABLES
Table 1-1 Nomenclature of Mixture Combinations (Treatments) ........................................... 6
Table 2-1: Binder Selection Guidelines for RAP Mixtures from NCHRP Report 452 ......... 12
Table 2-2: Summary of WMA Technologies Used ............................................................... 26
Table 2-3: Summary of NCAT In-Situ Studies ..................................................................... 39
Table 3-1: Aggregate Gradation ............................................................................................ 45
Table 3-2: Aggregate Gradation of RAP Fractions ............................................................... 48
Table 3-3: Calculation of Bulk Specific Gravity (Gsb) for RAP Fractions ............................ 49
Table 3-4: Asphalt Additives Summary................................................................................. 51
Table 4-1: Design Aggregate Gradation as Obtained from JMF (9.5B Mix) ........................ 53
Table 4-2: Mixing and Compaction Temperatures ................................................................ 55
Table 4-3: Volumetric Properties for 0% RAP Mixtures with 6% PG 64-22 Binder ............ 57
Table 4-4: % Air voids in 20% RAP Mixtures with PG 64-22 Binder .................................. 58
Table 4-5: Volumetric Properties for 20% RAP Mixtures with 5.9% PG 64-22 Binder ....... 59
Table 4-6: Air voids in 40% RAP Mixtures with 6% Binder Content .................................. 59
Table 4-7: Volumetric Properties for 40% RAP Mixtures with 5.8% PG 64-22 Binder ....... 60
Table 4-8: Volumetric Properties for 40% RAP Mixtures with 5.8% PG 58-28 Binder ....... 61
Table 5-1: N92 Values for HMA Mixtures ............................................................................ 63
Table 5-2: N92 Values for Evotherm Mixtures ..................................................................... 65
Table 5-3: N92 Values for Foamer Mixtures ......................................................................... 66
Table 5-4: N92 Values for 40% RAP Mixtures ..................................................................... 69
Table 6-1: Tensile Strength Values for Virgin (0% RAP) Mixtures ..................................... 74
xi
Table 6-2: Tensile Strength Values for 20% RAP Mixtures ................................................. 75
Table 6-3: Tensile Strength Values for 40% RAP HMA Mixture......................................... 76
Table 6-4: Summary of TSR test results of all the mixtures .................................................. 78
Table 6-5: Multi-factor ANOVA for Indirect Tensile Strength ............................................. 80
Table 6-6: Bonferroni (Dunn) t Tests for ITS ........................................................................ 82
Table 6-7: One-Way ANOVA Statistics for Indirect Tensile Strength ................................. 83
Table 6-8: One-Factor ANOVA Comparisons ...................................................................... 84
Table 7-1: E* Stiffness Ratio Test Results ............................................................................ 94
Table 7-2: Comparison of TSR and ESR Test Results ........................................................ 100
Table 8-1: Dynamic Moduli of All Mixtures—Specimens at 4% Target Air Voids ........... 103
Table 8-2: ANOVA of Dynamic Modulus .......................................................................... 106
Table 9-1: E* Data from Master Curves for Use as M-E PDG Input .................................. 119
Table 9-2: Fatigue and Rutting Failure Prediction for NCDOT Pavement Structure .......... 121
Table 9-3: Fatigue and Rutting Failure Prediction for a Weak Pavement Structure ........... 122
Table 9-4: Summary of Costs and Benefits with Usage of WMA and RAP ....................... 125
Table 9-5: Material Cost Estimates for Mixture Production................................................ 126
Table 9-6: Costs per Ton of Each Mixture Type ................................................................. 129
xii
LIST OF FIGURES
Figure 2-1: Classification of Asphalt Concrete by Approximate Temperature Ranges from
2007 WMA Scan Summary Report ........................................................................................ 19
Figure 2-2: “The Foamer” Device and Its Schematic Representation ................................... 23
Figure 2-3: Schematic Representation of Control Panel Displays in “The Foamer” ............ 23
Figure 2-4: Foamed Asphalt Produced by “The Foamer” ..................................................... 25
Figure 2-5: Photograph of Evotherm 3G Used in the Study.................................................. 26
Figure 4-1: Design Gradation: Percentage Passing vs. 0.45 Power of Sieve Size ................ 54
Figure 5-1: %Gmm Evolution Curves for HMA Mixtures...................................................... 64
Figure 5-2: %Gmm Evolution Curves for Evotherm Mixtures ............................................... 65
Figure 5-3: %Gmm Evolution Curves for Foamer Mixtures ................................................... 66
Figure 5-4: %Gmm Evolution Curves for Virgin (0% RAP) Mixtures ................................... 67
Figure 5-5: %Gmm Evolution Curves for 20% RAP Mixtures ............................................... 68
Figure 5-6: %Gmm Evolution Curves for 40% RAP Mixtures ............................................... 69
Figure 5-7: Number of Gyrations to Achieve 92% Gmm (N92 Values) ................................. 70
Figure 6-1: Median and Range of Indirect Tensile Strength Values ..................................... 79
Figure 6-2: Tensile Strength Ratio Values of All Mixtures................................................... 79
Figure 6-3: Effects of Mixture Type, %RAP and Conditioning Treatment on Mean ITS .... 85
Figure 6-4: Interaction Effects between Mixture Type, %RAP and Conditioning Treatment
................................................................................................................................................. 86
Figure 7-1: Schematic Diagram of Stress and Strain in Asphalt Concrete ............................ 90
Figure 7-2: Arrangement of LVDTs on Dynamic Modulus Test Specimen ......................... 90
xiii
Figure 7-3: Average ESR Values at Each Test Temperature ................................................ 96
Figure 7-4: Average and Range of Dynamic Modulus at 20°C and 1 Hz for ESR ............... 97
Figure 7-5: Average and Range of Storage and Loss Moduli for All Mixtures .................... 97
Figure 7-6: Ratios of Storage Modulus (SMR) and Loss Modulus (LMR)........................... 99
Figure 8-1: E* Master Curves for All Mixtures (reference temperature 70°F) ................... 102
Figure 8-2: Average and Range of Dynamic Modulus at 1 Hz Loading Frequency ........... 104
Figure 8-3: Influence of All Factors on Dynamic Modulus ................................................ 105
Figure 8-4: Influence of Mixture Type and RAP Content on Dynamic Modulus ............... 105
Figure 8-5: Interactions between Mixture Type, %RAP and Testing Conditions on |E*| ... 107
Figure 8-6: Change in Dynamic Modulus with Mixture Type and % RAP ........................ 107
Figure 8-7: E* Master Curves for Virgin (0% RAP) Mixtures (Reference Temperature 70°F)
............................................................................................................................................... 109
Figure 8-8: E* Master Curves For 20% RAP Mixtures (Reference Temperature 70°F) .... 109
Figure 8-9: E* Master Curves For 40% RAP Mixtures (Reference Temperature 70°F) .... 110
Figure 8-10: E* Master Curves for HMA Mixtures (Reference Temperature 70°F) .......... 110
Figure 8-11: E* Master Curves for Evotherm Mixtures (Reference Temperature 70°F).... 111
Figure 8-12: E* Master Curves for Foamer Mixtures (Reference Temperature 70°F) ....... 111
Figure 8-13: Phase Angle Master Curve of Virgin HMA Mixtures (70°F) ........................ 112
Figure 8-14: Phase Angle Master Curve of Virgin Evotherm Mixtures (70°F) .................. 112
Figure 8-15: Phase Angle Master Curve of Virgin Foamer Mixtures (70°F) ...................... 112
Figure 8-16: Phase Angle Master Curve of 20% RAP HMA Mixtures (70°F) ................... 113
Figure 8-17: Phase Angle Master Curve of 20% RAP Evotherm Mixtures (70°F) ............ 113
xiv
Figure 8-18: Phase Angle Master Curve of 20% RAP Foamer Mixtures (70°F) ................ 113
Figure 8-19: Phase Angle Master Curve of 40% RAP HMA Mixtures (70°F) ................... 114
Figure 8-20: Phase Angle Master Curve of 40% RAP Evotherm Mixtures (70°F) ............ 114
Figure 8-21: Phase Angle Master Curve of 40% RAP Foamer Mixtures (70°F) ................ 114
Figure 9-1: NCDOT Pavement Layer Structure for Performance Prediction ..................... 116
Figure 9-2: Weaker Pavement Layer Structure ................................................................... 122
Figure 9-3: Rutting Predictions Normalized with Respect to HMA ................................... 124
Figure 9-4: Percentage Fatigue Predictions Normalized with Respect to HMA ................. 124
xv
CHAPTER 1.
1.1.
INTRODUCTION
Background
With a road network spanning around 3.7 million miles, the United States of
America has the longest road network in the world. Around two-thirds of these roads
are paved with asphalt or concrete. 94% of all paved roads in the USA have asphalt
surfaces [1], [2]. Roads are the lifeline of the country’s economy bearing around 3
trillion annual vehicle miles of travel [2].
Since they occupy such a large surface area, the environmental impact of roads is
substantial throughout their construction, use and maintenance. Today’s asphalt is
processed from nonrenewable crude oil. Asphalt plants and pavement construction
practices require tons of fuel and contribute heavily to atmospheric pollution. Since
sustainable practices are the need of the day, it is imperative that the pavement
industry take serious steps to address the impact of roads on the environment.
Numerous studies have attempted to find solutions to these environmental
challenges. The United States Department of Transportation has designated
“environmental stewardship” as a major focus area of its strategic plan [3]. Adopting
the use of Reclaimed/Recycled Asphalt Pavement (RAP) and Warm Mix Technology
(WMA) are two potentially sustainable practices that can enable us to “Reduce, Reuse
and Recycle” our way to “green” pavements.
1
1.1.1. Recycled Asphalt Pavement (RAP)
Reclaimed or recycled asphalt pavement material is processed from the material
removed during resurfacing, rehabilitation or reconstruction of asphalt pavements. The
oil crises in 1970s with the Arab crude oil embargo with simultaneous increased
demand and decreased domestic oil production all popularized the use of RAP material
in construction applications [4]. Utilizing RAP helps divert tons of waste from landfills,
reduces material costs and of course, provides great environmental benefits. It can lead
to further savings in material transportation costs in areas where local aggregate
quarries and asphalt plants are not readily available.
Many research projects have tested the performance of RAP mixtures. Since the
RAP material has undergone long-term aging and oxidation in the field, the RAP binder
is stiffer than typical virgin binders. The higher stiffness can help combat permanent
deformation problems however, there are concerns with fatigue-resistance and
moisture-susceptibility of RAP mixtures, particularly when higher amounts of RAP are
used (greater than 30-40%). The higher stiffness of RAP mixtures can also lead to
workability issues during compaction. NCHRP Report 452 includes recommendations
for the use of softer binder grades in mixtures with high RAP to overcome problems
associated with increased stiffness [5]. Mandates to use softer binder grades may
discourage contractors from incorporating higher amounts of RAP. This is because the
softer binder grade may not be available locally and thus increases the cost of
construction.
2
1.1.2. Warm Mix Asphalt (WMA)
As the name suggests, Warm Mix Asphalt—in contrast to the conventional Hot
Mix Asphalt—utilizes technologies that allow production and placement of asphalt
mixtures at lower temperatures. Unlike RAP, the usage of WMA is relatively new to the
USA; the first field demonstration of WMA in the USA was in 2004 in Charlotte, NC [6].
Numerous WMA technologies exist in the market today and are either additives or work
by foaming the asphalt binder. Reduction in production temperatures can translate to
economic and environmental benefits due to lower energy consumption required for
heating the materials. Harmful emissions from spent fuel and construction materials
can be reduced. The lower mixing and compaction temperatures also lessen short-term
oxidative hardening of asphalt, leading to softer mixtures, which can improve the
fatigue resistance of the pavement.
The use of WMA technologies have also has raised some concerns. Their
production temperatures may not be high enough to ensure complete removal of
moisture from the aggregates. Foaming technologies are water-based and can cause
more residual moisture in the mixture. This moisture can be detrimental to the
durability of the mixture. The softer WMA mixtures may be more prone to permanent
deformation.
1.2.
RAP and WMA in Conjunction
The properties of the stiffer RAP and softer WMA mixtures contrast each other.
Thus, it is believed that their use in conjunction with each other may help combat the
3
performance issues that arise with workability, moisture-susceptibility, permanent
deformation and fatigue when they are used individually. Reduced oxidative hardening
with use of WMA technologies may eliminate the need for use of softer binder grades in
RAP mixtures and can even help in incorporating greater amounts of RAP into the
mixtures. Using these technologies together will lead to even greater sustainability in
pavements. The potential for significant economic gain, if verified, will encourage
mainstream adoption into construction.
1.3.
Need for Study
There is a huge potential for economic and environmental benefits when RAP and
WMA technologies are used together. A systematic study that evaluates different
material performance characteristics of WMA-RAP mixtures using local materials and
identifies the effect of addition of RAP and incorporation of WMA technologies on these
characteristics would be greatly beneficial.
There is a need for a study that can address the following needs:
i.
Identify appropriate types of WMA technologies that work well RAP.
ii.
Quantify the ability of WMA technologies to incorporate higher amounts of RAP.
iii.
Determine effects of lower mixing and compaction temperatures on workability
and moisture susceptibility of RAP mixtures.
iv.
Determine effects of addition of RAP and any lowering of binder grade on
workability and moisture susceptibility.
4
v.
Evaluate fundamental material properties of WMA-RAP mixtures that can be
used in pavement performance prediction.
vi.
Compare the costs and benefits of using WMA technologies and RAP in asphalt
concrete mixtures.
1.4.
Research Objective
The objective of this study was to compare the performance Warm Mix Asphalt
(WMA) with Hot Mix Asphalt (HMA) mixtures and evaluate the effect of incorporation
of Reclaimed Asphalt Pavement (RAP) material on this performance. Specific
performance characteristics that were evaluated include: workability, moisture
susceptibility and pavement performance prediction based on dynamic modulus. A
cost-benefit analysis was also conducted.
1.4.1. Experimental Design
Two WMA technologies were used in this study: PTI Foamer and Evotherm® 3G
additive. Two different RAP contents were used in addition to the virgin mix (no RAP):
20% and 40% RAP. Thus, the basic experimental design consisted of two factors, each
with three levels: three mixture types (HMA, Foamer and Evotherm®) and three RAP
contents (0%, 20% and 40%), resulting in nine treatments. Table 1-1 shows the
experimental design and nomenclature employed for each treatment. All mixtures were
designed to be Asphalt Concrete Surface Course Type S 9.5 B as specified by the North
Carolina Department of Transportation (NCDOT) [7].
5
Table 1-1 Nomenclature of Mixture Combinations (Treatments)
RAP Content
Mixture Type
HMA
Evotherm
Foamer
0%
H0R
E0R
F0R
20%
H20R
E20R
F20R
40%
H40R
E40R
F40R
1.4.1. Sequence of Tasks
Specific objectives of this research study were met using the following series of
tasks:
i.
Produce S9.5B HMA and WMA mixtures that meet Superpave mix design
volumetric criteria for all RAP contents.
ii.
Determine workability of the mixtures using %Gmm evolution curves and
evaluate the need for softer binder grade for 40% RAP mixtures.
iii.
Determine moisture susceptibility of the mixtures using Tensile Strength Ratio
(TSR) and observe the effects of employing WMA technologies and using RAP on
resistance to moisture-damage.
iv.
Use dynamic modulus values on moisture-conditioned and unconditioned
specimens to determine the E* Stiffness Ratio.
v.
Predict pavement performance of all mixtures using dynamic modulus master
curves.
vi.
Compute costs and benefits of using WMA and RAP in asphalt concrete mixtures.
6
CHAPTER 2.
LITERATURE REVIEW
A comprehensive literature review on use of Recycled Asphalt Pavement (RAP)
material, Warm Mix Asphalt (WMA), and use of RAP with WMA technology is presented
in this section. The section focuses on the effect of workability and moisture
susceptibility of WMA and RAP mixtures separately. Results from studies on the
performance of mixtures incorporating WMA and RAP in conjunction are also
summarized.
2.1.
Recycled Asphalt Pavement
Asphalt pavements are regularly removed for reconstruction, resurfacing or in the
process of digging for access to buried utility lines. This material when removed and/or
processed is called Reclaimed/Recycled Asphalt Pavement (RAP) [8].
RAP has achieved great success in the pavement industry. Its history dates back to
over a century ago; the first reported hot in-place recycling work reported in the 1930’s
[8]. In the 1970s, there was huge oil crisis with the Arab crude oil embargo at a time of
increased domestic demand in the USA. This spurred the use of RAP material in
construction applications and led to an evolution in modern recycling practices [4], [8].
The Federal Highway Administration estimated that 45 million tons of RAP are
produced annually in the USA of which less than 20% is disposed, mostly in landfills.
Thus, 80-85% of the RAP that is generated every year is incorporated into asphalt
paving, aggregate base/subbase construction or bases/fills of embankments.
7
Sometimes, the RAP that is generated is not used in the same construction season but is
stockpiled and eventually reused [8], [9].
The different techniques of incorporating RAP either rely on processing prior to
construction or use equipment capable of in-place recycling. At a central processing
facility, RAP is crushed and screened to produce high quality graded stockpiles. This
material is subsequently incorporated into hot or cold mixes. In-place recycling into hot
or cold mixes involves single or multiple pass operations using specialized plants or
trains that heat, mill, mix, rejuvenate and compact RAP into new pavements [8], [9].
Since RAP material generally comes from pavement surfaces that have been
exposed to weathering over many years, its binder is aged and oxidized. The process of
milling and crushing affects the aggregate gradation. Thus, asphalt and aggregate
properties of RAP can change significantly from the original materials used to construct
the source pavement. Li and Gibson analyzed RAP material milled from an 8-year
naturally-weathered pavement section that was a part of FHWA full-scale accelerated
pavement test [10]. The original material characteristics were well-documented and
thus the authors could identify changes in material properties. The RAP material was
crushed and processed from three different sections: a control section with PG 70-22
binder, one with SBS modified binder and a third one with air blown binder. Extracted
binder of RAP from control and air blown sections were much stiffer with PG 94-11 and
94-21, respectively. The extracted binder of RAP sourced from SBS modified section
showed the almost no change from original performance grading. This proves that the
properties of RAP binder are highly dependent on the original construction materials.
8
Since the RAP material contains aged and oxidized binder, its effect and interaction
with virgin binder in a mixture is of interest. At low levels of RAP—less than 20
percent—the behavior of RAP is likened to a “black rock”; it acts completely like
aggregate and influences the mixture only through gradation and aggregate properties.
At higher levels, this analogy is erroneous as the aged binder casts an influence on
mixture properties.
With looming global shortage of crude oil, it is necessary that new pavement
construction projects use higher amounts of RAP. Material, energy and transportation
costs are significantly lower with use of RAP. A study by Hajj et al. estimated the value
of RAP with 4.7% asphalt content at $46.5 per ton. They estimated a savings of
approximately $5 per ton by incorporating 15% RAP into the mixture. At high amounts
of RAP (50%), these savings can go up to $17 per ton [11].
To facilitate the incorporation of higher amounts of RAP, three techniques exist: use
of rejuvenating agents, softer virgin binder or extra binder above the optimum level.
Rejuvenating agents can help revitalize RAP binder and improve degree of blending
between virgin and RAP binders. Using a softer binder grade tries to compensate for the
stiffness of the aged RAP binder. Extra amount of virgin binder above the optimum level
can improve the workability of the stiff RAP mixtures.
Hajj el al. evaluated the influence of a bio-rejuvenating agent, BituTech RAP (brand
name Hydrogreen) on the viscoelastic properties of 15% and 50% RAP HMA mixtures
from Canada and compared their performance with control mixtures containing 0, 15%,
50% and 100% RAP [11]. The different mixture combinations were compared based on
9
their storage modulus (elastic component) and loss modulus (viscous component)
using dynamic modulus tests on unconditioned and moisture-conditioned specimens.
Higher loss modulus is attributed to better resistance against fatigue and low
temperature cracking. Use of BituTech RAP significantly increased loss modulus
without affecting storage modulus and assisted in maintaining it upon moisture
conditioning. The moisture-conditioned to unconditioned storage and loss moduli
ratios were around 80% for virgin and medium RAP mixtures but dropped down to
65% for high RAP mixtures. Upon using the rejuvenating agent, these values were
almost 100% for both RAP mixtures.
Vahidi et al. investigated performance of high RAP mixtures containing ground tire
rubber that has been treated with rejuvenating additives [12]. They designed
combinations of HMA with 10% and 15% ground tire rubber (with and without
treatment) and 40% RAP. Based on the dynamic modulus results, they observed an
increase in stiffness with addition of RAP and ground tire rubber (both treated and
untreated). They noted that mixtures with treated ground tire rubber were more
temperature sensitive than those with untreated rubber—at 40°C, the mixtures with
treated ground tire rubber had lower stiffness than those with untreated ground tire
rubber, their stiffness values were similar at 20°C and treated ground tire rubber
mixtures were stiffer than those with untreated rubber at 4°C. However, the authors did
not try to isolate the effect of using RAP and ground tire rubber on elastic and viscous
properties of the mixtures.
10
However, the use of rejuvenating agents can be costly. In the same study by Hajj et
al. the savings resulting from using 15% RAP in the mixture reduced by half with use of
BituTech RAP. Savings resulting from using 50% RAP in the mixture fell from around
$17 per ton to around $7 per ton [11].
Willis et al. evaluated the latter two techniques: increasing amount of virgin binder
and using a softer binder grade in the mixture [13]. They designed mixtures with 25%
and 50% RAP along with a control HMA mixture using PG 67-22 virgin asphalt binder.
They also prepared RAP mixtures with 0.25% and 0.5% higher asphalt contents than
the optimum and along with those with a softer virgin binder (PG 58-28) at optimum
asphalt content. They used energy ratio and overlay tester to determine top-down and
reflection cracking, respectively, of all mixtures. A high energy ratio is desirable, for
which the mixture needs to perform well in fracture (not too stiff) as well as a creep
(not too soft). Softer binder grade was not desirable in both mixtures in this aspect and
the only higher RAP mixture needed 0.25% more virgin binder than optimum to have
good energy ratio. However, the 25% RAP mixture with softer binder performed best in
the overlay tester but at the same time failed to meet the rutting criterion in Asphalt
Pavement Analyzer tests.
They also tested the fatigue properties of virgin and blended binders using Linear
Amplitude Sweep (LAS). LAS results show that binder fatigue life improved for both
RAP contents upon using a softer binder grade. Increased virgin binder content also
improved fatigue performance only in 25% RAP mixtures but not as much as lowering
the PG binder grade [13]. Thus, the authors recommend increasing virgin binder
11
content above the optimum level for up to 30% RAP and using a softer binder grade
when RAP content exceeds 30% to improve cracking resistance provided they meet
rutting specifications.
As per guidelines created by National Cooperative Highway Research Program
(NCHRP), low amounts of RAP can be used without characterizing the recovered binder
in RAP and it can be considered solely a part of the aggregate structure. At greater
amounts of RAP, they recommend testing the extracted RAP binder and using either a
softer virgin binder grade or blending charts to determine the PG grade of virgin binder
that should be used in the mixture [5]. The binder selection guidelines from NCHRP are
shown in Table 2-1.
North Carolina Department of Transportation leaves it up to the engineer to decide
the PG grade of virgin binder that needs to be used in mixtures incorporation greater
than 30% RAP. They generally do not require lowering of binder grades for lesser
amounts of RAP except in RS9.5C, RS 12.5C and RI19.0D mix types where they require
usage of PG 64-22 in lieu of PG 70-22 binder with 20-30% RAP mixtures [7].
Table 2-1: Binder Selection Guidelines for RAP Mixtures from NCHRP Report 452
RAP Percentage
Recovered RAP Grade
Recommended Virgin Asphalt Binder Grade
No change in binder selection
Virgin binder one grade softer than normal
Follow recommendations from blending charts
12
PG xx-22
or lower
PG xx-16
PG xx-10
or higher
<20%
<15%
<10%
20-30%
15-25%
10-15%
>30%
>25%
>15%
Use of softer binder grades with high RAP mixtures requires laboratory
evaluation of the mixtures for rutting and fatigue susceptibility. Results from dynamic
modulus tests performed by Hajj et al. indicated that addition of RAP material increased
storage modulus and reduced loss modulus; both these values were reduced when a
softer binder grade was used [11]. Thus, while softer binder grades may make the RAP
mixtures more fatigue resistant, they could compromise their ability to resist
permanent deformation.
RAP aggregate properties also heavily affect mixture performance. With higher
amounts of RAP, there may be more variability in RAP gradation. Crushing and
screening processes before stockpiling can help control for this variability. NCDOT
recommends fractionating RAP into two or more sized stockpiles when more than 30%
RAP is used [7]. Shannon et al. conducted a study in Iowa on how RAP stockpile
fractionation affects volumetric properties of high RAP mixtures [14]. Their analysis of
three unique RAP stockpiles indicated that milling and crushing of RAP degraded the
aggregate significantly and produced excessive amounts of fine particles. They created
two fractionation processes: “Fractionated RAP” involving complete removal of all RAP
passing #30 or #16 sieve and “Optimum FRAP” where either #4 or 3/8” sieve was used
to divide the RAP into a coarse and fine fraction after which the amount of coarse RAP
was increased to control for excessive fine particles. The critical sieve on which a
stockpile was fractionated depended on sieve analyses of the original RAP stockpile.
The authors found that asphalt content varied with size of RAP particles and that the
particles passing #200 contained very little asphalt. Also, fine particles coated nearly all
13
of the larger particles. They designed mixtures with 40% RAP using the original as well
as the fractionated stockpiles and compared their volumetric properties. When the
original stockpile contains heavy amounts of fine particles and dust, both fractionation
methods worked well in improving the volumetric properties. On the other extreme,
with heavily coarse RAP stockpiles, fractionation methods improved the aggregate
structure but adversely affected the asphalt film thickness due to reduction in optimum
asphalt content. With a RAP stockpile situated somewhere in between, i.e. if it contained
more coarse particles and lesser amounts of dust, only complete removal of fine
particles (“Fractionated RAP”) was successful in improving the volumetric properties.
Thus, the authors recommend using some form of fractionation with high RAP mixtures.
Numerous studies have been done to evaluate the performance of RAP mixtures
in laboratory tests. Vahidi et al. found based on results from performance tests
including Multiple Strain Creep Recovery, Texas Overlay Tester, Hamburg Wheel
Tracking Device and Thermal Stress Restrained Specimen Test that incorporation of
RAP and ground tire rubber (both treated with rejuvenating agents and untreated)
improved resistance to rutting, fatigue cracking and moisture susceptibility but made
them more prone to reflective cracking [12]. Li and Gibson conducted dynamic modulus
and cyclic direct tension fatigue tests on mixtures with 20% and 40% RAP [10]. With
increased RAP, dynamic modulus results showed increased stiffness and decreased
phase angle and the mixture becomes more susceptible to fatigue cracking. The authors
theorize that with addition of RAP, lesser micro cracking is needed to cause fatigue
failure.
14
However, results were different when Ajideh et al. compared the fatigue
performance of 50% RAP mixture with a control HMA mixture of same gradation and
PG 64-10 virgin binder [15]. They prepared cylindrical test specimens and tested their
fatigue under haversine loads of three different stress amplitudes under the same
temperature and frequency of loading. For each stress mode, they tested replicates of
two different air void contents. They observed fatigue failure using three approaches:
number of cycles to 50% reduction in initial dynamic modulus, peak energy ratio and
dissipated energy ratio and compared the results with a newly proposed approach
involving laser light scattering using scanning laser detection. For all stress levels, the
50% RAP mixture showed greater fatigue resistance than control. They also found that
the fatigue resistance of the RAP mixture remained same with increase in air voids
while the control HMA was sensitive to change in air void content. This may indicate
that the performance of RAP varies with source, aggregate gradation, binder and virgin
material properties.
Several studies have focused on analyzing the characteristics of extracted RAP
binder and its blends with virgin binders. Huang and Turner blended RTFO-aged
binders with 15 and 50% RAP and conducted rheological tests to under aging effects on
RAP on fresh binders [16]. They aged the blended binders for various durations ranging
from 2 days to 12 weeks and measured their respective complex moduli and phase
angles using dynamic shear rheometer. They focused on oxidative hardening effects
with respect to time and temperature. In general, they found that the PG high
temperature grade of the blended binder increased linearly with increasing amounts of
15
RAP. With higher amounts of aging, this linear relationship exhibited more scatter.
Addition of RAP also reduced the crossover frequency (where storage and loss moduli
are equal) indicating increasing hardness of the binder. The authors noted a strong
linear relationship between log G* and phase angle at all aging times and RAP contents,
independent of material sources. They advocate using this modified Black diagram as
an alternative manner of characterizing the material flow property.
There have also been many field studies with RAP mixture pavement sections. In
general, they have performed as well as virgin HMA sections. Anderson and Daniel
surveyed state agencies in Washington, Wyoming, New Hampshire and Florida to
evaluate the long term performance (in place for at least 10 years) of pavement sections
with varying amounts of RAP and compared their performance with suitable virgin
pavement sections nearby [17]. They collected data on rutting, cracking, ride quality,
maintenance costs as well as other local performance indices. Ride quality and amount
of cracking are generally more for RAP sections; there were no clear trends with
respect to rutting. The severity of distress factors differ from virgin sections by less
than 5%. Thus, the authors conclude that high RAP sections perform at an equivalent
level as their virgin counterparts.
Bennert and Maher presented results from coring and forensic analysis on
pavement sections in New Jersey that were a part of the Study of Rehabilitation of
Asphalt Concrete Pavements (SPS-5) experiment under the Long Term Pavement
Performance (LTPP) program [18]. They observed fatigue and transverse cracking
performance of virgin and 30% RAP sections in the field and tried to explain the
16
differences using binder and mixture performance tests (binder PG, stiffness, LAS,
overlay tester, disk shaped compact tension, creep compliance and indirect tensile
strength). They found that for the same overlay thickness and paving surface condition,
30% RAP sections did not perform as well as the virgin sections even though a softer
binder grade was used in the RAP mixture. They noted that crack initiation was around
the same time in both but crack propagation rates were higher in RAP sections. The
results of mixture performance tests corresponded well with those observed in the
field. However, the binder fatigue results from LAS test indicated better performance in
30% RAP mixtures than virgin, in contrast to what occurred in the field. Based on
results from other studies the authors advocate increasing in effective asphalt content
in RAP mixtures over utilizing a softer binder grade.
Amongst all the studies, the overarching conclusion is that inclusion of RAP leads
to an increase in stiffness of the mixtures. However, how this increased stiffness affects
performance varies widely between the studies, particularly with higher amounts of
RAP. This suggests that local agencies should evaluate RAP mixtures based on locally
available materials before making major construction decisions.
2.2.
Warm Mix Asphalt
Unlike RAP, use of Warm Mix Asphalt (WMA) in the USA is relatively new. Warm
mix asphalt technologies have been used for more than a decade in Europe with good
results. Initial reports of WMA technologies in Europe were presented in 1999-2001
[19]. In 2002, the first European Scan tour was conducted, which introduced WMA
17
technologies to USA. In 2003, NAPA featured WMA at its Annual Convention and the
National Center for Asphalt Technology (NCAT) began research on the performance of
WMA produced using Sasobit® and aspha-Min®. Evotherm® was later added to this
list. In 2004, the first field demonstrations and pilot installations of WMA in USA were
conducted in Charlotte, North Carolina; Nashville, Tennessee; and Orlando, Florida [6].
In 2005, a Warm Mix Asphalt Technical Working Group was formed jointly by NAPA
and FHWA to provide national guidance on the implementation of WMA in USA [20].
NCAT published reports on its evaluation of asphaMin® Zeolite and Sasobit® in 2005
and on Evotherm® in 2006. A second WMA Scan tour to Europe sponsored by United
State Department of Transportation (USDOT), American Association of State Highway
and Transportation Officials (AASHTO) and National Cooperative Highway Research
Program (NCHRP) was conducted in 2007 [21]. Since then, numerous studies and field
trials on WMA have been conducted all over North America.
WMA mixtures are produced at temperatures ranging from 20 – 30°C lower than
HMA to slightly above 100°C. This temperature range distinguishes WMA from “half
warm” and “cold mix” asphalt. Figure 2-1 shows the classification of asphalt concrete
mixtures by production temperature as shown in the WMA scan report [21].
WMA mixtures employ an additive or a process that facilitates their production at
lower temperatures than the conventional HMA that is manufactured normally between
300 to 350°F. Reductions of mixing and compaction temperatures by 50 to 100°F have
been documented [19]. The major advantages of implementing WMA include reduced
emissions, lower energy consumption and increased workability, among others. In fact,
18
development of WMA in Europe was driven by the need for green construction and
improvement in field compaction and worker safety [21].
Figure 2-1: Classification of Asphalt Concrete by Approximate Temperature Ranges from
2007 WMA Scan Summary Report
2.2.1. Production of WMA
A summary of popular technologies used to produce WMA in Europe has been
prepared by the FHWA [20]. The summary also includes a description of various
additives used in WMA production and projects completed/currently being undertaken
in the United States to study the properties of warm mix asphalt concrete. FHWA
described the various warm-mix additives in terms of their physical and chemical
properties, recommended percentage of additive and mechanism by which the
additives modify the asphalt binder. The products listed by FHWA include asphaMin®,
19
WAM-Foam®, Sasobit®, Evotherm®, Advera® WMA and Asphaltan B®. A review of
WMA prepared by the Texas Transportation Institute has identified eight WMA
technologies available in USA [22].
Vaitkus et al have compiled a list of various warm mix asphalt production
technologies used world-wide [23]. They have divided them into four categories based
on the mechanism of production. WMA is generally produced by either foaming the
asphalt or reducing its viscosity. These processes allow the asphalt binder to coat the
aggregates at lower temperatures. A modified list of some WMA technologies compiled
by Vaitkus et al is shown below:
I.
Foaming Asphalt using Water: These technologies are based on either spraying
water in the hot binder or mixing wet sand into the asphalt mix. They include:

WAM Foam® (joint venture of Shell, UK and Kolo-Veidekke, Norway)

Terex® Warm Mix Asphalt System (Terex, USA)

Double Barrel® Green (Astec Industries, USA)

LEA – Low Energy Asphalt (McConnaughay Technologies, USA)

EBT (EiffageTP, USA)

LEAB (Royal BAM Group, Netherlands)

Ultrafoam GX (Gencor Industries, USA)

LT Asphalt (Nynas, Sweden)

The Foamer (Pavement Technology, Inc. USA)
20
II.
Foaming Asphalt using Zeolites: Zeolites are aluminosilicate minerals
containing microscopic pores, in which water can he held. This internally held
water is released upon heating. This property of zeolites has been used to foam
asphalt binders. Commonly used zeolites for this purpose are:

asphaMin® (asphaMin GmbH, Germany) – synthetic zeolite

Advera® WMA Zeolite (PQ Corporation, USA) – synthetic zeolite

Natural zeolite
III.
Organic Additives: This group of WMA technologies is based on organic
compounds that can modify certain properties of asphalt binder to improve its
workability at reduced temperatures. They are added to the hot asphalt binder
and the following ones have been used to produce WMA:

Sasobit® – a Fischer-Tropsch wax (Sasol Wax GmbH, Germany)

Asphaltan B® – a low molecular weight esterified wax (Romonta GmbH,
Germany)

IV.
Licomont BS 100 – a mixture fatty acid derivatives (Clariant, Switzerland)
Chemical Additives: These are inorganic chemicals that are also used to modify
the properties of asphalt. The following ones have been successfully used to
produce WMA.

Iterlow T (Iterchimica SRL, Italy)

Rediset® WMX (AkzoNobel, The Netherlands)

Cecabase RT® (CECA, France)
21

Evotherm® (MeadWestvaco, USA)

Revix arba Evotherm® 3G (MeadWestvaco Mathy-Ergon license, USA)
In the present study, two WMA technologies were used: The Foamer and
Evotherm® 3G. These technologies are described below.
2.2.1.1.
The Foamer
The Foamer is a device manufactured by Pavement Technology, Inc. (PTI) based
in Covington, Georgia, USA [24]. The device feeds hot asphalt cement and water into a
reaction chamber. The cold water acts on the hot binder, producing foam. The foamed
asphalt comes out of the chamber at desired temperature. A photograph of The Foamer
and its diagrammatic representation are shown in Figure 2-2.
The device is capable of accepting standard 1 quart and 1 gallon cans of binder at
room temperature and heating it as per requirement in its reservoir. The reservoir is
lined with a disposable bag made with high temperature polymer to facilitate easy
clean-up. In this study, pre-heated asphalt was poured into the lined reservoir.
Temperature of the reservoir and exit pipe is controlled using the electronic display
control panel mounted on the device, a schematic representation of which is shown in
Figure 2-3.
22
Figure 2-2: “The Foamer” Device and Its Schematic Representation
Figure 2-3: Schematic Representation of Control Panel Displays in “The Foamer”
23
The “setup” menu allows the necessary information including target
temperatures and required weight of binder to be entered and the “control” menu
shows the current status of the device. The “Foam” button pops up when the set
temperature parameters have been achieved and the device is ready to produce foamed
asphalt. The water used for producing foam is stored in a water storage chamber at the
bottom of the device. The manufacturer recommends addition of 2% water content by
weight of asphalt to produce the best foaming action. Due to cooling effect of water on
the binder, it is recommended that the exit temperature be set higher than the reservoir
temperature.
In foamed asphalt, the presence of bubbles makes the binder more workable at
lower temperatures. This allows the binder to evenly coat aggregate particles while
mixing. It has been observed that by foaming the asphalt, the volume of the binder is
increased, further increasing the workability. The larger bubbles dissipate fast and the
effect is temporary, thus, delay in mixing once the foamed asphalt is produced should
be avoided. Figure 2-4 shows a photograph of foamed asphalt produced in this study
using this device. For ease in mixing, the foamed asphalt was made to fall directly on to
the heated aggregate from the exit chamber. The weight of the binder was controlled by
using an external weighing scale underneath the exit chamber of the device.
24
Figure 2-4: Foamed Asphalt Produced by “The Foamer”
2.2.1.1.
Evotherm®
Evotherm® is a chemical additive manufactured by WestRock Company
(formerly MeadWestvaco Corporation). It was invented in 2003 and several versions of
Evotherm® have been released since then. These include, Evotherm® ET (a waterbased asphalt emulsion), Evotherm® DAT (dispersed asphalt technology) and
Evotherm® 3G (third generation) [25].
The latest version, Evotherm® 3G was released in 2008 and is in the form of a
dark amber liquid as shown in Figure 2-5. It is a water-free system. The recommended
dosage rate of Evotherm® 3G is 0.25 to 0.75% by weight of total binder (including the
binder contributed by RAP) [26]. The manufacturer says that it can either be blended
with liquid asphalt or added at the mix plant. There are two types of Evotherm® 3G
products: M1 and J1. These products differ mainly in viscosity and exhibit slight
25
differences in physical properties like density and specific gravity. Evotherm® M1 was
used in this study.
Figure 2-5: Photograph of Evotherm 3G Used in the Study
Several advantages of Evotherm® as claimed by the manufacturer include upto
75°F reduction in mix temperatures, anti-stripping properties, ability to help
incorporate high amounts of RAP, shingles and improvement in foam [27].
Table 2-2 summarizes information about the two WMA technologies that were
used in this study. The manufacture recommendations were taken into consideration
during experimental design.
Table 2-2: Summary of WMA Technologies Used
Technology
Manufacturer
The Foamer
Pavement Technology,
Inc., USA
Evotherm®
3G
WestRock Company
(formerly
MeadWestvaco)
Recommended
Amount of Additive
Mixture
Production
Temperature
2% water
by weight of binder
~ 275°F
0.25% to 0.75%
by weight total
binder
>220°F
26
2.2.2. Economic and Environmental Benefits of WMA
Reduced fuel costs and decrease harmful emissions are the two major potential
advantages of WMA over conventional HMA. A report by Ball as a part of New Zealand
Transport Agency Research quotes that 44% of the total energy needed for constructing
an HMA pavement is consumed by fuel and electricity expended in heating the
aggregate and bitumen (approximately 277MJ/ton of mix) [28]. Any reduction in this
value can lead to tremendous savings.
Savings in fuel costs are quoted from 20 to 35% with some technologies
reporting possible economy of 50% or higher [21]. In a low volume pavement
construction project in Alaska, the mixing plant operator reported a consumption of 0.5
gallons of fuel per ton of WMA produced as opposed to 1.5 gallon of fuel per ton needed
to produce HMA [29]. The higher the cost of fuel used, the greater the savings.
However, savings in terms of fuel costs is offset by initial investment for plant
modification and/or continuous additional costs for additives. Considering equipment
modification or installation costs, royalties and material costs approximate cost per ton
of mix can range from 30 cents for WAM-Foam to $4 for Evotherm® mixtures [30]. The
Alaska study reported that the mix to binder costs for Sasobit® and HMA were 60/750
and 70/720 US Dollars/ton, respectively [29] showing that WMA modified binders can
be more expensive while the mixture production can be cheaper. Ball reports that
currently, the balance of savings and costs of manufacturing WMA using available
technologies results in warm mixes being more expensive than the equivalent standard
hot mixes [28].
27
Field demonstrations of WMA have shown noticeable reduction in dust, odor
and blue smoke [19], [29]. Expected reductions in specific emissions as compiled by
FHWA are 30-40% for carbon dioxide (CO2), 35% for sulfur dioxide (SO2), 50% for
volatile organic compounds (VOC), 10-30% for carbon monoxide (CO), 60-70% for
nitrogen oxides (NOX) and 20-25% reduction in dust [21]. Emission control can be large
part of the budget for a construction project. As emission controls become more and
more stringent, WMA mixtures will become more economical.
All these statistics emphasize the need for further research to lower the
production costs of WMA. Systematic studies on life cycle cost assessment of WMA are
needed to give credence to these values.
With reduced air pollution, working conditions for the plant/paving crew are
improved and the industry can garner support from the ecologically-conscious citizens
of today [22]. With lower amounts harmful emissions, asphalt plants can be
increasingly located in urban and non-attainment areas and lead to easier obtaining of
permits. Paving can be carried out on days previously out-of-bounds due to air quality
restrictions. These benefits are definitely advantageous to the contractor.
Due to lesser difference between ambient and mixture temperatures, hauling
distances can potentially be increased and paving operations can be carried on in cooler
weather, extending the paving season. It expands the market for pavement construction
industry and contractors can reap in additional monetary benefits.
28
2.2.1. Performance of WMA Mixtures
WMA has also shown material performance benefits over HMA. With lower
mixture production temperatures, binder ageing and oxidative hardening of mixture
during production and placement can be reduced. This lengthens the pavement service
life by decreasing susceptibility to cracking with increased pavement flexibility. The
lower production temperatures and shorter heating times can also reduce thermal
segregation problems.
Since WMA technologies focus on making the mixture more workable, they have
very high potential in benefitting stiff mixtures like those with higher percentages of
Recycled Asphalt Pavement (RAP) content. The conjunction of two sustainable
technologies like WMA and RAP can be very beneficial to the contractors and
simultaneously help the green construction industry. The increased workability of
these mixtures can also reduce the compaction effort required in pavement
construction.
Many research studies have been carried out on different WMA mixtures both in
the laboratory as well as in-situ, comparing their performance with conventional HMA
mixtures. Questions still exist, particularly with respect to rutting and moisture
susceptibility. Since WMA is produced at lower temperatures, it is thought that water
(already existing in the aggregate and/or introduced during foaming) may remain
trapped in the aggregate, thus increasing moisture susceptibility of these mixtures.
Also, reduction in oxidative hardening leading to increased pavement flexibility and
decreased susceptibility to cracking has raised additional concerns over increased
29
rutting potential of these mixtures. Clarity in designing a WMA mixture and deviations
from HMA mix design procedure, if any, is also needed.
2.2.1.1.
Mixture Design and Volumetric Properties
National Cooperative Highway Research Program (NCHRP) project 09-43 was
conducted to provide recommendations on mix design practices for WMA [31]. A
preliminary procedure for designing WMA mixtures was subsequently tested and
modified based on results obtained from two phases of testing, including field
validation. In 2011, the project’s findings were published in NCHRP Report 691. The
research project made the key finding that a stand-alone WMA mix design procedure
was not necessary. The project also compiled a set of special considerations while
designing mixtures using WMA, given in Appendix A of NCHRP Report 691 and
published as separate report NCHRP Report 714 [32].
Regarding selection of binder grade for WMA, there are indications that WMA
additives can change the performance grade of the base asphalt that is used. NCAT
study on Sasobit® [33] recommends engineering the modified binder to meet the
Performance Grade requirements. The study used a base binder of PG 58-28, which
upon modification with 2.5% Sasobit® yielded a grade of PG 64-22. NCHRP 09-43 did a
series of tests on binder grade and its results from a comparative study of recovered
binders from the field show only a small difference between WMA and HMA sections.
Thus, they recommend using the same grade of binder for the same environmental and
traffic conditions even for WMA mixtures with 100°F lower production temperatures.
30
However, even in their study, Sasobit® modified binders showed an increase in high
temperature grade with minute loss/no change in low temperature grade [32]. They
also stressed the need for more study in this area.
Findings from the three NCAT studies on asphaMin® [34], Sasobit® [33] and
Evotherm® [35] was summarized in 2006 by Hurley and Prowell [36]. In their study,
the optimum asphalt content used for WMA mixture design was determined using
unmodified binder. Their findings showed that the resulting air voids were lower than
estimated values and surmised that the optimum asphalt content for WMA binders may
be different from that of HMA. They also performed a statistical analysis which showed
that the Superpave Gyratory Compactor (SGC) is insensitive to compaction
temperature. NCHRP 09-43 project’s work on binder content concluded that there was
no statistically significant difference in average design binder content between HMA
and WMA mixtures made with the same aggregates and binder [32]. However, they
found that binder absorption in WMA is 10% lesser than that of HMA mixtures.
2.2.1.2.
Binder Characterization
Numerous studies have evaluated the effect of warm mix additives and
technologies on binder rheological properties and performance characteristics. Biro et
al. studied the effect of asphaMin and Sasobit® on properties of PG 64-22 binders at
intermediate temperatures [37]. Both the additives increased the viscosity of binders at
60°C and lowered compliance values but did not have any significant effect on shear
modulus. aspha-min® modified binders showed no significant change in flow
31
properties, stiffness and response to creep as compared to virgin binders. Sasobit® was
found to affect the flow, and increase the stiffness and penetration resistance and lower
the phase angle of the virgin asphalt binders. Increased stiffness of Sasobit® binders
was also observed by Liu and Li [38]. They investigated the low-temperature
performance of WMA produced using three Sasobit® contents (0.8%, 1.5% and 3% by
weight of binder) with PG 58-28 polymer-modified binder. They found that with
increase in Sasobit® content, creep stiffness of the binder increased while m-value,
tensile strength and strain at failure decreased, indicating increased susceptibility to
low temperature cracking. However, Asphalt Binder Cracking Device (ABCD) tests on
both unaged and PAV aged binders did not indicate any trends between cracking
temperatures and Sasobit® contents.
Different mechanisms employed by various types of WMA technologies cause
varying responses on binder properties. For example, aspha-min® can become a part
of the mineral filler and Sasobit® recrystallizes in the binder at mid-range
temperatures and forms a stiff lattice structure at lower temperatures. With use of
WMA on the rise, the existing binder test methods will need modification to take into
account their unique behaviors.
2.2.1.3.
Laboratory Mixture Characterization
Since WMA relatively new, there have a large number of research studies in this
area across the USA and North America. NCAT has been prominent in performing
studies on Warm Mix Asphalt mixtures. Their studies on WMA have found positive
32
findings in their performance evaluation of WMA mixtures. NCAT reports on the
investigation of and Sasobit® [33] and asphaMin® Zeolite [34] were published in June
2005 and on Evotherm® [35] in June 2006. Results from various studies indicate that
the behavior of warm mix asphalt is heavily dependent on the mechanism employed by
the technology.
WMA additives have been reported to have the same or even better workability
and compactability than conventional HMA mixtures. From Hurley and Prowell’s study
of Sasobit, aspha-min® and Evotherm®, densification data for all WMA mixtures
showed lower air void contents at lower compaction temperatures, indicating greater
compaction of mixes containing additives [36]. Bennert et al. used a prototype
workability device developed by University of Massachusetts, Dartmouth to measure
workability of WMA and HMA mixtures from torque values exerted on a paddle shaft.
[39]. Results from this test showed that with fall in mixture temperature below 230240°F, increased amounts of WMA additives resulted in lower torque values, indicating
better workability of these mixtures.
Moisture susceptibility is a major concern with use of warm mix technologies.
Since WMA is produced at lower temperatures, there are concerns of incomplete
removal of residual moisture from the aggregates. This concern is even more evident
with foaming technologies and additives. Hurley and Prowell found that addition of
anti-stripping agents or hydrated lime improved resistance to moisture susceptibility in
all WMA mixtures [36]. A study published by Xiao et al. in 2009 investigated moisture
damage in asphaMin® and Sasobit® mixtures containing moist aggregate [40]. Indirect
33
Tensile Strength (ITS) test was performed on these mixtures using both unconditioned
(dry) and conditioned (wet) specimens. Under identical conditions, no significant
differences in dry and wet ITS values were observed amongst the WMA mixtures.
Mixtures with moist aggregate exhibited a loss of dry ITS, which could be improved by
the addition of hydrated lime.
There are indications that WMA mixtures gain resistance to moisture damage
and become stiffer with time. Punith et al. studied the effects long-term aging in foamed
WMA and aspha-min® mixtures containing moist aggregate [41]. Half of the samples
prepared were subjected to long-term aging as specified by AASHTO R30. WMA
mixtures exhibited lesser dry ITS values as compared to the control mixture. With longterm aging, the dry ITS values increased. With ageing, the wet ITS of WMA exceeded
that of aged control mixtures. They also observed that WMA mixtures with the highest
moisture content (3% foaming water and ~0.5% aggregate moisture) had slightly
lower wet ITS than other WMA mixtures. Hydrated lime was found to be more effective
in increasing moisture resistance than liquid anti-strip additive.
Mogawer et al. also reported improvement in performance of Advera,
Evotherm®, Sasobit® and SonneWarmix mixtures with increase in aging times and
temperatures and use of anti-stripping additives [42]. They used Hamburg wheeltracking device (HWTD) to determine stripping inflection points; a value less than
10,000 load cycles was taken to indicate moisture susceptibility. The authors
recommended a minimum aging time of 4 hours for WMA mixtures to provide them
with adequate stiffness to resist moisture-induced damage.
34
However, the need for an ageing or curing time before testing WMA mixtures
depends on the type of technology being used. In their review of WMA, Chowdhury et
al. provide discussion about allowing curing of compacted specimens before testing
[22]. For HMA specimens, there are no curing requirements before testing a compacted
specimen. Chowdhury et al. cite a 1994 study conducted by Maccarrone et al. to
underscore the potential need for a cure-time for compacted specimens prepared using
moisture-dependent WMA technologies (foaming technologies) [22]. Meanwhile,
strength gain test results of NCAT studies on additive-based WMA technologies showed
no significant increase in strength over time (test conducted after 2 hours, 4 hours and
one-day increments up to five days) [36]. This is in line with the claim by Chowdhury et
al. that compacted specimens of additive-based WMA mixtures probably do not require
cure-time before testing.
All these results indicate that the presence of residual moisture in aggregates
may not be a big cause for concern. WMA mixtures, especially water-based ones may
exhibit better performance with time. A well-designed WMA mixture along with a
compatible anti-stripping agent can exhibit good resistance to moisture susceptibility.
Rutting susceptibility of WMA mixtures has been another major focus area in
terms of performance characteristics. Since WMA additives typically make mixtures
softer, it is important to assess their potential for permanent deformation. However,
unlike moisture susceptibility, laboratory evaluations have found lower incidences of
WMA mixtures failing to meet rutting criteria. Hurley and Prowell found that Sasobit®
and Evotherm® mixtures exhibited a decrease in rutting potential as compared to
35
conventional HMA mixtures as measured using the Asphalt Pavement Analyzer (APA)
[36]. In Michigan, APA tests on plant-produced HMA and WMA with 1.5% Sasobit®
have shown similar rutting potential for both mixtures [43]. In a study by Goh et al.,
evaluations using ME-PDG based on the dynamic modulus results predicted lower rut
depths in asphaMin® mixtures than HMA [44]. However, ME-PDG analysis uses many
assumptions and this this prediction may not be reliable. Previous research at North
Carolina State University did not find any practical difference between APA rut depths
of WMA and HMA mixtures, even after running the tests on moisture-conditioned
specimens [45].
Stiffness of asphalt concrete mixtures can be measured using different
parameters. Hurley and Prowell found that as compared to the control mixture,
resilient modulus values were not affected in Sasobit® and Zeolite mixtures, while
Evotherm® mixtures exhibited a significant increase in resilient modulus [46]. Goh et
al. did not find a significant difference in dynamic modulus values of aspha-min®
mixtures and HMA, but the stiffness value of aspha-min® increased with increase in
dosage and compaction temperature [43].
Results from IDT of Sasobit® modified WMA mixtures conducted by Liu and Li
indicated that low temperature tensile strengths (0°C, -10°C and -20°C) decreased with
increased Sasobit® content [38]. This is in line with binder tests performed in the same
study. However, thermal cracking analysis of the mixtures indicated that there was only
a slight decrease in cracking temperature with increase in Sasobit® content.
36
Further research on fundamental material properties is needed to correctly
quantify the performance of WMA and predict the behavior of WMA mixtures in the
field.
2.2.1.4.
In-Situ Studies
In association with the respective state DOTs, NCAT has conducted in-situ
studies on WMA in Ohio [47], Michigan [48], Tennessee [49], Missouri [50], Wisconsin
[51], Colorado [52] and Washington [53] between 2006 and 2010. Field sections with
Sasobit® mixtures were constructed in the first six projects, Evotherm® in five
projects, Advera® WMA and asphaMin® in two projects. Astec Double Barrel Green™
system was also used in Tennessee and Maxam Equiment’s AquaBlack™ foaming system
was used in Washington. Generally, all these studies have indicated a positive
performance of WMA mixtures in the field.
Table 2-3 provides a brief summary of the major results obtained from these
studies. In this table, the performance of each WMA mixtures is compared to that of the
control HMA and reported as same, higher or lower than HMA.
As described before, WMA mixtures may be subject to a curing effect, improving
their performance with time. However, these mixtures may exhibit failing values in the
laboratory. This was evident in a field trial of WMA conducted in Alabama using
Evotherm® Dispersed Asphalt Technology (DAT) [54]. RAS and RAP were also
incorporated in these mixtures. These plant-produced mixtures were compared to HMA
mixtures by laboratory evaluation. Characterization of the mixtures using conditioned
37
and unconditioned indirect tensile strengths, APA rut depths, Hamburg wheel tracking
results, dynamic modulus, flow number and creep compliance test results predicted
better performance of HMA as compared to WMA. However, the ITS values of WMA
field cores showed comparable values to that of HMA after one year. This indicated that
WMA mixtures
Saboundjian et al. described paving a low volume road with WMA in Alaska in
the late-season of 2008 [29]. This was Alaska’s first roadway construction to use WMA.
The project utilized PG 58-28 polymer modified binder with 1.5% Sasobit® by weight
of binder. An adjacent roadway project was constructed using HMA. The production
temperatures of WMA and HMA were 265°F and 315°F respectively. The WMA mixture
was shipped 800 miles to the project site on barges in heated containers. Addition of
Sasobit® changed the binder grade to PG 70-22, decreasing its low temperature
reliability. Since the construction was on a low volume road, this was thought to have
no detrimental effects. The WMA mixtures required lesser compactive effort than HMA
in the field. Dynamic modulus and flow number tests conducted on specimens
compacted from these mixtures revealed an increase in |E*| and F N values for Sasobit®
mixtures. Thus, the WMA mixtures were expected to have higher resistance to
permanent deformation. The TSR values of HMA and Sasobit® mixtures were also
found to be similar. No problems in pavement performance were reported during
construction or after 1 year of construction and field evaluations of smoothness and rut
depth turned up similar results for both pavements.
38
Table 2-3: Summary of NCAT In-Situ Studies
State
OH
WMA
Technology
APAbased
Rutting
Aspha-min
Same
In-Situ Study Results
TSRHWTD
based
Stripping Dynamic
Moisture
Inflection Modulus
Damage
Points
Higher
Higher
Lower
Same
Evotherm
Higher
Higher
Mostly
Higher
Sasobit
Same
Higher
Higher
Lower
MI
Sasobit
Same
Same
Same
Same
TN
Advera
Astec DBG
Evotherm
Same
Higher
Lower
Lower
Higher
Lower
Lower
Same
Lower
Lower
Lower
Lower
Sasobit
Higher
Lower
Higher
Lower
Evotherm
Sasobit
Aspha-min
Evotherm
Sasobit
Advera
Same
Lower
Higher
Higher
Lower
--
Same
Same
Lower
Same
Same
Lower
Lower
Same
Same
Same
Same
--
Sasobit
--
Same
--
Same
Same
Lower
Lower
Same
Lesser
Lesser
Evotherm
--
Lower
--
Lesser
Aquablack
Same
Same
Same
Same
MO
WI
CO
WA
General
Comments
As-constructed,
in-place densities
higher for WMA
No evidence of
rutting or moisture
damage in the
field
Bleeding,
Raveling in HMA
and Advera after 1
year. WMA
binders aged more
than HMA.
Minimal rutting
and cracking in all
No difference in
field rutting
Same field
performance for
all, even with very
cold climate
No rutting,
cracking observed
after 13 months.
Similar to HMA
A field study of WMA was conducted in Virginia to evaluate the installation and
initial performance of three trial sections of WMA, two produced using Sasobit® and
39
one using Evotherm® ET [55]. WMA and HMA field cores were extracted at set
intervals, whose air voids were generally not different. Recovered binder tests from
these cores showed lower rate of stiffness gain in Sasobit® cores than HMA with
Evotherm® ET cores mostly exhibiting no difference.
2.3.
Studies on RAP-WMA
It is clear that combining the two sustainable pavement technologies—WMA and
RAP—will lead to further economic as well as environmental benefits. With lower
production temperatures, WMA binders undergo lesser ageing and help in offsetting the
adverse effects of using stiff RAP material in the mixtures.
Numerous laboratory studies have focused on the ability of different types of WMA
technologies in augmenting higher amounts of RAP in asphalt concrete mixtures.
Rashwan and Williams compared the performance Advera, Evotherm® J1 and Sasobit®
mixtures containing 0 and 30% RAP against control HMA mixtures [56]. They found
that dynamic moduli of WMA mixtures was lower than HMA at low and intermediate
temperatures, indicating improved resistance to fatigue and low temperature cracking.
Mogawer et al. also found that use of WMA technology reduced stiffness and overlay
tester cracking high-performance thin overlay mixtures containing 40% RAP, thus
improving fatigue life [57]. Energy ratio values from a laboratory evaluation of plantmixed foamed asphalt mixtures with varying amounts of RAP conducted by Zhao et al.
found that addition of RAP makes WMA mixtures more resistant to fracture but
reduced the fatigue life of HMA mixtures [58].
40
However, Tao and Mallick found contrasting results where 100% RAP mixtures
with WMA additives were stiffer than a 100% RAP with no additives [59]. They
prepared WMA mixtures using Sasobit® and Advera zeolite at a mixing temperature of
125°C. They compared the seismic modulus and indirect tensile strength at 0°C of these
mixtures with a control mixture with no WMA additives. While they found that use of
Advera and Sasobit® improved workability of the mixtures, they found a stiffening
effect of these additives at lower temperatures that caused an increase in both seismic
modulus as well as ITS values. This is interesting because WMA additives are generally
though to soften the mixtures. However, it should be noted that the control mixture in
this case was also produced at the same mixing temperature as those with WMA
additives. If the HMA mixture were heated to the usual high temperatures, the
comparative ranking may be different. Also, the samples were produced in height
control mode, and thus the improved compactability of WMA mixtures could create
denser test specimens that yield higher stiffness.
Rutting and moisture susceptibility is a concern when WMA technologies are
used. In the same study by Rashwan and Williams, WMA mixtures had significantly
lower flow numbers, showing signs that they may be more susceptible to rutting [56].
With inclusion of RAP, the flow numbers of WMA mixtures improved; the effect of
addition of RAP was higher in HMA than WMA mixtures. Thus addition of RAP can make
WMA mixtures less susceptible to rutting and use of WMA technologies can help
mitigate the high stiffness of HMA RAP mixtures. Based on TSR tests and Hamburg
41
Wheel Testing Device results, Zhao et al. found that addition of RAP improved the
ability WMA mixtures to resist moisture damage [58].
Several field studies have also been conducted using combinations of RAP and
WMA. Two paving demonstrations in California with HMA and foamed asphalt sections
containing 15% RAP indicated similar workability and in-place densities for WMA and
HMA despite 50°F difference in production temperatures [60]. Copeland et al. described
Florida Department of Transportation’s first large production using a high RAP (45%)
mixture with foamed asphalt [61]. They took samples during production and
characterized binder and mixture properties for comparison with a control 45% RAP
HMA mixture. All test results—extracted binder PG, dynamic moduli at all
temperatures, flow number—indicated that the WMA RAP mixtures were softer than
their HMA counterparts. There were indications of incomplete blending in the RAPWMA mixture, but the authors felt that complete blending was unnecessary for
acceptable performance of these mixtures. A full-scale structural evaluation of RAPWMA pavement sections against WMA and HMA control sections at National Center for
Asphalt Technology (NCAT) indicated that the RAP-WMA section had the best fatigue
performance at 68°F in comparison to sections that contained only WMA or only HMA
[62]. Thus, the softness of RAP-WMA may not be detrimental to their ability to resist
strain under wheel-path loads.
A major selling point for using combinations of RAP and WMA in asphalt
mixtures are their environmental and economic benefits. Vidal et al. performed a
detailed life cycle analysis on HMA and zeolite mixtures containing 0 and 15% RAP. It
42
considered material, production, transportation, construction, usage and salvage costs
as well as assessment of environmental impacts like climate change, fossil depletion
and damage to human health, ecosystem and resource availability [63]. They found that
any reduction in fuel consumption by WMA mixtures, especially those based on
additives, is offset by production and transportation of materials and additives involved
in preparing them. On the other hand, addition of 15% RAP significantly reduced all
negative impacts on resources and the environment. HMA and WMA mixtures with 15%
RAP had similar impacts. Thus, they conclude that WMA’s key economic and
environmental benefit is its potential to increase usage of RAP.
43
CHAPTER 3.
MATERIAL CHARACTERIZATION
This chapter provides information about the materials used to prepare test
specimens for this study. The source and properties of aggregates, asphalt binder,
additives, and RAP material are described. These materials were chosen in accordance
with a Job-Mix Formula (JMF) for Asphalt Concrete Surface Course Type S 9.5 B as
specified by the North Carolina Department of Transportation (NCDOT).
3.1.
Aggregates
Granite aggregates sourced from Martin Marietta Materials Quarry at Garner, North
Carolina were used in this study. Aggregates from three stockpiles were combined to
achieve the target gradation. These included #78M Coarse Aggregates, Manufactured
Sand and Dry Screenings. In addition to these aggregates, pond fines were incorporated
by replacing the mineral filler at 1.5% of the total aggregate weight.
3.1.1. Gradation
Firstly, washed sieve analysis was conducted on representative samples from
the three stockpiles as per standard procedures described in ASTM C136-06, “Standard
Test Method for Sieve Analysis of Fine and Coarse Aggregates” and ASTM C117-04,
“Standard Test Method for Materials Finer than 75-µm (No. 200) Sieve in Mineral
Aggregates by Washing”. The average gradation from three sieve analysis samples for
each aggregates aggregate stockpile was computed. These values were compared with
the corresponding gradations specified in the JMF as shown in Table 3-1.
44
The sieve analysis results from the laboratory tests do not match the
specifications in the JMF. Thus, the blend gradation from the JMF was used as the design
gradation. Instead of blending aggregates from the stockpiles, all aggregates were ovendried and separated into individual sieve-size fractions. Requisite amount of aggregates
from each fraction were combined as per the target design gradation. This ensured that
there was the least amount of variability in virgin aggregate structure of the specimens.
Table 3-1: Aggregate Gradation
Sieve Size
U.S.
S.I.
1/2”
3/8”
#4
#8
12.5 mm
9.5 mm
4.75 mm
2.36 mm
#16
1.18 mm
#30
#50
#100
#200
600 μm
300 μm
150 μm
75 μm
Percentage Passing
Laboratory Measured Average
Manuf.
Dry
#78M
Sand
Screengs.
Stone
100.0
100.0
100.0
93.0
73.0
49.0
24.0
8.0
3.0
100.0
100.0
97.0
77.0
59.0
44.0
30.0
19.0
12.0
100.0
93.0
36.0
13.0
7.0
5.0
3.0
2.0
2.0
Percentage Passing
Specified in JMF
Manuf.
Dry
#78M
Sand
Screengs.
Stone
100.0
100.0
100.0
82.0
55.0
38.0
23.0
9.0
2.6
100.0
100.0
99.0
87.0
65.0
48.0
32.0
19.0
10.6
100.0
93.0
41.0
7.0
2.0
1.0
1.0
1.0
0.4
Pond fines were oven-dried and sieved such that only the portion passing
through #200 (75 μm) sieve size was used. They were added as a replacement for virgin
aggregate at this sieve-size fraction.
45
3.1.2. Bulk Specific Gravity
In order the determine the bulk specific gravity of the aggregates (G sb), firstly,
the aggregates from each aggregate stockpile were divided into coarse and fine
aggregates using the #4 sieve (4.75 mm). The bulk specific gravities of the coarse and
fine aggregate portions were then determined separately as per AASHTO T 85-88,
“Standard Method of Test for Specific Gravity and Absorption of Coarse Aggregate” and
AASHTO T 84-88, “Standard Method of Test for Specific Gravity and Absorption of Fine
Aggregate”. The values were combined to determine the bulk specific gravity of each
stockpile. For pond fines, specific gravity value provided by the supplier was used.
Next, the blend ratio of the aggregates to achieve the design gradation was
computed by trial and error. This value, along with the individual bulk specific gravity
values were combined to obtain the Gsb value for the aggregate blend. The combined
aggregate bulk specific gravity was determined to be 2.640.
3.2.
Asphalt Binder
Standard North Carolina binder grade PG 64-22 [7] was used in all nine mixture
combinations. At higher RAP contents, it is suggested that a softer binder grade be used
to counteract the effects of hardened RAP binder [5], [7]. Thus, for 40% RAP mixtures,
mixtures with PG 58-28 binder were prepared in addition to those with PG 64-22
binder.
46
Both the asphalt binders used in this study were supplied by NuStar Asphalt
Refining Company located in Wilmington, NC. The specific gravity of the binders (Gb)
was reported as 1.034 by the manufacturer.
3.3.
RAP Material
To control variability in RAP aggregate gradation in the test specimens, RAP
material used in the study was first separated into two size fractions based on the #4
sieve (4.75 mm) [5], [7]. The aggregate gradation and binder content for these two RAP
fractions were calculated individually. RAP fraction retained on the #4 sieve will be
referred to as coarse RAP while the fraction passing it will be referred to as fine RAP.
3.3.1. Ignition Oven Test
Asphalt contents (Pb) of the two RAP fractions were determined using an
ignition oven according to the procedure outlined in AASHTO T 308-05, “Standard
Method of Test for Determining the Asphalt Binder Content of Hot-Mix Asphalt (HMA)
by the Ignition Method.” From the ignition oven test results, the asphalt contents of
coarse and fine RAP were determined to be 3.2% and 6.4%, respectively. Based on a
previous study conducted at NC State University on the same RAP, the high temperature
grade of the extracted RAP binder was estimated to be PG 92 [64].
The gradation of the residual aggregate of each fraction leftover in the ignition
oven was analyzed as per AASHTO T 30-13, “Standard Method of Test for Mechanical
Analysis of Extracted Aggregate.” Using these gradations, an appropriate blend ratio of
coarse and fine RAP fractions to meet the JMF RAP gradation was calculated. A blend of
47
one-third (33%) coarse RAP and two-thirds (67%) fine RAP by weight was found to be
ideal. Gradations of the RAP fractions, target RAP gradation from JMF as well as the
blend gradation are shown in Table 3-2.
Table 3-2: Aggregate Gradation of RAP Fractions
Percentage Passing
Blend
Sieve Size
Coarse
RAP
Fine RAP
Target
JMF RAP
½” / 12.5mm
100
100
100
100.0
3⁄ ” / 9.5mm
8
89
100
96
96.3
#4 / 4.75mm
42
100
81
80.7
#8 / 2.36mm
28
84
65
65.3
#16 / 1.18mm
23
66
51
51.7
#30 / 600µm
18
48
38
38.0
#50 / 300µm
13
33
26
26.3
#100 / 150µm
8.4
21
17
16.8
#200 / 75µm
5.3
13.3
10.3
10.6
(1/3 Coarse RAP +
2/3 Fine RAP)
3.3.1. Bulk Specific Gravity of RAP
Back calculation process mentioned in AASHTO R35-04, “Standard Practice for
Superpave Volumetric Design for Hot-Mix Asphalt (HMA)” was used to determine the
bulk specific gravity (Gsb) of each RAP fraction.
48
Firstly, the theoretical maximum specific gravity (Gmm) of the each fraction was
determined as per AASHTO T 209 “Standard Method of Test for Theoretical Maximum
Specific Gravity and Density of Hot Mix Asphalt.” Using these values and the asphalt
contents from the ignition oven tests, the effective specific gravity (Gse) of each fraction
was computed using the following equation:
𝐺𝑠𝑒 =
100 − 𝑃𝑏
100 𝑃𝑏
𝐺𝑚𝑚 − 𝐺𝑏
Assuming binder absorption (Pba) of 0.1% (as specified by NCDOT), the bulk
specific gravity (Gsb) for each fraction was determined from the Gse values using the
equation below:
𝑃𝑏𝑎 × 𝐺𝑠𝑒
𝐺𝑠𝑏 = 𝐺𝑠𝑒 ÷ [(
) + 1]
100 × 𝐺𝑏
The computed specific gravity values for each RAP fraction are shown in Table 3-3.
Table 3-3: Calculation of Bulk Specific Gravity (Gsb) for RAP Fractions
RAP
Fraction
Max. Spf. Gr.
(Gmm)
%Binder
(Pb)
Eff. Spf. Gr.
(Gse)
Bulk Spf. Gr.
(Gsb)
Coarse
2.540
3.2
2.672
2.665
Fine
2.435
6.4
2.690
2.683
49
3.3.1. RAP Batching
Total weight of RAP includes both the aggregates and binder. So, when a
batching RAP fraction, it was necessary to increase the RAP quantity to accommodate
the added weight of RAP binder [5]. This ensured that the correct amount of aggregates
was weighed out. At the same time, the amount of new binder being added to RAP
mixtures was reduced. The calculations were based on the binder content values (Pb) of
each fraction determined from the ignition oven tests.
Once the weights were determined, quartering was done as per AASHTO T 248,
“Reducing Samples of Aggregate to Testing Size” to obtain representative testing
samples of each RAP fraction.
3.3.1. RAP Heating Procedure
A two-step RAP heating procedure as recommended by the Texas Transportation
Institute was followed [65]. Sample RAP fractions for each test specimen were weighed
and heated at 60°C for 12 hours in individual trays. Then, they were heated to the target
mixing temperature for two hours. After the two hours of heating at mixing
temperature, the two fractions were mixed with the virgin aggregate and virgin binder
to prepare the test specimens.
50
3.4.
Asphalt Additives
3.4.1. Liquid Anti-Strip
Liquid anti-strip additive AD-here® LOF 65-00, manufactured by ArrMaz
Custom Chemicals, Inc. was used in this study. It was incorporated at 0.75% by weight
of binder. It is a brown, viscous liquid derived from amidoamines and increases the
adhesion between asphalt and aggregate surfaces [66].
3.4.2. Warm Mix Additive
Two WMA technologies were considered in this study: PTI Foamer and
Evotherm® 3G. Evotherm® 3G is chemical additive manufactured by MeadWestvaco
Corporation. A dosage of 0.5% by weight of binder of the additive was mixed with the
binder.
WMA mixtures prepared using the PTI Foamer did not require any additives, but
2% water by weight of binder was used for foaming the binder.
A summary of the additives and their dosage is shown in Table 3-4.
Table 3-4: Asphalt Additives Summary
Additive
Amount
Liquid Anti-strip 0.75% by weight of binder
Mixtures Modified
All
Evotherm® 3G
0.50% by weight of binder
WMA using Evotherm® 3G
Water
2% by weight of binder
WMA using The PTI Foamer
51
3.4.1. Additive Weight Calculation
Required weight of additives was calculated based on the total amount of binder in
the mixture, which includes virgin binder added as well as the old binder contributed
from RAP. Thus, in RAP mixtures, the weight of the additives to be added was calculated
after including binder contribution from RAP. This entire amount was dissolved into the
virgin binder before mixing the virgin aggregates, binder and RAP material together.
52
CHAPTER 4.
SUPERPAVE MIX DESIGN
This chapter describes the design of mixture combinations to determine the
optimum asphalt content. It was ensured that the volumetric properties were within
specifications. %Gmm computed from compaction heights of mix design specimens was
used to compare workability of the mixtures.
4.1.
Aggregate Gradation
All the HMA and WMA mixtures were designed as per the JMF for an Asphalt
Concrete Surface Course, Type S 9.5 B. These aggregates have a nominal maximum
aggregate size of 9.5 mm. As mentioned in chapter 3, the aggregate blend gradation in
the JMF was the target design aggregate gradation. Individual size fractions met the
permissible control points established by NCDOT for 9.5 mm mixes. Table 4-1 and
Figure 4-1 show the design aggregate gradation as well the control points as specified
by NCDOT.
Table 4-1: Design Aggregate Gradation as Obtained from JMF (9.5B Mix)
Sieve Size
% Passing Control Points
½ “ 12.5 mm
100
100
3/8” 9.5 mm
97
90 - 100
#4 4.75 mm
76
90
#8 2.36 mm
55
32 - 67
#16 1.18 mm
40
#30 600 μm
29
#50 300 μm
20
#100 150 μm
11
#200 75 μm
5.8
4.0 – 8.0
53
AGGREGATE GRADATION
SIEVE SIZES RAISED TO .45 POWER
100
90
80
% Passing
70
60
50
40
30
20
#30 600µm
#50 300µm
0
#200 75µm
#100 150µm
10
#16
1.18mm
#8
2.36mm
#4
4.75mm
3/8"
1/2"
9.5mm 12.5mm
Sieve Sizes
Figure 4-1: Design Gradation: Percentage Passing vs. 0.45 Power of Sieve Size
4.2.
Mixing and Compaction Temperatures
Mixing and compaction temperature ranges provided by the binder supplier were
taken into consideration for determining the production temperatures for the control
HMA mixture. These values are based on equi-viscous temperatures, i.e. temperatures
corresponding to a viscosity of 0.15 - 0.19 Pa.s for mixing and 0.25 – 0.31 Pa.s for
compaction [7]. Mixing and compaction temperatures for HMA mixture were fixed at
163°C (325°F) and 149°C (300°F), respectively. The same temperatures were used for
HMA-RAP mixtures as well, including the 40% RAP mixtures prepared with PG 58-28
binder.
54
For WMA mixtures, production temperatures cannot be established based on binder
viscosity [67]. Based on manufacturer recommendations and previous research studies,
mixing and compaction temperatures of 135°C (275°F) and 120°C (248°F), respectively
were used for WMA mixtures, which is a reduction of 30°C (50°F) from HMA production
temperatures [45], [46], [68]. The same mixing and compaction temperatures were
used for both Evotherm® 3G and Foamer mixtures. These temperatures were not
changed with addition of RAP, including the ones with 40% RAP and PG 58-28 binder.
The mixing and compaction temperatures for each mixture type have been
tabulated below in Table 4-2.
Table 4-2: Mixing and Compaction Temperatures
RAP Content Binder Grade
0%
20%
40%
40%
4.3.
PG 64-22
PG 64-22
PG 64-22
PG 58-28
Production Temperature: Mixing/Compaction (°C)
HMA
Evotherm
Foamer
163/149
135/120
135/120
163/149
135/120
135/120
163/149
135/120
135/120
163/149
135/120
135/120
Optimum Asphalt Content
6.0% binder content by weight of total mix (as specified in the JMF) was used as a
starting point for determining the optimum asphalt content for all mixture
combinations. With each amount of RAP, the optimum asphalt content for the HMA
mixture was first determined. Using this binder content, the volumetric properties for
the corresponding Evotherm® and Foamer mixtures were verified [69].
55
Volumetric verification for each RAP amount involved the following steps:
i.
Compact two HMA specimens 4500 g total aggregate and design binder content
to 65 gyrations using the Superpave Gyratory Compactor (initial gyrations, N ini =
7 and design gyrations, Ndes = 65 are specified 9.5B mix types [7]).
ii.
Prepare two loose mixtures with 2000 g of aggregates and same binder content.
iii.
Use the compacted specimens to obtain the average bulk specific gravity (Gmb) as
per AASHTO TP 69-04, “Standard Method of Test for Bulk Specific Gravity and
Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing
Method”.
iv.
Test the loose mixtures to obtain average theoretical maximum specific gravity
(Gmm) as per AASTO T 209-05, “Standard Method of Test for Theoretical
Maximum Specific Gravity and Density of Hot-Mix Asphalt Paving Mixtures.”
v.
Calculate the volumetric properties of the mixture including %VTM (Voids in
Total Mixture/Air Voids), %VMA (Voids in Mineral Aggregate), %VFA (Voids
Filled with Asphalt), %Gmm at Nini and %Gmm at Ndes using the above two
measurements and determine Gsb of aggregate blend using aggregate and RAP
bulk specific gravities from chapter 3.
vi.
Verify the volumetric properties against NCDOT Superpave mix design criteria
for S9.5B mix type [7].
56
vii.
If the volumetric properties for HMA are within specification, repeat the process
for Evotherm® and Foamer mixtures.
viii.
Adjust design binder contents if WMA mixtures did not meet volumetric
specification. For a given RAP amount, the binder content is to be kept constant
to ensure comparability between the mixtures.
4.3.1. Volumetric Data of Virgin (0% RAP) Mixtures
Table 4-3 compiles the volumetric data for the virgin HMA, Evotherm® and
Foamer mixtures with 6% design binder content. All mixtures used PG 64-22 binder
grade. Volumetric properties for all three mixtures were within the specified limits and
thus, the optimum asphalt content for 0% RAP mixtures was fixed at 6%.
Table 4-3: Volumetric Properties for 0% RAP Mixtures with 6% PG 64-22 Binder
Mix Type
Volumetric
Properties
HMA
Evotherm
Foamer
Gsb
2.640
2.640
2.640
Gmb @ Ndes
2.330
2.325
2.316
Gmm
2.425
2.420
2.410
% VTM
3.9
3.9
3.9
4.0 ± 0.5
% VMA
17.0
17.2
17.5
> 15.0%
% VFA
64.8
65.1
65.8
65-78%
% Gmm at Nini (7)
89.5
89.3
89.5
≤ 89.0%
% Gmm at Ndes (65)
96.1
96.1
96.1
96%
57
Volumetric
Requirements
4.3.1. Volumetric Data of 20% RAP Mixtures
Using the same target aggregate gradation and PG 64-22 binder, Gmb and Gmm for
mixtures with 20% RAP were determined. % Air voids (VTM) of these mixtures are
shown Table 4-4. While HMA mixtures had air voids within the specified limit, air voids
WMA mixtures were low. Hence, new set of mixtures were with reduced asphalt
content of 5.8% were prepared and measured for air voids. % Air voids from these
specimens are also shown in Table 4-4. As can be seen, the air voids were higher than
specified limit for HMA and Evotherm® mixtures.
Thus, a third set of specimens with 5.9% binder content were prepared and
tested. For this binder content, all volumetric properties of HMA, Evotherm® and
Foamer mixtures with 20% RAP met specification limits. These values are shown in
Table 4-5. Thus, the optimum asphalt content of 20% RAP mixtures was fixed at 5.9%.
Table 4-4: % Air voids in 20% RAP Mixtures with PG 64-22 Binder
Mixture Type % Binder % VTM Specification
HMA
Evotherm
3.8
6
3.4
Foamer
3.4
HMA
4.8
Evotherm
5.8
4.8
Foamer
4.4
58
4.0 ± 0.5
Table 4-5: Volumetric Properties for 20% RAP Mixtures with 5.9% PG 64-22 Binder
Mix Type
Volumetric
Properties
Volumetric
Requirements
HMA
Evotherm
Foamer
Gsb
2.647
2.647
2.647
Gmb @ Ndes
2.320
2.317
2.316
Gmm
2.422
2.414
2.415
% VTM
4.2
4.0
4.1
4.0 ± 0.5
% VMA
17.5
17.6
17.7
> 15.0%
% VFA
76.0
77.3
76.8
65-78%
% Gmm at Nini (7)
88.9
89.2
89.1
≤ 89.0%
% Gmm at Ndes (65)
95.7
95.9
96.0
96%
4.3.2. Volumetric Data of 40% RAP Mixtures
Similar the previous mix designs, the virgin aggregates blended with 40% RAP
were mixed with PG 64-22 binder at 6.0% binder content. The same mixtures were also
prepared using a softer binder grade, PG 58-28 due to their high RAP content [5], [7].
The air voids determined from these mixtures are shown in Table 4-6.
Table 4-6: Air voids in 40% RAP Mixtures with 6% Binder Content
Mixture Type Binder Grade % VTM Specification
HMA
Evotherm
3.7
PG 64-22
3.4
Foamer
3.3
HMA
4.0
Evotherm
PG 58-28
Foamer
3.4
3.4
59
4.0 ± 0.5
Similar to 20% RAP mixtures, for 6% binder content, the WMA mixtures
exhibited low air void contents while the HMA mixtures were within specification. A
second set of mix design specimens of all these six mixtures were prepared with 5.8%
design binder content. This time, the volumetric properties for all mixtures were within
the specified limits for both PG 64-22 and PG 58-28 binders. The volumetric data are
summarized in Table 4-7 and Table 4-8. The optimum asphalt content of 40% RAP
mixtures was fixed at 5.8%.
Table 4-7: Volumetric Properties for 40% RAP Mixtures with 5.8% PG 64-22 Binder
Mix Type
Volumetric
Properties
Volumetric
Requirements
HMA
Evotherm
Foamer
Gsb
2.655
2.655
2.655
Gmb @ Ndes
2.316
2.330
2.334
Gmm
2.425
2.437
2.430
% VTM
4.4
4.4
3.9
4.0 ± 0.5
% VMA
17.5
17.3
17.2
> 15.0%
% VFA
66.9
66.5
66.2
65-78%
% Gmm at Nini (7)
88.9
89.2
89.4
≤ 89.0%
% Gmm at Ndes (65)
95.6
95.6
96.1
96%
In 40% RAP mixtures, HMA mixture exhibits the greatest difference in
volumetric properties at optimum asphalt content with change in binder grade. Air
voids reduced by 0.6% for HMA while the difference in air voids for both WMA mixtures
60
were within 0.2%. This indicated better compactability in HMA when softer binder
grade was used.
Table 4-8: Volumetric Properties for 40% RAP Mixtures with 5.8% PG 58-28 Binder
Mix Type
Volumetric
Properties
HMA
Evotherm
Foamer
Gsb
2.655
2.655
2.655
Gmb @ Ndes
2.325
2.330
2.325
Gmm
2.415
2.436
2.424
% VTM
3.8
4.3
4.1
4.0 ± 0.5
% VMA
17.6
17.3
17.5
> 15.0%
% VFA
66.9
66.5
66.9
65-78%
% Gmm at Nini (7)
89.9
89.2
89.2
≤ 89.0%
% Gmm at Ndes (65)
96.2
95.7
95.9
96%
61
Volumetric
Requirements
CHAPTER 5.
EVALUATING WORKABILITY USING %Gmm
In this chapter, the compaction heights of mix design specimens at optimum asphalt
content were used to evaluate the workability of the mixtures. The Superpave Gyratory
Compactor records compaction heights against the number of gyrations; using this data,
%Gmm corresponding to gyration levels were computed. This compactability was used
to compare and rank mixture workability.
5.1.
Procedure
During mix design, each specimen was compacted to the design number of
gyrations, Ndes = 65. The height of the specimen was recorded at every gyration level.
Based on these heights and weight of the specimen, bulk specific gravity (Gmb) was
estimated for every 5 gyrations. Since the mixtures were not coarse, correction factors
generally used to account for surface irregularities while calculating Gmb was ignored.
From these values, the %Gmm achieved at that gyration level was calculated as shown
below. For each mixture, average %Gmm values based on two mix design specimens
were used.
𝐺𝑚𝑏 =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 (𝑔)
𝜋𝑟 2 ℎ
𝑤ℎ𝑒𝑟𝑒, 𝑟 = 15𝑐𝑚 𝑎𝑛𝑑 ℎ = 𝑐𝑜𝑚𝑝𝑎𝑐𝑡𝑖𝑜𝑛 ℎ𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑐𝑚
%𝐺𝑚𝑚 =
𝐺𝑚𝑏
𝐺𝑚𝑚
𝑤ℎ𝑒𝑟𝑒 𝐺𝑚𝑚 𝑖𝑠 𝑅𝑖𝑐𝑒 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑔𝑟𝑎𝑣𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑚𝑖𝑥𝑡𝑢𝑟𝑒
62
Workability of mixtures was compared based on N92, the number of gyrations to
reach 92% Gmm. Smaller N92 indicated better workability. ±0.5% error interval was
used to check for significant differences in N92 values. Mix design specimens prepared
at optimum asphalt content were used for workability evaluation.
5.2.
Effect of RAP in Each Mixture Type
5.2.1. HMA Mixtures
Figure 5-1 shows average %Gmm attained at different gyration levels for HMA
mix design specimens prepared at optimum asphalt content. Virgin, 20% RAP as well as
40% RAP mixtures are shown. The horizontal red band indicates 92% Gmm line to
obtain N92 with ±0.5% error interval. Table 5-1 shows these N92 values.
With no change in binder grade, N92 values for virgin HMA and 20% RAP HMA
were not significantly different (~18); however, HMA mixtures with 40% RAP had a
significantly higher N92 value (23). By using the softer PG 58-28 binder in lieu of PG 6422, the 40% RAP mixture N92 value could be reduced to 16, similar to the virgin and
20% RAP HMA mixtures. Thus, for high RAP HMA, lowering the binder grade is
necessary to ensure the same workability as virgin mixtures.
Table 5-1: N92 Values for HMA Mixtures
Mixture
H0R
H20R
H40R
H40R
Softer
Binder
N92 Value
17
18
23
16
63
100.0
98.0
96.0
94.0
% Gmm
92.0
H40R PG 58-22
H40R PG 64-22
90.0
H20R PG 64-22
88.0
H0R PG 64-22
86.0
84.0
82.0
N initial = 7
N design = 65
80.0
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-1: %Gmm Evolution Curves for HMA Mixtures
5.2.2. Evotherm® Mixtures
Figure 5-2 and Table 5-2 show the %Gmm evolution and N92 values,
respectively, for Evotherm® mixtures. Adding 20% RAP to Evotherm® mixtures did
not significantly increase the N92 value (~16). N92 values of 40% RAP Evotherm®
mixtures (~20) were significantly different from that of virgin Evotherm®; however,
the difference in these values was not as high as seen in HMA. Even with use of softer
binder, there was no significant difference between the N92 values of 40% RAP
Evotherm® mixtures. Thus, using a softer binder in high RAP Evotherm® mixtures
does not improve workability.
64
Table 5-2: N92 Values for Evotherm Mixtures
Mixture
E0R
E20R
E40R
E40R
Softer
Binder
N92 Value
15
17
19
20
100.00
98.00
96.00
94.00
92.00
% Gmm
E40R PG 58-22
90.00
E40R PG 64-22
E20R PG 64-22
88.00
E0R PG 64-22
86.00
84.00
82.00
N initial = 7
N design = 65
80.00
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-2: %Gmm Evolution Curves for Evotherm Mixtures
5.2.3. Foamer Mixtures
For Foamer mixtures, %Gmm evolution and the resulting N92 values are shown in
Figure 5-3 and Table 5-3, respectively. There were no significant differences in N92
values between any Foamer mixtures. Thus, addition of RAP did not significantly affect
65
the workability of Foamer mixtures. Similar to Evotherm mixtures, lowering the binder
grade for high RAP mixtures did not lead to improved workability.
Table 5-3: N92 Values for Foamer Mixtures
Mixture
F0R
F20R
F40R
F40R
Softer
Binder
N92 Value
16
16
16
18
100.0
98.0
96.0
94.0
% Gmm
92.0
F40R PG 58-22
F40R PG 64-22
90.0
F20R PG 64-22
88.0
F0R PG 64-22
86.0
84.0
82.0
N initial = 7
N design = 65
80.0
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-3: %Gmm Evolution Curves for Foamer Mixtures
In general, it can be inferred that intermediate amounts of RAP (20%) do not affect
workability of HMA or WMA mixtures. Incorporating high amounts of RAP (40%) in
66
HMA reduces workability, which can be improved using a softer binder grade. Softer
binder grades are not required in high RAP WMA mixtures.
5.3.
Effect of Mixture Technology in Each RAP Content
5.3.1. 0% RAP Mixtures
%Gmm evolution curves of virgin mixtures can be seen in Figure 5-4. PG 64-22
binder was used in all mixtures. As seen in section 5.2, the N92 values for virgin HMA,
Evotherm® and Foamer mixtures are 18, 15 and 16, respectively, and are not
significantly different. Thus, despite the 30°C (50°F) reduction in production
temperatures, WMA mixtures have similar workability as HMA.
100.00
98.00
96.00
94.00
% Gmm
92.00
F0R PG 64-22
90.00
H0R PG 64-22
88.00
E0R PG 64-22
86.00
84.00
82.00
N initial = 7
N design = 65
80.00
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-4: %Gmm Evolution Curves for Virgin (0% RAP) Mixtures
67
5.3.2. 20% RAP Mixtures
%Gmm evolution of HMA, Evotherm® and Foamer mixtures with 20% RAP is
shown in Figure 5-5. Softer binder grades were not used to prepare 20% RAP
mixtures; all mixtures were prepared with PG 64-22 binder. The evolution curves are
not significantly different for all three mixtures with N92 values at 18, 17 and 16,
respectively for HMA, Evotherm® and Foamer 20% RAP mixtures (from section 5.2). It
can be seen again that the workability of WMA mixtures is similar to HMA mixtures.
100.00
98.00
96.00
94.00
% Gmm
92.00
F20R PG 64-22
90.00
H20R PG 64-22
88.00
E20R PG 64-22
86.00
84.00
82.00
N initial = 7
N design = 65
80.00
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-5: %Gmm Evolution Curves for 20% RAP Mixtures
68
5.3.3. 40% RAP Mixtures
%Gmm evolution curves for 40% RAP mixtures are shown below in Figure 5-6.
40% RAP HMA, Evotherm® and Foamer mixtures were prepared with the softer PG 5828 binder in addition to the standard PG 64-22, yielding six different types of 40% RAP
mixtures. The N92 values of these six mixtures are show in Table 5-4.
Table 5-4: N92 Values for 40% RAP Mixtures
Mixture Technology 
Binder Type ↓
PG 64-22
PG 58-28
HMA
Evotherm
Foamer
23
16
19
20
16
18
100.00
98.00
96.00
94.00
F40R PG 64-22
92.00
% Gmm
H40R PG 64-22
90.00
E40R PG 64-22
H40R PG 58-28
88.00
E40R PG 58-28
86.00
F40R PG 58-28
84.00
82.00
N initial = 7
N design = 65
80.00
0
10
20
30
40
No. of Gyrations
50
60
70
Figure 5-6: %Gmm Evolution Curves for 40% RAP Mixtures
69
The N92 values for all mixtures are represented in Figure 5-7. In HMA with 40%
RAP, the N92 value significantly reduces with use of the softer binder grade. However in
both WMA mixtures, using a softer binder did not affect the N92 values significantly.
Evotherm® and Foamer mixtures have similar N92 values. To have comparable
workability as WMA, 40% RAP HMA mixtures require the use of a softer binder grade.
25
N92
20
15
10
5
0
Mixture Type
Figure 5-7: Number of Gyrations to Achieve 92% Gmm (N92 Values)
5.4.
Conclusions
Since there was no significant difference in workability of 40% RAP WMA mixtures
with lowering of binder grade, these mixtures were henceforth prepared using the
standard PG 64-22 binder for all material and performance tests. Specimens for HMA
with 40% RAP were prepared using PG 58-28 binder for all future tests. Thus, test
specimens for eight mixture combinations (all virgin, all 20% RAP and 40% RAP WMA
mixtures) were prepared using PG 64-22 binder while those for only one mixture (40%
RAP HMA) were prepared using the softer PG 58-28 binder.
70
CHAPTER 6.
TENSILE STRENGTH RATIO
This chapter focusses on characterizing mixtures for moisture-susceptibility based
on Tensile Strength Ratio (TSR). The test was performed according to the guidelines
specified by NCDOT [7], which is a modification of AASHTO T 283, “Standard Method of
Test for Resistance of Compacted Hot Mix Asphalt (HMA) to Moisture-Induced
Damage.”
6.1.
Specimen Preparation
The test requires two sets of specimens for every mixture: a control,
“unconditioned” set and a vacuum-saturated, “conditioned” set. For each mixture
combination, 10 specimens were prepared. All specimens included liquid anti-stripping
additive, LOF 6500 at 0.75% by weight of binder. As mentioned in section 5.4, all
mixtures were prepared using PG 64-22 binder, except HMA with 40% RAP, which
contained PG 58-28 binder.
In accordance with the specifications, aggregates were first mixed with binder at the
designated mixing temperature (163°C for HMA and 135°C for WMA). Then, the loose
mixtures were cooled at room temperature for two hours, after which they were cured at
60°C for 16 hours. After curing, the mixtures were heated for 2 hours to their respective
compaction temperatures (149°C for HMA and 120°C for WMA) and then compacted to
a height of 95 ± 5 mm and 7 ± 0.5% air voids using the Superpave gyratory compactor.
The air void content of each specimen was determined as per AASHTO T 331, “Standard
Method of Test for Bulk Specific Gravity and Density of Compacted Hot Mix Asphalt (HMA)
71
Using Automatic Vacuum Sealing Method”. From the 10 specimens prepared for each
mixture, two specimens whose air voids had the most deviation from the targeted value
of 7.0% were eliminated. The remaining 8 specimens were randomly divided into two
sets of 4 specimens each. One set was kept as is and tested at room temperature i.e.
25°C (77°F), while the other set was subject to a conditioning process. They were first
vacuum-saturated with water to a saturation level of 70–80% and then immersed in a
water bath at 60°C for 24 hours. Lastly, they were cooled for two hours in a water bath
at 25°C (77°F) before testing.
6.2.
Test Procedure
The specimens were loaded diametrically at a rate of 2 inches (50 mm) per minute
until failure. The load at failure was noted and the indirect tensile strength of the
specimen was calculated using this value using the following equation:
𝐼𝑇𝑆 =
𝑤ℎ𝑒𝑟𝑒,
2𝑃
𝜋𝑑ℎ
𝐼𝑇𝑆 = 𝐼𝑛𝑑𝑖𝑟𝑒𝑐𝑡 𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ,
𝑃 = 𝑃𝑒𝑎𝑘 𝐿𝑜𝑎𝑑,
𝑑 = 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑎𝑛𝑑 ℎ = 𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 ℎ𝑒𝑖𝑔ℎ𝑡
The median value of the indirect tensile strengths of each set of specimens
(conditioned and unconditioned) was taken as the representative indirect tensile
strength value of that set. For each mixture, the tensile strength ratio was then
calculated for each mixture by taking a ratio of indirect tensile strength (ITS) value of
72
conditioned specimens to that of unconditioned specimens, as shown below. NCDOT
requires all its mixtures to pass a minimum TSR value of 85%.
𝑇𝑆𝑅 =
6.3.
𝐼𝑇𝑆𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑
𝐼𝑇𝑆𝑢𝑛𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛𝑒𝑑
Test Results
Nomenclature as shown in Table 1-1 was used to label the mixtures (H–HMA, E–
Evotherm®, F–Foamer, #R–amount of RAP).
6.3.1. Virgin Mixtures
Table 6-1 shows the TSR values for virgin mixtures. The TSR values for the
HMA, Evotherm® and Foamer are 101.4%, 93.8% and 94.4%, respectively. All of these
values are above the minimum limit for TSR value of 85% as specified by the NCDOT.
Hence, as all the virgin mixtures meet the minimum TSR criterion, none of them are
expected to exhibit significant moisture damage in the field.
6.3.2. 20% RAP Mixtures
TSR results for mixtures with 20% RAP are shown in Table 6-2. The TSR values
of HMA, Evotherm® and Foamer are 87.7%, 89.9% and 87.4%, respectively. As with
virgin mixtures, all of them met the NCDOT passing TSR value of 85%. TSR values for all
20% RAP mixtures are very close to each other. Compared with virgin mixtures, the
TSR values reduced for all three.
73
Table 6-1: Tensile Strength Values for Virgin (0% RAP) Mixtures
(a) HMA
Moisture
Conditioning
Dry
Wet
Specimen#
Air Void
Content
ITS
(psi)
ITS
(kPa)
H0R 2
H0R 3
H0R 6
H0R 7
H0R 4
H0R 5
H0R 8
H0R 9
7.1
6.9
6.8
6.7
7.1
6.9
6.8
6.7
150.39
154.75
154.75
152.57
161.28
156.92
154.75
152.57
1037
1067
1067
1052
1112
1082
1067
1052
Median
Subset ITS
(kPa)
TSR
(%)
1059
101.4
1074
(b) Evotherm
Moisture
Conditioning
Dry
Wet
Specimen#
Air Void
Content
ITS
(psi)
ITS
(kPa)
E0R 3
E0R 4
E0R 5
E0R 7
E0R 1
E0R 2
E0R 9
E0R 10
6.7
6.6
6.6
6.6
6.6
6.6
6.5
6.7
122.05
124.23
126.41
122.05
115.51
119.87
111.16
115.51
842
857
872
842
796
826
766
796
Median
Subset ITS
(kPa)
TSR
(%)
849
93.8
796
(c) Foamer
Moisture
Conditioning
Dry
Wet
Specimen#
Air Void
Content
ITS
(psi)
ITS
(kPa)
F0R 3
F0R 4
F0R 8
F0R 9
F0R 5
F0R 6
F0R 7
F0R 10
6.9
6.9
6.9
7.0
6.9
6.8
6.9
6.9
159.10
154.75
159.10
154.75
148.21
141.67
148.21
148.21
1097
1067
1097
1067
1022
977
1022
1022
74
Median
Subset ITS
(kPa)
TSR
(%)
1082
94.4
1022
Table 6-2: Tensile Strength Values for 20% RAP Mixtures
(a) HMA
Moisture
Specimen
Conditioning
#
Dry
Wet
H20R 4
H20R 5
H20R 7
H20R 9
H20R 1
H20R 2
H20R 8
H20R 10
Air Void
Content
ITS
(psi)
ITS
(kPa)
6.6
6.6
6.7
6.6
6.8
6.7
6.7
6.7
217.95
209.23
215.77
211.41
180.9
187.44
217.95
187.44
1503
1443
1488
1458
1247
1292
1503
1292
Median
Subset
ITS (kPa)
TSR (%)
1473
87.7
1292
(b) Evotherm
Moisture
Specimen
Conditioning
#
Dry
Wet
E20R 3
E20R 4
E20R 6
E20R 10
E20R 1
E20R 2
E20R 5
E20R 9
Air Void
Content
ITS
(psi)
ITS
(kPa)
6.8
6.7
6.6
6.7
6.8
6.7
6.6
6.7
215.77
217.95
224.49
200.51
196.16
207.05
193.98
185.26
1488
1503
1548
1383
1352
1428
1337
1277
Median
Subset
ITS (kPa)
TSR (%)
1495
89.9
1345
(c) Foamer
Moisture
Specimen
Conditioning
#
Dry
Wet
F20R 6
F20R 8
F20R 9
F20R 10
F20R 1
F20R 2
F20R 3
F20R 5
Air Void
Content
ITS
(psi)
ITS
(kPa)
6.7
6.8
6.7
6.8
6.7
6.8
6.7
6.7
198.34
200.51
200.51
193.98
170.00
178.72
174.36
174.36
1368
1382
1382
1337
1172
1232
1202
1202
75
Median
Subset
ITS (kPa)
TSR (%)
1375
87.4
1202
Table 6-3: Tensile Strength Values for 40% RAP HMA Mixture
(a) HMA
Moisture
Conditioning
Dry
Wet
Specimen
#
Air Void
Content
ITS
(psi)
ITS
(kPa)
H40R 1
H40R 4
H40R 7
H40R 10
H40R 2
H40R 5
H40R 6
H40R 9
7.1
6.7
6.7
6.9
6.9
6.9
6.7
6.7
193.98
211.41
211.41
220.13
172.18
191.8
196.16
189.62
1337
1458
1458
1518
1187
1322
1352
1307
Median
Subset
ITS (kPa)
TSR
(%)
1458
90.2
1315
(b) Evotherm
Moisture
Conditioning
Dry
Wet
Specimen
#
Air Void
Content
ITS
(psi)
ITS
(kPa)
E40R 3
E40R 4
E40R 7
E40R 8
E40R 1
E40R 2
E40R 9
E40R 10
7.0
6.8
6.7
6.7
7.1
7.0
6.9
7.0
235.39
226.67
228.85
233.21
185.26
202.69
200.51
193.98
1623
1563
1578
1608
1277
1397
1382
1337
Median
Subset
ITS (kPa)
TSR
(%)
1593
85.4
1360
(c) Foamer
Moisture
Conditioning
Dry
Wet
Specimen
#
Air Void
Content
ITS
(psi)
ITS
(kPa)
F40R 2
F40R 4
F40R 7
F40R 9
F40R 1
F40R 3
F40R 6
F40R 8
6.6
6.6
6.6
6.5
6.7
6.5
6.5
6.5
294.23
287.7
296.41
285.52
198.34
222.31
215.77
220.13
2029
1984
2044
1969
1368
1533
1488
1518
76
Median
Subset
ITS (kPa)
TSR
(%)
2006
74.9
1503
6.3.3. 40% RAP Mixtures
Table 6-3 shows the TSR test results for mixtures with 40% RAP. The TSR
values HMA, Evotherm® and Foamer mixtures with 40% RAP are 90.2%, 85.4% and
74.9%, respectively. As was mentioned before, the HMA mixtures were prepared using
a softer binder (PG 58-28) while there was no change in binder grade for Evotherm®
and Foamer mixtures (PG 64-22).
The 40% RAP HMA mixtures exhibited the highest TSR value amongst all 40% RAP
mixtures and comfortably met the minimum required 85% value. Evotherm® mixtures
just met the NCDOT criterion while with Foamer mixtures, with the least value out of all
nine mixture combinations, failed to meet the NCDOT specification.
6.4.
Analysis of Test Results
The ITS and TSR values of all mixtures are summarized in
Table 6-4, Figure 6-1 and Figure 6-2. Within each group of same RAP content, the
HMA mixtures had the highest TSR values. For WMA mixtures, addition of RAP resulted
in decreasing TSR values. In HMA mixtures, the RAP mixtures had lower TSR values
than the virgin mixtures although the TSR did not decrease when RAP content was
doubled.
In general, TSR values of Evotherm® and Foamer mixtures were similar. However,
40% RAP Foamer mixtures exhibited low TSR values and the only failing value. This is
due to the high ITS values of their unconditioned set.
77
Table 6-4: Summary of TSR test results of all the mixtures
Mixture Type
Median Indirect Tensile Strength
(kPa)
Conditioned
Unconditioned
TSR
(%)
Pass/Fail
(Min 85%)
HMA 0% RAP
1074
1059
101.4
PASS
EVO 0% RAP
796
849
93.8
PASS
FOAM 0% RAP
1022
1082
94.4
PASS
HMA 20% RAP
1292
1473
87.8
PASS
EVO 20% RAP
1345
1495
89.9
PASS
FOAM 20% RAP
1202
1375
87.4
PASS
HMA 40% RAP
(PG 58-28)
1315
1458
90.2
PASS
EVO 40% RAP
1360
1593
85.4
PASS
FOAM 40% RAP
1503
2006
74.9
FAIL
TSR only gives an indication of how much the moisture conditioning affects the
ITS value of a mixture. Thus, it is of interest to examine the ITS values themselves. The
indirect tensile strength results were analyzed as a multi-factor experiment with 3x3x2
factorial treatment structure. The three factors used in the design were: type of mixture
technology with 3 levels (“HMA”, Evotherm®—“EVO” and Foamer—“FOAM”), RAP
content with 3 levels (“0%”, “20%” and “40%”) and moisture conditioning treatment
with 2 levels (“Unconditioned” and “Conditioned”).
78
Indirect Tensile Strength (kPa)
2500
2000
1500
1000
500
0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
F40R
Mixture Type
Conditioned
Unconditioned
Figure 6-1: Median and Range of Indirect Tensile Strength Values
Tensile Strength Ratio (%)
120
100
85%
80
60
40
20
0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
Mixture Type
Figure 6-2: Tensile Strength Ratio Values of All Mixtures
79
F40R
The dependent variable, indirect tensile strength values “ITS” were modeled as a
function of Type, RAP and Treatment using multi-factor ANOVA (analysis of variance)
as shown in Table 6-5. First and second order interactions between the three factors
were also observed.
Table 6-5: Multi-factor ANOVA for Indirect Tensile Strength
Dependent Variable: ITS
Class Level Information
Class
Levels
Values
Treatment
2
Conditioned Unconditioned
Type
3
EVO FOAM HMA
RAP
3
0% 20% 40%
Number of Observations Read and Used 72
Source
Model
Error
Corrected Total
DF Sum of Squares Mean Square F Value Pr > F
17
5642132.70
331890.16 125.21 <.0001
54
143130.75
2650.57
71
5785263.45
R-Square Coeff Var Root MSE ITS Mean
0.98
3.99
51.48
1291
Source
Treatment
Type
Treatment*Type
RAP
Treatment*RAP
Type*RAP
Treatment*Type*RAP
DF Sum of Squares Mean Square F Value Pr > F
1
477015.16
477015.16 179.97 <.0001
2
180349.94
90174.97
34.02 <.0001
2
86390.16
43195.08
16.30 <.0001
2
3803687.33 1901843.67 717.52 <.0001
2
226343.01
113171.50
42.70 <.0001
4
787090.85
196772.71
74.24 <.0001
4
81256.24
20314.06
7.66 <.0001
80
Comparing the p-values (Pr>F) at a significance level of 5%, the results indicate that
all factors as well as their first and second order interactions significantly affect the ITS
values. This implies that, as is expected, the indirect tensile strength value is dependent
on the type of mixture technology, moisture-conditioning as well as the amount of RAP.
The first order interactions are also significant, indicating:
i.
The effect of mixture type on ITS depends on whether the specimens were
unconditioned or conditioned and vice versa (Treatment*Type).
ii.
The effect of RAP content on ITS depends on whether the specimens were
unconditioned or conditioned and vice versa (Treatment*RAP).
iii.
The effect of mixture type on ITS depends on the amount of RAP and vice versa
(Type*RAP).
iv.
The effect of each factor depends on the levels of the other two factors
(Treatment*Type*RAP).
From the test results, it can be noted that the median ITS value for Foamer mixtures
with 40% RAP is much higher than that for all other mixture combinations. This
significantly higher ITS value may be the reason that the three-way interaction between
the factors (second order interaction) is statistically significant.
Pairwise difference in mean ITS values were analyzed using the Bonferroni t tests.
This test calculates the minimum significant difference, i.e. the amount by which two
ITS values have to differ to be declared statistically significant. The mixture
81
combinations are grouped by letters; groups that share a letter do not have statistically
significant differences. The results of this pairwise comparison are shown in Table 6-6.
Table 6-6: Bonferroni (Dunn) t Tests for ITS
Alpha
0.05
Error Degrees of Freedom
54
Error Mean Square
2650.57
Critical Value of t
3.84
Minimum Significant Difference 139.74
Means with the same letter are not significantly different
Bon Grouping
Mean
N
Mixture
A
2006.1
4
F40R Unconditioned
B
1592.9
4
E40R Unconditioned
C
B
1480.2
4
E20R Unconditioned
C
B
1476.4
4
F40R Conditioned
C
B
D
1472.7
4
H20R Unconditioned
D
1442.6
4
H40R Unconditioned
C
C
E
D
1367.5
4
F20R Unconditioned
C
E
D
1348.7
4
E20R Conditioned
C
E
D
1348.7
4
E40R Conditioned
F
E
D
1333.7
4
H20R Conditioned
F
E
1292.4
4
H40R Conditioned
F
G
1202.2
4
F20R Conditioned
H
G
1082.0
4
F0R Unconditioned
H
G
1078.2
4
H0R Conditioned
H
1055.7
4
H0R Unconditioned
H
1010.6
4
F0R Conditioned
I
852.8
4
E0R Unconditioned
I
796.4
4
E0R Conditioned
82
In order to compare the mean ITS values between certain mixture combinations,
all the values were analyzed using one-factor ANOVA. The results are shown in Table
6-7.
Table 6-7: One-Way ANOVA Statistics for Indirect Tensile Strength
Dependent Variable: ITS
Class Level Information
Class Levels
Values
E0R Cond, E0R Uncond, E20R Cond, E20R Uncond, E40R Cond, E40R
Uncond, F0R Cond, F0R Uncond, F20R Cond, F20R Uncond, F40R
Mixture
18
Cond, F40R Uncond, H0R Cond, H0R Uncond, H20R Cond, H20R
Uncond, H40R Cond, H40R Uncond
Number of Observations Read and Used 72
Source
Model
Error
Corrected Total
R-Square
0.975259
DF
17
54
71
Sum of Squares Mean Square F Value Pr > F
5642132.700
331890.159 125.21 <.0001
143130.749
2650.569
5785263.449
Coeff Var
3.987623
Root MSE
51.48368
ITS Mean
1291.087
Source DF Sum of Square Mean Square F Value Pr > F
Mixture 17 5642132.700 331890.159 125.21 <.0001
Using the one-factor analysis, statistical significance of difference in ITS values
between difference combinations of mixtures can be isolated. It is of interest to see how
moisture conditioning affects ITS values for different RAP contents, how addition of
RAP affects ITS and how each mixture technology type affects ITS values. The results of
these comparisons are summarized in Table 6-8.
83
Table 6-8: One-Factor ANOVA Comparisons
Parameter
Estimate
Unconditioned vs Conditioned: Evotherm
Mixtures
Unconditioned vs Conditioned: Foamer
Mixtures
Unconditioned vs Conditioned: HMA
Mixtures
Unconditioned vs Conditioned: Virgin
Mixtures
Unconditioned vs Conditioned: 20% RAP
Mixtures
Unconditioned vs Conditioned: 40% RAP
Mixtures
Virgin vs RAP Evotherm Mixtures
Conditioned
Virgin vs RAP Evotherm Mixtures
Unconditioned
Virgin vs RAP Foamer Mixtures
Conditioned
Virgin vs RAP Foamer Mixtures
Unconditioned
Virgin vs RAP HMA Mixtures Conditioned
Virgin vs RAP HMA Mixtures
Unconditioned
HMA vs WMA Virgin Mixtures
Conditioned
HMA vs WMA Virgin Mixtures
Unconditioned
HMA vs WMA 20%RAP Mixtures
Conditioned
HMA vs WMA 20%RAP Mixtures
Unconditioned
HMA vs WMA 40%RAP Mixtures
Conditioned
HMA vs WMA 40%RAP Mixtures
Unconditioned
84
Standard
Error
t Value
Pr > |t|
-144.01
21.02
-6.85
<.0001
-255.46
21.02
-12.15
<.0001
-88.91
21.02
-4.23
<.0001
-35.07
21.02
-1.67
0.101
-145.25
21.02
-6.91
<.0001
-308.06
21.02
-14.66
<.0001
552.26
31.53
17.52
<.0001
683.75
31.53
21.69
<.0001
328.70
31.53
10.43
<.0001
604.84
31.53
19.18
<.0001
234.81
31.53
7.45
<.0001
401.94
31.53
12.75
<.0001
-174.69
31.53
-5.54
<.0001
-88.32
31.53
-2.8
0.007
-58.23
31.53
-1.85
0.0702
-48.83
31.53
-1.55
0.1273
120.20
31.53
3.81
0.0004
356.91
31.53
11.32
<.0001
Main factor and interaction plots between mixture type, RAP content and
moisture conditioning levels on mean ITS values are shown below in Figure 6-3 and
Figure 6-4. Since the interactions between the factors (mixture type, RAP content and
moisture conditioning) are statistically significant, it is important to note that the main
effects plot in Figure 6-3 averages over dissimilar trends and thus, it is important to
consider the first-order interactions shown in Figure 6-4. The second-order
interactions were also statistically significant, but these interactions have not been
shown in detail since they are caused due to a high degree of variation in ITS values of
each mixture combination.
Figure 6-3: Effects of Mixture Type, %RAP and Conditioning Treatment on Mean ITS
85
Figure 6-4: Interaction Effects between Mixture Type, %RAP and Conditioning Treatment
6.5.
Inferences
6.5.1. Virgin Mixtures (0% RAP)
In virgin mixtures, the moisture conditioning process does not have a significant
effect on the ITS values; for all three mixture types (HMA, Evotherm and Foamer), the
difference between mean ITS values of the two moisture treatment subsets is not
statistically significant. So, in general, the HMA as well as the WMA mixtures with no
RAP are expected to show good resistance to moisture damage.
86
While comparing the ITS values of virgin mixtures based on mixture type, there
was no significant difference between results of HMA and Foamer mixtures.
Evotherm® mixtures had softer unconditioned and conditioned ITS than HMA and
Foamer. Overall, the virgin mixtures had the lowest indirect tensile strength values with
Evotherm® virgin mixtures exhibiting the least values amongst all mixture
combinations.
6.5.2. 20% RAP Mixtures
With addition of 20% RAP, HMA and Evotherm® mixtures still did not exhibit
significant difference between unconditioned and conditioned ITS. However, moisture
conditioning treatment led to a significance difference in ITS values for Foamer
mixtures with 20% RAP. Overall, all the ITS values of 20% RAP mixtures, both
unconditioned as well as conditioned, were higher than those of the virgin mixtures.
This is as expected due to the addition of stiff RAP material in these mixtures.
There was no significant difference in unconditioned ITS values of the 20% RAP
HMA and WMA mixtures. Evotherm® and HMA had similar conditioned ITS values.
Foamer mixtures had the lowest conditioned ITS value amongst the 20% RAP mixtures.
6.5.3. 40% RAP Mixtures
In high RAP mixtures (40% RAP), there were significant differences in
unconditioned and conditioned ITS for all types of mixtures—HMA, Evotherm® as well
as Foamer. Even though the moisture conditioning process seems to significantly lower
the ITS values in high RAP mixtures, all ITS values are still much larger than the virgin
87
mixtures, even in the conditioned state. Thus, while the TSR values are lower in 40%
RAP mixtures, moisture susceptibility may not increase with addition of RAP.
From the Bonferroni tests, it is clear that the 40% RAP Foamer mixture had a
significantly higher unconditioned ITS value than all the other mixture combinations.
The unconditioned Evotherm® had the second highest ITS value. The ITS value of
unconditioned HMA was significantly lower. This is probably due to the use of softer PG
grade in this mixture (PG 58-28) while the WMA mixtures still used the standard binder
(PG 64-22). Thus, while workability may not be affected by change in binder grade in
WMA, the indirect tensile strength values may be sensitive to binder grade changes.
It should be noted that Evotherm additive has anti-stripping properties. The
research project included testing of this aspect of Evotherm; these results are shown in
Appendix A.
88
CHAPTER 7.
E* STIFFNESS RATIO
This chapter details the performance test based on dynamic modulus ratio of
specimens that were specifically prepared to evaluate moisture damage. These tests
were conducted on specimens with 7±0.5% air voids, whose dynamic moduli were
measured in unconditioned and moisture-conditioned states.
7.1.
Background on Dynamic Modulus
Dynamic modulus is a fundamental material property used in various performance
prediction models, such as the Mechanistic-Empirical Pavement Design Guide, to
predict pavement distresses. Here, it was used to directly compare different mixtures
using the E* stiffness ratio (ESR) parameter. Dynamic modulus testing was performed
using the Asphalt Mixture Performance Tester (AMPT) device.
The AMPT device is a computer-controlled hydraulic machine that can be used to
apply stress-controlled cyclic loading on cylindrical asphalt concrete specimens over a
range of test temperatures and loading frequencies. Dynamic modulus (E*) is the ratio
of peak stress applied (𝜎0 ) to peak strain in the specimen (𝜀0 ) as shown in the equation
below:
|𝐸 ∗ | =
89
𝜎0
𝜖0
Since asphalt concrete has viscoelastic properties, there is a time lag (Δt) between the
stress and strain cycles. For each frequency of loading (𝑓), this is used to calculate the
phase angle, 𝜑 as shown below. Figure 7-1 shows a sinusoidal loading cycle applied
using the AMPT device.
𝜑 = 2𝜋𝑓Δ𝑡
Figure 7-1: Schematic Diagram of Stress and Strain in Asphalt Concrete
Gauge length 70 mm
4 LVDTs
at 90° angles
Top View
Side View
Figure 7-2: Arrangement of LVDTs on Dynamic Modulus Test Specimen
90
Peak stress for each test temperature and loading frequencies is fixed in order to
achieve strain levels less than 90 microstrains. The axial strain on the specimen is
measured by placing linear variable displacement transducers (LVDTs) along the
vertical length of the specimen. Figure 7-2 shows a schematic representation of LVDTs
mounted on a dynamic modulus test specimen. The strain amplitude is ideally reported
as the average of the four LVDTs. If any LVDT malfunctions, values from the opposite
LVDT are also discarded and the average of the remaining two LVDTs is reported.
7.2.
ESR Test Description
Moisture susceptibility of virgin and warm mix asphalt mixtures based on Tensile
Strength Ratio (TSR) has been described in CHAPTER 6. Previous studies indicated that
WMA, especially those produced using moisture-inducing technology such foamed
asphalt, perform poorly when subjected to the TSR test [45]. Some researchers have
tried to use E* stiffness ratio (ESR) to evaluate moisture susceptibility [11], [70].
The ESR test is conducted on conditioned and unconditioned subsets of specimens,
which are subjected to a conditioning procedure similar to the TSR test. ESR is defined
as the ratio of average dynamic modulus of conditioned specimens to the average
dynamic modulus of unconditioned specimens.
ESR 
Average | E* | of wet specimens at any test temperature and frequency
Average | E* | of dry specimens at any test temperature and frequency
Since dynamic modulus using the AMPT is measured at three temperatures and six
frequencies for each specimen, ESR values have been reported as averages for each test
91
temperature. However, for the purposes of comparing with TSR results, the |E*| results
at 20°C and 1 Hz loading frequency were used.
7.3.
Specimen Preparation and Conditioning
Specimens for ESR test were prepared according to the procedure described in
AASHTO TP 79-09, "Standard Method of Test for Determining the Dynamic Modulus
and Flow Number for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance
Tester (AMPT)". The specimens were initially compacted to a height of 178 mm with
diameter of 150 mm using the Superpave gyratory compactor, and were cut and cored
to dimensions of 150 + 2.5 mm height and 100 + 1 mm diameter for testing. The target
air void content for ESR test was selected as 7 + 0.5 % for the finished (cut and cored)
specimens to ensure adequate saturation for testing in the moisture-conditioned state.
For preparing the moisture-conditioned subset of specimens, specimens were
vacuum-saturated with water to 35- 45%. The saturated specimens were placed in a
water bath at 60oC for 24 hours. After removal, the specimens were surface-dried and
left to air-dry at room temperature for a period of 24 hours. This was to ensure that the
surface of the specimens was completely dry to allow proper adhesion of brass targets
for mounting LVDTs.
Since the dynamic modulus test is a non-destructive test unlike the AASHTO T-283
test, the same specimens were used for testing in both dry and wet conditions. First, the
dynamic modulus testing of unconditioned specimens for all mixtures were measured.
92
Tests on conditioned specimens were conducted exactly one week later for to allow
recovery of residual viscoelastic strains in specimens from the unconditioned test.
7.4.
ESR Test Results
Table 7-1 shows the results of ESR test for all mixtures. The dynamic modulus
values shown in the table are averages of two specimens tested for each mix type. The
table has three subgroups based on the mixture technology used. The first part shows
all the HMA mixtures with increasing RAP contents, followed by Evotherm® mixtures
and finally the Foamer mixtures. The dynamic moduli obtained from moistureconditioned specimens are highlighted. For each of the three test temperatures, an
average ESR value using the dynamic moduli values of the six frequencies of loading
was computed.
ESR values are summarized in Figure 7-3. The average ESR value across all test
temperatures and loading frequencies was greater than 85% for seven out of the total
nine mixtures. For Foamer mixtures with 0% RAP, the average was only slightly below
85%. The HMA mixture with 40% RAP with 64% ESR value behaved substantially
differently from the other mixtures.
93
Table 7-1: E* Stiffness Ratio Test Results
7-1 (a) ESR for HMA Mixtures
Mix
Type
Temp
(oC)
Specimen
State
Frequency (Hz)
4
HMA
0%
RAP
20
40
4
HMA
20%
RAP
20
40
4
HMA
40%
RAP
20
40
Dynamic Modulus (MPa)
25
10
5
Dry
15,199
13,736
12,584
Wet
15,313
13,674
Dry
7,084
Wet
1
0.5
0.1
9,994
8,943
6,610
12,575
9,965
8,867
6,532
5,844
4,995
3,235
2,710
1,666
6,672
5,407
4,547
2,842
2,290
1,330
Dry
1,728
1,310
1,058
647
534
384
Wet
1,377
991
771
445
370
263
Dry
14,469
13,298
12,343
10,150
9,253
7,209
Wet
14,839
13,589
12,568
10,346
9,443
7,532
Dry
7,595
6,435
5,604
3,893
3,299
2,145
Wet
7,188
5,975
5,138
3,480
2,914
1,850
Dry
2,014
1,535
1,244
755
637
478
Wet
1,558
1,194
987
663
619
523
Dry
18,614
17,074
15,946
13,142
12,239
9,516
Wet
13,533
12,266
11,302
9,136
8,215
6,286
Dry
9,977
8,936
7,829
5,541
4,943
3,376
Wet
6,564
5,454
4,699
3,164
2,653
1,664
Dry
2,782
2,088
1,691
998
833
514
Wet
1,812
1,361
1,083
633
517
345
Average
ESR
(%)
99.7
88.3
72.4
102.5
90.5
88.2
69.7
57.8
64.5
7-1 (b) ESR for Evotherm Mixtures
4
EVO
0% RAP
20
40
Dry
13,613
12,105
10,937
8,346
7,274
5,062
Wet
13,138
11,685
10,550
8,016
7,020
4,851
Dry
5,773
4,544
3,724
2,183
1,730
962
Wet
5,341
4,175
3,405
1,956
1,540
835
Dry
1,198
862
672
410
362
283
Wet
1,040
732
560
327
276
201
94
96.3
90.2
80.4
Table 7-1 Continued
4
EVO
20%
RAP
20
40
4
EVO
40%
RAP
20
40
Dry
13,336
12,014
10,991
8,666
7,725
5,680
Wet
13,136
11,828
10,824
8,530
7,579
5,515
Dry
6,234
5,039
4,269
2,691
2,178
1,282
Wet
5,753
4,652
3,878
2,399
1,921
1,097
Dry
1,512
1,106
860
495
410
292
Wet
1,276
917
714
411
347
249
Dry
16,022
14,796
13,826
11,542
10,601
8,352
Wet
15,086
13,806
12,762
10,435
9,423
7,249
Dry
7,865
6,624
5,731
3,871
3,221
2,008
Wet
7,379
6,153
5,299
3,504
2,900
1,711
Dry
2,071
1,538
1,209
685
553
367
Wet
1,742
1,262
976
541
433
287
98.2
89.7
83.9
91.0
90.8
80.4
9-1 (c) ESR for Foamer Mixtures
4
FOAM
0% RAP
20
40
4
FOAM
20%
RAP
20
40
4
FOAM
40%
RAP
20
40
Dry
13,895
12,432
11,236
8,593
7,554
5,274
Wet
13,190
11,704
10,557
7,991
7,007
4,883
Dry
5,785
4,584
3,771
2,227
1,773
980
Wet
2,561
3,927
3,171
1,799
1,407
771
Dry
1,131
813
633
381
321
242
Wet
973
675
517
301
253
190
Dry
13,958
12,642
11,597
9,192
8,192
6,141
Wet
13,708
12,327
11,281
8,862
7,891
5,826
Dry
6,492
5,330
4,532
2,907
2,401
1,461
Wet
5,836
4,684
3,905
2,410
1,943
1,116
Dry
1,417
1,063
860
521
438
327
Wet
1,266
903
700
403
341
263
Dry
15,734
14,524
13,559
11,242
10,295
8,057
Wet
13,714
12,541
11,645
9,518
8,565
6,567
Dry
7,681
6,418
5,549
3,742
3,084
1,877
Wet
6,816
5,632
4,817
3,129
2,572
1,500
Dry
1,887
1,362
1,047
584
468
301
Wet
1,691
1,225
950
528
427
286
95
93.6
75.5
81.2
96.8
84.0
81.9
84.8
85.0
91.2
120.0
100.0
85%
ESR (%)
80.0
60.0
4C
20 C
40 C
40.0
20.0
0.0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
E40R
F40R
Mixture Type
Figure 7-3: Average ESR Values at Each Test Temperature
Figure 7-4 shows the average dynamic moduli values of unconditioned and
conditioned specimens at a test temperature of 20°C and 1 Hz loading frequency. The
error bars indicate the range of values. In all mixture types, the virgin mixtures had the
lowest |E*| values; dynamic moduli values increased with addition of RAP.
The 40% RAP HMA mixture had the highest unconditioned dynamic modulus
value as well as a high variability in the values. This high unconditioned dynamic
modulus value has led to a low ESR for this mixture. Since this mixture contains a high
amount of RAP and is being produced at high temperatures, the aged RAP material may
have been exposed to higher amount of oxidation. This could explain the high
unconditioned |E*|.
96
7000
Unconditioned
Conditioned
6000
|E*| (MPa)
5000
4000
3000
2000
1000
0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
F40R
Mixture Type
Figure 7-4: Average and Range of Dynamic Modulus at 20°C and 1 Hz for ESR
7000
Uncond Storage Modulus
Uncond Loss Modulus
6000
Cond Storage Modulus
Cond Loss Modulus
MPa
5000
4000
3000
2000
1000
0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
F40R
Mixture Type
Figure 7-5: Average and Range of Storage and Loss Moduli for All Mixtures
97
The high unconditioned stiffness of HMA mixture with 40% RAP is in contrast to
the behavior exhibited during TSR tests, where the WMA mixtures had higher
unconditioned ITS than HMA. The TSR test procedure includes a 16 hour curing period
before specimen compaction. This curing period may have caused stiffening of the TSR
mixtures. Since the HMA 40% RAP mixture was prepared with softer binder, the
unconditioned ITS value was lower than WMA. However, in preparing ESR specimens,
there is no curing period for the mixtures before compaction. This could explain why
the WMA mixtures do not have as high unconditioned dynamic moduli as HMA.
Thus, it can be inferred that even with a softer binder grade, the HMA mixtures
with higher amounts of RAP may have an undesirably high stiffness value that could
exacerbate fatigue damage. Upon moisture conditioning, these mixtures experience a
large loss of stiffness. WMA mixtures work very well in this scenario where the need for
binder grade bump is eliminated while simultaneously preserving the mixture’s
moisture damage resistance.
While comparing the complex moduli of different mixtures is useful in ranking
them based on stiffness, these values only show their total viscoelastic behavior. To
isolate the effects of elasticity and viscous behavior, the time lag (phase angle) between
stress and strain should be considered. Storage modulus (|𝐸 ∗ |𝑐𝑜𝑠𝜑) measures the
stored energy in the specimen that results from its elastic behavior. Loss modulus
(|𝐸 ∗ |𝑠𝑖𝑛𝜑) represents the dissipated energy, representing the viscous behavior of the
material.
98
The average storage and loss modulus values for both unconditioned and
conditioned specimens are shown in Figure 7-5. The error bars indicate the range of
values. Average ratios of storage and loss moduli (SMR and LMR) in the moistureconditioned to those in the unconditioned state for all mixtures are shown in Figure
7-6.
Percentage
SMR
LMR
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
F40R
Mixture Type
Figure 7-6: Ratios of Storage Modulus (SMR) and Loss Modulus (LMR)
For virgin mixtures, SMR and LMR values remain similar to the ESR. In 20% RAP
mixtures, the storage modulus ratio behavior similar to ESR but there is a large increase
in loss modulus ratios in HMA and Evotherm. Increase in loss modulus ratio indicates
that the mixtures will be more resistance to cracking distresses.
99
Table 7-2 shows a comparison of TSR and average ESR values for the nine
mixtures. For the purposes of comparing with TSR, the ESR values in this table were
obtained from ratios of dynamic moduli at 20°C and 1 Hz loading frequency.
Table 7-2: Comparison of TSR and ESR Test Results
Mix Technology RAP Content Binder Grade TSR (%)
HMA
Evotherm
Foamer
ESR (%)
[20°C, 1 Hz]
0%
PG 64-22
101.4
87.9
20%
PG 64-22
87.7
89.4
40%
PG 58-28
90.2
57.1
0%
PG 64-22
93.8
89.6
20%
PG 64-22
89.9
89.2
40%
PG 64-22
85.4
90.5
0%
PG 64-22
94.4
80.8
20%
PG 64-22
87.4
82.9
40%
PG 64-22
74.9
83.6
One additional concern to note is that the TSR test measures properties in tension
(indirect tension), whereas the E* value is measures in compression mode of loading.
Thus, the E* ratio (ESR) may not be appropriate in evaluating the moisture sensitivity of
mixtures, because compression testing measures more of aggregate structure
properties as opposed to the adhesive properties in tensile or flexural mode of loading.
Further investigation is needed with E* values evaluated using tensile or flexural tests.
100
CHAPTER 8.
DYNAMIC MODULUS
In this chapter, results of dynamic modulus tests conducted as per AASHTO TP 7909, "Standard Method of Test for Determining the Dynamic Modulus and Flow Number
for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT)" are
described.
8.1.
Specimen Preparation and Test Description
Dynamic modulus (|E*|) is an important fundamental material property that is used
in performance prediction models to predict pavement distresses over a specified
design period. In this study, dynamic modulus testing was performed using the AMPT
device according to specimen preparation procedure is similar to that used for
preparing ESR test specimens, except that the target air voids for the specimens was 4 +
0.5%.
Dynamic modulus test was conducted on HMA and WMA mixtures at three
temperatures: 4, 20 and 40°C and six frequencies: 25, 10, 5, 1, 0.5 and 0.1 Hz.
8.2.
Results and Master Curves
Table 8-1 shows the average dynamic modulus of three specimens for each mix
type at the three test temperatures and six loading frequencies. This table lists the
mixtures in increasing order of RAP content (0%, 20% and then 40%) and is colorcoded for each WMA technology (grey for HMA, white for Evotherm® and orange for
Foamer, respectively). Note that H, E and F in these tables refer to HMA, Evotherm®
101
and Foamed mixtures, respectively. The letter R and the number preceding it (0, 20 or
40) refers to the amount of RAP present in the mixture.
The data obtained from the test were used to develop E* master curves at a
reference temperature of 21°C (70°F) using a non-linear optimization procedure
according to AASHTO PP 61-09, "Standard Practice for Developing Dynamic Modulus
Master Curves for Hot Mix Asphalt (HMA) Using the Asphalt Mixture Performance
Tester (AMPT)".
Developing a master curve is based on the thermo-rheologically simple property of
asphalt concrete—the behavior of asphalt concrete in low temperature is equivalent to
its behavior under high frequency of loading and vice versa. Figure 8-1 shows
sigmoidal fit of E* master curves for all mixtures at a reference temperature of 70°F.
2.50E+07
2.00E+07
H0R
E0R
Dynamic Modulus (kPa)
F0R
1.50E+07
H20R
E20R
1.00E+07
F20R
H40R PG 58-28
5.00E+06
E40R
F40R
0.00E+00
1.00E-07
1.00E-05
1.00E-03
1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07
log Reduced Frequency
Figure 8-1: E* Master Curves for All Mixtures (reference temperature 70°F)
102
Table 8-1: Dynamic Moduli of All Mixtures—Specimens at 4% Target Air Voids
Mix Temp
Type (oC)
Frequency
(Hz)
H0R
E0R
F0R
H20R
E20R
F20R
H40R
E40R
F40R
Dynamic Modulus (MPa)
25
10
5
1
0.5
0.1
4
20
40
4
20
40
4
20
40
20,962
10,783
2,942
16,984
8,243
1,805
17,846
8,412
1,866
19,398
9,172
2,205
15,530
6,757
1,274
16,349
6,926
1,317
18,165
8,013
1,740
14,329
5,746
968
15,099
5,887
1,017
15,231
5,587
975
11,504
3,618
517
12,279
3,704
563
13,879
4,755
783
10,319
2,906
424
10,994
3,004
478
11,134
3,047
502
7,694
1,614
327
8,279
1,673
338
4
20
40
4
20
40
4
20
40
18,461
10,125
2,986
18,650
9,211
2,283
18,695
8,805
1,586
17,057
8,653
2,264
17,104
7,686
1,681
17,228
7,377
1,785
15,888
7,575
1,828
15,961
6,608
1,325
16,019
6,344
1,442
13,338
5,374
1,061
13,102
4,371
757
12,972
4,095
798
12,201
4,578
878
11,933
3,610
625
11,611
3,385
658
9,763
3,000
589
9,295
2,189
433
8,769
2,027
492
4
20
40
4
20
40
4
20
40
16,787
9,233
3,139
16,616
9,518
2,606
17,486
9,434
3,037
16,129
7,206
2,388
15,814
8,071
1,938
16,189
8,179
2,280
15,211
5,979
1,841
15,773
7,058
1,537
15,225
7,372
1,821
11,698
3,945
996
13,008
4,897
865
12,732
5,438
1,040
10,779
3,525
623
11,867
4,153
697
11,656
4,769
849
8,745
2,478
508
9,439
2,694
437
9,326
3,055
556
103
For each of the nine mixture combinations, three replicates were tested. Figure
8-2 compares the average dynamic modulus tested at a loading frequency of 1 Hz for all
the mixtures. It is clear that dynamic moduli values reduce with increase in
temperature and there is a large different in values at 4°C and 40°C.
18000
|E*| at 4C, 1 Hz
|E*| at 20C, 1 Hz
|E*| at 40C, 1 Hz
16000
|E*| (MPa)
14000
12000
10000
8000
6000
4000
2000
0
H0R
E0R
F0R
H20R
E20R
F20R
H40R
PG58_28
E40R
F40R
Mixture Type
Figure 8-2: Average and Range of Dynamic Modulus at 1 Hz Loading Frequency
There are four factors that influence the value of dynamic modulus: test
temperature, loading frequency, mixture technology (HMA or WMA) and amount of
RAP. Figure 8-3 is a factorial plot that shows the effects of all these factors on dynamic
modulus values. However, interest lies in identification of effects of mixture type and
%RAP on these values. Figure 8-4 shows how dynamic modulus changes with these
two factors. Overall, HMA mixtures have the highest dynamic moduli, followed by
104
Evotherm. While virgin mixtures are the softest, 20% RAP mixtures have the highest
modulus values.
Figure 8-3: Influence of All Factors on Dynamic Modulus
Figure 8-4: Influence of Mixture Type and RAP Content on Dynamic Modulus
105
A multi-factor ANOVA was performed as shown Table 8-2. Results showed that
while mixture type was statistically significant, there was no significant difference
between dynamic moduli with change in RAP content. However, the first order
interaction between mixture type and RAP content was significant.
Table 8-2: ANOVA of Dynamic Modulus
Dependent Variable: DynMod
Source
DF
Sum of Squares Mean Square F Value Pr > F
Model
15
15717685217
Error
469 806008751
1047845681
609.72
<.0001
1718569
Corrected Total 484 16523693969
R-Square
Coeff Var
Root MSE
DynMod Mean
0.951221
18.58390
1310.942
7054.182
Source
DF Type I SS
Mean Square F Value Pr > F
Type
2
61703370
30851685
17.95
<.0001
RAP
2
4868845
2434422
1.42
0.2436
Type*RAP
4
108430327
27107582
15.77
<.0001
Temperature 2
13499810028 6749905014
3927.63 <.0001
Frequency
2042872648
237.74
5
408574530
<.0001
An interactions plot of all factors is shown in Figure 8-5. Interactions between
mixture type and RAP content have been separately shown in Figure 8-6. Addition of
RAP makes WMA mixtures stiffer. Use of softer binder grade may have caused the 40%
RAP HMA mixture to behave differently from its WMA counterparts.
106
Figure 8-5: Interactions between Mixture Type, %RAP and Testing Conditions on |E*|
Figure 8-6: Change in Dynamic Modulus with Mixture Type and % RAP
107
For ease of comparison, the dynamic modulus master curves were grouped into
threes by mixture type (Figure 8-10, Figure 8-11 & Figure 8-12) and RAP contents
(Figure 8-7, Figure 8-8 &Figure 8-9). From Figure 8-7, we can observe that the
dynamic modulus values are higher for HMA mixture as compared to the WMA
mixtures. As expected, the virgin WMA mixtures are softer than HMA. However, we can
observes that the E* master curve behavior is similar for all RAP mixtures, at both 20%
(Figure 8-8) and 40% (Figure 8-9) RAP contents. Note that 40% RAP HMA mixtures
have used a softer PG 58-28 binder grade while all other RAP mixtures were prepared
using PG 64-22. Despite lower production temperatures, HMA and WMA RAP mixtures
have similar E* behavior. This reinforces the evidence that lowering of binder grade use
in 40% RAP HMA mixture (PG 58-28 from PG 64-22) can be avoided when WMA
mixtures are used. Unlike the results from ESR, there is no appreciable difference
between HMA and WMA RAP mixture dynamic moduli. In ESR specimens, there was a
higher air void content, which may have exposed more surface area to oxidative
hardening effects.
In Figure 8-10, the fitted E* master curves for all HMA mixtures are shown. The
40% RAP HMA mixture has the lowest E* values. This may be because of the softer
binder grade (PG 58-28) used in this mixture while the 0% RAP and 20% RAP mixtures
were prepared using PG 64-22 binder. When the E* master curve behavior is compared
for the WMA mixtures (Figure 8-11 and Figure 8-12), no specific trend is observed,
especially in the test range of loading frequencies (0.1 Hz to 25 Hz).
108
2.50E+07
Dynamic Modulus (kPa)
2.00E+07
1.50E+07
H0R
E0R
1.00E+07
F0R
5.00E+06
0.00E+00
1.00E-07
1.00E-05
1.00E-03
1.00E-01
1.00E+01
log Reduced Frequency
1.00E+03
1.00E+05
1.00E+07
Figure 8-7: E* Master Curves for Virgin (0% RAP) Mixtures (Reference Temperature 70°F)
2.50E+07
Dynamic Modulus (kPa)
2.00E+07
1.50E+07
H20R
E20R
1.00E+07
F20R
5.00E+06
0.00E+00
1.00E-09 1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09
log Reduced Frequency
Figure 8-8: E* Master Curves For 20% RAP Mixtures (Reference Temperature 70°F)
109
2.00E+07
1.80E+07
Dynamic Modulus (kPa)
1.60E+07
1.40E+07
1.20E+07
H40R PG 58-28
1.00E+07
E40R
8.00E+06
F40R
6.00E+06
4.00E+06
2.00E+06
0.00E+00
1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09
log Reduced Frequency
Figure 8-9: E* Master Curves For 40% RAP Mixtures (Reference Temperature 70°F)
2.50E+07
Dynamic Modulus (kPa)
2.00E+07
1.50E+07
H0R
H20R
1.00E+07
H40R PG 58-28
5.00E+06
0.00E+00
1.00E-07
1.00E-05
1.00E-03
1.00E-01
1.00E+01
1.00E+03
1.00E+05
1.00E+07
log Reduced Frequency
Figure 8-10: E* Master Curves for HMA Mixtures (Reference Temperature 70°F)
110
2.50E+07
Dynamic Modulus (kPa)
2.00E+07
1.50E+07
E0R
E20R
1.00E+07
E40R
5.00E+06
0.00E+00
1.00E-07
1.00E-05
1.00E-03
1.00E-01 1.00E+01 1.00E+03
log Reduced Frequency
1.00E+05
1.00E+07
Figure 8-11: E* Master Curves for Evotherm Mixtures (Reference Temperature 70°F)
2.50E+07
Dynamic Modulus (kPa)
2.00E+07
1.50E+07
F0R
F20R
1.00E+07
F40R
5.00E+06
0.00E+00
1.00E-07
1.00E-05
1.00E-03
1.00E-01 1.00E+01 1.00E+03
log Reduced Frequency
1.00E+05
1.00E+07
Figure 8-12: E* Master Curves for Foamer Mixtures (Reference Temperature 70°F)
111
H0R
Phase Angle (deg)
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-13: Phase Angle Master Curve of Virgin HMA Mixtures (70°F)
Phase Angle (deg)
E0R
50
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-14: Phase Angle Master Curve of Virgin Evotherm Mixtures (70°F)
Phase Angle (deg)
F0R
50
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-15: Phase Angle Master Curve of Virgin Foamer Mixtures (70°F)
112
Phase Angle (deg)
H20R
30
25
20
15
10
5
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-16: Phase Angle Master Curve of 20% RAP HMA Mixtures (70°F)
Phase Angle (deg)
E20R
50
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-17: Phase Angle Master Curve of 20% RAP Evotherm Mixtures (70°F)
Phase Angle (deg)
F20R
30
25
20
15
10
5
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-18: Phase Angle Master Curve of 20% RAP Foamer Mixtures (70°F)
113
Phase Angle (deg)
H40R
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-19: Phase Angle Master Curve of 40% RAP HMA Mixtures (70°F)
Phase Angle (deg)
E40R
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-20: Phase Angle Master Curve of 40% RAP Evotherm Mixtures (70°F)
Phase Angle (deg)
F40R
40
30
20
10
0
1.00E-06
1.00E-04
1.00E-02
1.00E+00
1.00E+02
Reduced Frequency (Hz)
Figure 8-21: Phase Angle Master Curve of 40% RAP Foamer Mixtures (70°F)
114
The phase angles of all the mixtures are also shown in Figure 8-13 to Figure
8-21. Near low loading frequencies (or high temperature), a characteristic hump can be
observed. This occurs because at low loading frequencies or high temperature, the
binder behaves like a fluid and the aggregate structure controls the response. This
hump is prominent in virgin mixtures while it is not as prominent in RAP mixtures. In
RAP mixtures, the aged binder is much stiffer than the virgin binder. 40% RAP mixtures
exhibit a high variability from the general trend. This may be due to variability in RAP
aggregate gradation.
Using a sigmoidal function, the master curves were used to extrapolate and
obtain |E*| data at five temperatures: -10, 5, 20, 40 and 54°C (14, 40, 70, 100 and 130oF)
and six frequencies: 0.1, 0.5, 1, 5, 10 and 25 Hz for each mix as shown in the next
chapter. The sigmoidal function is given by:
𝑏
log|𝐸 ∗ | = 𝑎 +
1+
1
𝑒𝑥𝑝
𝑑+𝑒𝑙𝑜𝑔𝑓𝑇0
𝑤ℎ𝑒𝑟𝑒, 𝑎, 𝑏, 𝑑 𝑎𝑛𝑑 𝑒 𝑎𝑟𝑒 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡𝑠 𝑎𝑛𝑑 𝑓𝑇0 𝑖𝑠 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 𝑎𝑡 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒.
This data was used in AASHTOWare pavement ME software to predict the
performance of a typical pavement section with respect to two primary distresses:
fatigue cracking and rutting.
115
CHAPTER 9.
PERFORMANCE PREDICTION AND ECONOMICS
In this chapter, the dynamic modulus data from the previous chapter are used to
predict pavement performance. A cost-benefit analysis of the mixtures is also described.
9.1.
Pavement Performance Prediction
The AASHTOWare Pavement ME software, which is based on Mechanistic-Empirical
Pavement Design Guide (M-E PDG), was used to predict pavement performance in this
study. A typical flexible pavement section for 9.5B mixtures as recommended by
NCDOT, in addition to a weaker pavement section was used for evaluating the
performance of the mixtures.
NCDOT recommended pavement section is a five-layer flexible pavement with layer
dimensions and properties as shown below in Figure 9-1.
AC (Design Mixture)
3 in.
Asphalt Concrete
2.5 in.
Asphalt Concrete
4 in.
Chemically Stabilized Base
(Soil Cement)
Resilient Modulus = 2 x 106 psi
8 in.
Subgrade (AASHTO A-7-5)
Resilient Modulus = 10,000 psi
Figure 9-1: NCDOT Pavement Layer Structure for Performance Prediction
116
Traffic parameters, base and subgrade properties typically used for design of
NCDOT traffic level B pavements were used as inputs for AASHTOWare analysis. The
assumed pavement section was a four-lane highway with two lanes in each travel
direction, having a two-way average annual daily truck traffic (AADTT) of 900,
operating at 45 mph and increasing at an annual linear growth rate of 3%. Climatic data
provided in the software for Raleigh-Durham Airport weather station was used.
A reliability of 90% was targeted for all distresses including fatigue (bottom-up
and top-down), rutting (permanent deformation) and thermal cracking for all the
analysis. Failure criteria were defined as 25% bottom-up cracking and 0.75 inches for
total pavement rutting.
Analysis runs were conducted using the E* data from
117
Table 9-1 as Level 1 inputs for the topmost AC layer. For the topmost layer,
virgin and recycled binder data was obtained from a previous study conducted using
the same materials at NC State University for the NC Department of Transportation
[64].
Default Level 3 inputs were used for bottom two AC layers. Using a design life of
20 years for the pavement, months to failure was evaluated with respect to fatigue
cracking and rutting for all nine mixtures. Table 9-2 shows the failure predictions as
obtained from the analysis for the NCDOT pavement structure.
118
Table 9-1: E* Data from Master Curves for Use as M-E PDG Input
Frequency (Hz) →
Temperature (oC)
↓
H0R
25
10
5
1
0.5
0.1
Dynamic Modulus (Values in MPa)
-10
20962.0
20962.0
20962.0
20962.0
20962.0
20962.0
5
20232.8
18708.5
17138.0
14474.7
12918.2
10177.7
20
10481.9
8910.4
7671.5
5399.7
4514.6
2953.1
40
2839.8
2128.7
1661.4
942.9
753.5
502.0
54
1024.9
774.1
612.0
502.0
502.0
502.0
E0R
Dynamic Modulus (Values in MPa)
-10
16983.8
16983.8
16983.8
16983.8
16983.8
15833.6
5
16316.1
14878.5
13364.0
10854.9
9447.3
6881.3
20
8272.4
6791.3
5711.3
3596.8
2892.1
1622.2
40
1759.0
1249.7
947.4
511.2
420.6
327.5
54
521.0
405.6
354.7
326.9
326.9
326.9
F0R
Dynamic Modulus (Values in MPa)
-10
17846.3
17846.3
17846.3
17846.3
17846.3
17738.5
5
17358.9
15860.1
14410.4
11776.3
10310.4
7720.6
20
8420.7
6939.2
5898.4
3710.1
3011.4
1676.6
40
1790.0
1271.7
977.2
549.3
468.2
338.4
54
578.0
456.0
380.6
338.4
338.4
338.4
H20R
Dynamic Modulus (Values in MPa)
-10
19073.1
19073.1
19073.1
19073.1
19073.1
18945.0
5
18445.3
17030.1
15606.0
13256.4
11893.4
9489.5
20
10307.2
8624.2
7467.9
5342.6
4481.9
3032.6
40
2877.1
2183.4
1738.1
1040.3
861.6
621.3
54
1037.6
800.1
676.5
621.3
621.3
621.3
E20R
Dynamic Modulus (Values in MPa)
-10
18650.2
18650.2
18650.2
18650.2
18650.2
18441.4
5
17863.3
16402.6
14865.7
12387.4
10932.0
8294.3
119
Table 9-1 Continued
20
9149.3
7578.8
6468.9
4294.8
3528.1
2144.9
40
2272.4
1678.2
1321.4
755.8
623.6
433.2
54
822.8
644.3
524.8
432.9
432.9
432.9
F20R
Dynamic Modulus (Values in MPa)
-10
19661.3
19661.3
19661.3
19661.3
19661.3
19661.3
5
19125.2
17682.1
16128.5
13178.9
11556.5
8610.2
20
9126.2
7667.3
6544.8
4261.3
3497.2
2101.7
40
2353.1
1722.2
1353.6
783.3
652.2
470.1
54
960.8
751.5
623.8
471.6
470.1
470.1
H40R
Dynamic Modulus (Values in MPa)
-10
16790.3
16790.3
16790.3
16790.3
16790.3
16307.4
5
16217.6
14792.2
13411.7
11066.4
9875.3
7613.8
20
8618.2
7235.7
6270.1
4290.3
3586.2
2296.8
40
2521.6
1893.9
1510.8
869.9
712.4
462.4
54
944.1
739.2
582.1
461.0
461.0
461.0
E40R
Dynamic Modulus (Values in MPa)
-10
16615.7
16615.7
16615.7
16615.7
16615.7
16404.8
5
16242.2
15685.5
14944.8
12456.8
11157.3
8971.3
20
9480.5
8037.4
7009.3
4870.9
4131.8
2690.9
40
2646.5
1967.6
1562.7
876.6
709.4
442.4
54
976.3
753.5
587.9
437.0
437.0
437.0
F40R
Dynamic Modulus (Values in MPa)
-10
17341.2
17341.2
17341.2
17341.2
17341.2
16599.4
5
16582.5
15231.8
13817.1
11598.4
10283.8
8066.3
20
9054.0
7651.8
6633.8
4566.1
3859.1
2417.2
40
2502.8
1938.0
1535.3
887.6
725.5
499.4
54
917.2
705.7
571.8
496.6
496.6
496.6
120
Table 9-2: Fatigue and Rutting Failure Prediction for NCDOT Pavement Structure
Mix Type
Rutting (in.)
Fatigue (%)
Pass/Fail
Target: 0.75 in. Target: 25%
H0R
0.31
1.45
Pass
E0R
0.34
1.45
Pass
F0R
0.33
1.45
Pass
H20R
0.32
1.45
Pass
E20R
0.33
1.45
Pass
F20R
0.32
1.45
Pass
H40R
0.34
1.45
Pass
E40R
0.33
1.45
Pass
F40R
0.34
1.45
Pass
As can be seen from the failure predictions, none of the mixtures fail the target
criteria for either fatigue or rutting. The amount of permanent deformation predicted in
the total pavement varies slightly between the mixtures while the fatigue predictions
are uniform throughout. The differences in rutting predictions are not practically
significant, in the order of 0.01 inches. As such, the pavement performance between
these mixtures could not be distinguished using this thick and rigid pavement structure.
In order to facilitate observation of trends in the performance of the mixtures, a
weaker pavement structure with three layers as shown in Figure 9-2 was also
analyzed.
121
AC (Design Mixture)
3 in.
Non-Stabilized Base
(Crushed Stone)
Resilient Modulus = 30,000 psi
8 in.
Subgrade (AASHTO A-7-5)
Resilient Modulus = 10,000 psi
Figure 9-2: Weaker Pavement Layer Structure
Table 9-3: Fatigue and Rutting Failure Prediction for a Weak Pavement Structure
Mix Type
Rutting (in.)
Fatigue (%)
Pass/Fail
Target: 0.75 in. Target: 25%
Years to
Failure
H0R
0.63
23.44
Pass
No Failure
E0R
0.68
25.90
Fail
18
F0R
0.67
25.16
Fail
19
H20R
0.65
24.44
Pass
No Failure
E20R
0.66
24.74
Pass
No Failure
F20R
0.65
24.11
Pass
No Failure
H40R
0.68
27.17
Fail
18
E40R
0.66
25.59
Fail
18.5
F40R
0.67
25.42
Fail
18.5
122
The same inputs as used in the previous analysis were given and the rutting and
fatigue failure criteria were evaluated. The results from AASHTOWare analysis with this
structurally weaker pavement structure are shown in Table 9-3.
The resulting rutting and fatigue predictions were normalized for virgin HMA
mixture performance as shown in Figure 9-3 and Figure 9-4. By observing difference
in pavement performance in rutting and fatigue from that of a virgin HMA mixture, we
can distinguish the effect of using WMA and RAP in the mixtures. Since rutting depths of
all nine mixtures were below the threshold value of 0.75 inches, failure of the pavement
was controlled by fatigue cracking. However, in pavements that did experience fatigue
failure, it occurred only towards the end of the design life of the pavement: 18-19 years
out of a design life of 20 years.
Since the only difference in input values between all the analyzed pavements
were the Level 1 inputs for the first layer of asphalt concrete, the performance of the
pavement in this analysis is dependent on the dynamic modulus values.
The predicted number of months to failure with respect to rutting follows the
same trend as the variation in stiffness observed in the master curves. Trends of
normalized rutting depths and fatigue failure values of pavements with WMA and RAP
mixtures (with respect to the virgin HMA mixture) are similar.
As compared to virgin HMA, the virgin WMA mixtures experience higher rut
depths and fatigue. Adding intermediate amounts of RAP (20%) improved the WMA
mixture performance. Of all mixtures, the high RAP HMA mixture (40%) exhibited the
123
highest susceptibility to failure. In comparison, the high RAP WMA mixtures performed
better, again showing that it is more desirable to use WMA technology to help
incorporate higher amounts of RAP rather than using a softer binder grade.
Rutting Difference (in.)
0.06
0.05
0.04
0.03
0.02
0.01
0
H0R
E0R
F0R
H20R E20R F20R H40R E40R F40R
Mixture Type
Figure 9-3: Rutting Predictions Normalized with Respect to HMA
Fatigue Difference (%)
4
3.5
3
2.5
2
1.5
1
0.5
0
H0R
E0R
F0R
H20R E20R F20R H40R E40R F40R
Mixture Type
Figure 9-4: Percentage Fatigue Predictions Normalized with Respect to HMA
124
9.2.
Economic Analysis
An economic analysis of the different mixture combinations is based on the
pavement performance predictions. The AASHTOWare analysis was conducted for a
design period of 20 years. For a typical pavement structure used for an NC 9.5B type
mix, there were no predicted failures due to rutting or fatigue cracking within this
design life. Thus, sole factors that influence the economics of these mixtures were
assumed arise from initial material and equipment costs and savings that result from
reduced fuel consumption and usage of RAP. The costs and benefits of using WMA and
RAP are summarized in Table 9-4.
Table 9-4: Summary of Costs and Benefits with Usage of WMA and RAP
Costs
Benefits
WMA equipment purchase, installation Reduced energy consumption from
and modification costs
lowering of production temperature
Manufacturer Royalties
Savings in virgin material costs with use of
RAP
Additive costs based on dosage
RAP processing, quality control and plant Savings from reduced emission control
costs
Virgin material costs include the cost of asphalt binder and aggregates. The cost
of asphalt was assumed to be $500 per ton based on asphalt cement prices published by
NCDOT [71]. Virgin aggregates were priced at $15 per ton [11]. At 6% asphalt content,
the cost of asphalt concrete mix was calculated to be $45 per mix. Factoring in
additional overheads like transportation and quality control, the cost of mix was
125
estimated to be $50 per ton. This is similar to values used in previous studies conducted
for NCDOT [72].
Use of Evotherm® involves material as well as plant modification costs. In this
study, 0.5% Evotherm® by weight of binder was used with a maximum 6% asphalt
binder (for virgin mixtures) by weight of the mix. For this dosage, 0.3 kg of Evotherm®
will be needed per ton of the mix. Based on literature review, the cost of Evotherm®
was estimated to be $2.00 per ton of mix [11], [73]. The Foamer device does not employ
any additives, and thus is not associated with material costs. However there is a onetime equipment purchase and installation fee as well as yearly maintenance costs. The
cost of producing Foamer mixtures were estimated at $0.5 per ton [30], [73]. These
values are summarized in Table 9-5.
Table 9-5: Material Cost Estimates for Mixture Production
Material
Cost (USD per ton)
Asphalt concrete surface coarse mix (S9.5B)
50.0
Evotherm mixtures: additive and plant modification
3.0
Foamer - purchase, installation and maintenance
0.5
Energy costs associated with asphalt concrete production include fuel and
electricity. Energy is consumed in the process of heating of aggregate and asphalt
cement, material transportation to and from the plants and during in-place compaction.
Energy costs vary widely depending on demand and supply of fuel, site and plant
location and distances, climatic conditions, productivity and efficiency of the
126
equipment, etc. To aid in comparison, a $10 per ton estimate was used to account for
energy costs of HMA. Use of WMA technologies was assumed to conservatively reduce
energy consumption by 25% as compared to HMA [30]. Thus, energy costs associated
with WMA mixtures was estimated to be $7.5 per ton.
No purchase costs were associated with use of RAP since state and federal
agencies generally mandate or encourage its use. Thus, it was assumed that using RAP
would result in a complete deduction of asphalt concrete material costs. However, there
are extra equipment, processing and handling and quality control costs associated with
RAP. These costs in total were estimated to be $7.0 per ton of RAP [11].
With weaker pavement sections, rehabilitation and salvage values may need to
be taken into account. Based on the results from AASHTOWare analysis on the weak
pavement section, at the maximum, only one rehabilitation course will be required for
some surface mixtures, particularly at high RAP contents.
Using all these values, total production costs of all mixtures were estimated and are
summarized in
127
Table 9-6. With respect to HMA, Evotherm® mixtures were approximately 1%
more expensive to produce. Despite the additional equipment costs, using the Foamer
leads to approximately 3% savings. Usage of RAP is highly economical and leads to
around 14% savings for every 20% replacement of virgin mix. Using both sustainable
technologies led to a summation in savings.
128
Table 9-6: Costs per Ton of Each Mixture Type
Mixture
Type
Material
cost
per ton
(USD)
Energy
cost
per ton
(USD)
Technology
cost
per ton
(USD)
Processing
cost
per ton
(USD)
Total
cost
per ton
(USD)
%
Savings
H0R
50
10
0
0
60
0.0
E0R
50
7.5
3
0
60.5
-0.8
F0R
50
7.5
0.5
0
58
3.3
H20R
40
10
0
1.4
51.4
14.3
E20R
40
7.5
3
1.4
51.9
13.5
F20R
40
7.5
0.5
1.4
49.4
17.7
H40R
30
10
0
2.8
42.8
28.7
E40R
30
7.5
3
2.8
43.3
27.8
F40R
30
7.5
0.5
2.8
40.8
32.0
Without using warm mix technologies, it would be difficult to achieve higher
rates of recycling. Thus, the key economic benefit of warm mix asphalt is its ability to
help replace virgin material at 30% or higher with RAP. In this study, the Foamer
mixtures with 40% RAP turned out to be most economical in terms of initial production
costs with savings exceeding 30% as compared with virgin HMA.
In addition to economic benefits, WMA, RAP and WMA-RAP mixtures also result in
lower emissions and great environmental benefits during the entire construction
process, thereby having a less severe impact on the environment. Even though the
savings from using WMA technology is not very significant, it will still lead to extensive
environmental benefits. This reduction in carbon footprint of roads and improvement
129
in sustainability will transfer to benefits that cannot be easily monetized but are
invaluable nonetheless.
130
CHAPTER 10.
SUMMARY, CONCLUSION and RECOMMENDATIONS
10.1. Summary
This research evaluated two WMA technologies (Evotherm® 3G and PTI Foamer)
with varying RAP contents (0%, 20% and 40%) and compared their performance with
virgin, 20% RAP and 40% RAP HMA mixtures. A job mix formula for Asphalt Concrete
Surface Course type 9.5B was provided by the NCDOT. This was used to design all the
mixtures.
During mix design, %Gmm evolution curves were used to evaluate workability. Based
on workability evaluations, PG 64-22 asphalt was used in all the mixtures except for
40% RAP HMA that used the softer PG 58-28 asphalt. For WMA mixtures with 40%
RAP, adequate compactability indicated that lowering of binder PG grade was not
necessary.
To determine moisture susceptibility, modified AASHTO T283 method was used to
obtain Tensile Strength Ratios for all mixture combinations. All the mixtures passed the
minimum TSR criteria of NCDOT specifications, expect for 40% RAP Foamer mixture.
Dynamic modulus tests were used to compute the E* master curves for all mixtures.
They were also used to compute the ratio of dynamic modulus between conditioned
and unconditioned specimens, analogous to the Tensile Strength Ratio, called the E*
Stiffness Ratio (ESR).
Based on the dynamic modulus values, pavement performance was analyzed using
AASHTOWare Pavement ME software. For a typical S9.5B section used by the NCDOT,
131
none of the mixtures exhibited rutting or fatigue failure within a design life of 20 years.
By estimating the total cost per ton to produce a mixture by taking into account
material, WMA technology costs, RAP screening and processing costs as well as energy
benefits, savings for each mixture was calculated.
The conclusions based on the results of this study are listed below.
10.2. Conclusions
i.
Volumetric properties for HMA and WMA mixtures with the same amount of
RAP were similar. For every addition 20% RAP by weight of mix, the optimum
binder content reduced by 0.1%.
ii.
In 40% RAP mixtures, the workability of HMA improved when a softer grade
binder, PG 58-28 was used. The 40% RAP-WMA mixtures did not show any
difference in workability with the change of binder grade, i.e. the compactability
was similar for WMA mixtures with PG 64-22 binder and those with PG 58-28
binder. Thus, using WMA can eliminate the need for binder grade change in
mixtures with high RAP contents.
iii.
In Evotherm® and Foamer mixtures, TSR values decreased as the amount of
RAP increased. However, in HMA mixtures, TSR values for 20% and 40% RAP
contents were similar but lesser than the virgin mixture.
iv.
With addition of RAP, both unconditioned and conditioned Indirect Tensile
Strength (ITS) values increased. Thus, despite lower TSR values, it is not
necessary that moisture conditioning is adversely affected with addition of RAP.
132
v.
WMA mixtures with 40% RAP had the highest unconditioned ITS values. Since
these mixtures were cured for 16 hours before compaction, it is possible that
removal of excess foaming water in Foamer mixtures and oxidation of
Evotherm® additive caused this increase. In HMA mixtures, where a softer
binder grade was used with 40% RAP, there was no significant difference in ITS
value after doubling the amount of RAP.
vi.
The E* stiffness ratios of most mixtures were higher than or close to 85%. Only
the high RAP HMA mixture with 40% RAP with PG 58-28 binder behaved
substantially differently from other mixtures with 57% ESR value. This was due
to a significantly higher unconditioned dynamic modulus value. Even though a
softer binder was used, the higher production temperatures may have
accelerated oxidation of the already aged RAP material in HMA mixtures. Since
there was no curing period while preparing the ESR test specimens, the WMA
mixtures were not oxidized to that extent due to their lower production
temperatures.
vii.
Since dynamic modulus is a compression test, it may not be appropriate to test
moisture susceptibility using this test as moisture damage is controlled by the
adhesive property of asphalt.
viii.
As expected, the virgin WMA mixtures are softer than HMA. Despite lower
production temperatures, HMA and WMA RAP mixtures show similar E*
133
behavior. This reinforces the evidence that lowering of binder grade use in high
RAP HMA mixtures can be avoided with the use of WMA technologies.
ix.
For a typical 9.5B mix pavement structure, none of the mixtures exhibited failure
within the design period. As compared to virgin HMA, the virgin WMA mixtures
experience higher rut depths and fatigue, which improved with addition of 20%
RAP. Of all mixtures, the high RAP HMA mixture (40%) exhibited the highest
susceptibility to failure. In comparison, the high RAP WMA mixtures performed
better, again showing that it is more desirable to use WMA technology to help
incorporate higher amounts of RAP rather than using a softer binder grade.
x.
For every 20% of the HMA mix that is replaced with RAP, saving of around 14%
can be expected in the initial cost of production. Cost of WMA depends on the
technology being used; additive based technologies may be more expensive to
produce. The key economic advantage of WMA mixtures is their ability to help
incorporate higher amount of RAP.
10.3. Recommendations for Further Studies
i.
Correlation between field workability and laboratory measured parameters
should be conducted for mixtures with RAP materials and WMA technologies.
ii.
The possibility of the use of higher RAP contents has to be investigated by either
increasing the dosage of the Evotherm® additive, or by using different WMA
technology, and or by reducing the asphalt binder grade.
134
iii.
The effect of curing periods on oxidative hardening on RAP mixtures should be
further studied.
135
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142
APPENDIX
143
APPENDIX A
Evotherm as an Anti-Strip Additive
Evotherm is an established Warm Mix Additive that helps in reducing the
production and compaction temperatures of the mixtures. MeadWestvaco, the
manufacturers of Evotherm have proposed that it can also be used as an anti-strip
additive. However, as Evotherm is a chemical additive, its thermal stability is important
when subject to extended heating as with any other chemical anti-strip additive. The
amount of additive left in the mixture due to volatilization, after heating the mixture for
a certain amount of time is an important measure for effectiveness of the additive as an
anti-strip. As the volatility of an additive increases, the amount of additive left in the
mixture decreases, and hence the effectiveness of the additive in mitigating moisture
sensitivity is severely affected.
In 2005 an NCDOT study showed that the effectiveness of amine based anti-strip
additive deteriorated in as little as 8 hours of prolonged heating of mixes containing
these additives and the moisture susceptibility of mixtures (in terms of TSR) was
severely affected [ (Tayebali, Knappe, & Chen, 2005)]. Therefore, thermal analysis of
Evotherm as an anti-strip additive is important.
Thermal analysis of Evotherm vis-à-vis LOF 6500 anti-strip additive
Figures A1 and A2 show the degradation of Evotherm through volatilization, oxidation
and chemical degradation due to prolonged exposure to heat. The figures show the
144
changes after 2, 8 and 24 hours of heating at 120°C for Evotherm in comparison to the
anti-strip additive LOF 6500 that was used in this study. It is apparent that there is
degradation in chemical composition that may affect the moisture susceptibility of
mixtures for which Evotherm is used as an anti-strip additive. The question is, what
percentage of the Evotherm additive remains in the mix after prolonged heating. In
order to evaluate the thermal effect on the mixes, HMA mixture samples were prepared
and tested using the Strip Scan device.
Figure A1: Evotherm and LOF Additives with 0hr (Left) And 2hr (Right) Heating at 120°C
Figure A2: Evotherm and LOF Additives with 8hr (Left) and 24hr (Right) Heating at 120°C
145
Litmus Test using StripScan
The strip scan uses the color difference caused in the litmus paper when exposed
to the fumes from the mixture containing the additive to measure the amount of
additive left in the mixture. Initial calibration is necessary using asphalt concrete
mixtures with varying additive content. The color difference for litmus paper is
determined using a spectrophotometer that takes quantitative readings of the litmus
paper before exposure to the fumes from the asphalt concrete mix and after exposure.
The difference in color measured by the spectrophotometer is the color index and a
measure of the amount of anti-strip additive present.
A calibration curve was developed using four different Evotherm additive
contents – 0.0%, 0.25%, 0.5% and 1.0% by the weight of asphalt binder as shown in
Figure A3. Two sets of mixtures were prepared for each additive percentage and their
color counts were measured. The average count values of each additive percentage
were used to develop the calibration equation. The mixtures were preheated for an
hour to 120°C (248°F) before their readings were taken during calibration. 120°C
simulates the compaction temperature of mixtures when Evotherm is used as a WMA
additive.
Once the calibration curve was established, mixtures containing 0.5% Evotherm
by weight of asphalt were heated for 2, 8 and 24 hours. Each time period simulates
different field or laboratory conditions. The 2 hour heating represents the normal
compaction time in the field, while the 8 hour and 24 hour represent delayed
146
compaction in the field, or storage of the mix in silo overnight if necessary. After the
appropriate heating duration, the level of Evotherm amount in the mix was determined
using the Strip Scan device.
1.2
Additive Content (%)
1
0.8
0.6
0.4
0.2
0
350
400
450
500
550
600
650
Spectrophotometer Count
Figure A3: Calibration Curve for Evotherm Additive Content
The Calibration Equation obtained for Evotherm presented in Figure A3 is –
𝑨𝑪 = 𝟏. 𝟕𝟕𝟏 × 𝟏𝟎−𝟔 × 𝒄𝟐 + 𝟎. 𝟎𝟎𝟑𝟑𝟑𝟓 × 𝒄 − 𝟏. 𝟕𝟑𝟔,
where, AC = Additive Content and c = Count from the sample.
147
𝑹𝟐 = 𝟎. 𝟗𝟖
Table A1 shows the average Strip Scan count values for the mixtures for
different heating times and the estimated amount of Evotherm additive present after
the prolonged heating duration. The additive content was estimated using the
calibration equation.
Table A2 indicates that the mixture with Evotherm additive content of 0.5% as
used in this study but without additional LOF 6500 anti-strip additive still had the same
amount after two hours heating time. The two hours of heating is important as it
corresponds to the transport for a mixture before compaction. But when the mixture is
heated to 8 hours, the additive content falls to 0.33%, and to 0.10% when heated to 24
hours. Therefore, when Evotherm is used as anti-strip additive it is important to realize
this fact that it may not act as an effective anti-strip.
To test the effectiveness of Evotherm as an anti-strip additive, TSR test was
conducted on virgin mixtures by adding just Evotherm and not adding any additional
anti-strip additive. The results of the TSR test with only Evotherm and no additional
anti-strip additive is shown in Table A2. The TSR ratio for mixtures with only Evotherm
is 88% that passes the minimum required TSR value of 85% as specified by NCDOT; and
therefore for the mix under consideration, Evotherm does act as an anti-strip additive.
However, based on TSR results from chapter 6, it must be emphasized that when RAP is
used with the mix, especially higher contents, it may be necessary to either increase the
Evotherm dosage or add an additional anti-strip additive such as LOF 6500 as used in
this study.
148
Table A1: Estimated Evotherm Additive Content after Prolonged Heating
Heating Time
Count Additive Content
2 hours
527
0.51%
8 hours
491
0.33%
472
0.23%
445
0.10%
TSR conditioning
(16-hour curing at 600C)
24 hours
Table A2: TSR Test Results For 0% RAP Mixtures with Evotherm only (without LOF 6500)
Moisture
Conditioning
Dry
Wet
Specimen
#
Air Void
Content
ITS
(psi)
ITS
(kPa)
EVO 2
6.8
137.31
947
EVO 6
7.0
135.13
932
EVO 3
7.2
135.13
932
EVO 5
7.1
104.62
721
149
Average
Subset
ITS
(kPa)
TSR
(%)
939
88
827