September 2007
UCPRC- SR- 2007- 02
Summary and Recommendations toward
Implementing Innovations Based on
Selected Presentations at the Asphalt
Rubber Pavement Conference ( AR2006),
October 2006
Work Conducted as part of California Center for Innovative Transportation
( CCIT) Task Order 1018 and Partnered Pavement
Research Center Strategic Plan Element No. 2.3 – Provide Pavement
Technology Advice to Caltrans
PREPARED FOR:
California Department of Transportation
( Caltrans),
Division of Maintenance,
Office of Pavement Preservation
PREPARED BY:
J. M. Signore, R. Wu, L. Santucci, and
J. T. Harvey
University of California
Pavement Research Center
UC Davis and Berkeley
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DOCUMENT RETRIEVAL PAGE UCPRC- SR- 2007- 02
Title: Summary and Recommendations toward Implementing Innovations Based on Selected
Presentations at the Asphalt Rubber Pavement Conference ( AR2006), October 2006
Authors: J. M. Signore, R. Wu, L. Santucci, and J. T. Harvey
Prepared for:
Caltrans Division of
Research
Client Reference No:
TO 1018
FHWA No.:
CA081077A
Date:
November 6,
2007
Strategic Plan Element No:
2.3
Status:
Final
Version No:
Stage 5
Abstract:
UCPRC staff attended the 2006 RPA conference in Palm Springs, CA. This document contains brief
summaries of the technical papers/ presentations given in both the technical and practical portions of the
event. Items within specific papers that may be implementable by Caltrans are noted. Suggestions for
future research, development, and information dissemination are given.
Keywords:
Asphalt Rubber, Crumb Rubber, SAMI, Binder, Overlay
Related documents:
Signatures:
J. Signore
1st Author
C. Monismith
Technical Review
D. Spinner
Editor
C. Monismith
Principal
Investigator
M. Samadian
Caltrans Contract
Manager
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DISCLAIMER
The contents of this report reflect the views of the authors who are responsible for the facts and accuracy
of the data presented herein. The contents do not necessarily reflect the official views or policies of the
State of California or the Federal Highway Administration. This report does not constitute a standard,
specification, or regulation.
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TABLE OF CONTENTS
List of Tables ............................................................................................................................... ............... iv
Glossary ............................................................................................................................... ........................ v
1.0 Background and Overview .............................................................................................................. 1
2.0 Review of Papers from Technical Sessions and Application Track ................................................ 3
2.1 Review and Discussion of Technical Session Papers.................................................................. 3
2.2 Application Track ( Practical Sessions) Overview..................................................................... 18
3.0 Summary and Recommendations .................................................................................................. 23
3.1 Additional Suggestions and Recommendations from AR2006................................................. 27
LIST OF TABLES
Table 1: Summary Table of Selected Papers and Recommendations......................................................... 24
Table 2: Suggested Actions and Agents to Aid AR Implementation.......................................................... 29
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GLOSSARY
Below are basic definitions for commonly found terms in the summary and the proceedings. The reader
is urged to do further research on any of these terms if interest is warranted.
Asphalt Concrete Friction Course ( AR-ACFC)
An open- graded friction course utilized by the state of Arizona
as a noise reducing layer.
Asphalt rubber
( AR)
An asphalt concrete mix produced with crumb rubber through
the wet process – see wet process.
Asphalt Rubber Stress Absorbing
Membrane Interlayers
( AR- SAMI)
A reflection crack and waterproofing layer composed of
asphalt rubber binder with a single layer of uniform chips.
Similar to a chip seal.
American Society for Testing and
Materials ( ASTM)
Governing body for standards development in the United
States
Binder
Asphalt cement. May be modified with polymers or rubber.
Conventional asphalt concrete Asphalt concrete made with unmodified asphalt cement.
Other names commonly used for this are conventional DGAC
and conventional HMA.
Crumb rubber modifier ( CRM)
Small rubber particles produced from shredding rubber tires.
Various gradations of this material are used worldwide.
Cement Treated Base ( CTB) CTB is a mix generally used with asphalt concrete pavements,
composed of aggregate, portland cement, and water and is
specified as either Class A or Class B by Caltrans. It can be
either plant mixed or road mixed.
Dense Graded Asphalt Concrete
( DGAC)
Another name for conventional asphalt concrete.
Dynamic Modulus ( E*) Dynamic modulus ( E*) refers to the bending stiffness under
repeated loading of a viscoelastic material which has both
magnitude and phase components. With viscoelastic
materials, there is a time lag between the applied stress and
resulting strain ( and vice- versa) in the material. The time lag
is represented by the phase component of E*, noted as φ.
Dry process
Modification of asphalt concrete mixtures by use of granulated
or ground rubber/ crumb rubber. The rubber is used as a
portion of the fine aggregate.
Elastic modulus
The stiffness of an asphalt concrete mix as measured by the
ratio of applied stress to resulting strain within the material.
Extender Oil
An aromatic oil used to supplement the reaction of the asphalt
and the crumb rubber modifier
Indirect Tensile Test ( IDT)
An asphalt concrete performance test in which tensile
capabilities are determined from compressive forces placed
upon an edge loaded asphalt concrete core.
International Roughness Index ( IRI) A measurement unit used to illustrate the relative “ roughness”
of a pavement. It is measured in inches/ mile or mm/ km and is
a summation of deviations from the horizontal plane over a
given distance.
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Marshall test
An asphalt mix design procedure commonly used in the
eastern and middle portions of the United States.
Mechanistic- Empirical ( ME)
Refers to the concept of designing and characterizing
pavements using both the mechanical/ physical properties of
the materials as well as the actual measured field and
laboratory performance.
Modified Binder ( MB) Generic term describing asphalt binders that have incorporated
into them either polymers, rubbers, or other “ modifiers” in
order to enhance performance.
Open- graded Asphalt Concrete
( OGAC)
An asphalt layer primarily used to minimize standing water
and water splash and for drainage when used as a base layer.
Open- graded Friction Course ( OGFC) An asphalt surfacing layer placed to aid friction and water
splash issues. Arizona has placed this material ( AR- ACFC)
on many of their PCC pavements.
Polyphosphoric Acid ( PPA)
An additive extensively used in the chemical modification of
both conventional and polymer modified binders.
Porous pavements
Pavements that are designed to promote water runoff and/ or
capture. A wide range of performance capabilities exist for
this type of pavement, ranging from open- graded surfaces to
fully permeable pavements.
RAC Caltrans name for Rubberized Asphalt Concrete
RAC- G
Caltrans name for Rubberized Asphalt Concrete with a gap
gradation
RAC- O Caltrans name for Rubberized Asphalt Concrete with an open
gradation. Binder content is typically about 20% higher than
conventional DGAC.
RAC- O- HB Caltrans name for Rubberized Asphalt Concrete with an open
gradation and even higher binder content. Binder content is
typically about 65% higher than conventional DGAC.
Raveling
The pavement distress in which surface aggregate is dislodged
or worn away due to aging and hardening of the asphalt binder
and/ or tire wear.
RUMAC Dry process CRM modified mix Caltrans uses for pavement
rehabilitation projects.
SBS and SBR
Refers to the polymer additive or mixes modified with Styrene
Butadiene Styrene and Styrene Butadiene Rubber
SGC Superpave Gyratory Compactor
Spread rate
Refers to the amount of binder placed per unit area in chip
seals or SAMIs. Also refers to amount of chips placed per
unit area.
Stiffness
Another term for Modulus. Stiffness refers to the degree of
bending of an asphalt mix for a given load.
Strain
Change of length per unit length. Permanent strain in asphalt
concrete results in rutting within an asphalt mix
Stress Applied load per unit area
Stress Absorbing Membrane Interlayer
( SAMI)
A reflection crack and waterproofing layer composed of
conventional asphalt binder with a single layer of uniform
chips. Similar to a chip seal.
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Terminal blend
A rubber modified asphalt concrete in which the crumb rubber
is blended with the asphalt at an asphalt distribution terminal.
This process allows for holding the mix for extended periods
of time. Typical the blend contains less than 10% crumb
rubber along with extender oils and small amounts of polymer
modifiers.
Texas Gyratory Compactor ( TGC)
A specific compactor used by TxDOT for mix testing.
Traffic Index ( TI)
A measure of traffic count over the course of a pavement
lifetime. Higher TIs indicate higher traffic levels.
Voids in the Mineral Aggregate
( VMA)
The volume of voids as a percent of the entire specimen
between the aggregate particles in a compacted asphalt
concrete mix, including air voids and effective ( but not
absorbed) binder.
Wet process
Modification of asphalt cement by blending crumb rubber into
the cement through agitation. The rubber reacts through
absorption of aromatic oils from the asphalt cement into the
polymer chains of the rubber. Typically a set mixing or
agitation time is required for “ digestion.”
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1.0 BACKGROUND AND OVERVIEW
The Asphalt Rubber Pavement Conference ( AR2006) held in Cathedral City, California from October 25–
27, 2006, provided a forum for the world’s leading experts on asphalt rubber in government, industry, and
academia to present information and recommendations on innovations in the use of this material.
Increased use of crumb rubber is mandated in California under Assembly Bill 338. This conference was
sponsored by the Rubber Pavement Association ( RPA), the California Department of Transportation
( Caltrans), and other organizations.
One of the driving forces behind AR2006 was the Caltrans Office of Pavement Preservation
( OPP) in the Headquarters Division of Maintenance, which had identified a need to scan available
practices and new technologies for the use of asphalt rubber in pavements. As part of this effort, OPP
asked the University of California Pavement Research Center ( UCPRC) to attend AR2006 and to provide
services as outlined in an agreed- upon work plan ( Work Plan for: “ Summary and Recommendations
Toward Implementing Innovations Based on Selected Presentations at the Asphalt Rubber Pavement
Conference [ AR2006], October 2006”). Services provided by the UCPRC focused on reviewing papers,
attending the conference ( talking with authors and other participants), and preparing a summary and
recommendations for OPP. These services are funded largely under Task Order 1018 between the
Caltrans Division of Research and Innovation ( DRI) and the California Center for Innovative
Transportation ( CCIT). Some additional funding was provided under Strategic Plan Element 2.3 of the
UCPRC’s contract with DRI.
The focus of this document is to provide an independent and objective summary and a set of
recommendations to Caltrans on implementable— as well as promising— innovations for further use of
asphalt rubber based upon selected presentations at AR2006. Of key interest is highlighting those
innovations and recommendations that:
( i) Are implementable,
( ii) Need further development, and
( iii) Need further research.
Specifically implementable techniques or materials are those that may be readily utilized by
Caltrans with minimal effort. It is important to keep in mind that many of the implementable techniques
recommended in this summary are already being used by Caltrans at some level. Innovations that need
further development are those that have shown promise in California or other states, but are not fully
proven in the judgments of the authors. Those concepts identified as needing further research are
promising and novel research ideas that are in the early stages and with additional work may be very
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effective. A fourth category that reviewers called “ general information,” was identified after reviewing
the papers ( which were not available before the conference due to copyright concerns).
The professional judgment of the UCPRC reviewers serves as the basis of the comments made in
this paper. Their review revealed that seven of the papers presented have direct implementable content for
Caltrans, seven of the papers contain ideas that with further development could be utilized, and the ideas
in the rest of the papers need further research or provided general information. All the papers have merit,
but the content and processes they discuss are in various stages of progress. Where appropriate, details of
these papers are highlighted.
This summary may be considered a stand- alone document. However, it may also be used in
conjunction with the DVD from the conference ( which the Rubber Pavements Association expects to
distribute to Caltrans in February 2007) for further detail and education. It is recommended that those
interested in specific papers or information read the official proceedings from the conference.
Asphalt Rubber ( AR) is defined by ASTM as a material that contains a minimum of 15%
reclaimed tire rubber. California in its specifications, calls for 20+/- 2% crumb rubber modifier by total
binder mass ( includes 75% CRM and 25% HNR). The type and size of rubber particles as well as method
of manufacture are also defined. The State of Arizona has successfully used AR in its pavement
surfacings for several years. The Arizona Department of Transportation ( ADOT) attributes its use of thin
AR overlays for noise reduction for the entire highway network in Phoenix because of its confidence in
the material. The State of California has also had good success with AR and over a longer period of time
( beginning in the late 1970s). Texas, Florida, and other states have also had experience with AR. Many
of the papers presented in this conference were based upon work by or for ADOT and by or for Caltrans.
Although AR is the most widely used rubber modified asphalt pavement system ( used in chip
seals and mixes), there are many other civil engineering uses for scrap tire rubber ( some of these were not
included in the conference). For example, small particle size rubber used in terminal blend binders
shows significant promise and allows for the use of rubber in dense graded asphalt mixes. Scrap rubber
shreds have been used as embankment fill for landfill projects, particularly on weak compressive soils due
to the lightweight nature of the material. Again, due to the lightweight nature of the material, scrap
rubber has been used as backfill for retaining walls and bridge abutments, yielding thinner less costly wall
construction projects. Scrap rubber has also been used for drainage backfill as well as for numerous
surfacing applications.
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2.0 REVIEW OF PAPERS FROM TECHNICAL SESSIONS AND APPLICATION
TRACK
2.1 Review and Discussion of Technical Session Papers
During the course of the 3- day AR2006 conference, UCPRC staff reviewed the papers, attended the
sessions, and spoke with authors. For this document, the 34 papers that were reviewed have been divided
into sections that closely align with the sessions held at the conference.
• General Usage and Applications
• Pavement Performance and Design
• Recycling and Environmental Aspects
• Rehabilitation and Noise Reduction
• Mix Properties
• Binder Properties
Pertinent papers from each of these topics are presented in this section. Implementation
recommendations for selected papers are presented where pertinent. Papers noted to be readily
implementable are denoted with an asterisk (*).
2.1.1 General Usage and Applications
35 Years of Asphalt- Rubber Use in Arizona
Authors: A. Zareh, G. B. Way
The emphasis of this presentation and paper was on the use of asphalt rubber ( AR) in Arizona and how
the authors have assessed its success. When using crumb rubber in asphalt binders, Mr. Way stated the
importance of using base binder 1 to 2 grades softer than that used in conventional asphalt concrete
pavement. For example if 76- 16 is needed, then use 64- 16 as the base stock for the AR. The authors felt
that AR provided good skid resistance and lower maintenance costs. From a graph in the paper,
maintenance costs for AR and conventional dense graded mix were comparable for the first 5 years,
however after that the dense graded mix costs rose considerably faster than the AR costs. Also shown are
results from Mu Meter skid testing. AR performed better than HMA initially and over the life of the
pavements tested. However, both AR and HMA showed good skid resistance test values. An additional
benefit of this material mentioned by the speaker is that CO2 emission studies have shown that lower rates
of CO2 are emitted into the atmosphere from the use of AR. The lower CO2 emissions could be from the
use of lower quantities of aggregates, but the paper makes no specific reference to this claim.
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The Urban Heat Island Effect and Impact of AR- ACFC Overlays on PCC Pavements
Authors: M. Belshe, K. Kaloush, J. Golden
The authors cite the importance of minimizing the urban heat island effect and the part that AR plays in it.
In using the darker AR- OGFC there is an increased surface temperature during the daytime, but reduced
surface temperature at night time ( explained by the porosity and lower thermal mass). Traffic is felt to
have an aeration effect and the relative coolness of the pavement is due to the high air- void content and
the lower thermal mass of the surface mix. Without the AR surfacing, the PCC surface would be 3– 8 º F
higher. The authors believe this increased temperature would result in increased truck and
warping/ curling damage due to greater slab stresses than would be experienced without the overlay.
While the insulating effects of overlays on PCC is not a new concept, the idea that the AR surface could
provide such benefits is fairly dramatic. The increase in PCC lifetime is unknown and this is a complex
issue that could use further research. In conducting this research, the authors deployed a network of
“ iButton” thermal sensors to record pavement temperatures. Caltrans may want to follow up with these
researchers for further conclusions on this work.
Influence of Temperature Variation on the Reflective Cracking Behaviour of Asphalt
Overlays
Authors: M. Minhoto, J. Pais, P. Periera
The paper evaluated the performance of AR mix vs. conventional AC mix in fatigue using finite element
method ( FEM) modeling considering primarily temperature but also traffic loading. It was found that the
difference in damage rate between AR and conventional AC mix seems to be larger in winter than in
summer, due to the reduced stiffness of the AR and more closed joints in the lower pavement layer.
Further research is needed because it is not apparent whether this is a cyclic/ dynamic loading situation or
just static loading at different temperatures. There may be other questions about the model, as well.
Asphalt Rubber in New Pavement Construction and Design
Authors: J. Sousa, G. Way, S. Shatnawi, C. Dantas
This paper advocates the use of asphalt rubber in new pavements and that, as designers, we are not taking
proper advantage of these materials. The authors emphasize the need to develop new design
methodologies ( based on minimizing reflective cracking) and laboratory tests for AR since material
performance is not well characterized by traditional models, such as fatigue models or in tests such as the
Marshall test.
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When designing new pavements with an AR surface layer, it is usually necessary to provide
sound base support to control the strain and use AR as the shield to stop cracks from propagating to the
surface. However traditional design for new pavements uses strain at the bottom of the underlying AC
layer to control fatigue and strain at the top of subgrade to control subgrade rutting. This usually leads to
very thick AC layers. However, since thin AR overlays in Arizona perform very well in both fatigue and
rutting, it is necessary to have a better way to incorporate the AR surface layer into the design. One way
is to convert the AR surface layer into an equivalent conventional AC layer. Another alternative is to
design the AR surface as an overlay and assume the underlying AC will crack soon. Again, AZ does not
allow reduced thickness when using AR mixes.
Evaluation of Tire Wear Emissions, Roughness and Friction Characteristics of Asphalt
Rubber Friction Course Pavements
Authors: O. Alexandrova, J. Allen, K. Kaloush
This paper evaluated tire wear emissions from portland cement concrete ( PCC) and Asphalt Rubber–
Asphalt Concrete Friction Course ( AR- ACFC), which was overlaid on the same PCC, surfaces within the
Deck Park Tunnel in Phoenix, Arizona. Vehicle emissions of PM10 and PM2.5 particulates were
measured as they have been found to affect human health. It was found that AR- ACFC has higher
friction and lower International Roughness Index ( IRI) than PCC, but resulted in less tire wear emissions.
* Multi- Layer Pavement Strategies Utilizing Asphalt Rubber Binder
Author: J. Van Kirk
As the title suggests, this paper looks at the use of multi- layer systems to produce long- life rehabilitation
projects with the use of asphalt rubber binder. The author has found that an AR SAMI interlayer
followed by an AR gap- graded overlay has provided good performance over badly distressed pavements.
In combination with conventional asphalt concrete leveling course the performance is further enhanced.
In the presentation the author mentioned that this added performance was at least in part due to the
waterproofing benefit of the SAMI. The base materials studied in this paper did not break down as much
as expected because they had been kept relatively dry.
The SAMI suggested by the author contains 9.5 mm chips with 2- 3 l/ m2 of binder, which is
considerably higher than conventional chip binder spread rate of 1.4- 1.8 l/ m2. The author claims
significant cost savings to agencies along with the potential benefit of minimizing grade and height
problems, commonly encountered with thick conventional overlays. Two examples are cited in the paper
where the use of multi- layer strategies saved the client 40% over the cost of a conventional treatment.
The author provides references to previous papers where he discusses potential cost savings in more
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detail. While these savings are substantial, it is helpful to look at multi- generational costs when making
comparison between alternatives. This strategy may still be less expensive, but a longer term analysis
would verify this.
Implementable Items
• Use of this design strategy. This pavement strategy has been shown to perform well in lower
volume, lower truck traffic roadways provided that proper construction is ensured. Adequate
construction controls and monitoring are required for success.
2.1.2 Pavement Performance and Design
Asphalt Rubber Mixtures in Portugal: Practical Application and Performance
Authors: M. De Lurdes Antunes, F. Batista, P. Fonseca
The authors studied the performance of field and laboratory produced wet process AR specimens. Four
point bending beam tests along with rolling wheel rutting tests were performed. The findings indicate
that significantly higher strain levels are allowable for a given number of cycles to failure than in
conventional asphalt mixes. It was found that the AR stiffness increased significantly during the first six
years of service and that the authors’ laboratory aging process was able to mimic this performance. The
authors felt that both the new and aged AR specimens performed better than conventional mixtures. With
respect to rutting, AR deformation resistance increased with aging, as expected with the binder stiffening.
In constructing these pavements, it is suggested that the compaction equipment closely follow
laydown as these materials cool quickly and that strict compaction temperature guidelines be followed.
Asphalt plants using this technique must be specially adapted to control the reaction between the asphalt
and the crumb rubber and this reaction must be kept under strict control.
Performance of a Test Track Using Crumb Rubber Asphalt and Other Modifiers
Authors: G. Martinez, B. Caicedo, D. Gonzalez, L. Celis
This paper looks at ways to modify Colombian asphalt binders with asphalt rubber techniques. Test track
studies were performed using wheel loading ( 1.5E6 ESALS [ 80kN]). It was found that while the dry
process performed better in the laboratory, its field performance was not comparable to the wet process.
According to the authors, the wet process “ presented huge advantages” compared to unmodified mixtures
and had “ better performance than the commercial polymer modified mixes with SBS and SBR.”
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Structural Design Considerations for Rubberized Asphalt Mixes
Authors: T. Bressette, H. Zhou, S. B. Seeds, R. G. Hicks
This study examined performance and design issues regarding conventional and AR mixtures through
modeling numerous structural sections with mechanistic techniques. Well established fatigue ( University
of California) and rutting ( Asphalt Institute) models were used to determine appropriate strains and
performance predictions. It was found that for low traffic index ( TI) pavements, RAC- G fatigue
performance was better than DGAC regardless of subgrade soil strength. For higher traffic levels ( TI > 8),
DGAC mixes outperformed RAC- G mixes in terms of fatigue and rutting. For structural overlays, the
structural benefit of the RAC overlay varies with thickness. RAC- G overlays provide the most benefit in
thin layers and Caltrans recommendations regarding reduced thickness are valid. For new designs, the
authors recommend that a structural design be developed that is necessary to provide adequate cover and
that thickness not be reduced when using asphalt rubber mixes.
It should be noted that UCPRC provided the AR and conventional beam fatigue equations
( repetitions to 50% loss of stiffness) to the authors and reviewed the results.
Conventional ME analysis using fatigue failure criteria of 50% loss of stiffness was used in this
paper along with a converted shift factor to account for crack propagation and boundary conditions.
There were strong doubts from some conference participants whether the approach used captures the full
benefits of AR performance. Questions were asked whether this approach ignored the crack propagation
benefits of AR. This suggests that additional research should be done to understand the differences in
crack propagation between conventional asphalt and AR.
Laboratory Characterization and Full- Scale Accelerated Performance Testing of Crumb
Rubber Asphalts and Other Modified Asphalt Systems
Authors: X. Qi, A. Shenoy, G. Al- Khateeb, T. Arnold, N. Gibson, J. Youtcheff, T. Harman
This paper tries to relate laboratory performance of binder and mix to rutting and fatigue performance of
different mixes in Accelerated Loading Facility ( ALF) tests. The purpose is to find a performance test to
help characterize AR binder. No SHRP binder test was found that gives good correlation to rutting or
fatigue performance in ALF tests.
ALF test results showed that the AR section provided the largest number of cycles to failure of all
12 sections tested. The authors also note that the AR section had a significantly stiffer base layer
supporting it than the other test sections. Therefore, a concern regarding this work is that fatigue cracking
is not only determined by mix property, but structural capacity as well. The authors state that it is difficult
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and possibly unwise to try to find a direct correlation between mix property and ALF fatigue cracking
performance without considering the in- situ strain. Again, AR tends to defy or at least stretch many of
the conventional “ rules” of pavement performance, so further research in this regard is relevant. It was
also felt that healing in the modified binder seems to be significant because of the higher binder content
used.
2.1.3 Recycling and Environmental Aspects
* Study and Application of Asphalt Rubber and Rubberized Pavement in China
Authors: H. Wen- Yuan, C. Yung- Chieh, X. Li- Ting
This paper summarizes trends of using AR mixtures in China, where the authors used the combination of
conventional and AR mixes. For an existing PCC pavement or CTB base, an AR- SAMI can be placed
first to prevent water infiltration, then followed by a conventional DGAC structure layer, with an AR
friction layer at the top.
In the dry process, the authors found great success with the use of very fine rubber powder ( 40,
80, 120 mesh), even on high traffic roads. The rubber proportion used was 20 percent of the binder or 1
percent of the total mix by weight. Based on their observations of 30 km of ( mostly) dry process
pavement they suggest that this very fine material allows for better reaction with asphalt and hence better
compaction properties, mix stability, and integrity.
Implementable Items
• Use of fine rubber powder in dry process with less than 1 percent by weight of total mix after
validation with materials available in California.
• Use of thin AR overlay over a conventional structural overlay on Rubber SAMI to minimize
reflection cracking.
• Use of precoated chips.
Energy Consumption of Alternative Scrap Tire Uses
Authors: J. Sousa, G. B., Way, D. Carlson
The authors state that using scrap tires in asphalt rubber is by far the most beneficial method of reuse to
society in terms of energy savings, when compared to alternate daily cover ( ADC) and tire derived fuel.
The authors compared these three end uses and indicated potential energy savings gained by using
granulated tire rubber as a modifier to asphalt pavement. The speaker cited the potential for
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$ 200,000,000 in annual cost savings should the entire network ( 11,000 lane miles) be rehabilitated with
AR. The author states that this savings is equivalent to the entire Caltrans yearly Maintenance budget.
Taiwan’s Experience on Asphalt Rubber Pavement
Authors: C. Chiu, C. Pan
This paper presents experiences in Taiwan using Arizona DOT ( ADOT) gap- graded and open- graded AR
mixes for flexible pavement rehabilitation. The climate for this area was high temperatures, no freeze, and
significant summer rain, leading to concerns about moisture damage. The AR- GG mix had as- built air-void
content of around 4 percent, which is much lower than that of conventional mix in the control
section due to differences in compaction controls. The mix contained 1 percent portland cement as
mineral filler. Overall material cost was 30 percent higher than conventional asphalt concrete. After 42
to 46 months, the AR mixes are performing well with insignificant pop- off for AR- GG and some oil
spotting ( bleeding) for AR- OG. Future plans for use of AR will be based upon political decisions.
2.1.4 Rehabilitation and Noise Reduction
* Asphalt Rubber Chip Seal Evaluation in California ( scheduled but not presented)
Authors: L. Rouen, B. Toepfer
This paper first presented findings from a review of five AR chip seal projects that had exhibited bleeding,
primarily in the number two truck lane, then discussed field validation of these findings. It was
concluded that the following factors may have caused the bleeding: high truck volume, high binder spread
rate in wheelpaths of truck lanes, aggregate embedment into the underlying pavement, and highway
geometry, notably tight curves and steep grades, which lead to vehicle acceleration and deceleration.
The use of extender oil was also found to increase bleeding in hot areas.
Implementable Items
Recommendations taken directly from this paper for changes in AR chip seal practice include:
• Use more uniformly graded aggregate,
• Use PG70- 10 asphalt in high temperatures,
• Reduce or eliminate the use of extender oil,
• Establish a maximum ADT for chip seal applications,
• Specify that contractors use a variable rate spreader bar that can be adjusted to apply binder at a
reduced rate in the wheelpath areas.
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* Asphalt- Rubber Gap Graded Mix Design Concepts
Authors: J. Sousa, G. B. Way, A. Zareh
This paper presented the concepts and evolution of gap- graded mix designs for AR in Arizona. Arizona
went from Hveem designs in the 1960s to Marshall designs from the 1980s to the 2000s. The speaker
cited the balancing act that mix design entails – balancing the two opposing forces of cracking and rutting
along with mechanical damage such as raveling. Arizona had experimented with open- graded friction
course ( OGFC) mixes in the 1950s and 1960s. Inconsistent performance of these mixes and their design
being an “ art form” led to the idea of using a new mix type. It was found that OGFCs were prone to drain
down at high temperatures as well as raveling.
AR does not drain down, but these mixes need good quality aggregate to avoid rutting potential.
Consequently, gap- graded mixes were developed. These mixes contain higher binder contents ( 7%- 8%)
than conventional mixes and have high VMA ( minimum 19%), but not consistently high stabilities. The
authors are confident in the performance of this mix and it has been used throughout the State of Arizona
to overlay their interstate system.
Implementable Items
• ARFC ( open- graded) and ARAC ( gap- graded) mixes have shown good success in Arizona. They
are also used in California.
• California uses Type II binder in their mixes and the use of these gap- graded mixes is
recommended.
Tire/ Road Noise Relationship with Viscoelastic Properties of Asphalt Mixtures
Authors: K. Biligiri, A. Sotil, K. Kaloush, J. Golden
The objective of the study was to analyze the dynamic modulus test parameters for dense- graded
conventional asphalt and asphalt rubber ( AR) mixtures to assess if any of the parameters are indicative of
the mix’s tire/ road noise characteristics. The paper examined dynamic modulus ( E*), phase angle ( φ), 
relationship between E* and φ, use of phase angle as a noise discriminator, relationship between E’
( storage modulus) and E’’ ( loss modulus), and correlation of phase angle and highway noise sound
intensity. It was found that the phase angle is potentially a noise discriminate factor between DGAC and
open- graded/ gap- graded asphalt rubber mixtures. The average peak phase angles for each mix type
correlated well with the sound intensity measured for each mix/ surface type.
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* Asphalt- Rubber Open Graded Mix Reduces Tire Pavement Noise
Authors: A. Zareh, G. Way, K. Kaloush
The State of Arizona has developed a Quiet Pavement Program where conventional AC and PCC surfaces
are overlaid by a 25 mm ARFC. The ARFC was found to reduce noise by 3 to 5 dB compared to DGAC,
and by 6 to 12 dB compared to PCC pavement. It was decided to overlay around 11,000 lane- km of PCC
pavement with 25 mm of ARFC to reduce noise. The authors feel this program has been very successful.
Implementable Items
• Mix corresponding to ADOT ARFC, RAC- O has been used as part of Caltrans Quieter
Pavement Program.
Performance Evaluation of Asphalt Rubber and Its Use in Old Concrete Pavement
Overlays
Authors: R. Cao, Z. Guo, Q. Bai ( presented by J. Sousa)
In this paper, the wet- process production method for asphalt rubber is described. The use of 10 mm
Asphalt Rubber Stress Absorbing Membrane Interlayers ( AR- SAMI) layers were studied in laboratory
fatigue and shear as well as in field conditions. Mix designs were produced with the SHRP ( Superpave)
Dynamic Shear Rheometer ( DSR) and Bending Beam Rheometer ( BBR) tests to select the AR binder.
However, SuperPave binder tests may not be a good indicator for mix performance, as shown in the paper
by Xicheng Qi. According to Superpave binder test results, CRM from truck tires is better than car tires,
the # 20 particle size is better than # 40 and # 60, and Pen 70 is better than Pen 90. Recommended levels of
CRM are between 16% and 18%. From the field experience of 10 months, the asphalt rubber has
outperformed conventional mixes when used as overlays on old concrete pavements. This strategy, used
in Arizona, could potentially be used elsewhere after further validation.
* An Update on the Asphalt Rubber Pavement Preservation Strategies Used in California
Authors: S. Shatnawi, A. Stonex, R. G. Hicks
This paper focused on asphalt rubber chip seals, asphalt rubber hot mixes ( both open- graded and gap-graded
surface mixtures), and thin bonded overlay as preservation strategies. Specifically of interest were
strategies utilizing wet process, Type II binder materials. The paper also emphasized specifications, mix
design, material requirements, job selection criteria, and application parameters.
While the specifics of these techniques are beyond the scope of this document, a few highlights
are discussed here. These changes are also being incorporated into the new Caltrans specifications. The
materials and systems identified were asphalt rubber chip seals, composed of asphalt rubber binder with
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0.55– 0.65 g/ sy binder and chips spread at 28– 40 lb/ sy. It is recommended these chips be precoated to
minimize dust problems and enhance binder adhesion.
Asphalt rubber hot mixes with both RAC- O and RAC- G surfaces have shown good success in the
state. Increased performance of Rubberized Asphalt Concrete – Open- graded ( RAC- O) and Rubberized
Asphalt Concrete – Open- graded – High Binder ( RAC- O- HB) is due to the higher level of binder present
in these mixes than in conventional RAC mixes. RAC- O has 20% more binder and RAC- O- HB has 65%
more binder than conventional, leading to thicker films and added durability. Open- graded mixes can last
10 years, but the noise benefits may dissipate with time. There is the additional benefit of rumble factor
effects in that the noise emitted from the pavements is lower in frequency if the higher frequencies are
absorbed by the pavements. It is mentioned that sound walls can reflect noise to up to 1 mile away and
that removing noise at the source is helpful.
Caltrans has seen good performance with RAC- G surfaces as evidenced by studies that looked at
the performance history of over 200 RAC- G projects in California. These surfaces were constructed with
50 percent thickness reduction compared to conventional asphalt concrete. However, delamination was
reported in a recent project constructed in nighttime hours.
Thin bonded overlays using AR materials have been used successfully in California. To
construct these systems, a heavy polymer modified emulsion is used as a tack coat to foster adhesion of
the polymer modified gap- graded or open- graded hot mix. In order to use RAC- G as the surfacing
however, further development is required.
Implementable Items
• Changes to the wet process specification
• Potentially expanded use of RAC- O and RAC- G as surfacings
• Chip seal specification modifications
Items for Further Development
• Use of RAC- G in thin bonded overlays.
• Requirement for developing an asphalt rubber binder design profile that includes results of tests
on samples obtained at designated intervals over a 24- hour period to establish specification
compliance.
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2.1.5 Mix Properties
Effect of Crumb Rubber on the Aging of Asphalt Binders
Authors: S. Lee, S. Amirkhanian, K. Shatanawi
The author used gel- permeation chromatography ( GPC) to detect the molecular size distribution change
of CRM binders and unmodified binders before and after aging. Based on the change in large molecular
size ( LMS), virgin binder and rubber modified binders were found to have very similar aging levels. In
binders with rubber modification, the rubber absorbed the lower molecular weight maltenes leaving the
residual asphalt binder containing a higher proportion of the heavier weight asphaltenes. This result
requires more validation so the authors can generalize the results of this study to various rubber particle
sizes and binder sources.
* Laboratory Evaluation of Asphalt Rubber Modified Mixes
Authors: M. C. Cook, T. Bressette, S. Holikatti, H. Zhou, R. G. Hicks
Rutting and fatigue performance of asphalt rubber modified mixes were evaluated using SST ( AASHTO
T320), flexural beam bending fatigue ( AASHTO T321), and Hamburg Wheel Track Test ( AASHTO
T324).
Mixes used include:
• Rubberized Asphalt Concrete – Gap- graded ( RAC- G),
• Rubber Modified Asphalt Concrete – Gap- graded ( RUMAC- GG),
• Modified Binder – Gap- graded ( MB- G),
• Modified Binder – Dense- graded ( MB- D), and
• Conventional Dense- graded Asphalt Concrete ( DGAC).
It was found that all asphalt rubber modified mixes except for the MB- G and MB- D mixes in the
Hamburg Wheel Track Device performed at least equally well as if not better than the conventional
DGAC mix.
Implementable Item
• Asphalt rubber modified mixes, including RAC- G, RUMAC- GG, MG- G, and MB- D should
continue to be used in applications that are most cost- effective.
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Effect of Compaction Temperature on Rubberized Asphalt Mixes
Authors: S. Lee, S. Amirkhanian, K. Shatanawi
It was found in this laboratory experiment that gyratory compaction temperatures used ( 116° C, 135° C,
154° C, and 173° C) significantly affected the volumetric properties of the rubberized mixtures, but not
those of conventional mixtures with virgin and SBS- modified binders. It is recommended to conduct
another study with higher rubber percentages ( greater than 15%) and various crumb rubber sizes to
evaluate the mechanical properties of rubberized mixes and correlate the volumetric properties of the
mixes at different compaction temperatures. As shown by other authors in this conference, an adequate
and sustained temperature level is imperative for proper compaction.
Mix Design Procedure for Crumb Rubber Modified Hot Mix Asphalt Concrete
Authors: V. Swami, V. Tandon
The paper dealt with evaluating and modifying the existing Tex- 232- F procedure and determining its
suitability for performance testing of CRM- HMAC. The paper also identified problems and provided
solutions to the current procedure ( Tex- 232- F) and identified the influence of compaction devices.
Currently, the state specifies Static Creep Test and Hamburg Wheel Tracking Device to evaluate such
mixtures. Differences in compaction procedures, laboratory- versus plant- prepared mixes, and plant-versus
plant- produced mixes have shown varying performance. It was found that in comparing
specimens produced from the SGC and TGC, that the TGC did show signs of influence on specimen
performance based upon increased compaction temperature. Specifically, different mixes may not meet
the acceptance criterion for static creep testing; however, these mixes may meet the requirements
specified by the TxDot specification with the Hamburg Wheel Tracking Device. The authors recommend
further research in this area.
Predictive Equations to Evaluate Thermal Fracture of Asphalt Rubber Mixtures
Authors: A. Zborowski, K. Kaloush
The existing thermal cracking model in AASHTO Mechanistic Empirical Pavement Design Guide
( MEPDG) using tensile strength was found to underestimate the crack resistance of AR mixtures in
Arizona. While the AR has lower tensile strength it possesses a much greater ductility than conventional.
This added ductility offsets the lower tensile strength and results in better long- term thermal cracking
performance in the field. It was recommended to use total fracture energy and initial creep compliance in
addition to tensile strength and the slope of the creep compliance to predict thermal cracking. The IDT
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was modified so that the additional parameters can be measured. Subsequent research will focus on
developing a new thermal cracking model.
Performance of Wet Process Method Alternatives: Terminal or Continuous Blend
Authors: L. Fontes, P. Periera, J. Pais, G. Triches
Binders in wet process ( called “ continuous blend” in this paper) had 21% CRM, while binders in terminal
blend had 15% or 20% CRM. In contrast to mixtures produced in wet process, it was found that asphalt-rubber
mixtures produced with terminal blend process presented better rutting performance but worse
fatigue performance. More study of terminal blends is recommended.
Mechanical Properties of Hot Mixture Asphalts with Crumb Rubber and Other Modifiers
Authors: G. Martinez, B. Caicedo, D. Gonzalez, L. Celis
The paper reports laboratory performance of asphalt rubber mixtures using Colombian asphalt binder.
Compared to polymer modified binder ( PMB) mixture, AR mixtures showed the same performance in
fatigue and rutting, but with less moisture susceptibility. One exception was that AR mixture produced
with dry process was less resistant to fatigue than the mixture made with unmodified binder. These
findings are not consistent with the other papers in the conference.
2.1.6 Binder Properties
Influence of Crumb Rubber Gradation on Asphalt- Rubber Properties
Authors: S. A. Dantas Neto, M. Farias, J. Pais, P. A. A. Pereira
Crumb rubber gradation can have a tremendous influence on the performance of asphalt rubber mixture
performance. In this paper, the authors studied this issue and found that by using smaller particles with
higher specific surface, increases in rotational viscosity, softening point, and resilience of the samples
were noted. These smaller particles allowed for greater contact area between the rubber particles and the
lighter fraction of the binders, leading to greater absorption. Larger particles may also lead to problems in
the field likely due to compaction difficulties. While not cited in the paper, the author discussed the use
of rubber from truck tires, which contains more natural rubber and interacts better with the asphalt binder.
Optimizing the Use of Crumb Rubber by Binary Modification
Authors: H. Ajideh, B. Burris, H. Bahia
The proprietary method of modification described by the authors has shown increases in performance life
of crumb rubber modified binders. This very specific oxidation technique adjusts the molecular structure
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of the asphalt, allowing a reduction in the amount of rubber required to achieve high performance
properties. The authors have found that binary modification of the asphalt under repeated cyclic loading
with the DSR does not change the short- term aging properties but it does increase the binder fatigue life
significantly. Also noted was the decreased permanent strain at 100 cycles for binary modified mixes
versus mixes with no additives. The speaker noted that a 76- 22 binary mix “ feels like” a 58- x mix. This
technique allows for the use of 2- 5% rubber in the binder.
There were concerns from some conference participants about the techniques and data presented
in this paper. The fact that such very low concentrations of rubber apparently produced such significant
performance gains was questioned. While this technique shows potential, the authors do not yet have
supporting field data. Further research into this technique is recommended.
Laboratory Investigation of Dimensional Changes of Crumb Rubber Reacting with
Asphalt Binder
Authors: F. Xiao, B. J. Putman, S. Amirkhanian
This paper presents the study on permanent physical dimensional changes of CRM particles after
blending with asphalt binder. It was found that blending led to decreases in both size and mass of CRM
particles, and liberation of short fibers and filler materials included in the rubber compounds. This is
attributed to the breakdown, or depolymerization of the rubber particles in hot asphalt binder.
Crumb Rubber Modification of Binders: Interaction and Particle Effects
Authors: B. J. Putman, S. N. Amirkhanian
The authors explain how crumb rubber modifiers interact with the asphalt binders through a two- fold
process: rubber particle swelling and the removal of light fractions of binder. Tests were performed using
a modified rotational viscometer ( with larger plate spacing) and shear rheometer to establish these
interaction effects. With increased rubber contents regardless of size, viscosity increased. For a given
rubber content, smaller sized rubber particles showed higher viscosities than larger particles.
The authors referred to the binders being “ drained” of their crumb rubber content ( crumb rubber
was extracted from binder) and compared them with unmodified binders. It is reported that the crumb
rubber particles absorb some of the lighter fractions of the binders due to the particle- binder interaction.
This is evidenced by increased viscosity of drained versus unmodified binders. The authors called this
phenomenon the “ interaction effect” ( IE). It was realized that the IE alone could not account for the
increased viscosity of modified binders, but a “ particle effect” ( PE) was occurring as well. The PE
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represented the filler nature of the crumb rubber particles, so the higher the crumb rubber content, the
higher the PE was found to be.
Creep Flow Behavior of Asphalt Rubber Binder: The Zero- Shear Viscosity Analysis ( ZSV)
Authors: F. Guiliani, F. Merusi, I. Antunes
This paper pointed out the different creep behaviors demonstrated by binders modified with various
amounts of CRM. The test was able to distinguish between traditional binders and rubber modified
binders through a difference in the ZSV numeric value. However, measuring ZSV for binders with 20
percent of CRM required some approximation because they show non- Newtonian creep flow. This is a
promising technique and can potentially be used to optimize the percentage of CRM in binders.
Role of Polyphosphoric Acid in the Transition from Asphalt Rubber to Terminal Blend of
Rubberized Asphalt
Authors: I. Antunes, F. Guiliani
The authors start this paper by noting the ease of production of terminal blend binders compared to
asphalt rubber binders. They focus their work on whether it is possible to make terminal blend binders
that show performance comparable to asphalt rubber binders. The terminal blend binders possess a
smaller fraction of rubber ( 10%) plus extender oil and perhaps additives such as SBS. The authors found,
for a given rubber content, adding Polyphosphoric Acid ( PPA) resulted in higher viscosity, resilience, and
softening point but lower penetration values. The authors found that the inclusion of PPA in terminal
blends, while providing a significant improvement in performance over conventional asphalt concrete, it
did not measure up to that of asphalt rubber. While the addition of the PPA modifier shows promise, the
authors caution about the potential hazards and constraints of working with this material.
Aging Studies of Asphalt- Rubber Binder ( scheduled but not presented)
Authors: C. Lima, L. Tome, C. Filho, S. Soares, J. Soares
The authors use a DSR and a Fourier Transform Infrared ( FTIR) spectrometer to study the aging of
unmodified and rubber- modified binder, considering the effect of extender oil. It was found that CRM
acted as an anti- oxidant in the aging process, perhaps due to the carbon black added to the rubber. The
extender oil seemed to reduce aging by reacting with the asphalt rubber binder. DSR results showed that
AR binders have better fatigue performance than unmodified binder.
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2.2 Application Track ( Practical Sessions) Overview
The Application Track at the Asphalt Rubber Conference contained several presentations on the practical
aspects of asphalt rubber systems. The UCPRC staff attended these sessions as well as the tour that
included stops at Granite’s mining and crushing operations, hot mix facilities, and asphalt rubber project
sites. No written documentation from the practical program was available to UCPRC staff to review
following the conference.
Some of the presentations in the Application Track were promotional in nature, but most offered
useful information on the experiences of contractors and agencies involved in the construction and
performance of asphalt rubber systems. Helpful information on scrap tire processing as well as RAC
grants and grant applications was also presented. In the opinion of the UCPRC staff, the most significant
contributions were the presentations by:
• Anne Stonex of MACTEC on “ Asphalt Rubber Binder and Mix Design and Testing”
• Jordan Main of Granite Construction on “ Emission Studies”
• Terrie Bressette of Caltrans on “ Caltrans Asphalt Rubber Design Guide”
• Jack Van Kirk of Basic Resources on “ Long Term Performance”
• Bruce Rymer of Caltrans on “ Noise Reduction”
• K. Brian Rickey of Sully- Miller Contracting Co. on “ Recyclability”
It is recommended that the reader view these presentations on the AR2006 DVD, released by the
RPA in February 2007, for complete details. Brief summaries of these selected presentations are given
below. Specific implementable items are marked with an asterisk *.
Asphalt Rubber Binder, Mix Design and Testing
Anne Stonex, MACTEC
This presentation focused primarily on general information and best practices for using CRM modified
materials. Selected items of importance based upon her experience include:
• * In field conditions it is essential not to directly substitute a no- agitation binder for a high
viscosity binder in RAC due their different viscosities and behavior. The general appearance
difference, described as “ tapioca” versus smooth should be readily recognized by inspectors and
field personnel. Proper training of these individuals should be conducted.
• Caltrans is updating Section 39 to include RAC- G with high viscosity asphalt rubber binders to
facilitate information retrieval.
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• * Do not use a modified asphalt cement as a base for CRM mixtures.
• With submittals it is important to gather as much information as possible, particularly if there are
field discrepancies.
• * It is recommended that RAC grant applicants obtain the Binder Design Profile which ensures
that the binder has been tested for 24 hour stability. This is very important should there be
unforeseen field delays.
• With the binder profile, it is important not to design near the limits because it is as likely as not to
be outside these limits when produced.
• * Viscosity is the “ go – no go” test in the field.
• * Resilience is a very good way of determining how well the material will perform, particularly in
regard to fatigue and reflection cracking concerns.
Emission Studies
Jordan Main, Granite Construction
Concerns over emission air quality from RAC projects prompted a recent small sample study in Caltrans
District 1. Relevant findings in their comparison of rubber modified mixes with conventional mixes are:
• Overall there is no significant difference between the emissions from RAC projects and
conventional projects.
• Many contaminants and parameters were evaluated including particulate matter, PAH, VOC,
NOx, CO, O2, CO2, stack flow rates, and moisture contents.
• In general, for the four mixes tested ( DGAC, RAC- G, Type D MB, and RUMAC), all had much
lower emissions than required by AP- 42 ( standard emission factors).
• Specifically, however, benzene, which is highly toxic, showed an emission level for wet process
materials twice that recommended in AP- 42 and roughly 1.5 times that of the conventional
process. A couple of explanations for this high level include that a diesel fueled plant wasn’t
fully combusting in the dryer and that the wet process mix was the only one to use asphalt
extender oil with high volatile content. However, the presenter stated that the commenters from
the local air districts were not concerned with the levels measured.
• When comparing odor production from the four materials, it was found that the odor was “ not
significantly stronger than conventional asphalt production” and was most prevalent near loaded
haul trucks and load out areas.
• Results may be viewed in the Lake County Air Quality Management District ( LCAQMD)
Summary Report dated 10/ 11/ 06.
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Rubber in Reclaimed Asphalt Pavement
K. Brian Rickey, Sully- Miller Contracting Co.
The presenter discussed the use of recycled rubber materials in future rehabilitation and construction
projects. He did not present specific data, but stated that:
• Nine states have or are undertaking studies into the use of rubber asphalt materials as RAP. The
presenter said there are limited reproducible opportunities for data analysis because of the
differences in rubber percentages, gradations, and applications.
• Due to the limited data available, conclusions are also limited, but he stated that the general
direction of using rubber pavements as RAP, based upon the pilot projects to date is positive,
however further research is necessary. The industry is not ready to develop specifications based
upon the current data.
• Caltrans is updating Section 39 to include the use of up to 15% RAP for HMA. This is expected
to go into effect in 2007. However, the presenter mentions it is premature to assume that this
same amount will be specified for rubber mixes.
• The presenter referred to a Caltrans pilot project report as saying that “ hot plant recycling is an
excellent candidate for development and implementation, but laboratory and field evaluation are
necessary to promote success”.
Caltrans Asphalt Rubber Design Guide
Terrie Bressette, Caltrans
The presentation discussed the updated “ Caltrans Asphalt Rubber Design Guide,” which the presenter
sees as a tool to convey information, to serve as a marketing device, and as a means of spreading
knowledge of the subject. Ultimately, the purpose of the guide is to improve the success rate of asphalt
rubber projects. A few key points that were made in this presentation include:
• In projects where a high level of quality control was provided, Caltrans has successfully placed
these materials in the most extreme climates in the state from Death Valley to Mono Lake to
Donner Summit. As Caltrans has compelled districts to use AR and as contractors have become
more familiar with these materials the quality of the projects has increased. In general however,
these mixes don’t work well in cold or wet weather or for long hauls because temperature control
is crucial, and they don’t tolerate considerable handwork.
• * When there are failures with these projects, these failures can also be attributed to strategy
selection errors and not necessarily material issues. Personnel should be trained in the proper use
and application of these materials.
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• * There are definite tradeoffs between added cost and enhanced performance/ longer service lives
for the AR pavements that can help engineers in selecting appropriate pavement strategies. Unit
costs for AR overlay are about 1.5 times the cost of conventional AC for overlays and about 3
times the cost for chip seals. However the additional costs are countered by added durability and
increased service life. There is substantial evidence that these materials perform well when
applied correctly.
Long Term Performance of Asphalt Rubber
Jack Van Kirk, Basic Resources
The presenter strongly believes in asphalt rubber and shows tremendous confidence in the success and
long- term performance of the product. He noted that while many people feel this is a “ new” product, it
has been around since the 1960s in varying capacities, with gap- graded usage starting in the early 1990s.
A few key points to consider from his talk:
• * The presenter believes that crumb rubber binders can be used in dense graded mixes. Much of
the success shown in the early projects, particularly the I- 80 study in the 1980s has been with
dense graded mixes.
• * The multi- layer strategies have shown promise. Two layer systems ( 45 mm RAC- G over AR
Chip Seal) and three layer systems ( 45 mm RAC- G over AR Chip Seal over 25 mm leveling
course) have been successfully constructed and have performed well as evidenced by examples
cited of pavements performing well for over 15 years. Traffic loading on these pavements was
not discussed.
• From a performance standpoint, the presenter discussed how cracked rubber pavements in winter
will heal with hot temperatures and the kneading of vehicle traffic.
• Additionally, maintenance on these asphalt rubber sections was minimal to none in comparison to
adjacent pavement sections. He states that this lower maintenance cost results in increased safety
to the agency and users.
• * Due to reduction in layer thicknesses and use of SAMIs and other thin layers, there is a savings
in energy and natural resources when using these products.
Noise Reduction Studies
Bruce Rymer, Caltrans
The presenter discussed the effects of pavement material type on pavement noise and how noise can be
reduced by deliberately selecting and designing surfacing materials. These quiet pavements have shown
considerable noise reduction benefits that can last up to 7 to 10 years. Wayside ( side of the road at 50 ft
away) and onboard sound intensity ( OBSI, next to tire) measurements were used in the research.
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Automobile road noise measurements have been well researched and documented while truck road noise
requires further work. Key points from his presentation include:
• * Noise reductions of up to 6dB were noted when using RAC- O when compared to DGAC.
• * Pavement acoustics may be considered as a design parameter when designing mixes.
• * It was found that transverse texture, regardless of whether it is in the pavement or the tire is a
main cause of high pavement noise. Removing transverse texture on the pavement has a more
marked effect ( about 8dB) on noise than removing transverse texture from the tire ( about 1dB).
• Additional research is essential to quantify the noise reduction due to quiet pavements under
traffic from truck tires.
• * The use of quiet pavements to replace sound walls is a controversial issue because the FHWA
does not recognize quiet pavements as a solution to reducing noise levels. The quiet pavement
pilot project is showing solid evidence of a noise reduction method. However, it is imperative
that if noise walls are to be replaced by quiet pavements maintenance plans be put in place to
keep this noise reduction in perpetuity.
Granite Construction Plant Tour
The tour of facilities and project sites convinced UCPRC staff that material preparation and construction
capabilities are available to build successful asphalt rubber systems. Contractors experienced in the
construction of AR systems using approved QC/ QA procedures should have little difficulty producing
quality products. However, a mechanism such as test strip requirements should be in place to assure
Caltrans and other agencies that quality work can be achieved by contractors with less experience
working with asphalt rubber systems.
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3.0 SUMMARY AND RECOMMENDATIONS
The AR2006 Conference highlighted a significant number of advances in the use of crumb rubber to
modify asphalt binder and its subsequent use in asphalt mix applications. Overall, the conference
provided information that could increase competence and build confidence in the use of asphalt rubber for
a variety of pavement strategies. Additionally, some of the implementable items presented in the
conference will aid in the implementation of AB338. A sampling of these items includes:
• Continue to use of Asphalt rubber modified mixes, including RAC- G, RUMAC- GG, MB- G in
applications that are most cost- effective,
• Use RAC- O for quiet pavement applications,
• Develop modifications to the wet process and chip seal specifications to further enhance
performance and promote usage of asphalt rubber materials, and
• Use more RAC- O- HB strategies.
Approximately 34 papers were presented in this conference with another 14 or so papers
published in the conference proceedings. It is evident from these papers that a significant amount of work
went into the research presented and that the authors are fully committed to their findings.
Based upon the printed papers, speaker presentations, and conversations with authors and
conference delegates, many recommendations may be made regarding the use of crumb rubber
modification in asphalt concrete pavements. Recommendations based on the UCPRC review and
subsequent discussions with authors are shown in Table 1, which shows that several topics look
implementable ( 7), some warrant development ( 7), and a few papers ( 3) gave general information. The
remaining ( 17) need further research.
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Table 1: Summary Table of Selected Papers and Recommendations
Section Topic
Paper Title and Author
Implementable Need Further
Development
Need
Further
Research
General
Info.
Function
**
( see end of
table)
General Usage and Applications
35 Years of Asphalt- Rubber Use in Arizona
Authors: A. Zareh, G. B. Way
x
The Urban Heat Island Effect and Impact of AR-ACFC
Overlays on PCC Pavements
Authors: M. Belshe, K. Kaloush, J. Golden
x
Influence of Temperature Variation on the
Reflective Cracking Behaviour of Asphalt Overlays
Authors: M. Minhoto, J. Pais, P. Periera
x
Asphalt Rubber in New Pavement Construction
and Design
Authors: J. Sousa, G. Way, S. Shatnawi, C. Dantas
x
Evaluation of Tire Wear Emissions, Roughness
and Friction Characteristics of Asphalt Rubber
Friction Course Pavements
Authors: O. Alexandrova, J. Allen, K. Kaloush
x
Multi- Layer Pavement Strategies Utilizing Asphalt
Rubber Binder ( not in program)
Author: J. Van Kirk
x
Implementable Items
• Use of this design strategy. This pavement strategy has been shown to perform well in lower
volume, lower truck traffic roadways provided proper construction is ensured. Without adequate
construction controls and monitoring, success is questionable.
1
Pavement Performance and Design
Asphalt Rubber Mixtures in Portugal: Practical
Application and Performance
Authors: M. De Lurdes Antunes, F. Batista, P.
Fonseca
x
Performance of a Test Track Using Crumb Rubber
Asphalt and Other Modifiers
Authors: G. Martinez, B. Caicedo, D. Gonzalez, L.
Celis
x
Structural Design Considerations for Rubberized
Asphalt Mixes
Authors: T. Bressette, H. Zhou, S. B. Seeds, R. G.
Hicks
x
Laboratory Characterization and Full- Scale
Accelerated Performance Testing of Crumb
Rubber Asphalts and Other Modified Asphalt
Systems
Authors: X. Qi, A. Shenoy, G. Al- Khateeb, T. Arnold,
N. Gibson, J. Youtcheff, T. Harman
x
Recycling and Environmental Aspects
Study and Application of Asphalt Rubber and
Rubberized Pavement in China
Authors: H. Wen- Yuan, C. Yung- Chieh, X. Li- Ting
x
Implementable Items
• Use of fine rubber powder in dry process with less than 1 percent by weight of total mix after
1, 2
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Section Topic
Paper Title and Author
Implementable Need Further
Development
Need
Further
Research
General
Info.
Function
**
( see end of
table)
validation with materials available in California.
• Use of thin AR overlay and structural overlay on Rubber SAMI to minimize reflection cracking.
• Use of precoated chips.
Energy Consumption of Alternative Scrap Tire
Uses
J. Sousa, G. B., Way, D. Carlson
x
Taiwan’s Experience on Asphalt Rubber Pavement
Authors: C. Chiu, C. Pan
x
Rehabilitation and Noise Reduction
Asphalt Rubber Chip Seal Evaluation in California
( scheduled but not presented)
Authors: L. Rouen, B. Toepfer
x
Implementable Items
Recommendations taken directly from this paper for changes in AR chip seal practice include:
• more uniformly graded aggregate,
• a PG70- 10 asphalt in high temperatures,
• reduce or eliminate the use of extender oil,
• establish a maximum ADT for chip seal applications, and
• specify that contractors use a variable rate spreader bar that can be adjusted to apply binder at a
reduced rate in the wheel path areas.
3
Asphalt- Rubber Gap- graded Mix Design Concepts
Authors: J. Sousa, G. B., Way, A. Zareh
x
Implementable Items
• ARFC ( open- graded) and ARAC ( gap- graded) mixes have shown good success in Arizona.
California uses Type II binder in their mixes and the use of these gap- graded mixes is
recommended.
1
Tire/ Road Noise Relationship with Viscoelastic
Properties of Asphalt Mixtures
Authors: K. Biligiri, A. Sotil, K. Kaloush, J. Golden
x
Asphalt- Rubber Open Graded Mix Reduces Tire
Pavement Noise
Authors: A. Zareh, G. Way, K. Kaloush
x
Implementable Items
• Mix corresponding to ARFC, RAC- O has been used for as part of Caltrans Quieter Pavement
Program.
1, 2
Performance Evaluation of Asphalt Rubber and Its
Use in Old Concrete Pavement Overlays
Authors: R. Cao, Z. Guo, Q. Bai ( presented by J.
Sousa)
x
An Update on the Asphalt Rubber Pavement
Preservation Strategies Used in California
Authors: S. Shatnawi, A. Stonex, R. G. Hicks
x
Implementable Items
• Changes to the wet process specification
• Continued use of RAC- O and RAC- G as surfacings
• Chip seal specification modifications
Items for further development
• Use of RAC- G in thin bonded overlays.
• A requirement for developing an asphalt rubber binder design profile that includes results of tests
1, 2, 3
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26
Section Topic
Paper Title and Author
Implementable Need Further
Development
Need
Further
Research
General
Info.
Function
**
( see end of
table)
on samples obtained at designated intervals over a 24 hour period to establish specification
compliance.
Effect of Crumb Rubber on the Aging of Asphalt
Binders
Authors: S. Lee, S. Amirkhanian, K. Shatanawi
x
Mix Properties
Laboratory Evaluation of Asphalt Rubber
Modified Mixes
Authors: M. C. Cook, T. Bressette, S. Holikatti, H.
Zhou, R. G. Hicks
x
Implementable Items
• Asphalt rubber modified mixes, including RAC- G, RUMAC- GG, MG- G, and MB- D should
continue to be used in applications that are most cost- effective.
2, 3
Effect of Compaction Temperature on Rubberized
Asphalt Mixes
Authors: S. Lee, S. Amirkhanian, K. Shatanawi
x
Mix Design Procedure for Crumb Rubber
Modified Hot Mix Asphalt Concrete
Authors: V. Swami, V. Tandon
x
Predictive Equations to Evaluate Thermal
Fracture of Asphalt Rubber Mixtures
Authors: A. Zborowski, K. Kaloush
x
Performance of Wet Process Method Alternatives:
Terminal or Continuous Blend
Authors: L. Fontes, P. Periera, J. Pais, G. Triches
x
Mechanical Properties of Hot Mixture Asphalts
with Crumb rubber and other Modifiers
Authors: G. Martinez, B. Caicedo, D. Gonzalez, L.
Celis
x
Binder Properties
Influence of Crumb Rubber Gradation on Asphalt-
Rubber Properties
Authors: S. A. Dantas Neto, M. Farias, J. Pais, P. A. Pereira
x
Optimizing the Use of Crumb Rubber by Binary
Modification
H. Ajideh, B. Burris, H. Bahia
x
Laboratory Investigation of Dimensional Changes
of Crumb Rubber Reacting with Asphalt Binder
Authors: F. Ziao, B. J. Putman, S. Amirkhanian
x
Crumb Rubber Modification of Binders:
Interaction and Particle Effects
Authors: B. J. Putman, S. N. Amirkhanian
x
Creep Flow Behavior of Asphalt Rubber Binder.
The Zero- Shear Viscosity Analysis
Authors: F. Guiliani, F. Merusi, I. Antunes
x
Role of Polyphosphoric Acid in the Transition
from Asphalt Rubber to Terminal Blend of
Rubberized Asphalt
Authors: I. Antunes, F. Guiliani
x
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27
Section Topic
Paper Title and Author
Implementable Need Further
Development
Need
Further
Research
General
Info.
Function
**
( see end of
table)
Rheological Behavior of Asphalt with Crumbed
Rubber and other Modifiers
Authors: G. Martinez, B. Caicedo, L. Celis, D.
Gonzalez
x
Aging Studies of Asphalt- Rubber Binder
( scheduled but not presented)
Authors: C. Lima, L. Tome, C. Filho, S. Soares, J.
Soares
x
** Function:
1) Helps advocate potential increase in use of Asphalt Rubber as directed in AB338.
2) Builds confidence in using Asphalt Rubber.
3) Increases technical competence in using Asphalt Rubber.
3.1 Additional Suggestions and Recommendations from AR2006
The comments below are additional research and development applications that were put forth by various
presenters at AR2006 either during presentations or in the papers. Since this summary focused primarily
on implementable items, it was felt that a brief summary of some of the areas that need further research
would be helpful. Note that much of this research is being conducted by universities and other state
agencies.
Applications, Design, and Testing
• Caltrans and ADOT could help promote asphalt rubber OGFCs as many states have had bad
experiences with these materials.
• There is a need to develop new design methodologies and laboratory tests for AR since material
performance is not well- characterized by traditional models ( such as fatigue models) or in tests
( such as the Marshall test). This research is needed in applications ranging from new
construction to maintenance and rehabilitation.
• Others have reported satisfactory performance on projects with AR SAMI interlayers. Additional
controlled full- scale studies ( HVS testing) could be performed to enhance design and construction
details.
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Materials Evaluation
• Further study into CO2 emission studies. The previous studies have shown that lower rates of CO2
are emitted into the atmosphere from the use of AR.
• Further study into the insulating effects of overlays on and the potential increase in PCC lifetime
gained. This is a complex issue.
• Further research into terminal blends should be explored due their relative ease of manufacture.
Terminal blends are used but to a limited extent in California. The intent is to use them more in
dense graded mixes in the future. The PG Task Force is working on specifications for terminal
blend asphalts that will be very similar to those of PG polymer modified asphalts. The question is
how they perform in comparison to polymer modified mixes. Caltrans plans a laboratory study,
possibly with the Asphalt Institute, to measure fatigue and rut resistance of mixes using terminal
blend and polymer modified binders. A similar laboratory study is being planned to evaluate
PPA. Studies into these potential modifiers such as PPA and others could enhance performance.
• Continued research on new thermal cracking models for AR mixtures to help explain their
performance relative to thermal cracking. A few specific items to address are:
o Addressing the effect of binder content versus the effect of CRM modification.
Specifically, how do different binder contents change the thermal behavior of
mixtures with CRM modifications? This is essential to separate out the effects ( and
benefits) of increased binder content with CRM modified mixtures.
o Study thermal fatigue under controlled stress testing ( closer to field conditions) and
with long term oven aged ( LTOA) specimens, as thermal fatigue is more likely after
several years of service
o Look at thermal fatigue resistance with different base asphalt cements.
• Further study into environmental assessment of vehicle emissions of particulates from AR
pavements, since particulates affect human health.
• Further study into fatigue and rutting models as they pertain to AR mixtures. There was enough
debate at the conference to at least warrant looking into this. Testing for reflection cracking is the
major need
• Moisture sensitivity research into AR performance that would include laboratory test
development, which then would be calibrated to field performance.
• Peak phase angles have been well- correlated to sound intensity of each mix. Further study could
perhaps lead to a priori knowledge of mix noise characteristics.
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• Further research into the usage and effects of extender oil. The benefits, performance, and risks
with using this material should be looked into.
• Application of binary modified binders with crumb rubber modifiers has shown promise. While
this is proprietary, the long- term potential for this product may be significant.
Each of the categories specified in Table 2 reflects actions that should be carried forward for the
benefit of the State of California. Based upon the core competencies of the principal groups that address
research and training needs for the state, we suggest the following task delineation to best take advantage
of each activity.
Table 2: Suggested Actions and Agents to Aid AR Implementation
Item Group
Research UCPRC, Chico State, PPTG, Consultants,
national research programs such as NCHRP,
State DOT pooled fund studies
Implementation to local governments Chico State, PPTG
Technology transfer to industry Chico State, PPTG, Industry Organizations
Disseminate knowledge internally within Caltrans Caltrans, ITS Tech Transfer, CCIT,
Consultants