Scrap Tire Rubber in Asphalt: Materials

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Scrap Tire Rubber in AsphaltMaterials Usage
Basics

• by
• Anne Stonex
• MACTEC Engineering Consulting
• 3630 East Wier Avenue
• Phoenix, Arizona 85040
• [phone]

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Presentation Outline

• Definitions
• History
• Components
• Products
• Applications
• Hot Mixes (Types and Appropriate Uses)
• Spray Applications (Seals and Interlayers)
• Benefits

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Definitions

• Crumb Rubber Modifier (CRM)
• CRM is produced from grinding up whole scrap
  tires, tread buffings, and other waste rubber
  products.
• CRM comes in a variety of grades and
  designations, typically described by passing
  sieve size and source/manufacturer.

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Definitions

• Wet Process Method of modifying asphalt cement
  with CRM and other components
• Most widely used approach (AZ, CA, TX, FL,
  others)
• Thoroughly mix CRM other components with hot
  (400-425ºF) asphalt cement
• Interact at 350-375ºF for designated period
  (typical minimums 45-60 minutes)
• CRM particles swell, exchange oils with AC
• Can produce wide variety of CRM modified binders
• Rotational Viscosity is discriminator for
  appropriate use

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Definitions

• Asphalt Rubber ASTM D 8
• A blend of asphalt cement, reclaimed tire rubber
  and certain additives in which the rubber
  component is at least 15 by weight of the total
  blend and has reacted in the hot asphalt cement
  sufficiently to cause swelling of the rubber
  particles.
• Related Specification ASTM D 6114
• Standard Specification for Asphalt Rubber Binder
• High viscosity material that typically requires
  agitation to keep CRM particles dispersed.

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Definitions

• Rubber-modified asphalt
• A blend of asphalt cement, recycled tire rubber,
  and other optional additives, in which the
  recycled tire rubber is the major additive or
  modifier, and which has been interacted in the
  hot asphalt cement sufficiently to cause swelling
  or digestion of the rubber particles.
• Includes Terminal Blends
• Low viscosity, no agitation.
• Typically 10 CRM content or less, some _at_ 15
• May include polymers and/or other modifiers
• Content in hot mixes is similar to neat asphalt
  cement

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Definitions

• Dry Process
• Substitutes CRM for 1 to 3 of aggregate in hot
  mix
• Not considered to modify binder, although some
  interaction with CRM may occur in place over time
  (absorbs light fractions)
• CRM gradations have ranged from coarse (-1/4) to
  fine (-80) currently between 16 and 50
• Mixed performance history limited current use
• May be related to mix design
• CRM specific gravity 1.100-1.200

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History

• Development dates back to 1930s joint sealers
  and membranes
• 1954 Bureau of Public Roads studies
• Effects of Various Rubbers on the Properties of
  Petroleum Asphalts
• Laboratory Study of Rubber-Asphalt Paving
  Mixtures
• 1960 TAI Symposium on Rubber in Asphalt
• 1960s Charles McDonald City of Phoenix developed
  wet process also called McDonald process

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History

• 1970s McDonald pioneered routine use of AR in
  hot mix patching and chip seals
• AR chip seals effective as citys primary
  strategy for maintenance and preservation of
  arterial streets for 20 yrs
• 1975 Type 1 patent issued
• 1978 Patents issued for Types 1 2
• Increasing traffic volumes in 1980s forced change
  to thin overlays using AR hot mixes
• Gap-graded AR mixes developed as substitute for
  chip seals

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History

• California
• 1975 Caltrans started experimenting with AR chip
  seals
• Caltrans tries dry process
• 1984 Caltrans Ravendale project - wet dry, gap
  dense-graded, full half thickness, AR
  interlayers, single and double AR chip seals,
  dense-graded conventional HMA control
• 1980s continued use and development in AZ, CA.
• 1988 FL State Legislature mandates use of scrap
  tires in HMA. DOT selects terminal-blend type
  system

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History

• Intermodal Surface Transportation Efficiency
  Act (ISTEA) Federal mandate
• Required CRM in 5 of HMA by 1994
• CRM in 20 of HMA by 1997
• Spurred frantic research and enormous backlash by
  DOTs throughout US
• Fragmented approach - each agency did own way,
  often without understanding how or where to use,
  or how to optimize materials
• Wet and dry processes confusion between systems

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History Bitter Legacy of ISTEA

• Studies varied widely, little basis for
  comparisons
• Results mixed differences in performance due to
• Project Selection
• Specifications
• Project design including materials selection
• Mix design
• Contractors unfamiliar with highly modified
  materials
• Issues with materials handling, production,
  placement compaction
• Many of these factors ignored in analyses
• Nearly destroyed the asphalt-rubber industry

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History Bitter Legacy of ISTEA

• Other Issues COST
• Proprietary materials
• High costs due to mobilization from AZ, CA, TX,
  RI, MS for small test sections, demo projects
• Mandate was repealed
• Development continued in areas with local
  suppliers primarily AZ, CA, FL, TX

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Asphalt Rubber Binder

• Components
• Crumb Rubber
• Asphalt Cement
• Additives

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Crumb Rubber Modifier (CRM)

• Primarily from Passenger Cars Light Trucks
• Heavy Truck tires for high natural rubber content
  
• Chip Seals
• Caltrans binders
• Whole Tires
• Buffings/ Tread Peel
• Recycled Tires
• Waste Rubber from Tire Production
• Example gradation specifications on following
  slide

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CRM Gradation Specifications
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Asphalt Cements Asphalt cements come in a
variety of grades and designations. Typically
softer grades are used in blending asphalt rubber
binder for a particular region than are used for
production of conventional hot mix.
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• Asphalt Cements
• CRM can be used in Performance-graded (PG)
  binders
• CRM Modification bumps high temperature grade, as
  do polymers
• Rule of thumb
• 15 CRM or more ? 2 high temp grades
• Little effect on low temperature grade
• Use softer base asphalts, stiffen for high
  operating temps

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• Additives
• Sometimes used in conjunction with CRM to enhance
  interaction or produce desirable property.
• Extender oils - aid in the reaction of the
  crumb rubber by providing aromatics which are
  absorbed by the rubber, and help with dispersion
  by chemically suspending the rubber in the
  asphalt. Required only in California.
• Anti-stripping agents - used to increase or
  improve adhesion to aggregates. Mineral
  admixtures preferred
• High natural rubber - used to improve adhesion
  and flexibility, chip seal aggregate retention,
  and to compatibilize interactions. Required by
  Caltrans.
• Polymers typically limited to terminal
  blends

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Interactions Depend On

• Asphalt Cement Source and Grade
• Rubber Type/Source
• Amount of Rubber
• Gradation of Rubber
• Interaction Time
• Interaction Temperature

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Advantages of High ViscosityAsphalt Rubber Binder

• Allows higher binder content and increased film
  thickness
• Enhances aggregate retention
• Minimizes drain-down problems
• Increases resistance to moisture damage
• Increases resistance to fatigue
• Increases resistance to bleeding, flushing and
  deformation
• Reduces aging of the mix
• Improves durability

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Specifications for High-Viscosity Wet Process CRM
Modified Binders with Minimum Viscosity 1.5
Pasec, 1500 cPs
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Specifications for High-Viscosity Wet Process
CRM Modified Binders with Minimum Viscosity 1.5
Pasec, 1500 cPs
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• Asphalt Rubber Blend Design Profile
• For High-viscosity AR Binders
• A design profile is typically developed to
  evaluate the compatibility between materials
  used, component interaction and to check for
  stability of the blend over time. Should be
  required for each project, for hot mix and spray
  applications.

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Example Design Profile
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Applications for High Viscosity Binders

• Hot Mixes
• Increases cost so use where most effective
• Most effective in gap-graded and open-graded
  mixes where aggregate matrix has room to
  accommodate CRM particles and high binder content
• Most effective in relatively thin surface lifts
  (max 60 mm)
• Gap-graded is used as structural layer,
  equivalent to DG
• Open-graded is used as surface friction course
• Increased resistance to rutting, fatigue and
  reflective cracking a function of binder content
• Not suitable for use in dense-graded mixes

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Applications for High Viscosity Binders

• Chip Seals and Stress Absorbing Membrane
  Interlayers (SAMI-R)
• Use over severely cracked but structurally sound
  pavements
• SAMI-R may be covered with asphalt rubber mix,
  terminal blend mix, or conventional HMA

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Specifications for No Agitation Wet Process CRM
Modified Binders with Minimum Viscosity Less Than
1.5 Pasec, 1500 cPs
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Specifications for No Agitation Wet Process CRM
Modified Binders with Minimum Viscosity Less Than
1.5 Pasec, 1500 cPs
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Applications for No Agitation Binders

• Hot Mixes
• Suitable for use in dense-graded mixes
• May also be used in gap-graded and open-graded
  mixes, but binder content is limited by drain-off
• Content increase over neat or polymer-modified is
  small
• Chip Seals and Stress Absorbing Membrane
  Interlayers
• Suitable for use and often more convenient than
  high viscosity binders, but resistance to
  reflective cracking may vary with CRM content

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Benefits of No Agitation Binders

• Similar to asphalt rubber in quality, contents
  between conventional HMA and hi-viscosity
• Fewer impacts on mix design, production and
  placement than high viscosity AR
• Has worked very well in Florida for over 15 yrs
• Does not require major shifts in aggregate
  gradation to accommodate CRM particles
• Mixing and placement temperatures comparable to
  polymer-modified HMA

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Considerations for CRM with PG

• PG binder system was developed for unmodified
  asphalt cements use for base asphalt
• Not intended for two-phase systems
• AASHTO M320 includes sieve test for particles
  retained, but cites no test method for
  determination
• Solvents affect size of CRM particles
• How to test?
• Is test necessary?
• PG modified to add elastic recovery
• Waive particle size for two-phase systems?

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Benefits and Limitations of Asphalt Rubber

• Benefits
• Improved durability as a surface layer
• Improved resistance to fatigue and reflective
  cracking
• Limitations
• Increased cost must be offset by increased
  performance
• Requires attention to detail and experienced
  contractors
• Higher temperatures for placement and compaction