This document summarizes Caltrans' use of asphalt rubber binder made from recycled waste tires in pavement materials. It discusses Caltrans' goals of sustainability and climate leadership. Crumb rubber modifier is produced from grinding waste tires and is mixed with asphalt at plants. Caltrans specifications require minimum recycled tire content. Implementation involved research on mix design, testing variability reduction, and industry partnerships. Caltrans aims to divert waste tires from landfills while producing more durable and sustainable pavements.

1. Materials Engineering and Testing Services
Asphalt Rubber
State Perspective
Tim Greutert
State Materials Engineer

2. 1) Caltrans Sustainability and Lead Climate Action
2) Asphalt Rubber Binder Management
3) How did Caltrans Implement?
Outline
Crumb Rubber
Waste
Tire

3. Caltrans Goals
OUTCOMES
Mitigation Resilience Leadership
Reduce greenhouse
gas emissions from the
transportation sector.
Strengthen our
capability to withstand
and recover from the
impacts from climate
change.
Demonstrate
leadership in climate
action.
Caltrans Commitment to People and The Planet
• Caltrans Strategic Plan integrates sustainability principles across all goals
Goal: Lead Climate Action
• Develop & Implement a Caltrans Climate Action Plan
• Pilot New Materials and Practices w/Greenhouse Gas Reduction in
mind and Recyling
• Life Cycle Analysis – Performance & Sustainability

4. > 51M
Waste Tires Each Year
Recycling Tires: Diverting Landfill Waste
28M
Tires Recycled
in 2021
• In 2021:
• 28M Tires Recycled
• ~38% of Asphalt Placed Contained
Crumb Rubber (~1.28M Metric Tons)
• CalRecycle Waste Tire Recycling
Management Program
California Challenge: Diverting
and Managing Waste Tires
246M
Total Tires
Recycled

5. Crumb Rubber Modifier (CRM) Manufacturing
Tire Stockpile Grinding
or
Cryogenic
Processing
Reduced to a
Particular Size
Packaging
Belts
(Fiber/
Metal)
Removed
Removed
Fibers
Crumb Rubber Modifier (CRM)
Mixing at Asphalt
Blending Facility

6. First California Tire Recycling Act
SB-876 (Further Augmentation)
Public Resource Code §42703
Legislative Mandates
PRC § 42703 (e) & (f)
The Department of Transportation and the board shall
develop procedures for using crumb rubber. Only crumb
rubber manufactured in the United States may be used.
CalRecycle regulates and manages waste tires.
1989
2000
2006

7. Benefits & Challenges
Benefits Challenges
• More Sustainable Pavement
• Higher Durability
• Less Pavement Noise
• Improved Safety (Higher Surface
Skid)
• Improved Resistance to Fatigue
and Reflective Cracking
• Protecting the Environment
• Asphalt Modified at the Plant
• Recycling Tires
• Compacting RHMA-G Specimen
• Variable Design Parameters
• Gyrations, Hold Time, Ram
Pressure, Mix/Compact Temps
• Drying the Samples
• Higher Field Compaction Temps
Compared to HMA

8. Additional Challenges
Sticky
• Rubber does not melt when mixing with binder
• Surface pores of the CRM absorb binder light ends
• Rubber crumbs swells, become soft and stickier
• Sticks to boots, tools, equipment, etc.
Fumes
• More aromatics with the rubber binder
• Research on safety impact of rubber construction
• 2001 CDC Report findings:*
• No definitive indications that CRM exposures
are more hazardous than conventional
asphalt
• Higher occurrence of eye, nose, and throat
irritation in those exposed to CRM asphalt
*Report: Crumb-Rubber Modified Asphalt Paving: Occupational Exposures and Acute Health Effects (HETA #2001-0536-2864)
Asphalt Rubber
Binder
Neat/Virgin
Binder

9. Asphalt Rubber Binder Management
Wet Process (Field Blend):
• Added into the binder before mixing
• Rubber swells and softens during mixing & handling
Asphalt Rubber Binder Management Considerations:
• Blending Process and Proportioning
• Reaction Time
• Monitoring rubber binder properties

10. Two Hoppers
Crum Rubber Source Requirements
• 75% scrap tire crumb rubber / 25% high natural crumb rubber
• Delivered in separate bags
• Proportion and mix asphalt binder, asphalt modifier, and CRM
to within their own ratio limits
• Material Plant Quality Program (MPQP) verification

11. Gap-Graded vs Open-Graded
Mix Type
2022 Caltrans
Specification
Optimum Binder
Content (OBC)
Responsible Party
Notes
RHMA-G
Section
39-2.03
Contractor
• Contractor submits a Job Mix Formula
• Asphalt rubber binder design profile
• Materials and proportions
• The asphalt rubber binder blending plant must
be MPQP authorized
• Caltrans performs JMF Verification
RHMA-O
Section
39-2.04
Caltrans
• Contractor submits a JMF without specifying an
asphalt binder content
• Caltrans determines the OBC
• JMF Prequalification Program (JMFPP)

12. Independence Assurance (IA) Program
• IA program certifies QC testers and labs testing asphalt rubber binder
• Viscosity Test ASTM D7741
• At each reaction time in the
asphalt rubber binder design
profile
• QC each lot before use
• Daily for Acceptance

13. Testing Variability
Interlaboratory Studies (ILS)
• ILS have shown high variability for rubberized mixes compared to national data
• Rutting (Hamburg Wheel Test Track)
• Binder Content (Higher variability for ¾” compared to ½”)
• Developed California Test (CT) 389 to reduce rutting variability of test results
California Test (CT) Methods
• Leverage CTs to reduce sampling
and testing variability
• Currently using CTs for
• Sampling
• Sample reduction
• Crumb rubber modifier properties
• Hamburg wheel-track testing, etc.

14. Method Approach
Method Approach
• 80% Asphalt Binder and Asphalt Modifier / 20% CRM
• Asphalt Rubber Binder Design Profile
• Specified Production Constraints
• Mixing, and integration time
• Temperature limits
• Limits on heating time

15. How did Caltrans Implement?
Caltrans Implementation
• Crumb Rubber Usage Memo
• Guidance for use
• Exceptions require District Director approval
• Highway Design Manual
• Preferred surface course
Lessons Learned
• Increased test variability
• Reduced thickness for reflective cracking
• Rubber pavement is more resilient
• Compaction (Time/Temperature in the oven)

16. How did Caltrans Implement?
Research
• Caltrans has conducted several studies
• 1992 Heavy Vehicle Simulator (HVS)
• UCPRC Publications
• Pilot projects

17. How did Caltrans Implement?
Partnership with Industry
• Pavement and Materials Partnering
Committee (PMPC)
• Awesome research partner UCPRC
• Hamburg Working Group
• Emphasis on RHMA-G consideration
• CT 389 ILS demonstrated reduced
variability
• 10% RAP and RHMA-G Pilots

18. Thank you!