This document summarizes research being conducted by the University of California Pavement Research Center on increasing the use of recycled materials in asphalt pavements. It outlines 8 ongoing research topics, including comparing the effects of rubberized RAP versus conventional RAP, determining the optimal thickness for RHMA-G surface layers, adding RAP to RHMA-G mixes, and developing a pavement life cycle assessment tool to quantify environmental impacts. The goal of the research is to advance the use of recycled materials like rubber and RAP in ways that improve pavement performance and sustainability.

1. California Research Perspective & Tools
Where the RUBBER meets the ROAD
CalAPA Workshop
Sacramento
April 18, 2023
Angel Mateos, David Jones, John Harvey,
Jeff Buscheck, Rongzong Wu, Ali Butt
University of California Pavement Research Center

2. 1. Rubberized RAP (RRAP) versus conventional RAP
2. RHMA-G surface layer... How thick can we go?
3. Adding RAP to RHMA-G
4. Adding recycled tire rubber to (dense-graded) HMA
5. ARB PG specifications
6. RHMA-G lift thickness limit
7. Use of RHMA-G for concrete pavement bases
8. Pavement life cycle assessment (LCA)
Presentation Outline

3. Research Motivation:
• Increasing amounts of rubberized RAP (RRAP)
• Do RRAP and RAP produce different effects on the asphalt mix?
• Do RRAP and RAP require different stockpiling and processing?
Background:
• Research funded by CalRecycle (2014-2017):
o RRAP and RAP produce similar effects on HMA (somewhat better
cracking resistance with RRAP)
o “...there appears to be no reason or justification for separating RRAP
and RAP millings or maintaining separate stockpiles at asphalt plants.”
(Research Report: UCPRC-RR-2016-03)
1. Rubberized RAP versus conventional RAP

4. Research Goal:
• Compare the effect of RRAP versus conventional RAP on HMA properties
Research Approach:
• Research funded by Caltrans (2020-2023)
• Sample one RRAP and one RAP
• Select control HMA; add 25% RRAP and 25% RAP
• Test the two mixes in the lab:
o Volumetrics
o Stiffness
o Fatigue resistance
o Rutting resistance
o Moisture susceptibility
1. Rubberized RAP versus conventional RAP

5. Research Motivation:
• Caltrans Highway Design Manual sets a maximum limit to RHMA-G layer
thickness: 0.2 ft (2.4 in.)
• The limit is based on past economic cost of RHMA-G versus HMA and some
concerns with rutting
Background:
• The optimum layer thickness can be determined by the designer (using CalME)
• Mechanistic-empirical modelling with CalME:
o Use of 0.2-0.3 ft of RHMA-G provides best performance for overlays on
flexible pavements
o Use of 0.2-0.5 ft of RHMA-G provides best performance for overlays on rigid
pavements
• Possible approach: set up maximum limit (~0.3-0.5 ft) and let the designer
choose thickness to provide most economical and constructable design
2. RHMA-G surface layer... How thick can we go?

6. Research Goal:
• Determine maximum RHMA-G surface thickness limit based on mechanistic-
empirical principles
Research Approach:
• Research funded by Caltrans (4.75 Project; 2020-2023)
• CalME modelling of RHMA mixes (mixes evaluated in the laboratory: stiffness,
fatigue resistance, and rutting resistance)
• Heavy Vehicle Simulator (HVS) testing
2. RHMA-G surface layer... How thick can we go?
RHMA-G ½” 0.20’
RHMA-G ½” w/RAP RHMA-G ½”
RHMA-G ½” 0.20’
RHMA-G ¾”
0.2’ RHMA-G ¾”
RHMA-G ¾”
0.25’
0.25’
Thickness
effect

7. Research Motivation:
• Caltrans does not allow any RAP in RHMA-G
• Are there applications where RAP can be added to RHMA-G and obtain similar or
better performance?
Background:
• Research funded by CalRecycle:
o Adding of 10% RAP (<3/8”) to RHMA produces these effects: stiffness ↑,
rutting resistance ↑, and fatigue resistance ↓
o The amount of RAP that can be added to RHMA-G is limited to ~10% because
of gradation requirements
• Despite possible drop in fatigue resistance, the addition of RAP may be positive in
certain applications (e.g. intermediate layers) because of the increase in stiffness
3. Adding RAP to RHMA-G

8. Research Goal:
• Determine effects of RAP addition to RHMA-G
• Determine applications where the RAP addition results in improvement in
performance
Research Approach:
• Research funded by Caltrans (4.75 and 4.76A Projects; 2020-2023); expected
CalRecycle support 2023-2025
• Experimental data:
o Heavy Vehicle Simulator (HVS) testing ~ 10% coarse RAP (>3/8”)
o Monitoring field pilots (RHMA-G with RAP) ~ 10% fine RAP (<3/8”)
• CalME modelling to determine the best applications for the RHMA-G with RAP
3. Adding RAP to RHMA-G

9. • Experimental data:
o Heavy Vehicle Simulator (HVS) testing
3. Adding RAP to RHMA-G
RHMA-G ½” 0.20’
RHMA-G ½” w/RAP RHMA-G ½”
RHMA-G ½” 0.20’
RHMA-G ¾”
0.2’ RHMA-G ¾”
RHMA-G ¾”
0.25’
0.25’
RAP

10. • Experimental data:
o Monitoring field pilots (RHMA-G with RAP)
3. Adding RAP to RHMA-G
✓ Sonoma 101 (04-1W880)
✓ Done
✓ Sonoma 1 (04-1W890)
✓ Done
✓ Ventura 1 (07-0w140)
✓ Ongoing
✓ Imperial 186 (11-2N116)
✓ Planned Apr-2023
• Each project includes several lane-miles
• Control and test sections (0.4 miles each)
• Sampling of production mixes for lab evaluation
• Monitoring of test sections performance up to 5 years

11. Research Motivation:
• Caltrans HMA (dense graded) mixes do not contain rubber other than terminal blend
• If Caltrans builds ~60% HMA and ~40% RHMA-G, adding 10% rubber (% binder; ~0.5% mix) to HMA
would increase recycled tire rubber use in ~50%
Background:
• Research funded by Caltrans and CalRecycle (2017-2020):
o Overall, consistent positive results with 5% rubber addition (% binder)
o Varying results for 10% rubber addition
• Improvements in crumb rubber modifier (CRM) production techniques: possibility to produce small-
sized particles (passing #40 or finer)
• Variety of products that provide a straight-forward way to add rubber to dense-graded mixes:
o Engineered CRM
o Concentrated terminal blend
o New approaches to add regular CRM particles
4. Adding recycled tire rubber to (dense-grade) HMA

12. Research Goal:
• Increase use of rubber in (dense graded) HMA
Research Approach:
• Research funded by Caltrans (4.76A Project; 2020-2023); expected CalRecycle support 2023-2025
• Target:
o Structural layers below the surface (assume RHMA-G will remain Caltrans surface of
preference)
o Mixes currently using plain binder (one of the questions: can we balance an increase in RAP
content with rubber addition?)
o Constructability/implantability: without changing mix design, production, or construction
processes; storability
o Looking at all means of rubber inclusion
• Experimental data:
o Ongoing lab study
o Possible pilot implementation in the field in Summer 2024
4. Adding recycled tire rubber to (dense-grade) HMA

13. • Experimental data:
o Phase 1: Ongoing lab study (volumetrics, IdealCT)
o Phase 2: Comprehensive lab characterization (few
selected mixes)
o Phase 3: Field pilots (Summer 2024)
4. Adding recycled tire rubber to (dense-grade) HMA
Untreated CRM
< #40 (0.425 mm)
Untreated CRM
< #60 (0.25 mm)
Engineered CRM
Untreated CRM
MTOA
Control mix
(25% RAP)
Control mix
MTOA
Diluted wet, with
agitation
TB
(Terminal Blend)
Advanced wet
< #40 (0.425 mm)

14. Research Motivation:
• AASHTO PG system is not conceived for asphalt rubber binder (ARB)
Background:
• Caltrans QC/QA of ARB focuses on viscosity
• Challenge: large (“undigested”) rubber particles whose size is comparable to the gap
between dynamic shear rheometer (DSR) plates
• Increasing DSR parallel plate gap (to 3 mm) and concentric cylinder testing have been
suggested
• Research funded by Caltrans (4.63 Project; 2017-2020):
o Large (“undigested”) rubber particles dominate results
o Different results between the two geometries: parallel plate and concentric cylinder
o High variability between results, regardless of the testing geometry
o High-temperature PG appeared unrealistically high (>82°C)
5. ARB PG specifications

15. Research Goal:
• Develop and recommend testing procedures and criteria for PG specifications of
ARB used in gap- and open-graded mixes
Research Approach:
• Research funded by Caltrans (4.77 Project; 2020-2023)
• 19 plant-produced ARB and 5 RHMA-G mixes
• Proposed approach:
o Centrifuge/filter to remove largest rubber particles (>0.5 mm)
o Test filtered ARB using regular PG testing
o California PG map for ARB
• Round robin study to develop precision and bias statements for the proposed PG
testing procedure (if requested by Caltrans)
5. ARB PG specifications

16. Research Motivation:
• Caltrans Std. Specs. Section 39 requires nominal maximum aggregate size (NMAS)
of ¾ in. for 0.20 ft RHMA-G lift
Background:
• Some concerns with constructability (compaction and smoothness) due to low
thickness/NMAS ratio
• Use of NMAS of ½ in. for 0.20 ft RHMA-G lifts has been suggested by construction
experts
6. RHMA-G lift thickness limit

17. Research Goal:
• Determine optimum NMAS for 0.20 ft RHMA-G lifts (½ in. or ¾ in. ?)
Research Approach:
• Research funded by Caltrans (4.75 Project; 2020-2023)
• Heavy Vehicle Simulator (HVS) testing
6. RHMA-G lift thickness limit
RHMA-G ½” 0.20’
RHMA-G ½” w/RAP RHMA-G ½”
RHMA-G ½” 0.20’
RHMA-G ¾”
0.2’ RHMA-G ¾”
RHMA-G ¾”
0.25’
0.25’
NMAS
effect

18. Research Motivation:
• The only asphalt concrete option for concrete pavement bases is HMA
Background:
• Research funded by Caltrans (Project 4.58B; 2014-2017)
o Excellent performance of RHMA-G as base for thin concrete overlay,
evaluated under Heavy Vehicle Simulator
• No main reason why RHMA-G would not work as a concrete pavement base
7. Use of RHMA-G for concrete pavement bases

19. Research Goal:
• Evaluate the alternative of using RHMA-G (rather than HMA) for concrete
pavement bases
Research Approach:
• Research funded by Caltrans (4.76B Project; 2020-2023)
• Laboratory study
o Develop test procedure for asphalt mix used as concrete pavement base
o Compare RHMA-G versus HMA
• Build test section and monitor performance
7. Use of RHMA-G for concrete pavement bases

20. Research Motivation:
• Number of state legislation bills to bring down the greenhouse gas emissions in California
• Quantify greenhouse gas emissions and other environmental impacts of pavements,
considering all life cycle stages (material, construction, maintenance, use, and end of life)
Background:
• Research funded by Caltrans (4.66 Project; 2017-2020):
o Start development of eLCAP (Environmental Life Cycle Assessment of Pavements), a
tool that can quantify environmental impacts of pavements
8. Pavement life cycle assessment (LCA)

21. Research Goal:
• Finalize eLCAP and its database (life cycle inventory database)
Research Approach:
• Research funded by Caltrans (3.54 and 4.80 Projects; 2020-2023)
• Collected life cycle inventory database (new rubber commercial products not included yet)
• Develop web-based eLCAP
• Evaluate strategies that are expected to bring down environmental impacts:
o Rubberized hot mix asphalt
o Cold in-place recycling
o Warm mix asphalt
o ...
8. Pavement life cycle assessment (LCA)

22. • Research on recycle tire rubber use in asphalt mixes has been active in California
for years, mainly supported by Caltrans and CalRecycle
• Main targets of ongoing research includes:
o Increase use of recycle tire rubber
o Bring RAP and rubber together
o Improve constructability and QC/QA of ARB and asphalt mixes with rubber
o Approaches that are easy to implement
o Quantify environment impact
• Past, current, and possible future research/implementation projects included in
UCPRC Rubber Research Roadmap (www.ucprc.ucdavis.edu)
Conclusions