This document discusses the key aspects of designing rubberized hot mix asphalt (RHMA). RHMA uses asphalt rubber binder and gap-graded aggregates. The mix design process involves selecting aggregates and an asphalt rubber binder that meet specifications. The binder is produced by adding crumb rubber to heated asphalt cement. Optimum binder content is a minimum of 7.5% by total weight. Performance tests include the Hamburg wheel track test for rutting resistance and tensile strength ratio test for moisture susceptibility. Proper compaction at high temperatures and aggregate gradation are critical to meeting volumetric and performance requirements.

1. Rubberized Hot Mix Asphalt
(RHMA) Mix Design
By
Jack Van Kirk
Director of Asphalt
Technology
George Reed Inc.
CalAPA Fall Conference
October 26 - 27, 2016
Sacramento, CA

2. Rubberized Hot Mix Asphalt
(RHMA)
• What is rubberized HMA?
• Binder, aggregate requirements ?
• How do you design it?
• How do you test it?
• What are the differences from
conventional HMA?
• Critical factors?

3. What is RHMA?
• Completely different than
conventional HMA
• Uses asphalt rubber binder
• Uses gap-graded or open graded
aggregate gradation
• Used in reduced thickness
pavement design

4. Asphalt Rubber Specifications
Rubberized hot mix asphalt (RHMA)
Type “G” *
Type “O”
Type “O-HB”
* For this presentation will
only discuss Type G

5. Extremely Important
• RHMA mixes are more
temperature sensitive
• Lab and field compaction must be
achieved at higher temperatures

6. Parameters of the Mix Design
• Binder requirements
• Aggregate requirements
• Volumetrics and performance tests
• Mix and compaction requirements
• Major differences from
conventional HMA

7. Quality characteristic Test method Requirement
Air voids content (%) AASHTO T 269a
Ndesign = 4.0
Gyration compaction (no. of gyrations) AASHTO T 312 Ndesign = 50–
150b
Voids in mineral aggregate (min, %) SP-2
Asphalt
Mixture
Volumetricsc
18.0–23.0
Hamburg wheel track (min, number of passes at
0.5-inch rut depth)
Binder grade:
PG 58
PG 64
PG 70
AASHTO T
324 (Modified)d
15,000
20,000
25,000
Hamburg wheel track (min, number of passes at the inflection
point)
Binder grade:
PG 58
PG 64
PG 70
AASHTO T 324
(Modified)d
10,000
10,000
12,500
Moisture susceptibility, dry strength (min, psi) AASHTO T 283d
100
Moisture susceptibility, wet strength (min, psi) AASHTO T 283d,
70

8. Selection of Materials
• Binder
–Asphalt rubber binder
–PG 58-22, PG 64-16, PG 70-10
base asphalt
• Aggregate
–Quality requirements
–Gap-graded aggregate gradation

9. Asphalt Rubber Binder
• Uses a minimum of 20 +/- 2 % crumb
rubber
• Uses 10 mesh (2mm) maximum size
crumb rubber
• Reacts/interacts crumb rubber for a
minimum of 45 min. at elevated
temperatures
• Modifies original properties of asphalt
cement

10. Asphalt
Rubber
Conventional
Asphalt

11. Laboratory Binder Design
• Asphalt heated to 400 to 425°F
• Asphalt modifier added to asphalt
• Crumb rubber (18-22 %) blended into
asphalt/asphalt modifier blend
• Reacted for a minimum of 45 minutes
• Agitated (stirred) frequently during
reaction period
• Properties tested over 24 hour period
Compatibility of components

12. Optimum Binder Content
(OBC) (by total weight)
• Minimum 7.5% for RHMA Type G
• OBC target value cannot go below 7.5%
• For best practice - field produced AR
binder should be used for the mix design

13. Minimum OBC (by total weight)
• Minimum 7.5% for RHMA Type G is
extremely important for good
performance in the field (resistance to
reflective cracking and raveling)
• Some aggregate sources encounter
difficulty meeting the minimum binder
content, volumetrics and performance
requirements

14. Aggregate Requirements
• High quality aggregate is required
because of the reduced thickness
pavement design
–High % fractured faces
–Low % flat and elongated
–Sound durable particles

15. Aggregate Gradation
• Uses a gap-graded aggregate gradation
• Limits of proposed gradation tolerances
much tighter
• For gap graded and open graded mixes -
40% higher binder content (7.5 -8.5% by
total weight)
• Use of asphalt rubber allows for higher
binder contents and thicker film thickness

16. Aggregate Gradations for RHMA-G
(Percentage Passing)
1/2 inch
Sieve size Target value limit Allowable
tolerance
3/4" 100 --
1/2" 90–98 TV ± 6
3/8" 83–87 TV ± 5
No. 4 28–42 TV ± 6
No. 8 14–22 TV ± 5
No. 200 0.0–6.0 TV ± 2.0

17. Gap-Graded Aggregate

18. Gap-Graded
Aggregate
Dense Graded
Aggregate

19. Rubber particles
in the binder help fight cracking

20. Mixing and Compacting
• Select a combined grading
• Prepare samples at 4 binder contents
 7.5 % – 9.0 %
• Compact using the gyratory compactor
 Select gyrations (50-150) and pressure (600-
825 kPa)
 30 - 90 minute dwell or squaring time (use of
fan to cool optional)
• Analyze volumetric properties for each binder
content
 Air voids (4.0 or 5.0 % and VMA (18 – 23 %)

21. SuperPave
Gyratory
Compactor
Fan to Aid in Cooling
6 “ Diameter Specimen

22. Aggregate/Binder Combination
Selection
• Select best aggregate and binder
combination that meets the
requirements
• Select optimum binder content
(OBC)

24. OBC Verification
• Mix and compact 3 specimens at
the OBC
• Verify mix volumetrics
• If volumetrics meet requirements
proceed with performance testing

25. Performance Tests of the
RHMA Mix
AASHTO T-283
Tensile TestHamburg Wheel
Tracking Device
(HWTD)

26. HWTD Performance Test
• Hamburg Wheel Tracking Device
(HWTD) for rut resistance
• 15,000 – 25,000 minimum passes for
maximum rut depth of 0.5 “depending
on grade of binder
• No stripping inflection
point requirement
(pending)

27. Stripping Inflection Point
Diagram
Normal SIP
For HMA Type A
No discernable SIP
For RHMA

28. AASHTO T-283 Performance
Test
• AASHTO T-283 for moisture
induced damage resistance
• Only dry and wet strengths required
(100 psi and 70 psi)
• No tensile strength
ratio (TSR) required
(except for selected areas)

29. Critical Issues With RHMA
• 7.5 % binder content (by total wt.)
– For RHMA mixes the voids and VMA must
be met for the 7.5 % binder content (this is
why we have a range for gyration and
pressure)
– For HMA Type A the binder content is
adjusted to meet the voids and VMA Dwell or
squaring time critical for cooling specimens to
eliminate swelling of specimens

30. Critical Issues With RHMA
• Voids and VMA requirements
– Must adjust grading, gyrations and pressure to
achieve requirements
– 18 vs. 20 % CRM
– The higher the CRM % and the coarser the CRM the
more difficult to compact and achieve volumetric
requirements (longer dwell time is required for higher
% of CRM)
• HWTD minimum number of passes are higher for the
0.5” rut requirement because of the higher viscosity
(5000 higher than HMA Type A for each grade)

31. Summary
• Mix design is similar to
conventional HMA
• But there are some significant
differences
• Industry continues to work
together with agencies in a
partnering effort with the goal of
improving the mix design process

32. Thank You