Rubberized Hot Mix Asphalt (RHMA) Mix Design

By

Jack Van Kirk

Director of Asphalt Technology

George Reed Inc.

CalAPA Fall ConferenceOctober 26 - 27, 2016Sacramento, CA

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?

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

Asphalt Rubber Specifications

Rubberized hot mix asphalt (RHMA)Type “G” *Type “O”Type “O-HB”

* For this presentation will only discuss Type G

Extremely Important

• RHMA mixes are more temperature sensitive

• Lab and field compaction must be achieved at higher temperatures

Parameters of the Mix Design

• Binder requirements • Aggregate requirements• Volumetrics and performance tests• Mix and compaction requirements• Major differences from

conventional HMA

Quality characteristic Test method RequirementAir voids content (%) AASHTO T 269a Ndesign = 4.0Gyration compaction (no. of gyrations) AASHTO T 312 Ndesign = 50–

150b

Voids in mineral aggregate (min, %) SP-2Asphalt Mixture

Volumetricsc

18.0–23.0

Hamburg wheel track (min, number of passes at 0.5-inch rut depth)Binder grade:PG 58PG 64PG 70

AASHTO T 324 (Modified)d

15,00020,00025,000

Hamburg wheel track (min, number of passes at the inflection point)Binder grade:PG 58PG 64PG 70

AASHTO T 324 (Modified)d

10,00010,00012,500

Moisture susceptibility, dry strength (min, psi) AASHTO T 283d 100Moisture susceptibility, wet strength (min, psi) AASHTO T 283d, 70

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

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

Asphalt Rubber

ConventionalAsphalt

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

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

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

Aggregate Requirements

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

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

Aggregate Gradations for RHMA-G(Percentage Passing)

1/2 inchSieve size Target value limit Allowable

tolerance3/4" 100 --1/2" 90–98 TV ± 63/8" 83–87 TV ± 5No. 4 28–42 TV ± 6No. 8 14–22 TV ± 5

No. 200 0.0–6.0 TV ± 2.0

Gap-Graded Aggregate

Gap-Graded Aggregate

Dense Graded Aggregate

Rubber particles in the binder help fight cracking

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 %)

SuperPave Gyratory

Compactor

Fan to Aid in Cooling

6 “ Diameter Specimen

Aggregate/Binder Combination Selection

• Select best aggregate and binder combination that meets the requirements

• Select optimum binder content (OBC)

OBC Verification

• Mix and compact 3 specimens at the OBC

• Verify mix volumetrics• If volumetrics meet requirements

proceed with performance testing

Performance Tests of the RHMA Mix

AASHTO T-283Tensile TestHamburg Wheel

Tracking Device (HWTD)

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)

Stripping Inflection Point Diagram

Normal SIPFor HMA Type A

No discernable SIPFor RHMA

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)

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

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)

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

Thank You