Rubberized Hot Mix Asphalt (RHMA) Mix Design
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Transcript of Rubberized Hot Mix Asphalt (RHMA) Mix Design
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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
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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?
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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
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Asphalt Rubber Specifications
Rubberized hot mix asphalt (RHMA)Type “G” *Type “O”Type “O-HB”
* For this presentation will only discuss Type G
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Extremely Important
• RHMA mixes are more temperature sensitive
• Lab and field compaction must be achieved at higher temperatures
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Parameters of the Mix Design
• Binder requirements • Aggregate requirements• Volumetrics and performance tests• Mix and compaction requirements• Major differences from
conventional HMA
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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
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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
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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
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Asphalt Rubber
ConventionalAsphalt
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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
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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
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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
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Aggregate Requirements
• High quality aggregate is required because of the reduced thickness pavement design–High % fractured faces–Low % flat and elongated–Sound durable particles
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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
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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
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Gap-Graded Aggregate
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Gap-Graded Aggregate
Dense Graded Aggregate
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Rubber particles in the binder help fight cracking
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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 %)
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SuperPave Gyratory
Compactor
Fan to Aid in Cooling
6 “ Diameter Specimen
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Aggregate/Binder Combination Selection
• Select best aggregate and binder combination that meets the requirements
• Select optimum binder content (OBC)
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OBC Verification
• Mix and compact 3 specimens at the OBC
• Verify mix volumetrics• If volumetrics meet requirements
proceed with performance testing
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Performance Tests of the RHMA Mix
AASHTO T-283Tensile TestHamburg Wheel
Tracking Device (HWTD)
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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)
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Stripping Inflection Point Diagram
Normal SIPFor HMA Type A
No discernable SIPFor RHMA
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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)
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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
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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)
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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
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Thank You