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Rubberized Hot Mix Asphalt (RHMA) Mix Design

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  1. 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. 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. 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. 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. 5. Extremely Important RHMA mixes are more temperature sensitive Lab and field compaction must be achieved at higher temperatures
  6. 6. Parameters of the Mix Design Binder requirements Aggregate requirements Volumetrics and performance tests Mix and compaction requirements Major differences from conventional HMA
  7. 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 18.023.0 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. 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. 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. 10. Asphalt Rubber Conventional Asphalt
  11. 11. Laboratory Binder Design Asphalt heated to 400 to 425F 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. 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. 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. 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. 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. 16. Aggregate Gradations for RHMA-G (Percentage Passing) 1/2 inch Sieve size Target value limit Allowable tolerance 3/4" 100 -- 1/2" 9098 TV 6 3/8" 8387 TV 5 No. 4 2842 TV 6 No. 8 1422 TV 5 No. 200 0.06.0 TV 2.0
  17. 17. Gap-Graded Aggregate
  18. 18. Gap-Graded Aggregate Dense Graded Aggregate
  19. 19. Rubber particles in the binder help fight cracking
  20. 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. 21. SuperPave Gyratory Compactor Fan to Aid in Cooling 6 Diameter Specimen
  22. 22. Aggregate/Binder Combination Selection Select best aggregate and binder combination that meets the requirements Select optimum binder content (OBC)
  23. 23. OBC Verification Mix and compact 3 specimens at the OBC Verify mix volumetrics If volumetrics meet requirements proceed with performance testing
  24. 24. Performance Tests of the RHMA Mix AASHTO T-283 Tensile TestHamburg Wheel Tracking Device (HWTD)
  25. 25. 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)
  26. 26. Stripping Inflection Point Diagram Normal SIP For HMA Type A No discernable SIP For RHMA
  27. 27. 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)
  28. 28. 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
  29. 29. 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)
  30. 30. 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
  31. 31. Thank You

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