Lecture Notes on Marshall and Superpave Mix Design Process

Overview

• Previous discussion on design aggregate gradation concluded.
• Focus on further steps in Marshall and Superpave mix design processes.
• Aim: Understand differences between the two methods.

Steps in Mix Design Process

1. Material Selection

   • Binder and aggregates chosen.
   • Design aggregate gradation completed.

2. Mixing and Compaction

   • Producing a cylindrical bituminous mixture for laboratory testing.
   • Marshall Method:
     • Uses impact compaction.
     • Standard: 75 blows on both sides.
     • Impact rate: 64 ± 4 blows/min.
     • Adjustments based on mix type (e.g., stone mastic asphalt requires only 50 blows).
   • Superpave Method:
     • Uses gyratory compactor.
     • Controlled by vertical pressure, angle of gyration, and number of gyrations.
     • Standard values:
       • Vertical pressure: ±18 kPa
       • Angle of gyration: 1.16 ± 0.02 degrees
       • Gyration rate: 30 ± 0.05 gyrations/min.

Differences in Compaction

• Aggregate Orientation:
  • Varies between methods, affecting final properties.
• Compaction Techniques:
  • Marshall uses impact compaction; Superpave uses gyratory compaction.

Sample Preparation

• Marshall Method:

  • Mold and collar are used for compaction.
  • Standard sample height: 63.5 mm; diameter: 101.6 mm.
  • Samples cooled overnight before removal.

• Modified Marshall Method:

  • Used for aggregates > 25 mm in size.
  • Adjusted hammer weight and sample dimensions.
  • Higher number of blows required.

• Superpave Method:

  • Samples prepared at 3-5 binder contents.
  • Standard sample height: 115 mm or 100 mm diameter.

Testing Procedures

1. Marshall Mix Design:

   • Stability and Flow Tests:
     • Conducted at 60°C, load applied at 51 mm/min.
     • Stability: Maximum load before failure.
     • Flow: Vertical deformation at peak load.
   • Correction factors for height differences.

2. Superpave Mix Design:

   • No specific strength tests.
   • Focus on moisture sensitivity and volumetric parameters.
   • Indirect tensile strength ratio (ITS) for moisture sensitivity.

Air Voids and Design Checks

• Marshall Method:
  • Aim for 4% air voids; check various parameters.
• Superpave Method:
  • Three checks: N initial, N design, and N max:
    • N initial: Ensures mix is not too tender (air voids 89-91.5% GMM).
    • N design: Simulates field density at 4% air voids.
    • N max: Limits air voids to no less than 2%.

Final Checks and Conditions

• Moisture Sensitivity:
  • Minimum tensile strength ratio > 80% in Marshall method.
  • Performance testing suggested for confidence in mix design.

Conclusion

• Marshall and Superpave mix design methods have distinct processes and testing requirements.
• Importance of performance-based mix design to enhance durability and field performance.
• Next presentation will cover concepts related to performance-based mix design.