CCIL Superpave Mix Design 2 — Questions and Answers
Question 1: In Superpave mix design, what does the term 'N_design' refer to?
- The number of gyrations used to compact specimens to the design air void level (Correct answer)
- The number of aggregate gradation points required
- The nominal maximum aggregate size in millimeters
- The number of binder performance grade tests required
Correct answer: The number of gyrations used to compact specimens to the design air void level
N_design is the number of gyrator compaction cycles at which the mix must achieve 4% air voids, representing the expected traffic loading.
Question 2: Which Superpave aggregate property measures resistance to polishing under traffic?
- Coarse Aggregate Angularity (CAA)
- Sand Equivalent (SE)
- Flat and Elongated particle content (Correct answer)
- Los Angeles Abrasion value
Correct answer: Flat and Elongated particle content
The Los Angeles Abrasion (LAA) test measures aggregate resistance to degradation, but polishing resistance is assessed via the British Pendulum or other polish tests; however, in Superpave, CAA and uncompacted void content address angularity, not polishing — polishing resistance is evaluated through aggregate source approval.
Question 3: What is the primary purpose of the Superpave gyratory compactor (SGC)?
- To simulate field compaction by applying a kneading action with a fixed angle and pressure (Correct answer)
- To measure the resilient modulus of asphalt mixtures
- To determine the optimum binder content by measuring density at failure
- To compact specimens for indirect tensile strength testing only
Correct answer: To simulate field compaction by applying a kneading action with a fixed angle and pressure
The SGC simulates field compaction by rotating specimens at a 1.25° angle under 600 kPa pressure, replicating the kneading action of rubber-tired rollers.
Question 4: In Superpave volumetric design, VMA stands for:
- Volume of Mineral Aggregate
- Voids in the Mineral Aggregate (Correct answer)
- Volumetric Mix Analysis
- Void Maximum Allowance
Correct answer: Voids in the Mineral Aggregate
VMA (Voids in the Mineral Aggregate) is the intergranular void space in the compacted aggregate skeleton, including air voids and effective binder content.
Question 5: A Superpave mix design shows VFA = 58% at the design air void content. What does this indicate?
- The mix has too much binder and may bleed under traffic
- The binder fills only 58% of the VMA, which may be too low for adequate durability (Correct answer)
- The mix meets typical requirements for high-traffic roadways
- The aggregate gradation passes through the restricted zone
Correct answer: The binder fills only 58% of the VMA, which may be too low for adequate durability
VFA below 65% is generally below the minimum threshold, indicating insufficient binder to coat aggregates adequately, which can reduce durability and fatigue life.
Question 6: Which parameter is used to select the Superpave performance grade (PG) binder for a given project?
- Annual average daily traffic (AADT) and aggregate gradation
- Design pavement temperature range at the project location and traffic level (Correct answer)
- Marshall stability and flow values at the site
- The sand equivalent value of the local aggregate source
Correct answer: Design pavement temperature range at the project location and traffic level
PG binder selection is based on the high pavement design temperature (rutting) and low pavement design temperature (cracking) at the project site, adjusted for traffic speed and volume.
Question 7: In Superpave, what is the significance of the 'restricted zone' on the 0.45-power gradation chart?
- Aggregates must pass through this zone to ensure adequate VMA
- Gradations passing through this zone were found to produce tender mixes prone to rutting, though its use is now optional (Correct answer)
- It defines the maximum allowable amount of natural sand in the mix
- It marks the boundary between coarse-graded and fine-graded mixes
Correct answer: Gradations passing through this zone were found to produce tender mixes prone to rutting, though its use is now optional
The restricted zone was originally established to avoid tender, rut-prone mixes from excess fine sand, but NCHRP research showed it was not always necessary and AASHTO made it optional.
