CCIL Superpave Mix Design 3 — Questions and Answers
Question 1: What does the Superpave parameter 'Dust Proportion' (DP) measure and what is its typical acceptable range?
- The ratio of P0.075 to effective binder content; typically 0.6–1.2 (Correct answer)
- The percentage of aggregate passing the No. 200 sieve; typically 3–8%
- The ratio of coarse to fine aggregate by weight; typically 1.0–2.0
- The ratio of natural sand to total aggregate; typically 0–15%
Correct answer: The ratio of P0.075 to effective binder content; typically 0.6–1.2
Dust Proportion equals the percentage passing the 0.075 mm sieve divided by the effective binder content, and values between 0.6 and 1.2 indicate a balanced mix.
Question 2: During Superpave mix design, what condition must be met at N_initial gyrations?
- Air voids must equal exactly 4%
- The relative density must not exceed 89% of theoretical maximum density (Gmm) (Correct answer)
- The VMA must be at its minimum specified value
- The specimen height must equal 115 mm
Correct answer: The relative density must not exceed 89% of theoretical maximum density (Gmm)
At N_initial, the mix density must not exceed 89% Gmm; exceeding this limit suggests the mix may over-compact under traffic and become unstable.
Question 3: How is the Theoretical Maximum Density (Gmm) of an asphalt mixture determined?
- By measuring the bulk specific gravity of compacted specimens in a water bath
- By the Rice test (ASTM D2041), dissolving air voids in a vacuum-sealed container with water (Correct answer)
- By dividing the dry weight of the mix by the volume of the compacted specimen
- By calculating from aggregate gradation and binder specific gravity only
Correct answer: By the Rice test (ASTM D2041), dissolving air voids in a vacuum-sealed container with water
Gmm is determined by the Rice method (ASTM D2041/AASHTO T209), which removes air voids under vacuum and measures the void-free density of the loose mixture.
Question 4: Which Superpave aggregate test evaluates the angularity of fine aggregates?
- Uncompacted Void Content of Fine Aggregate (AASHTO T304) (Correct answer)
- Coarse Aggregate Angularity (CAA) test
- Los Angeles Abrasion test
- Flat and Elongated ratio determination
Correct answer: Uncompacted Void Content of Fine Aggregate (AASHTO T304)
The Uncompacted Void Content test (AASHTO T304) measures the void space in loosely placed fine aggregate, with higher values indicating more angular particles that provide better shear resistance.
Question 5: What is the effect of increasing the nominal maximum aggregate size (NMAS) in a Superpave mix?
- VMA decreases, allowing for less binder while maintaining durability (Correct answer)
- VMA increases, requiring more binder to fill the larger void space
- Air voids increase proportionally with aggregate size
- The required N_design increases to accommodate larger particles
Correct answer: VMA decreases, allowing for less binder while maintaining durability
Larger NMAS mixes generally produce lower VMA because coarser gradations pack more efficiently, which can reduce binder content requirements.
Question 6: In Superpave design, what does 'effective binder content' (Vbe) represent?
- The total binder content by weight of the mix
- The portion of binder remaining after absorption into the aggregate, contributing to the binder film (Correct answer)
- The binder content required to achieve 4% air voids at N_design
- The volume of binder per unit volume of aggregate
Correct answer: The portion of binder remaining after absorption into the aggregate, contributing to the binder film
Effective binder content is the total binder minus absorbed binder; it coats aggregate surfaces and fills the portion of VMA not occupied by air voids.
Question 7: A Superpave design trial shows the mix does not meet the minimum VMA requirement. Which adjustment is most appropriate?
- Increase the compaction effort by adding more gyrations
- Change the aggregate gradation to move away from maximum density line (Correct answer)
- Reduce the design binder content to increase aggregate contact
- Increase the aggregate absorption to reduce effective binder
Correct answer: Change the aggregate gradation to move away from maximum density line
VMA is controlled by aggregate gradation; moving the gradation away from the maximum density line (e.g., coarser or finer) increases interparticle void space and VMA.
CCIL Superpave Mix Design 3
What does the Superpave parameter 'Dust Proportion' (DP) measure and what is its typical acceptable range?