CCIL Superpave Mix Design 4 — Questions and Answers
Question 1: What traffic parameter determines the N_design level in Superpave mix design?
- Annual average daily traffic (AADT)
- Design Equivalent Single Axle Loads (ESALs) over the pavement design life (Correct answer)
- Posted speed limit on the roadway
- Number of lanes on the facility
Correct answer: Design Equivalent Single Axle Loads (ESALs) over the pavement design life
N_design is selected based on the 20-year design ESALs, with higher traffic volumes requiring more gyrations to simulate greater compaction under loading.
Question 2: In Superpave, the 'coarse aggregate angularity' (CAA) test measures:
- The percentage of coarse aggregate particles with one or more fractured faces (Correct answer)
- The void content of loosely packed coarse aggregate
- The ratio of elongated to flat particles by weight
- The LA abrasion loss after 500 revolutions
Correct answer: The percentage of coarse aggregate particles with one or more fractured faces
CAA is expressed as the percentage of aggregate particles by count (or weight) with at least one or two mechanically fractured faces, which improve shear strength and resistance to rutting.
Question 3: Which test method is used to determine the bulk specific gravity (Gmb) of a compacted Superpave specimen?
- AASHTO T166 (saturated surface-dry method) or T331 (CoreLok) (Correct answer)
- AASHTO T209 (Rice maximum specific gravity test)
- ASTM D1559 (Marshall stability and flow)
- AASHTO T283 (moisture-induced damage test)
Correct answer: AASHTO T166 (saturated surface-dry method) or T331 (CoreLok)
Gmb is typically measured by AASHTO T166 (SSD weighing in water) or T331 (CoreLok vacuum-sealed bag) for permeable specimens, providing the compacted specimen density.
Question 4: What is the purpose of the AASHTO T283 (modified Lottman) test in Superpave mix design evaluation?
- To determine the optimum binder content from stability and flow measurements
- To evaluate the susceptibility of the mix to moisture-induced damage (stripping) (Correct answer)
- To measure the dynamic modulus of the compacted mixture at multiple temperatures
- To assess the fatigue cracking resistance of the design mix
Correct answer: To evaluate the susceptibility of the mix to moisture-induced damage (stripping)
AASHTO T283 compares the tensile strength of conditioned (freeze-thaw cycled, vacuum-saturated) specimens to unconditioned specimens; a TSR ≥ 0.80 is typically required.
Question 5: In Superpave, what is 'binder absorption' and how does it affect mix design?
- Absorption is binder lost to oxidation; it requires a higher PG grade selection
- Absorption is binder drawn into permeable aggregate pores; it reduces the effective binder and must be accounted for in volumetric calculations (Correct answer)
- Absorption is the increase in binder viscosity during mixing; it requires higher mixing temperatures
- Absorption is the amount of binder retained on aggregate surfaces after compaction
Correct answer: Absorption is binder drawn into permeable aggregate pores; it reduces the effective binder and must be accounted for in volumetric calculations
Absorbed binder fills permeable pores in the aggregate and is not available for film formation; failing to account for it leads to underestimating the needed total binder content.
Question 6: At what air void content is the optimum binder content selected in Superpave volumetric design?
- 2% air voids
- 3% air voids
- 4% air voids (Correct answer)
- 5% air voids
Correct answer: 4% air voids
Superpave selects the optimum binder content at which the mix achieves exactly 4% air voids at N_design gyrations, then verifies all other volumetric criteria are met.
Question 7: Which of the following best describes a 'coarse-graded' Superpave mix on the 0.45-power chart?
- The gradation plots above the maximum density line through most of its range
- The gradation plots below the maximum density line through most of its range (Correct answer)
- The gradation passes through the center of the restricted zone
- The gradation is entirely within the allowable band without a restricted zone
Correct answer: The gradation plots below the maximum density line through most of its range
A coarse-graded mix plots below the maximum density line, meaning there are more large particles and fewer intermediates, creating more void space (higher VMA) in the aggregate skeleton.
CCIL Superpave Mix Design 4
What traffic parameter determines the N_design level in Superpave mix design?