CCIL Superpave Mix Design 5 — Questions and Answers
Question 1: What does 'Percent Gmm at N_design' represent, and what is the target value?
- The ratio of bulk density to maximum density expressed as a percentage; target is 96% (4% air voids) (Correct answer)
- The percentage of binder by volume at the design gyration count; target is 5%
- The compaction ratio after N_design gyrations compared to N_max; target is 98%
- The percentage of aggregate passing No. 4 sieve at the design traffic level; target is 50%
Correct answer: The ratio of bulk density to maximum density expressed as a percentage; target is 96% (4% air voids)
Percent Gmm at N_design equals (Gmb/Gmm)×100; reaching 96% means 4% air voids remain, which is the Superpave design target.
Question 2: How does the Superpave system account for very slow-moving or stationary traffic loads when selecting a PG binder grade?
- By specifying a lower high-temperature PG grade to reduce stiffness
- By bumping up the high-temperature PG grade one or two grades to account for increased rutting risk (Correct answer)
- By requiring a higher N_design to simulate the additional loading cycles
- By mandating polymer modification regardless of the pavement temperature
Correct answer: By bumping up the high-temperature PG grade one or two grades to account for increased rutting risk
AASHTO M320 specifies that slow-moving traffic (< 70 km/h) requires the high-temperature PG grade to be increased by one grade, and stationary loads require an increase of two grades.
Question 3: In Superpave mix design, what is the function of the N_maximum gyration criterion?
- To ensure the mix does not over-compact under extended traffic to below 2% air voids (Correct answer)
- To define the minimum compaction effort needed to achieve 4% air voids
- To establish the mixing temperature for the production plant
- To determine the maximum allowable binder film thickness
Correct answer: To ensure the mix does not over-compact under extended traffic to below 2% air voids
At N_maximum, the mix must not compact to less than 2% air voids (≥98% Gmm), ensuring it will not become unstable or exhibit bleeding under the maximum anticipated traffic loading.
Question 4: What aggregate property is evaluated by the 'flat and elongated' particle test in Superpave?
- The percentage of particles with a length-to-thickness ratio exceeding a specified limit (typically 5:1) (Correct answer)
- The percentage of particles that pass the 0.075 mm sieve after washing
- The void space in loosely packed aggregate measured by rodding
- The number of fractured faces on coarse aggregate particles
Correct answer: The percentage of particles with a length-to-thickness ratio exceeding a specified limit (typically 5:1)
Flat and elongated particles with a ratio > 5:1 are prone to breakage during compaction and traffic, reducing the mix's long-term structural integrity.
Question 5: A Superpave mix design trial at the estimated optimum binder content shows VFA = 82%. For a high-traffic roadway, this result indicates:
- The mix is acceptable since VFA exceeds the minimum of 65%
- The VFA is likely too high, indicating insufficient air voids and potential rutting risk (Correct answer)
- The mix needs more fines to increase the dust proportion
- The mix should have a lower N_design to reduce over-compaction
Correct answer: The VFA is likely too high, indicating insufficient air voids and potential rutting risk
For high-traffic levels, Superpave specifies a maximum VFA of 75–80%; a value of 82% suggests excess binder, which can lead to flushing and rutting under heavy loads.
Question 6: Which laboratory test is used in Superpave to evaluate a compacted mix specimen's resistance to permanent deformation (rutting)?
- Indirect Tensile Strength (IDT) at -10°C
- Hamburg Wheel-Track Test (AASHTO T324) or Flow Number (AASHTO T378) (Correct answer)
- Marshall Stability and Flow (ASTM D1559)
- Beam fatigue test at intermediate temperature
Correct answer: Hamburg Wheel-Track Test (AASHTO T324) or Flow Number (AASHTO T378)
Performance-related rutting tests such as the Hamburg Wheel-Track Test or Flow Number test are used in Superpave-based specifications to verify resistance to permanent deformation.
Question 7: In Superpave, what is the purpose of selecting trial binder contents bracketing the estimated optimum?
- To identify the binder content that maximizes Marshall stability at a fixed flow value
- To construct a volumetric property curve and interpolate the binder content that achieves 4% air voids at N_design (Correct answer)
- To determine the mixing and compaction temperatures from viscosity measurements
- To evaluate moisture susceptibility at multiple binder contents before selecting the design
Correct answer: To construct a volumetric property curve and interpolate the binder content that achieves 4% air voids at N_design
By testing three or more binder contents, the designer plots air voids, VMA, and VFA versus binder content and interpolates the optimum at 4% air voids, then checks all other criteria.
CCIL Superpave Mix Design 5
What does 'Percent Gmm at N_design' represent, and what is the target value?