CCIL Marshall Asphalt Mix Design — Questions and Answers
Question 1: In the Marshall mix design procedure, how many blows per face are applied to each specimen when designing a mix for heavy traffic conditions?
- 35 blows
- 50 blows
- 75 blows (Correct answer)
- 100 blows
Correct answer: 75 blows
The Marshall method (ASTM D1559 / AASHTO T 245) uses 75 blows per face for mixes designed for heavy traffic loads (equivalent single axle loads exceeding 3 million). Medium traffic uses 50 blows per face, and light traffic uses 35 blows per face. The compaction energy level affects the void structure and volumetric properties of the compacted specimen.
Question 2: In Marshall mix design, what does 'Marshall Stability' measure?
- The tensile strength of the asphalt mix at 25°C
- The maximum load resistance of a compacted specimen at 60°C under a constant loading rate (Correct answer)
- The resistance of loose asphalt mix to oxidative aging
- The penetration of a standard needle into the compacted mix
Correct answer: The maximum load resistance of a compacted specimen at 60°C under a constant loading rate
Marshall Stability is the maximum load (in kN or lbs) that a 101.6 mm (4-inch) diameter compacted specimen can resist when loaded diametrically at a rate of 50.8 mm/min (2 in/min) at 60°C. The 60°C test temperature simulates the maximum pavement surface temperature in summer service conditions.
Question 3: What is 'Marshall Flow' and what does an excessively high flow value indicate?
- The rate of binder drainage from the mix; high flow indicates inadequate binder viscosity
- The deformation of the specimen at maximum load in 0.25 mm units; high flow suggests the mix may be prone to rutting (Correct answer)
- The air void content at optimum binder content; high flow indicates over-compaction
- The rate of creep under sustained load; high flow indicates thermal cracking susceptibility
Correct answer: The deformation of the specimen at maximum load in 0.25 mm units; high flow suggests the mix may be prone to rutting
Marshall Flow is the total deformation of the specimen (measured in 0.25 mm increments) that occurs from initial loading to the point of maximum load. An excessively high flow value (above the specified maximum, typically 16–20 units) indicates a mix that is too plastic and may be susceptible to permanent deformation (rutting) under traffic.
Question 4: In the Marshall design procedure, what is the standard specimen dimensions used for mix design?
- 100 mm diameter × 63.5 mm height
- 150 mm diameter × 95 mm height
- 101.6 mm diameter × 63.5 mm height (Correct answer)
- 50 mm diameter × 50 mm height
Correct answer: 101.6 mm diameter × 63.5 mm height
Standard Marshall specimens are 101.6 mm (4 inches) in diameter and approximately 63.5 mm (2.5 inches) in height after compaction. This height is critical: specimens outside the range of 57.2–69.9 mm require stability correction factors. The standard mold dimensions and compaction procedure are defined in ASTM D6926.
Question 5: At what test temperature is Marshall Stability typically determined, and why?
- 25°C, to simulate average annual pavement temperature
- 40°C, to simulate the softening point of the binder
- 60°C, to simulate maximum summer pavement surface temperature (Correct answer)
- 135°C, to simulate mixing temperature conditions
Correct answer: 60°C, to simulate maximum summer pavement surface temperature
Marshall Stability tests are performed at 60°C (140°F) because this temperature approximates the maximum pavement surface temperature experienced during hot summer conditions. Testing at this temperature ensures the mix has adequate stiffness and deformation resistance under the most critical service conditions, when binders are softest.
Question 6: Which Marshall volumetric property describes the percentage of total mix volume occupied by voids in the compacted mineral aggregate skeleton?
- Air Voids (Va)
- Voids in the Mineral Aggregate (VMA) (Correct answer)
- Voids Filled with Asphalt (VFA)
- Effective Binder Content (Pbe)
Correct answer: Voids in the Mineral Aggregate (VMA)
Voids in the Mineral Aggregate (VMA) represents the total void space between aggregate particles in the compacted mix, including both the air voids and the space occupied by the effective asphalt binder. VMA must be sufficiently high (minimum values range from 13–17% depending on nominal maximum aggregate size) to allow adequate binder film thickness for durability.
CCIL Marshall Asphalt Mix Design
In the Marshall mix design procedure, how many blows per face are applied to each specimen when designing a mix for heavy traffic conditions?