Marshall Asphalt Mix Design Flashcards

In the Marshall mix design procedure, how many blows per face are applied to each specimen when designing a mix for heavy traffic conditions?

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.

In Marshall mix design, what does 'Marshall Stability' measure?

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.

What is 'Marshall Flow' and what does an excessively high flow value indicate?

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.

In the Marshall design procedure, what is the standard specimen dimensions used for mix design?

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.

At what test temperature is Marshall Stability typically determined, and why?

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.

Which Marshall volumetric property describes the percentage of total mix volume occupied by voids in the compacted mineral aggregate skeleton?

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.