CCIL Marshall Asphalt Mix Design

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?

Accepted Answer

75 blows

Answer

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?

Accepted Answer

The maximum load resistance of a compacted specimen at 60°C under a constant loading rate

Answer

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?

Accepted Answer

The deformation of the specimen at maximum load in 0.25 mm units; high flow suggests the mix may be prone to rutting

Answer

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?

Accepted Answer

101.6 mm diameter × 63.5 mm height

Answer

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?

Accepted Answer

60°C, to simulate maximum summer pavement surface temperature

Answer

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.

CCIL - Canadian Council of Independent Laboratories Practice Test

CCIL - Canadian Council of Independent Laboratories Practice Test

CCIL Marshall Asphalt Mix Design