Marshall stability test procedure

The Marshall stability test finds the load-carrying capacity and deformation of a compacted cylindrical bituminous specimen. It is the basic test of the Marshall mix design method (ASTM D6927, Asphalt Institute MS-2, adopted in NRS 2070).

Apparatus

• Cylindrical mould 101.6 mm dia and 75 mm high, with base plate and collar.
• Marshall compaction hammer (4.536 kg, free fall 457 mm) and compaction pedestal.
• Breaking head (two semicircular segments of 101.6 mm inside diameter), Marshall testing machine with a proving ring/load cell and a flow dial, loading rate 50 mm/min.
• Water bath at 60 degrees C, ovens, mixing bowl, thermometers.

Procedure

1. Heat aggregates (blended to the specified gradation, about 1200 g per specimen) and bitumen (grade 60/70 or VG-30) to the mixing temperature (about 150-160 degrees C for aggregates, 150 degrees C for bitumen); mix thoroughly.
2. Place in a preheated mould (and a paper disc at the bottom) and compact with 75 blows on each face for heavy traffic (50 for medium; 35 for light) at about 135-140 degrees C.
3. Cool, extrude, and measure height; find weight in air and in water and calculate the bulk density and voids.
4. Immerse the specimens in the 60 degrees C water bath for 30-40 minutes (to represent the hottest condition).
5. Remove, place in the breaking head, and put it in the Marshall machine; apply load at 50 mm/min until the maximum load is reached, then the load drops.
6. Record the maximum load (Marshall stability, kN) with correction for the specimen thickness, and the flow value (vertical deformation at the max load in 0.25 mm units or mm). The test must be done within 30 seconds after removing from the bath.

Significance

• Stability (typically not less than 8 kN for heavy traffic; or as per NRS 2070/IRC:111 for BC) shows the resistance to rutting and shoving.
• Flow (2 to 4 mm) indicates the plasticity and flexibility; high flow means a plastic mix that tends to rut; low flow means a brittle mix that cracks.
• Marshall quotient = stability/flow (kN/mm), an indicator of the stiffness of the mix.
• Together with density and voids data, the results are used to find the optimum binder content, to check the mix against standards, and for quality control of the plant mix.