Marshall method of bituminous mix design

The Marshall method (developed by Bruce Marshall and further by the US Army Corps of Engineers) designs a bituminous mix by testing compacted cylindrical specimens for stability, flow, density and voids at different binder contents, and then selects the optimum binder content (OBC). It is the method used in Nepal (NRS 2070, the Asphalt Institute MS-2 and IRC:111).

Specimen

Cylinders 101.6 mm (4 in) in diameter and 63.5 mm (2.5 in) high, made from about 1200 g of aggregate plus bitumen.

Procedure

1. Select materials - aggregates that pass the quality tests, with grading within the specified envelope (for example, BC or DBM), and the bitumen (VG-30/60-70 penetration grade).
2. Prepare the trial mixes - at least 5 binder contents in 0.5% steps (for example, 4.5, 5.0, 5.5, 6.0 and 6.5% by weight of mix), with 3 specimens each.
3. Mixing and compaction - heat the aggregates to about 150-160 degrees C and the bitumen to 140-150 degrees C; mix; place in a mould at the compaction temperature (about 138 degrees C) and compact with the Marshall hammer (4.5 kg falling 457 mm) with 75 blows on each face (heavy traffic; 50 for medium).
4. Density - cool, extrude, and find the bulk specific gravity $G_b$ (from the weights in air and in water).
5. Stability and flow test - immerse the specimens in a 60 degrees C water bath for 30-40 minutes, then load them at 50 mm/min in the Marshall testing head. The maximum load in kN is the Marshall stability; the deformation at maximum load in mm is the flow value.
6. Voids analysis - calculate $V_v$ (air voids), VMA (voids in mineral aggregate) and VFB (voids filled with bitumen), and apply the stability correction for specimen volume/height.
7. Plot graphs (binder content on the X-axis): stability, flow, unit weight, $V_v$, VMA and VFB.

Optimum binder content

Take the binder contents corresponding to:

• maximum stability ($B_1$),
• maximum bulk density ($B_2$),
• 4% air voids, the middle of 3-5% ($B_3$).

$$\text{OBC} = \frac{B_1 + B_2 + B_3}{3}$$

The mix is then checked against the criteria (for 75 blows: stability not less than about 8 kN; flow 2-4 mm; air voids 3-5%; VFB 65-75%; VMA as per the nominal aggregate size). If any criterion is not satisfied, the mix is redesigned (by changing the grading or the binder).