Desirable properties of bituminous mixes

A bituminous mix (aggregate plus bitumen) should have the following properties.

1. Stability - resistance to deformation (rutting, shoving) under traffic load, which comes from the internal friction and interlock of the aggregates and the cohesion of the binder. It depends on the aggregate shape, grading and binder content.
2. Durability - the mix must resist weathering, ageing of the bitumen, water, and the abrasive action of traffic for a long time. This needs a sufficient binder content with adequate film thickness, low voids and good compaction.
3. Flexibility - the ability to adjust to settlement and deflection of the base and subgrade without cracking; it requires a higher binder content and a proper binder grade.
4. Fatigue resistance - resistance to cracking under repeated load applications.
5. Skid resistance - a rough texture and angular aggregate give a good grip on wet and dry surfaces.
6. Impermeability - protects the lower layers from water; air voids should be adequate (3-5%) but not interconnected.
7. Workability - ease of mixing, spreading and compaction; it depends on the grading and temperature.
8. Resistance to moisture damage (stripping) - good adhesion between aggregate and bitumen.
9. Resistance to low-temperature cracking and high-temperature rutting.
10. Economy - using local materials at minimum cost.

These properties conflict (for example, more binder gives durability and flexibility but reduces stability), so the mix design (Marshall) finds an optimum balance.