Crushed aggregate mixed with bitumen (semi-solid hydrocarbon fraction of crude oil)
- Used to surface roads, roads are constructed in layers with a base-course overlain by a top wearing-course.
Must be:
- Strong, high impact and crushing strength, withstand traffic weight
- Resistant to abrasion and mechanical weathering (freeze-thaw)
- Impermeable and resistant to chemical corrosion (gritting and chemical spills)
- Skid resistant, every chipping must be made of more than one mineral with different hardness so they wear down at different rates and do not polish
- Bond well with bitumen
Basalt and dolerite are best, Granite is bad as it polishes, local sources used mean variety
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