Highway materials

A road is a layered structure built mostly from soil, crushed stone and binder. Each layer has a job: the subgrade is the prepared natural soil or fill that ultimately carries the load; the sub-base, often granular, spreads load, drains water and protects the subgrade; the base, of wet mix macadam, cement-treated or bituminous material, provides most of the structural strength; and the surface course gives a smooth, skid-resistant, waterproof riding surface.

Soil for subgrade and embankment is tested for grading, plasticity, maximum dry density and California Bearing Ratio. Aggregates must be strong, tough, durable, clean and well shaped. The aggregate impact value and Los Angeles abrasion value measure resistance to impact and wear; flakiness and elongation indices limit thin and long pieces that break under rollers; water absorption and soundness test durability; stripping tests check adhesion to bitumen.

Bitumen, the binder for flexible pavements, is graded in India by viscosity at 60 °C into VG10, VG20, VG30 and VG40, the stiffer grades for hotter climates and heavier traffic. Penetration, softening point, ductility and viscosity tests characterise it. Modified binders with polymers or crumb rubber resist rutting and cracking better. Emulsions and cutbacks allow cold application for tack and prime coats and patching. For rigid pavements, the materials are paving quality concrete with a design flexural strength, dowel and tie bars and joint sealants. Increasingly, recycled asphalt, industrial by-products such as slag and fly ash, and waste plastic are used where tests show they perform.