Aggregates are the fraction of rock which are inert or non-reactive. These are obtained from rocks by means of crushing.
Main constituents of the wearing surface are bitumen and aggregates. Mostly 90 % of the road consists of aggregates.
There are two problems to use non-inert materials.
- Cost will increase. As non-inert materials are expensive than inert materials.
- Non-inert materials will form weak bond with bitumen.
Desirable properties of aggregates
- Strength is a measure of resistance to crushing. Crushing value depends on strength.
- Soundness is another property of aggregates. It is the measure of resistance to weathering. Weathering includes variation of temperature, rainfall, and wind forces etc.
- Their Shape must be angular for roads. They should not be rounded, flaky or elongated.
- Toughness should be more. Toughness is the measure of resistance of aggregates to fracture. Fracture is the splitting of aggregates into pieces.
- Hardness should be more. Hardness is the measure of resistance of aggregates to abrasion. Abrasion is the rubbing action between road and tire.
- They must be non-porous. They should absorb minimum water as much as possible.
- They should have more affinity for bitumen. Hydrophobic have less affinity with water and more affinity for bitumen. Whereas Hydrophilic having more affinity for water and less affinity for bitumen.
- They must be economical.