Aggregates make up 70-80% of concrete by volume and can be natural materials like sand, gravel, granite or artificial like slag or fly ash. They are classified based on weight as normal, light or heavy; size as fine or coarse; and shape as rounded, irregular, angular or flat. Good aggregates are hard, durable, free of organic materials and have low crushing, impact and abrasion values. Tests are conducted to evaluate aggregates for use in concrete.