Concrete is a mixture of paste and aggregates. Aggregates make up 60-75% of concrete and include sand, gravel, or crushed stone. Aggregates are classified as fine or coarse based on their size, and can be natural or manufactured. Tests are performed on aggregates to determine properties like grading, shape, density, moisture content, and durability which influence the properties of fresh and hardened concrete. Proper aggregate selection and testing is important for producing high quality, high strength concrete.