What is the difference between coarse and fine aggregates?

Fine aggregate (sand) passes through a 4.75mm sieve and fills voids between coarse aggregates, providing workability. Coarse aggregate (gravel or crushed stone) is retained on a 4.75mm sieve and provides bulk and strength. Maximum aggregate size is typically 20-25mm, limited to 1/4 of member thickness or 3/4 of clear spacing between bars. Both must be clean, hard, durable, and free from harmful materials like clay, silt, or organic matter.