AASHTO soil terminology comes from AASHTO M 145, “Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes”. Aggregate terminology comes from AASHTO M 147, “Materials for Aggregate and Soil-Aggregate Subbase, Base and Surface Courses”. Basic terms include:
- Boulders & Cobbles: Material retained on a 75-mm (3-inch) sieve.
- Gravel: Material passing a 75-mm (3-inch) sieve and retained on a 2.00-mm (No. 10) sieve.
- Coarse Sand: Material passing a 2.00-mm sieve (No. 10) and retained on a 0.475-mm (No. 40) sieve.
- Fine Sand: Material passing a 0.475-mm (No. 40) sieve and retained on a 0.075-mm (No. 200) sieve.
- Silt-Clay: Material passing a 0.075-mm (No. 200) sieve.
- Silt Fraction: Material passing the 0.075 mm and larger than 0.002 mm.
- Clay Fraction: Material smaller than 0.002 mm.
- Silty: Material passing a 4.75-mm (No. 4) sieve with a PI ≤ 10
- Clayey: Material passing a 4.75-mm (No. 4) sieve with a PI ≥ 11
- Coarse Aggregate: Aggregate retained on the 2.00 mm sieve and consisting of hard, durable particles or fragments of stone, gravel or slag. A wear requirement (AASHTO T 96) is normally required.
- Fine Aggregate: Aggregate passing the 2.00 mm (No. 10) sieve and consisting of natural or crushed sand, and fine material particles passing the 0.075 mm (No. 200) sieve. The fraction passing the 0.075 mm (No. 200) sieve shall not be greater than two-thirds of the fraction passing the 0.425 mm (No. 40) sieve. The portion passing the 0.425 mm (No. 40) sieve shall have a LL ≤ 25 and a PI ≤ 6. Fine aggregate shall be free from vegetable matter and lumps or balls of clay.
Note that these definitions are AASHTO definitions and are slightly different than those of the Unified Soil Classification system (ASTM).