AASHTO Terminology

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:

Note that these definitions are AASHTO definitions and are slightly different than those of the Unified Soil Classification system (ASTM). The table below shows the AASHTO soil classification system (from AASHTO M 145).

AASHTO Soil Classification System (from ASTM M 145)

General Classification	Granular Materials 35% or less passing the 0.075 mm sieve							Silt-Clay Materials >35% passing the 0.075 mm sieve
Group Classification	A-1		A-3	A-2				A-4	A-5	A-6	A-7
Group Classification	A-1-a	A-1-b	A-3	A-2-4	A-2-5	A-2-6	A-2-7	A-4	A-5	A-6	A-7-5 A-7-6
Sieve Analysis, % passing
2.00 mm (No. 10)	50 max	---	---	---	---	---	---	---	---	---	---
0.425 (No. 40)	30 max	50 max	51 max	---	---	---	---	---	---	---	---
0.075 (No. 200)	15 max	25 max	10 max	35 max	35 max	35 max	35 max	36 min	36 min	36 min	36 min
Characteristics of fraction passing 0.425 mm (No. 40)
Liquid limit	---		---	40 max	41 min	40 max	41 min	40 max	41 min	40 max	41 min
Plasticity index	6 max		N.P.	10 max	10 max	11 min	11 min	10 max	10 max	11 min	11 min^a
Usual types of significant constituent materials	stone fragments, gravel and sand		fine sand	silty or clayey gravel and sand				silty soils		clayey soils
General rating as a subgrade	excellent to good							fair to poor
^aPlasticity index of A-7-5 subgroup is equal to or less than the LL - 30. Plasticity index of A-7-6 subgroup is greater than LL - 30

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 ³ 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.