1. Pavement Base & Subbase CEE 320 Steve Muench 8/13/2015 1 The subbase course is between the base course and the subgrade. It functions primarily as structural support. The Base course is defined as the layer of material that lies immediately below the wearing surface of a pavement

2. Subbase Material Subbase: May consist of selected materials, such as natural gravels, that are stable but have characteristics which make them not completely suitable as base course (less CBR, M R ) They may also be of stabilized soil or simply select borrow. The quality of subbase can vary within wide limits as long as the thickness design criteria (SN) are fulfilled.

3. Base Material Base: Consist of selected good quality Aggregate. In the case of asphalt pavement, the base course lies close to the surface; and therefore, it must possess high resistance to deformation in order to withstand the high pressure imposed upon it. Base material should be of the same quality as that used in the Surface layer. The aggregate should me hard, sound and durable, resistant to abrasion, and angular.

4. Purpose & Importance of Subbase The purpose of a subbase is to permit the building of relatively thick pavement at low cost. It functions primarily as structural support but it can also: Minimize the intrusion of fines from the subgrade into the pavement structure. Improve drainage. Minimize frost action damage.frost action damage Provide a working platform for constructionconstruction

5. Purpose & Importance of Base The purpose and function of the base varies according to type of pavement. Base course is used under rigid pavements for: 1- Prevention of pumping, 2- Protection against frost action, 3- Drainage, 4- Prevention of volume change of the subgrade, 5- Increase structural capacity, and 6- Expedition of construction Base course is used under flexible pavement to: 1- increase the structural capacity by providing added stiffness and resistance to fatigue, 2- Additionally, it distribute the traffic load on larger area of the subbase and therefore, prevent failure, 3- It also provide drainage, 4- prevent frost and volume change in the base, and 5- Expedition of construction

6. Some Representative Aggregate Gradation for pavement layers. Percent Passing Sieve SizeSubbase CourseBase CourseSurface Course 63 mm2.5-inch-100- 50 mm2-inch10097 – 100- 37.5 mm1.5-inch97 – 100-- 25.0 mm1-inch--100 19.0 mm0.75- inch --97 – 100 12.5 mm0.5-inch-40 – 60 (8)- 4.75 mmNo. 440 – 60 (8)-41 – 71 (7) 0.425 mmNo. 40-9 – 17 (4)12 – 28 (5) 0.075 mmNo. 2000 – 12 (4)4 – 8 (3)5 – 16 (4) Notes: 1.Number in parentheses indicates the allowable deviations (± ) from the target value. 2.These are only representative gradations and do not represent a comprehensive list of FHWA specified gradations.

7. Some Representative Gradation Specifications for Aggregate Courses from the 1996 FHWA 19 mm 0.075 mm 4.75 mm No.200 ¾-in 1 ½ in2.0 in 50 mm Subbase Base