The composition and structure of flexible pavement consists of surface course, binder course, base course, subbase course, frost protection course, subgrade. Flexible pavements contain bitumen or asphalt layer as wearing course and supports loads through bearing. They have low flexural strength.
Composition and Structure of Flexible Pavement
Fig 1: Layers of Flexible Pavement
1. Surface Course
Surface course or wearing course is the top most layer of flexible pavement which has direct contact with the vehicular loads. Since it is directly in contact with traffic, good quality aggregates and high dense bitumen or asphalt is recommended for the construction of surface course.
The main function of surface course is to provide skid-resistance surface, friction and drainage for the pavement. It should be water tight against surface water infiltration. The thickness of surface course generally provided is 25 to 50 mm.
Fig 2: Surface Course
2. Binder Course
Binder course is also constructed using aggregates and bitumen but with less quality than materials used for surface course. In general, its thickness is about 50 to 100 mm.
If economy is not a problem, binder course and surface course can be constructed monotonically using good quality materials with 100 to 150 mm thickness. The function of binder course is to transfer the loads coming from surface course to the base course.
Fig 3: Binder Course
3. Base Course
The base course is important layer of pavement structure and it distributes the loads from top layers to the underneath Subbase and sub-grade layers. It provides structural support for the pavement surface. It is constructed with hard and durable aggregates which may either stabilized or granular or both.
The thickness of base course must be great enough to reduce the load capacity on sub-grade and Subbase courses. The minimum base course thickness recommended is 100 mm. sub surface drainage system can be provided with in the base course.
Fig 4: Laying Base Course
4. Subbase Course
The Sub-base course is provided beneath the base course and it also functions as same as base course. If the sub-grade soil is strong and stiff, then there is no need to sub-base course.
Granular aggregates are used to construct sub-base course. If sub-grade is weak minimum 100 mm thick sub-base course should be provided.
Fig 5: Laying Sub-base Course
5. Frost Protection Layer
Frost protection layer is provided for the pavements in colder regions where temperatures are very low. It is generally provided between Subbase and sub-grade soil. The function of frost protection layer is to prevent damage of pavement from frost heaves, which are formed by freezing of groundwater. A good quality base course and Sub-base courses provided can also serves frost protection layer.
6. Subgrade
Subgrade is the bottom most layer which is nothing but natural soil layer compacted up to required depth generally about 150 to 300 mm to receive the loads coming from top layers. This layer is termed as foundation for the pavement system.
The sub-grade should be strong enough to take the stresses and also it is important to keep the stresses coming from top layers should be within the limit of sub-grade capacity. To reduce the amount of stress on soil sub-grade, provide thick layers of base course, Sub-base course and surface course.
Fig 6: Compacting Sub-grade
Apart from the above layers, three types of coats or finishes are provided in flexible pavement system which are as follows
· Seal Coat
· Tack Coat
· Prime Coat
a. Seal Coat
Seal coat is provided directly on the top of surface course to make it watertight and to provide skid resistance to the surface. Mixture of Emulsified asphalt, mineral fillers and water is used as seal coat material.
Fig 7: Seal Coat
b. Tack Coat
Tack coat is provided on the top of binder course to develop strong bond between the binder course and surface course. Asphalt emulsion diluted with water is used as tack coat material.
c. Prime Coat
Prime coat is provided between base course and binder course to develop strong and water tight bong between them. Low viscous cutback bitumen is sprayed on the top of base course as prime coat material.
