Structure of Flexible Pavement
A. Surface Course: Surface course or wearing course is the topmost 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.
B. 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.
C. Base Course: The base course is important layer of pavement structure and it distributes the loads from top layers to the underneath Sub-base and sub-grade layers. 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 Sub-base courses. The minimum base course thickness recommended is 100 mm.
D. Sub-base 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.
E. 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. 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.
Apart from the above layers, three types of coats or finishes are provided in flexible pavement system which are as follows
1. 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.
2. 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.
3. 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.
Structure of Rigid Pavement
A. Concrete Slab: The concrete slab is the top most layer of rigid pavement which is in direct contact with the vehicular loads. This is also called as surface course. It is water resistant and prevents the water infiltration into the base course. It offers friction to the vehicles to provide skid resistance. The thickness of concrete slab is kept between 150 mm to 300 mm.
B. Granular Base or Stabilized Base Course: The base course or granular base or stabilized base is the second layer from the top and is constructed using crushed aggregates. This course helps the surface course to take additional loads. It is also useful to provide sub surface drainage system. In frost areas, the frost action can be controlled by the stabilized base course. It helps to control swelling of subgrade soil. The base course thickness should be minimum 100mm.
C. Granular Subbase or Stabilized Subbase Course: It is the third layer from the top and is in contact with the subgrade soil and base course. It is constructed by using low quality aggregates than the base course but they should be better quality than subgrade. Generally subbase course is not required when the traffic loading is light. The minimum thickness of sub base course is 100mm. The drainage facility will also improve when there is a subbase course.
D. Subgrade Soil: The subgrade is nothing but the existing soil layer which is compacted using equipment to provide stable platform for rigid pavement. The stresses coming from the top layers is received by different soils in different manners. Some soils may resist them and some may not. It is depends upon the interrelationship of texture, density, moisture content and strength of subgrade. So, proper examination should be done on subgrade before construction.
E. Joints: expansion contraction and wrapping of concrete slabs resulting from temperature and moisture changes may cause pavement failure. The provision of joints in concrete pavement
- Prevent breaks in the construction at the end of days work
- Construction of pavements in lanes of convenient width.
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