Composition and Structure of Rigid Pavement
Rigid pavements support loads through rigidity and high modulus of elasticity of concrete slab. The loads will distribute to natural soil layer through different layers of rigid pavement. The compostion and structure of rigid pavement tells us about the function of each layer of rigid pavement as explained below.Composition of Rigid Pavement
In general, Portland cement concrete is used as primary structural element for rigid pavement. The reinforcement is provided in the slab depending upon the soil strength and loading conditions. Pre-stressed concrete slabs can also be used as surface course. The concrete slab usually lies on a compacted granular or treated subbase, which is supported, in turn, by a compacted subgrade. Better results of pavement are obtained when the support layers under the pavement are uniform. The strength of rigid pavement is Rigid pavement is mostly depends upon the concrete slab so, it should be laid strongly while the bottom layers are constructed using low cost materials to make it economical.
Fig 1: Transfer of Wheel Load to Subgrade in Rigid Pavement
Structure of Rigid Pavement
The structure of a rigid pavement consists following layers.- Concrete slab or surface course
- Granular base or stabilized base course
- Granular subbase or stabilized subbase course
- Frost protection layer
- Subgrade soil
Fig 2: Typical Rigid Pavement Structure
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.
Fig 3: Concrete Slab Laying
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 provides stable platform to construct rigid pavement 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.
Fig 4 : Providing Base Course
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. When the loading exceeds 100000 pounds it should be constructed. Its primary function is to provide support for the top layers and it also serves as frost action controller and prevents the intrusion of fines from subgrade to top layers. The drainage facility will also improve when there is a subbase course.
Fig 5 : Laying of Subbase Course
Frost Protection Layer
In low temperature regions there is a problem of frost action on the pavements. If the soil contains high ground water table, during low temperatures the water will freeze and frost heave will formed under the subgrade which will cause the pavement to rise because of non-uniform formation of ice crystals. Similarly, when the ice melts the pavement will penetrate into the subgrade when load comes on it. To overcome this frost protection layer should be provided. Generally a good base course and subbase course themselves acts as frost protection layers.
Fig 6: Formation of Ice Crystals in Frost-Susceptible Soil
Subgrade Soil
The subgrade is nothing but the existing soil layer which is compacted using equipment to provide stable platform for rigid pavement. The subgrade soils are subjected to lower stresses than the top layers since the stresses will reduce with depth. Subgrade soils may vary considerably. 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. At the same time the pavement layers above the subgrade should be capable of reducing stresses imposed on the subgrade soil to prevent the displacement of subgrade soil layers.
Fig 7 : Preparing Subgrade Soil