Flexible Pavements

Flexible pavement is constructed with asphaltic cement and aggregates and usually consists of several layers. The lower layer is called subgrade (the soil itself). The upper 6 to 8 inches of the subu\grade are usually stained and mixed to provide uniform material before it compacted to maximum density.

The next layer is the subbase, which usually consists of crushed aggregate (rock). This material has better engineering properties (higher modulus values) than the subgrade material in terms of its bearing capacity. The next layer is the base layer and is also often made of crushed aggregates (of a higher strength than those used in the subbase), which are either unstabilized or stabilized with cementing material.

The cementing material can be portland cement, lime fly ash, or asphaltic cement. The top layer of flexible pavement is referred to as the wearing surface. It is usually made of asphaltic concrete, which is a mixture of asphalt cement and aggregates. The purpose of the wearing layer is to protect the base layer from wheel friction and to waterproof the entire pavement structure.

It also provides a skid-resistant surface that is important for safe vehicle stops. These thicknesses vary with the type of axle loading, available materials, and expected pavement design life, which is the number of years the pavement is expected to provide adequate service before it must undergo major rehabilitation.

Flexible pavements are those with completely low or negligible flexural strength and are flexible in their structural action under load. The flexible pavement layers reflect the deformation of the lower layers on-to the surface old): layer. Thus if the bottom layer of the pavement or the lower layer of soil subgrade is untouched. The surface of the flexible pavement also becomes unobstructed.

Components of typical flexible pavement :

1. soil subarade
2. sub-base course
3. base course and 11% surface course.

The flexible pavement layers transmit the vertical or compressive stresses to the tosser layers by grain to grain transfer through the points of contact in the granular structure A compacted granular structure consisting of a strong graded aggregate (interlocked composite structure with or without binder material) can transfer compressive stress through a wide area and thus forms a good flexible pavement layer.

The load spreading capacity of this layer therefore depends on the type of material and the mixture design factors. bituminous concrete is one of the best flexible pavement materials Other materials which fall under the group are all granular materials with or without bituminous binder, granular sub-base course materials like the Water Bound Macadam, crushed aggregate, gravel, soil-aggregate mixes etc.

1. Smoother ride, Again the bounce.
2. Less wear/maintenance to vehicle and fuel consumption.
3. Less wear to the tyre, the difference in hardness.
4. Repairs are cheaper, require less downtime and easier.
5. Initial construction cost is low in medium to hard soil

Rigid pavement is constructed with portland cement concrete (PCC) and aggregates. As with flexible pavements, the subgrade (the lower layer) is often sacrificed, blended, and compacted to maximum density. In rigid pavements, the base layer is optional, depending on the engineering properties of the subgrade. If the subgrade soil is poor and erodable, then it is advisable to use a base layer. However, if the soil has good engineering properties and drains well, a base layer need not be used. The top layer (wearing surface) is the portland cement concrete slab.

Rigid pavements are those with remarkable flexibility or malleable hardness. Stress is not transferred from grain to grain in lower layers, as in layers of flexible pavement. The rigid pavements arc made of Portland cement’ concrete-either plain, reinforced or prestressed concrete. The plain cement concrete slabs are expected to take-up about 40 kg/cm² flexural stress.

Rigid pavement has slab action and is capable of transmitting wheel load stress through a wide area below. The main point of difference in the structural behavior of rigid pavement compared to flexible pavement is that the critical state of stress in rigid pavement is the maximum flexural stress occurring in the slab,  Wherever this compressive stress is distributed in flexible pavements due to change in wheel load and temperatures. As rigid pavement slabs have tensile strength, develop due to tensile stresses.

Slab bending under wheel load and temperature variations. Thus the types of stress developed and their distribution within the cement concrete slab are quite different.The rigid pavement does not deform in the shape of the lower surface. it can bridge the minor variations of the lower layer. The cement concrete pavement slab can very well serve as a wearing surface as well an effective base course.

Therefore usually the rigid pavement structure consists of a cement concrete slab. below which a granular base or sub-base-course may be provided Though the cement concrete slab can also be laid directly over the soil subgrade, this is not preferred particularly when the subgrade, consists of fine-grained soil. Providing a good base or sub-base course layer under the cement concrete slab increases the pavement life considerably and therefore works out more economical in the long run. The rigid pavements are usually designed and the stresses are analyzed using the elastic theory assuming the pavement as an elastic plate resting over an elastic or a viscous foundation.