Types of Highway pavements | What Is the Difference Between Flexible Pavement and Rigid Pavement?

Types of Highway pavements | What Is the Difference Between Flexible Pavement and Rigid Pavement?

Types of Highway pavements

Types of highway pavement
 

In general, there are two types of pavement structures flexible pavements and rigid pavements.

There are, however, many variations of these pavement types. Composite pavements (which are made of both rigid and flexible pavement layers), continuously reinforced pavements, and post-tensioned pavements are other types, which usually require specialized designs and are not covered in this chapter.As with any structure, the underlying soil must ultimately carry the load that is placed on it. A pavement’s function is to distribute the traffic load stresses to the soil (subgrade) at a magnitude that will not shear or distort the soil.

Flexible Pavements

A flexible pavement is constructed with asphaltic cement and aggregates and usually consists of several layers. The lower layer is called the subgrade (the soil itself). The upper 6 to 8 inches of the subgrade is usually scarified and blended to provide a uniform material before it is 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 a cementing material.

The cementing material can be portland cement, lime fly ash, or asphaltic cement. The top layer of a 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 abrasion 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, which on the whole have low or negligible flexural strength and are rather flexible in their structural action under the loads. The flexible pavement layers reflect the deformation of the lower layers on-to the surface old): layer. Thus if the lower layer of the pavement or soil subgrade is undulated. the flexible pavement surface also gets undulated.

A typical flexible pavement consists of four components :

(i) soil subarade
(ii) sub-base course
(iii) 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 well compacted granular structure consisting of strong graded aggregate (interlocked aggregate structure with or without binder materials) can transfer the compressive stresses through a wider area and thus forms a good flexible pavement layer. The load spreading ability of this layer therefore depends on the type of the materials and the mix 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 base and sub-base course materials like the Water Bound Macadam, crushed aggregate, gravel, soil-aggregate mixes etc.

Types of Highway pavements
Types of Highway pavements
Flexible and Rigid pavement

Advantages of flexible pavements

  1. Smoother ride. Again the bounce.
  2. Less wear/maintenance to vehicle and fuel consumption. Again bounc
  3. Less wear to the tyre. Difference in hardness.
  4. Repairs are cheaper, requires less down time and easier.
  5. Initial construction cost is less in medium to hard soils

Disadvantage of flexible pavements

  1. Their maintenance cost is high.
  2. Shorter lifespan than rigid pavement.
  3. Frequent maintenance required, which drives up the cost.
  4. Susceptible to oil stains and damage from other chemicals.
  5. Edges are weak and therefore require curb structures or edging.
Rigid Pavements
A rigid pavement is constructed with portland cement concrete (PCC) and aggregates, . As with flexible pavements, the subgrade (the lower layer) is often scarified, 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 which possess note worthy flexural strength or flexural rigidity. The stresses are not transferred from grain to grain to the lower layers as in the ease of flexible pavement layers. 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/cm` flexural stress.
The rigid pavement has the slab action and is capable of transmitting the wheel load stresses through a wider area below.The main point of difference in the structural behaviour of rigid pavement as compared to the flexible pavement is that the critical condition of stress in the rigid pavement is the maximum flexural stress occurring in the slab due to wheel load and the temperature changes where-as in the flexible pavement it is the distribution of compressive stresses. As the rigid pavement slab has tensile strength, tensile stresses are developed due to the. bending of the slab under wheel load and temperature variations. Thus the types stresses developed and their distribution within the cement concrete slab are quite different. The rigid pavement does not get deformed to the shape of the lower surface . it can bridge the minor variations of 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 analysed using the elastic theory` assuming the pavement as an elastic plate resting over an elastic or a viscous foundation.

Advantages of Rigid Pavement

  1. Rigid pavement have a long life and durable.
  2. They provide an excellent riding surface under all weather condition.
  3. No corrugations are developed in cement concrete roads.
  4. They can be laid on any sub-grade.
  5. They provide good visibility for traffic during night hours.
  6. They are practically non-slippery and offer less tractive resistance.

Disadvantage of Rigid Pavements 

  1. The initial cost of Rigid pavement construction is very high.
  2. They require skilled supervision and labour for their construction.
  3. Concrete pavements are liable to crack and warp due to temperature variations..
  4. They may cause glare due to reflected sunlight.

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