Wednesday, January 30, 2013

Types of Pavements: Highway Engineering

The pavement is the structure which takes up all the traffic loads coming on the road. According to their behavior they can classified in these major categories:
(1) Flexible pavements: These are the pavements which have very less flexural strength. They transfer the load coming over them directly to the lower layers through grain to grain contact. So the load transfer largely depends upon the type of the grains/aggregates that are used in the construction of the pavement.  If the aggregates are well compact/ well graded then their grain to grain contact will be good and they can transfer the load to a wider area. The flexible pavement has the following layers:
(a) Wearing course
(b) Sub base course
(c) Base course
(d) Sub grade
The stress intensity is maximum at the top layer of the pavement and it decreases with depth. Due to ability of the pavement to transfer the loads to lower layers in the truncated form, the layer concept has been introduced for its construction. The pavements is made of layers of different materials. The top layer is made of superior material, because it has to face the largest stresses, and the lower layers are constructed with with inferior materials.  So the wearing course is made of the superior quality material and sub-grade can be constructed of the locally available earth too. The materials like bituminous concrete, is best to use as the surface course and the others like water bound macadam, and other crushed aggregates, gravels can be used for the base and sub base courses. The flexible pavements are designed with the help of the empirical design charts or some equations taking into account the various design factors. Some semi-empirical and theory methods are also available for the design of the flexible pavements.

(2) Rigid Pavements: These kind of pavement possess a high flexural strength and can transfer the loads coming over it to the lower layers with the slab action of the various layers. The reason behind the flexural strength is the kind of material used in its construction. The various materials like plain cement concrete, reinforced cement concrete or the pre-stressed concrete are the good materials to be used as the rigid pavement materials. There is no need to construct the sub-base course or the base course but for the precautionary purpose the base is generally provided. A good base course of course aggregates makes it more stable, and is more economical in the long run. The base course is more important when the sub grade is formed of the finer material. The rigid pavements are analysed and designed by using the elastic theory, in which the surface layer is considered as the elastic layer, which is supported by another elastic plate called the subgrade. The rigid pavement will not deform like the flexible pavements if the undulating surface is present at the base.

(3) Semi Rigid Pavement: These pavements are constructed with the materials like the pozzolana  concretes,the lime-fly ash-aggregate mix, or lean cement concrete and thus have sufficient flexure strength. They possess the flexural strength in between those of the rigid pavement and that of the flexible pavements. They are designed as the flexible pavements but with some correction factors applied to it, or they are designed by some other methods. These materials don't have the good resistant against the impact and the abrasion loads so the surface course is constructed with the flexible pavement materials.

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