In general, highway pavement may be defined as a structure made up of superimposed layers of processed materials built over the soil sub-grade and primarily designed to transmit the imposed vehicular load to the soil sub-grade. An ideal pavement must be such that it provides a surface with adequate skid resistance, suitable light reflecting characteristics, acceptable riding quality and emit minimum noise. The two most common types of highway pavement constructed widely are rigid pavement and flexible pavement.
A brief overview of flexible pavement is presented below:
The type of highway pavement that transmits the imposed wheel load to the underlying layers by grain-to-grain transfer mechanism through the point of contact in their granular structure is known as flexible pavement. In simple terms, these are the pavements that transfer the wheel load to a wider area beneath by grain-to-grain contact. The load transfer mechanism is depicted in fig 1.
Fig 1: Load transfer mechanism of flexible pavement
According to this transfer mechanism, the wheel load is transferred from the uppermost layer to the underlying wider area and the imposed stress decreases with the increase in depth. Owing to this concept, flexible pavement is constructed in a number of layers with the topmost layer of the best quality as it is subjected to maximum stress, wear and tear.
General Cross-Section of Flexible Pavement
A typical cross-section of a flexible pavement consists of the following layers:
⦁ Surface Course: The surface course is the topmost layer of the flexible pavement and is generally the layer of the best quality as it has to withstand maximum stresses, wear and tear. It is primarily designed to resist the imposed loads as well as to prevent the ingress of water to the underlying layers and to ensure skid-resistant riding surface.
⦁ Binder Course: Binder course is the intermediate layer between the surface course and the base course and duly transmits the wheel load from the surface to the base course. It caters for most of the bulk of the pavement.
⦁ Base Course: The base course acts as a medium to transmit the imposed stresses to a wider underlying area. It also plays an important role in facilitating sub-surface drainage.
⦁ Sub-base Course: The sub-base course is the layer beneath the base course that provides additional structural support and boosts sub-surface drainage. It is usually an optional layer and may not be constructed if the base course is made up of superior quality materials.
⦁ Sub-grade: The sub-grade is the lowermost layer of the flexible pavement and usually consists of a compacted layer of natural soil. Its primary function is to bear all the imposed stresses from the upper layers.
A typical cross-section of flexible pavement is shown below in fig.2.
Fig 2: Cross-section of flexible pavement
Types of Flexible Pavement
Basically, flexible pavements may be classified into the following categories:
⦁ Full-Depth Asphalt Pavement:
The type of flexible pavements that consist of a directly placed bituminous layer over the sub-grade layer is known as full-depth asphalt pavement. Such type of flexible pavements can withstand the considerable intensity of stresses and are usually preferred in areas where local construction materials are not available.
⦁ Conventional Layered Flexible Pavements:
The type of flexible pavements that consist of multiple layers with the upper layers made up of superior quality and the lower layers made up of relatively inferior quality materials. This is due to the fact that the upper layers have to withstand maximum stresses due to direct contact with the wheel load while the lower layers have to withstand the lower intensity of stresses.
⦁ Contained Rock Asphalt Mat:
Commonly abbreviated as CRAM, these are the type of flexible pavements that are constructed by placing a layer of aggregate between two asphalt layers. The aggregate used may be dense or open-graded.
Construction of flexible pavement
Since a flexible pavement consists of a number of layers its construction also as a whole consists of erecting the subsequent layers. The general construction can be sub-divided as follows:
⦁ Sub-grade Course Construction: Generally, the materials used for the construction of the sub-grade course includes natural selected soil, murram and aggregates. The construction procedure consists of spreading of the natural soil layer uniformly. It is the compacted so as to ensure optimum moisture content.
⦁ Sub-base Course Construction: The materials used for the construction of this layer include crushed stones, gravel and coarse sand. The first step in the construction of this layer includes spreading of the selected materials over the prepared sub-grade layer maintaining the specified thickness and cross slope. In order to maintain the optimum moisture content; water is sprinkled from the top. Finally, the layer is compacted by the use of rollers.
⦁ Base Course Construction: Mostly, hard crushed aggregates are used in the construction of this layer. Initially, the crushed aggregates are mixed with water in a suitable proportion in a mixing plant so as to produce a wet mix macadam (WMM). The mix is then transported to the site and spread uniformly over the sub-base course usually with the aid of a paver. The layer is then compacted by means of a roller and allowed to dry for a period of at least 24 hours.
⦁ Surface Course Construction: The materials used for the construction of surface course include coarse and fine aggregates, a filler such as lime, cement and binders such as bitumen, asphalt, tar etc. The materials are mixed in definite proportions, uniformly spread and duly compacted so as to achieve a uniform surface.
Advantages of Flexible Pavements
The advantages of flexible pavement can be listed as follows:
⦁ It provides the possibility of low-cost type construction.
⦁ The construction process consists of a series of simple steps and can be easily constructed.
⦁ It has a wider resistance to temperature fluctuations.
⦁ Repair works can be carried out easily in case of flexible pavement.
⦁ The thickness can be increased easily as per the need.
Disadvantages of Flexible Pavements
The disadvantages of flexible pavements can be listed as follows:
⦁ The life span is decreased with repeated prolonged use.
⦁ Maintenance cost is relatively higher.
⦁ The problem of weakening of edges may arise.