What Is Pavement & Types of Pavement Used In Road Construction

What Is a Pavement?

Pavement is one type of hard surface made from durable surface material laid down on an area that is intended to carry vehicular or foot traffic.

its main function is to distribute the applied vehicle loads to the subgrade through different layers. The road Pavement should provide sufficient skid resistance, proper riding quality, favorable light-reflecting characteristics, and low noise pollution.

Its goal is to reduce the vehicle-transmitted load so that it will not exceed the bearing capacity of the subgrade.

Road Pavements are playing a crucial role in the development of any construction. There are mainly two types of road pavement used namely flexible and rigid pavements road.

Ideal Road Pavement Requirement

A good pavement should possess the following requirement,

• it should have required thickness to distribute the wheel load stresses to a safe value on the sub-grade soil,
• It should be structurally strong to resist all types of stresses imposed upon it.
• To prevent the skidding of vehicles, it should have a sufficient coefficient of friction.
• It should have a smooth level surface that offers comfort to road users even at high speed.
• Ensure less noise when the vehicle moving on it.
• It should be dustproof so that there is no danger to traffic safety.
• It must provide  an impervious surface, so that sub-grade soil is well-protected, and
• It should offer low maintenance with long life.

Read More: Types of Road Pavement & Difference between Flexible and Rigid Pavement

Types of Pavement

The following are two major pavement types used in road construction,

• Flexible pavement
• Rigid Pavement

In flexible pavements, vehicular stress is transferred to subgrade through gain-to-gain contact of the aggregate through the granular structure.

These types of roads have less flexural strength and act like flexible sheets (e.g. bituminous roads).

In the case of rigid pavement, vehicle loads are transferred to sub-grade soil by the flexural strength of the pavement and the pavement acts as a rigid plate (e.g. cement concrete roads).

A combined pavement is also used which is known as semi-rigid pavement. In this, a rigid pavement is provided with a thin layer of flexible pavement over it and is an ideal pavement with the most desirable characteristics.

However, these types of combinations of pavements are rarely used in new construction because of the high cost and complex analysis required.

1. Flexible Pavements

In Flexible Pavement, wheel loads are transferred to subgrade by grain-to-grain transfer through the points of contact in the granular structure.

The wheel load stresses acting on the pavement are distributed to a larger area and the stress decreases with depth.

Considering this load distribution characteristic of flexible pavements, it has many layers.

Hence, a flexible pavement design system uses the concept of a layered system. By considering this the flexible pavement should have better quality to sustain maximum compressive stress, in addition, to wear and tear.

Below layers are accepted to experience the magnitude of stress and low-quality material can be used.

In the construction of flexible roads mainly bituminous materials are used. Defects in the flexible road can be seen on the surface if there is a settlement of the lower layer.

The design of flexible pavement is done by considering the overall performance of the road, and the stresses produced should be kept well below the allowable stresses of each road layer.

Types of Flexible Pavement

The following are major types of flexible pavement,

• Conventional layered flexible pavement
• Full-depth asphalt pavement
• Contained rock asphalt mat (CRAM)

1. Conventional Flexible Pavements

Conventional flexible pavement uses the layered system. High-quality materials are placed at the top of the pavement layer to resist maximum stress and low-quality cheap materials are placed in lower layers.

2. Full-depth Asphalt Pavements

It is constructed by placing bituminous layers directly on the soil subgrade. These types of pavement are most suitable when there is high traffic and local materials are not available.

3. Contained Rock Asphalt Mats

It is constructed by placing dense/open-graded aggregate layers in between two asphalt layers. Properly designed asphalt concrete is placed above the subgrade.

This asphalt concrete will reduce the vertical compressive strain on the soil subgrade and protect it from surface water.

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Road Construction Layers (Road Pavement Layers)

Following are pavement layers in road construction,

1. Compacted subgrade (150 – 300mm).
2. Sub-base Course (100 – 300 mm)
3. Base Course (100 – 300 mm)
4. Prime Coat
5. Binder Coat (50 -100 mm)
6. Tack Coat
7. Surface Course (25 – 50 mm)
8. Seal Coat.

1. Compacted Sub-grade (150 – 300 mm)

The compacted subgrade is the base of all pavement layers. All the above pavement layers transfer the stress to this layer.

So, it is essential to ensure that soil subgrade is not overstressed. Therefore, it should be properly compacted to the desired density, near the optimum moisture content.

2. Sub-base Course (100 – 300 mm)

It is a layer of material below the base course. It is a major role to provide structural support, improve drainage, and reduce the intrusion of fines from the subgrade in the pavement structure.

In the case of the base, the course is open graded then the sub-base course with more fine material can be utilized as filler between the sub-grade and the base course.

A sub-base is not strictly needed or used. For example, road pavement made on high-quality, hard subgrade may not need the additional features offered by a sub-base course. In such situations, the sub-base course can be avoided.

3. Base Course (100 – 300 mm)

It is a layer of materials just below the surface of the binder course and it provides additional load distribution and contributes to the sub-surface drainage.

Different materials are used for base courses such as crushed stone, crushed slag, and other untreated or stabilized materials.

4. Prime Coat

A prime coat is applied by spreading low viscous cutback bitumen to an absorbent surface like granular bases on which the binder layer is placed.

It is primly used to provide a bond between two layers.  A prime coat is able to penetrate into the below layers, plug the voids, and form a watertight surface.

5. Binder Coat (50 -100 mm)

The Binder coat has the bulk of the asphalt concrete structure. Its main function is to distribute the load to the base course.

The binder course primly consists of aggregate mixed with low asphalt and doesn’t require quality as high as the surface course.

Replacement of some parts of the surface course by the binder course results in a more economical design.

6. Tack Coat:

In the tack coat, a little amount of asphalt is applied to the surface. It is generally an asphalt emulsion diluted with water.

Its main function is to provide proper bonding between two layers of binder course and must be thin, uniformly cover the entire surface, and set very fast.

7. Surface Course (25 – 50 mm)

It is the main layer that bears the direct traffic load and generally contains superior-quality materials. The surface course is generally constructed with graded asphalt concrete (AC). The functions and requirements of this layer are:

This course offers major characteristics like friction, smoothness, drainage, etc. Also, it prevents the entry of excess water into the underlying base, sub-base, and sub-grade,

It should be provided with a hard surface to resist distortion under traffic and provide a smooth and skid-resistant riding surface,

8. Seal Coat

The seal coat is a thin layer with a water-proof surface and provides skid resistance.

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2. Rigid Pavements

Rigid pavements are cable to transfer wheel load to a wider area as it has good flexural strength. In rigid pavement, there are not many layers of materials as in the case of flexible pavement.

In rigid directly placed on a well-compacted subgrade or on a single layer of granular or stabilized material.

As there is only a single layer between the concrete and the subgrade, this layer can be called a base or sub-base course.

In rigid, the vehicular traffic load is transferred through slab action, and the road behaves like an elastic plate resting on a viscous medium. It is constructed by using plain cement concrete.

Generally, its design is analyzed by plate theory instead of layer theory, assuming an elastic plate resting on a viscous foundation.

The plate theory assumes that the road pavement slab is a medium-thick plate that is plane before loading and remains plane after loading. Pavement slab experiences bending due to wheel load and temperature variation and the resulting tensile and flexural stress.

Types of Rigid Pavements

Rigid pavements can be classified into four types,

• Jointed plain concrete pavement (JPCP),
• Jointed reinforced concrete pavement (JRCP),
• Continuous reinforced concrete pavement (CRCP), and
• Pre-stressed concrete pavement (PCP).

1. Jointed Plain Concrete Pavement

This type of rigid pavement is constructed using plain cement concrete with closely spaced contraction joints. In this dowel, steel bars are generally used for load transfer across joints. It has a joint spacing of around 5 to 10m.

2. Jointed Reinforced Concrete Road

It is clear in the rigid pavement that placing reinforcement does not increase its structural stability, but it can drastically increase the joint spacing to 10 to 30m.

The main function of the dowel bar is to transfer load only and reinforcement is used in the slab to keep the pavement together even after cracks.

3. Continuous Reinforced Concrete Road

In this type, there is no joints are provided and it is achieved by providing reinforcement.

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