Although there are many different types of pavement, pavements consist of superimposed layers of treated materials applied above a natural soil subgrade, the main function of which would be able to withstand and distribute vehicle loads applied to the subgrade without excessive deformation.
What is a Pavement?
Pavement is an integral part of our lives. We use them as roads, tracks, parking lots, driveways, etc. Different types of pavement are engineered structures and are essential for our daily life, commerce, and defense.
Land transport is the world’s most common mode of transportation, and the total mileage of paved roads often measures a country’s development.
Road construction is and will be a significant industry in developing countries, and as infrastructure matures, it will also be a prominent industry in developed countries.
In fact, hard-surface pavements make up about 67% of US highways and 70% of Washington state highways.
Like any other engineered structure, the pavements must be strong and durable enough for their expected lifespan.
Pavement is a type of hard surface consisting of a durable surfacing material laid in an area intended for vehicular or pedestrian traffic and has a set of layers or materials laid on top of the natural ground (subgrade).
The ultimate goal is to ensure that the stresses transmitted due to wheel loads are reduced sufficiently so as not to exceed the bearing capacity of the subgrade. It is able to distribute the vehicle load applied to the base through the different layers.
The pavement must be able to provide a surface of acceptable ride quality, sufficient slip resistance, favorable light reflective characteristics, and low noise pollution.
The challenge for the pavement engineer designing a highway is to select the appropriate material and layer thickness so that the pavement can serve its full life. Pavement plays a crucial role in the development of any construction.
Different Types of Pavement
There are generally two main categories of pavement based on design considerations and are usually recognized to serve these purposes: Flexible pavements and Rigid pavements.
To understand the different types of pavement better, one should know that both categories have their subgroup, which includes the different types of pavement.
Also, the main difference between flexible and rigid pavements is based on how their loads are distributed on the subgrade.
In flexible pavements, the wheel load is transferred by contact of aggregates between grains through the granular structure. The flexible pavement, which is less resistant to bending, acts like a flexible slab (for example, a bituminous road).
On the contrary, in rigid pavements, the wheel loads are transferred to the subgrade soil due to the bending resistance of the pavement, and the pavement behaves like a rigid slab (e.g., cement concrete driveways).
In addition to this, composite pavements are also available. A thin layer of flexible pavement over a rigid pavement is the ideal pavement with the most desirable characteristics.
However, these pavements are rarely used in new construction due to the high cost and complex analysis required.
1. Flexible Pavement
Flexible pavements are generally the most commonly used among the two different types of pavement. They usually have multiple layers.
With flexible pavements, wheel load stresses will be transferred on the subgrade to lower layers by grain-to-grain transfer through contact points in the granular structure.
The load acting on the pavement will be distributed over a wider area, and the stress decreases with depth. Using this stress distribution characteristic, flexible pavements typically have multiple layers.
Therefore, the design of flexible pavements uses the concept of a layered system. Based on this, flexible pavement is usually built in several layers.
The top layer must be of the highest quality to withstand the maximum compressive stresses and wear or tear. Lower layers will experience a lesser impact of stress, and lower-quality materials may be used.
As a result, the flexible pavement must be of overall better quality to resist enough compressive stresses and wear as much as possible.
Flexible pavements are constructed from bituminous materials. These can take the form of surface treatments (such as bituminous surface treatments typically found on lightly trafficked roads) or asphalt concrete surface courses (typically used on high-traffic roads such as interstate highways).
The flexible pavement layers reflect the deformations of the lower layers onto the surface layer (for example, if there is a wave in the subgrade, it will be transferred to the surface layer).
In flexible pavement, the design is based on the overall performance of the flexible pavement, and the stresses produced must be well below the allowable stresses of each layer of pavement.
Types Of Flexible Pavements
The following pavement types have been used in the construction of flexible pavements:
- Conventional layered flexible pavement,
- Full-depth asphalt pavement,
- Contained Rock Asphalt Mat (CRAM).
Conventional flexible pavements: are layered systems with high-quality, expensive materials placed on top where stresses are high, and low-quality inexpensive materials placed on lower layers.
Full-depth asphalt pavements: are made by placing bituminous layers directly on the subgrade of soil. This is more appropriate when traffic is heavy and local materials are not available.
Contained rock asphalt mats: are made by placing layers of dense/open-graded aggregate between two layers of asphalt. Densely graded modified asphalt concrete placed directly over the subgrade will significantly reduce vertical compressive stresses in the subgrade and protect it from surface water.
Typical Layers Of Flexible Pavement
Typical layers of conventional flexible pavement include the seal coat, tack coat, surface course, binder course, prime coat, base course, sub-base course, compacted subgrade, and natural subgrade.
- Seal Coat:
A seal coat is a thin surface treatment used to waterproof the surface and provide slip resistance.
2. Surface Course (25-50mm):
The surface course is the layer that is in direct contact with the traffic load and generally contains higher quality materials.
They are usually constructed of graded asphalt concrete (AC). The functions and requirements of this layer are:
- It provides major characteristics such as friction, smoothness, drainage, etc. It will also prevent excessive amounts of surface water from entering the underlying base, sub–base, and subgrade.
- The surface course should be resistant to warping in traffic and provide a smooth, non-slip riding surface.
- It must also be waterproof in order to protect the entire base and sub-base from weakening by the effect of water.
3. Tack Coat:
A tack coat is a very light application of asphalt, usually an asphalt emulsion diluted with water. It ensures a good bond between the two layers of the binder course and must be thin, cover the entire surface evenly, and set very quickly.
4. Binder Course (50-100mm):
This layer constitutes the bulk part of the asphalt concrete structure. Its main purpose is to distribute the load on the base layer.
The intermediate layer is generally made of an aggregate containing less asphalt and not requiring as high a quality as the surface course, so replacing part of the surface course with a binder course is more economical.
5. Prime Coat:
Priming is the application of dilute low viscosity bitumen to an absorbent surface such as granular bases over which a binder coat is placed. It allows the bond between two layers. Unlike the tack coat, the prime coat penetrates the layer below it, fills gaps, and creates an impermeable surface.
6. Base Course (100-300mm)
The base course is a layer of material located immediately below the surface of the binder course and provides additional load distribution and aids in sub-surface drainage. It can be crushed stone, crushed slags, or other raw untreated or stabilized materials.
7. Sub-Base Course (100-300mm)
The sub-base course is a layer of material below the base course. Its main functions are to provide structural support, improve drainage, and reduce the intrusion of subgrade fine particles into the pavement structure.
If the base course is open graded, the sub-base course with more fines can be used as a filler between the subgrade and the base course.
A sub-base course is not always necessary or used. For example, a pavement built on a high-quality, rigid subgrade may not need the additional characteristics offered by a sub-base course. In such situations, a sub-base course may not be required.
8. Compacted Subgrade (150-300mm)
The subgrade is a layer of natural soil that is prepared to receive the stresses of the upper pavement layers. It is essential that the floor subgrade is not overstressed at any time. Therefore, it should be compacted to the desired density, close to the optimum moisture content.
2. Rigid Pavement
For rigid pavements, a concrete slab is poured in-situ, reinforced or not, and is placed on a thin granular subbase.
The loads are carried by the bending resistance of the pavement, which acts as a rigid plate, transferring the load over a wider area of subgrade.
Concrete roads are of the rigid pavement type, and there are usually joints in the concrete to control cracking.
Compared to flexible pavements, rigid pavements do not have many layers of materials installed; instead, they are directly on a prepared subgrade or a single layer of granular or stabilized material.
Therefore, since there is only one layer of material between the concrete and the subgrade, this layer can be called the base or sub-base course.
In a rigid pavement, the vehicular traffic load is distributed by the action of the slab, and the pavement acts like an elastic plate resting on a viscous medium.
Rigid pavements are constructed using Portland cement concrete (PCC) and should be analyzed using plate theory rather than layer theory, assuming an elastic plate resting on a viscous medium.
Plate theory is the simplified version of layer theory which assumes that a concrete slab is a medium-thickness slab that is flat before loading and remains flat after loading.
The bending of the slab can be due to wheel load, temperature variations, or resulting tensile and bending stresses.
Concrete pavements can withstand very intense traffic flows and high axle loads. They are now more common in urban areas, ports, and cities, where heavy goods vehicles travel slowly.
The design life of a rigid pavement is typically 40 years, and failure usually occurs due to slab cracking or joint degradation.
Types Of Rigid Pavements
Rigid pavements can be divided into four types:
- Jointed plain concrete pavement (JPCP),
- Jointed reinforced concrete pavement (JRCP),
- Continuous reinforced concrete pavement (CRCP), and
- Prestressed concrete pavement (PCP).
1.Jointed Plain Concrete Pavement (JPCP)
These are simple cement concrete pavements constructed with closely spaced contraction joints. Dowel or steel bars are typically used to transfer loads across joints. They usually have a distance between the joints of 5-10 m.
2. Jointed Reinforced Concrete Pavement (JRCP)
Although reinforcements do not significantly improve the bearing capacity of the structure, they can significantly increase joint spacing from 10 to 30 m. Dowel bars are needed for the transfer of load. Reinforcement help hold the pavement together even after cracks.
3. Continuous Reinforced Concrete Pavement (CRCP)
The complete elimination of transverse contraction joints is achieved by reinforcement. The CRCP is designed with enough embedded reinforcing steel (approximately 0.6-0.7% of the cross-sectional area) to hold the cracks tightly together.
4. Prestressed Concrete Pavement (PCP)
If all weight stresses on a reinforced concrete structure are supported by steel reinforcement. Prestressed concrete induces stresses throughout the entire structure.
The end result is a product that is more resistant to vibration and impact than conventional concrete. Additionally, it allows for the design of more extended, thinner structures that can still support these heavier loads.
Pavements are the basic load-bearing structure of road traffic. Each pavement layer must perform a multitude of functions that must be carefully considered during the design process.
According to the traffic requirements, different types of pavements can always be adopted in construction. Improper pavement design will lead to premature pavement failure, which will also affect riding quality.
There are undoubtedly many different reasons to stick with one of the different types of pavements, some practical, some economical, and some political. In fact, 95% of paved roads in the United States are covered with bituminous materials (asphalt).