Although there are many 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.
Hard-surface pavements comprise about 67% of US 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 hard surface consisting of a durable surfacing material laid in an area intended for vehicular or pedestrian traffic.
It 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 not to exceed the subgrade’s bearing capacity.
The subgrade can distribute the vehicle load applied to the base through the different layers.
The pavement must provide a surface with 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
Based on design considerations, there are generally two main categories of pavement, which are usually recognized to serve these purposes: flexible pavements and rigid pavements.
It is important to know that both categories have their subgroups, including the different pavement types, to better understand 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 the 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, wheel loads are transferred to the subgrade soil due to the pavement’s bending resistance, 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 ideal and has 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, which usually have multiple layers, are the most commonly used type of pavement.
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 less 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 be 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 a wave is in the subgrade, it will be transferred to the surface layer).
The design of flexible pavement is based on its overall performance, and the stresses produced must be well below the allowable stresses of each layer of pavement.
Different 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 soil’s subgrade. This is more appropriate when traffic is heavy and local materials are unavailable.
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.
1. Seal Coat
A seal coat is a thin surface treatment to waterproof the surface and provide slip resistance.
2. Surface Course (25-50mm)
The surface course is the layer 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 surface water from entering the underlying, sub–base, and subgrade.
- The surface course should resist warping in traffic and provide a smooth, non-slip riding surface.
- It must also be waterproof 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. It 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 does not require 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 impervious surface.
6. Base Course (100-300mm)
The base course is a layer of material located immediately below the surface of the binder course.
It provides additional load distribution and aids in subsurface drainage.
The base course 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, and it may not be required in such situations.
8. Compacted Subgrade (150-300mm)
The subgrade is a layer of natural soil prepared to receive the stresses of the upper pavement layers.
The floor subgrade mustn’t be 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, reinforced or not, is poured in situ and placed on a thin granular subbase.
The loads are carried by the pavement’s bending resistance, 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 layer theory, which assumes that a concrete slab is a medium-thickness slab that is flat before loading and remains flat after loading.
The slab can bend due to wheel load, temperature variations, or the 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.
Different 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.
Dowels or steel bars are typically used to transfer loads across joints, which are usually 5-10 m apart.
2. Jointed Reinforced Concrete Pavement (JRCP)
Although reinforcements do not significantly improve the structure’s bearing capacity, they can significantly increase joint spacing from 10 to 30m.
Dowel bars are needed to transfer the load. Reinforcement helps hold the pavement together even after cracks.
3. Continuous Reinforced Concrete Pavement (CRCP)
Reinforcement completely eliminates transverse contraction joints.
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 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.
Conclusion
Pavements are the basic load-bearing structure of road traffic. Each pavement layer must perform many functions that must be carefully considered during the design process.
Different types of pavements can always be adopted in construction according to the traffic requirements.
Improper pavement design will lead to premature pavement failure, affecting riding quality.
There are undoubtedly many different reasons to choose one of the different types of pavement, some practical, some economical, and some political.
95% of paved roads in the United States are covered with bituminous materials (asphalt).
