Flexible pavements have gained popularity due to their durability, performance and value. This versatile form of pavement has been used for several years now and is considered one of the most important developments in modern road construction history. In this article, we will go over the many kinds of flexible pavements, their functions, and other specifics that will assist you in gaining a better understanding of this method used in the building and construction sector.
Flexible pavement: What is it?
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Flexible pavement is the most prevalent form of pavement in contemporary times. A flexible pavement comprises the subgrade, many layers of lower levels of an aggregate of a suitable quality, and an upper surface of bitumen-bound aggregate (also known as asphalt concrete). Because of the layers, the pavement is able to “bend,” which lessens the likelihood of damage and the frequency of maintenance and repair needs over time.
Flexible pavement: Characteristics
Flexible pavement has almost little flexural strength, and its other features and characteristics include the following:
- Deformations in the subgrade or other layers below may be reflected in the top layers and on the pavement surface if the pavement is sufficiently flexible.
- When a car rolls over an area of pavement, the contact pressure beneath the tire causes vertical compressive stress that is equal to the weight of the car.
- Because they are farther from the surface, the pavement’s lower layers are subject to less intense pressures and less direct wear from vehicle traffic and weather. So, cheaper and lesser-quality materials may be utilised lower down.
- For flexible pavements, the top layer must be the most durable since it must withstand the greatest compressive loads, as well as the effects of traffic and weather.
Flexible pavement: Functions
There are three main roles of flexible pavements, and each one is best served by a different layering scheme.
- A flexible pavement’s primary purpose is to offer a medium that is secure, smooth, and long-lasting enough to support the volume of traffic expected to be travelling over it during its design life.
- The second major purpose is to prevent repetitive deformation of the subgrade by distributing tyre loads over a larger area below the roadway.
- Last but not least, it is essential to shield the subgrade and the lower pavement levels from any eroding impacts that can be caused by water.
Flexible pavement: Different types of flexible pavement
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Conventional layered
They consist of layered systems, with the highest quality, most costly materials located at the top (where stresses are minimal) and the highest quality, most cost-effective materials located at the bottom.
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Full-depth asphalt pavements
They are built with bituminous layers endorsed on the soil subgrade. This is preferable when both heavy traffic and materials are unavailable.
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Contained rock asphalt mats
They are built by sandwiching asphalt layers with either open- or dense-graded aggregate layers. When modified thick-graded asphalt concrete is laid on top of the subgrade, the subgrade is subjected to a greater vertical compression load and is less protected from surface water.
Flexible pavement: Standard layers of flexible pavement
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The following are examples of typical layers that are found in the traditional flexible pavement:
Seal Coat
This is a very thin treatment that is applied to the surface in order to give a skid-resistant and waterproof finish.
Tack Coat
In this case, the asphalt is applied extremely thinly, and the asphalt emulsion actually comes into contact with water. A tack coat, which ensures appropriate bonding between two layers, must be slender and binder course, set extremely quickly, and cover the surface evenly.
Prime Coat
The binder layer is applied to a road layer that is made of absorbent material, such as granular bases and has a low viscosity cutback bitumen. Primer is used to glue different materials together. creates and covers a waterproof surface, unlike tack.
Surface course
This layer is in direct touch with the loads caused by traffic, and it is constructed out of high-quality materials. In most cases, thick-graded asphalt concrete is used in the construction of surface layers. The following are the prerequisites for this layer:
- The surface course is responsible for providing qualities like friction, suppleness, and permeability, among other things. Additionally, the surface course will restrict excessive amounts of surface water from entering the subgrade and sub-base, as well as the underlying base.
- Surface courses need to be resistant to wear and tear if they are going to produce a riding surface that is both smooth and resistant to slipping underfoot activity.
- They have to be waterproof so that the whole subgrade and base are shielded from the influence that water has on weakening things.
Binder course
The binder course is responsible for the majority of the structure’s asphalt concrete content. The primary objective of the binder course is to evenly distribute the weight throughout the base course. In most cases, the binder course is made up of sediments that do not need to be of as good of a quality as the surface course, in addition to having less asphalt.
Base course
It is placed directly under the level of the binder course and offers extra load distribution in addition to contributing to the sub-surface drainage. It is possible for it to be composed of crushed rock, other unprocessed or stabilised materials, or crushed slag.
Sub-Base course
The sub-base course is the layer of material underneath the base course that serves to give structural support, enhance drainage, and lessen the infiltration of particles from the subgrade into the pavement structure.
Sub-grade
The subgrade is the lowest layer, and it consists of nothing more than a layer of natural soil that has been compacted to the necessary depth, usually between 150 and 300 mm, in order to support weights from higher levels. The word “foundation” is used to describe this level of pavement.
Flexible pavement: Standard Construction Procedures
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Surface cleansing
The first step is to clear the dust and grime from the existing pavement layer. It may be accomplished by hand, with the use of a mechanical sweeper and high-pressure air stream from a compressor, or in any other manner deemed appropriate.
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Prime Coat
This is the coat which is put on the freshly cleaned surface of the existing pavement. Prime Coat serves as the first layer of a low-viscosity liquid bituminous substance that is laid down on top of an already present permeable or porous pavement surface.
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Tack Coat
Applying a more viscous bituminous substance, such as hot bitumen, is the next step after the Prime Coat has dried for a full day. IRC:16, “Standard Specification and Code of Practice for Prime and Tack Coat,” details the requirements for both the Tack Coat and the Prime Coat.
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Transmission and blending
Hot mix plants are where bituminous materials are mixed, and both pre-and post-mixing laboratory testing takes place. The results of the tests must be in accordance with the requirements outlined in the “Specification for Dense Graded Bituminous Mixes” (IRC: 111-2009).
After the Bituminous Mix has been loaded onto a truck or dump truck, it is transported to the location where it will be used as paving material. Once the material has been loaded into the truck, it cannot be delayed any further before being paved. The loaded bituminous mix is heated to temperatures between 150 and 1650 C. In order to pave using the bituminous mix, temperatures must be between 1400C for laying and 900C for rolling.
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Paving of Bituminous mix
Bituminous mixtures may be paved using pavers. Pavers are used to put down DBM or SDBC/BC layers, as well as to provide the desired thickness and slope/camber, during a paving process.
Flexible pavement: Failure types
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Alligator or map ruptures
This is a typical example of the failure that may occur with flexible pavements. Failure due to weariness is another name for this. The most common factors contributing to this kind of failure are
- The relative motion of the pavement’s layers
- Intense wheel loads are repeatedly applied
- Subgrade or even another layer swelling/shrinking as a result of moisture changes
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Accumulation of pavement layers
This sort of failure results in the formation of ruts. A rut is a groove or depression in the road surface caused by repeated tire motion. The following factors contribute to this form of failure.
- The continual application of load in the same direction as the wheel path causes longitudinal ruts to form.
- Shallow ruts are created as a consequence of the surface course being worn away along the wheel path.
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Shear failure cracking
Shear failure leads to the upheaval of the material that makes up the pavement by generating a crack or fracture. The following factors are the most significant contributors to shear failure cracking.
- Unacceptable levels of wheel loading
- A relatively low shear resistance in the pavement mixture
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Longitudinal cracking
Cracks of this sort extend all the way through the thickness of the pavement. The following factors are the most significant contributors to longitudinal cracking.
- Variations in the subgrade soil’s volumetric distribution
- The settling of the materials used for filling
- The wobbling of side slopes
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Frost heaving
An upheaval of a limited part of pavement may be caused by frost heaving. The depth of frost heaving may vary depending on the water table in the ground as well as the weather conditions.
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Lack of inhibition with the lower layer
Some of the surface course loosens up its materials, resulting in patches and potholes when there is insufficient binding between the surface course and the layer underneath it. One kind of this failure is called slippage cracking. This failure often occurs because there was no primer or tack coat applied in between the layers.
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Reflection cracking
This form of failure takes place when a bituminous surface layer is installed on top of an old cement concrete pavement that already has some fractures. On the bituminous surface, this fissure may be seen mirrored in the same pattern.
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Creation of ripples & corrugation
Because of the unsteady surface course that is created by traffic that is constantly stopping and going, transverse undulations emerge at regular intervals.
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Bleeding
Bleeding is caused when there is an excess of bituminous binder on the surface of the pavement. The surface of the wound will become glossy, glass-like, and reflecting after bleeding, and it may also feel sticky to the touch. Wheel pathways are the typical location for this.
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Pumping
Pumping refers to the process by which water and particles are expelled from underneath the pavement via cracks in the pavement.
Flexible pavement: Benefits
- By creating a thin coating of bituminous on the surface, one may lower their initial costs.
- When flexible pavement is used, maintenance and repairs are not only simple but also inexpensive.
- The standard design load is in accordance with Cumulative Standard Axles.
- Overlaying for added strength
- High salvage value
- It can be accessible to traffic after a period of twenty-four hours.
Flexible pavement: Limitations
- Degradation under stagnant water
- Routine maintenance
- Filling up gaps and potholes is a costly endeavour.
- The thickness is rather significant.
- The visibility throughout the night is pretty poor.
FAQs
Why is flexible pavement used?
Pavements that bend or deflect in response to traffic loads are called flexible pavements, and they are less likely to sustain damage. For the road to bend and twist without cracking, engineers design flexible pavement structures with many layers of various materials.
Why is flexible pavement referred to as flexible?
Since the whole pavement structure bends or deflects in response to the weight of passing vehicles, this kind of pavement is referred to as flexible. In most cases, the structure of flexible pavement is made up of many layers of different kinds of materials that are able to allow flexing.