A pavement layer comprising a mixture of aggregates and bitumen, heated and mixed correctly and then laid and compacted on a bed of granular layer, is called flexible pavement.
Or,
The type of highway pavement that transmits the imposed wheel load to the underlying layers by a grain-to-grain transfer mechanism and is commonly constructed utilizing bitumen & aggregates is known as flexible pavement.
The load transfer mechanism is depicted in fig 1.
Fig 1: Load transfer mechanism of flexible pavement
✔ According to this transfer mechanism, the imposed stress decreases with the increase in depth.
Owing to this concept, flexible pavement is constructed in several layers, with the topmost layer of the best quality as it is subjected to maximum stress, wear, and tear.
1. How is Flexible Pavement Different From Rigid Pavement?
| S.N. | Flexible Pavement | Rigid Pavement |
| 1. | It transfers the wheel load to subgrade by grain to grain mechanism. | It transfers the wheel load to subgrade by slab action. |
| 2. | The initial construction cost is low. | The initial construction cost is high. |
| 3. | It doesn’t require joints. | It requires joints. |
| 4. | Durability is low. | Durability is high. |
| 5. | It doesn’t distribute load uniformly. So, a good subgrade is required. | It distributes wheel load uniformly. So, there is no requirement for a good subgrade. |
| 6. | There is no effect of temperature variation on stress variation. | Temperature variation affects the stress variation. |
| 7. | The lifespan of flexible pavement is approximately 10 to 15 years. | The maximum lifespan of rigid pavement is approximately 20 to 30 years. |
| 8. | Repair work is simple. | Repair work is complex. |
| 9. | The maintenance cost is high. | The maintenance cost is low. |
| 10. | It doesn’t require curing. | It requires curing. |
| 11. | Poor night visibility due to the use of asphalt. | Good night visibility due to the use of concrete. |
| 12. | No glare due to sunlight. (Glare: shine with a strong or dazzling light.) | High glare due to sunlight. |
| 13. | Easy to locate and perform underground works like repairing or finding pipes. | Hard to perform underground works. |
| 14. | Its thickness is more. | Its thickness is less. |
| 15. | The bearing capacity of the subgrade influences design. | The bearing capacity of the subgrade doesn’t influence its design. |
| 16. | Aggregate and bitumen are used as the key materials. | Concrete and steel are used as key materials. |
| 17. | Stability depends upon the interlocking of aggregates, particle friction, and cohesion. | Stability depends upon joints between the slabs of concrete. |
| 18. | It is capable of resisting settlements to some extent. | The settlement is permanent. It is not capable of resisting settlement. |
| 19. | It is susceptible to heat, oils, greases, and chemicals. | It is highly resistive to heat, oil, greases, and chemicals. |
| 20. | Rolling ( Compacting ) of the surface is needed. | Rolling ( Compacting ) of the surface is not needed. |
| 21. | It has a low flexural strength. | It has high flexural strength. |
| 22. | The force of friction is less. | The force of friction is high. |
| 23. | It is more comfortable for travel. | It is less comfortable for travel. |
2. General Cross-Section of Flexible Pavement
A typical cross-section of a flexible pavement consists of the following layers:
Fig 2: Cross-section of flexible pavement
a. 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 stress and wear and tear.
It is primarily designed to resist the imposed loads, prevent water ingress to the underlying layers, and ensure a skid-resistant riding surface.
It is generally made with bitumen bonded with aggregates graded 25 mm – 0.75 mm ( filler ).
Its thickness varies between 25 to 50 mm.
b. Binder Course:
The 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 is a bitumen-bound aggregate ( nominal size ) layer.
This course is also called a levelling course.
Its thickness ranges from 50 mm to 100 mm.
c. Base Course:
Mainly, hard crushed aggregates are used in the construction of this layer.
The base course is the backbone of flexible pavement.
Its thickness ranges from 100 mm to 300 mm.
d. 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 comprises superior quality materials.
Its thickness ranges from 100 mm to 300 mm.
e. 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.
3. Types of Flexible Pavement
Flexible pavements may be classified into the following categories:
a. Full-Depth Asphalt Pavement:
The type of flexible pavement that consists of a directly placed bituminous layer over the sub-grade layer is known as full-depth asphalt pavement.
Such flexible pavement is usually preferred in areas where local construction materials are unavailable, and there is low traffic.
b. Conventional Layered Flexible Pavement:
The type of flexible pavement consists of multiple layers, with the upper layers of superior quality and the lower layers of relatively inferior quality materials.
This is because the upper layers must withstand maximum stresses due to direct contact with the wheel load, while the lower layers must resist the lower intensity of pressures.
c. Contained Rock Asphalt Mat:
It is commonly abbreviated as CRAM. It is the type of flexible pavement constructed by placing a layer of aggregate between two asphalt layers. The aggregate used may be dense or open-graded.
4. Construction of flexible pavement
The general construction can be sub-divided as follows:
a. Sub-grade Course Construction:
Generally, the materials used to construct the sub-grade course include natural soil or murram.
The construction procedure consists of uniformly spreading the natural soil layer and then compacting it at optimum moisture content.
b. Sub-base Course Construction:
The materials used to construct this layer include crushed stones, gravel, and coarse sands.
The first step in constructing this layer includes spreading the selected materials over the prepared sub-grade layer maintaining the specified thickness and cross slope.
The layer is compacted by the use of rollers at optimum moisture content.
Its thickness ranges from 100 mm to 300 mm.
c. Base Course Construction:
Mainly, 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 to produce a wet mix of 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 utilizing a roller and allowed to dry for at least 24 hours.
Its thickness ranges from 100 mm to 300 mm.
d. Application of Prime Coat
A prime coat is an application of a low viscosity cutback bitumen above the base course.
It is commonly prepared by mixing bitumen ( 80%) and kerosene (20%).
To make one liter of prime coat, we need 1.05 kg bitumen and 0.10 kg kerosene and burn it to an adequate temperature.
After preparing the prime coat, it is spread over the base course surface and allowed to dry for about three days.
It is applied at a rate of 6-9 kg per 10 sqm.
The purposes of Prime Coat are:
~ To fill the capillary voids present in the base course.
~ To bind the materials into a single unit.
~ To harden or toughen the surface.
~ To develop sufficient adhesion between base and binder course.
e. Binder Course Construction
The 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 is constructed with a bitumen-bound aggregate ( nominal size ).
This course is also called a levelling course.
Its thickness ranges from 50 mm to 100 mm.
f. Application of Tack Coat
A tack coat is provided between the binder course and the surface course. Asphalt emulsion diluted with water is used as tack coat material.
It is applied at 6.0 to 7.5 kg per 10 sqm area; this quantity may be increased to 7.5 to 10 kilograms of a non-bituminous base.
g. Surface Course Construction:
A surface course is also known as a wearing course. The main aim of the wearing course is to provide skid-resistance surface, friction, and drainage for the pavement.
It is the topmost layer that comes in direct contact with wheel loads.
Its thickness ranges from 25 mm to 50 mm.
5. Advantages of Flexible Pavement
The advantages of flexible pavement can be listed as follows:
a. The initial construction cost is low.
b. It doesn’t require joints.
c. There is no effect of temperature variation on stress variation.
d. Repair works can be carried out quickly.
e. The thickness can be increased easily as per the need.
f. No glare due to sunlight. (Glare: shine with a strong or dazzling light.)
g. It is more comfortable for travel.
6. Disadvantages of Flexible Pavement
The disadvantages of flexible pavements can be listed as follows:
a. The life span is decreased with repeated prolonged use.
b. Maintenance cost is relatively higher.
c. It is susceptible to heat, oils, greases, and chemicals.
d. Poor night visibility due to the use of asphalt.
| Read Also: Earth Road |
| Read Also: Rigid Pavement |
