Flexible pavements are those pavements which reflect the deformation of subgrade and the subsequent layers to the surface. Flexible, usually asphalt, is laid with no reinforcement or with a specialized fabric reinforcement that permits limited flow or repositioning of the roadbed under ground changes.

The design of flexible pavement is based on load distributing characteristic of the component layers. The black top pavement including water & gravel bound macadam fall in this category.

Flexible pavement on the whole has low or negligible flexible strength flexible in their structural action). The flexible pavement layers transmit the vertical or compressive stresses to the lower layers by grain transfer through contact points of granular structure.

The vertical compressive stress is maximum on the pavement surface directly under the wheel load and is equal to contact pressure under the wheels. Due to the ability to distribute the stress to large area in the shape of truncated cone the stresses get decreased in the lower layer.

As such the flexible pavement may be constructed in a number of layers and the top layer has to be strongest as the highest compressive stresses.

To be sustained by this layer, in addition to wear and tear, the lower layer have to take up only lesser magnitude of stress as there is no direct wearing action due to traffic loads. Therefore, inferior material with lower cast can be used in the lower layers. 

The sub-grade material should be clean and free from organic matter and should be able to be compacted by roller, to form stable sub-base. The material should have the following characteristics:

ii) DEPTH 

It transmits load to the subgrade through a combination of layers. Flexible pavement distributes load over a relatively smaller area of the subgrade beneath. The initial installation cost of a flexible pavement is quite low which is why this type of pavement is more commonly seen universally. However, the flexible pavement requires maintenance and routine repairs every few years. In addition flexible pavement deteriorates rapidly; cracks and potholes are likely to appear due to poor drainage and heavy vehicular traffic.

A valuable advantage of flexible pavement is that it can be opened for traffic within 24 hrs after completion. Also the repair and maintenance of flexible pavement is easy and cost effective.

Today 96% of all paved roads and streets in Pakistan are surfaced with asphalt. Almost all paving asphalt used today is obtained by processing crude oils. Man-made asphalt consists of compounds of hydrogen and carbon with minor proportions of nitrogen, sulfur and oxygen.

The design of flexible pavement relies heavily on the entire structural section, meaning the subgrade, subbase, base and surface course. Under loading, the flexible pavement transfers and spreads the loading downward and outward. For this reason, the base course and subbase course (if used) and subgrade must be well compacted and have the ability to handle the additional loading. The strength of flexible pavement relies on all layers, especially the subgrade.

Rigid pavements, such as concrete pavement, do not rely as heavily on the lower base material because it can bridge the material. The strength of rigid pavement is in the concrete itself.

Flexible pavements are not heavily stressed by temperature variations. It is designed to be flexible and move. Rigid pavement does not do as well under temperature variations.

Flexible pavement can be “self-healing” to some extend under heavy loads and stresses. Rigid pavement will fail if the load or stress is too much.

Flexible pavement can be patched. Rigid pavement generally cannot be patched, but must be repaired. For this reason, where there are numerous underground utilities, it is much better to have flexible pavement that can be replaced quickly.

Fexible pavement tends to cost less in the short term, but may not last as long as ridid pavement if it is not well maintained.

Now, in your question, you talk about flexible pavement consisting of 6 things.

There are various types of flexible asphalt pavement that are used for various purposes.

  

 

A true flexible pavement yields “elastically” to traffic loading. It is constructed with a bituminous surface treatment or a relatively thin surface of hot-mix asphalt (HMA) over one or more unbound base courses resting on a subgrade. Its strength is derived from the load-distributing characteristics of a layered system designed to ultimately protect each underlying layer including the subgrade from compressive shear failure.

Progressively better materials are used in the upper structure to resist higher near-surface stress conditions caused by traffic wheel loads. These materials include an all-weather surface that is resistant to erosion by the environment and traffic action. The bituminous/ HMA surface layer must also be resistant to fatigue damage and remain stable under traffic loads when pavement surface temperatures are in excess of 150ºF.

In this manual, the term “flexible pavements” is used in a more generalized way to describe any “black-topped” structure (other than HMA-overlaid concrete). These pavements range in composition from true flexible pavement to semi-rigid systems (including the full-depth or perpetual design). This chapter is applicable to the design of these types of structures.

The fundamental difference between a flexible, semi-rigid, and rigid pavement is the load distribution over the subgrade. The semi-rigid pavement has a higher composite modulus than a true flexible pavement and begins to resemble the rigid structure in terms of how the traffic loads are distributed over the subgrade. The elements contributing to the higher modulus may be:

The higher modulus adds to the structural capacity of the pavement layers. As a result, the load is distributed over a wider area of the subgrade. 

These pavements may fall into one of the following categories:

Stabilization of the subgrade layer can be used with any of the above pavement types. Typical stabilizers include asphalt cement (for base only), lime, cement, fly ash, or lime-fly ash combinations. 

There are two types of pavements based on design considerations i.e. flexible pavement and rigid pavement. Difference between flexible and rigid pavements is based on the manner in which the loads are distributed to the subgrade.

Before we differentiate between flexible pavements and rigid pavements, it is better to first know about them. Details of these two are presented below:

Flexible pavement can be defined as the one consisting of a mixture of asphaltic or bituminous material and aggregates placed on a bed of compacted granular material of appropriate quality in layers over the subgrade. Water bound macadam roads and stabilized soil roads with or without asphaltic toppings are examples of flexible pavements.

The design of flexible pavement is based on the principle that for a load of any magnitude, the intensity of a load diminishes as the load is transmitted downwards from the surface by virtue of spreading over an increasingly larger area, by carrying it deep enough into the ground through successive layers of granular material.

1. Granular Base Course:

It is a mixture of soil particles ranging in size from coarse to fine. Processing involves crushing oversized particles and screening where it is necessary to secure the desired grading. The requirements of a satisfactory soil aggregate surface are;

2. Macadam Base:

Successive layers of crushed rock mechanically locked by rolling and bonded by stone screening (rock duct, stone chips etc).

3. In-water bound Macadam:

The crushed stones are laid, shaped and compacted and then finer materials are added and washed into surface to provide a dense material.

4. Treated Bases:

Compose of mineral aggregate and additive to make them strong or more resistant to moisture. Among the treating agents is bitumen.

The sub-base has already been designed on the foundation page, a link to which is at the bottom of this page.

The total thickness of the combined bituminous layer is then determined from
 Figure 2.1 - Design Thickness for Flexible Pavements.

This is on a separate page so as to reduce the page download time. It may take a few seconds for the page to download but please be patient. It is essential that you look at the chart before progressing any further.

The final design of the pavement is then dependent on the material chosen for the wearing course. It is standard to use a wearing course consisting of either

If PA is used then it's contribution to the bituminous design thickness is 20mm. A 60mm base course of either HRA , Dense Bitumen Macadam (DBM), or High Density Macadam (HDM) is required below a PA wearing course.

If HRA is used then a base course is optional. If used it may be of any permitted material and should be at least 50mm thick.

Base courses over HDM should also be HDM

Design traffic | 75 msa
Roadbase | HDM
Design Thickness | 320mm

Design Options given that HDM is the chosen material and thus from the design chart the total thickness is 320mm

Consists of combination of mineral aggregate with bituminous binder ranging from inexpensive surface treatment ¼ in or less thick to asphaltic concrete. For good service throughout the full life, bituminous pavement must retain the following qualities:

Rigid pavement is the technical term for any road surface made of concrete. Concrete roads are called rigid while asphalt-covered roads are flexible. These terms refer to the amount of deformation created in the road surface itself when in use and over time.)

There are four types of concrete pavement:

Surface course or wearing course is the top most layer of flexible pavement which has direct contact with the vehicular loads. Since it is directly in contact with traffic, good quality aggregates and high dense bitumen or asphalt is recommended for the construction of surface course.

The main function of surface course is to provide skid-resistance surface, friction and drainage for the pavement. It should be water tight against surface water infiltration. The thickness of surface course generally provided is 25 to 50 mm.
  

Binder course is also constructed using aggregates and bitumen but with less quality than materials used for surface course. In general, its thickness is about 50 to 100 mm.

If economy is not a problem, binder course and surface course can be constructed monotonically using good quality materials with 100 to 150 mm thickness. The function of binder course is to transfer the loads coming from surface course to the base course.

  

The base course is important layer of pavement structure and it distributes the loads from top layers to the underneath Subbase and sub-grade layers. It provides structural support for the pavement surface. It is constructed with hard and durable aggregates which may either stabilized or granular or both.

The thickness of base course must be great enough to reduce the load capacity on sub-grade and Subbase courses. The minimum base course thickness recommended is 100 mm. sub surface drainage system can be provided with in the base course. 
 

The Sub-base course is provided beneath the base course and it also functions as same as base course. If the sub-grade soil is strong and stiff, then there is no need to sub-base course.

Granular aggregates are used to construct sub-base course. If sub-grade is weak minimum 100 mm thick sub-base course should be provided.

  

Frost protection layer is provided for the pavements in colder regions where temperatures are very low.  It is generally provided between Subbase and sub-grade soil.

The function of frost protection layer is to prevent damage of pavement from frost heaves, which are formed by freezing of groundwater. A good quality base course and Sub-base courses provided can also serves frost protection layer.

Subgrade is the bottom most layer which is nothing but natural soil layer compacted up to required depth generally about 150 to 300 mm to receive the loads coming from top layers. This layer is termed as foundation for the pavement system.

The sub-grade should be strong enough to take the stresses and also it is important to keep the stresses coming from top layers should be within the limit of sub-grade capacity. To reduce the amount of stress on soil sub-grade, provide thick layers of base course, Sub-base course and surface course. 

Surface course is the layer directly in contact with traffic loads and generally contains superior quality materials. They are usually constructed with dense graded asphalt concrete(AC). The functions and requirements of this layer are:

This layer provides the bulk of the asphalt concrete structure. It's chief purpose is to distribute load to the base course The binder course generally consists of aggregates having less asphalt and doesn't require quality as high as the surface course, so replacing a part of the surface course by the binder course results in more economical design.

The base course is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage It may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.

The sub-base course is the layer of material beneath the base course and the primary functions are to provide structural support, improve drainage, and reduce the intrusion of fines from the sub-grade in the pavement structure If the base course is open graded, then the sub-base course with more fines can serve as a filler between sub-grade and the base course A sub-base course is not always needed or used. For example, a pavement constructed over a high quality, stiff sub-grade may not need the additional features offered by a sub-base course. In such situations, sub-base course may not be provided.

The top soil or sub-grade is a layer of natural soil prepared to receive the stresses from the layers above. It is essential that at no time soil sub-grade is overstressed. It should be compacted to the desirable density, near the optimum moisture content.

The primary functions of a pavement are to:

Functions of each layer are described as below:

·         Sub-Grade :

·         Functions: The load is transferred by the sub-grade effectively to the earth mass. However the locally available earth is used to construct the sub-grade but it becomes necessary that the sub-grade should be of required strength.

·         Testing: This makes it necessary to check the strength characteristics of the sub-grade by using the various methods. Here is the list of the tests that are employed to check the strength of the sub-grade:

a.  California bearing ratio test.

b.  California resistance value test.

c.  Tri-axial shear test.

d.  Plate load test.

California bearing ratio test is a kind of penetration test, in which we get the value of the California bearing ratio of the given soil sample. The test can also be performed in the field to take the in-situ measurements. 

The value is used to design the thickness of the flexible pavements by using some empirical charts.This test is also used to calculate CBR value for the other layers too. 

California resistance value is got by using the Hveem stabilometer. This test is used in empirical methods of the flexible design methods, based on the soil strength.

Tri-axial shear test: This test gives the value of the shear strength of the soil, but it is not done, because the values obtained from this test are used only in a very few theoritical methods of flexible pavement design.

Plate Load Test:    In this test we make use of the plates of some larger diameter. In this test we get the values of the elastic modulus of sub-grade. These values are used in the methods of flexible pavement designs like that in Mcleaod method and in method of layer system approach for analysis by Burmister. 

The modulus of sub-grade reaction is also calculated for the rigid pavement analysis by the Westergaard's approach.

·         Sub-base and base course: 

Base course and sub-base course is used in the flexible pavement to disperse the upcoming loads to large area through a finite thickness, so as to increase the load bearing capacity of the pavement. The material used is the broken stones, or gravels, or bound or unbound aggregates. 

Sometimes the bricks can also be used as the sub-base and base materials. Generally the sub-base layers have the inferior quality of aggregates, than that used in the base course.  

Generally smaller aggregates are used because the larger gravels or aggregates have the tendency to sink to the lose sub-grade soil under the heavy loading.

1.  Check the pumping action.

2.   Protecting the sub-surface against the frost action.

The basic purpose of the sub-base and base layer is to transfer the load through a dispersed/larger area to the sub-grade. So it must of sufficient strength to do its job. These layers are evaluated using a suitable strength or stability tests like the plate load test, California bearing ratio test, and the Hveem stabilometer test, where each test has its own significance and importance. 

The Flexible Pavements Program focuses on issues related to the design, construction, and maintenance of asphalt pavements.

Currently studies are underway that include design of long-life full-depth pavements, design of overlays that 🤬 reflection cracking, rubblization of existing concrete structures, full-depth reclamation, and use of new technologies to measure the quality of flexible pavements. The program conducts research into the optimal properties of each layer within flexible pavements. Soil stabilization and methods of mitigating sulfate heave problems continue to be a major focus area, together with quantifying the benefits of heavy-duty flexible-base materials. In the Flexible Pavements Program researchers make use of innovate nondestructive testing equipment and advanced laboratory testing equipment in order to understand and document pavement performance.

Researchers work closely with sponsoring agencies to ensure that the products of the research program are implemented. Developing and teaching training schools are ongoing activities.