Flexible Pavement: Layers, Components, and Advantages

Flexible pavements or roads are areas of asphalt that “bend” or “deflect” due to traffic loads, making them less susceptible to damage and requiring fewer repairs over time. A flexible pavement structure is composed of several layers of different materials, which together enable the road to accommodate this flexing.

In this guide, we’ll discuss:

• What is flexible pavement?
• Where is a flexible pavement used?
• Functions of flexible pavement
• Types of a flexible pavement 
• Layers of flexible pavement and what they do
• The advantages of flexible pavement

We’ll also look at a flexible pavement cross section and explain how Tensar geogrids can be used to provide flexible pavement stabilisation - assisting in the construction of a more robust and longer-lasting structure. 

What is a flexible pavement?

Flexible pavement consists of a surface layer of bitumen-bound aggregate (asphalt concrete), several lower layers of appropriate quality aggregate and the subgrade beneath. The layers enable the pavement to “bend”, reducing damage and the need for repairs over time.

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Where is flexible pavement used?

Flexible pavement is used in road pavement construction, highways, airport runways, and car parks where traffic loads are moderate to heavy. It is well-suited to areas with varying climates and soil conditions, as it resists cracking and deformation.

Functions of a flexible pavement

Flexible pavements serve three functions, and in order to achieve these effectively, several layers are required:

• The initial (visible) function of flexible pavements is to provide a surface that is safe, smooth and durable enough for the traffic anticipated to be using it, over the design life.
• The second key function is to distribute loads from the vehicle tyres onto a wider area underneath (subgrade – see later) so that the subgrade does not deform under repeated loading.
• Lastly, it is important to protect the lower pavement layers and subgrade from any of the weakening effects of water.
  

Types of flexible pavement

There are many types of flexible pavement in use currently, the most common of which include:

• Full-depth asphalt pavement: In this type, the entire pavement structure is made from asphalt materials without a separate base or sub-base layer. Hot mix asphalt (HMA) is a paving material used to construct full-depth asphalt pavements. It is often used in areas with heavy traffic, as it provides strong load distribution and is quicker to construct. 
• Conventional layered flexible pavement: This type consists of several layers, usually consisting of a base course, a sub-base layer, and an asphalt surface course. Layer interlocking distributes the weight and each layer has a unique strength and material composition.
• Contained rock asphalt mat (CRAM): This kind of flexible pavement is made by sandwiching two layers of asphalt with an aggregate layer. Open-graded or dense aggregate may be utilised.
• Composite pavement: This is a hybrid of flexible and rigid pavement with an asphalt layer over an existing concrete layer. This combination enhances the structural capacity and durability of the pavement, reducing maintenance needs.

Layers of a flexible pavement

The structure of flexible pavement is typically made up of the following layers:

• Subgrade layer
• Capping layer
• Subbase layer
• Base layer
• Binder layer
• Surfacing layer

The various layers of a flexible pavement can be divided into two overlapping sections: the pavement section and the foundation section.

The foundation section of a flexible pavement comprises the subgrade and subbase with possibly a capping layer between, whilst the pavement section includes the subbase, the base, the binder course and the surface course layers. Together, these two overlapping sections form the overall structure of flexible pavement.

The foundation section enables the structural road layers to be properly placed and compacted. In addition, the foundation layers of a flexible pavement must support the construction traffic, distributing the load to protect the subgrade from deformation.

A flexible pavement's foundation section may have either a single layer or double layer above the subgrade, comprising either a subbase and capping layer or simply the subbase. The layers must be sufficiently thick to prevent frost damage in the flexible pavement's structural layers. Provided they are thick enough, water is unable to rise up into the base layers where it could freeze and weaken the layers, also known as freeze thaw weathering. 


Layers of a flexible pavement shown in cross section, divided into two overlapping sections: the pavement section and the foundation section. 

The subgrade layer

The deepest element of the foundation section in flexible pavements is the subgrade. The upper surface of the subgrade is known as the formation. If the road surface is to be constructed at or below the original ground level, the formation will be in a cutting, where material has been removed – excavating to the required depth. Alternatively, if the road surface is above the original ground level, the formation will be the top surface of an embankment formed from imported soil. Either way, the formation is usually graded to a cross fall to facilitate the drainage to the side of the pavement.

The subgrade is mostly compacted natural soil, and the design engineer will need to know its strength and condition as it impacts pavement design. Water will affect the post-construction strength, and drainage is therefore essential to maintaining consistent strength.

The capping Layer

Capping materials are typically secondary aggregates or recycled demolition materials. They may also be lime or cement-stabilised soils. The thickness required by the capping or subbase layer in a flexible pavement may be reduced by the inclusion of soil stabilisation geogrids, such as the Tensar geogrid at the base. The geogrid aperture size should be compatible with the different types of aggregate used. For lower-strength subgrade soils, a geotextile separator may also be incorporated to prevent the migration of fines upward into the pavement layers.