What are the functions of layers in a flexible pavement?

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?
• The functions of flexible pavement
• The 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 assist in the construction of a more robust and longer-lasting flexible pavement.

What Is 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.

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.
  

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.


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. The post-construction strength will be affected by water and drainage is therefore essential to maintain 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 aggregate used. For lower strength subgrade soils, a geotextile separator may also be incorporated to prevent migration of fines upward into the pavement layers.

The Subbase Layer

The subbase layer of a flexible pavement will usually be a higher quality, well-graded aggregate. The grading may be designed to provide a drainage function, carrying water that may ingress the surface to the side of the pavement.

The Base Layer

The structural layer is often called the base layer and it is the primary load-spreading layer – the backbone of the flexible pavement. The base layer's function is to distribute the traffic-induced stresses at the bottom of the surfacing layers widely and evenly onto the foundation section. It can be formed from either compacted, high-quality unbound aggregate, or a bitumen bound aggregate layer. In some cases, cement bound aggregate is used for the base layer, though there can be issues with cracking, which require continual maintenance.

The base layer must be designed to perform adequately over the 20-year service life of the flexible pavement, without excessive rutting, deformation or cracking. The inclusion of Tensar stabilisation geogrid in the subbase or unbound base layer can increase the design life for a given thickness of pavement, or enable a reduction in thickness reducing the whole life cost of the pavement.

The Binder Course Layer

The binder course is an intermediate, bitumen bound aggregate layer placed between the base layer and the surfacing of asphalt pavement, sometimes referred to as a levelling course. The binder course's function is to distribute the load from the surfacing into the base course, strengthening the pavement.The thickness of the binder course layer should be bulkier than the surface layer, meeting the requirements of 40mm thickness, however 50mm thickness is ideal for creating a strong pavement.

The Surfacing Layer

The surfacing course (layer) is the upper layer of the flexible pavement and provides the smooth, durable, abrasion-resistant characteristics of a good roadway, retaining adequate friction for road safety. It is normally made from bitumen bound aggregate – asphalt concrete. It will be waterproof and prevent ingress of water into the lower pavement layers, which would adversely affect their strength. These layers must be strong enough not to rut under the action of traffic.

The surfacing may be susceptible to cracking due to fatigue or from cracks reflecting upward from cracked base layers. Asphalt Interlayers such as Glasstex or Tensar ARG-GN reinforcement can be incorporated at the base of the surfacing or maintenance overlay to delay cracking.

The Advantages of Flexible Pavement

Thanks to the various layers of flexible pavement, it offers the following advantages:

• With flexible pavement, the subgrade beneath is less likely to deform than with rigid pavement.
• Repairs are easy and inexpensive with flexible pavement.
• The installation process does not require joints and materials are cheaper.
• Its short curation time (typically less than 24 hours) makes for quick installations, reducing traffic disruption.
• Black ice is less likely to form on the surface of flexible pavement.
• Water is unable to rise up through the lower layers, preventing the pavement from becoming weakened.

 

To learn more about the differences between flexible and rigid pavements, see our guide to types of pavement and road construction methods.

Next steps

This guide has explained what flexible pavement is, its functions, the layers of flexible pavement, and the advantages it offers. If you’ve found this post useful, you may also want to read some of our other articles:

• What is rutting in roads?
• Types of road cracks and how to avoid them
• Geogrids in temporary access roads

To find out more about how Tensar products support road design and construction processes, visit our pages on geogrids and other geosynthetic construction solutions. Also Tensar+, our design software that allows you to design with and evaluate the costs and time savings when using geogrids.