What are the function of layers in a flexible pavement?

by Andrew Lees, on 08-Apr-2021 04:14:25

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 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 in a Flexible Pavement

There are actually typically five or six layers in a flexible pavement, divided into two overlapping sections: the pavement section and the foundation section. The foundation section comprises the subgrade and subbase with possibly a capping layer between, whilst the pavement section also includes the subbase, as well as the base, binder course and surfacing course layers. The foundation section enables the structural layers to be properly placed and compacted. In addition, the foundation layers must support the construction traffic, distributing the load to protect the subgrade from deformation. The 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 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.

Pavement Layers Diagram-1
Layers in a flexible pavement, divided into two overlapping sections: the pavement section and the foundation section. 

The Subgrade layer

The deepest element of the foundation section 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 on 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 may be reduced by the inclusion of a stabilisation geogrids, such as the Tensar geogrid at the base. The 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

Subbase 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 pavement. Its 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. 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 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 Layer

The binder course (layer) is an intermediate, bitumen bound aggregate layer placed between the base layer and the surfacing, sometimes referred to as a levelling course. Its function is to distribute load from the surfacing into the base course.

Surfacing Layer

The surfacing course (layer) is the upper layer of the 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 reinforcement can be incorporated at the base of the surfacing or maintenance overlay to delay cracking.

To find out more about Tensar geogrids, you can visit our website here. 

Want to know more about Pavement Design incorporating geogrids? 

Tensar will be holding an Academy Webinar on “Pavement Design Incorporating Geogrids” on Thursday 29th April at 11.00BST.

The session will be presented by Piotr Mazurowski – Application Technology Manager, who will be looking at the benefits of incorporating geogrids into pavement design – stabilised vs non stabilised, tried and tested.  

To find out more information and to register your place, please visit the link here