The ‘pyramid’ of strength

We often mention different layers in our articles that make up a road structure, we thought we should explain these layers a little further.

The pyramid of strength

Broken down into layers as opposite, the overall thickness of each layer is reduced as the road construction reaches the surface.

This gives an overall ‘pyramid’ of strength, with the lower layers spreading the weight and providing durability, it is the surface course ultimately being responsible for transferring load to the lower layers and resisting the effects of the traffic application.

Tack coat solutions can strengthen the bond between the top layers. Tack coat is a very light application, it provides strength in bonding between two layer of binder course and must be thin, uniformly cover the entire surface, and sets extremely fast.

Sub-Grade Course

The subgrade is the compacted natural soil below the pavement layers and it is the finished or compacted surface on which the pavement rests. They are also known as the formation and serves as the foundation of pavement layers.

Sub-base Course

The sub-base layer enhances the bearing capacity of the subgrade and improves the load distribution capacity of the base course and wearing course.

Binder course

Binder Course Layers are generally the first levels of asphalt to be laid. This is a load-bearing, strengthening layer of the pavement and can range between 40mm-300mm thick depending upon trafficking. The binder course material should be laid and thoroughly compact.

Surface Course

The Surface Course is the final surface. A surface course should consist of a small, hard aggregate, usually 6mm or 10mm combined in a bitumen or asphalt binder. This layer should be a minimum of 40mm in thickness in accordance with BS 594987, the surface course can be applied in a range of material depending upon the weight of vehicles using the roadway.

Types of surface course:

 Close graded surface course/dense macadam

Typically found on most residential estate roads, footpaths and older car parks.

Stone Mastic Asphalt (SMA)

Typically found in car park applications and some highways particularly roundabouts.

Hot Rolled Asphalt (HRA)

Typically found in industrial applications and high traffic commercial routes.

High Stone Content HRA

Typically found in large industrial areas such as ports and industrial yards and estates.

HRA & Chippings

Typically found in petrol forecourts, carriageways and high traffic areas.