Types of pavement and road construction methods
by Andrew Lees, on 24-Mar-2021 10:01:27
In road construction, the two main types of pavement are rigid and flexible. These two types use different materials and methods of construction, resulting in different physical properties (more on this below).
In this guide, we’ll discuss the different road construction methods and the types of pavement involved. Read on to learn more about:
- Types of road construction methods.
- Types of pavement in road construction (including rigid pavements and flexible pavements).
- The future of pavements.
Use these links to find the section you’re most interested in.
What are the types of road construction methods?
One important point to note is that within our industry, what this question usually relates to is the different types of pavement construction.
The pavement is the part of the road that carries the traffic (not to be confused with the footpath), and has a set of layers or material placed over the natural ground (subgrade). The pavement layers spread the load of the vehicles so that it does not exceed the strength capacity of the subgrade.
The challenge for the pavement engineer designing the road is to select the right material and layer thicknesses so that the pavement will be serviceable for the full design life. When discussing the strength of pavements on roads, we usually measure this in the number of vehicles they can support.
Types of pavement in road construction
There are two main types of pavement: rigid pavement (consisting of one layer) and flexible pavement (typically made up of multiple layers). Pavements are often made of asphalt or concrete, but they can also be constructed from artificial stone, flagstone, cobblestone, bricks, tiles or even timber.
Cross section of modern pavements.
(Left) Flexible asphalt-based pavement.
(Right) Rigid Portland cement concrete pavement.
With rigid pavements a reinforced or unreinforced in-situ concrete slab is laid over a thin granular subbase. Loads are supported by the flexural strength of the pavement, which acts like a stiff plate, transferring load over a wider area of subgrade. Concrete roads are a rigid type, and there will usually be joints in the concrete to control cracking.
Much of the US Interstate network and European highway network use concrete construction, although it has been much less popular in the UK. Concrete pavements can handle very heavy traffic flows and high axle loads. They are now more common in urban areas, ports and locations, where heavy trucks travel slowly. The design life of rigid pavement is typically 40 years, with failure usually occurring due to cracking of the slabs, or degradation at the joints.
Flexible pavements are the most commonly used pavement type in the UK. They typically have multiple layers. With flexible pavements, wheel loads are transferred by particle-to-particle contact of the aggregate through the unbound granular layers. The pavement is supported by and protects the subgrade below the pavement.
Asphalt roads are constructed using flexible pavement. Also included within the flexible pavement type are gravel roads, which are common across the globe in rural areas, farm-to-market roads, quarries and access roads.
Flexible road construction
In flexible road construction, one or more layers of selected granular material are laid and compacted over the subgrade. As-dug, naturally occurring materials are often used, but greater performance is gained by using manufactured, crushed aggregates.
With crushed aggregate (unpaved) roads, one or more layers of manufactured, crushed aggregate will form the pavement, with the crushed rock having angular particle shape with a defined range and mix of particle size. The layers are wetted to aid compaction, allowing the particles to interlock and densify. Where multiple layers are used, the lower layer is of a coarser grading, while the upper layer is finer textured granular material.
Bituminous pavements are flexible. Bitumen is used to bind aggregate particles together to form a flexible, strong, and waterproof layer. The mixture of aggregate and bitumen is called asphalt concrete (or asphalt or tarmac for short). Aggregate roads can be given a thin, flexible surface of bitumen-bound stone that will prevent dust, provide waterproofing, reduce maintenance, and provide a safer driving surface. Bitumen is sprayed on and stone chippings spread over the surface and bound in place by the bitumen.
By far the most commonly adopted construction type for urban and network roads is the layered flexible bituminous pavement. The pavement is built up with one or more unbound aggregate layers, the subbase, followed by either an unbound granular base or asphalt base layer. Above this is the asphalt binder course and finally the asphalt surface course. The base course and binder course provide most of the structural support while the surfacing provides a smooth waterproof riding surface with high skid resistance.
The design life of bituminous flexible pavements is normally 20 years. Failure sometimes occurs due rutting, cracking, or ageing of the asphalt layers or failure of the subgrade leading to deep rutting. Regular maintenance of the surfacing layer can greatly prolong pavement life.
A highway constructed using flexible bituminous pavement
The future of pavements
As with all areas of the industry, there is a need for lower cost road construction methods.
One route would be reduced maintenance requirements, which would both reduce operating costs and minimise traffic disruption on congested networks.
A second option would be to increase pavement life, thereby also enhancing sustainability. This might be achieved by using less non-renewable materials, reducing the carbon emissions generated during construction and maintenance, as well as reducing traffic delays which themselves increase carbon emissions and local air pollution.
And a third, key possible way forward is that greater resilience of pavements would make them less susceptible to the effects of climate change, such as increased frequency and intensity of storms in addition to higher and lower air temperatures.
Stabilising the A556 improvement scheme in Cheshire, UK with Tensar Technology saving up to £2million in construction.
Opportunities for the use of geogrids
Incorporating TriAx stabilisation geogrids into the granular layer of pavements increases the strength of the granular layer. This then allows a reduction in thickness of granular layer, saving time, non-renewable aggregate materials, carbon emissions and cost. It also increases the design life of the pavement, reducing maintenance costs and traffic disruption.
For more information, visit our geogrids for road and pavement stabilisation page.
Got a burning question about geotechnical engineering?
Why not drop us a line and the answer to your question may feature in a future episode of Ground Coffee firstname.lastname@example.org