What are the different types of road construction methods?

by Andrew Lees, on 24-Mar-2021 10:01:27

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, usually measured in numbers of vehicles.

There are two types of pavement:

Rigid pavements

A reinforced or unreinforced in-situ concrete slab laid over a thin granular subbase, where 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. Design life is typically 40 years, with failure usually due to cracking of the slabs, or degradation at the joints.

Flexible Pavements

Flexible pavements are the most commonly used pavement type. 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 flexible type.

Included within the flexible pavement type are gravel roads– common across the globe in rural areas, farm to market roads, quarries, access roads, etc. 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.

Flexible vs Rigid Pavement
Cross section of modern pavements.
(Left) Flexible asphalt-based pavement.
(Right) Rigid Portland cement concrete pavement.
Source: https://www.britannica.com/technology/flexible-pavement 

The future of pavements

As with all areas of the industry, there is a need for lower cost 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, higher and lower air temperatures.

A556 case study
Stabilising the A556 improvement scheme in Cheshire, UK with Tensar Technology saving up to £2million in construction. 

Opportunities for the use of geogrids

Including 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.

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

Got a burning question about geotechnical engineering?

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