Most British roads are constructed from asphalt, also referred to as tarmac, but that depends on the nature of the highway, with some older roads built from concrete. Only around 4% of all UK roads are made from concrete, and these date back to the 60s and 70s when roads experienced far lower traffic volumes.
As roads become worn and deteriorate, they are typically replaced, resurfaced or renewed with asphalt, which has various properties that make it more durable, safer and more comfortable for road users due to skid resistance and low-noise performance.
The materials used in roadway design depend on the location of the highway. Some rural and semi-rural roads are still surfaced with gravel, whereas asphalt is now the primary material used elsewhere.
Asphalt has various names, depending on the composition of the raw materials used in surfacing, including tarmac, flexible pavement and blacktop, and is a well-established road surfacing material made from:
Roads within busy urban regions are most commonly surfaced with asphalt because its flexibility prevents the roadway from cracking and breaking under pressure, and it has the capacity to manage heavy traffic – think HGVs or the hundreds of thousands of vehicles using UK motorways every day.
However, the asphalt surface is one aspect of roadway design, and the longevity of the road will depend on the base and sub-base underneath.
Portland concrete, a type of cement, can be mixed with rocks, water and sand to create a road surfacing layer, which is very strong, and indeed stronger than asphalt. Hence some roads still stand seventy years after they were first constructed.
The challenge with concrete is that it can become brittle and is time-consuming and complex to pour. Concrete is also less cost-effective and more challenging to repair when older concrete cracks or wears down.
Chip seal is used for pavements rather than roads but is commonly used for footpaths, where the surface is not expected to experience continual and heavy traffic. This surfacing material is simple to lay and has a tar layer beneath gravel, steamrolled for compression. This versatile material can be used as a repair to fill cracks that appear in concrete or asphalt roads.
Modern roads are built in layers, which aids flexibility and natural movement in varying temperatures and weather conditions. Materials are granular and laid in layers which are then compacted, creating a firm, durable surface above the subgrade.
Adding particle aggregates help to boost performance, and highway engineers use different particle sizes, aggregate grades and mixes depending on the nature of the road and the traffic volumes expected.
Lower sublayers use coarse-graded aggregate with larger, rougher particles, decreasing in size until the upper layers are laid with finer material and compacted between applications to help the particles interlock and create a dense structure.
Bitumen is a binder, in liquid form, which keeps the asphalt adhered, used as a spray sealant to hold all the materials together, and usually topped with a fine aggregate suitable for surfacing use.
The reason bitumen is used across pavements and roads is that it adds strength, flexibility and waterproofing, preventing rainwater from penetrating the surface of the road and causing lifting, cracking or deviations.
Asphalt, or asphalt concrete, made with a mix of bitumen and aggregates, is normally laid with bitumen-bound surfacing, which is a thin layer that provides waterproofing and dust prevention and helps make roads safer for modern vehicles with an even, level surface that resists skidding and slipping.
As we have demonstrated, not all roads are built from the same materials to the same specifications or with the same techniques, and there are several aspects of the intended use of a road that are incorporated into the design process.
Traffic weight is important because the projected number of vehicles, and the proportion of HGVs, will affect the materials used in hardcore, laying the base, and designing the gradient of the camber at either side of the highway.
Major roads and motorways need thicker substrate layers, with compounds tightly packed to ensure the road is tough enough to function safely without requiring maintenance within a short period.
Larger traffic volumes increase friction, whereas braking, changing lanes and accelerating causes stress on the road surface. However, the faster vehicles travel, the less friction they create, which means the highway may need to be smoother to improve the efficiency of tyres against the surface.
Highways contractors also evaluate the natural environment because the ground beneath the substrate can be important; softer soil commands a thicker hardcore layer, or increased compacting, which might mean the finished road is slightly higher – whereas bedrock and harder earth need shallower substrates to achieve the same effect.
Road construction can be split into four components:
The subgrade is the first layer, usually the ground the road is built on and compacted using rollers. In other scenarios, the contractor will lay bulking materials to ensure the foundation is strong enough to bear the weight of the road.
Next, the sub-base is made from hard-wearing materials, usually aggregates, which form a buffer to the road, and help water to drain away faster. The gaps between the aggregate prevent frost and ice formation while providing slight padding that accommodates shrinkage and expansion.
Surface layers are then applied, normally asphalt, with varying compounds, aggregate grades and surface treatments to create a finished roadway that is tough, level and suitable for vehicles.
Finally, highway engineers need to apply road markings, applied with thermal paints in white, yellow, blue and red, with a hot mix technique that creates uniform marking that is instantly recognisable. Roadway marking includes lane markings, rumble strips, cat eyes, median lines, and other directional signage such as arrows and speed limits.
About our guides: We are experts in our field but sometimes it is just as important to explain to the public what we do and why. So with this in mind and because of the importance of the communities we work in, we have put together a series of blog posts explaining what we do for the public.
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