What are our roads made of?

It seems like a daft question, doesn't it? We all know that roads are made from tarmac: at least that's the answer your blogger got from his wife and from those colleagues who bothered to answer the question. The obvious follow-up question, "what is tarmac?", is met with various tentative guesses and attempted witticisms.

To be honest, "tarmac" pretty much summed up my knowledge of road construction too, until I researched this article. What's more, most roads aren't even made of tarmac anymore, although the term is widely used to refer to any blacktop road surface.

When you think about it, it's a little surprising we don't have a better idea what we're driving over; after all, we get to see under the surface often enough when potholes appear, or when one of those interminable roadworks digs our street up.

Materials used in road construction

There are several ways to build roads, and each method uses its own combination of materials. Here's a quick summary of just some of the materials found in our roads.

• Aggregate

  Aggregates are mixtures of particular materials, such as gravel, crushed rock or chippings and sand.

• Concrete

  Concrete is a robust building material made from water, aggregate and cement. It may also contain additional or alternative ingredients such as polymers or, for some construction applications, steel or carbon fibre reinforcement.

• Tar

  Tar is a brown or black, viscous hydrocarbon/carbon liquid. It can be manufactured industrially although it also occurs naturally. Manufactured tar is made by a process of destructive distillation. It can be made from wood during the production of charcoal and from coal during the production of coke. A famous example of naturally occurring tar is the La Brea Tar Pits in Los Angeles, USA.

  Tar contains more carbon than bitumen (see below) and has a higher melting temperature.

• Asphalt / bitumen

  This is a bit of a tricky one. Terminology varies geographically, and even some industry sources don't agree on the definitions of asphalt and bitumen. Asphalt and bitumen are regarded by some sources as interchangeable names for the same substance. Some construction industry sources state that the word asphalt means the product resulting from combining bitumen with aggregate such as sand. For the purposes of this article we will go with the opinion of the Encyclopædia Britannica website, that asphalt and bitumen are the same substance.

  Like tar, bitumen is a naturally occurring, black or brown, viscous material that can also be produced industrially. In a natural form it may be found at the Pitch Lake of southwest Trinidad. When manufactured, it is produced by the refining of oil or petroleum. Bitumen can be so viscous that it is almost solid.

  Tar and bitumen are both used as a binding/adhesive medium for aggregates in the road construction industry.

Different types of road construction

Roads come in a surprising number of variants. Perhaps the most obvious differences are those between concrete roads and those with a more flexible surface, such as asphalt or tarmac. All surface materials have their relative pros and cons, as we'll see.

Concrete

About 4% of Britain's motorway and A-road network has concrete surfaces. Most of the UK's concrete roads were built during the 1960s and 1970s and are noticeably ageing. Consequently, National Highways is currently executing a programme of repair and rebuilding. Where surface repairs are not the best option, roads will be rebuilt with a different surface material.

Concrete pros

• Longevity

  Concrete roads are expected to last from 20 to 40 years. Asphalt generally needs relaying after about 10 years.

• Suitability for recycling

  Concrete can be crushed to provide a reusable aggregate.

• Copes better with a high volume of very heavy vehicles

• Less adverse environmental impact

  Concrete is less polluting to produce than asphalt and gives better fuel efficiency to the vehicles that drive on it.

• Potentially quieter

  A smooth concrete pavement would give a very even, quiet ride. In practice, concrete road surfaces are rarely completely smooth, and would be less safe if they were, so the texture of most concrete surfaces is noisier than smooth asphalt.

Concrete cons

• Noise

  Concrete is up to 7 decibels noisier to drive on than asphalt. Because a concrete surface isn't as "grippy" as asphalt, texture has to be applied by brushing/raking to improve traction. This can make for a noisy ride.

• More expensive to produce

• Difficult to repair

  Unlike asphalt, concrete can't easily be patched. When damage occurs, entire slab sections may need to be replaced. When smaller repairs are possible they can take much longer than comparable repairs to asphalt surfaces.

• Water and fluid spills tend to lie on concrete for longer

  Concrete surfaces can present skid hazards, as spills are not absorbed as readily as they are with asphalt.

Asphalt

Asphalt as used in road construction is more technically known as asphalt concrete. It is much more common than the cement-based material we call concrete in everyday language. ("Concrete" is actually a term for any composite material composed of mineral aggregate adhered with a binder.)

Asphalt pros

• Cheaper to build and maintain than concrete

  As well as being less expensive to build, asphalt permits quick and easy patching and spot repairs.

• Recyclable

  Asphalt is 100% recyclable.

• Safer to drive on

  An asphalt surface offers better traction than concrete.

• Easier and less disruptive to repair

  As surfaces can be easily and quickly patched, repairs to asphalt roads tend to be quicker and cause less disruption to traffic.

• Absorbs spills faster

  Water, chemicals, oil etc, don't lie on asphalt as long as they do on concrete surfaces, so skid hazard are less of an issue.

• Smoother, quieter ride

  Despite an asphalt surface giving better traction, well-finished and maintained asphalt usually provides a smoother quieter ride.

Asphalt cons

• Emissions

  Melting asphalt releases greenhouse gases.

• Shorter lifespan

  Asphalt is not as durable or as long-lasting as concrete. The lifespan of asphalt before relaying may be a quarter that of concrete

• Not as resilient under high volume traffic of heavy vehicles.

Anatomy of a road

The structure and construction method chosen for a road is primarily dictated by the nature of the project: the terrain, the type and volume of traffic the road will carry etc. Although we have presented here the two main categories of road construction, both asphalt and concrete roads come in many varieties. Two asphalt-topped roads may contain different aggregates and binding mediums arranged in differing number of layers. The same is true of concrete-paved roads.

The following are simplified descriptions of the layers comprising a typical road.

• The road surface (commonly called the pavement in American English)

  Made of concrete or, more commonly in the UK, of a mix of bitumen-bound aggregates. The road surface usually features a "camber". This means the profile of the road isn't flat but, instead, slopes gently downwards from the centre "crown" of the road towards the shoulders or edges of the road. The camber is designed to allow water to drain from the road surface.

• The binding course (not required for concrete roads)

  Typically made of asphalt concrete with proportions of aggregates graded according to the requirements of the road that is being built. The binding course helps to spread load and to waterproof the layers beneath.

• Base course

  The base course is made from bitumen-bound aggregate, or from compacted, unbound aggregate.

• Sub-base course

  This layer is made from graded, crushed rock.

• Subgrade

  The subgrade is the prepared ground upon which the road is constructed. It is usually soil.

A road may look like little more than a flat, black strip of "tarmac" that's been visited by a steamroller, but it is a carefully engineered load-bearing system. When one thinks about the countless millions of tons of non-stop traffic our roads have to carry for decades, the length of time they take to build and repair suddenly doesn't seem quite so outrageous.

The disruption is still infuriating but, at the same time, kind of understandable.

You might also like

• What is wrong with our road network?
• The pothole menace
• 10 interesting facts about cars and driving