What Are Roads Made Of?

What Are Roads Made Of?
15 Feb 2019
By Richard Bolton

If you have ever wondered what are roads made of? Then you’re not alone and although there are many different types of road in the UK that vary depending on the type of traffic they need to support, the volume of traffic, environmental factors and the planning that is approved. From the most simple dirt tracks to the most complex smart motorways, all roads need to be fit for purpose.

What effects road materials?

Traffic Weight

Considerations for the weight of the vehicles using the road will determine how dense the material used for the base, hardcore, tarmacadam and how steep the camber of the road must be. Heavier vehicles require roads to be made of thicker layers and more tightly packed compounds to increase the life span of the road before maintenance is required.

Traffic

A major factor to be considered when constructing the road is the amount of daily traffic the road will be subject to. Large amounts of traffic will increase the friction caused by driving and braking this friction is what can cause track lines in a road, a simple example of this can be seen in winter particularly in snow or heavy rain with two tracks created by lots of cars driving on the same part of the road.

Speed

The faster a car is going the less friction is present between the tyre and road service, this requires a more abrasive surface, or in the case of motorways a smoother surface to increase the efficiency of the tyre on the road. Faster moving cars can cause more damage to the road surface when braking from high speed, this combined with heavy traffic volume and heavy and light goods vehicles can be detrimental to the road requiring repairs more often.

Environmental conditions

The ground a road is built on is as important as the cars driving on it, soft ground requires a thicker layer of hardcore or more compacting of the ground which can increase the curb and roadside height. Harder ground or shallow bedrock allows road builders to use fewer materials before adding a road surface. Examples of ground erosion and environmental conditions can cause potholes.

What roads are made of

How to make a road?

Subgrade

The subgrade is the very base of the road, in the UK and Europe this is typically the existing soil or ground the road is to be built on. Heavy rollers and industrial wacker plate machines are used to compact the ground to be prepared for the next layer, where a suitable hardness of the ground can’t be achieved by condensing the ground alone, various bulking materials or even concrete can be set to provide a strong foundation. This method of compacting the ground before laying a harder surface has been attributed to the ancient Romans who are credited with constructing the "first modern roads," some of which are still used today as either a surface road or foundation.

Subbase

Once the subgrade is fully prepared a thick layer of hard wearing material is used to serve several functions, this is typically a strong stone hardcore or another aggregate. This thick layer of about 5-inches is a buffer to the roadbed, allowing the road surface to drain faster than just running surface water. The gaps in this layer reduce the damage from frost on the top layer while providing padding that lets the road expand and shrink with the weather and; ground underneath to help prevent cracks or sinking from the weight of traffic or the road itself. This layer is often the surface layer used for low traffic roads like driveways or dirt tracks.

Surface

The surface texture varies significantly from road to road as the stiffest part of the road the tarmacadam; more commonly referred to as tarmac is a compound of tar and sand. Other countries including America and Australia use asphalt also known as bitumen. Asphalt concrete used for roads is a combination of refined bitumen and aggregate, the mixture of sand, stone and pitch can be varied according to the intended use of the road. Smoother roads use more sand and less stone or gravel such as motorways. Sliproads and high friction roads would use a rougher compound with more gravel or stone, this helps heavy and fast travelling traffic slow down faster by providing more grip. Tarmac is a waterproof layer that is tough wearing, the subbase prevents the mixture from seeping through the subgrade and creating a flat level to pour the molten liquid on.

Road Markings

Most roads are only completed when the road limit, separation and indications are completed, in the UK, a tarmac or bitumen mixture using colour wax or high thermal paint is used to colour the mix, white, yellow, red or blue are typical marking colours. The hot mix is poured to fill a tray that slowly dispenses the solution as the road marker follows a design. This process ensures all markings are uniform. The hardware compound is used for rumble strips, lane markings and no parking or stopping destinations that will give the driver some feedback particularly at speed. Cats eyes can also be used for the same effect. On lower traffic or slower roads a hardy paint or whitewash can be used with a spray lance and stencil for faster signage, often used for road indications before junctions or to suggest drivers slow for roundabouts, hard bends or hidden junctions.


Different types of roads

The different types of roads across the world have been designed for the traffic that was intended to travel across it, while one car will have no effect on a well-built road, 10,000 cars an hour will slowly erode the tarmacadam, heavier vehicles are estimated to cause 10,000 times as much damage as a standard passenger vehicle like the Volkswagen Golf. Some road compounds have higher friction that gives better grip, driver feedback and drainage, while smoother roads are quieter, more fuel efficient and are harder wearing.


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