What is asphalt?
Asphalt is a composite material that is used worldwide for construction, infrastructure, recreation, aviation, agriculture and industrial applications. Typically black, although available in coloured varieties too, asphalt is manufactured in a plant and becomes a hard surface when it dries. The UK produces approximately 25 million tonnes of asphalt annually and accounts for over 95% of our roads.
What is it used for?
The primary use for asphalt is the construction of roads and highways, including bus and cycle lanes, airport runways and taxiways, footpaths and pavements. Its versatility means it is also used for car parks, playgrounds, driveways, construction and agricultural flooring, sports surfaces such as running tracks or tennis courts, and motor racing circuits.
What is asphalt made of?
Asphalt is a composite material with two primary ingredients: aggregates and bitumen. Aggregates – usually crushed stone, sand and gravel – are bound together with bitumen (a viscous and waterproof substance) and this becomes asphalt.
There are three layers of asphalt in the typical construction of a pavement.
- The uppermost layer is the surface course, which is approximately 25-50mm thick and provides an even, skid-resistant and waterproof surface
- This is followed by the binder course, approximately 50-90mm thick, which is a load bearing layer designed to distribute the load from the surfacing into the base course. It is also used as a levelling course;
- Then the base course, which sits on top of a sub-base and provides the load-bearing for the road. Typically, the nominal aggregate size increases as you move down the layers from the surface to the base, while the bitumen content decreases.
The different types of asphalt
Depending on the application, asphalt is designed to achieve varied strength and durability. While there are seven general asphalt mixture types in total, there are three common asphalts which form 95% of the product we produce. These are:
Asphalt Concretes (ACs) In this asphalt, aggregate particles are continuously graded or gap-graded to form an interlocking structure. Gap-graded means that the asphalt tends to have one coarse aggregate and one fine aggregate, without incremental gradings in between.
Hot Rolled Asphalts (HRAs) A dense, gap-graded bituminous mixture in which the mortar of fine aggregate, filler and high viscosity binder are major contributors to the final performance of the laid material.
Stone Mastic Asphalts (SMAs) Gap-graded asphalt mixture, composed of a high blend of single size coarse crushed aggregate skeleton bound with bitumen.
What is the difference between asphalt and tarmac?
Whilst they have a similar appearance and properties, asphalt and tarmac are different products and thus it’s a misconception that they are the same product. Tarmac (originally tarmacadam) is made from aggregates and tar (which comes from coal) whereas asphalt’s ingredients are aggregates and bitumen (which derives from crude oil). Since the late 1980s and early 1990s, tar has ceased to be used in the production of roads, as it is a carcinogenic substance, and therefore considered hazardous. Instead, it has been superseded by asphalt which is more hard-wearing and less prone to damage from diesel or petrol spills.
Why do we use asphalt for roads?
Asphalt is cost-effective, low maintenance and extremely durable, making it an ideal road surfacing material. It is a popular choice because:
- It is fast to construct
- It can withstand significant weight and temperature fluctuations
- It is easy to maintain and repair
- It produces a smooth surface for maximum tire grip
- It reduces noise pollution
- It rapidly disperses surface water, reducing spray, the risk of aquaplaning and improving visibility for drivers
- It increases the visibility of road markings
Is asphalt sustainable and can it be recycled?
As we collectively move towards Net Zero, the production of asphalt has evolved to adopt greener techniques for production. Asphalt is one of the most sustainable construction materials, as we can use construction waste and recycled asphalt planing (known as RAP - whereby the recycled road surface is put back into the production process) as replacement aggregates. This is effectively put into practice throughout the road construction industry, and optimising the use of RAP is key in new asphalt innovations such as our Foamix ® range.
We can also adapt the manufacturing method to reduce the carbon footprint of the product, and therefore the entire project. There are three key production methods:
Hot Mix Asphalt (HMA) HMA is made under a high heat, between 150-180⁰C and is used for heavy duty applications such as airport runways, car parks and roads with a high volume of traffic.
Warm Mix Asphalt (WMA) Mixed at temperatures 20° to 40°C lower than HMA, Warm Mix Asphalt is the greener choice as it requires significantly less energy during manufacturing and therefore releases less emissions. WMA is also considered safer for construction crews as the temperature of the material is lower and it thereby emits minimal odour and steam. All of our WMA are tested to meet industry specifications and are suitable for the same applications as HMAs, without compromising performance.
Cold Mix Asphalt (CMA) Most useful for repairs and maintenance of small cracks, potholes or patching of existing pavements, Cold Mix Asphalt is produced without any heat and, instead, water is used for emulsification.
To go one step further, we’ve also developed other ways to reduce embodied carbon during the manufacturing process and have launched our SuperLow-Carbon range, which includes a biogenic material within the bitumen that effectively locks carbon within the asphalt.
Asphalt is one of the most widely used, reliable products with a virtually unparalleled versatility when it comes to road surfacing, commercial and domestic applications. View our full range of nearly 30 asphalt products here.