Asphalt vs Tarmac: What’s the difference?

Asphalt is familiar to all as the most common road surfacing material: 95% of UK roads, and more than 90% of European roads, have an asphalt surface and most of us drive on it every day.  Another familiar term for road surfacing is tarmac and the question is: is there a difference, and if so, what it is? And what about tar? What is that and where is it used? This blog explores these familiar terms and explains the differences.  

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• What is asphalt?
• Why is asphalt used for roads? 
• The pros and cons of asphalt vs concrete roads 
• What is tarmac? 
• What is tar? 
• Summary of asphalt vs tarmac
• Improving asphalt roadways 
• What are the most common types of asphalt? 

What is asphalt? 

The term ‘asphalt’ does not just mean one material: it is an umbrella term that covers several mixes used within the construction industry. Over 25 million tonnes of asphalt are produced every year in the UK alone, and they all use bitumen as binder to cement together the aggregate particle components of the mix. Other components such as polymers, fibres, or crumb rubber may be added to the mix to modify its properties. The most commonly asphalt materials are listed in more detail later in this article.  

In the US, the term asphalt is used to describe what the rest of the world calls bitumen:  the sticky black viscous liquid derived from petroleum that forms the binder in asphalt mixes. 

Why is asphalt used for roads? 

Roads need strength and durability and asphalt is the most economical option for road surfacing. It is generally cheaper than concrete and more popular with road users due to its smoother ride and lower noise levels. Asphalt roads are constructed in layers and open to traffic quickly, minimising disruption; it can be milled, reprocessed, and recycled into new road surfacing, minimising aggregate usage and reducing whole-life costs. 

The pros and cons of asphalt vs concrete roads 

• Installation time – Asphalt is quicker to install and with no curing time it can be open to traffic more quickly. 

• Cost – In most countries, asphalt roads are lower in cost to build 

• Flexibility – asphalt surfaces are able to accommodate minor settlement and deformation. 

• Ambient Temperature – In hotter climates asphalt can rut or bleed whilst concrete is more tolerant. However, where seasonal temperature vary widely, concrete roads can have expansion issues. 

• Durability – In general, concrete surfaces are more durable and tolerant of high loading. Asphalt loses flexibility as it ages leading to fatigue cracking if not re-sealed. However, there are some newly developed asphalt technologies, like perpetual pavement or long-lasting hot mix asphalt pavement (US) that can perform beyond its design life, even up to 50 years. 

• Ease of Maintenance – a well-constructed concrete surface will require less maintenance than asphalt and have a longer life. However, maintenance of asphalt is relatively straightforward and can be done in small sections and re-opened to traffic very quickly. 

• Ride quality – Road noise is lower for asphalt than for concrete (by about 3-6dB) and the ride quality is generally better. 

• Recyclability – Concrete can be broken up at end of life and recycled as construction aggregate. Asphalt can be milled and re-cycled into new road surfacing. In some European countries already over 90% of reclaimed asphalt is reprocessed and re-used in new pavements. 

  What is tarmac? 

  Rather like the ‘hoover’ or ‘thermos’, tarmac is a term used by the public, in the UK at least, to represent all ‘black top’ road surfacing materials regardless of the mix constituents. It is in fact a shortened version of ‘tarmacadam’, a road surfacing material made from a mix of aggregate and sand bound together with tar.  

  Tarmacadam was invented by Edgar Purnell Hooley and patented in in 1902. Adding tar to the aggregate enhanced the compacted macadam method introduced in the previous century, making the roads dust-free and more durable. 

  Tarmac materials will comprise of crushed rock aggregate particles coated with modified tar. When compacted in place this forms a bound, strong and durable surface. It is more likely to be used in small areas such as household driveways. Today, a bituminous binder will usually replace tar in the mix, and the material is then referred to a bitmac.  
  
  

  What is tar?

  The tar used for tarmacadam is a natural black liquid substance. It can be extracted from the ground where it is found in large tar pits.  It is also obtained as a by-product of coke production from the distillation of coal. 

  The use of tar for road making materials has receded with the introduction and wider availability of bituminous products obtained from petroleum. 

  Summary of asphalt vs tarmac

   

  	Asphalt	Tarmac
  Binder	Bitumen of various grades	Modified tar
  Types of mix	Multiple mix types with a range of properties,	Coated stone
  Modifiers and add mixtures	Add-mixtures and modifiers available to alter material characteristics	N/A
  Strength and durability	High strength and highly durable	Moderate strength and durability, prone to damage from petrol and diesel spills
  Current applications	Major and minor roadways, runways, parking areas, cycleways	Domestic driveways, small parking areas, footpaths
  Surface characteristics	Wide range from smooth to coarse. Porous mixes available	Coarse surface, impermeable
  Mix temperature	Hot mix, warm or half-warm mix options	Hot mix
  Recyclability	Fully recyclable, can be reprocessed into new road surfacing	Recyclable as unbound aggregate or as cold recycle technology in mineral cement emulsion mixtures

Improving asphalt roadways 

While asphalt roads have achieved proven success, they are not without problems: the surface is flexible but requires a stiff strong foundation, and when the foundation is insufficiently stiff, fatigue failure of the asphalt can occur much faster. The use of Tensar InterAx to stabilise road foundations improves the strength and stiffness of the road foundation, increasing pavement life and reducing whole-life cost. 

Existing asphalt roads may be resurfaced by milling off the upper asphalt layers and overlaying with new asphalt. In these circumstances any cracking in the lower layers would quickly reflect through to the new surface. Reflective and fatigue cracking can be controlled by introducing a Tensar asphalt interlayer product, such as Tensar AX5-GN, Tensar Glasstex or GlasstexPatch, prior to overlaying. 

What are the most common types of asphalt? 

• Asphalt concrete (AC), the most commonly used asphalt material in the UK.  Its strength comes from the use of an aggregate component with a smooth grading curve designed to have a high interlocking structure. 

• Bèton Bitumineuse Très Mince (BBTM), common in France, is asphalt concrete for very thin layers (AC-TL) of 20mm to 30mm, using a gap-graded aggregate to provide an open surface texture. 

• Hot Roiled Asphalt (HRA), found almost exclusively in the UK is used for higher speed roads. This material uses a gap-graded aggregate and the larger stones are bound together with a mix of fine aggregate and high viscosity binder. Stone chippings coated with a high viscosity binder are then rolled into the surface to form a durable running surface.  

• Stone Mastic Asphalt, sometimes called stone matrix asphalt (SMA). Developed in Germany and often used as a surface layer, this material is attractive due to its high deformation resistance and low tyre noise. It uses a gap-graded aggregate, and its strength predominately comes from the direct stone-to-stone contact in the structural skeleton of large stone particles. It is very dense mix with a high mastic binder content comprising, modified bitumen, filler, and fibre. 

• Porous Asphalt (PA) uses an open graded aggregate mix bound together, leaving a high proportion of interconnected voids that allow free passage of water. 

• Warm Mix Asphalt (WMA) has properties and performance levels similar to HMA, but with the advantage that the mix is prepared and laid at lower temperature, saving energy and carbon emissions. Because of this, there is growing interest in the use of these warm mix asphalts for environmental reasons. 
   

Next steps

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