As predictions of soaring summer temperatures began to feature in the news, so too did the perennial and pervasive story of melting roads. Motorists were warned that they would be mired in black goo or sliding around uncontrollably on slippery road surfaces.
Well, there were a few such incidents. Video footage of a small stretch of road in Stockport with liquid binder on its surface was republished and re-reported multiple times, even featuring in New Delhi’s Hindustan Times. Residents of a Swindon street, which had been recently treated with a micro asphalt surface dressing aimed to prolong its life, were reported to be ‘living in fear’ after the surfacing began sticking to shoes and tyres. And there were a few issues on our main road network, with a handful of locations requiring overnight closures for repairs due to heat, according to New Civil Engineer magazine. But the asphalt Armageddon we had been led to expect did not materialise.
The reason why roads don’t melt as much as the click-bait press might like is that the materials used for surfacing today are more highly engineered than they were. The properties of the bitumen used to bind the aggregate together is selected to suit the conditions a road will face, with temperature ranges as well as loading and traffic volumes taken into consideration. Of course, as our climate changes, temperature fluctuations will get bigger so roads must be designed with binders that can cope with that.
Bitumen comes in different grades of hardness, measured in penetration values (an old-fashioned test that sees a needle pushed into a sample at a given temperature and for a given time period). Harder – or lower penetration grade – bitumens, such as 40/50 (needle penetrates between 4 and 5mm in 5 seconds at 25 degrees C), tend to be used in warmer climates where heat can cause bitumen to soften, whereas softer bitumens are used in cold locations to avoid brittleness which can lead to cracking. In designing a mix for a road, the aim is always to get the balance right between toughness and ductility, given the loading and typical temperatures it will face.
When bitumen in a road surface softens, vehicles driving over it can cause rutting or ridges. Or a small amount of the binder at the surface may become liquid – like the road in Stockport. National Highways also reported issues this year with motorway surfaces distorting due to the movement of concrete beneath them.
One of the design changes that has helped roads perform better in warm summers is the use of polymer modified bitumen (PMB). Adding a polymer such as styrene-butadeine-styrene (SBS) to bitumen reduces the susceptibility of a road surface to temperature, as well as allowing it to be both flexible and rough. Bitumen can begin to soften at around 50 degrees C, a temperature that can be easily reached inside the black road material if the air temperature is in the high 20s. Polymer modified bitumen may not start to melt until it reaches 80 degrees C. Although PMB is generally only deployed in heavily trafficked roads in the UK, it is often used in surface dressings used to seal surfaces and restore skid resistance, according to the Road Surface Treatment Association.
It is worth noting that coal tar, which was used to bind road ingredients together until the mid-1980s starts to melt at 30 degrees C. Although there is unlikely to be any in the surfaces of major roads, it may be lurking in minor roads that haven’t been resurfaced (or in lower layers of other roads). As an aside, coal tar is classed as a hazardous waste so any planed road material containing more than 0.1% coal tar can’t be recycled in the same way as modern materials made with bitumen binders can.
In conclusion, the story of melting roads is never quite as sensational – or as simple – as some news outlets would have us believe. Roads have been designed and constructed at different times to suit traffic and climate conditions that may well have changed. Understanding these facts, and creating repair and maintenance programmes accordingly, will help maintain and improve the resilience of our road networks.
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