The current heatwave means that local highway authorities have one eye on the thermometer and the other one on their road surfaces as the current high temperatures are causing some to melt. With temperatures topping 30C, the bitumen in some road surfaces may soften and rise to the top. This makes the road surface sticky and more susceptible to pressure loads from heavy vehicles resulting in surface ridging and rutting.
Most roads will not begin to soften until they hit a temperature of around 50C. However, even a sunny day in the 20Cs can be enough to generate 50C on the ground as the dark asphalt road surface absorbs a lot of heat and this builds up during the day. The response for local highway authorities is to send out the gritters to spread granite dust or sand to absorb the soft bitumen and so stabilise the road surface and make it less sticky.
“Drivers may be bemused to see the gritters out in the summer when they are usually spreading grit and salt during the winter”, said Howard Robinson, chief executive of the Road Surface Treatments Association. “However, this is effective standard practice for keeping a road surface safe during extreme hot temperatures.”
He continued: “Asphalt is like chocolate – it melts and softens when it’s hot, and goes hard and brittle when it’s cold – it doesn’t maintain the same strength all year round.”
Following a heatwave in 1995, the road industry introduced a new asphalt specification introducing the use of polymer modified binders in hot rolled asphalt (HRA). These polymers raise the asphalt road surface softening point to around 80C which prevents it from softening under extreme hot weather. Other asphalt products such as thin surface course systems also normally contain polymer modified binders. Modified asphalts tend to be more expensive and are generally only used on heavily-trafficked roads. Robinson estimates that less than 5% of all the UK’s road surfaces contain polymer modified asphalt. Surface dressings which are used to seal road surfaces and restore skid resistance also now predominantly contain polymer modified binders which will resist softening during periods of hot weather.
“Melting of some roads is not surprising during this heatwave but they can be quickly treated and revert back to normal once temperatures decline,” said Robinson.