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Climate change: the bad news for potholes

Earlier this month, Scotland enjoyed its warmest day in 115 years. On Wednesday 5 September the temperature in Charterhall reached 28.6C, the highest the country had seen since 1906 when the temperature in Gordon Castle, Moray, rose to 32.2C.

While people up in Scotland – and many other parts of the UK – may be happy at the prospect of more Indian summers, those managing our road networks may be a little less enthusiastic. Because changing weather patterns in both summer and winter are likely to mean many of our less well cared for roads will deteriorate more quickly, with cracking and, ultimately, potholes.

According to the UN Intergovernmental Panel on Climate Change, we can expect warmer summers and warmer winters here in the UK. There won’t necessarily be more rainfall, but it is likely to be concentrated into shorter periods, with longer dry spells, particularly in the summer.

Here are five problems that the new weather norms could cause for roads, their users and their owners:

1. Rutting

When bitumen is heated up, it becomes softer and more malleable. This certainly isn’t news for Thermal Road Repairs (TRR) since our process involves heating up failed material in and around potholes so that it can be mixed with some new stuff and re- compacted. But a road that softens as vehicles are driving on it is bad news all round.

Although roads don’t tend to soften until they reach around 500C, a dark road surface will absorb heat so that even air temperatures in the 200s can lead to the road surface softening. Add some HGVs or buses to the equation and you could get some nasty rutting. For new road surfaces, this can be avoided by using polymer modified bitumen (PMB) in the asphalt mix – but that isn’t much help for existing roads.

2. Cracking

In the longer term, repeated high-temperature spells can lead to the bitumen in the road oxidising at a faster pace. And when bitumen oxidises, it becomes more brittle which in turn leads to cracking. Add a bit of wear and tear or water action, and a pothole will be just around the corner, unless action is taken.

The engineers behind the TRR process were careful to design in sensors and controls, so that the heater used to raise the temperature of the loose material and surroundings automatically turns itself off before the bitumen gets overcooked and brittle.

In the US, some states have reported cracking problems due to the use of recycled asphalt planings (RAP) in their asphalt mixtures. These problems can be overcome by using more equipment at the mixing plant so that the RAP is heated more gently and by adding rejuvenators, chemical additives designed to have a softening effect on the bitumen.

3. Subsidence

Depending on the type of ground beneath a road, alternating dry and wet spells can also lead to subsidence. Clay for instance shrinks when deprived of water and then swell when its water content rises again. The result of this ‘shrink-swell’ behaviour can be some pretty hefty cracks, which can extend up through the layers of the road.

Roads with grass verges may also suffer cracking due to changes in moisture. The verges are likely to be wetter in the winter and drier in the summer, with the extremes getting extremer thanks to climate change. Again, if the soil is clay-based, moisture migrating from the verges under the road can cause the ground beneath the edges to shrink and swell, leading to longitudinal cracking, often through the whole depth of the pavement.

4. Structural failure

Many rural roads have evolved over time, with tracks becoming small roads and then asphalt pavement being added as traffic volumes rise. Though we often think of water getting into roads through cracks in the surface, the opposite can be true for some of these rural roads. Water gets into the ground below the asphalt and loosens the bond over time, perhaps washing parts of the ground away causing structural failure of even leading to patches of road delaminating. The same can also happen in towns or cities where asphalt has been laid on top of old cobbled streets.

5. Stripping

Nowhere near as exciting as it sounds, stripping is the separation of the binder or bitumen from the pieces of aggregate in a road pavement. Certain types of aggregate are more susceptible to stripping than others, and it is an unwelcome phenomenon that is exacerbated by wet and warm winters.

Unfortunately, stripping tends to begin at the bottom of an asphalt layer, so by the time the effects are noticeable on the surface – as potholes or crumbling edges – it’s already too late. The use of anti-stripping additives and careful selection of aggregate helps protect against the problem.

The good news?

With all seasons getting warmer thanks to climate change, that should mean that there are fewer very cold spells here in the UK where the temperature drops below zero. When that does happen, cracked roads come under attack due to freeze-thaw: water runs into a crack, freezes and expands pushing the crack wider apart. However, although there may be fewer below-zero spells, they will still occur which means any cracks should be sealed ASAP to prevent the chance of them getting wider.

Clearly, well maintained roads are in far less danger of any of these ills befalling them than poorly maintained ones. This is what the 2021 Annual Local Authority Road Maintenance (ALARM) survey, commissioned by the Asphalt Industry Alliance (AIA), had to say about the impacts of climate change:

“Adverse weather conditions, particularly wetter winters with more intense downpours and hotter, drier summers, coupled with increased volume and weight of traffic and the age of the network, can result in accelerated deterioration and unpredicted failures. The impact is particularly acute on less well maintained – and therefore less resilient – roads, where water can penetrate existing cracks or defects, leading to the formation of potholes and, in time, undermine the entire structure of the road.”

The challenge for all local authorities is that they just don’t have the budget to get every road up to scratch. But the likely impacts of climate change on road condition and performance should be factored into risk management and asset management planning – and in many cases, already will be.

Thermal Road Repairs is a green technology company which supplies systems to improve the quality, cost and time efficiency of road repairs and paving – at a far lower environmental cost than traditional methods. We invest significantly in R&D, to create new technologies and to continuously improve our existing ones.

High output. Low emission. Permanent solution.


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