One of the biggest frustrations for road authorities and road users alike is for a pothole to be fixed, only to reform after it has been repaired. Sometimes it can be only a matter of months, or even weeks before the offending crater reappears.
We are often asked why repaired potholes come back. To answer that question, it helps to explain why potholes form in the first place. There can be several different reasons or factors involved. Here are some of them.
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Wear and tear
The mixes for the various layers that make up a road are designed to cope with the traffic that is expected to run over them in a given climate. A road surface must be ductile enough to move with the forces it receives but hard enough not to rut or move around too much.
Over time, the bitumen in the road surface – which is mixed with aggregates and fillers – starts to age which means it becomes more brittle. Small cracks start to form. At this point, the road could be maintained with a treatment such as surface dressing or slurry microsurfacing to fill in those little cracks before they can widen.
Without treatment, those cracks will get bigger as traffic runs over them and weather takes its toll. Eventually a pothole will form.
Utility reinstatement
Most people will have observed potholes forming along the edges of a utility trench that has been reinstated in the road. This shouldn’t happen because there is legislation and specifications which set out how roads should be repaired.
Faults in reinstatement include problems with the joints between new and old asphalt or too many voids in the asphalt. Tiny gaps at the joints will widen into cracks over time or the new material may compress too much so that it is at a different level to the main surface. Eventually potholes can form.
Utility companies are only responsible for sorting any problems with the reinstatements for two years after the work has been done. So, if the problems start to manifest themselves after that time, the local authority ends up picking up the work and the bill.
Manhole covers, gullies and the like
Another location where potholes often form is around manhole covers. This could be because the joint between manhole and road surfacing material has not been properly treated. Or it may be that the road has been resurfaced but some of the older material around the manhole has been left in place and it then wears at a different rate to the newer materials.
Any small gap, or difference in level or material, will only be worsened as traffic passes over the manhole and surrounding road materials. Over time and, aided by freeze-thaw and water, a pothole will form.
Bottom-up cracks
Potholes can sometimes originate from cracks that travel from the lowest layer of the road upwards. This can often be the case for country roads which may only be made up from one or two layers of asphalt.
Cracks can form when the ground beneath the road moves for some reason. This could be due to water in the ground freezing and causing it to expand upwards or heave. Or, if there is heavy rainfall, certain types of ground could be partially washed away leaving a void, which can again cause the road to crack.
Cracks at the bottom spread to the top surface of the road. And then they widen and can eventually become potholes if not treated.
Heavy loading
Obviously if a road is getting more traffic than it was designed for, or heavier vehicles, or both, it is going to start deteriorating sooner. For many of our older roads, this is definitely the case. With the growth of internet shopping, there has been a big increase in the number of vans using our roads, with two-thirds of these travelling regularly on local and rural roads.
Points at which vehicles turn, accelerate, decelerate, or maybe all of those are obviously receiving more of a beating than stretches of road where vehicles are just running at a constant speed. That’s why we often see several potholes around junctions.
This is going to become more of an issue as more electric vehicles take to the roads. Currently, their batteries make them heavier which means that they will be causing more damage.
But why do potholes come back?
Where potholes have formed, it is vital that the repair method does not leave any tiny gaps around it. This is, in effect, a crack which will widen over time, as in the cases above.
Unfortunately, most pothole repair methods do leave a joint or weak point. Even the most diligent treatment of the joint will still leave micro gaps in some places.
This fact is what drove Thermal Road Repairs to develop its technology in the first place. Rather than creating new joints which can fail, our system heats up the old asphalt, adds a little new hot material, and then mixes and compacts the lot. That means there is a seamless repair and hence no weak spot to fail.
Any repair method that leaves a seam, however slim, is inviting potholes to reoccur.
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Thermal Road Repairs: Decarbonising the asphalt repair industry
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