Freeze and Thaw Cycle: How Road Defects Form

A road pavement has a natural life cycle and at some point it will require maintenance, repair and replacement. While most of these repair and replacement projects can be planned, sometimes a road surface can fail prematurely. Such failure can happen over the course of months, or it can be very sudden.

Asphalt surfaces, like the ones that make up most of the trunk roads BEAR manage, are known for their durability and resilience. Asphalt’s strengths make it a highly used material for many road surfacing applications. It is by far the preferred material for the vast majority of roads across the UK, and indeed Europe.

Like all paved surfaces however, it is susceptible to deterioration due to the laws of nature. Despite the great longevity of a properly laid asphalt surface, it can be cut short simply due to long term exposure to the elements.

Freeze and thaw cycle

Over time, road pavement will naturally deteriorate as the materials that make up the road begin to break down and become affected by elements such as temperature (especially freezing followed by thawing), rain, sunlight and chemicals, such as diesel, that come into contact with the road surface.

The asphalt binder, the ‘glue’ of the road, begins to lose its natural resistance to water, allowing water to penetrate into and underneath the pavement. Once this happens, the surface can quickly fall prey to different types of deterioration.

Water expands and contracts when it freezes and thaws. The ice formed in freezing conditions expands in voids within the road carriageway’s matrix which can lead to tearing of the binder which holds the aggregate in the road together. This generally happens at surface level where the frost penetrates the upper layers. Repeated expansion and contraction can see the stones in the road surface begin to loosen as their bond is weakened. The more this happens, the greater the rate of deterioration. This is a major contributory factor into how potholes occur.

In a section of road where surfacing materials are all the same age and composition, sudden onset deterioration can occur on a rapidly spreading basis and water penetrates not only from the running surface down, but also through the looser stone which occurs on the sides of the pothole as it forms.


Fixing potholes in winter is difficult as to snow, ice, water and moisture naturally collect in the holes and cracks. The existing pavement needs to be dry for most asphalt mixtures to form a solid, permanent bond. If moisture remains in the hole it can start the deterioration process all over again: freezing, expanding and allowing the opportunity for more precipitation to enter and expand further.  It is therefore a real challenge to try to repair the road surfaces in winter conditions.

In such circumstances placing temporary repair material into such defects can help minimize the spread of potholes in the short term while a permanent solution can be delivered when warmer and drier conditions prevail.

Reporting defects

BEAR urges people to report potholes. This can usually be done through the BEAR website ‘report a defect’ function, or by calling Transport Scotland’s Freephone trunk road customer care line on 0800 028 1414.

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