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Freeze and Thaw Cycle: Tackling road defects in the North West of Scotland12 Feb 2018

BEAR Scotland is doing all it can to carry out repairs across trunk roads in the North West of Scotland after harsh weather conditions experienced earlier this year had a detrimental effect on sections of the network.

Now an ongoing £24M programme of surfacing improvements at over 200 locations across the North West is well underway. 

BEAR Scotland’s North West Planned Maintenance Manager explains the science behind the cause of this deterioration across the road surface and how BEAR teams are carrying out repairs.

 

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, a road surface can fail prematurely. Such failure can happen over the course of months, or it can be very sudden.

Asphalt (“flexible”) road surfaces, such as those that make up most of the trunk roads BEAR manage, are known for their durability and resilience. Its strengths make it a highly used material for many road surfacing applications and 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 actions of nature. Despite the great longevity of a properly laid asphalt surface, even under heavy traffic, its life can be rapidly cut short simply by exposure to exceptional weather conditions. 

 

Freeze and Thaw Cycle

 

Over time, a road pavement will naturally deteriorate as the materials that make up the road become affected by temperature, rain (especially freezing following after heavy rain), sunlight and chemicals (such as diesel) that come into contact with the surface. 

The asphalt binder that is the “glue” of the road slowly begins to lose its flexibility under traffic and the actions above, tiny cracks form, reducing its natural resistance to water, allowing the water to penetrate into and underneath the road pavement. Once this happens, the surface can quickly fall prey to a number of different types of deterioration. 

Due to the expansion and contraction of water when it freezes and thaws, in freezing water expands in the voids within the 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. With the repeated expansion and contraction the stones in the road surface begin to loosen as their bond with the binder (the “glue”) is weakened. The more often this happens, the greater the rate of deterioration. This is a major contributory factor to formation of potholes.

In a section of road where surfacing materials are all of the same age and composition, locally accelerated deterioration can occur on a rapidly spreading basis as water penetrates not only from the running surface downwards, but also through the areas of looser stone which occur on the sides of a pothole as it forms and grows in size under the combined action of traffic and the elements.

This freeze/thaw action happened across the North West network at an extraordinary rate over the winter 17-18 period due to the range and frequency of temperature fluctuations above and below the freezing point. Between December 17 and March 18 alone this freeze/thaw cycle occurred on 58 days - the highest number recorded in the last five years.

On the A85 in Connel, one of the many areas very badly damaged by the weather earlier this year, road surface temperatures in December 17 ranged from a high of 10.3C to a low of minus 6.5C degrees which, when coupled with heavy rain in the same month, meant that the road surface had begun to break down in some places due to this flux in temperature and weather conditions.

 

Challenges

 

Fixing potholes in winter is difficult as snow, ice, water and moisture naturally collect in the holes and cracks. The existing pavement needs to be dry for most hot asphalt mixtures to form a solid, permanent bond. If any moisture remains in or near the pothole during the repair it can start the deterioration process all over again: freezing, expanding and allowing room for more precipitation to enter and expand further. When air temperatures have dropped to below minus 12Co as they did in January this year, it is a real challenge to repair road surfaces in winter conditions.

As a result, and due to the accelerated rate of pothole development in the last few months, the fallback solution used to overcome this has been to place a special temporary repair material into potholes to minimize the risk of them enlarging until a permanent repair or general resurfacing can be delivered when warmer and drier conditions prevail.

That’s the reason the spring and summer months are ideal for conducting road repairs wherever and whenever possible.  However, at the same time we have to consider the condition of the deteriorated road surface, the resources available to us and the impact our work has on road users and therefore we strive to plan our works to minimise disruption to local communities and the important tourist economy.  

 

What are BEAR doing to carry out road repairs?

 

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

Our teams are now well underway with a £24M programme of surfacing improvements at over 200 locations across the North West Unit before the end of this year, with teams working to minimise any disruption to road users wherever they can.  

For all up to date traffic information, visit Traffic Scotland at www.trafficscotland.org, their new mobile site at my.trafficscotland.org or on twitter @trafficscotland.

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