Surface tension - pothole linked road deterioration

Materials World magazine
26 Nov 2013

A mutual hatred for potholes is one of the few things that unites drivers and cyclists. These holes in the road damage vehicles and can even endanger the lives of road users by causing accidents. Collectively, the potholes marking Britain’s roads take up enough space to cover the Isle of Wight twice over. Or to put it in context, that’s an average of one pothole on every mile of road.

But what causes these faults in the surfaces of the UK’s roads? Malcolm Simms, Director of Asphalt at the UK’s Mineral Products Association explains, ‘At a material level, there are various deterioration mechanisms, generally related to fatigue effects. Over time, road materials undergo millions of cycles of traffic loading and unloading as well as temperature related stresses. They will gradually become more brittle as they lose elasticity and their ability to recover after loading is compromised. This leads to the initiation of cracks.’

These cracks can eventually lead to the creation of potholes. Simms explains, ‘If the materials continue to be stressed, the cracks will propagate either between the mixture particles or through the matrix of the material itself. Cracks present an entry route for water. Under the pressure of traffic this water can strip the bond between aggregates and push the mixture’s particles apart.’ In winter conditions, this can cause further problems. ‘If water is held in cracks and then freezes, it will expand and further force the material apart, creating wider cracks, ’ says Simms. Sometimes the fault behind potholes lies further down within the road material. Simms continues, ‘If surface materials are laid directly onto substrates that are not stable or are already cracked or deformed, then the surface materials are unlikely to be able to absorb the additional stresses where those deficiencies pre-exist.’

Balancing act
Selecting materials for road surfaces often involves balancing the conflicting needs both for smooth, impermeable surfaces to prevent water damage and rough surfaces that provide adequate traction for vehicles. Simms says, ‘Road surfaces have to take the brunt of traffic effects, but are also the first point of exposure to the climate and weather. So a road needs to be both rough and smooth.’

How do materials currently used meet these criteria? The most commonly used materials on UK high-speed and local road networks are asphalts. The exact materials chosen are dictated by European Standards series EN13108, and this includes asphalt concretes, hot-rolled asphalt (HRA), stone mastic asphalt (SMA) and thin surface course systems (TSCS), which are variants of SMA. The properties of each of these varieties differ and should dictate their selection for each specific task. TSCS and SMA are the most commonly used materials over the past 15 years. Simms explains, ‘Asphalt materials are both thermoplastic and viscoelastic, which means they are harder at low temperatures and softer at high temperatures. They tend to deform microscopically when loaded slowly but recover almost immediately when loaded rapidly. Therefore the design of the mixture needs to be carefully assessed in order to deliver performance across the anticipated temperature and loading ranges. When hydraulically bound mixtures are used in lower layers, consideration must also be given to joint formation and positioning, as temperature changes and traffic-induced movement can cause reflective cracking.’

Simms points out that ‘It is important to specify the right material for the right site and thereby balance the range of performances of the available mixtures’. He says that the primary factors in determining the choice of material and its ingredients are therefore determined by the site characteristics, including:

  • volume of traffic
  • proportion of traffic made up by HGVs
  • traffic speed, alignment and layout
  • skid resistance requirements
  • ground substrate conditions
  • economics

‘All these considerations should be well understood and communicated by specifiers and procurers prior to materials selection. No single mixture will always work for all situations,’ says Simms.

For example, when constructing a new motorway engineers have much more control over the site and installation, giving them the best chance to implement best practice. Engineers working on existing roads must make use of temporary traffic management and may have to work at night when traffic levels are at their lowest. While both these sites might make use of the same materials, the different practical considerations affect how well each project is realised. Simms added, ‘The nature of maintenance can also be relevant to material considerations. For full depth reconstruction or new build, the selection of materials is increasingly determined by economics. The choice and source of materials may also be governed by both location and scale of the works which could, for example, permit setting up site production plants thereby minimising haulage time and costs.’

MPA Asphalt carried out its own survey into the number of potholes reported by Local Authority engineers on UK roads in association with the Refined Bitumen Association. The total number of potholes was 2.2 million in 2012 – a 30% increase on the previous year. Reasons behind this significant but on-trend increase were primarily put down to under-funding, as many authorities lacked the budget for preventative measures. Simms warns, ‘Without proper repairs and funding for maintenance, this situation will continue in a downward spiral. Highway authorities have a responsibility to make roads safe if and when potholes appear. Simply temporarily filling them does not generally address the underlying problems leading to their appearance – they are merely a symptom.’

Aside from regular maintenance, a long-term method for preventing potholes is much needed. One example is the improved guidelines for road surface materials now specified. Simms tells me, ‘Industry and companies continue to carry out R&D, both individually and in conjunction with key clients. This has enabled the Highways Agency to review and amend its skid resistance policy and related materials requirements to help enhance the safe durability of materials’. The Highways Agency is currently researching skid resistance by using different techniques for measuring its impact. While previous methods measured the forces generated when a rubber tyre or slider is forced to slide across a wetted surface, the new process characterises the skid resistance of road surfaces using high-resolution images of the materials.

But with a network of ageing, and often busy, roads, the industry needs a stop-gap repair solution. Simms adds, ‘There are many proprietary pothole repair systems on the market that aim to quickly and safely fill potholes, but in reality these may not always address the underlying reasons for failure. The thermoplastic and viscoelastic properties of asphalts are, to a degree, self-healing. The use of binder and mixture modifiers in asphalts aim to extend material performance at both ends of the temperature and stress scale.’

European Commission-funded research is currently looking for more radical solutions to road problems at the Forever Open road project. One current scheme is looking to improve materials specification and design to help infrastructure deal with the influx of hotter, drier summers and wetter, colder winters caused by climate change.

Hot rolled asphalt
Composed of approximately 5% bitumen and 95% aggregates including stone, sand and gravel, HRA has been used in the UK for almost 100 years and is the most common surface on the country’s roads. It is durable and fatigue resistant although it can be prone to deformation. Among other uses, it is widely used for airport runways as it is relatively sturdy and easy to repair.

Stone mastic asphalt
Developed in Germany in the 1960s, this material is made up of a coarse aggregate (57-80%) mixed with bitumen and filler, giving it high resistance to deformation and making suitable for residential streets as well as high-speed and high volume roads. SMA also has good spray and noise reduction properties.

Thin surface course systems
Developed in the 1990s, this mixture uses high levels of aggregates and is usually machine-laid as it is difficult to lay by hand. Its properties are generally reliant on the mixture of stones used but usually offer reduced amount of spray in wet conditions and can be quieter than other road surfaces.


In numbers

54,000 compensation claims made to UK councils for vehicle damage caused by potholes 2010–2012. 

£4.9m cost of compensation councils have paid to motorists for damages related to potholes from 2010–2012.

75% of drivers believe road surfaces are now in a worse state than in 2007. 

13% of UK drivers noticed damage to their cars as a result of poor road surfaces.