Deciding how to design the materials that go into making Britain’s roads is no simple matter. ‘It’s all about getting the right materials, with the right properties in the right locations and ensuring long term performance to deliver good asset value,’ Michael Wright, senior pavement engineer at a new research centre devoted to the science of road-making, points out. ‘There is no single material that will do everything.’
In the laboratories at the Jean Lefebvre (UK) technology centre in the leafy Hertfordshire town of Cheshunt an array of machines test combinations of materials for customers in the road-building industry. There is equipment to test the ‘stiffness’ of a sample created in the laboratory, others to gauge its durability and even more to find out how porous it will be in wet conditions.
Inside one particularly large, noisy machine, a piece of asphalt loaded with a pneunatic tyre is being tested by a large rubber wheel to see how easily it deforms under pressure - important to avoid potentially dangerous ‘rutting’ on heavily used roads. The wheel goes over the asphalt around 30,000 times, mimicking the conditions of a real road over several years.
The basic raw ingredients of most roads are aggregate - the stones that make up the bulk of the road - and bitumen - the sticky black substance created as a by-product of crude oil which binds the aggregate. Aggregate comes in different sizes and qualities depending on what it will be used for. The bitumen also varies and can be altered by the addition of various chemicals to change important factors such as its bond strength - its ability to glue the aggregate together - durability and temperature sensitivity.
Industry standard tests gauge various properties of aggregate and bitumen and the finished asphalts. These include, for example, the Aggregate Abrasion Value (AAV) that indicates how quickly the material will wear away and the Polished Stone Value (PSV) indicating how quickly stones will become shiny and dangerous to drive on. Tests for moisture absorption are important because an aggregate that absorbs a high level of moisture is relatively unstable and will deteriorate more quickly especially if subjected to freezing temperatures.
Road construction companies must consider all these issues when deciding which kind of asphalt to recommend - a more durable material is needed for busy intersections, for example, than that used for long, straight stretches of road in areas with light traffic. Which materials are used in specific conditions are subject to established industry standards.
According to the recent Annual Local Authority Road Maintenance survey produced by the Asphalt Industry Alliance, funding has improved in recent years, but the cost of bringing the local network in England and Wales up to scratch has leapt to a staggering £12.16bn. With one in six roads in poor condition the task of upgrading them would take 13 years to complete, while in the meantime local traffic is predicted to rise 40% by 2040.
The report points to the continuing trend of wet winters in the UK in recent years, with record rainfall and flooding a major reason for damage. The most immediate and visible problem and the cause of most complaints is potholes. The number of potholes filled, the ALARM report says, has increased dramatically in the last year to almost 2.7 million, a third higher than the previous twelve months.
It says money would be best spent on dealing with underlying structural problems rather than simply filling in potholes. Large-scale investment of the kind now being rolled out will require careful management to ensure that demand for materials does not outstrip supply. However supply and demand issues might be mitigated by new developments in the industry. Much of the latest thinking is on using materials and techniques offering better value for money.
Jonathan Core, divisional manager at Jean Lefebvre (UK), points to asset management approaches that measure the whole-life costs of the materials and treatments. Without adequate asset management processes in place, he says, local authorities cannot make informed choices when designing and specifying techniques and materials and in turn cannot effectively challenge the supply chain to reduce whole life costs.
‘Sweating the asset is a high risk when you are unclear on how assets deteriorate over time,’ he says. ‘It’s clear that prevention is the best cure, and the technical industry should use our material and treatment knowledge to support more robust asset management processes to add asset value for long-term performance.”
The need for a long-term approach is strongly advocated in a recent report by the Highways Term Maintenance Association. Invest to Save - Benefits of early intervention for highway maintenance says the all too common fire-fighting approach - filling in potholes - is the result of years of under-investment.
One of the report’s authors, Martin Towey of ToweyDuffy consultants, says the key to getting better value for money is collaboration between the different partners - companies that work together on different aspects of the process including surfacing, traffic management and white lining. All too often valuable time is lost because of a lack of alignment.
‘The collaboration agenda,’ he says, ‘is where the big opportunities lie in reducing costs, improving quality and increasing client satisfaction.’
The need to achieve long-term sustainability is made more pressing by the dangers of climate change: if temperatures rise, as widely predicted, deterioration rates are likely to increase, so materials will need to be more robust.
Potential solutions include capturing solar energy through asphalt aquifer systems and using it to maintain a constant road temperature, thus reducing the amount of maintenance needed.
Lighter-coloured surfaces could also reduce deterioration rates by reflecting the sun’s rays. The key is in using appropriate materials. If the structural layers within a road are sound –typically the total bituminous pavement thickness is around 260- 360mm in a strategic route and 200mm in a local road - they will add strength. That then means a thinner surface course, which is really only there to supply the relevant driving characteristics, can be used. A well-designed thin surfacing material just 22mm thick can last up to 20 years. An increase in the use of recycled aggregates and road asphalt pavement might also be part of the solution.
Roads, says Mr Core, are 100 per cent reusable. Recycling the materials used in their construction could prove extremely cost-effective. Warm mix asphalt – designed to be laid at lower temperatures than the traditional hot variety, thus saving working time and also improving durability as the bitumen properties are less affected during the production process - might also be part of the solution.
