The road to sustainability

There are many innovative projects across Europe and the UK where new materials and techniques are being used to deliver sustainable roads, reports Alan Mackenzie, director of the Asphalt Industry Alliance

The asphalt industry is operating today in a much different environment than it was even as recently as two years ago. And environment is the key word. The need to find more sustainable solutions is encouraging customers to look at new materials and techniques that previously they would not even have considered. The move away from a dogmatic rejection of any material not already proven to have lasted for 20 years is not just welcome for the industry, but gives both customer and supplier alike, an opportunity to benefit in environmental and financial terms.
The last two or three decades have seen the development of so many new products that customers are now able almost simply to specify the benefits they require from these materials, rather than having to define their detailed specification. Mixes such as porous and double layer porous asphalt, stone mastic asphalt, ultra thin asphalts and asphalt concrete, provide a wide choice of solutions to client requirements for noise reduction, enhanced durability, higher skid-resistance, or surface drainage to name only the most obvious benefits available.
Yet progress continues and across Europe there are some really innovative projects which, although still at testing stage, indicate very exciting possibilities for the future.

New ways to pave
Most new materials have been developed with the dual need for more sustainable products and to reduce the amount of time that roads are closed for maintenance or repair. High quality and more durable surfaces make longer life pavements realisable while reducing the need for carriageway possession and allowing faster resurfacing times. New ways of paving are also helping the process.
In the Netherlands, for example, the concept of “asphalt-on-a-roll” – a pre-fabricated layer of porous asphalt using a special bond layer activated by induction – emerged some years ago. This is currently being tested on a motorway service area so its routine use on roads is on a not too far horizon. The test section material incorporates re-used tyres, the resulting “poro-elastic” surface significantly contributing to the abatement of vibration and therefore traffic noise of up to 10db(A). It may also help to decrease vehicle rolling resistance and thus contribute to the reduction of emissions.
Another innovation is that of “compact asphalt” a technique developed in Germany which enables two layers of asphalt to be laid simultaneously by one paving machine which applies a thin top layer onto a thicker binder layer, creating a perfect bond. As compact asphalt employs the use of a much thinner surface layer, it reduces the amount, and therefore the cost, of the highway quality aggregate content.
More sophisticated hardware also plays a role in achieving longer-life roads. Intelligent compaction is a technique that employs electronic equipment on the paver roller to deliver instant information about the compaction achieved along the length of the laid material. The resulting more uniform compaction improves durability.

Energy producing pavements
Not surprisingly, energy reduction is top of the agenda. In the Netherlands, the excellent heat-absorbing properties provided by asphalt’s dark colour, are being used to store and retrieve energy. There are several techniques but all are based on the principle of running water through the warm pavement surface in the summer and storing it underground for heating buildings in the winter. The warmer water returned from buildings to the pavement during winter also helps reduce maintenance activities necessitated by icy weather.
On the other hand, several techniques have been developed to lower asphalt production and application temperatures, resulting in significant reductions in mixing temperatures by as much as 300C. In addition to much smaller carbon footprints, these Warm Mix Asphalts (WMAs) have several other advantages, such as reducing the wear and tear of the plant used in their production and enabling newly surfaced roads to be opened earlier to traffic.
In the UK, a number of innovative longer-life asphalts are already being put to use. A cold mix used on a project in Northamptonshire combines recycled road planings with an emulsion bitumen binder designed for cold production. The process reduces the energy consumption required by hot-mix asphalt by 99 per cent.
Following years of research and development, and trials for suitability of its use in the UK, a high performance asphalt (EME2) originally developed in France has been used here for the first time on trunk roads in Scotland. This type of asphalt employs a high content of hard bitumen binder with carefully graded aggregate to provide good resistance to fatigue and cracking, meaning less maintenance over a longer period of time. At the same time Transport Scotland has also been road-testing German-style stone mastic asphalts on the Glasgow-Edinburgh M8, which may be more durable than mixes previously used on Scottish roads.
Even with a good knowledge of all the latest technological developments, customers wanting to achieve the optimum solution, i.e. the one that will deliver the most sustainable and the most cost effective solution for their particular situation, should include a good proportion of evidence-based common sense within the decision-making mix.

The whole picture
The most sustainable option in the longer term often comes from looking at alternatives. Night maintenance work has become the norm in response to the need to keep traffic moving on our ever more congested road network.  However, the most recent research conducted by the Transport Research Laboratory into durable roads, resulting in Road Note 42, shows that they are best achieved when asphalt is laid in warm, dry weather. Daytime surfaces are, in fact, more durable. So perhaps on less trafficked roads night-time maintenance should not be an automatic decision.
It is understandable that specifiers want high quality materials to ensure the longevity of their roads but there are occasions when high – rather than highest – quality will do the job just as well and at a lower environmental and financial cost. For both these reasons the use of locally sourced materials should be considered as the first option wherever possible, especially if the alternative is to transport thousands of tonnes of only slightly better material 100 miles or more by road.
So, even a cursory review of how new technology can help provide more sustainable highways offers considerable food for thought. There are many ways of reducing cost and environmental footprint that can be achieved without the use of technology, and there are just as many ways of adding to these advantages by efficient, practical planning. By looking at projects holistically, considering common sense alternatives to common practices and making best use of new technology developed here and abroad, we can find the most sustainable solutions and reduce future costs.

Alan Mackenzie is a Director of the Asphalt Industry Alliance and President of the European Asphalt Pavement Association.

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