Hot Mix Asphalt – the sustainability case

Nynas Bitumen in common with others involved in the design, delivery and maintenance of asphalt roads is looking hard at the sustainability of the products it helps to produce. In this article, the company's Product Manager Steve Harris reports on the environmental credentials of Hot Mix Asphalt. He writes:

Pressures on all companies to operate in a sustainable fashion are being felt particularly acutely by those whose industrial sectors make high use of natural resources and energy. It follows then, that those which produce asphalt concrete are having to look keenly at ways to minimise their impact on the environment – by reducing their use of virgin materials and cutting emissions. A lot of resources are being directed towards delivering new developments that achieve lower or even cold mixtures.

However, today these developments still only account for a small proportion of the asphalt produced globally. Construction and maintenance of transport infrastructure requires the use of millions of tonnes of natural resources – by far the most of these going into Hot Mix Asphalt (HMA). When discussing the challenge of sustainability, it is HMA and its environmental credentials that are the principal matters for the road sector to address.

So, how does Hot Mix Asphalt meet the challenge of sustainability?

Perspective

Hot Mix Asphalt’s proven credentials of performance, durability and versatility provide compelling reasons to confirm the material’s position as a more than viable solution when it comes to the sustainable development of transport infrastructure, today and in the future. HMA has a long track record of producing very durable pavements. Ultimately the construction of smooth, long life, low maintenance, low noise pavements plays a significant role in delivering real sustainable development.

The cognitive connection between HMA, the environment and sustainability tends to draw attention to temperature related issues such as CO 2 emissions and energy usage – the climate change perspective. Hot mix asphalt production and application typically requires materials to be heated between 135°C and 180°C. The temperature is dictated by the viscosity of the bitumen binder, the need to ensure a homogeneous coating of aggregates by the bitumen and the requirement to ensure adequate workability.

But climate change is only one of the elements of the sustainable development challenge. Taking into account the need to develop and maintain transport infrastructure, HMA offers an important environmentally responsible solution.

HMA’s Green credentials

Performance
• Long life
The concept of Long Life or Perpetual Pavements is well established. HMA materials can be designed and constructed to provide pavements with a minimum life expectancy of 40 years. Building infrastructure that lasts a long time and requires minimal intervention for maintenance has a big impact on the environmental cost of the user. Faster construction times and less road works reduce traffic delays significantly with a resultant decrease in fuel use, exhaust emissions and journey times.

Ultimately the best way to achieve sustainable development using asphalt is to ensure that asphalt pavements are durable, that once built they last a life time and perhaps considerably longer than this.

• After life
Modern asphalt is 100% recyclable and the practice of recycling asphalt back into HMA is well established and understood. Material development and introduction of performance based specifications over the past decade have facilitated the re-use of premium quality surfacing material in HMA surfacing materials; this is significant progress as previously they were ‘downgraded’ to lower layers/performance levels.

The European asphalt industry produces around 350M tonnes of HMA per annum. Recycling RAP (Reclaimed Asphalt Pavement) has been a common practice for over 25 years. In Europe the average content of RAP in new asphalt currently is around 22% but this figure is growing. Most progressive countries like the Netherlands reportedly use around 65% of reclaimed material in new HMA production. Recent studies in the USA suggest 75% is now achievable.

Transport is by far the largest energy consumer when constructing and maintaining pavement infrastructure. Use of recycled material within or close by the construction site can have a significant impact on the total energy usage during construction.

Recycling and re-use of reclaimed asphalt are intrinsic parts of the HMA toolbox, helping the industry move towards sustainable development.

(*Footnote: the European Asphalt Paving Association (EAPA) distinguishes between recycling and re-using asphalt as: ‘Re-use means adding reclaimed asphalt to new asphalt mixes, with aggregates and old bitumen performing the same function as in their original application. Recycling means the use of reclaimed asphalt as foundation, fill or road base material, with the recovered aggregate and bitumen performing a lesser (or alternative) engineering role.’

Durability
• Whole life benefit
HMA is a very durable paving material which can be designed to last 40 years and more (see above) with a minimum of maintenance and the disruption that roadworks can cause. This can be viewed as a major attribute in terms of sustainable development. Energy use and emissions if measured against the whole life of the carriageway can be considerable less over that period than those of alternative means of surfacing which have to be replaced.

In short, initial higher emissions stemming from the heat required to mix and lay HMA are offset against well designed and produced HMA pavements’ longer lives.

Structural contribution
Asphalt has traditionally been characterised by its flexibility and its ability to accommodate flexural stresses within the pavement. Technological advances mean that the HMA toolbox of materials can now deliver far more in terms of a balanced performance spectrum. These allow engineers to design flexible pavements which are durable but with reduced construction thicknesses which maintain an equivalent structural capacity and life expectancy of thicker pavements.

Harder binders and enhanced flexibility can produce stiffer materials, for example EME. These developments mean in some cases a 50% increase in stiffness to allow a 20% reduced in pavement thickness.

Modified binders such as Polymer Modified Binders (PMBs) build on the performance attributes of HMA adding even greater versatility while significantly extending performance envelopes across all applications, traffic categories and environmental conditions

The results are tangible, demonstrable benefits in terms of preservation of natural resources and the continued drive towards sustainable development.

Versatility
1. Meeting needs
The versatility plus the toughness and durability of HMA allows the road engineer to select materials according to their needs.

• Porous and negatively textured materials reduce noise pollution and provide smoother running surfaces, reducing rolling resistance resulting in reduced fuel usage and exhaust emissions per km. They also improve the uniformity of loading which reduces wear and tear and minimises potential for premature failure. 
• Premium hot mix binder allows the use of thinner surface courses, reducing material use, construction times and user delays.
• Surface texture can be engineered to produce improved skidding resistance with aggregates that have lower polishing values which helps conserve aggregate use and encourages the re-use of reclaimed surfacing materials.

2. Adaptability
Engineering with HMA is a long established practice. The emergence of Warm Mix Asphalt (WMA) is a technology which builds on basic principles of HMA and allows material design, production and construction with the use of conventional HMA plant. One of the benefits of WMA is that production temperatures can be reduced by 20%; and studies show that every 5°C reduction in HMA production temperatures can equate to around a 2% reduction in fuel consumption.

Recently HMA has been used in the construction of porous pavements which in conjunction with drainage systems facilitate the managed removal of surface water – reducing the impact of flooding, helps protect water quality against contamination and create a more natural environment for wildlife.

Conclusion

Environmental conscience and legislation will demand that industry meets reduction targets. Politically speaking, reference today to “sustainable development” is an essential election message for any political party. From a corporate point of view, innovative solutions that add environmental responsibility can benefit a company’s credentials.

Energy and CO 2 reduction are important measures when considering impact on climate change. But taking an holistic approach to sustainable development, the proven credentials, durability and versatility of HMA provides compelling reasons for its use. An appropriate expression is ‘optimum whole life sustainable development’ – balancing environmental responsibility with realistic economic and social growth.

A bright future for bitumen
This perspective on bitumen’s future stems from a presentation by Nynas Bitumen’s Business Development Director Martin Carlson at the recent Nynas Bitumen Forum in France. He said:

Bitumen along with the advantages and benefits it brings will be with us for many years to come, provided we take sensible care over its production, use and development. Fundamental to a continuing demand for bitumen is the need for road transport, which has come under the spotlight because of concern about emissions of carbon dioxide. The most efficient methods of reducing CO 2 is by cutting road vehicle fuel consumption via better engine technology and improved road networks – improved in the sense of carriageway additions and modification to achieve good traffic flow but also in the sense of durability and maintainability. And what better material is there with which to achieve all of this than asphalt?

The majority of roads within the European Union are built of asphalt. The main use of asphalt today is for maintenance. And good flexible maintenance assists with good traffic flow and improved fuel consumption, while providing higher levels of safety and noise reduction. So, to cut emissions in the future, more asphalt will be used, which means of course more bitumen.

Will bitumen continue to be deemed an environmentally acceptable binder in the future? Again, the answer has to be yes. The refining process for high performance binder consumes relatively little energy, which means bitumen will always be price competitive against any synthetic binder that emerges. Synthetic products are likely to be highly energy intensive and thereby costly, both in terms of energy use and emissions generated. It is hard to see any realistic alternative replacing 100Mt of bitumen, without increasing CO 2 emissions.

Bitumen is an inert, non leaching material; and eminently recyclable. Hot, warm and cold mixes are being developed that are technically and commercially competent, but which also reduce further the energy consumption and impact of the bitumen sector. As road building materials go, bitumen can be – and will be, in the future – regarded as sufficiently sustainable. Its environmental credentials bear scrutiny.

Of course, none of the above is relevant if the raw material from which bitumen comes runs out in the next few years. This will not happen. There is certainly enough cude to satisfy demand for the next 50 years, almost certainly for the next 100 years. The perspective of governments and petroleum companies worldwide on reserves is that these have not yet peaked, despite increases in consumption.

All that said, the availability of crude will diminish at some stage in the future. Just when depends on how soon the world’s consumers reduce their dependence on oil feedstocks for fuel. In the end, alternative materials will have to be developed for road building but until this time, bitumen will remain the construction material for roads. Our responsibility is to use the material wisely, through recycling, energy efficiency, long life pavements, good maintenance and the provision of good working environments.

Performance Programme: focusing on the future
Nynas has introduced a new concept for realigning its products to demonstrate how it is meeting clients’ need for delivery of value and functional requirements. The company has isolated trends that it believes are (or soon will be) affecting the way the bitumen industry does business. More of its customers are now seeking long term, cost effective solutions that are based on functional needs, requiring bitumen that can match varying demands of performance.

To find out more about the Performance Programme please visit the campaign pages on our website, at www.nynas.com/performanceprogramme.

Nynas Bitumen: the items below are extracted from Performance, the Nynas bitumen magazine.