Asphalt is Green

Raising environmental concerns, fuel costs and the steadily declining natural resources affect not only the cost of asphalt mixes per se, but is forcing the industry to go green. Today, every professional in the field understands that going green is not just a fashion statement or eyewash, but it is posing a very crucial question about survival. It is impossible to be profitable without being green.

Sustainability is no longer a ‘nice to have’ accessory for business – it is an essential requirement, and must be built into all our goals and policies. The benefits of working responsibly and sustainably are immediate as well as long term. Many of our customers are looking for us to demonstrate publicly our commitment to working sustainably and report on performance.

There are numerous methods that the road building industry has adapted to reduce their energy consumption and to be more carbon friendly. The urgency is so great that at each level starting form equipment design to production and laying techniques, focus has to be consistently placed on reducing the carbon emission.

This technical note describes the various techniques available and presently being implemented world over for production of Low energy asphalts or Carbon friendly mixes. Production techniques for warm and half warm mix asphalt either use chemical or mechanical solutions. The modifications made to existing plants in order to produce low energy asphalt by chemical methods (additives) are minor, whereas more significant modifications must be made to mechanical methods (foam bitumen or sequential coating). This paper covers the possible solutions and provides A summary on its advantages.

Beginnings

It is highly unlikely that we can put an exact date on the emergence of the concept. Since the mid 20th century, and the industrial development of hot mix asphalt (HMA), engineers have had a cheap energy source at their disposal with the environment only a minor concern. Good Practice stipulated that the asphalt had to be around 150°C. Since bitumen is hydrophobic, coating is that much easier when the aggregates are dry. The European production standard nevertheless states that the maximum residual water content of HMA is 0.5%. In the 1970s, when TSMs (dryer drum mixers at parallel flow) developed, excellent results at 130°C were nevertheless noted, with a residual water content of 1%, especially during compaction.

Coating attempts with foam bitumen subsequently developed in the 1990s in the UK, South Africa, the Netherlands and France. The concept of warm and half warm mix asphalt was first mentioned in the late 1990s, particularly by JENKINS [1], and alternative techniques to foam bitumen flourished in all countries, notably Scandinavia, with sequential coating techniques using several types of bitumen.

A typology of the mixes was officially described by Dr. Jacques Bonvallet from FAYAT Group in 2000 [2], with reference to the heating and drying energy used during the process. The 100°C border is widely discussed and there are two schools of thought: one that recommends staying shy of the 100°C mark for the sake of making vaporisation energy savings (1/3 of the total energy), and the other which recommends vaporising water while remaining within the 110 to 130°C range, which also saves around 30%.


The last decade, in Europe at least, has been characterised by an increase in methods – some of which are experimental. We have recently been seeing a segregation of processes that are becoming simpler and can be split into two families:

• chemical processes in which the use of an additive makes it possible to reduce the bitumen viscosity and perform high-quality coating;
• mechanical processes, which either involve sequential coating or coating with foam bitumen.

We can suppose that processes will become more industrial and develop considerably over the coming years. In France they certainly accounted for some 500,000 t in 2009 for 40 million tons of asphalt; the room for progression may be significant, but certain.

Low energy production

One trend that seems to be emerging is the production of warm mix asphalt around 120/130°C using bitumen with additives. The hypotheses put savings at between 10 and 20%.

Without recycling and depending on the humidity (ignoring any water additions), the calculated consumption levels of the burner are as follows:


Some techniques are based on temperatures below 130°C, and even below 100°C. Reductions of 40 to 50% in energy consumption or emissions are claimed, especially during low-temperature and/or low energy processes (LEP and LTP) [3], [4].

Several studies conducted jointly by inventors and the French Public Works Research Laboratory are under way, some equipping a production plant so as to be able to compare calculations to the test data.

Chemical solutions

These solutions require the injection of either solid or liquid additives into the mixture. These can be incorporated into the bitumen or during the mixing stage.

In all cases, the plant must be fitted with systems for storing, transporting and metering additives (flowmeter).


All of these processes implement coating of the same quality as HMA, whereas under standard conditions, the bitumen viscosity would not allow the aggregates to be properly coated. Adding water to monitor the water content is always necessary, and it is water that bestows workability and therefore the aptitude for compaction.

Mechanical solutions

These solutions are perhaps more complex and based either on the process using foam bitumen, injected with a system of injectors placed above the mixer, or on the creation of foam in the mixer over a very short timeframe in dispersed form. To do this, the company applies a sequential process, bringing the water and hot bitumen into contact in the mixture.

Several methods co-exist – the injection of foam directly by injector seems simpler and less expensive to implement, for less investment. Fairly exhaustive laboratory studies were published in 2008 on this matter [5].

The peripheral equipment required is fairly simple and shown in photograph 3 and 4.

The Ultimate sustainable mix production

The combination of low energy with recycling is a worthwhile solution likely to be pushed to the extent of obtaining high recycling levels. It seems to us that the most suitable equipment and – in all cases – the most flexible for meeting these objectives is that which comprises two drums and an external mixer, like the RMS TSR in photograph 5.


Since the energy consumed for the RAP on the one hand and the virgin materials on the other is controlled in two drums, there is total independence and optimised rational management.

Advantages of Low energy mix production

• T° decrease by 30 or 40°C
• Substantial reduction in CO₂ emissions
• Combustible savings: 15 to 30%
• No bitumen vapour emissions
• Hot-mix easy-to-use
• Possible high recycling rate

Is India ready?

Most of the mechanical components required for various techniques used for low energy mix production can be added on to any advanced asphalt plant as an add-on kit. Processing of RAP requires that the plant is supplied with adequately designed dryer drum, bag house filter, burner system, mixer unit, process automation and control systems. The challenging part of the operation is to add and mix the special chemical / sequence in the already fully automatic process of asphalt mix production.

Marini’s global experience and expertise in the field allowed them to anticipate the raise of these technologies in India and hence the plants supplied in India are already configured with the required accessories and process automation, making production of these energy saving mixes instantly adaptable in India.

Comment on warm mix asphalt compaction

The influence of the temperature on asphalt workability has long been known, and summarised in the Table below from the manufacturer BOMAG. We can particularly see that the necessary energy for obtaining an equivalent compaction between 120°C and 150°C is multiplied by 4.


In reality this means a higher compaction requirement, either through more passes or by using a more powerful compactor.

The solution adopted in Europe to monitor the subject firstly involves controlling the residual water – which improves workability and therefore compaction in a small amount – and secondly using variable-energy compactors belonging to the category of so-called smart compactors, such as Asphalt Manager [6].

Conclusion

We are going through a pivotal period in which the significant rise in sustainable development is completely changing the way we perform analyses. Carbon footprints are still incomplete and our profession is aware of the long way we still have to go. That said, use of eco-software is on the rise, allowing for more accurate analyses. The next step is unquestionably the experimental validation of these analyses. Manufacturers of both continuous and batch mix plants are now offering tried and tested solutions, and are willing to improve them. It seems to us that French techniques, particularly combining recycling, integrated humidity management, low energy and – for major projects – in-place continuous production, are producing an arsenal of adapted solutions that have been developed over the last decade and from which our Indian customers can benefit usefully and above all swiftly today. With these new types of mixes, the color of asphalt can be green, both in terms of profitability and sustainability.

References

• Mix design considerations for cold and half Warm bituminous mixes with emphasis on foamed bitumen. Kim Jenkins, Thesis September 2000, Stellenbosch
• IRF Congress in PARIS 2000, Les mousses de bitume un technique émergente incontournable. J. Bonvallet
• Enrobage à basse énergie (EBE) aux performances des enrobés à chaud. A. Romier, M. Audéon, J. David, Y. Martineau. RGRA No. 831 September 2004
• Les enrobés à basse température: EBT. F. Ollard, C. Le Noan, P. Huon. RGRA No. 846. February 2006
• Effect of process parameters on foam bitumen based Road Material production at ambient temperature Road Materials and pavement Design journal Cazacliu, Peticila, Colange, Leroux, Bonvallet, Blaszczyk. 2008
• BOMAG Compaction guideline. Handbook Boppard.